Literature review on terrestrial broadband VHF radio channel models

www.B-VHF.org

Project co-funded by the European Community within the 6th Framework Programme (2002-2006)

REPORT D-15 Literature Review on Terrestrial Broadband VHF Radio Channel

Models

PROJECT NUMBER: AST3-CT-2003-502910

PROJECT ACRONYM: B-VHF

PROJECT TITLE: BROADBAND VHF AERONAUTICAL COMMUNICATIONS

SYSTEM BASED ON MC-CDMA

INSTRUMENT: SPECIFIC TARGETED RESEARCH PROJECT

THEMATIC PRIORITY: AERONAUTICS AND SPACE

PROJECT START DATE: 01.01.2004

DURATION: 30 MONTHS

PROJECT CO-ORDINATOR: FREQUENTIS GMBH (1) (FRQ) A

PRINCIPAL CONTRACTORS: DEUTSCHES ZENTRUM FÜR LUFT UND RAUMFAHRT E.V. (2) (DLR) D

NATIONAL AIR TRAFFIC SERVICES (EN ROUTE) PLC (3) (NERL) UK

LUFTHANSA GERMAN AIRLINES (4) (LH) D

BAE SYSTEMS (OPERATIONS) LTD (5) (BAES) UK

SCIENTIFIC GENERICS LTD (6) (SGL) UK

UNIVERSITEIT GENT (7) (UGent) B

UNIVERSIDAD POLITECNICA DE MADRID (8) (UPM) E

PARIS LODRON UNIVERSITAET SALZBURG (9) (UniSBG) A

DEUTSCHE FLUGSICHERUNGS GMBH (10) (DFS) D

UNIVERSIDAD DE LAS PALMAS DE GRAN CANARIA (11) (ULPGC) E

DOCUMENT IDENTIFIER: D-15

REVISION: 1.1

DUE DATE: 26.10.2004

SUBMISSION DATE: 31.01.2005

LEAD CONTRACTOR: BAES

DISSEMINATION LEVEL: PU - PUBLIC

DOCUMENT REF: GSY/040340/106329

Contract number: AST3-CT-2003-502910 Report number: D-15 Issue: 1.1

File: D15_Lit-Review_11.doc Author: BAES

Copyright B-VHF Consortium Page: I

History Chart

Issue Date Changed Page (s) Cause of Change Implemented by

Draft 0.1 04/10/2004 All sections New Document BAES

Draft 0.2 18/10/2004 All sections Editorial corrections based on internal and project partner comments

BAES

Draft 0.3 10/11/2004 Sections 5,6 & 8 Further references added

BAES

1.0 22/11/2004 None Review within Consortium

BAES

1.1 13/12/2004 Front Page Released FP6 reporting guidelines

BAES

Authorisation

No. Action Name Signature Date

1 Prepared R. Larsen and J.D. Milsom 09/12/2004

2 Approved J.C. Currie 10/12/2004

3 Released C. Rihacek 13/12/2004

The information in this document is subject to change without notice.

All rights reserved.

The document is proprietary of the B-VHF consortium members listed on the front page of this document. No copying or distributing, in any form or by any means, is allowed without the prior written agreement of the owner of the proprietary rights.

Company or product names mentioned in this document may be trademarks or registered trademarks of their respective companies.

CCMU



Copyright B-VHF Consortium Page: II

Contents

1. Introduction .................................................................1-1

2. Executive Summary.......................................................2-1

3. Scope..........................................................................3-1

4. General Approaches to Time-Varying Radio Channels .........4-1

5. Terrestrial Mobile Radio Channels ....................................5-1

6. Aeronautical Mobile Channel Models.................................6-1

7. Satellite-Ground Channel ...............................................7-1

8. Air-Ground Propagation Measurements.............................8-1

9. Implications for B-VHF Channel Modelling .........................9-1

10. Summary & Conclusions...............................................10-1

11. References .................................................................11-1

12. Abbreviations .............................................................12-1



Copyright B-VHF Consortium Page: 1-1

1. Introduction

Air transport has been identified as a dominant factor for sustainable economic growth of the European Union. The "Vision 2020" clearly points out the cornerstones of a future air transport system and the Advisory Council for ATM Research in Europe (ACARE) elaborates these requirements in depth in their "Strategic Research Agenda".

A/G communication is the key enabler for achieving an Air Transport System that is capable of meeting future demands. The communications in the VHF aeronautical communications (COM) band (118 - 137 MHz) are particularly attractive as they provide adequate coverage with moderate equipment power and acceptable price.

Today, an analogue VHF voice communications system is still used for tactical aircraft separation and guidance. This communications technology has been introduced in the '40s and generally utilises the available VHF spectrum in an inefficient and inflexible manner. A small part of the COM spectrum is used by several types of aeronautical data links (ACARS, VDL Mode 2, VDL Mode 4) for safety-related data link communications.

After 2010, the VHF COM band in Europe is expected to become progressively saturated. This is expected to happen in spite of the recent introduction of the 8.33 kHz DSB-AM voice system and the VDL Mode 2 data link that both use the VHF spectrum in a more efficient manner than the "old" solutions. The main reason for the saturation is the traditional ATM operational concept based on the tactical control of aircraft that generates increased demand for voice communications channels proportional to the increase in air traffic itself.

The problem can only be solved by adopting new ATM concepts. Strategic European documents and recent studies indicate that a relief after 2010 may be achievable with intensive usage of the aeronautical data link. The tactical Air Traffic Control (ATC) will shift towards strategic Air Traffic Management (ATM), and at the same time the demand for new VHF voice communications channels would be reduced.

Today�s VHF solutions � including VDL Mode 2 data link - cannot fulfil performance and capacity requirements of future data link applications.

As there are no plans to deploy VDL Mode 3 system in Europe, VDL Mode 4 remains as only European option to replace VDL Mode 2 data link in the future. VDL Mode 4 as a pure data link technology without support for voice communications is capable to solve only a part of the congestion problem. In order to provide expected data link capacity, VDL Mode 4 would require multiple VHF channels that are difficult to find and co-ordinate. As there are still some unresolved architectural issues, there is no guarantee that VDL Mode 4 airborne radio can be operated without interference with analogue VHF voice radios.

EUROCONTROL�s Communications Strategy clearly points out the need for alternative communications systems. Air Traffic Service Providers (ATSPs) prefer keep on using their existing ground communications facilities, so an integrated voice-data system in the VHF range would be highly appreciated, being capable of using same physical locations of ground stations and same interconnecting infrastructure as the current VHF system. Therefore, more and more attention in Europe is directed towards broadband VHF technologies.

Within the course of the B-VHF project bottom up research on multi-carrier technology (MC) for aeronautical communications is carried out. This work will result in the definition of a new future MC broadband VHF (B-VHF) system, which is able to support Single European Sky, Free Flight and other advanced concepts and programmes, leading far




beyond 2015 into Vision 2020. The B-VHF project is conducted under Priority #4/ Aeronautics and Space of the Sixth Framework Programme (FP6) of the European Commission (EC).

The target technology is MC-CDMA, a highly innovative, high capacity technology that is also discussed for fourth generation (4G) mobile communications systems. However, the project will investigate possible implementation outside the VHF range, as well as non-CDMA access schemes.

The B-VHF system has the potential to exploit the mobile VHF aeronautical channel better than any currently discussed VHF communication alternative. It increases voice and data capacity and addresses security and safety issues, promising a service level that is today unknown to the aeronautics user. Moreover, it has the potential to preserve the excellent inherent cost-range characteristics of the VHF band. It may eventually be applied as an overlay system and co-exist with the available VHF infrastructure, providing smooth transition and rollout scenarios.

The proposed B-VHF system will support both voice and data link communications. The main expected benefits of the future B-VHF communications system are:

! High spectral efficiency - the broadband B-VHF system uses VHF spectral resources more efficiently than today's narrowband VHF communications systems

! High communication capacity - the total capacity of the B-VHF system is higher than the aggregate capacity of VHF systems deployed today or planned for a near future

! Flexibility - the B-VHF system may be easily adapted to provide support for new operational and communications requirements

! Security - the B-VHF system is inherently resistant against narrowband jamming and provides mechanisms supporting end-to-end data security

! Sound transition path - the B-VHF system uses the knowledge about the current usage of VHF spectrum and may be able to share the VHF spectrum with legacy narrowband VHF systems without adverse interfering effects

The high-level goal of the B-VHF project - proving the feasibility of the broadband MC-CDMA technology and demonstrating its benefits to the aeronautical community - requires a series of interrelated tasks that have been encapsulated as five separate workpackages in the B-VHF project:

! WP 0 � "Project Management and Quality Assurance"

! WP 1 � "B-VHF System Aspects"

! WP 2 � "VHF Band Compatibility Aspects"

! WP 3 � "B-VHF Design and Evaluation"

! WP 4 � "B-VHF Testbed"




Figure 1-1 summarises the detailed work breakdown of the B-VHF project, including main work packages and all sub-work packages:

Research, technologicaldevelopment and innovation

ProjectManagement

WP 1B-VHFSystemAspects

WP 2VHF BandCompatibilityAspects

WP 3B-VHFDesign andEvaluation

WP 1.1B-VHFOperationalConcept

WP 1.2ReferenceEnvironment

WP 1.3B-VHFDeploymentScenario

WP 2.1Theoretical VHFBandCompatibilityStudy

WP 2.2VHF ChannelOccupancyMeasur.

WP 2.3InterferenceModelling

WP 3.2PHY LayerDesign & SWImplementation

WP 3.3DLL LayerDesign & SWImplementation

WP 3.5B-VHFEvaluation

WP 3.4Protocol Design& SWImplementation

WP 0ProjectManagementand QualityAssurance

WP 0.1ProjectManagement

WP 0.2Validation andQM

WP 0.3KnowledgeManagement

WP 3.1VHF ChannelModelling

WP 4B-VHFTestbed

WP 4.1BasebandImplementation

WP 4.2VHF FrontendDevelopment

WP 4.3B-VHF TestbedEvaluation

Figure 1-1: B-VHF Project Work Breakdown Structure Overview

WP 0 "Project Management and Quality Assurance" comprises all activities that are essential to all work packages. It takes care of achieving high quality results throughout the whole project. It covers all management activities on Consortium level, in particular the information exchange and co-ordination with the European Commission and with the partners. A separate sub-work package has been destined for the validation and quality control which reflects the importance of maintaining high quality outputs in all project phases. Another sub-work package is dedicated to manage new knowledge generated within the B-VHF project in terms of intellectual property rights and dissemination strategies.

WP 1 "B-VHF System Aspects" establishes the necessary connection between the scope and goals of the B-VHF project and the high-level objectives of the EC, European and global aeronautical community. Starting at the very beginning of the B-VHF project, this work package will produce high-level requirements for the B-VHF system, describe the reference aeronautical environment and produce the B-VHF Operational Concept document. By the end of the B-VHF project, the WP 1 will produce the B-VHF Deployment




Scenario document, describing technological, operational and institutional issues of the B-VHF initial deployment, transition and operational usage.

WP 2 "VHF Band Compatibility Aspects" assesses by theoretical (modelling) and practical (measurements) means probably the most critical aspect of the future B-VHF broadband channel: its capability to be installed and operated "in parallel" with legacy narrowband channels, sharing the same part of the VHF spectrum, but remaining robust against interference coming from such legacy narrowband VHF systems. The investigations will also address the conditions for interference-free operation of the B-VHF system towards legacy narrowband VHF systems. The Theoretical VHF Band Compatibility Study developed in the WP 2 will provide inputs to the WP 1 required for the development of the B-VHF Deployment Scenario. Together with the B-VHF Interference Model developed in the WP 2, the Theoretical VHF Band Compatibility Study will also be used as input for the B-VHF system design and evaluation (WP 3).

