PERFORMANCE ANALYSIS OF LTE PHYSICAL LAYER USING SYSTEM VUE

PROJECT REPORT

Entitled

“PERFORMANCE ANALYSIS OF LTE PHYSICAL LAYER USING

SYSTEM VUE”

Submitted in partial fulfillment ofthe requirement

For the Degree of

: Presented & Submitted By:

PRAVAT KARKI (Roll No.U09EC410)

BIPLAV BHURTEL (Roll No. U09EC428)

KISHOR BHANDARI (Roll No. U09EC429)

AVI GUPTA (Roll No. U09EC435)

B. TECH. IV (Electronics & Communication) 8th Semester

: Guided By:

Prof. (Ms.) SHILPI GUPTA

Associate Professor, ECED.

(MAY - 2013)

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

SardarVallabhbhai National Institute of Technology

Surat-395 007, Gujarat, INDIA.

SardarVallabhbhai National Institute of Technology

Surat-395 007, Gujarat, INDIA.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

This is to certify that the B. Tech. IV (8th Semester) PROJECT REPORT entitled “Performance Analysis of LTE Physical Layer using System Vue” is presented & submitted by Candidates Mr. Pravat Karki, Biplav Bhurtel, Kishor Bhandari, Avi Gupta, bearing Roll No.U09EC410,U09EC428,U09EC429,U09EC435 respectively in the fulfillment of the requirement for the award of B. Tech. degree in Electronics & Communication Engineering.

They have successfully and satisfactorily completed their Project Exam in all respect. We, certify that the work is comprehensive, complete and fit for evaluation.

Prof. (Ms.) Shilpi Gupta Prof. P.K. SHAH

Project Guide Head of the Dept., ECED

Assistant Professor Associate Professor

SEMINAR EXAMINERS:

Name Signature with date

1. Prof. (Ms.) Jigisha N. Patel

2.Prof. Mehul C. Patel

3.Mrs. Kirti Inamdar

DEPARTMENT SEAL

MAY-2013.

ACKNOWLEDGEMENT

This Project was not possible to be completed without the help and able guidance of many

great people. We, with profound veneration and reverence would like to thank our guide Prof.

(Ms) SHILPI GUPTA (ECED,SVNIT) for her amiable attitude and motivation .She provided

constant guidance and support throughout semester for this project preliminary despite her tight

academic schedule.

We too take the opportunity to express our gratitude to our affable peers and venerable staff

members for the encouragement they have shown towards the work of ours and for their direct

and indirect assistance.

We would also like to express our appreciation to all the people who have been supporting us for

our project on “Performance Analysis of LTE physical layer using System Vue” and paved the

way to a better completion of this Project.

ABSTRACT

Technological advancement aims to make wireless communication efficient in terms of data

speed and QoS of service. Despite commercial 3G networks are starting to be fully operational

and High Speed Data Packet Access (HSDPA) is on its way to be deployed, operators and

manufacturers are already in a race towards 4G technologies.The road to 4G has a mandatory

milestone in Long Term Evolution (LTE) as it is a promising technology which will allow

backwards compatibility besides a higher performance. LTE, which is mainly deployed in a

macro/microcell layout, provides improved system capacity and coverage, high peak data rates,

low latency, reduced operating costs, multi-antenna support, flexible bandwidth operation and

seamless integration with existing systems.

In this project report, we present an overview of the techniques being used for LTE and explain

different technological advancement scenario of Long Term Evolution (LTE).For this we have

implemented and observed the OFDM modulation or Multiplexing Scheme that is basis of

implementation of physical layer of LTE.This report presents anintroduction to the performance

evaluation of LTE downlink physical layer according to the latest 3GPP specifications and

describes the various parameters which are important to analyze the performance of the LTE

Network. Our work aims to make a comprehensive investigation of the maximum data

throughput under different conditions and scenarios and calculate the Bit Error rates for the same

conditions by implementation in the System Vue software. Accordingly we have implemented

MIMO and SISO schemes of LTE and Measured Throughput and BER Under different

Modulation Schemes.

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Table of Contents

List of Figures ……………………………………………………………………………….......iii

List of Tables…………………………………………………………………………………….vii

CHAPTER – 1 LTE Introduction………………………………………………………………...1

1.1 Historical Context …………………………………………………………………....1

1.2 Introduction of LTE in Mobile Radio………………………………………………..2

1.3 Requirements and Targets for the Long Term Evolution…………………………….3

1.4 System Performance Requirements………………………………………………......4

1.5 Multi carrier Technology…………………………………………………………......9

1.6 Multiple Antenna Technology…………………………………………………….…10

CHAPTER – 2 LTE BASIC CONCEPTS…………………………………………………........11

2.1 Single Carrier Modulation and Channel Equalization…………………………..…...11

2.2 OFDM…………………………………………………………………………....…..14

2.3 OFDMA……………………………………………………………………………...16

2.3.1 Comparison of OFDMA with Packet-Oriented Protocols……………..…..16

2.3.2 OFDMA and LTE Generic Frame Structure………………………….…...18

2.4 MIMO and MRC……………………………………………………………..….…..19

2.5 SC-FDMA……………………………………………………………………….…..21

CHAPTER – 3 LTE Physical Layer …………………………………………..………………...24

3.0.1 Generic Frame Structure …………………………………………………….….…24

3.1 Downlink………………………………………………………………….…….…....24

3.1.1 Modulation Parameters………………………………………………………..…...25

3.1.2 Downlink Multiplexing…………………………………………………….……....26

3.1.3 Physical Channels………………………………………………………….….…...26

3.1.4 Physical Signals…………………………………………………………….….…..28

ii

3.1.5 Transport Channels…………………………………………………………….......30

3.1.6 Mapping Downlink Physical channels to Transport channels……………….…….31

3.1.7 Downlink Channel Coding……………………………………………….…….….32

CHAPTER – 4 OFDM……………………………………...……………………………………33

4.1 OFDM for LTE……………………………………………………………..….…….33

4.2 OFDM architecture ……………………………………………………………...…..34

4.3 FFT Implementation……...……………………………………………………….…36

4.4 OFDMA BASICS……………………………………………………………………37

CHAPTER – 5 OFDM IMPLEMENTATION………………………………………….……....40

5.1 OFDM Block Diagram……………………………………………………………....40

5.2 Components used in OFDM Implementation …………………………...………….41

5.3 Measurement of signal at different points…………………………………………...56

CHAPTER –6 PERFORMANCE ANALYSIS FOR LTE……………………………………...62

6.1 Quantitative factors in LTE …………………………………………………………62

6.2 Implementation and Results For Different Schemes………………………..……….63

6.3 Results and Analysis…………………………………………………………………73

CHAPTER-7 CONCLUSION………………………………………………………………..…74

REFERENCES …………………………………………………………………………….……75

ACRONYM……………………………………………………………………………………..76

iii

List of figures

Fig 1.1.Approximate timeline of the mobile communications standards landscape…………...…3

Fig 2.1 Multipath Caused by Reflections off Objects Such as Buildings and Vehicles…………12

Fig 2.2 Multipath-Induced Time Delays Result in ISI………………….............…….…………12

Fig 2.3 Longer Delay Spreads Result in Frequency Selective Fading……………..………........12

Fig 2.4 Transversal Filter Channel Equalizer…………………………………………..…..........13

Fig 2.5 OFDM Eliminates ISI via Longer Symbol Periods and a Cyclic……………………….14

Fig 2.6 FFT of OFDM Symbol Reveals Distinct Subcarriers…………………..……………….15

Fig Preamble and Header………………………………………………………………………..16

Fig 2.8 LTE Generic Frame Structures……………………………………………….…............18

Fig 2.9 MRC/MIMO Operation Requires Multiple Transceivers………………………..….......19

Fig. 2.10 MRC Enhances Reliability in the Presence of AWGN and Frequency

Selective Fading………………………………………………………………………………..20

Fig 2.11 Reference Signals Transmitted Sequentially to Compute Channel Responses for

MIMO Operation……………………………………………………..………………..………..21

Fig 2.12 SC-FDMA and OFDMA Signal Chains Have a High Degree of

Functional Commonality…...………………………………………………………………..….22

Fig 2.13 SC-FDMA Subcarriers Can be Mapped in Either Localized or

DistributedMode…...……………………………………………………………...……….….....23

Fig 3.1 LTE Generic Frame Structures………………………………………………..…………24

Fig 3.2 Resource Elements Mapping of Reference Signals………………………………..….…29

iv

Fig 3.3 Mapping DL Transport Channels to physical channels ……………………………..…..31

Fig 4.1 .Effect of channel on signals with short and long symbol duration………………….….34

Fig 4.2 Simplex Point-to-point transmission using OFDM………………………………….….34

Fig 4.3 OFDM Cyclic Prefix (CP) insertion………………………………………………….….35

Fig. 4.4 cyclic extension and windowing of OFDM………………………………………….….35

Fig 4.5 OFDM Transmitter…………………………………………………………………........36

Fig 5.1 : OFDM Block Diagram…………………………………………………………………40

Fig. 5.2 Parameter change box of a RandomBits block………………………………………….41

Fig.5.3 Data sink block…………………………………………………………………………..41

Fig. 5.4 Parameter box of a Complex Symbol Mapper…………………………………………42

Fig.5.5 Parameter block for OFDM Subcarrier Multiplexing block…………………………....43

Fig.5.6 Parameter box ofa Complex Fast Fourier Transformation block…………………….….44

Fig.5.7 Parameter box of an OFDM guard interval insertion block……………………………..45

Fig. 5.8 Parameter box of a Complex to Envelope converter block…………………………….45

Fig.5.9 Parameter box of an Add Noise Density to Input Block………………………………...46

Fig. 5.10 An envelope to complex converter block……………………………………………..47

Fig. 5.11 Parameter box of an OFDM guard interval removal block…………………………....48

Fig. 5.12 A Complex Fast Fourier Transform block…………………………………………….49

Fig. 5.13 Parameter box of an OFDM subcarrier demultiplexing block………………………..50

Fig.5.14 Parameter box is a Complex Symbol Demapper/Slicer block………………………....51

Fig.5.15 Parameter box of a Delay block………………………………………………………..52

v

Fig. 5.16 Parameter box of a Bit and Frame Error Rate Measurement Block……………….….53

Fig 5.17 OFDM Block…………………………………………………………………………...53

Fig 5.18 .Output of Random bit Generator………………………………………………………54

Fig 5.19: Scatterplot diagram for 16 QAM………………………………………………………54

Fig 5.20: Polar plot after sub carrier allotment…………………………………………………..55

Fig 5.21: Output of FFT………………………………………………………………………….55

Fig 5.22: OFDM Signals after Guardband Insertion…………………………………………….56

Fig 5.23: OFDM signal after noise addition……………………………………………………..56

Fig 5.24: Spectrum at receiver side………………………………………………………………57

Fig 5.25: Scatter plot after Demodulation………………………………………………………..57

Fig 5.26: Output Signal…………………………………………………………………………..58

Fig 6.1 Basic Block for calculating BER of LTE SISO Scheme…………………………….…..60

Fig 6.2 BER vs SNR plot of LTE SISO Scheme………………………………………………..61

Fig 6.3 BLER vs SNR plot of LTE SISO Scheme……………………………………………....61

Fig 6.4 Block Diagram for calculating MIMO BER with QPSK Modulation……………….…62

Fig 6.5 : BER vs SNR Graph of MIMO LTE System…………………………………………...62

Fig 6.6 Block Diagram for MIMO BER plot with 16 QAM Modulation……………………….63

Fig 6.7 :BER vs SNR Graph of MIMO LTE System with 16 QAM Modulation……………....63

Fig 6.8 Basic blocks of SISO Throughput Calculation…...………………………………….….64

Fig 6.9 Throughput vs SNR plot of LTE SISO Scheme……………………………………........64

Fig 6.10 SISO Throughput fraction vs SNR…………………………………………………......65

Fig 6.11 Block Diagram for plotting MIMO Throughput for QPSK Modulation………....…….66

