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Future Transmitter/Receiver Diversity Schemes in Broadcast Wireless Networks

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Future Transmitter/Receiver Diversity Schemes in Broadcast Wireless Networks ZHANG LAYOUT 9/20/06 1:50 PM Page 120 CORE ACCEPTED FROM OPEN CALL Metadata, citation and similar papers at core.ac.uk Provided by Brunel University Research Archive Future Transmitter/Receiver Diversity Schemes in Broadcast Wireless Networks Yue Zhang, John Cosmas, and YongHua Song, Brunel University Maurice Bard, Broadreach Systems ABSTRACT well documented technique where each pair of transmit and receive antennas provides a differ- An open diversity architecture for a cooperat- ent signal path from the transmitter to the ing broadcast wireless network is presented that receiver. By sending signals that carry the same exploits the strengths of the existing digital information through these different paths, multi- broadcast standards. Different diversity tech- ple independently faded replicas of the data niques for broadcast networks that will minimize symbol can be obtained at the receive end. Max- the complexity of broadcast systems and improve imum diversity gain is achieved when fading is received SNR of broadcast signals are described. uncorrelated across antenna pairs. Resulting digital broadcast networks could More recent work has focused on investigat- require fewer transmitter sites and thus be more ing how techniques such as trellis-based space- cost effective with less environmental impact. time codes [4, 5] and orthogonal designs [6] can Transmit diversity is particularly investigated be applied to multiple transmit antennas to max- since it obviates the major disadvantage of imize the diversity gain. Unfortunately, space- receive diversity being the difficulty of locating time coding is not compliant with current two receive antennas far enough apart in a small standards for broadcast systems; therefore, only mobile device. The schemes examined here are spatial diversity techniques can be applied. If compatible with existing broadcast and cellular multiple signals are received with short delay telecom standards, and can be incorporated into spread, the OFDM signal is faded equally across existing systems without change. the channel in a flat fade; this induces long error bursts that are hard to correct since there is INTRODUCTION insufficient frequency selectivity to be exploited by the coding and interleaving schemes. Cyclic The vision for future mobile communications delay diversity (CDD) [7] is a very simple and networks is of an open architecture supporting elegant method that, when combined with a multiplicity of international standard wire- MIMO, improves frequency selectivity, thereby less network technologies ranging from sec- randomizing the channel response. The compu- ond-/third-/fourth-generation (2G/3G/4G) tational cost of CDD is very low, as all signal cellular radio systems (GSM, GPRS, UMTS) processing needed is performed on the OFDM to wireless local area networks (WLANs), signals in the time domain. broadband radio access networks (BRANs) to This article focuses on an open wireless diver- digital video and audio broadcast networks sity structure. First, we introduce the system (DVB, DAB) [1]. model and then discuss different diversity tech- Future mobile radio systems are expected to niques, including CDD for transmitter and maxi- deliver services, which inherently require high mum ratio combining (MRC) for receiver. The data rates. Orthogonal frequency-division multi- application of CDD to DVB-H systems is dis- plexing (OFDM) [2] is an ideal technique for cussed, and results of simulations are presented. broadband transmission in multipath fading Finally, we present the conclusions of this study. environments and is implemented in broadcast standards like DAB or DVB as well as WLAN SYSTEM MODEL standards [3] such as HIPERLAN/2 or IEEE 802.11a/g and WiMAX. Spatial diversity can be OFDM used to improve the error performance and OFDM is a proven technique for achieving high channel capacity in these systems by overcoming data rates and overcoming multipath fading in degradations caused by scattering environments. wireless communication. OFDM can be thought The use of multiple transmit and receive anten- of as a hybrid of multicarrier modulation (MCM) nas (multiple-input multiple-out, MIMO) is a and frequency shift keying (FSK) modulation. 