WP 3 "B-VHF Design and Evaluation" covers B-VHF system design tasks, starting with developing the model of the broadband VHF channel, and proceeding with the development and implementation of the SW representing the physical (PHY) B-VHF layer, DLL layer, higher protocol layers and representative aeronautical applications. The design and implementation tasks will be augmented by the development of detailed evaluation plans and corresponding simulation scenarios. The B-VHF Evaluation Reports produced in the WP 3 will provide necessary feedback to the B-VHF Deployment Scenario task of the WP 1. The WP 3 will also produce as a deliverable a complete set of the B-VHF System Design and Specification documents.

The prime objective of the B-VHF project - demonstrating the capabilities of the MC-CDMA technology - will be achieved within the scope of the WP 3 by using intensive and layered simulation trials. This task will start with investigating the capabilities and performance of the B-VHF physical layer and will proceed by adding/integrating the DLL and upper protocol layers, respectively. The "generic" B-VHF technology validation will be concluded by considering specific requirements coming from the aeronautical environment and applications. The WP 3 will develop and implement a SW set of representative communications applications and verifies by simulation means that the B-VHF system can support a mix of such applications under nearly-realistic loading, while fulfilling the Quality of Service (QoS) and other requirements of each particular application.

WP 4 "B-VHF Testbed" covers the baseband implementation and evaluation of a first B-VHF testbed for both the forward- and the reverse-link. The implementation is carried out in DSP technology and is restricted to the physical layer, which is the most critical part of the B-VHF development. The B-VHF baseband implementation is interfaced to the low-power broadband VHF frontend, thus, enabling testbed evaluation not only in the baseband but also in the VHF band. Testbed evaluation in the baseband is performed using channel and interference models, which are also implemented in DSP technology. The VHF band evaluation is carried out in the laboratory using actual VHF systems as interference sources and victim receivers, respectively.

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2. Executive Summary

The subject of wideband radio channel modelling and simulation has a long history and is based on substantial theoretical foundations. The rapid growth of terrestrial mobile radio systems in recent years (and its huge commercial importance) has meant that much of the recent work has been applied in this field. There have also been significant efforts in the area of satellite-based land mobile communications.

In the field of cellular mobile radio, and especially in the context of within-building communication, there has been much effort recently to develop models that characterise the angular dependence of scattered energy, and also to describe the clustering of scattering centres. Whilst this degree of rigour is to be admired, it is questionable whether the added complexity is justified by the improvement in the results that could be expected in the context of B-VHF. Somewhat simpler approaches using geometrically-based stochastic modelling would allow us to include basic angular information, and we suggest that the channel simulator for B-VHF could adequately represent the multipath channel using this approach.

For the implementation of channel simulators the tapped delay-line structure is widely used. For the control of the delay tap coefficients recent work has tended to move away from the use of filtered Gaussian white noise sources, and instead the Rician �sum-of-sinusoids� technique has found favour. There are a number of techniques for implementing such an approach, each with claimed merits and drawbacks; further investigation of this topic is proposed for the development of the B-VHF channel simulator.

The use of the wideband channel simulator to represent various scenarios of interest to the B-VHF project (e.g. taxiing, air-ground, etc) will require the specification of time delay spectra and Doppler spectra for those scenarios. Much of the measured wideband data that has been published is either for other scenarios, for other frequencies, or both of these. This means that, although the information in the published literature can provide a useful starting point, some engineering judgement will still be required in the use of this data in our simulations.

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3. Scope

This document is the Literature Review on Terrestrial Broadband VHF Radio Channel Models prepared for the project (Broadband VHF). The document describes a literature review of publications in the field of wideband radio channel modelling in order to identify approaches, modelling techniques and results that may be advantageously applied to the simulation model of the proposed B-VHF system being developed within WP3.

This Literature Review on Terrestrial Broadband VHF Radio Channel Models report is structured as follows:

! Chapter 1 provides an overview of the project, its goals and position within European ATM research framework and provides a brief summary of the deliverable D28 itself.

! Chapter 2 is the Executive Summary of this document.

! Chapter 3 (this section) presents the structure of the D15 deliverable.

! Chapter 4 gives a description of the general approaches to time-varying radio channels.

! Chapter 5 describes the work carried out recently on modelling of terrestrial wideband radio channels for the development of mobile radio, cellular systems, 3G, MIMO systems, etc.

! Chapter 6 reviews work that has been carried out specifically for aeronautical radio channel modelling, primarily at VHF, but also work in other bands considered relevant.

! Chapter 7 considers the relevance of published papers on satellite to ground radio channel modelling.

! Chapter 8 reviews papers providing data from actual measurements of air-ground radio link parameters such as path loss, fading rates and depths, Doppler shifts etc.

! Chapter 9 considers the results of this literature review, and extracts from it the points of most relevance to the B-VHF project.

! Chapter 10 summarises the main points of this D15 report, and the conclusions to be drawn.

! Chapter 11 lists the reference documents used for producing this deliverable D15.

! Chapter 12 lists abbreviations used throughout this document.

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4. General Approaches to Time-Varying Radio Channels

This Chapter briefly reviews a few key papers describing the essential points of radio channel modelling.

Bello [Bell 63]

This is one of the seminal papers on radio channel modelling, and much of more recent work is based to large extent on the concepts and principles proposed here. At the outset it is shown that time-varying linear channels (which may be regarded as filters) may be characterised in a useful symmetrical manner in the time and frequency domains by writing the system functions (which describe the behaviour of the received signal in terms of the transmitted signal and the channel characteristics) in (time-frequency) dual pairs. From this representation is developed a statistical characterisation of randomly time-varying channels by the use of the correlation functions of the system functions. The author gives separate discussion of channels which (a) are wide-sense stationary (WSS), (b) comprise a number of components which have uncorrelated scattering (US), or (c) are both stationary and uncorrelated, and hence are described as WSSUS. It is demonstrated that the WSS and the US channels are time-frequency duals. Previous work had mainly considered the WSSUS channel only, and so the approach used here which considers separately the properties of the WSS and the US channels is rather more general than previous treatments.

The author notes that when trying to apply these principles to practical radio channels, it is often found that the channel will have large-scale slow fluctuations superimposed on the small-scale rapid variations that are most readily described by the system functions. For this he introduces the concept of the quasi-WSSUS channel, in which the transmission characteristics (e.g. system error rate) may be determined over sufficiently short intervals that they may be considered stationary, and the long term behaviour is captured by averaging the short-term error rate behaviour over the long-term fading statistics of the channel. He then resorts to further mathematical fundamentalism in order to put these concepts onto an unshakeable foundation.

The paper then gives considerable space to the problems of adequately representing radio channel models when (as in most practical situations) there is discrete sampling in either time, or frequency (or both) and there may be additional constraints, e.g. of finite time duration or limited bandwidth, for the observations. A number of sampled channel models are discussed, and the paper also introduces channel models which use a power series representation in the time or frequency domains.

Applicability to B-VHF

This is a very meaty paper, which, even though it is now 40 years old, still provides the basis for much current work, and deserves the effort and time required to digest it.

Bello [Bell 64]

This paper is really a companion to [Bell 63] and provides an expansion of some of the concepts outlined there. It is applicable to communication signal processing networks which may comprise various basic elements such as filters, mixers, delay lines etc. The time-frequency duality concept is useful because two situations which may be physically different may have identical behaviour except for the interchange of the role played by time and frequency. As a result, it is sometimes possible to find the solution to a problem




by replacing variables and quantities by their duals, if the solution in the alternative domain is known or more tractable. The application of these concepts is illustrated by considering the problem of measuring the transfer function of a scattering medium by means of an optimal gating operation prior to spectrum analysis. New ideas and additional insights may be generated by the use of this concept. The example is given of time-varying linear channels, which may be characterised symmetrically in the time and frequency variables by defining dual system functions.


Perhaps this paper is not immediately applicable to B-VHF, but it is useful background information.

Hoeher [Hoeh 92]

The starting point for this paper is the WSSUS channel description, which is taken as given; the first section is a brief overview of the channel functions and their inter-relations as described by [Bell 63]. For the channel model simulator it then describes a Monte Carlo approximation to a discrete representation of the system impulse response. In contrast to the more usual simulation approaches which use a white Gaussian noise source followed by filtering to achieve the desired spectrum, this approach essentially assumes a number of discrete scattering sources and generates a signal component for each one; the individual components are then summed to give the resultant received signal. In order to give the desired characteristics a number of scatterers are assumed (up to 25 is suggested as a useful starting point), and for each one a random phase, Doppler shift and delay are calculated. These are generated from uniform random numbers, which are then filtered through a non-linearity which is designed to be the inverse of the required cumulative distribution function. The simulator described does not involve a specific amplitude term for each of the multipath components, and thus gives a phase modulation form of simulator, whereas the more conventional form of simulator (involving multipath components with varying amplitudes) would essentially generate a quadrature-amplitude form of simulator. Using these principles the author derives some example discrete-time channel models for receivers with an optimum matched-filter and also for an unmatched filter.

Compared with the conventional filtered Gaussian noise approach, this type of simulation is claimed to be faster to run, easier to implement, the realisation is closer to reality, and the result is more flexible to use. One possible disadvantage is that more effort is required to specify the non-linear filters which convert uniform random variates into the coefficients that will generate the required distribution functions; however references are given which provide guidance on ways of doing this.


This paper provides the basis for the simulation methods described in [Hoeh 99] for aeronautical channel simulation, and thus it is of direct importance to the B-VHF project.

Yip & Ng [Yip 95]

This paper describes another Monte Carlo method (MCM) of simulating Rayleigh-fading WSSUS channels. It is a development of the methods described in [Hoeh 92], but here, instead of generating the MCM variables for all the Rayleigh-faded paths in the whole multipath delay profile, the authors propose a technique whereby the MCM approximation can be applied directly to the tap gains. As the number of delay taps is expected to be




significantly smaller than the total number of multipaths required for a good simulation, the authors claim significant reductions in the amount of computation required. In their comparisons of the two approaches they also claim better accuracy for their new simulation approach, but this comparison appears to be made under conditions of �roughly the same computation effort� � the precise basis for this comparison is not entirely clear.


This paper provides an interesting alternative to the methods of [Hoeh 92] for implementing a channel simulator, and deserves consideration. However, it is not clear if sufficient information is provided to give it an unconditional endorsement.

Patzold et al [Patz 02]

This paper starts with the assumption that the discrete-time tapped delay-line model is a suitable form for the simulation of wideband multipath fading channels. Such a model can be viewed as a transversal filter with time varying tap gains, and the problem addressed here is the determination of the tap parameters in such a way as to model the required (either measured or specified) multipath power-delay characteristics as accurately as possible. Five different methods of calculating the tap coefficients are investigated - the method of equal distances (MED), the mean-square error method (MSEM), the method of equal areas (MEA), the Monte Carlo method (MCM), and the Lp-norm method (LPNM) (most of these methods have been discussed by the author in greater detail in other papers). Analytical procedures are derived for each of the methods to show how the coefficients are obtained for an exponential delay profile (representing an urban mobile scenario), and the performance is evaluated with respect to the frequency correlation function, the average delay and the delay spread. Under the test conditions the best performance was given by the LPNM method, and the worst (quite significantly) was the MCM. (In discussing the Monte Carlo method the authors cite both [Hoeh 92] and [Yip 95]; it is not clear from the text exactly which form of MCM method the authors have implemented for his testing.) The behaviour of the methods with varying number of delay taps was also investigated, and again the results were quite striking - the LPNM method clearly converged to the required parameters with only a few taps, whereas some of the alternatives performed poorly even with many delay taps. It was suggested that the LPNM method works so well because it exploits the full flexibility of the tapped delay-line structure by optimising 2L parameters (where L is the number of taps), whereas in most other methods L parameters are pre-defined and they optimise only the remaining L parameters.

