Investigation of Uplink and Downlink Performance of ... · PDF fileInvestigation of Uplink and Downlink Performance of Directivity Controlled Constrained Beamforming ... wavelength

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Investigation of Uplink and Downlink Performance ofInvestigation of Uplink and Downlink Performance ofInvestigation of Uplink and Downlink Performance ofInvestigation of Uplink and Downlink Performance of

DirectivityDirectivityDirectivityDirectivity

Controlled Constrained Beamforming Algorithms forControlled Constrained Beamforming Algorithms forControlled Constrained Beamforming Algorithms forControlled Constrained Beamforming Algorithms for

CDMA-Based SystemsCDMA-Based SystemsCDMA-Based SystemsCDMA-Based Systems

Holger Boche and Martin SchubertHolger Boche and Martin SchubertHolger Boche and Martin SchubertHolger Boche and Martin Schubert

Heinrich-Hertz-Institut fr Nachrichtentechnik Berlin GmbHHeinrich-Hertz-Institut fr Nachrichtentechnik Berlin GmbHHeinrich-Hertz-Institut fr Nachrichtentechnik Berlin GmbHHeinrich-Hertz-Institut fr Nachrichtentechnik Berlin GmbH

Broadband Mobile Communication NetworksBroadband Mobile Communication NetworksBroadband Mobile Communication NetworksBroadband Mobile Communication Networks

Einsteinufer 37, D-10587 Berlin/GermanyEinsteinufer 37, D-10587 Berlin/GermanyEinsteinufer 37, D-10587 Berlin/GermanyEinsteinufer 37, D-10587 Berlin/Germany

E-mail: [email protected]hi.de, [email protected] / Tel: +49 (0)30-31002-399E-mail: [email protected], [email protected] / Tel: +49 (0)30-31002-399E-mail: [email protected], [email protected] / Tel: +49 (0)30-31002-399E-mail: [email protected], [email protected] / Tel: +49 (0)30-31002-399

AbstractAbstractAbstractAbstract

This paper investigates the applicability of the blind DoA-based maximumThis paper investigates the applicability of the blind DoA-based maximumThis paper investigates the applicability of the blind DoA-based maximumThis paper investigates the applicability of the blind DoA-based maximum

directivity (MD) beamformer to CDMA-based systems in up- and downlink. Ourdirectivity (MD) beamformer to CDMA-based systems in up- and downlink. Ourdirectivity (MD) beamformer to CDMA-based systems in up- and downlink. Ourdirectivity (MD) beamformer to CDMA-based systems in up- and downlink. Our

approach is based on DoA estimation and consequently does not require any pilotapproach is based on DoA estimation and consequently does not require any pilotapproach is based on DoA estimation and consequently does not require any pilotapproach is based on DoA estimation and consequently does not require any pilot

signal or training sequence. It is shown how knowledge of the DoA can be usedsignal or training sequence. It is shown how knowledge of the DoA can be usedsignal or training sequence. It is shown how knowledge of the DoA can be usedsignal or training sequence. It is shown how knowledge of the DoA can be used

to generate a most robust beam pattern in order to perform spatial filtering ofto generate a most robust beam pattern in order to perform spatial filtering ofto generate a most robust beam pattern in order to perform spatial filtering ofto generate a most robust beam pattern in order to perform spatial filtering of

multipath components in up- and downlink. Robustness is achieved bymultipath components in up- and downlink. Robustness is achieved bymultipath components in up- and downlink. Robustness is achieved bymultipath components in up- and downlink. Robustness is achieved by

maximising the directivity of the beam pattern as well as by generating broadmaximising the directivity of the beam pattern as well as by generating broadmaximising the directivity of the beam pattern as well as by generating broadmaximising the directivity of the beam pattern as well as by generating broad

nulls. Analytical results are presented and different aspects of directivitynulls. Analytical results are presented and different aspects of directivitynulls. Analytical results are presented and different aspects of directivitynulls. Analytical results are presented and different aspects of directivity

controlled beamforming are discussed.controlled beamforming are discussed.controlled beamforming are discussed.controlled beamforming are discussed.

I INTRODUCTIONI INTRODUCTIONI INTRODUCTIONI INTRODUCTION

Wireless cellular communication based on DS-CDMA has experienced tremendous

growth in markets, technology and range of services throughout the last decade.

