Praise for Fundamentals of WiMAX

5.9 Advanced Techniques for MIMO 1912 Transmitters and 2 ReceiversAlamouti, Linear (16 QAM)SM, ZF (4 QAM)SM, ML (4 QAM)STBC, ML (4 QAM)10 –1 SNR (dB) per Receive Antenna10 –2Bit Error Rate10 –310 –410 –50 5 10 15 20 25Figure 5.20 BER versus SNR for configurations of 2×2 MIMO. The simple Alamouti code outperformsspatial multiplexing at the same data rate owing to its superior diversity. Figure from [26],courtesy of IEEE.users can use the spatial channels simultaneously, which can be advantageous versus users havingto take turns sharing the channel. For example, imagine a downlink scenario with N t=4 transmitantennas at the BS and N r=1 antennas at each MS. Using the techniques presented thus farin this chapter, only a single stream could be sent to single user. Using multiuser encoding techniques,though, four streams could be sent to the four users. Since each MS has only a singlereceive antenna, it received signal-processing capabilities are quite limited: In this example, it isnot possible for the MS to cancel out the interfering three streams and successfully receive itsown stream. Therefore, in a multiuser MIMO system, the base station must proactively cancel thespatial interference so that the mobile stations can receive their desired data streams.Multiuser MIMO has generated a large amount of recent interest; see [29] for a summary. Themain idea emerging from this research is that multiple users can be simultaneously multiplexed totake simultaneous advantage of multiuser and spatial diversity. The optimal BS interferencecancellationstrategy is the so-called dirty paper coding [7, 36], but it is not directly practical. Morerealistic linear multiuser precoding techniques have been developed [11, 56, 68], but these toorequire accurate channel knowledge for all the candidate MSs. In addition to these downlink strategies,it is possible for multiple users to transmit in the uplink at the same time, using a subset of theirantennas, to create a virtual MIMO system. For example, if three spatially distributed users eachtransmitted on a single antenna, as long as there were N r≥ 3 antennas at the base station, thiscould be treated as a 3 × N rvirtual MIMO channel, the significant difference from conventional