The application of the LPNM method to some other example channels was illustrated, and again good results were obtained. It was noted that the computation effort required for the LPNM method was somewhat higher than for the alternative simpler methods, but this was not considered to be a significant problem with currently available computing facilities.


The study and conclusions of this paper are highly relevant to the questions facing us in the B-VHF project, and deserve very serious consideration in our selection of an appropriate technique for implementing the channel simulation.




Patzold [Patz 03]

This paper is concerned with the accuracy of representation achieved by various types of simulation model in which the Rician �sum of sinusoids� technique is employed (the Monte Carlo method used in the wideband model of [Hoeh 92] is one such technique). In the early days of channel simulation efforts there was widespread use of filtered Gaussian white-noise generators in order to control the tap gains of a conventional tapped-delay-line channel simulator. However the �sum of sinusoids� principle is becoming more popular in preference to the filtered Gaussian noise technique for controlling the taps, as it enables the design of more efficient and flexible simulators. It can be used for simple time-varying channels, frequency selective channels or for elaborate space-time multipath fading wideband channels. However, these various techniques are not without some potential drawbacks in terms of simulation fidelity, as discussed in some of the author�s (many) previous papers. In this paper the author looks at the �sum of sinusoid� technique and investigates the conditions under which such a procedure can result in a channel simulator which is both stationary and ergodic.

Depending on whether the model parameters are random variables or constant quantities, altogether seven different classes of simulator can be defined. It is shown that if, and only if, the phases are random variables and the other parameters (Doppler frequency and time delay) are constants, then the channel simulator is both stationary and ergodic. If the frequencies are also random variables, then the simulator is stationary but non-autocorrelation ergodic. If the gains are random variables but the other parameters are constants then the simulation is non-stationary.

A number of published simulation techniques are then evaluated for their simulation fidelity. Several of these methods are ones that have been described by the author in previous publications.


This work provides an important input to the B-VHF project in terms of selecting an appropriate method for implementing the channel simulator.

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5. Terrestrial Mobile Radio Channels

Much work has been carried out recently on modelling of wideband radio channels for the development of mobile radio, cellular systems, 3G, MIMO systems, etc. This chapter will survey some of these papers that may offer insights for the modelling of the B-VHF system.

Gans [Gans 73]

The author notes that the mobile radio channel can be viewed as a super-position of slow variations ('shadowing') and rapid fading due to scattering / multipath interference. Slow variations may be countered by an appropriate choice of transmitter powers and fade margins etc, but rapid deep fading will radically affect communications quality. The author is only concerned with rapid fading - the short term 'local' effects - in this paper. He derives basic statistical properties of short-term variations in the transmission coefficient of the mobile channel as functions of time, space and frequency, and gives some example applications.

The initial sections review the results previously given by [Clar 68] for the specification of the channel transmission coefficient, but use derivations based on Rice fading statistics (1st and 2nd order moments of power spectrum [Rice 44] ), rather than summations of plane waves.

An expression is derived for the fading power spectrum for an arbitrary angular distribution of received waves, and then (for uniform angle of arrival) it is shown how the results are obtained for various assumed antenna polar diagrams on the mobile receiver. The author notes that one may think of the fading spectra with different antennas as filters through which the same white noise is passed. Autocorrelations in time may be translated into spatial correlations via the distance = speed * time relation.

The paper also calculates the cross-correlations of the three orthogonal components (Ez, Hx and Hy) of the electromagnetic field, and shows that they are all uncorrelated, and hence could be used for different branches of a diversity system.

The Doppler power spectrum of the signal received at the base station of a mobile system is computed. In this case, in order to derive the spatial correlation from the time correlation, one must assume that the mobile is fixed and the base station is moving; with the appropriate angular conditions, one obtains a double U-shaped fading spectrum for the signal received at the base station.

Following a discussion of signal level crossing rates and fade durations, the paper notes that variations of phase in the transmission coefficient appear as random FM in the receiver; the probability density and the spectrum of the random FM are shown.

The distribution of delays may be seen as the dual of the frequency power spectrum.

The paper quotes measurements showing a delay spectrum that could be approximated by a Maxwell distribution (observed in NY city); there are also simpler approximations to observed delay spectra using exponential or delta functions.


Useful background information, but perhaps not of immediate importance to the B-VHF project.




Parsons & Bajwa [Pars 82]

The authors note that in the mobile scenario the radio waves may be considered to undergo a two-stage process - (i) reflection / diffraction from large non-local terrain obstacles, followed by (ii) scattering from local surfaces near to mobile. The radio waves thus arrive at a local cluster of buildings with random amplitudes, phases and delays. In the local environment these waves are then broken up (scattered) into multiple paths having approximately the same differential delay but random carrier phases. The mathematical models describing the propagation mechanisms are referred to [Clar 68] and [Gans 73].

The channel characterisation starts from an assumption that it can be represented by a superposition of randomly varying multiple paths; this leads to a 2-port filter with a randomly time-varying transmission characteristic, and one may use the Bello representation of the impulse response [Bell 63]. The paper also discusses the other system functions of Bello, and how they are related by Fourier transforms. The authors note that in mobile radio the received signal shows temporal stationarity only over distances of a few wavelengths; changing environments as the mobile moves can lead to gross non-stationarity. It is possible to deal with this by postulating a mixture of amplitude distributions and an event-dependent modified delay distribution (as discussed in [Turi 72], [Suzu 77]). However it is simpler to devise a 2-stage model in which the characterising functions are stationary over a small area, and the variations over a larger area are taken into account by allowing the moments of the distributions (mean, correlation functions) to vary.

A study of the forms of the scattering functions (especially the delay / Doppler autocorrelation fn), shows that the spectral content at different Dopplers is uncorrelated, implying that scattering is from a distinct and uncorrelated angular distribution of sources.

WSSUS channel descriptions are taken from [Bell 63]. The authors note that delay / Doppler power spectral density can be identified with a 2-D scattering function (which is a function of time delay and Doppler), analogous to the scatter cross-section (σ) used in radar scattering. Observation of σ not only allows speculation on the location of the scattering centres, but also on their nature - a scattering function with spikey delta-functions in the delay-Doppler domain will indicate a channel which is WSSUS - it can then be modelled with randomly distributed discrete scatterers.

The remainder of this paper is about channel sounding techniques; the authors note that although the channel system functions are equivalent, in practice it is only useful to do sounding in the time / time delay (channel impulse response) or frequency / time (swept CW) domains.


Useful background information on channel modelling techniques.

COST207 [C207 89]

The COST207 project was set up to co-ordinate research activities into digital land mobile radio communications in preparation for the introduction of the GSM mobile radio system. The project investigated various aspects of radio propagation and communications modelling, and developed channel models suitable for studying the behaviour of radio links in the land mobile environment. A conventional WSSUS channel




representation and a discrete-time tapped delay line simulator structure was employed, and measurements were undertaken in a variety of environments and countries in order to establish appropriate parameters to describe the Doppler spectra and the time delay spectra.

For the Doppler spectra four different representative shapes were identified:

(i) A classical ( 'Jakes type' [Jake 74]) spectrum to be used at low time delays (< 500 nsec)

(ii) A sum of two Gaussian functions to be used for path delays between 500 nsec and 2µsec

(iii) An alternative sum of two Gaussians to be used when the path delay exceeded 2µsec

(iv) For the shortest paths in a rural scenario a Rice spectrum is used - this is a combination of a classical Doppler spectrum and one direct path, equal in power to the total multipath power.

Various power-delay profiles were specified, again based on the measurements obtained during trials. These took the form of an exponential decay, with either a single maximum at zero relative delay, or, for the bad cases of hilly terrain or hilly urban areas, there would be a second peak at longer time delays (typically in the range 5 - 15µsec), again followed by exponential decay.

Examples were given of the recommended parameter values (i.e. the values of time delay and class of Doppler spectrum) to be used on each tap of a simulator. Parameter set were specified for rural area (4 or 6 taps), urban area (6 or 12 taps), hilly urban area (6 or 12 taps), and hilly terrain (6 or 12 taps). It was noted that an irregular spacing of the taps in time was to be preferred in order to prevent the frequency transfer function from having large periodicities, but this recommendation did not appear to be always followed in the given parameter sets.


This report provides the channel model parameter measurements that are invoked by many later workers in the implementation of their (otherwise purely theoretical) models. It is thus an important source of scarce environmental parameters.

Parsons [Pars 92]

This book provides a useful introduction to the characterisation of multipath phenomena in mobile radio systems in terms of the vector combination of a number of scattered / reflected components. It discusses short-term fading of the received signal in terms of the local path geometry and considers both the 2-D and the 3-D scattering cases. The author notes that the distribution of the angles of arrival of the received signal components may have a significant effect on the fading spectra of the resultant signal, and describes several possible models for the distribution of elevation arrival angle and their consequences for the RF spectrum. He gives a systematic development of the envelope and phase characteristics of the received signal, as they may be derived from the system functions that describe the multipath fading process. There is a useful discussion of the signal dynamics (level crossing rates, fade durations, distributions of phase differences etc) as they may be derived from the system autocorrelation functions. The author also considers the signal received at the base station of a mobile radio system, and discusses how the signal characteristics may differ from those of the signal received at the mobile due to the smaller range of arrival angles and the generally more distant centres of scattering.




The book gives a methodical development of the characterisation of wideband fading radio channels in terms of the systems functions, firstly for deterministic channels and then extends the treatment to randomly time-varying channels and the use of channel correlation functions. The treatment essentially follows that of [Bell 63], but in a tutorial style that makes the subject more readily accessible.


This work provides valuable background to the general task of channel modelling.

Fuhl et al [Fuhl 98]

The authors note that conventional channel models which describe the received field strength, power delay profiles, and Doppler spectra, whilst important for the analysis of systems with omni-directional antennas, do not give any information about the directions of arrival of signal components. The purpose of this paper is to give a generalised and unified model for macro-, micro- and pico-cell channels, with or without line-of-sight, and with or without movement of the mobile station.

The authors distinguish two classes of situation and models to describe them. Low-rank channel models are cases where the time dispersion is much smaller than the bit-period and the angular spread of arriving signals is much smaller than the beamwidth of the receiving antenna. High rank channel models are required for situations where either the time dispersion is comparable to (or larger than) the bit period, and/or the angular spread of arriving signals is comparable to (or larger than) the beamwidth of the antenna. For low-rank cases the channel model may comprise a line-of-sight component together with local scattering around the mobile. For high rank cases the channel model must take account of local scattering around both the base-station and the mobile, and must also consider signal components from distant scatterers; there may or may not be any direct line-of-sight component or specular reflections.

Distribution functions are suggested for the incidence of scatterers, and the resulting angular power spectra and correlation functions are derived. Sets of typical parameters are suggested for macro-cells, micro-cells and pico-cells; the parameters were adjusted so that the derived Doppler and delay-power spectra agreed approximately with the values reported in COST207. A methodology is also suggested for describing the effects of mobile terminal movement within a simulation, by keeping track of the scattering elements which enter and leave the active scattering regions.