Fig 6.12 Throghputvs SNR plot MIMO scheme with QPSK

Modulation………………….…....................................................................................................66

vi

Fig 6.13 MIMO Throughput Fraction vsSNR for QPSK Modulation……………………..........67

Fig 6.14 Block Diagram for plotting MIMO Throughput for 16 QAM Modulation…………….68

Fig 6.15 MIMO Throughput vsSNR for 16QAM modulation………………………………….68

Fig 6.16 MIMO Throughput Fraction VSSNRfor 16QAM modulation………………………..69

vii

LIST OF TABLES

Table 1: Available Downlink Bandwidth is divided into Physical Resource Blocks………....14

Table 2: Downlink OFDM Modulation Parameters…………………………...……………....23

Table 3: Cyclic Prefix Duration……………………………………………………………….23

Performance Analysis Of LTE Physical layer using System Vue

1

CHAPTER 1 LTE INTRODUCTION

1.1 Historical Context

The Long Term Evolution of UMTS is one of the latest steps in an advancing series of mobile

telecommunications systems. Arguably, at least for land-based systems, the series began in

1947 with the development of the concept of cells by Bell Labs, USA. The use of cells enabled

the capacity of a mobile communications network to be increased substantially, by dividing the

coverage area up into small cells each with its own base station operating on a different

frequency.[1]

The early systems were confined within national boundaries. They attracted only a small

number of users, as the equipment on which they relied was expensive, cumbersome and

power-hungry, and therefore was only really practical in a car.

The first mobile communication systems to see large-scale commercial growth arrived in the

1980s and became known as the ‗First Generation‘ systems. The First Generation used

analogue technology and comprised a number of independently developed systems worldwide

(e.g. AMPS (Analogue Mobile Phone System, used in America), TACS (Total Access

Communication System, used in parts of Europe), NMT (Nordic Mobile Telephone, used in

parts of Europe) and J-TACS (Japanese Total Access Communication System, used in Japan

and Hong Kong)).[1]

Global roaming first became a possibility with the development of the ‗Second Generation‘

system known as GSM (Global System for Mobile communications), which was based on

digital technology. The success of GSM was due in part to the collaborative spirit in which it

was developed. By harnessing the creative expertise of a number of companies workingtogether

under the auspices of the European Telecommunications Standards Institute (ETSI), GSM

became a robust, interoperable and widely accepted standard.

Fuelled by advances in mobile handset technology, which resulted in small, fashionable

terminals with a long battery life, the widespread acceptance of the GSM standard exceeded

initial expectations and helped to create a vast new market. The resulting near-universal


2

penetration of GSM phones in the developed world provided an ease of communication never

previously possible, first by voice and text message, and later also by more advanced data

services. Meanwhile in the developing world, GSM technology had begun to connect

communities and individuals in remote regions where fixed-line connectivity was nonexistent

and would be prohibitively expensive to deploy.

This ubiquitous availability of user-friendly mobile communications, together with increasing

consumer familiarity with such technology and practical reliance on it, thus provides the

context for new systems with more advanced capabilities. In the following section, the series of

progressions which have succeeded GSM is outlined, culminating in the development of the

system known as LTE – the Long Term Evolution of UMTS (Universal Mobile

Telecommunications System).

1.2 Introduction of LTE in Mobile Radio

In contrast to transmission technologies using media such as copper lines and optical fibers, the

radio spectrum is a medium shared between different, and potentially interfering, technologies.

As a consequence, regulatory bodies – in particular, ITU-R (International Telecommunication

Union – Radio communication Sector) [1], but also regional and national regulators play a key

role in the evolution of radio technologies since they decide which parts of the spectrum and

how much bandwidth may be used by particular types of service and technology. This role is

facilitated by the standardization of families of radio technologies – a process which not only

provides specified interfaces to ensure interoperability between equipment from a multiplicity

of vendors, but also aims to ensure that the allocated spectrum is used as efficiently as possible,

so as to provide an attractive user experience and innovative services.

The complementary functions of the regulatory authorities and the standardization

organizations can be summarized broadly by the following relationship:

Aggregated data rate = bandwidth × spectral efficiency

On a worldwide basis, ITU-R defines technology families and associates specific parts of the

spectrum with these families. Facilitated by ITU-R, spectrum for mobile radio technologies is

identified for the radio technologies which meet ITU-R‘s requirements tobe designated as


3

members of the International Mobile Telecommunications (IMT) family. Effectively, the IMT

family comprises systems known as ‗Third Generation‘ (for the first time providing data rates

up to 2 Mbps) and beyond.

Figure 1.1.Approximate timeline of the mobile communications standards landscape[12].

1.3 Requirements and Targets for the Long Term Evolution

Discussion of the key requirements for the new LTE system led to the creation of a formal

Study Item‘ in 3GPP with the specific aim of ‗evolving‘ the 3GPP radio access technology to

ensure competitiveness over a ten-year time-frame. Under the auspices of this Study Item, the

requirements for LTE were refined and crystallized, being finalized in June 2005.

They can be summarized as follows:

• reduced delays, in terms of both connection establishment and transmission latency;

• increased user data rates;

• increased cell-edge bit-rate, for uniformity of service provision;

• reduced cost per bit, implying improved spectral efficiency;


4

• greater flexibility of spectrum usage, in both new and pre-existing bands;

• simplified network architecture;

• seamless mobility, including between different radio-access technologies;

• reasonable power consumption for the mobile terminal.

It can also be noted that network operator requirements for next generation mobile systems

were formulated by the Next Generation Mobile Networks (NGMN) alliance of network

operators, which served as an additional reference for the development and assessment of the

LTE design.

1.4 System Performance Requirements

Improved system performance compared to existing systems is one of the main requirements

from network operators, to ensure the competitiveness of LTE and hence to arouse market

interest. In this section, we highlight the main performance metrics used in the definition of the

LTE requirements and its performance assessment.

Improved system performance compared to existing systems is one of the main requirements

from network operators, to ensure the competitiveness of LTE and hence to arouse market

interest. In this section, we highlight the main performance metrics used in the definition of the

LTE requirements and its performance assessment.

It can be seen that the target requirements for LTE represent a significant step from the capacity

and user experience offered by the third generation mobile communications systems which

were being deployed at the time when the first version of LTE was being developed. As

mentioned above, HSPA technologies are also continuing to be developed to offer higher

spectral efficiencies than were assumed for the reference baseline. However, LTE has been able

to benefit from avoiding the constraints of backward compatibility, enabling the inclusion of

advanced MIMO schemes in the system design from the beginning, and highly flexible

spectrum usage built around new multiple access schemes.


5

Peak Rates and Peak Spectral Efficiency

For marketing purposes, the first parameter by which different radio access technologies are

usually compared is the peak per-user data rate which can be achieved.

This peak data rate generally scales according to the amount of spectrum used, and, for MIMO

systems, according to the minimum of the number of transmit and receive antennas. The peak

data rate can be defined as the maximum throughput per user assuming the whole bandwidth

being allocated to a single user with the highest modulation and coding scheme and the

maximum number of antennas supported. Typical radio interface overhead (control channels,

pilot signals, guard intervals, etc.) is estimated and taken into account for a given operating

point. For TDD systems, the peak data rate is generally calculated for the downlink and uplink

periods separately. This makes it possible to obtain a single value independent of the

uplink/downlink ratio and a fair system comparison that is agnostic of the duplex mode. The

maximum spectral efficiency is then obtained simply by dividing the peak rate by the used

spectrum allocation.

The target peak data rates for downlink and uplink in LTE Release 8 were set at 100 Mbps and

50 Mbps respectively within a 20 MHz bandwidth,7 corresponding to respective peak spectral

efficiencies of 5 and 2.5 bps/Hz. The underlying assumption here is that the terminal has two

receive antennas and one transmit antenna. The number of antennas used at the base station is

more easily upgradeable by the network operator, and the first version of the LTE specifications

was therefore designed to support downlink MIMO operation with up to four transmit and

receive antennas.

When comparing the capabilities of different radio communication technologies, great

emphasis is often placed on the peak data rate capabilities. While this is one indicator of how

technologically advanced a system is and can be obtained by simple calculations, it may not be

a key differentiator in the usage scenarios for a mobile communication system in practical

deployment. Moreover, it is relatively easy to design a system that can provide very high peak

data rates for users close to the base station, where interference from other cells is low and

techniques such as MIMO can be used to their greatest extent. It is much more challenging to


6

provide high data rates with good coverage and mobility, but it is exactly these latter aspects

which contribute most strongly to user satisfaction.

In typical deployments, individual users are located at varying distances from the basestations,

the propagation conditions for radio signals to individual users are rarely ideal, and the

available resources must be shared between many users. Consequently, although the claimed

peak data rates of a system are genuinely achievable in the right conditions, it is rare for a

single user to be able to experience the peak data rates for a sustained period, and the envisaged

applications do not usually require this level of performance.

A differentiator of the LTE system design compared to some other systems has been the

recognition of these ‗typical deployment constraints‘ from the beginning. During the design

process, emphasis was therefore placed not only on providing a competitive peak data rate for

use when conditions allow, but also importantly on system level performance, which was

evaluated during several performance verification steps. System-level evaluations are based on

simulations of multicell configurations where data transmission from/to a population of mobiles

is considered in a typical deployment scenario. The sections below describe the main metrics

used as requirements for system level performance.

Cell Throughput and Spectral Efficiency

Performance at the cell level is an important criterion, as it relates directly to the number of cell

sites that a network operator requires, and hence to the capital cost of deploying the system. For

LTE , it was chosen to assess the cell level performance with full-queue traffic models (i.e.

assuming that there is never a shortage of data to transmit if a user is given the opportunity) and

a relatively high system load, typically 10 users per cell.

The requirements at the cell level were defined in terms of the following metrics:

• Average cell throughput [bps/cell] and spectral efficiency [bps/Hz/cell];

• Average user throughput [bps/user] and spectral efficiency [bps/Hz/user];


7

• Cell-edge user throughput [bps/user] and spectral efficiency [bps/Hz/user] (the metric used for

this assessment is the 5-percentile user throughput, obtained from the cumulative distribution

function of the user throughput).

For the UMTS Release 6 reference baseline, it was assumed that both the terminal and the base

station use a single transmit antenna and two receive antennas; for the terminal receiver the

assumed performance corresponds to a two-branch Rake receiver with linear combining of the

signals from the two antennas. For the LTE systems, the use of two transmit and receive

antennas was assumed at the base station. At the terminal, two receive antennas were assumed,

but still only a single transmit antenna. The receiver for both downlink and uplink is assumed to

be a linear receiver with optimum combining of the signals from the antenna branches.

Voice Capacity

Unlike full queue traffic (such as file download) which is typically delay-tolerant and does not

require a guaranteed bit-rate, real-time traffic such as Voice over IP (VoIP) has tight delay

constraints. It is important to set system capacity requirements for such services – a particular

challenge in fully packet-based systems like LTE which rely on adaptive scheduling. The

system capacity requirement is defined as the number of satisfied VoIP users, given a particular

traffic model and delay constraints. The details of the traffic model used for evaluating LTE can

be found. Here, a VoIP user is considered to be in outage (i.e. not satisfied) if more than 2% of

the VoIP packets do not arrive successfully at the radio receiver within 50 ms and are therefore

discarded. This assumes an overall end-to-end delay (from mobile terminal to mobile terminal)

below 200 ms. The system capacity for VoIP can then be defined as the number of users present

per cell when more than 95% of the users are satisfied

Mobility and Cell Ranges

LTE is required to support communication with terminals moving at speeds of up to 350 km/h,

or even up to 500 km/h depending on the frequency band. The primary scenario for operation at

such high speeds is usage on high-speed trains – a scenario which is increasing in importance

across the world as the number of high-speed rail lines increases and train operators aim to offer

an attractive working environment to their passengers. These requirements mean that handover


8

between cells has to be possible without interruption –in other words, with imperceptible delay

and packet loss for voice calls, and with reliable transmission for data services.

