Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

WiMAX 265M-QAMMappingSTC withSpatialMultiplexer..SubchannelMappingSubchannelMappingIFFTIFFTD/AD/ARFRF1..NStandardizedSynchronization & Channel EstimationM ≥ NChannel, N/LOSDe-Mapping&Equaliz.STCDecoder &SpatialDe-MultiplexerSubchannelDe-MappingFFTAD//DARFRF1..MFigure 5-39Space diversity in WiMAXSimilar to two antennas, in the case of four antennas we can define the followingmatrices:A =⎛⎜⎝s 1 −s ∗ 20 0s 2 s ∗ 10 00 0 s 3 −s ∗ 40 0 s 4 s ∗ 3⎞ ⎛⎟⎠ ,B = ⎜⎝s 1 −s ∗ 2 s 5 −s 7s 2 s ∗ 1s 6 −s ∗ 8s 3 −s ∗ 4 s 7 s ∗ 5s 4 s ∗ 3s 8 s ∗ 6⎞ ⎛ ⎞s 1⎟⎠ ,C = ⎜ s 2⎟⎝ s 3⎠ . (5.3)s 4With matrix A, the encoder becomes a pure STC with full diversity (gain of 4) andwithout an increase of the data rate. With matrix B we have a combination of STC with adiversity gain of 2 and a data rate doubling. Matrix C allows a pure spatial multiplexingoffering up to 4 times more data rate without any diversity gain.Note that in the last case the number of used receive antennas is at least the same asat the transmitter side. The main advantage of this scheme is the increase of the transmitdata rate. Here, as in the STC case, the receiver estimates for each MIMO channel thecorresponding channel state information that is needed for the diversity combiner.– Adaptive antenna system (AAS). AAS is foreseen as an option for bothWirelessMAN-OFDM and WirelessMAN-(S)-OFDMA modes. The idea behind theAAS is to concentrate for a given period of time all radiated power toward agiven user by using, for instance, the so-called phased array antennas, where theantenna gain can be adjusted towards a given direction. Several antennas will beused at the transmitter (or receiver) side. Note that the combination or the steeringof the antennas can be done at the transmission or reception side. The examplegiven in Figure 5-40 takes into account the fact that beamforming is done at the