Applicability to B-VHF: Whilst the treatment of pico-cells may not be of direct interest to B-VHF, the methods suggested for larger area cells are of more general applicability, and provide some useful insights into techniques that we could perhaps utilise. The paper does not go into great detail, but it is nevertheless of considerable interest.

Kuchar et al [Kuch 00]

The authors describe measurements of the angle-resolved power-delay-spectra for a 900MHz test signal in an urban location. They used a novel aperture synthesis technique which allows them to obtain very fine angular resolution (roughly 1º) in azimuth, and also good resolution of the elevation angle of arrival. The results showed that there was strong guiding of the signals along street canyons, nearly omni-directional local scattering, and scatter/reflection from more distant isolated structures. The elevation angle measurements also showed instances of strong fairly directional signals that arrived at high elevation angles, indicating that the principal energy in these cases was




being diffracted over nearby buildings. There were also convincing examples where multiple scattering must have made a significant contribution to the resultant signal.

Applicability to B-VHF: This work is at a much higher frequency than B-VHF, and it is concerned only with a dense urban environment. However it provides some very useful insights into the detailed behaviour of multipath propagation in cluttered environments, and should prove helpful in the B-VHF channel modelling. Note also that this paper is a companion to [Fuhl 98], and that these measurements may be used to support and justify the modelling approach described there.

COST259 [C259 01]

The COST259 project was directed towards the study of flexible wireless personal communications; in some respects it may be regarded as an extension of the work carried out in COST207. The channel model of COST207 was further developed to include signal component directional information as well as clustering of the multipath components. The modelling was intended to cover all macrocells, microcells and picocells as well as their corresponding environments. The introduction of angle of arrival information means that the channel impulse response is now a function of three variables - time, time delay and angle.

The model attempts to take account of the fact that radio propagation conditions are very dependent on the local environmental conditions. In order to generalise this, the COST259 directional channel model defines a 3-level structure. At the largest scale various cell types are identified � macrocells, microcells and picocells; these may be characterised by the so-called external parameters (EP), such as frequency band, base-station height and mobile-station height, building height etc. Within each cell type a number of Radio Environments can be specified; these may be characterised by a set of probability distribution functions and/or statistical moments. Since they describe the propagation conditions for the entire Radio Environment, they are called Global Parameters; typically their values will be obtained from measurements. Within a particular Radio Environment there may be a whole class of propagation constellations that exhibit similar features; each constellation will be described by a set of Local Parameters, the statistical properties of which may be related to the Global Parameters.

An approach to modelling the clustering effect of scattering in the mobile environment when directional antennas are employed assumes that signals received from a very local scattering cluster will be distributed uniformly in azimuth, but the components received from a more distant cluster will arrive from a limited range of azimuths, for which a Laplacian distribution has been suggested. In principle the methods of COST259 accommodate the possibility of varying elevation angles as well as azimuthal variations, although most of the examples in the literature consider only azimuth. Some methods of dealing with the simulation of such scenarios are outlined in [Yong 02] and [Aspl 02]. The latter paper also considers how to treat the dynamic aspects of the visibility of the scattering regions as the mobile moves, and the appearance and disappearance of cluster regions.

Further development of the concepts of clustered scattering, and the 'birth and death' of scattering clusters as the movement of the mobile causes them to become visible and then non-visible is given in [Piec 01]. The model described here is a combination of two statistical channel models, a geometrically-based single reflection model (GBSR) and a Gaussian WSSUS channel model; the model is also extended to include temporal variations. The scattering clusters are assumed to be small enough that there is effectively no time dispersion within them, and it is assumed for algebraic simplicity that




the angular variations are only in azimuth, although elevation angles could be included if circumstances required.

One of the problems with such models is the characterisation of the scattering centres, and the specification of their area density, locations, sizes and shapes. There are relatively few measurements which allow the determination of these characteristics for different environments, and the published examples which show the results from the models are usually based on a fairly speculative parameter sets.


This report provides an important view of more advanced channel modelling efforts; however the lack of appropriate environmental data describing e.g. scattered signal clustering, will make this difficult to apply in the B-VHF context.

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6. Aeronautical Mobile Channel Models

This Chapter reviews work that has been carried out specifically for aeronautical radio channel modelling, primarily at VHF, but also reviews relevant work in any other bands.

Bello [Bell 73]

This paper is concerned with cases of air-air, air-ground, air-space and ground-space communications, but the main emphasis is on the air-space geometry. The author notes that although the propagation mechanisms of surface scatter, ionospheric scintillation, ionospheric refraction, tropospheric refraction and tropospheric scatter may, depending on radio frequency and path geometry, be important contributory factors to the signal behaviour, in this paper only surface scattering is addressed.

The description of the channel characteristics in terms of the system functions (transfer function and impulse response) follows that of his earlier paper [Bell 63]. These are developed into the correlation functions for the complex signal envelope, which for the assumption of wide-sense stationary uncorrelated scattering (WSSUS) may be separated into one-dimensional autocorrelation functions which depend only on the fading frequency or time delay.

The basic channel modelling is then developed into a model specifically for the surface scattering applicable to the aeronautical communications channel. For plane flat surfaces the reflection is derived using Fresnel reflection coefficients (and divergence factor for low angle reflections from the curved Earth, where relevant). The author notes that for scattering from irregular surfaces, the character of the fading caused will depend on the scale of the fluctuations relative to the size of the first Fresnel zone on the ground; for large scale surface variations there may be phase modulation of the carrier at the same rate that the reflection point moves over the ground, but for smaller scale fluctuations the scattered signal will approximate to Rayleigh fading.

The treatment of scattering from rough surfaces is based on the concept of scatter cross-section per unit area of ground, as used in the evaluation of bistatic radar. For rough surfaces the scatter cross-section may involve the distributions of surface heights and surface slopes. From this description of the surface may be derived a definition of the glistening area in terms of the normalised roughness and the scatter geometry.

These surface scattering concepts are then combined into a system function model for the airborne communications channel. The received signal will be a combination of the direct, specular reflected and diffuse scattered components; it is noted that the delay of the direct signal itself will be time-varying, giving rise to rapid phase variations but slow Doppler changes. The importance of the specular component will depend on radio frequency, complex surface reflection coefficient, antenna discrimination and surface roughness. At VHF the specular component reflected from the sea will often be significant, but over land the complex reflection coefficient may vary from negligible to near-unity.

The author develops expressions for the Doppler power spectrum, time correlation function, delay power spectrum, frequency correlation function and scattering function for the case under study. However he notes that the model as described is restricted to cases with the terminals far from the glistening zone, with high elevation angles, to rough surfaces with small surface curvature, and does not include the possibility of wave shadowing in the treatment of surface reflections.





The model should thus be applied with caution in situations which do not match the assumptions in the underlying model. In particular the constraints that terminals should be far from the glistening zone and that elevation angles should be high describe only a few of the scenarios of interest in the B-VHF Project.

Painter et al [Pain 73]

This paper considers ground-air communications at VHF and L-band, and deals with surface reflection as a complex process that includes an arbitrary modulation function. It uses the textbook result [Beck 63] that the resultant signal is a complex Gaussian process with mean and variance determined by the physical properties of the scattering surface. Derives expressions for the received signal as the resultant of a direct + reflected / scattered component, which is represented by a complex reflection coefficient multiplied by modulation functions describing inter-symbol interference. This representation is described as a general model for selective multipath fading, but for the remainder of the discussion the assumption is made that the direct and reflected paths are sufficiently similar in time delay that there is no significant inter-symbol interference i.e. the discussion is restricted to the narrow-band case.

The paper then describes flight measurements (at 1436 MHz) over a calm ocean, swamps and fields, with grazing angles between 0.5 and 45°. His simulation model input parameters were adjusted until the predicted signal fading patterns looked rather similar to the measured data, and then noted that �the fitted parameters were consistent with visual observation of the reflection areas�. (Typical model parameters were: ground antenna height = 10-12m, ground conductivity σ = 0.5 S/m, dielectric constant of ground ε = 11, RMS roughness of ground = 0.1m, RMS surface slope = 0.16 rad.)


The theoretical element of this paper is a fairly standard exposition of textbook results. Unfortunately the experimental work was at L-band and it was also essentially a narrow-band trial with little environmental data. Hence this paper provides only limited supporting information for the B-VHF project.

Elnoubi [Elno 92]

The author is concerned with air-ground communications at VHF, and proposes a statistical model of surface scattering / reflection combined with a direct signal. The power spectrum of the diffuse component depends on the angular distribution of the scattered waves, and the paper gives an expression for the diffuse component fading power spectrum for the case where there is a uniform angular distribution of multipath components over a finite range of angles. The autocorrelation of the signal also involves the angular distribution of multipath; when the angle of arrival is uniformly distributed the signal autocorrelation is a Bessel function, but when the angle has other distributions there may not be a closed form for the signal correlation functions. In these cases it will be necessary to approximate with trigonometric functions and use Taylor expansion, although often this will lead to a satisfactory result. The paper then considers some particular examples, including a small range of angles (representing the use of a narrow-beam antenna), and 3π/4; however the notation is poorly explained and the discussion is a little too cryptic to be helpful.




Trials at 118-137 MHz (narrow-band) are briefly described, including taxiing, take-off, flight near the airport and landing. The author notes that the signal underwent Rician fading with a strong line-of-sight component for most of the time, but there was severe fading during take-off. He also gives a diagram of the measured fading spectrum, but unfortunately the annotation of the diagram seems to be at variance with the associated text.


It is unfortunate that the author's presentation of his measured data is so brief, because the trials addressed the band and application of direct interest to the B-VHF Project. Nevertheless the brief description of narrowband experiments might be a useful check on some aspects of the B-VHF Projects adopted model.

Chateau et. al. [Chat 97]

This paper describes an analysis of the received signal level and bit-error-rate performance of VDL2 and VDL3 links using a theoretical model of a D8PSK receiver developed by Thomson CSF. The paper considers different methods for modelling the effects of multipath fading and ducting on the received signal. The authors are concerned only with low data-rate systems, and so they neglect the effects of multipath delay (their symbol period is 95 microseconds, corresponding to a path length difference of 30 km).

Their simple model of BER performance depends only on S/N at the receiver input; it assumes that bit error rate is 1/3 of symbol error rate. Thomson-CSF had developed an SPW simulation tool, which included the transmit system, the propagation channel and the receive parts of the signal chain. The propagation part included multipath fading and ducting; it was not explained why the latter was considered to be an important topic for air-ground communications.

The authors describe various methods of characterising radio propagation phenomena, mostly in general terms without mathematical detail. Their discussion includes:

(i) A standard atmosphere above a dielectric ground - including the effects of a lateral surface wave - this is only relevant if both antennas are low and the distance is short, but it could be of significance for a layer of vegetation or snow.

This Geometrical Optics (GO) method can only be applied to (relatively) smooth surfaces. For rough terrain it is necessary to use the Uniform Theory of Diffraction (UTD), which takes account of corner and wedge diffraction as well as creeping waves. The authors have developed a ray-optical method (using phase addition of all identified ray path components) for dealing with irregular terrain profiles. UTD is more accurate than simple GO, but requires more computation effort, and all ray paths must be correctly identified.

(ii) Distortion of the antenna polar diagram by the presence of nearby surfaces.

(iii) Methods for estimating the field strength for a receiver amongst buildings; no details are given of their computation methods, but a diagram shows apparently good agreement between predictions and measurement.

(iv) Refractive ducting: Ray-tracing gives a useful qualitative picture, but breaks down in areas of low field-strength and near to caustics. Parabolic equation (PE) methods can give quantitative results, and are valid in the regions where ray-optics breaks down, but they are more difficult to apply at high elevation angles. Thus the two techniques (ray-tracing and PE) are, to some degree, complementary.