These targets are to be achieved by the LTE system in typical cells of radius up to 5 km, while

operation should continue to be possible for cell ranges of 100 km and more,to enable wide-

area deployments.

Broadcast Mode Performance

The requirements for LTE included the integration of an efficient broadcast mode for high rate

Multimedia Broadcast/Multicast Services (MBMS) such as mobile TV, based on a Single

Frequency Network mode of operation . The spectral efficiency requirement is given in terms

of a carrier dedicated to broadcast transmissions –i.e. not shared with unicast transmissions.In

broadcast systems, the system throughput is limited to what is achievable for the users in the

worst conditions. Consequently, the broadcast performance requirement was defined in terms of

an achievable system throughput (bps) and spectral efficiency (bps/Hz) assuming a coverage of

98% of the nominal coverage area of the system. This means that only 2% of the locations in

the nominal coverage area are in outage – where outage for broadcast services is defined as

experiencing a packet error rate higher than 1%. This broadcast spectral efficiency requirement

was set to 1 bps/Hz [10]. While the broadcast mode was not available in Release 8 due to

higher prioritization of other service modes, Release 9 incorporates a broadcast mode

employing Single Frequency Network operation on a mixed unicast-broadcast carrier.

User Plane Latency

User plane latency is an important performance metric for real-time and interactive services. On

the radio interface, the minimum user plane latency can be calculated based on signaling

analysis for the case of an unloaded system. It is defined as the average time between the first

transmission of a data packet and the reception of a physical layer acknowledgement. The

calculation should include typical HARQ8 retransmission rates (e.g. 0–30%). This definition

therefore considers the capability of the system design, without being distorted by the

scheduling delays that would appear in the case of a loaded system. The round-trip latency is

obtained simply by multiplying the one-way user plane latency by a factor of two. LTE is also

required to be able to operate with IP-layer one-way data-packet latency across the radio access


9

network as low as 5 ms in optimal conditions. However, it is recognized that the actual delay

experienced in a practical system will be dependent on system loading and radio propagation

conditions.

Control Plane Latency and Capacity

In addition to the user plane latency requirement, call setup delay was required to be

significantly reduced compared to previous cellular systems. This not only enables a good user

experience but also affects the battery life of terminals, since a system design which allows a

fast transition from an idle state to an active state enables terminals to spend more time in the

low-power idle state. Control plane latency is measured as the time required for performing the

transitions between different LTE states.

1.5 Multi carrier Technology

Adopting a multicarrier approach for multiple access in LTE was the first major design choice.

After initial consolidation of proposals, the candidate schemes for the downlink were

Orthogonal Frequency Division Multiple Access (OFDMA) and Multiple WCDMA, while the

candidate schemes for the uplink were Single-Carrier Frequency-Division Multiple Access (SC-

FDMA), OFDMA and Multiple WCDMA. The choice of multiple-access schemes was made in

December 2005, with OFDMA being selected for the downlink, and SC-FDMA for the uplink.

Both of these schemes open up the frequency domain as a new dimension of flexibility in the

system.

This resulting flexibility can be used in various ways:

Different spectrum bandwidths can be utilized without changing the fundamental system

parameters or equipment design;

Transmission resources of variable bandwidth can be allocated to different users and

scheduled freely in the frequency domain;

Fractional frequencies re-use and interference coordination between cells is facilitated.


10

Extensive experience with OFDM has been gained in recent years from deployment of digital

audio and video broadcasting systems such as DAB, DVB and DMB. This experience has

highlighted some of the key advantages of OFDM, which include:

Robustness to time-dispersive radio channels, thanks to the subdivision of the wideband

transmitted signal into multiple narrowband subcarriers, enabling inter-symbol

interference to be largely constrained within a guard interval at the beginning of each

symbol

Low-complexity receivers, by exploiting frequency-domain equalization

Simple combining of signals from multiple transmitters in broadcast networks

1.6 Multiple Antenna Technology

The use of multiple antenna technology allows the exploitation of the spatial-domain as another

new dimension. This becomes essential in the quest for higher spectral efficiencies. With the

use of multiple antennas the theoretically achievable spectral efficiency scales linearly with the

minimum of the number of transmit and receive antennas employed, at least in suitable radio

propagation environments. Multiple antenna technology opens the door to a large variety of

features, but not all of them easily deliver their theoretical promises when it comes to

implementation in practical systems. Multiple antennas can be used in a variety of ways, mainly

based on three fundamental principles.

Diversity gain: Use of the spatial diversity provided by the multiple antennas to improve the

robustness of the transmission against multipath fading.

Array gain: Concentration of energy in one or more given directions via precoding or beam

forming. This also allows multiple users located in different directions to be served

simultaneously (so-called multi user MIMO).

Spatial multiplexing gain: Transmission of multiple signal streams to a single user on multiple

spatial layers created by combinations of the available antennas.


11

CHAPTER 2 LTE BASIC CONCEPTS

Before jumping into a detailed description of the LTE PHY, it‘s worth taking a look at some of

the basic technologies involved. Many methods employed in LTE are relatively new in cellular

applications. These include OFDM, OFDMA, MIMO and Single Carrier Frequency Division

Multiple Access (SC-FDMA). LTE employs OFDM for downlink data transmission and SC-

FDMA for uplink transmission. OFDM is a well-known modulation technique, but is rather

novel in cellular applications. A brief discussion of the basic properties and advantages of this

method is therefore warranted. When information is transmitted over a wireless channel, the

signal can be distorted due to multipath. Typically (but not always) there is a line-of-sight path

between the transmitter and receiver. In addition, there are many other paths created by signal

reflection off buildings, vehicles and other obstructions as shown in Figure 2.1. Signals

travelling along these paths all reach the receiver, but are shifted in time by an amount

corresponding to the differences in the distance travelled along each path.

2.1 Single Carrier Modulation and Channel Equalization

To date, cellular systems have used single carrier modulation schemes almost exclusively.

Although LTE uses OFDM rather than single carrier modulation, it‘s instructive to briefly

discuss how single carrier systems deal with multipath-induced channel distortion. This will

form a point of reference from which OFDM systems can be compared and contrasted. The

term delay spread describes the amount of time delay at the receiver from a signal traveling

from the transmitter along different paths. In cellular applications, delay spreads can be several

microseconds. The delay induced by multipath can cause a symbol received along a delayed

path to ―bleed‖ into a subsequent symbol arriving at the receiver via a more direct path. This

effect is depicted in Figure 2.1-1 and is referred to as inter-symbol interference (ISI). In a

conventional single carrier system symbol times decrease as data rates increase. At very high

data rates (with correspondingly shorter symbol periods), it is quite possible for ISI to exceed

an entire symbol period and spill into a second or third subsequent symbol.


12

Figure 2.1 Multipath Caused by Reflections Off Objects Such as Buildings and Vehicles[13]

Figure 2.2 Multipath-Induced Time Delays Result in ISI [13]

It‘s also helpful to consider the effects of multipath distortion in the frequency domain. Each

different path length and reflection will result in a specific phase shift. As all of the signals are

combined at the receiver, some frequencies within the signal passband undergo constructive

interference (linear combination of signals in-phase), while others encounter destructive

interference (linear combination of signals out-of-phase). The composite received signal is

distorted by frequency selective fading. (See Fig. 2.2)

Figure 2.3 Longer Delays Spreads Result in Frequency Selective Fading [13]


13

Single carrier systems compensate for channel distortion via time domain equalization. This is a

substantial topic by itself, and beyond the scope of this paper. Generally, time domain

equalizers compensate for multipath induced distortion by one of two methods:

1. Channel inversion: A known sequence is transmitted over the channel prior to sending

information. Because the original signal is known at the receiver, a channel equalizer is able to

determine the channel response and multiply the subsequent data-bearing signal by the inverse

of the channel response to reverse the effects of multipath.

2. CDMA systems can employ rake equalizers to resolve the individual paths and then

combine digital copies of the received signal shifted in time to enhance the receiver signal-to-

noise ratio (SNR). In either case, channel equalizer implementation becomes increasingly

complex as data rates increase. Symbol times become shorter and receiver sample clocks must

become correspondingly faster. ISI becomes much more severe possibly spanning several

symbol periods.

Figure 2.4: Transversal Filter Channel Equalizer [13]

The finite impulse response transversal filter (see Fig. 2.4) is a common equalizer topology. As

the period of the receiver sample clock (Δ) decreases, more samples are required to compensate

for a given amount of delay spread. The number of delay taps increases along with the speed

and complexity of the adaptive algorithm. For LTE data rates(up to 100 Mbps) and delay

spreads (approaching 17 μsec), this approach to channel equalization becomes impractical. As

we will discuss below, OFDM eliminates ISI in the time domain, which dramatically simplifies

the task of channel compensation.


14

2.2 OFDM

Unlike single carrier systems described above, OFDM communication systems do not rely on

increased symbol rates in order to achieve higher data rates. This makes the task of managing

ISI much simpler. OFDM systems break the available bandwidth into many narrower sub-

carriers and transmit the data in parallel streams. Each subcarrier is modulated using varying

levels of QAM modulation, e.g. QPSK, QAM, 64QAM or possibly higher orders depending on

signal quality. Each OFDM symbol is therefore a linear combination of the instantaneous

signals on each of the sub-carriers in the channel. Because data is transmitted in parallel rather

than serially, OFDM symbols are generally MUCH longer than symbols on single carrier

systems of equivalent data rate.

There are two truly remarkable aspects of OFDM. First, each OFDM symbol is preceded by a

cyclic prefix (CP), which is used to effectively eliminate ISI. Second, the sub-carriers are very

tightly spaced to make efficient use of available bandwidth, yet there is virtually no interference

among adjacent sub-carriers (Inter Carrier Interference, or ICI). These two unique features are

actually closely related. In order to understand how OFDM deals with multipath distortion, it‘s

useful to consider the signal in both the time and frequency domains.

To understand how OFDM deals with ISI induced by multipath, consider the time domain

representation of an OFDM symbol shown in Fig 2.5. The OFDM symbol consists of two major

components: the CP and an FFT period (TFFT). The duration of the CP is determined by the

highest anticipated degree of delay spread for the targeted application. When transmitted

signals arrive at the receiver by two paths of differing length, they are staggered in time as

shown in Fig. 2.5.[6]

Figure 2.5: OFDM Eliminates ISI via Longer Symbol Periods and a Cyclic Prefix [13]


15

Within the CP, it is possible to have distortion from the preceding symbol. However, with a CP

of sufficient duration, preceding symbols do not spill over into the FFT period; there is only

interference caused by time-staggered ―copies‖ of the current symbol. Once the channel

impulse response is determined (by periodic transmission of known reference signals),

distortion can be corrected by applying an amplitude and phase shift on a subcarrier-by-

subcarrier basis. Note that all of the information of relevance to the receiver is contained within

the FFT period. Once the signal is received and digitized, the receiver simply throws away the

CP. The result is a rectangular pulse that, within each subcarrier, is of constant amplitude over

the FFT period. The rectangular pulses resulting from decimation of the CP are central to the

ability to space subcarriers very closely in frequency without creating ICI. Readers may recall

that a uniform rectangular pulse (RECT function) in the time domain results in a SINC function

(sin(x) / x) in the frequency domain as shown in Fig. 2.6. The LTE FFT Period is 67.77 μsec.

