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Chapter 8. Multiuser MIMO Receiver Proce... > 8.2 Multiuser MIMO Systems - Pg. 338

338 CHAPTER 8 Multiuser MIMO Receiver Processing for Time-Varying Channels . . apply the Krylov subspace method for reducing the complexity of LMMSE filters; and investigate an alternative nonlinear detector using the sphere decoding algorithm. The rest of this chapter builds on these four tools. We present time-varying multiuser MIMO chan- nel estimation in Section 8.4. In Section 8.5, we adapt the parallel interference cancelation in user space such that it can be combined with the Krylov subspace method for data detection. A sphere decoding algorithm exploiting the reduced-rank model of time-varying channels is presented in Section 8.6. In Section 8.7, simulation results are used to compare the bit error rate (BER) versus signal-to-noise ratio (SNR) performance of the presented algorithms as well as their computational complexity. 8.2 MULTIUSER MIMO SYSTEMS We consider a multiuser MIMO uplink system. U users, having M T transmit antennas each, transmit to a receiver with M R receive antennas (see Fig. 8.1). The transmission is based on OFDM with L subcarriers. We uniquely identify the data symbols sent from antenna t {1, . . . , M T } of user u {1, . . . , U} by a random spreading sequence s u,t C L . The elements of each spreading sequence are independent