(v) Statistical methods for more complex environments: rather than use the deterministic approaches described above, Rayleigh or Rice fading models may be used, as appropriate.

Applicability to B-VHF: This paper is concerned only with the modelling of narrow-band path-loss for air-ground communications. A range of possible modelling techniques is outlined, but no details are given here of any of them. Whilst propagation loss characterisation is one component of channel modelling, the authors here seem to focus on a few rather specialised topics that are of secondary interest to VHF air-ground communications. This paper is only of passing interest to the B-VHF project.

Roturier et. al. [Rotu 97]

This paper is a companion to [Chat 97], which outlined some methods of estimating field-strength for VHF air-ground communications; this paper describes some experimental validation of the theoretical models.

A vehicle-mounted receiver was used to make measurements of the signal transmitted from the ATIS transmitter (118 MHz) at Toulouse airport. Signal level data were collected whilst the vehicle drove around a circular path that enclosed both the runways at the airport - a route which included areas with a clear line-of-sight to the transmitter, and also areas which were partially shielded by airport buildings. The observed signal levels ranged from close to free-space to almost 30dB below free-space.

The general slow variation of signal-level with vehicle position (so-called 'slow' or 'shadow' fading) was reasonably well characterised by the geometrical optics-type models. The general character of the fast fading was also fairly well approximated by the statistical description of Rayleigh fading.

As there were only a few large airport buildings, an attempt was made to use deterministic modelling to describe the detailed fast fading amongst the buildings. This was less successful, and there were discrepancies between measurement and modelling of up to 15 dB; it was thought that this was due to the simplifications that were necessary in order to run the model.

It was concluded that geometrical optics and statistical fading models were adequate when the receiver and transmitter were approximately within line-of-sight, but when the receiver was in a severely shadowed region then more advanced techniques would be required for a good estimate of received signal level.

Applicability to B-VHF: The measured data are of interest as they give practical examples of the signal characteristics to be expected when an aircraft is taxiing around an airport. The signal level estimation techniques are also of some interest, although they embody fairly standard textbook procedures.

Hoeher & Haas [Hoeh 99]

The authors propose a class of aeronautical channel models for use at VHF for various scenarios (parking, taxi, take-off etc) which use a channel simulator approach derived from the work described in [Hoeh 92]. Parameter sets suitable for the different scenarios are suggested. The models are wideband and stochastic, and characterise multipath propagation based on the Doppler power spectrum and the time delay power spectrum (i.e. the scattering function). The models take account only of small-area statistics, and it is questioned whether alternative models that attempt to also capture the influence of large-area statistics are really necessary for physical layer validation � it is suggested




that large-area behaviour may only be necessary for link budget design or the validation of traffic models.

Channel model parameters are proposed for various scenarios by assuming that the Doppler and delay spreads are independent, so that separate 1-D spectra can be specified for each. For the parking / taxi scenario the Doppler spectrum is taken from [C207 89]; 2-D isotropic scattering would give a classical �bath-tub� spectrum, whereas 3-D scattering would give a rectangular spectrum. Parameters for the delay spectrum are also taken from COST207 � the parking case corresponds to COST207 urban, whilst the COST207 rural case is assumed to apply to the taxiing case. For the worst case (parking) there is assumed to be no line-of-sight component and so the fading is Rayleigh, whilst for taxiing (represented by the rural) there will be some direct component and the fading will be Rician. The en-route case is also assumed to have a strong direct component, and the fading is again Rician with Rice K-factors (i.e. the ratio of direct-diffuse power) between 2-20 dB. The Doppler fading will be fast, depending on air-speed, with rates typically up to a few hundred Hz. For this scenario the scattering will no longer be isotropic, and the authors follow [Elno 92] in assuming that the angular distribution is uniform over a small range of angles. In the worst case the direct component arrives from straight ahead, whilst the diffuse scatter comes from directly behind the aircraft, giving maximum spread of the Doppler shifts. A wide range of maximum echo delays is quoted from the literature (up to 1msec is claimed), and some simple geometrical arguments are used to estimate representative values of delay � they claim 33 µsec for air-ground and 66 µsec for air-air are typical maximum delays for an aircraft at 10,000m height. The take-off and landing scenario is assumed to be somewhere between the taxiing and en-route cases. It is reasoned that, since this will not be a worst case, it is not necessary to give it more detailed attention.

The implementation of these channel models in a practical simulator of the type described in [Hoeh 92] is then described. First a flat-fading simulator is given (all time delays are taken to be zero), and then a frequency selective simulator is described. Examples are given showing how to generate the non-linear filter functions that are needed in order to create the desired cumulative distribution functions of Doppler and delay; cases include Doppler spreads for 2-D isotropic scattering, 2-D narrow angle scattering, 2-D scattering over an arbitrary angle, and a delay spread that is one-sided exponential. The claimed advantages of this simulation method over the traditional �white Gaussian noise + filtering� approach (speed, accuracy, ease of implementation, flexibility etc) are again enumerated, as in [Hoeh 92].


This paper is a major contribution. It builds on the much larger body of published work for land mobile services and applies it to frequencies and scenarios of direct interest in the B-VHF Project.

Eurocontrol [Euro 99]

This paper commences with a general discussion of the characteristics of air-ground radio communications, and explains in simple terms the causes and effects of signal fading, Doppler shift, multipath delay etc. The qualitative effects of various propagation scenarios on the delay and Doppler spectra are illustrated, and the paper gives some representative values of Doppler shift, Doppler rate of change (Hz/sec), Doppler spread and delay spread for typical air-ground and air-air links.




The second half of the paper describes an aeronautical channel model for VHF communications; it appears to be a re-statement of [Hoeh 99].


The work of [Hoeh 99] is the underpinning reference, but this paper does contain some channel and terminal parameters that might prove convenient in checking aspects of the future work in the B-VHF Project. The figures for rate of change of Doppler are a reminder that Doppler changes due to overflying aircraft could be a significant challenge to maintaining uninterrupted communication, see also [Haas 02].

Roturier & Chateau [Rotu 99]

Commences with a brief review of channel modelling work in the published literature, and notes that other studies have variously considered large scale variations (both stochastic and deterministic), small scale variations (both stochastic and deterministic), ground-ground (with and without line-of-sight), air-ground etc. However, no-one has covered all situations of interest and also validated their model against measured data. The aim of the paper is to synthesise a general model which combines a deterministic large-scale model with a stochastic model for small-scale variations.

The channel model used for the system characterisation includes both the transmit and receive antenna descriptions; this has the advantage that the analysis can be carried out using scalar variables for the transmitter output and receiver input, rather than time-varying vector fields which would have to be used if antennas were excluded from the channel model. The channel impulse response is written as a sum of separate impulse responses for the large-scale and small-scale behaviour. These may be transformed to get equivalent expressions in the frequency domain.

The models of large-scale fading for geometries involving a direct line-of-sight path were based on deterministic ray geometry, Fresnel zone sizes and Fresnel surface reflection coefficients. These cases included satellite-ground, air-ground and ground-ground where a clear line-of-sight existed between the two terminals. (It was assumed that for the case of ground-ground communication with no direct line of sight the large-scale variation of power would be log-normally distributed, and stochastic methods would be required.) Examples of measured power variations with distance in the LOS cases are compared with the predictions of the deterministic models that have been developed; generally good agreement is shown.

For the small-scale fading it is assumed that the channel may be characterised as WSSUS; it would then be sufficient to specify the delay spread & time variance of channel in order to fully describe it. A characterisation of the channel impulse response as a low-pass filter equivalent is introduced, this representation is taken from [Hoeh 99]. The author notes that when using this approach one needs to specify adequate probability distributions for the delay spread and Doppler. However, the paper points out that these models require little computing overhead, and so they are to be preferred over simpler representations. The paper notes that the adopted value of maximum time-delay will have a significant impact on the simulation results, and observations of this parameter seem to be either (a) sparse, (b) contradictory or (c) both of these.

The combination of large and small scale fading is accomplished essentially by adding together the two impulse responses. The paper notes that the signal is Rician, but the Rice K-factor itself varies with the evolution of the deterministic large-scale mean power (e.g. due to strong ground reflections, the deterministic signal may have rapid variations




of 20dB or more as the mobile terminal moves through peaks and troughs in the interference pattern).

The recommended steps in the construction of such a hybrid model are thus:

i) Determine (by experiment or scattering models) the small-scale average power

ii) From a deterministic model of mobile motion, calculate the mean power of large-scale fluctuations

iii) Calculate the value of K at each point in the path taken by the mobile (although one could take a short cut here by only looking at worst cases - areas where the large-scale power is low).


The emphasis of this paper is on the implementation of the large-scale fading model, although the small-scale fading is also briefly described. The experimental validation of the large-scale fading models is impressive, and we need to give serious thought to the question of whether this approach might bring benefits to the B-VHF project.

Haas [Haas 02]

This paper describes aeronautical channel models, giving relevant model parameters that are essentially for use at VHF. The paper appears to be an expanded version of [Hoeh 99]. A little more mathematical detail is given, the landing and take-off scenarios are addressed in more detail than previously, and taxiing and parking are treated separately, now being given somewhat different delay and Doppler spectra. The author also notes that the case of an aircraft over-flying a ground station is likely to produce a worst case for rate of change of Doppler, and gives some estimates of the values to be expected.

The channel model simulator of [Hoeh 99] is also described again in somewhat greater detail. In addition to the discrete-time realisation discussed previously, this paper describes a discrete-frequency implementation that may be useful for examining the performance of multi-carrier modulation systems. Examples are given of the calculated BER performance of an OFDM system under the various aircraft movement scenarios described (this used the time-domain implementation of the channel simulator). For the parameters used it was found that the parking scenario gave the worst performance; this was due to the absence of a direct LOS path, and the consequent deep Rayleigh fading of the signal.


A useful update to the material in [Hoeh 99] with a frequency domain model implementation and an application to the case of an OFDM waveform. This reference is especially relevant to the B-VHF Project.

Newhall & Reed [Newh 02]

The authors develop a 3-D geometric model of scattering from the ground, which takes account of aircraft elevation angle and evaluates the region of the horizontal ground surface which may contribute to the multipath scattering. Complicated 3-D ellipsoid geometry is used to derive expressions for the scattered intensity as a function of elevation angle, delay and angle of arrival (assumed to be in the horizontal plane). In principle the model could be extended to include (i) ground-air propagation losses ( eg free-space), (ii) ground - ground propagation losses (eg log-distance with log-normal




shadowing), (iii) ground scatter efficiency (cross-section), and (iv) a non-uniform distribution of scatterers in the area, but this is not done here.

The results of the modelling show that (even without these factors) the scattering region becomes more concentrated in azimuth towards the a/c location as the a/c elevation angle decreases towards the horizon. The paper gives graphs of intensity vs delay and angle-of-arrival. For a low path there is very strong local scatter, with narrow angular distribution for short delays, and weak scatter at longer delays but with a much wider spread in azimuth. For higher elevation angles there is a much larger range of delays which have significant scattering; the peak scattering comes from longer delays and there is a moderately wide angular spread at all delays.


This paper gives a very useful discussion of the geometrical factors involved in surface-scatter multipath. It may prove helpful in deriving the model parameters for scattering geometries for which we have little or no measured data to guide us.

Rice et al [Rice 04]

This paper presents a multipath channel model for wideband aeronautical telemetry links, and describes the results of some limited flight trials to validate the model. The model was a deterministic geometrical model that assumed a direct path and either one or two specular reflections; there was no diffuse reflected or scattered component in the model description.