Note that this is simply the inversion of the carrier spacing (1 /Δf). This results in a SINC

pattern in the frequency domain with uniformly spaced zero-crossings at 15 kHz intervals—

precisely at the center of the adjacent subcarrier. It is therefore possible to sample at the center

frequency of each subcarrier while encountering no interference from neighboring subcarriers

(zero-ICI).[6]

Figure 2.6 FFT of OFDM Symbol Reveals Distinct Subcarriers [13]


16

2.3 OFDMA

OFDMA is employed as the multiplexing scheme in the LTE downlink. Perhaps the best way to

describe OFDMA is by contrasting it with a packet-oriented networking scheme such as

802.11a. In 802.11a, Carrier-Sense Multiple Access (CSMA) is the multiplexing method.

Downlink and uplink traffic from the fixed access point (AP) to mobile user stations (STAs) is

by means of PHY layer packets. As explained below, OFDMA makes much more efficient use

of network resources.

2.3.1 Comparison of OFDMA with Packet-Oriented Protocols

Like 3GPP LTE, IEEE 802.11a uses OFDM as the underlying modulation method. However,

802.11a uses CSMA asthe multiplexing method. CSMA is essentially a listen-before-talk

scheme. For example, when the AP has queued traffic for a STA, it monitors the channel for

activity. When the channel becomes idle, it begins to decrement an internal timer that is

randomized within a specified window. The timer will continue to be decremented as long as

the network remains idle. When the timer reaches zero, the AP will transmit a PHY layer packet

of up to 2000 bytes addressed to a particular STA (or all STAs within the cell in the case of

broadcast mode). The randomized back-off period is designed to minimize collisions, but it

cannot eliminate them entirely.[7]

Figure 2.7Conventional Packet Oriented Networks Like IEEE 802.11a Precede Each Data

Transmission with a PHY Layer Preamble and Header [13]

Each 802.11a PHY packet utilizes all of the PHY layer bandwidth for the duration of the

packet. Consider the 802.11a PHY packet format shown in Figure 2.7. Each 802.11a packet has


17

a data payload of varying length from 64 to 2048 bytes. If the packet transmission is successful,

the receiving station transmits an ACK. Unacknowledged packets are assumed to be dropped.

Note that each packet is preceded by a PHY preamble which is 20 µsec in duration. The

purposes of the PHY preamble are:

• Signal detection

• Antenna diversity selection

• Setting AGC

• Frequency offset estimation

• Timing synchronization

• Channel estimation

The address of the intended recipient is not in the PHY preamble. It is actually in the packet

data and is interpreted at the MAC layer. From a networking perspective, the packet-oriented

approach of 802.11a has the advantage of simplicity. Each packet is addressed to a single

recipient (broadcast mode not withstanding). However, the randomized backoff period of the

CSMA multiplexing scheme is idle time and therefore represents an inefficiency. The PHY

preamble is also network overhead and further reduces efficiency, particularly for shorter

packets. The typical real-world efficiency of an 802.11a system is approximately 50 percent. In

other words, for a network with a nominal data rate of 54 Mbps, the typical throughput is about

25 – 30 Mbps. Some of the inefficiencies can be mitigated by abandoning the CSMA

multiplexing scheme and adopting a scheduled approach to packet transmission. Indeed,

subsequent versions of the 802.11 protocol include this feature. Inefficiencies due to dedicated

ACK packets can also be reduced by acknowledging packets in groups rather than individually.

In spite of potential improvements, it remains difficult to drive packet-oriented network

efficiency much beyond 65 to 70 percent. Further, because each packet completely consumes

all network resources during transmission and acknowledgement, the AP can provide addressed

(non-broadcast) traffic to user terminals only on a sequential basis. When many users are active

within the cell, latency can become a significant problem. Clearly, the objective of cellular


18

carriers is to create as much network demand as possible for a wide variety of traffic that

includes voice, multimedia, and data. Efficiency and low latency are therefore paramount. As

we will see in the following section, OFDMA is superior to packet-oriented schemes in both of

these critical dimensions.

2.3.2 OFDMA and the LTE Generic Frame Structure

OFDMA is an excellent choice of multiplexing scheme for the 3GPP LTE downlink. Although

it involves added complexity in terms of resource scheduling, it is vastly superior to packet-

oriented approaches in terms of efficiency and latency. In OFDMA, users are allocated a

specific number of subcarriers for a predetermined amount of time. These are referred to as

physical resource blocks (PRBs) in the LTE specifications. PRBs thus have both a time and

frequency dimension. Allocation of PRBs is handled by a scheduling function at the 3GPP base

station (eNodeB).[7]

Figure 2.8 LTE Generic Frame Structures [13]

In order to adequately explain OFDMA within the context of the LTE, we must study the PHY

layer generic frame structure. The generic frame structure is used with FDD. Alternative frame

structures are defined for use with TDD. However, TDD is beyond the scope of this paper.

Alternative frame structures are therefore not considered. As shown in fig. 2.8, LTE frames are

10 msec in duration. They are divided into 10 subframes, each subframe being 1.0 msec long.

Each subframe is further divided into two slots, each of 0.5 msec duration. Slots consist of

either 6 or 7 ODFM symbols, depending on whether the normal or extended cyclic prefix is

employed.


19

Table 1: Available Downlink Bandwidth is Divided into Physical Resource Blocks [13]

The total number of available subcarriers depends on the overall transmission bandwidth of the

system. The LTE specifications define parameters for system bandwidths from 1.25 MHz to 20

MHz as shown in Table 1. A PRB is defined as consisting of 12 consecutive subcarriers for one

slot (0.5 msec) in duration. A PRB is the smallest element of resource allocation assigned by

the base station scheduler.

2.4 MIMO and MRC

The LTE PHY can optionally exploit multiple transceivers at both the basestation and UE in

order to enhance link robustness and increase data rates for the LTE downlink. In particular,

maximal ratio combining (MRC) is used to enhance link reliability in challenging propagating

conditions when signal strength is low and multipath conditions are challenging. MIMO is a

related technique that is used to increase system data rates.

Figure 2.9 MRC/MIMO Operation Requires Multiple Transceivers [13]

Figure 2.9a shows a conventional single channel receiver with antenna diversity. This receiver

structure uses multiple antennas, but it is not capable of supporting MRC/MIMO. The basic


20

receiver topology for both MRC and MIMO is shown in Figure 2.9b. MRC and MIMO are

sometimes referred to as ―multiple antennas‖ technologies, but this is a bit of a misnomer. Note

that the salient difference between the receivers shown in Figures 2.9a and 2.9b is not multiple

antennas, but rather multiple transceivers. With MRC, a signal is received via two (or more)

separate antenna/transceiver pairs. Note that the antennas are physically separated, and

therefore have distinct channel impulse responses.

Channel compensation is applied to each received signal within the baseband processor before

being linearly combined to create a single composite received signal. When combined in this

manner, the received signals add coherently within the baseband processor. However, the

thermal noise from each transceiver is uncorrelated. Thus, linear combination of the channel

compensated signals at the baseband processor results in an increase in SNR of 3 dB on average

for a two-channel MRC receiver in a noise-limited environment.

Fig. 2.10 MRC Enhances Reliability in the Presence of AWGN and Frequency Selective Fading

[13]

Aside from the improvement in SNR due to combining, MRC receivers are robust in the

presence of frequency selective fading. Recall that physical separation of the receiver antennas

results in distinct channel impulse responses for each receiver channel. In the presence of

frequency selective fading, it is statistically unlikely that a given subcarrier will undergo deep

fading on both receiver channels. The possibility of deep frequency selective fades in the

composite signal is therefore significantly reduced. MRC enhances link reliability, but it does

not increase the nominal system data rate. In MRC mode, data is transmitted by a single


21

antenna and is processed at the receiver via two or more receivers. MRC is therefore a form of

receiver diversity rather than more conventional antenna diversity. MIMO, on the other hand,

does increase system data rates. This is achieved by using multiple antennas on both the

transmitting and receiving ends.

Figure 2.11 Reference Signals Transmitted Sequentially to Compute Channel Responses for

MIMO Operation [13]

In order to successfully receive a MIMO transmission, the receiver must determine the channel

impulse response from each transmitting antenna. In LTE, channel impulse responses are

determined by sequentially transmitting known reference signals from each transmitting

antenna as shown in Figure 2.11.

2.5 SC-FDMA

LTE uplink requirements differ from downlink requirements in several ways. Not surprisingly,

power consumption is a key consideration for UE terminals. The high PAPR and related loss of

efficiency associated with OFDM signaling are major concerns. As a result, an alternative to

OFDM was sought for use in the LTE uplink. Single Carrier – Frequency Domain Multiple

Access (SC-FDMA) is well suited to the LTE uplink requirements. The basic transmitter and

receiver architecture is very similar (nearly identical) to OFDMA, and it offers the same degree

of multipath protection. Importantly, because the underlying waveform is essentially single-

carrier, the PAPR is lower.


22

Figure 2.12 SC-FDMA and OFDMA Signal Chains Have a High Degree of Functional

Commonality [13]

The block diagram of Figure 2.12 shows a basic SC-FDMA transmitter / receiver arrangement.

Note that many of the functional blocks are common to both SC-FDMA and OFDMA, thus

there is a significant degree of functional commonality between the uplink and downlink signal

chains. The functional blocks in the transmit chain are:

1. Constellation mapper: Converts incoming bit stream to single carrier symbols (BPSK,

QPSK, or 16QAM depending on channel conditions)

2. Serial/parallel converter: Formats time domain SC symbols into blocks for input to FFT

engine

3. M-point DFT: Converts time domain SC symbol block into M discrete tones

4. Subcarrier mapping: Maps DFT output tones to specified subcarriers for transmission. SC-

FDMA systems either use contiguous tones (localized) or uniformly spaced tones (distributed)

as shown in Figure 2.13. The current working assumption in LTE is that localized subcarrier

mapping will be used. The trades between localized and distributed subcarrier mapping are

discussed further below.

5. N-point IDFT: Converts mapped subcarriers back into time domain for transmission

6. Cyclic prefix and pulse shaping: Cyclic prefix is pre-pended to the composite SC-FDMA

symbol to provide multipath immunity in the same manner as described for OFDM. As in the

case of OFDM, pulse shaping is employed to prevent spectral regrowth.


23

7. RFE: Converts digital signal to analog and up-convert to RF for transmission

In the receive side chain, the process is essentially reversed. As in the case of OFDM, SC-

FDMA transmissions can be thought of as linear summations of discrete subcarriers. Multipath

distortion is handled in the same manner as in OFDM systems (removal of CP, conversion to

the frequency domain, then apply the channel correction on a subcarrier-by- subcarrier

basis).Unlike OFDM, the underlying SC-FDMA signal represented by the discrete subcarriers

is—not surprisingly—single carrier. This is distinctly different than OFDM because the SC-

FDMA subcarriers are not independently modulated. As a result, PAPR is lower than for

OFDM transmissions. Analysis has shown that the LTE UE RFPA can be operated about 2 dB

closer to the 1-dB compression point than would otherwise be possible if OFDM were

employed on the uplink [2].

Figure 2.13 SC-FDMA Subcarriers Can be Mapped in Either Localized or Distributed Mode

[13]

As mentioned above, SC-FDMA subcarriers can be mapped in one of two ways: localized or

distributed as shown in Figure 2.13. However, the current working assumption is that LTE will

use localized subcarrier mapping. This decision was motivated by the fact that with localized

mapping, it is possible to exploit frequency selective gain via channel-dependent scheduling

(assigning uplink frequencies to UE based on favorable propagation conditions).