However, radio spectrum is a limited resource. The resulting challenge is to develop

enhanced transmission techniques in order to realise emerging broadband services and

applications. One promising way to significantly increase the spectral efficiency is the

deployment of antenna arrays at the base station in order to perform space-time

processing (STP)[1, 2, 3]

. While the deployment of antenna arrays in 3rdgeneration

systems is still optional, they will be an essential part of future systems[4].

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Exploitation of the spatial dimension can be used to reduce co-channel interference

(CCI) and inter-symbol interference (ISI), while improving resistance to fading and

thermal noise. Reducing intra- and inter-cell CCI can be traded for improved coverage,

capacity or quality. Thus, spatial filtering, also referred to as beamforming[5], will play

an important role in future broadband wireless networks.

In this paper, we focus on so-called blind DoA-based beamforming techniques, assuming

that the direction-of-arrival (DoA) of the dominant transmission paths are known. Blind

techniques do not need any training sequence or pilot signal. Thus, they consume no

additional spectrum resource (note that in GSM 20% of the bits are dedicated for

training). This makes them promising candidates for various types of wireless networks.

DoA estimates can be obtained with second order statistics of the communication

signals[6], which are assumed to be stationary within the coherence time of the channel.

Improvement can be achieved by mobility models, which help predicting the movement

of the mobile unit by considering the slowly time varying nature of the user location.

However, the wireless radio channel poses a great challenge as a medium for reliable

high speed communications and accurate DoA-estimation is difficult to realize. First field

trials [7, 8] have shown that DoA-based methods are very sensitive to error effects.

Consequently, DoA-based beamforming must take into account a DoA mismatch of

several degrees. Conventional DoA-based beamforming has been shown to perform poor

in this case. This is mostly due to beam pattern distortion caused by hyper-sensitive

algorithms in presence of DoA errors. Thus, the deployment of DoA-based beamforming

in mobile environments demands for more robust techniques being able to cope with

numerous error effects like inter-cell CCI, scattering effects or DoA estimation errors.

Consequently, the investigation of the beam pattern is an important aspect and new

performance parameters are needed to assess the quality of the beam pattern.

In this paper we will focus on the impact of directivity and broad nulls on STP

architectures for CDMA-based systems. It will be shown how these parameters can be

used to generate a beam pattern having maximum directivity. Robust beam pattern

control is envisaged which must compensate for DoA errors, angle spread and CCI.

We assume a single cell scenario without inter-cell CCI, where all users are separated

by quasi-orthogonal spreading codes. The cell is divided in three sectors of 120. For

each sector a uniform linear antenna array(ULA) is deployed at the base station. Linear

arrays have been developed vigorously during the last decades mainly for radar and

sonar signal environments in military applications. Its application to mobile

communications is subject of ongoing world wide research and development activity [1, 2].

The paper is organised as follows. In Section II we will briefly introduce the underlying

vector channel model and discuss the angle spread of dominant transmission paths.

Next, in Section III we discuss different aspects of directivity controlled beamforming

and broad nulls with respect to space-time processing. The maximum directivity (MD)

beamformer is presented and its application up- and downlink processing is discussed.

Finally, we conclude with a summary in Section IV.

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Some notational conventions are: scalars in lower case, matrices in upper case and

vectors in boldface lowercase. The expectation operator is written as E[ ]. The complex

conjugate and the complex conjugate transpose are given by ( ) and ( )H, respectively.

II SIGNAL AND CHANNEL MODELII SIGNAL AND CHANNEL MODELII SIGNAL AND CHANNEL MODELII SIGNAL AND CHANNEL MODEL

Consider a narrowband signal s(t) = u(t)ejw 0t, where u(t) denotes the complex

baseband envelope and w0 the carrier frequency. The signal source is assumed to

lie in in the far field of a ULA consisting of M isotropic antenna elements with half

wavelength element spacing. In this case, a plane wave front crosses the array with

the angle of incidence array elements with the azimuth angle , as depicted in Fig.

1. For convenience it is assumed that all users and the array lie in a horizontal

plane, but all results can be extended by the elevation angle.

Fig. 1: Plane wave front crossing a ULAFig. 1: Plane wave front crossing a ULAFig. 1: Plane wave front crossing a ULAFig. 1: Plane wave front crossing a ULA

If the ratio of the array aperture to the velocity of light is much smaller than the

inverse of the bandwidth of the signal, then u(t) can be regarded as constant during the

propagat