The airborne measurements were obtained by transmitting a 10MHz pseudo-random sequence from the airborne terminal. Trials took place using both L-band and S-band transmissions. Only a few flights appear to have been undertaken, at heights of 2700 ft and 7700ft above ground. At the ground receiving station various large parabolic dishes were used to track the aircraft; these had beamwidths between about 3° and 6°, depending on frequency and dish size. This would have limited the scope for receiving multipath signals from large angles from the direct path. The received signal was sampled at 100MHz using a high-speed sampling oscilloscope, and the data was processed to obtain snapshots of the channel impulse response every 20msec.

The trials took place over a dry lake bed that formed a good reflecting surface. The measurements were compared with the model predictions by performing a least-squares fit to the model parameters, and selecting those parameters that gave the best results. For the geometry used it was found that a 2-ray model (direct + 1 reflection) gave a reasonable fit, but a 3-ray model gave the best results; little was accomplished by attempting to use more reflected components. The first reflection was roughly specular from the lake bed, but the second reflection appeared to be due to scattering or diffuse reflection from the foothills of a nearby mountain range. Delays from 11 to 87 nsec were observed for the first echo, and delays up to 300 to 400 nsec for the second (more diffuse) component.


This paper is of limited interest. The channel model was simplified to the point that any diffuse scatter components were excluded. In addition, the experimental validation used very high gain ground terminal antennas that are likely have suppressed signal components that would be of interest in the B-VHF scenarios.

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7. Satellite-Ground Channel

This Chapter reviews a number of published papers on satellite to ground radio channel modelling. Whilst the frequencies will generally be much higher than VHF, nevertheless a satellite may be regarded as a (very) high-flying aircraft, and some aspects of dealing with surface-reflection multipath and directional antennas may have lessons for this project.

Bergmann & Kucera [Berg 69]

Signals at 136 MHz are received from the NASA Applied Technology Satellites (ATS-1 and ATS-2) on board commercial jet aircraft. The aim was to acquire additional data for the validation of analytical channel models.

Some helpful graphs are included that present, based on simple geometrical considerations, selective fading null separation and delay separation as a function of aircraft position. Worst case multipath separation is 100 µs. Also of interest are the presented examples for installed VHF antenna radiation patterns. Significant (i.e. ~ 5-15 dB) nulls appear at relatively low elevation angles.

Theoretical results for fade depth due to multipath are presented. Assuming a smooth earth reflection the worst-case fade depths are estimated to be ~30 dB. Uncertainties arising from ionospheric propagation effects (polarisation rotation etc.) are acknowledged but ignored.

Signals received on trans-North American and trans-Pacific routes totalling 35 hours in duration are processed in order to estimate multipath delay spread information. Path geometries were as constrained by the available routes and satellite positions, but the following results were obtained:

! Occurrence and severity of fading was more marked on paths with elevation angles below 30 degrees than those above. Some typical values:

Elevation angles range (degrees)

(1 � 99)% fading depth (dB)

0 � 30 13.2

30 � 50 6.9

50 � 70 5.9

! One reported multipath delay separation was close to the two ray theoretical value (39 µs), implying that others would have proved likewise.

! Antenna noise temperatures were typically observed in the range (289 � 2135) °K. A typical value was around 1000 °K. Precipitation static discharge produced values as high as 7000 °K.

! Antenna lobing and differences in polarisation response are estimated to produce significant fading for small changes in elevation angle.


This work is particularly useful in the body of material on satcom channel modelling mainly because it addresses a VHF system, rather than one in L or S band. Some of the




figures for observed fading range and system noise might be useful in establishing the B-VHF channel simulations.

The material on lobing in the installed radiation pattern of airborne antennas is a useful reminder of this additional factor to be addressed in B-VHF�s modelling activities.

Bello & Boardman [Bell 73b]

The authors perform a theoretical analysis of the effect of multipath due to rough surface scattering on the accuracy of satellite ranging techniques using �tone ranging� modems. The technique was being considered for improving the accuracy of aircraft navigation. Antenna directivity could not be relied upon to suppress the ground reflected signal and so some assessment of its effect on navigation error was necessary.

No particular frequency band is addressed. The analysis draws on various available (but partially validated) model components, such as the theoretical reflection coefficients for diffuse surface scattering proposed by [Beck 63].

Mercifully the authors preface the ten or so pages of mathematical analysis with the numerical results of their efforts.


The paper contains no significant experimental work, but the authors� choice of some representative parameters might be of some modest value to future work on the B-VHF project:

! RMS sea surface slopes lie in the range 0.02 to 0.2

! Typical RMS sea surface slope 0.11

! RMS multipath delay spread of the ground reflected path varies between 0.13 and 13.25 µs.

Neul et al [Neul 87]

In this relatively old investigation by the German Aerospace Research Establishment (DFVLR) channel measurements were made on an L band (1540 MHz) transmission from a MARECS satellite and recorded on a twin engined jet aircraft. In all some 1000 minutes of data was collected during 17 flights. Flight paths were chosen in order to hold path elevation angles constant for the duration of each test.

It appears that no temporal resolution of multipath could be achieved with the satellite waveforms available. Instead an assumption is made that the path consisted of direct and ground reflected components only. Multipath resolution was achieved spatially by the use of directional antennas on the aircraft. In particular the ground reflected signal was captured via a downward-pointing helix antenna with 10 dB gain. A dual channel receiving system on the aircraft recorded separated direct and ground reflected signal paths. These recordings could be used in the laboratory as part of a "stored channel simulator".

The results reported are all for paths where reflections would be from the sea surface.

Results indicate a reasonably good correspondence with Fresnel specular reflection theory (with divergence factor) for the ratio of powers in the direct and reflected signal components.




A deterministic Doppler shift due to aircraft motion was not present, due to the constant-elevation raypath routes flown. However the residual Doppler power spectrum of the reflected component were observed to have a Gaussian p.d.f. in all cases, as predicted by theory. The authors present an expression for the RMS Doppler spread, but no sea-state information is offered with which to complete the comparison with measurement.

Fading of the reflected signal component was "always exactly Rayleigh distributed"

The remainder of the paper presents results for theoretical, simulated and measured BER for a particular transmission format. It is encouraging to see the good correspondence of measured and simulated performance, as one would expect given that the simulator uses stored channels.


If the author�s assumption of a two ray (two delay tap) model is accepted then this paper lends weight to the adoption of the following ingredients for B-VHF channel model for air-air communications when both aircraft are clear of takeoff/landing:

! A constant amplitude direct signal

! A Rayleigh fading ground reflection with:

# Relative time delay based on simple geometrical considerations

# Relative amplitude calculated from Fresnel theory

# Gaussian Doppler power spectrum with RMS spread (possibly) based on rough surface scattering theory.

! Deterministic Doppler shifts due to relative motion of the two aircraft.

Should this approach be adopted then the author's assumption that VHF ground reflected signals will exhibit Gaussian Doppler power spectra should be checked.

Jahn et al [Jahn 96]

This is a paper by authors from DLR and presents work aimed at extending the range of wideband channel measurements available at the time, with a view to establishing a standard set of parameters. The work was partly funded by ESA.

The measurement campaign was undertaken at 1820 MHz and involved a mobile terminal (handheld or car) and an aircraft substituting for a satellite. Right hand circularly polarised antennas were used at the ground terminal. The channel sounder on the aircraft was capable of recording approx. 16 channel impulse responses per second at a spatial resolution of 10 m. Generally, echoes were appearing with small delays in the range up to 600 ns. Their powers decreased exponentially with delay (linear in dB). A relatively small number of long-delayed echoes were sometimes evident. The major differences between the various environments explored (open, rural, suburban, urban and highway) appeared in the degree of shadowing of the direct signal and the number of near echoes.

The authors interpret the measurements within the framework of a wideband channel model described the previous year by Jahn et al. (the authors' ref. 9). One of Jahn's co-authors was Lutz, and the model is mentioned elsewhere in the literature as the "Lutz model". The delay profile is partitioned into three regimes:

! The direct path (A combination of: (i) Rician fading to represent times when the LOS path is present, and (ii) Rayleigh fading for periods when the LOS path is




blocked or shadowed. In the Rayleigh case the mean received power is assigned a mean power with log-normal p.d.f. References are made to papers that quantify the proportion of LOS/non-LOS states).

! "Near echoes" - 0<delay<~600 ns. (A random number of these was assumed, the number being selected by a Poisson distribution. In the near-echo regime the taps are distributed along the delay line with an exponential p.d.f. and the average absolute power of the echoes decreases exponentially with delay. All taps exhibit Rayleigh fading). The foreground environment is reckoned to determine the characteristics of these echoes.

! "Far echoes" - 0.6 µs<delay<15 µs. (A random number of these is assumed, the number being selected by a Poisson distribution. In this regime the taps are placed uniformly in the far echo delay range. All taps exhibit Rayleigh fading with identical average powers). The background environment is reckoned to determine the characteristics of these echoes.

Tables are presented giving the various input parameters, assuming a handheld ground terminal, for the three regimes of the Lutz model. Parameter sets are available for most combinations of environment (open, rural, suburban, urban, highway, hilly and mountainous) and elevation angle (15, 25, 35, 45 and 55 degrees).

The Clarke classical Doppler spectrum [Clar 68] is assumed to apply to the various taps.


This paper presents a good account of a relatively elaborate channel model attributable to Lutz. The valuable contribution provided by the authors are their estimates of model parameters based on a large body of measurements.

Some elements of the model might be re-used in the present application. In particular the rural/suburban environments might be judged sufficiently similar to an aircraft taxiing situation to be relevant.

A significant obstacle to making extensive use of this published work is likely to be the order of magnitude difference in radio frequency between VHF and measurement data used by the authors to parameterise their model.

Parks et al [Park 96]

This paper predates a related one [Bell 00], which is discussed below. Generally this paper is more concerned with details of the channel model, rather than the measurement technique and analysis. In the following commentary only the items not covered in the later paper are highlighted.

At its highest level of abstraction the mobile satellite systems channel model is divided into just two components: the Satellite Process and the Terrestrial Process.

Mention of the Satellite Process is a useful reminder that the modelling of even an ideal path which is devoid of scattering or multipath will require some essential elements. These must address: air-ground propagation delay, air-ground attenuation, antenna gains, and Doppler effects associated with the relative motion of the aircraft and ground terminal (in the air-ground case). All these effects are deterministic and can be fairly easily estimated.

The Terrestrial Process represents the complicating factors of multipath and fading/Doppler. A tapped delay line representation is adopted. The taps are placed to




represent "resolvable multipath propagation". The tap weights are time-varying complex factors. The process used to derive the tap weights has four components:

! A constant (coherent) term of amplitude (KRice) 0.5 relative to the incoherent scattered signal components.

! Bulk Doppler shift for the mobile ground station's motion relative to the source of coherent power in the tap-gain process.

! Addition of a random variable with a Rayleigh amplitude variation and "classical" (i.e. Clarke) spectrum.

! Multiplication by a shadowing factor associated with radio blocking near the mobile ground station.

Parameters for the wideband channel model are presented for L-band reception in urban environments. Note however that the shadowing factor is not quantified. The emphasis is on the "near echoes" that appear with delays less than 600 ns. Six equi-spaced taps covering 0 - 500 ns are suggested. The first two taps are assigned Rician p.d.fs, and the other four are assumed to be Rayleigh. "Far echoes" (with delays > 600 ns) are mentioned as being few in number, low power and Rayleigh in nature.