24

CHAPTER 3 LTE PHYSICAL LAYER

The capabilities of the eNodeB and UE are obviously quite different. Not surprisingly, the LTE

PHY DL and UL are quite different. The DL and UL are treated separately within the

specification documents. Therefore, the DL and UL are described separately in the following

sections.

3.0.1 Generic Frame Structure

One element shared by the LTE DL and UL is the generic frame structure. As mentioned

previously, the LTE specifications define both FDD and TDD modes of operation. This paper

deals exclusively with describing FDD specifications. The generic frame structure applies to

both the DL and UL for FDD operation.

Figure 3.1 LTE Generic Frame Structure [13]

LTE transmissions are segmented into frames, which are 10 msec in duration. Frames consist of

20 slot periods of 0.5 msec. Sub-frames contain two slot periods and are 1.0 msec in duration.

[8]

3.1 Downlink

The LTE PHY specification is designed to accommodate bandwidths from 1.25 MHz to 20

MHz OFDM was selected as the basic modulation scheme because of its robustness in the

presence of severe multipath fading. Downlink multiplexing is accomplished via OFDMA. The

DL supports physical channels, which convey information from higher layers in the LTE stack,

and physical signals which are for the exclusive use of the PHY layer. Physical channels map to


25

transport channels, which are service access points (SAPs) for the L2/L3 layers. Depending on

the assigned task, physical channels and signals use different modulation and coding

parameters.

3.1.1 Modulation Parameters

OFDM is the modulation scheme for the DL. The basic subcarrier spacing is 15 kHz, with a

reduced subcarrier spacing of 7.5 kHz available for some MB-SFN scenarios. Table 3.2

summarizes OFDM modulation parameters.[8]

Table 2 Downlink Ofdm Modulation Parameters[13]

Depending on the channel delay spread, either short or long CP is used. When short CP is used,

the first OFDM symbol in a slot has slightly longer CP than the remaining six symbols, as

shown in Table 3. This is done to preserve slot timing (0.5 msec). [8]

The cyclic prefix is actually a copy of the last portion of the data symbol appended to the front

of the symbol during the guard interval. By adding a cyclic prefix, the channel can be made to

behave as if the transmitted waveforms were from time minus infinite, and thus ensure

orthogonality, which essentially prevents one subcarrier from interfering with another (called

intercarrier interference, or ICI). This is accomplished because the amount of time dispersion

from the channel is smaller than the duration of the cyclic prefix. After discovering the process

for OFDM, a cyclic prefix has been proposed for other modulations to improve the robustness

to multipath.


26

Table 3 Cyclic Prefix Duration[13]

Note that the CP duration is described in absolute terms (e.g. 16.67 μsec for long CP) and in

terms of standard time units, Ts. Ts is used throughout the LTE specification documents. It is

defined as Ts = 1 / (15000 x 2048) seconds, which corresponds to the 30.72 MHz sample clock

for the 2048 point FFT used with the 20 MHz system bandwidth.

3.1.2 Downlink Multiplexing

OFDMA is the basic multiplexing scheme employed in the LTE downlink. OFDMA is a new-

to-cellular technology and is described in detail in Section 2.3 above. As described in Section

2.3, groups of 12 adjacent subcarriers are grouped together on a slot-by-slot basis to form

physical resource blocks (PRBs). A PRB is the smallest unit of bandwidth assigned by the base

station scheduler. A two dimensional (time and frequency) resource grid can be constructed to

represent the transmitted downlink signal. Each block in the grid represents one OFDM symbol

on a given subcarrier and is referred to as a resource element. Note that in MIMO applications,

there is one resource grid for each transmitting antenna.

3.1.3 Physical Channels

Three different types of physical channels are defined for the LTE downlink. One common

characteristic of physical channels is that they all convey information from higher layers in the

LTE stack. This is in contrast to physical signals, which convey information that is used

exclusively within the PHY layer.


27

LTE DL physical channels are:

• Physical Downlink Shared Channel (PDSCH)

• Physical Downlink Control Channel (PDCCH)

• Common Control Physical Channel (CCPCH)

Physical channels are mapped to specific transport channels as described in Section 3.1.5

below. Transport channels are SAPs for higher layers. Each physical channel has defined

algorithms for:

• Bit scrambling

• Modulation

• Layer mapping

• CDD precoding

• Resource element assignment

Layer mapping and pre-coding are related to MIMO applications. Basically, a layer corresponds

to a spatial multiplexing channel. MIMO systems are defined in terms of N transmitters x N

receivers. For LTE, defined configurations are 1x 1, 2 x 2, 3 x 2 and 4 x 2. Note that while there

are as many as four transmitting antennas, there are only a maximum of two receivers and thus

a maximum of only two spatial multiplexing data streams. For a 1 x 1 or a 2 x 2 system, there is

a simple 1:1 relationship between layers and transmitting antenna ports.

However, for a 3 x 2 and 4 x 2 system, there are still only two spatial multiplexing channels.

Therefore, there is redundancy on one or both data streams. Layer mapping specifies exactly

how the extra transmitter antennas are employed. Precoding is also used in conjunction with

spatial multiplexing. Recall that MIMO exploits multipath to resolve independent spatial data

streams. In other words, MIMO systems require a certain degree of multipath for reliable

operation. In a noise-limited environment with low multipath distortion, MIMO systems can

actually become impaired.[8]


28

Physical Downlink Shared Channel

The PDSCH is utilized basically for data and multimedia transport. It therefore is designed for

very high data rates. Modulation options therefore include QPSK, 16QAM and 64QAM. Spatial

multiplexing is also used in the PDSCH. In fact, spatial multiplexing is exclusive to the

PDSCH. It is not used on either the PDCCH or the CCPCH.

To guard against propagation channel errors, convolutional turbo coder is used for forward

error correction. The data is mapped to spatial layers according to the type of multi-antenna

technique.

Physical Downlink Control Channel

The PDCCH conveys UE-specific control information. Robustness rather than maximum data

rate is therefore the chief consideration. QPSK is the only available modulation format. The

PDCCH is mapped onto resource elements in up to the first three OFDM symbols in the first

slot of a subframe.

Common Control Physical Channel

The CCPCH carries cell-wide control information. Like the PDCCH, robustness rather than

maximum data rate is the chief consideration. QPSK is therefore the only available modulation

format. In addition, the CCPCH is transmitted as close to the center frequency as possible.

CCPCH is transmitted exclusively on the 72 active subcarriers centered on the DC subcarrier.

Control information is mapped to resource elements (k, l) where k refers to the OFDM symbol

within the slot and l refers to the subcarrier. CCPCH symbols are mapped to resource elements

in increasing order of index k first, then l.

3.1.4 Physical Signals

Physical signals use assigned resource elements. However, unlike physical channels, physical

signals do not convey information to/from higher layers. There are two types of physical

signals:


29

• Reference signals used to determine the channel impulse response (CIR)

• Synchronization signals which convey network timing information

Reference Signals

Reference signals are generated as the product of an orthogonal sequence and a pseudo-random

numerical (PRN) sequence. Overall, there are 510 unique reference signals possible. A

specified reference signal is assigned to each cell within a network and acts as a cell-specific

identifier.

Figure 3.2 Resource Element Mapping of Reference Signals[13]

As shown in Figure 3.2, reference signals are transmitted on equally spaced subcarriers within

the first and third-from-last OFDM symbol of each slot. UE must get an accurate CIR from

each transmitting antenna. Therefore, when a reference signal is transmitted from one antenna

port, the other antenna ports in the cell are idle. Reference signals are sent on every sixth

subcarrier. CIR estimates for subcarriers that do not bear reference signals are computed via

interpolation


30

Changing the subcarriers that bear reference signals by pseudo-random frequency hopping is

also under consideration.

Synchronization Signals

Synchronization signals use the same type of pseudo-random orthogonal sequences as reference

signals. These are classified as primary and secondary synchronization signals, depending how

they are used by UE during the cell search procedure. Both primary and secondary

synchronization signals are transmitted on the 72 subcarriers centered around the DC subcarrier

during the 0th and 10th slots of a frame (recall there are 20 slots within each frame).

3.1.5 Transport Channels

Transport channels are included in the LTE PHY and act as service access points (SAPs) for

higher layers. Downlink Transport channels are:[6]

Broadcast Channel (BCH)

• Fixed format

• Must be broadcast over entire coverage area of cell

Downlink Shared Channel (DL-SCH)

• Supports Hybrid ARQ (HARQ)

• Supports dynamic link adaption by varying modulation, coding and transmit power

• Suitable for transmission over entire cell coverage area

• Suitable for use with beam forming

• Support for dynamic and semi-static resource allocation

• Support for discontinuous receive (DRX) for power save


31

Paging Channel (PCH)

• Requirement for broadcast over entire cell coverage area

• Mapped to dynamically allocated physical resources

Multicast Channel (MCH)

• Requirement for broadcast over entire cell coverage area

• Support for MB-SFN

• Support for semi-static resource allocation

3.1.6 Mapping Downlink Physical Channels to Transport Channels

Transport channels are mapped to physical channels as shown in Figure 3.3. Supported

transport channels are:[6]

1. Broadcast channel (BCH)

2. Paging channel (PCH)

3. Downlink shared channel(DL-SCH)

4. Multicast channel (MCH)

Figure 3.3 Mapping DL Transport Channels to physical channels[13]


32

Transport channels provide the following functions:

• Structure for passing data to/from higher layers Under Consideration

• Mechanism by which higher layers can configure the PHY

• Status indicators (packet error,DL Physical Channels CCPCH PDSCH PDCCHCQI

etc.) to higher layers.

• Support for higher-layer peer-to-peer signalling

3.1.7 Downlink Channel Coding

Different coding algorithms are employed for the DL physical channels. For the common

control channel (CCPCH), modulation is restricted to QPSK. The PDSCH uses up to 64 QAM

modulation. For control channels, coverage is the paramount requirement. Convolutional

coding has been selected for use with the CCPCH, though a final determination regarding code

rate has not yet been made. On the PDSCH, higher-complexity modulation is employed to

achieve the highest possible downlink data rates. The PDSCH uses QPSK, 16QAM, or 64QAM

depending on channel conditions. As a result, coding gain is emphasized over latency. Rate 1/3

turbo coding has been selected for the PDSCH.

The cyclic prefix is actually a copy of the last portion of the data symbol appended to the front

of the symbol during the guard interval. By adding a cyclic prefix, the channel can be made to

behave as if the transmitted waveforms were from time minus infinite, and thus ensure

orthogonality, which essentially prevents one subcarrier from interfering with another (called

intercarrier interference, or ICI). This is accomplished because the amount of time dispersion

from the channel is smaller than the duration of the cyclic prefix. After discovering the process

for OFDM, a cyclic prefix has been proposed for other modulations to improve the robustness

to multipath.


33

CHAPTER 4 OFDM

4.1 OFDM for LTE

The choice of an appropriate modulation and multiple-access technique for mobile wireless data

communications is critical to achieving good system performance. In particular, typical mobile

radio channels tend to be dispersive and time-variant, and this has generated interest in

multicarrier modulation.

In general, multicarrier schemes subdivide the used channel bandwidth into a number of

parallel sub channels. Ideally the bandwidth of each sub channel is such that they are, ideally,

each non-frequency selective (i.e. having a spectrally flat gain); this has the advantage that the

receiver can easily compensate for the sub channel gains individually in the frequency domain.

