TAILIEUCHUNG - Adaptive WCDMA (P13)

Multiuser CDMA receivers In this chapter we present a number of methods for multiple-access interference (MAI) cancelation. MAI is produced by the presence of the other users in the network, which are located in the same bandwidth as our own signal. The common characteristic of all these schemes is some form of joint signal and parameter estimation for all signals present in the same bandwidth. It makes sense to implement this in a Base Station (BS) of a cellular system because all these signals are available there anyway. At the same time this concept will considerably increase the complexity of the. | Adaptive WCDMA Theory And Practice. Savo G. Glisic Copyright 2003 John Wiley Sons Ltd. ISBN 0-470-84825-1 13 Multiuser CDMA receivers In this chapter we present a number of methods for multiple-access interference MAI cancelation. MAI is produced by the presence of the other users in the network which are located in the same bandwidth as our own signal. The common characteristic of all these schemes is some form of joint signal and parameter estimation for all signals present in the same bandwidth. It makes sense to implement this in a Base Station BS of a cellular system because all these signals are available there anyway. At the same time this concept will considerably increase the complexity of the receiver. Although very complex these schemes are being standardized already because they offer significantly better performance. Details can be seen in Chapter 17. Much simpler but less effective solutions feasible for implementations in mobile units are also considered minimum mean square error MMSE type of algorithms . OPTIMAL RECEIVER If user k transmits bit stream bk with bit interval T using spreading sequence sk then the low-pass equivalent of the overall signal received in the BS can be represented as 1 2 drt St b di o drnt t e R M K 5t b E Ebk i sk t - iT - Tk i -M k 1 where K is the number of users b b1 b2 . bK T is the vector of bits of all users and the signal is observed in time interval MT MT . The noise component is represented by the second term of equation and Tk is the delay of signal from user k. On the basis of the likelihood principle described in Chapter 3 the detector selects the vector of bits b that maximizes P rt t g Ä C exp Q 2o2 456 MULTIUSER CDMA RECEIVERS where C is a positive scalar independent of b and b 2p St b drt -T S2 b dt So the joint maximum likelihood ML decision estimate for vector b is obtained as b max Q b all b e 1 1 In other words vector b that jointly gives the maximum of equation .

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