Clarke's "classical" Doppler spectrum is recommended as a worst case. However the spectra presented in the paper seem to bear no resemblance to the model, being more or less flat apart from additional noise-like variations.

Some validation of the model is made using independent data from DLR.


This paper contains some useful detail on the channel model architecture that is absent from the later publication by the same team [Bell 00]. However the same main reservation applies, i.e. application of the results is hampered by the order of magnitude difference in radio frequency.

Abdel-Hafez & Safak [Abde 98]

Although the ultimate aim of the authors is the estimation of bit error rates for DPSK/NCFSK modulation schemes on S-UMTS channels, much of the paper is devoted to the description of the channel model adopted en route. The chosen model appears to be a lengthy re-iteration of that described in [Jahn 96], see above.

The model is used to demonstrate that satellite elevation, near echoes and far echoes all have significant effects on the BER of the waveforms investigated.


Nothing is available in this paper that cannot be extracted from the earlier work by Jahn.

Dottling et al [Dott 99]

The authors claim to introduce a novel approach to the problem of generating and processing realistic wideband time series of land mobile satellite propagation channels. The motivation for the work appears to be for the modelling of Wideband CDMA radio links in the Satellite-UMTS standard. The method addresses the effects of elevation angle, shadowing correlation, signal delay, imperfect power control and user mobility.

There are three main component steps in the overall modelling process:




(i) A raytracing technique which is used to estimate the time-variant channel transfer function. The algorithm uses digital terrain height and land usage maps. In addition a digital synthesis of buildings and roadside obstacles is created. A satellite orbit generator steps the satellite position along defined trajectories at each stage of the simulation. Each satellite step gives rise to a new instantaneous scattering function. Propagation calculations incorporate diffraction/transmission through vegetation, ground reflection, scattering from rough surfaces, and reflections from buildings and other obstacles.

Some validation of the propagation model has taken place.

(ii) Post-processing of the instantaneous scattering function is then performed in order to derive a time series of "instantaneous effective signal-to-noise ratio". The S/N values incorporate an "equivalent noise due to inter-symbol interference" and further background noise due to multiple access transmissions.

(iii) Further processing of S/N time series to assess diversity techniques.


This paper is not as useful for the current application as originally anticipated. The authors adopt an approach to wideband channel modelling that does not appear to feature the usual ingredients, such as a tapped delay line and Doppler spectra etc. Instead, inter-symbol interference due to multipath effects is modelled as additional noise.

However, some consideration might be given to using a raytracing process, similar to that invoked here, to help derive representative scattering functions in environments where measured channel parameters are unavailable.

Belloul et al [Bell 00]

This paper post-dates the related one by Parks et al ([Park 96], see above). Although the frequency bands under consideration are significantly higher than VHF (L band ~ 1550 MHz, S band ~ 2325 MHz), the general approach and measured parameters are still of interest.

The emphasis is on the measurement of air-ground signals in order to test wideband channel models to be used in the development of Land Mobile Satellite services. In order to exercise greater control over the path geometries, helicopter-borne equipment was used instead of satellite signals. At the ground the terminal equipment was mounted in a road vehicle which was moved at modest speeds of ~ 9 km/hr along stretches of road of approximately 50 metres. The helicopter flew along a parallel track in order to maintain particular selected elevation angles. Wideband sounder hardware at each terminal was connected to omni-directional RH-circular polarised quadrifilar helix antennas. The sounder waveforms achieved a minimum delay resolution of 100 ns in both bands.

The study addressed elevation angles of 15, 30, 45, 60 and 80 degrees. The environments investigated were open, urban area, suburban area, light wooded and heavy wooded. Analyses concentrated on first order statistics of the signal amplitude and power delay distributions.

A Rician amplitude distribution fitted the measurements with a high (80%) statistical confidence. For many combinations of elevation angle and environment a dominant ray component was absent so that the simpler Rayleigh distribution applied. The authors' Table 3 summarises the measured K (Rice) factors. Normally the K factor at 1550 MHz is higher than at 2325 MHz, suggesting that Rician statistics might be prevalent at VHF and




that VHF signals might be less vulnerable to environmental effects than in these higher frequency bands.

When the ground terminal was in woodland Rayleigh statistics applied for path elevations below approximately 45 degrees. However, in built-up areas Rician statistics apply even at low elevations, where any LOS component was expected to be obscured. This unexpected result was attributed to strong reflections from nearby buildings.

For the B-VHF application the environments of greatest relevance are expected to be "open" "urban" and "suburban areas". For these the RMS delay spreads behaved as follows:

RMS delay spread (ns)

Environment L band (1550 MHz) S band (2325 MHz)

Open 35 - 45 30 - 55

Urban 35 - 75 45 - 85

Suburban 35 - 55 45 - 80

Proposals were made for a wideband channel model for L band links. This is of the tapped delay line type. Six taps is deemed sufficient to represent the observed delay profiles. However it is suggested that in "open" environments only 2 or 3 taps might be needed. In the example shown by the authors the delay profile extends over 600 ms (RMS ~ 90 ns), and this corresponds reasonably well to equivalent independent measurements from elsewhere. Tap weights with complex time-varying Rician amplitude distributions and "classical" Doppler spectra are proposed. The paper by Parks et al. [Park 96] gives a more complete description of the model parameters, but essentially they are derived from the measurements reported in this paper.


Application of the results from this paper for "open", "urban" and "suburban" environments to aircraft taxiing and parking situations is hampered by the order of magnitude difference in radio frequency.

The fact that the K factor at 1550 MHz is higher than at 2325 MHz suggests that Rician statistics might be prevalent at VHF, and that VHF signals might be less vulnerable to environmental effects than in these satcom bands.

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8. Air-Ground Propagation Measurements

Whilst the purpose of this report is to provide a review of radio channel models, it will nevertheless be helpful to have guidance from input data from actual measurements of air-ground radio link parameters - path loss, fading rates and depths, Doppler shifts etc. This Chapter reviews relevant measurements published in the literature.

Chamberlin [Cham 86]

The author considers the problem of communication from ground to air when the ground site is partially surrounded by extensive woodland. Trials were carried out at 136MHz, and continuous measurements of path loss were made as an aircraft was flown along radials from the ground station and also around circuits centred on the test site. The ground antenna was above the height of the tree canopy.

Significant variations in signal level were found to occur that were attributed to the presence of the trees (5-15 dB), and extensive propagation modelling was carried out in attempting to understand the mechanisms involved. Signals propagated through the trees, reflected from the ground and then propagated back through the trees were considered to be too weak to be significant, as were possible creeping waves along the interface between the tree canopy and the air above, but there were believed to be significant reflections from the top of the trees that would be able to interfere with the direct signal. The observed effects were modelled successfully using a modified GTD (Geometrical Theory of Diffraction) approach.


This problem may not be of direct interest to B-VHF, but nevertheless the results are of some interest.

Talvitie et al [Talv 92]

The paper describes the design and construction of a wideband channel measurement system for characterising air-ground radio links at VHF and UHF. The system provides consecutive estimates of the time-varying channel impulse response using a sliding correlator technique. By measuring consecutive samples of the impulse response the intention is to gather information about the transmission loss, the total path delay and Doppler spread. The authors were thus interested to study both the large-scale and the small-scale structure of the channel; the work was supported by the Finnish Air Force. Some laboratory trials of the equipment are briefly described, plus a few additional outdoor measurements using a van, but unfortunately no airborne measurements are presented. A search of the literature for further publications by the same authors has sadly revealed nothing of relevance to this topic.


So far, it has not been possible to identify any subsequent air-ground measurements made using the sounder described here. This paper alone is of little value to the B-VHF Project.




Darian & Wilson [Dari 96]

This report describes a programme of VHF pathloss and signal fading measurements at a small commercial airport (in Bedford, Massachusetts). The trials used a 136 MHz transmitter mounted in a van with a λ/4 monopole antenna on the roof at a height of about 3m above ground. Measurements were made of the signal received at another monopole antenna mounted on a ground plane in a clear area. To undertake the measurements the van drove along various taxi-ways around the airport. The taxi-ways generally had line-of-sight to the receiver, but there were scattered small buildings in the area, together with many parked small aircraft and a few large ones.

The received signals were analysed to obtain the pathloss-distance relationship for the various routes. It was found that most of the routes had a relatively uncluttered radio path, and the pathloss exponent (i.e the value of N in the relation: path loss = k.(distance) N where k is some constant ) was between 2 and 2.3. (The value 2 corresponds to the normal rate of signal decay in free space).

Signal fading behaviour (as a function of vehicle spatial position) was also analysed. Small amounts of signal fading were observed on most routes, and the observed fading was fitted to Rician distributions. Rician K-factors (ratio of direct power to scattered power) were found to be in the range 4.5 - 7.2 dB.


Empirical exponents for the path loss expression mentioned above have been published for many land mobile environments. However this paper is a useful additional measure for the B-VHF taxiing and parking scenarios. Similarly the measured Rician K-factors should be taken into account when setting B-VHF model parameters.

Paradie & Pernic [Para 96]

The authors describe measurements of noise levels at VHF observed onboard a C130 aircraft flying at 19,000ft over parts of Arizona. Measurements in the band 30-54 MHz showed that the noise level was quite variable with time and location: in rural areas the noise was about 4-5 dB above expected levels, and over a built-up area during the rush hour the observed noise was 10-12 dB above expected levels, and 17 dB greater than expected for a rural area. In the 108-125 MHz band the mean noise levels were too close to the measurement noise floor to be accurately measured, although some large signals were observed in parts of the band.


It is difficult to make quantitative use of this work because of the inadequate sensitivity of the receiving system in the frequency range of interest to the B-VHF Project.

Dyer & Gilbert [Dyer 97]

Notes the earlier measurements at Toulouse (& Massachusetts) on path loss and multipath fading for ground-ground paths. This paper describes trials to extend these data to cover ground-air paths and also to characterise path loss, fading distributions and time dispersion too.

Two independent sets of measurements - at Duluth and at Aspen airports; these represented two different environments - one suburban with rolling hills, and the other mountainous.




Measurements were made with both a CW source (for path loss and signal fading characteristics), and also with a spread-spectrum sliding correlation technique (for time delay dispersion measurements). Data were collected for a number of departure, arrival and low-approach manoeuvres at both airports; ranges out to 10-20 nm from the airport were measured.

Examples are shown of some of the measured data, including path loss vs. distance curves of a steady signal with slight Rician fading, another case with classical 2-path reflection fading (where the implied surface reflection coefficient exceeded 0.95), and a time-delay curve showing an echo at 15 microseconds after the direct signal (although the latter was stated to be much longer than most of the echoes observed).

The large-scale signal level variations were analysed to determine the best-fit path-loss vs. distance exponent; at Duluth a value of n = 2.27 was found, whilst at Aspen the best-fit values were n = 1.33 for distances less than 5 km, and n = 3.9 for distances greater than 5 km (note: for free-space propagation n = 2).

The majority of the small-scale signal variations were well fitted by a Rice distribution, with Rician K-factors (i.e. the ratio of the power in the direct component to the power of the diffuse scattered component) typically in the range 15-20 dB.

Mean time delay spreads were typically about 2.5 - 4 microseconds, with the great majority of data below 6 microseconds. The rms delay spreads were in a similar range of values.

Applicability to B-VHF: This paper is one of the very few that contain measured time dispersion data, and thus it is an important contribution to the channel modelling task within B-VHF. It would be very helpful to know whether these authors have described a more extensive measurement data-set in later publications.