Orthogonal Frequency Division Multiplexing (OFDM) is a special case of multicarrier

transmission where the non-frequency-selective narrowband sub channels, into which the

frequency-selective wideband channel is divided, are overlapping but orthogonal. This avoids

the need to separate the carriers by means of guard-bands, and therefore makes OFDM highly

spectrally efficient. The spacing between the sub channels in OFDM is such that they can be

perfectly separated at the receiver. This allows for a low complexity receiver implementation,

which makes OFDM attractive for high rate mobile data transmission such as the LTE

downlink.

It is worth noting that the advantage of separating the transmission into multiple narrowband

sub channels cannot itself translate into robustness against time-variant channels if no channel

coding is employed. The LTE downlink combines OFDM with channel coding and Hybrid

Automatic Repeat request (HARQ) to overcome the deep fading which may be encountered on

the individual sub channels. These aspects lead to the LTE downlink falling under the category

of system often referred to as ‗Coded OFDM‘ (COFDM). The primary advantage of OFDM

over single-carrier schemes is its ability to cope with severe channel conditions (for example,

attenuation of high frequencies in a long copper wire, narrowband interference and frequency-

selective fading due to multipath) without complex equalization filters


34

Figure 4.1 .Effect of channel on signals with short and long symbol duration.[12]

4.2 OFDM architecture

Fig 4.2 Simplex Point-to-point transmission using OFDM[12]

Figure shows the block diagram of a simplex point-to-point transmission system using OFDM

and FEC coding. The three main principles incorporated are as follows:

1. The IDFT and the DFT are used for, respectively, modulating and demodulating the data

constellations on the orthogonal SCs . These signal-processing algorithms replace the

banks of I/Q-modulators and demodulators that would otherwise be required. Note that at the

input of the IDFT, N data constellation points {xi,k} are present, where N is the number of DFT

points. (i is an index on the SC; k is an index on the OFDM symbol). These constellations can

be taken according to any phase shift keying (PSK) or QAM signaling set (symbol mapping).

The N output samples of the IDFT, being in TD, form the baseband signal carrying the data


35

symbols on a set of N orthogonal SCs. In a real system, however, not all of these N possible

SCs can be used for data Usually, N is taken as an integer to the power of two, enabling the

application of the highly efficient (inverse) FFT algorithms for modulation and demodulation.

2. The second key principle is the introduction of a cyclic prefix as a GI, whose length should

exceed the maximum excess delay of the multipath propagation channel . Due to the cyclic

prefix, the transmitted signal becomes periodic, and the effect of the time-dispersive multipath

channel becomes equivalent to a cyclic convolution, discarding the GI at the receiver. Due to

the properties of the cyclic convolution, the effect of the multipath channel is limited to a point

wise multiplication of the transmitted data constellations by the channel TF, or the FT of the

channel IR; that is, the SCs remain orthogonal This conclusion will also follow from the

derivation of the system model. The only drawback of this principle is a slight loss of effective

transmit power, as the redundant GI must be transmitted. Usually, the GI is selected to have a

length of one tenth to a quarter of the symbol period, leading to an SNR loss of 0.5 to 1 dB .

Fig 4.3 OFDM Cyclic Prefix (CP) insertion.[12]

Fig. 4.4 cyclic extension and windowing of OFDM[12]


36

The equalization (symbol demapping) required for detecting the data constellations is an

element wise multiplication of the DFT output by the inverse of the estimated channel TF

(channel estimation). For phase modulation schemes, multiplication by the complex conjugate

of the channel estimate can do the equalization. Differential detection can be applied as well,

where the symbol constellations of adjacent SCs or subsequent OFDM symbols are compared

to recover the data.

3. FEC coding and (FD) interleaving are the third crucial idea applied. The frequency-selective

radio channel may severely attenuate the data symbols transmitted on one or several SCs,

leading to bit errors. Spreading the coded bits over the bandwidth of the transmitted system, an

efficient coding scheme can correct for the erroneous bits and thereby exploit the wideband

channel‘s frequency diversity. OFDM systems utilizing error-correction coding are often

referred as coded OFDM (COFDM) systems. The complex equivalent baseband signals

generated by digital signal processing are in-phase/quadrature (I/Q)–modulated and up-

converted to be transmitted via an RF carrier. The reverse steps are performed by the receiver.

Synchronization is a key issue in the design of a robust OFDM receiver. Time and frequency

synchronization are paramount, respectively, to identify the start of the OFDM symbol and to

align the modulators‘ and the demodulators‘ local oscillator frequencies. If any of these

synchronization tasks is not performed with sufficient accuracy, then the orthogonality of the

SCs is(partly) lost. That is, ISI and ICI are introduced.

4.3 FFT Implementation

Figure 4.5 OFDM Transmitter[12]


37

As seen in OFDM block diagram the signal to be transmitted is defined in the frequency

domain.

A Serial to Parallel (S/P) converter collects serial data symbols into a data block Sk = [Sk [0] ,

Sk [1] , . . . , Sk [M 1]]T of dimension M, where the subscript k is the index of an OFDM

symbol (spanning the M sub-carriers).here M parallel data string can be differently modulated

or same modulation can be applied on it which results in complex vector Xk = [Xk [0] , Xk [1] ,

. . . , Xk [M 1]]T. Zero padding is done that makes processed carrier greater than modulated

subcarrier (N>M). To avoid ISI completely, the CP length G must be chosen to be longer than

the longest channel impulse response to be supported. The CP converts the linear (i.e.

aperiodic) convolution of the channel into a circular (i.e. periodic) one which is suitable for

DFT processing. The output of the IFFT is then Parallel-to-Serial (P/S) converted for

transmission through the frequency- selective channel. At the receiver, a highly efficient FFT

implementation may be used to transform the signal back to the frequency domain. i.e, the

reverse operations are performed to demodulate the OFDM signal. Assuming that time- and

frequency-synchronization is achieved, a number of samples corresponding to the length of the

CP are removed, such that only an ISI-free block of samples is passed to the DFT.

4.4 OFDMA BASICS

The additional tasks that the OFDMA receiver needs to cover are time and frequency

synchronization. Synchronization allows the correct frame and OFDMA symbol timing to be

obtained so that the correct part of the received signal is dropped (cyclic prefix removal). Time

synchronization is typically obtained by correlation with known data samples – based on, for

example, the reference symbols – and the actual received data. The frequency synchronization

estimates the frequency offset between the transmitter and the receiver and with a good estimate

of the frequency offset between the device and base station, the impact can be then

compensated both for receiver and transmitter parts. The device locks to the frequency obtained

from the base station, as the device oscillator is not as accurate (and expensive) as the one in the

base station.

Even if in theory the OFDMA transmission has rather good spectral properties, the real

transmitter will cause some spreading of the spectrum due to imperfections such as the clipping


38

in the transmitter. Thus the actual OFDMA transmitter needs to have filtering similar to the

pulse shape filtering in WCDMA. In the literature this filtering is often referred as windowing.

An important aspect of the use of OFDMA in a base station transmitter is that users can be

allocated basically to any of the sub-carriers in the frequency domain. This is an additional

element to the HSDPA scheduler operation, where the allocations were only in the time domain

and code domain but always occupied the full bandwidth. The possibility of having different

sub-carriers to allocated users enables the scheduler to benefit from the diversity in the

frequency domain, this diversity being due to the momentary interference and fading

differences in different parts of the system bandwidth. The practical limitation is that the

signaling resolution due to the resulting overhead has meant that allocation is not done on an

individual sub-carrier basis but is based on resource blocks, each consisting of 12 sub-carriers,

thus resulting in the minimum bandwidth allocation being 180 kHz. When the respective

allocation resolution in the time domain is 1 ms, the downlink transmission resource allocation

thus means filling the resource pool with 180 kHz blocks at 1 ms resolution. Note that the

resource block in the specifications refers to the 0.5 ms slot, but the resource allocation is done

anyway with the 1 ms resolution in the time domain. This element of allocating resources

dynamically in the frequency domain is often referred to as frequency domain scheduling or

frequency domain diversity. Different sub-carriers could ideally have different modulations if

one could adapt the channel without restrictions. For practical reasons it would be far too

inefficient to try either to obtain feedback with 15 kHz sub-carrier resolution or to signal the

modulation applied on a individual sub-carrier basis. Thus parameters such as modulation are

fixed on the resource block basis.

The OFDMA transmission in the frequency domain thus consists of several parallel subcarriers,

which in the time domain correspond to multiple sinusoidal waves with different frequencies

filling the system bandwidth with steps of 15 kHz. This causes the signal envelope to vary

strongly compared to a normal QAM modulator, which is only sending one symbol at a time (in

the time domain). The momentary sum of sinusoids leads to the Gaussian distribution of

different peak amplitude values. This causes some challenges to the amplifier design as, in a

cellular system; one should aim for maximum power amplifier efficiency to achieve minimum

power consumption.


39

There is a signal with a higher envelope variation (such as the OFDMA signal in the time

domain) requires the amplifier to use additional back-off compared to a regular single carrier

signal. The amplifier must stay in the linear area with the use of extra power back-off in order

to prevent problems to the output signal and spectrum mask. The use of additional back-off

leads to a reduced amplifier power efficiency or a smaller output power. This either causes the

uplink range to be shorter or, when the same average output power level is maintained, the

battery energy is consumed faster due to higher amplifier power consumption. The latter is not

considered a problem in fixed applications where the device has a large volume and is

connected to the mains, but for small mobile devices running on their own batteries it creates

more challenges.

An OFDMA system is also sensitive to frequency errors. The basic LTE sub-carrier spacing of

15 kHz facilitates enough tolerance for the effects of implementation errors and Doppler Effect

without too much degradation in the sub-carrier orthogonality.


40

CHAPTER 5 OFDM IMPLEMENTATION

5.1 OFDM Block Diagram

It shows the Block Diagram of the OFDM System including transmitter, receiver and channel.

Fig 5.1: OFDM Block Diagram


41

5.2 Components used in OFDM

1.RandomBits (Random Bit Generator)

The RandomBits block generates a random bit sequence in which the probability of a 0 bit is

ProbOfZero and the probability of a 1 bit is 1 – ProbOfZero. Parameters can be set in the

parameter box as shown in “Fig.5.2” where the RandomBits block is shown on the right top

corner.

Fig. 5.2 Parameter change box of a RandomBits block.

2.S INK ( DAT A S I NK)

Fig.5.3 Data sink block.

This block is an output data drop down box which provides three choices on how the

simulation data will be stored:


42

1. DataSet: simulation data is stored in a dataset; data in the dataset can be plotted in

graphs/tables as well as post processed in equation pages.

2. File: simulation data is written into one (or more) file(s).

3. Both: simulation data is stored in a dataset as well as written into one (or more)

file(s).

3.Mapper (Complex Symbol Mapper)

Fig. 5.4 Parameter box of a Complex Symbol Mapper.

Mapper block supports different types of modulation: BPSK, QPSK, PSK8, PSK16, QAM16,

QAM32, QAM64, QAM128, QAM256 or can be user defined. Input to this block is Boolean

bit sequence and output is complex symbols. These parameters can be set in the parameter

box shown in the “Fig. 5.4”. Mapper groups consecutive bits as specified by the

BITORDER parameter in the input to form a symbol value which is mapped to a complex

valued constellation point that is output.

A constellation point is a pair of real values (I,Q) that is expressed on the output as I + jQ.

Later in the modulation chain, I modulates the inphase part of the carrier, and Q modulates

the quadrature part of the carrier over a symbol period. Each modulation type has its

constellation and symbol length. For QPSK, PSK8, and PSK16 the mapping from bits to

symbols is using Gray encoding. For QAM16, QAM32, QAM64, QAM128, and QAM256,


43

Gray encoding is used inside each quadrant. When ModType is specified to

USER_DEFINED, a custom constellation is defined with Mapping Table. The input symbol

is mapped directly to a constellation point as a 0 based index into Mapping Table.