120 0163-6804/06/$20.00 © 2006 IEEE IEEE Communications Magazine • October 2006 ZHANG LAYOUT 9/20/06 1:50 PM Page 121 Transmitter The block interleaver can be described as Outer Inner QAM Data source Outer coder Inner coder interleaver interleaver mapping a matrix to which data is written in Pilot and TPS columns and read in OFDM signals modulator rows. The modulator MIMO channel NxM MIMO Frame process is equivalent encoder adaptation to an inverse OFDM discrete Fourier modulator transform, while the demodulation Channel equalization process is equivalent OFDM to a discrete Fourier Synchronization demodulation transform. MIMO decoder OFDM Synchronization demodulation Channel estimator Received Outer Outer Inner Inner QAM data decoder deinterleaver decoder deinterleaver demodulation Receiver MIMO scheme added Required part for DVB-H to DVB-H transmission systems I Figure 1. End-to-end MIMO system diagram of DVB-T/H transmission with coded data. Orthogonality among carriers is achieved by sep- coder is Reed-Solomon shortened code arating them by an integer multiple of the RS(204,188) derived from the original systematic inverse of symbol duration of the parallel bit- RS(255,239) code. The inner coder comprises a streams, thus minimizing intersymbol interfer- puncture coder and a convolutional coder. The ence (ISI). Carriers are spread across the puncture coder is based on the mother convolu- complete channel, fully occupying it and hence tional code of rate 1/2 with 64 states. The convo- using the bandwidth very efficiently. The OFDM lutional coder increases error correction receiver uses adaptive bit loading techniques capability and codes used are similar to those based on a dynamic estimate of the channel widely employed in digital communication sys- response, to adapt its processing and compen- tems such as GSM, IS95, and WLAN systems: sate for channel propagation characteristics and convolutional code generators of x6 + x4 + x3 + achieve near ideal capacity. This most important x + 1 and x6 + x5 + x4 + x3 + 1. A puncture advantage of OFDM systems over single-carrier coder is a common solution employed to reduce systems is obtained when there is frequency- the loss of data throughput capacity caused by a selective fading. The signal processing in the convolutional coder. This is done by not trans- receiver is rather simple; distortion of the signals mitting some of the bits output by the convolu- is compensated by multiplying each subcarrier by tional coder, but results in the receiver having to a complex transfer factor, thereby equalizing the implement different convolution puncture channel response. It is not feasible for conven- decoders for each of the puncture patterns used. tional single-carrier transmission systems to use An inner interleaver is used to distribute trans- this method. mitter bits in time or frequency or both to However, some disadvantages of OFDM sys- achieve an optimum distribution of bit errors tems are that the peak-to-average power ratio after modulation. The block interleaver can be (PAPR) of OFDM is higher than for a single- described as a matrix to which data is written in carrier system, and OFDM is sensitive to a flat columns and read in rows. The modulator pro- fading channel. cess is equivalent to an inverse discrete Fourier OFDM is employed in the DVB-T and DVB- transform (IDFT), while the demodulation pro- H systems; a block diagram of a coded OFDM cess is equivalent to a discrete Fourier transform system for DVB-H is shown in Fig. 1. The outer (DFT). IEEE Communications Magazine • October 2006 121 ZHANG LAYOUT 9/20/06 1:50 PM Page 122 900 MHzm 50km/h, Max Doppler=41.67Hz, 100echos 11 2 5 E(r )=0.05 dB 0 22 -5 Transmitter Receiver -10 33dB -15 M -20 Deep NM-25 fading 50 100 150 200 250 300 350 400 450 500 ms A B 900 MHzm 50km/h, Max Doppler=41.57Hz, 100echos 900 MHzm 30km/h, Max Doppler=25.00Hz, 100echos 5 35 E(r2)=0.60dB E(r2)=0.10dB 30 0 25 -5 20 -10 dB dB 15 -15 10 -20 5 2 0 E(r )=30.18 dB -25 50 100 150 200 250 300 350 400 450 500 100 200 300 400 500 ms ms C D I Figure 2. a) M × N i.i.d MIMO channel; b) Rayleigh fading channel (E = 0.05 dB); c) Rician fading channel with Rician K-factor = 0.17 (E = 30.18 dB); and d) time histories of two uncorrelated MIMO channels (Rayleigh fading). MIMO combined in such a way as to minimize the trans- A MIMO system utilizes multiple antennas at mission degradation that could be caused by per- the transmitter and receiver. Such systems have formance of each of the individual channels. demonstrated the ability to reliably provide high Effectiveness or “diversity gain” of an implemen- throughput in rich multipath environments. tation is dependent on the scenario for the prop- Spatial diversity employs the spatial proper- agation channels (e.g., rural or urban), ties of multipath channels, providing a new transmission data rate, Doppler spread, and dimension that can be exploited to enhance per- channel delay spread. formance. Coding and diversity are key elements In our modeling analysis we consider an of a successful MIMO implementation, while the equivalent baseband MIMO channel with N propagation channel and antenna installations transmit antennas and M receiver antennas have a major impact on system performance. illustrated in Fig. 2a. There are M × N channels Figure 1 also shows the additional compo- with independently distributed wide-sense sta- nents required for a typical coded MIMO sys- tionary uncorrelated scattered (WSSUS) tem. There are three kinds of MIMO techniques. Rayleigh-fading. If there is no direct line of The first aims to improve power efficiency by sight between the transmitter and the receiver, maximizing spatial diversity; such techniques the impulse response of the channel is modeled include delay diversity, space-time block codes by several paths, each with a Gaussian distribu- (STBC), and space-time trellis codes (STTC).
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