Zurk [Zurk 99]

This paper considers aspects of terrain scattering and multipath propagation on air-ground links, with particular reference to height-finding by the use of airborne radar. Trials were carried out using VHF and UHF airborne radars, and the theory of surface scattering and reflection was developed for comparison with the experimental results. One aircraft transmitted a 3MHz pulsed-CW signal (either at 141 MHz carrier for VHF or at 431MHz for UHF), and the signal was received on a second aircraft after reflection from the ground surface.

The surface scattering at VHF showed significantly wider time-spreading than at UHF, although there was much variation with terrain type. The number of observed reflections was found to decrease as the terrain roughness increased; the strength of these reflections also decreased. However, it should be noted that the author's principal interest was in the specular reflections rather than diffuse scattering, as the arrival-time of the first reflection was to be used for the purpose of system height-finding.


Although the author�s attention was focussed on the specular reflection and higher-angle paths, the paper provides some indication of the relative importance of different reflection processes at VHF and UHF.




Newhall et al [Newh 03]

This report describes wideband channel sounding measurements at 2GHz of the air-ground communications environment. An airborne transmitter was flown along constant-radius arcs at low altitudes (below 10,000ft) around the ground-based receiver to obtain measurements at 4 elevation angles - 7.5, 15, 22.5 and 30°. The receiver was located in a university campus environment of 4- to 6-storey buildings and rolling terrain. Power-delay profiles that approximated channel impulse responses were used to measure magnitude, phase and delay of multipath signal components. Analysis of the data generated RMS delay spreads, excess delay spreads, multipath fading cumulative distribution functions, antenna diversity gain etc.

Mean RMS delay spread was found to increase with decreasing elevation angle - mean RMS delay spreads ranged from about 18 nsec at 30° elevation to 98 nsec at 7.5° elevation, whilst the maximum spreads ranged from 75 nsec to 485 nsec for the same range of elevations. Excess delay spreads were also analysed - i.e. the maximum delay difference between two multipath components having strengths within a specified dB level of the strongest multipath component. Means of the excess delay spreads ranged from 89 to 169 nsec for the 10dB level to 284 -703 nsec for the 30dB level. At the 10dB level the maximum excess delays were more than 1.3 µsec.

The number of significant multipath components contributing to any one measurement was also analysed; it was found that typically there were about 7-9 components in a delay profile, and this number did not vary significantly with elevation angle. Since the mean delay and delay spread tended to increase at low elevations, it was concluded from this that the longer delayed echoes must tend to increase in strength as the elevation angle decreased.

Signal fading characteristics were also analysed to produce cumulative distribution functions of the fading envelopes. The fading was found to be Rician at all elevation angles, but the Rice K-factor was largest for the high elevations, i.e. multipath scatter was smaller in relation to the direct signal at the higher elevations, as might be expected.

Path loss variations in the received signal level were interpreted by separating the propagation paths of the multipath components into 'air legs' and 'ground legs'. The air legs were the paths from the aircraft to the first-encountered ground surface, and these legs were assumed to experience free-space propagation. The ground legs were the paths from the first ground surface to the receiver; these were generally obstructed paths, and were characterised by a log-distance path loss exponent. For the environment of these trials it was found that an exponent of about 4.1 fitted the measured data; this result was used in the channel simulation model described in [Newh 02].


Although the radio frequency used for the channel soundings is an order of magnitude higher than that of interest to the B-VHF Project, it is envisaged that some of the derived parameters such as RMS and maximum delay spreads might be used to supplement other published data in setting up the channel model(s).

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9. Implications for B-VHF Channel Modelling

This Chapter considers the results of the above literature reviews, and extracts from them the points of most relevance to the B-VHF project.

The task of generating a channel model for the B-VHF system simulation may be divided broadly into the following topic areas:

1. Determination of the functionality and features to be represented in the model (e.g. should it include time delay, Doppler spread, angle of arrival, clustering of scattering centres, etc.).

2. Definition of the methods and algorithms by which the model shall be implemented (e.g. should it be a time-domain or a frequency-domain model, how should the tap coefficients be generated, what input and output interfaces are required etc.).

3. Specification of the parameters that will be used to define the various scenarios of interest (e.g. what Doppler spectra are appropriate to a taxiing scenario, what time delay spectra should be used to represent a take-off/landing scenario etc.).

The papers reviewed above cover a range of different aspects of air-ground radio communication systems. Not all of them are directly relevant to the B-VHF project, and the papers do not always agree amongst themselves, but nevertheless some themes emerge:

! The basic theoretical foundations of radio channel modelling are fairly well established; the discrete-time tapped delay-line implementation of such models is commonly used for simulation.

! Whilst there has been a great deal of theoretical modelling and simulation work, there has been relatively little experimental validation of such models.

! Much of the work reported has been concerned with terrestrial mobile radio communications, some work describes satellite-ground or satellite-aircraft links, but there is relatively little work in the open literature that directly relates to the air-ground case.

! Information from the satellite-ground studies for open/rural/suburban situations might be useful for the modelling of air-ground scenarios, although the satellite data are almost all taken at much higher radio frequencies (L- and S-band), and careful re-scaling of the data will be necessary.

! Some measurements of noise levels at lower VHF frequencies found that this varied considerably with location and time of day. This suggests that the representation of noise in the model could be made environment-dependent; however the variation of noise at the frequencies of interest to B-VHF is expected to be small.

! There is some debate in the literature on the merits of small-scale models (that describe local rapid fading effects only) versus large-scale models (which include variations of signal level due to terrain shadowing, etc), and the value (or otherwise) of including the latter.

! The maximum time delay spread to be used in simulations is an important parameter, which may have a major impact on the results of system evaluation.




However the measured values of this parameter are sparse, and sometimes contradictory.

! Some of the channel models that have been more recently developed involve a high degree of complexity, by including angular dependence, clustering of scatterers, birth & death of scattering centres as the mobile terminal moves, etc. The effort required to develop these models is large, but there seems to be a lack of the supporting environmental data that would be required to run the models realistically. It is questionable whether the additional complexity is justified by the improvement in the reliability of the results obtained. However some recently developed geometrically-based stochastic models may offer a slightly simpler compromise approach that would allow the inclusion of some directional information.

! There are a number of methods available for modelling a specified power-delay profile in the discrete-time tapped delay-line form of simulator. The Monte Carlo method has been claimed to offer good accuracy, and computationally-efficient implementation; however a comparison of various alternative methods cast doubt on the accuracy of Monte Carlo methods in this application. Further discussion of the general use of the �sum-of-sinusoids� techniques indicated that there may be other and more preferable techniques for implementing a channel simulator.

In terms of the implementation of a simulator for the B-VHF project and the three principal topic areas identified above, we draw the following provisional conclusions:

1. At its simplest the channel simulator could be restricted to representing the time delay and Doppler frequency domains. The inclusion of functions describing angle of arrival, clustering of scattering elements, signal component birth and death etc would lead to considerable extra complication, and would be likely to have significant implications for simulation execution time. Furthermore it would be difficult to provide useful input parameters to realistically characterise such behaviour in the air-ground scenarios of interest here. However, a geometrically based stochastic approach would possibly provide a method of including some angle-of-arrival information without the full complexity of the COST259 models.

2. In terms of channel implementation techniques, the �sum-of-sinusoids� methods appear to be preferable to the older filtered Gaussian noise approaches in terms of simulation accuracy, flexibility of application and speed of execution. However, there are a number of competing techniques to realise such a method (e.g. Monte Carlo, harmonic decomposition, method of equal areas, method of exact Doppler spread, Lp-norm, etc), each having its own merits. Further work is required to evaluate these methods, and to select one for this project.

3. The parameters to be used in the model to characterise delay spreads, Doppler spectra etc will need to be derived from published measurements. The reviews above indicate that there is relatively little information available that relates specifically to the environments of interest to B-VHF, and thus we shall have to make use of data derived from different (but related) environments. There is a moderate amount of observational data in the mobile radio field (most of it at much higher frequencies), which may be utilised for the airport and taxiing scenarios. There are some satellite-ground measurements which give some insights to the propagation characteristics, but again these results are invariably at frequencies an order of magnitude greater than we desire, and so must be scaled with care. Direct measurements of the




wideband air-ground channel at VHF are scarce, and so the simulation parameters that we employ in the model will involve a degree of engineering judgement.

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10. Summary & Conclusions

This Chapter summarises the main points of the D15 report, and the conclusions to be drawn.

The subject of wideband radio channel modelling and simulation has a long history and is based on substantial theoretical foundations. The rapid growth of terrestrial mobile radio systems in recent years (and its huge commercial importance) has meant that much of the recent work has been applied in this field. There have also been significant efforts in the area of satellite-based land mobile communications.

For the implementation of channel simulators the tapped delay-line structure is widely used. For the control of the delay tap coefficients recent work has tended to move away from the use of filtered Gaussian white noise sources, and instead the Rician �sum-of-sinusoids� technique has found favour. There are a number of techniques for implementing such an approach, each with claimed merits and drawbacks; further investigation of this topic is proposed for the development of the B-VHF channel simulator.

In the field of cellular mobile radio, and especially in the context of within-building communication, there has been much effort recently to develop models that characterise the angular dependence of scattered energy, and also to describe the clustering of scattering centres. Whilst this degree of rigour is to be admired, it is questionable whether the added complexity is justified by the improvement in the results that could be expected in the context of B-VHF. Somewhat simpler approaches using geometrically-based stochastic modelling would allow us to include basic angular information, and we suggest that the channel simulator for B-VHF could adequately represent the multipath channel using this approach.

The use of the wideband channel simulator to represent various scenarios of interest to the B-VHF project (e.g. taxiing, air-ground, etc) will require the specification of time delay spectra and Doppler spectra for those scenarios. Much of the measured wideband data that has been published is either for other scenarios, for other frequencies, or both of these. This means that, although the information in the published literature can provide a useful starting point, some engineering judgement will still be required in the use of this data in our simulations.

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11. References

[Abde 98] Abdel-Hafez, M. & Safak, M., A BER Analysis for a Candidate Satcom Channel Model for the UMTS, IEEE Vehicular Technology Conference VTC-98, 1998, Vol. 3, pp. 2338-2342.

[Aspl 02] Asplund, H. et al, Clustering of Scatterers in Mobile Radio Channels - Evaluation & Modelling in the COST259 Directional Channel Model, IEEE International Conference on Communications, ICC2002, Vol. 2, pp.901-905.

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12. Abbreviations

A/G Air-Ground

ACARS Aircraft Communications Addressing and Reporting System

ATC Air Traffic Control

ATM Air Traffic Management

ATS Applied Technology Satellites

ATSP Air Traffic Service Provider

BER Bit Error Rate

CDMA Code Division Multiple Access

DLL Data Link Layer

DSB-AM Dual Side Band Amplitude Modulation

EP External Parameters

LOS Line Of Sight

LPNM LP-Norm Method

MC Multi-Carrier

MC-CDMA Multi-Carrier Code Division Multiple Access

MCM Monte Carlo Method

MEA Method of Equal Areas

MED Method of Equal Distances

MIMO Multiple Inputs - Multiple Outputs

MSEM Mean-Square Error Method

OFDM Orthogonal Frequency Division Multiplexing

PDC Pre-Departure Clearance

PDF Probability Density Function

PHY Physical Layer (OSI model)

QoS Quality of Service

RMS Root Mean Square

UHF Ultra High Frequency

UMTS Universal Mobile Telecommunication System

US Uncorrelated Scattering

VDL VHF Digital Link

VHF Very High Frequency

WSS Wide-Sense Stationary

WSSUS Wide-Sense Stationary Uncorrelated Scattering

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Documents

Literature review on terrestrial broadband VHF radio channel models