4.OFDM subcarrier multiplexing

Fig.5.5 Parameter block for OFDM Subcarrier Multiplexing block

Input to this block is multiple complex signals to be placed in subcarriers and there are two

outputs to this block: a) OFDM frequency domain format ready for applying IDFT and b)

EVM (Error Vector Magnitude) reference subcarriers. Both the outputs are in complex form.

The “Fig5. 5 ” shows a set of example values of the parameters to be set. This model

multiplexes different types of signals in frequency-domain to corresponding subcarrier

locations. The Output is ready to be processed by the Inverse Digital Fourier Transformation

(IDFT). Each branch of the input bus may be data of an OFDMA user or pilots.

44


5.FFT_Cx (Complex Fast Fourier Transform)

Input and output to this block is complex signals. This model computes the DFT (Discrete

Fourier Transform) of the input signal using a mixed radix FFT (Fast Fourier Transform)

algorithm. At every execution of this model, Size complex samples are read from the input.

This block of Size samples is zero padded (if Size < FFTSize) to create a block of FFTSize

Samples. The block of FFTSize samples is then processed by a mixed radix FFT algorithm to

produce FFT_Size equally spaced samples that is the DFT of the input signal. The Direction

parameter specifies whether a forward or inverse FFT will be performed

Fig.5.6 Parameter box of a Complex Fast Fourier Transformation block

The FreqSequence parameter specifies the order in which the frequency values are written to

the output (forward FFT case) or read from the input (inverse FFT case). The “Fig. 5.6” shows

a set of examples values for the parameters and defaults values.


45

6.OFDM_GuardInsert (OFDM guard interval insertion)

Fig.5.7 Parameter box of an OFDM guard interval insertion block.

This model inserts guard intervals to OFDM symbols. The inputs are consecutive OFDM

time-domain signals from IDFT (Inverse Digital Fourier Transformation) module. Both the

input and output are complex signals. Both prefix and postfix can be added to input signal.

The stuff signal may be cyclic shift (extension) of an IDFT period or zeros. Different guard

intervals may be added to different OFDM symbols. The “Fig. 5.7” shows an example

values of parameters along with their default values.

7.CxToEnv (Complex to Envelope)

Fig. 5.8 Parameter box of a Complex to Envelope converter block.


46

The CxToEnv block converts the complex signal at input to a complex envelope signal at

output using the characterization frequency associated with the complex envelope signal at

input fc. Thus there can be two input signals, a complex signal and a complex envelope

signal. This block reads 1 sample from both inputs and writes 1 sample to output. Output to

this block is a complex envelope signal. The input fc is optional. CxToEnv is a modulator

whose output obtains it’s I and Q values from the input and its carrier frequency from fc. If

the fc input is not a complex envelope signal, then the output will be made a real signal and

the imaginary part of input will be ignored.

8.Add NDensity (Add Noise Density to input)

Fig.5.9 Parameter box of an Add Noise Density to Input Block.

Both the input and output to this block is envelope signals. This model adds noise to the input

signal. At every execution, it reads 1 sample from the input and writes 1 sample to the output.

If NDensityType is set to Constant noise density, then the noise added is white Gaussian.

The noise density is specified in the NDensity parameter. Although the units for this

parameter are power units the value is interpreted as power spectral density, that is, power

per frequency unit (Hz). The total noise power added to the input signal is NDensity × BW,

where NDensity is the noise power spectral density in Watts/Hz and BW is the simulation

bandwidth in Hz. If NDensityType is set to Noise density vs freq, then the spectral profile


47

of the noise added can be specified using the NDensityFreq (values need to be in Hz) and

NDensityPower (values need to be in dBm/Hz) array parameters. Here used , Additive white

Gaussian noise is a channel model in which the only impairment to communication is

linear addition of wideband or white noise with a constant spectral density(expressed as

watts per hertz of bandwidth )and Gaussian distribution of amplitude.

9.EnvToCx (Envelope to Complex)

Fig. 5.10 An envelope to complex converter block.

Input to this block is an envelope and there are two outputs, a complex signal and its

characteristic frequency. EnvToCx decomposes input into a complex envelope and its

characteristic frequency. For every input sample, one sample is written to both outputs. If

input is a real baseband signal (v), then the output is real and set to the input value (v), and fc

is a complex envelope signal set to 0+j*0 with a zero characteristic frequency. If input is a

complex envelope signal (i+j*q with non-zero characterization frequency f1), then the output

is a complex envelope set to the input value (i+j*q with non-zero characterization frequency

f1), and fc is a complex envelope signal set to 0+j*0 with non-zero characteristic frequency

set to f1. The parameter box for this model is similar to that of a Complex to Envelope

converter as shown in “Fig. 5.10”.


48

10.OFDM_GuardRemove (OFDM guard interval removal)

Fig. 5.11 Parameter box of an OFDM guard interval removal block.

Both the input and output signals to this block is complex signals. This model removes the

guard interval of the OFDM symbol. It outputs OFDM time-domain signals to DFT (Digital

Fourier Transformation) module directly. It can remove different guard intervals for different

OFDM symbols. It can work in two modes: CyclicShift and Zeros for cyclic guard interval

and zero padding guard interval removal. The mode can be specified in parameter

GuardStuff. In CyclicShift mode, it can remove cyclic guard interval. Parameter CIRAdjust

can be specified to get the DFT signals with cyclic delay, which can shift the CIR (channel

impulse response) in the time domain.In Zeros mode, it can remove zero padding guard

interval. Parameter CIRLength is the non-oversampled samples number covered by

maximum multipath delay. In zero padding mode, it needs to add the CIRLengthOS samples

that follow DFT window to the first CIRLengthOS samples in DFT window. So the input

begins with the CIRLengthOSth

sample and ends in the CIRLengthOSth

sample of

nextOFDM symbol.


49

11.FFT_Cx (Complex Fast Fourier Transform)

Fig. 5.12 A Complex Fast Fourier Transform block.

This block is the same as the one used for IFFT purpose. Input and output to this block is

complex signals and parameter box is shown is “Fig. 6(f)”. In the block used previously, the

direction parameter was set as “inverse” to obtain IFFT of signals. But there the direction

parameter is set as “forward” to obtain the FFT of the signal. As mentioned earlier, this

model computes the DFT (Discrete Fourier Transform) of the input signal using a mixed

radix FFT (Fast Fourier Transform) algorithm. At every execution of this model, Size

complex samples are read from the input. This block of Size samples is zero padded (if Size <

FFTSize) to create a block of FFTSize samples. The block of FFTSize samples is

then processed by a mixed radix FFT algorithm to produce FFT_Size equally spaced

samples that is the DFT of the input signal. The Direction parameter specifies whether

a forward or inverse FFT will be performed. The FreqSequence parameter specifies the

order in which the frequency values are written to the output (forward FFT case) or read from

the input (inverse FFT case)


50

12. OFDM_SubcarrierDemux (OFDM subcarrier demultiplexing)

Fig. 5.13 Parameter box of an OFDM subcarrier demultiplexing block.

The input signal is OFDM frequency domain signals from DFT and the output is the signals

demultiplexed from input subcarriers. This model de-multiplexes different type of signals

from OFDM symbols in frequency-domain. The parameters of this model is similar to that

of OFDM_SubcarrierMux and should be set accordingly.

13. D EMA P P E R ( COM P LEX S YMBOL DEMAPP E R )

As in the mapper block, this demapper block also supports different types of modulation

schemes like, BPSK, QPSK, PSK8, PSK16, QAM16, QAM32, QAM66, QAM128, and

QAM256 or can be user defined. And just opposite to the mapper block, the input to this block

is a complex symbol sequence and the output is boolean bit sequence. Demapper inputs a

complex value, finds the nearest constellation node for the input, and outputs both the

constellation node and the symbol value for the constellation node in a bit sequence specified

by the BitOrder parameter. Thus this block has two outputs


51

Fig.5.14 Parameter box as a Complex Symbol Demapper/Slicer block.

For each input, one constellation node is output at Node and depending on the ModType

parameter Symbol Length number of bits is output at Bits. A constellation value is a pair of

real values (I,Q) that is expressed on the input as I + jQ. Earlier in the modulation chain, I

modulated the inphase part of the carrier, and Q modulated the quadrature part of the carrier

over a symbol period. The output symbols are assumed to be Gray coded. When ModType is

specified to USER_DEFINED, a custom constellation is defined with MappingTable. The

output symbol is mapped directly to a constellation point as a 0 based index into

MappingTable. The parameters are similar to that of a mapper block and have to be set

accordingly.

14. Delay.

Input and output to this block can be signals of anytype. This model introduces a delay of N

samples to the input signal. For every input, there is one output. The initial N output samples

have a null value. For scalar signals, a null value is 0. For matrix signals, a null value is a

matrix with the same size as the input matrix and with all its elements set to 0.


52

Fig.5.15 Parameter box of a Delay block.

15. BER_FER (Bit and Frame Error Rate Measurement)

There are two inputs to this block, a reference bit stream and a test bit stream. Both the inputs

are of integer type. The BER_FER model can be used to measure the BER (bit error rate) and

FER (frame error rate) of a system. In some systems, FER is referred to as PER (packet error

rate) or BLER (block error rate). The input signals to the reference (REF) and test (TEST)

inputs must be bit streams. The bit streams must be synchronized, otherwise the BER/FER

estimates are wrong. The Start parameter defines when data processing starts. The end of

data processing depends on the settings of the Stop and EstRelVariance parameters: If

EstRelVariance is 0.0, then data processing ends when Stop is reached. If EstRelVariance is

greater than 0.0, then data processing ends when EstRelvariance is met or when Stop is

reached. In this case, Stop acts as an upper bound on how long the simulation runs just in

case the simulation takes too long for EstRelVariance to be met. In this mode of operation,

messages are printed in the simulation log showing the value of estimation relative variance

as the simulation progresses. The EstRelVariance parameter can be used to control the

quality of the BER estimate obtained. The lower the value of EstRelVariance the more

accurate the estimate is. The BitsPerFrame parameter sets the number of bits in each frame.

A frame is considered to be in error if at least one of the bits in the frame is detected

incorrectly.


53

Fig. 5.16 Parameter box of a Bit and Frame Error Rate Measurement Block.

5.3 Measurement of signal at different points.

Fig 5.17 OFDM Block


54


54

1. Input signal

The input signal as the random bits is provided by the Random bit generator.The figure shows

the amplitude vs. time curve for the input signal

Fig 5.18 .Output of Random bit Generator

2. Output of the mapper

The mapper maps the input bits to the corresponding symbols according the maaping scheme

specified.The figure shows the mapping scheme for the 16 QAM mapping.

Fig 5.19: Scatterplot diagram for 16 QAM


55

3. Output of the subcarrier MUX

It shows the polar plot of the subcarrier

Fig 5.20: Polar plot after sub carrier allotment

4. Output of FFT

Fig 5.21: Output of FFT


56

5. After Guard Band Insertion

Cyclic prefix is added to prevent the ISI in OFDM

Fig 5.22: OFDM Signals after Guardband Insertion

6. After Noise addition

Noise is added to simulate the noise present in the diffrenet wireless environments.

Fig 5.23: OFDM signal after noise addition


57

7. Spectrum at Receiver Side

The output spectrum after demodulation and guard band removal is shown in the figure.

Fig 5.24: Spectrum at receiver side

8. Scatter plot after Demodulation

The Constellation plot at the receiver side contains the same input plot with little variation of

position due to noise or other variations.

