Praise for Fundamentals of WiMAX

292 Chapter 8 • PHY Layer of WiMAX8.8 Transmit Diversity and MIMOSupport for AAS is an integral part of the IEEE 802.16e-2005 and is intended to provide significantimprovement in the overall system capacity and spectral efficiency of the network. Expected performanceimprovements in a WiMAX network owing to multiantenna technology, based on link- andsystem-level simulations, are presented in Chapter 11 and 12. In IEEE 802.16e-2005, AAS encompassesthe use of multiple antennas at the transmitter and the receiver for different purposes, such asdiversity, beamforming, and spatial multiplexing (SM). When AAS is used in the open-loopmode—the transmitter does not know the CSI as seen by the specific receiver—the multiple antennascan be used for diversity (space/time block coding), spatial multiplexing, or any combinationthereof. When AAS is used in closed-loop mode, the transmitter knows the CSI, either due to channelreciprocity, in case of TDD, or to explicit feedback from the receiver, in the case of FDD, themultiple antennas can be used for either beamforming or closed-loop MIMO, using transmit precoding.In this section, we describe the open- and closed-loop AAS modes of IEEE 802.16e-2005.8.8.1 Transmit Diversity and Space/Time CodingSeveral optional space/time coding schemes with two, three, and four antennas that can be usedwith both adjacent and diversity subcarrier permutations are defined in IEEE 802.16e-2005. Ofthese, the most commonly implemented are the two antenna open-loop schemes, for which thefollowing space/time coding matrices are allowed:BS 1= A =S 2S 1– S∗2S 2 S∗ 1, (8.6)where S1 and S2 are two consecutive OFDM symbols, and the space/time encoding matrices areapplied on the entire OFDM symbol, as shown in Figure 8.15. The matrix A in Equation (8.6) isthe 2 × 2 Alamouti space/time block codes [1], which are orthogonal in nature and amenable to alinear optimum maximum-likelihood (ML) detector. 5 This provides significant performance benefitby means of diversity in fading channels. On the other hand, the matrix B as provided—seeEquation (8.6)—does not provide any diversity but has a space/time coding rate of 2 (spatial multiplexing),which allows for higher data rates. Transmit diversity and spatial multiplexing are discussedin more detail in Chapter 6. Similarly, space/time coding matrices have been defined withthree and four antennas. In the case of four antenna transmit diversity, the space/time coding matrixallows for a space/time code rate of 1 (maximum diversity) to a space-time code rate of 4 (maximumcapacity), as shown by block coding matrices A, B, and C in Equation (8.7). By using moreantennas, the system can perform a finer trade-off between diversity and capacity. For transmitdiversity modes with a space/time code rate greater than 1, both horizontal and vertical encoding5. For complex modulation schemes, the full-rate space/time block codes with more than two antennasare no longer orthogonal and do not allow a linear ML detection. More realistic detections schemesinvolving MRC or MMSE are suboptimal in performance compared to the linear ML detector.