Fig 5.25: Scatter plot after Demodulation.


58

9. Output Signal

The output of the mapper is taken to obtain the output signal. The demapper converts the

corresponding symbols into continous bit stream.

Fig 5.26: Output Signal.


59

CHAPTER 6

PERFORMANCE ANALYSIS FOR LTE

6.1 Quantitative factors in LTE

1. BER

In digital transmission, the number of bit errors is the number of received bits of a data

stream over a communication channel that have been altered due

to noise, interference, distortion or bit synchronization errors.The bit error rate or bit error

ratio (BER) is the number of bit errors divided by the total number of transferred bits during a

studied time interval. BER is a unitless performance measure, often expressed as a percentage.

The bit error probability is the expectation value of the BER. The BER can be considered as an

approximate estimate of the bit error probability. This estimate is accurate for a long time

interval and a high number of bit errors.The packet error rate (PER) is the number of incorrectly

received data packets divided by the total number of received packets. A packet is declared

incorrect if at least one bit is erroneous. The BER may be improved by choosing a strong signal

strength (unless this causes cross-talk and more bit errors), by choosing a slow and

robust modulation scheme or line coding scheme, and by applying channel coding schemes such

as redundant forward error correction codes.

2. BLER

Block Error Rate (BLER) is used in LTE/4G technology to know the in-sync or out-of-sync

indication during radio link monitoring (RLM). This is number of erroneous blocks / Total no of

Received Blocks. Normal in-sync condition is 2% of BLER and for out-of-sync is 10%.

3.THROUGHPUT

Throughput or network throughput is the average rate of successful message delivery over a

communication channel. This data may be delivered over a physical or logical link, or pass

through a certain network node. The throughput is usually measured in bits per second (bit/s or

bps), and sometimes in data packets per second or data packets per time slot.

http://en.wikipedia.org/wiki/Digital_transmission

http://en.wikipedia.org/wiki/Bit

http://en.wikipedia.org/wiki/Data_stream

http://en.wikipedia.org/wiki/Data_stream

http://en.wikipedia.org/wiki/Communication_channel

http://en.wikipedia.org/wiki/Noise_(telecommunications)

http://en.wikipedia.org/wiki/Interference_(communication)

http://en.wikipedia.org/wiki/Distortion

http://en.wikipedia.org/wiki/Bit_synchronization

http://en.wikipedia.org/wiki/Percentage

http://en.wikipedia.org/wiki/Expectation_value

http://en.wikipedia.org/wiki/Network_packet

http://en.wikipedia.org/wiki/Modulation

http://en.wikipedia.org/wiki/Line_coding

http://en.wikipedia.org/wiki/Channel_coding

http://en.wikipedia.org/wiki/Forward_error_correction

http://en.wikipedia.org/wiki/Network_node

http://en.wikipedia.org/wiki/Bit

http://en.wikipedia.org/wiki/Data_packets

http://en.wikipedia.org/wiki/Time-division_multiplexing


60

6.2 Implementation and Results For Different Schemes

1 .SISO BER

The block diagram for calculating the BER of the SISO Scheme is shown in the figure .The ber

is found out by comparing the received signal with the original input signal

Figure 6.1 Basic Block for calculating BER of LTE SISO Scheme


61

Figure 6.2 BER vs SNR plot of LTE SISO Scheme

Figure 6.3 BLER vs SNR plot of LTE SISO Scheme


62

2.MIMO BER

The diagram for finding the BER of the MIMO systems is shown in the figure.

Figure 6.4 Block Diagram for calculating MIMO BER with QPSK Modulation

Fig6.5 :BER vs SNR Graph of MIMO LTE System


63

Figure 6.6 Block Diagram for MIMO BER plot with 16 QAM Modulation

Fig6.7 :BER vs SNR Graph of MIMO LTE System with 16 QAM Modulation


64

3.SISO THROUGHPUT

Block diagram for implementation of the lte SISO systems is given in the figure.Constant noise

is added for simulation of the AWGN noise

Figure 6.8 Basic blocks of SISO Throughput Calculation

Figure 6.9 Throughput vs SNR plot of LTE SISO Scheme


65

Figure 6.10 SISO Throughput fraction vs SNR


66

4.MIMO THROUGHPUT

The block diagram for the implementation of the 2x2 MIMO scheme with HARQ erorr

correction is shown in the figure.

Figure 6.11 Block Diagram for plotting MIMO Throughput for QPSK Modulation

Figure 6.12 Throughput vs SNR plot MIMO scheme with QPSK Modulation


67

Figure 6.13 MIMO Throughput Fraction vs SNR for QPSK Modulation


68

Figure 6.14 Block Diagram for plotting MIMO Throughput for 16 QAM Modulation

Figure 6.15 MIMO Throughput vs SNR for 16QAM modulation


69

Figure 6.16 MIMO Throughput Fraction vs SNR for 16QAM modulation


70

6.3 Result and Analysis.

As compared from the MIMO and SISO Graph ,For SNR =13.3db.

Throughput for QPSK Modulation for SISO =0.6mbps

Throughput for QPSK Modulation for MIMO =9.5mbps

The throughput for the MIMO system increases due to the use of multiple antennas.

As seen from the figure in case of MIMO for SNR=15db

Throughput for QPSK Modulation =10.6 Mbps

Throughput for 16 QAM Modulation =15 Mbps

As the higher modulation schemes are used the throughput of the system increases .

From the graph, for SNR = 15db

BER of the SISO System for QPSK Modulation=nearly 1e-3

BER of the MIMO System for QPSK Modulation= 1e-6

As the number of antennas increases the quality of the channel increases due to diversity and

the BER Performance is improved.

For the MIMO Systems, For SNR= 25 db

BER of the MIMO System for QPSK Modulation= 1e-6

BER of the MIMO System for 16 QAM Modulation= 1e-3

As the higher modulation schemes are used The BER performance is degraded due to the less

redundancy in the higher modulation schemes.

So we recommend the use of Adaptive Modulation Schemes for balancing the need of the higher

throughput and the good signal quality and MIMO schemes for improving both throughput and

BER Performance.


71

CHAPTER 7 CONCLUSION

The 3GPP Long Term Evolution is the latest evolution of the wireless communication systems.

LTE is part of the UMTS standards but includes many changes and improvements identifiedby

the 3GPP consortium. The goal of LTE is to increase the data throughput and the speed of

wireless data using a combination of new methods and technologies like OFDM and MIMO

technics. The LTE downlink transmission is based on Orthogonal Frequency Division Multiple

Access (OFDMA).

In this project work, an effective study, analysis and evaluation of the LTE downlink

performance with 2x2MIMO techniques in comparison with the traditional SISO system has

been carried out. The performance is evaluated with respect to two definitive metrics namely

Throughput and BER,. In our research we analyze that for a fix value of SNR, the BER increases

for high order modulation (16-QAM and 64-QAM) used in LTE system. On the other hand, the

lower order modulation scheme ( QPSK) experience less BER at receiver thus lower order

modulations improve the system performance in terms of BER and SNR. If we consider the

bandwidth efficiency of these modulation schemes, the higher order modulation accommodates

more data within a given bandwidth and is more bandwidth efficient as compare to lower order

modulation. Thus there exists a tradeoff between BER and bandwidth efficiency among these

modulation schemes used in LTE. We also observed the increase in throughput with the use of

multiple antenna i.e. MIMO.

To obtain high performance under all the changing conditions use of adaptive Modulation and

Coding is suggested and use of MIMO systems is suggested for either increasing the reliability

of the network or to increase the throughput.


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References

[1]. 3GPP TR 25.913 - v7.3.0, Requirements for EUTRA and EUTRAN,

http://www.3gpp.org/ftp/Specs/archive/25%5Fseries/25.913/

[2]. Van Nee and Prasad, OFDM for Wireless Multimedia Communications, Artech House

Publishers,ISBN 0-890006-530-6, 2000

[3]. T Doc #R1-060023, Cubic Metric in 3GPP-LTE, Motorola, Helsinki, January 2006

[4. 3GPP TS 36.300 – v8.0.0, E-UTRA and E-UTRAN Overall Description,


[5]. 3GPP TS 36.201 – v1.0.0, LTE Physical Layer – General Description,


[6.] 3GPP TS 36.211 – v1.0.0, Physical Channels and Modulation,


[7]. 3GPP TS 36.212 – Multiplexing and Channel Coding,


[8]. 3GPP TS 36.213 – v1.0.0, Physical Layer Procedures,


[9]. 3GPP TS 36.214 – v0.1.0, Physical Layer – Measurements,


[10]. 3GPP TS 36.300 v8.0.0, E-UTRA and E-UTRAN Overall Description; Stage 2,


[12]. Ericssion white paper entitled LTE –a 4G solution

http://www.ericsson.com/res/docs/whitepapers/wp-4g.pdf/

[13]. Free scale white paper entitled Overview of the 3GPP Long Term Evolution Physical layer

[14]. Seminar Ausgewählte Kapitel der Nachrichtentechnik, WS 2009/2010LTE:

Der Mobilfunk der ZukunftScheduling & HARQ.

http://www.lmk.lnt.de/fileadmin/Lehre/Seminar09/Ausarbeitungen/Ausarbeitung_Schrage.pdf/

[15]. System Vue 2011 Electronic Design Software From Agilent Techonolgies


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Acronym

16QAM Sixteen point quadrature amplitude modulation

3GPP Third Generation Partnership Project

64QAM Sixty Four point quadrature amplitude modulation

ACK Acknowledgement

AGC Automatic gain control

AP Access point

ARQ Automatic repeat request

BCH Broadcast channel

BPSK Binary phase shift keying

BW Bandwidth

CCPCH Common control physical channel

CDD Cyclic delay diversity

CDMA Code Division Multiple Access

CIR Channel impulse response

CP Cyclic prefix

CQI Channel quality indication

CSMA Carrier sense multiple access

DC Direct current

DFT Discrete Fourier transform

DL Downlink

DL-SCH Downlink-shared channel

DRX Discontinuous receive

eNodeB Enhanced Node B


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FDD Frequency division duplexing

FFT Fast Fourier transform

GMSK Gaussian minimum shift keying

GT Guard time

HARQ Hybrid automatic repeat request

HSDPA High Speed Downlink Packet Access

HSUPA High Speed Uplink Packet Access

ICI Inter carrier interface

IDFT Inverse discrete Fourier transform

IEEE Institute of Electrical and Electronics Engineers

IFFT Inverse fast Fourier transform

MBMS Multimedia broadcast multicast service

MB-SFN Multicast/broadcast single frequency network

MCH Multicast channel

MIMO Multiple Input Multiple Output

MRC Maximal ratio combining

NACK Not acknowledgement

OFDM Orthogonal Frequency Division Multiplexing

PAPR Peak-to-average power ratio

PCH Paging channel

PDCCH Physical downlink control channel

PDSCH Physical downlink shared channel

PHY Physical layer

PRACH Physical random access channel

PRB Physical resource block


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PRN Pseudo random numerical sequence

PSK Phase shift keying

PUCCH Physical uplink control channel

PUSCH Physical uplink shared channel

QAM Quadrature amplitude modulation

QPSK Quadrature phase shift keying

RACH Random access channel

RFE Radio front end

RFPA Radio frequency power amplifier

S/P Serial-to-parallel

SAP Service access point

SC-FDMA Single Carrier – Frequency Division Multiple Access

SNR Signal-to-noise ratio

STA Station

TDD Time Division Duplexing

UE User equipment

UL Uplink

UL-SCH Uplink – shared channel

Documents

PERFORMANCE ANALYSIS OF LTE PHYSICAL LAYER USING SYSTEM VUE