Wireless Network Design: Optimization Models and Solution ...

13 Optimization of Wireless Broadband (WiMAX) Systems 305
13.3.2 MIMO Mode Selection
One of the key features of a WiMAX system is the use of multiple antennas at
the transmitter and receiver. WiMAX supports two MIMO (multi-input, multioutput)
modes: MIMO-A and MIMO-B. MIMO-A is a rank-1 diversity scheme,
and MIMO-B a rank-2 spatial multiplexing scheme [4, 8, 9] The architecture of
MIMO-A is illustrated in Fig. 13.5. As shown in the Fig., the symbols from the
encoder and modulator are encoded into two paths through STC encoding. Then
the symbols of each path are mapped onto the sub-channels of OFDMA downlink
space-time coding (STC) [2, 4]. The symbols from two paths are passed through the
IFFT and transmitted on two separate antennas.
Fig. 13.5 System Architecture for MIMO-A (STC) in WiMAX
The symbol flow of the MIMO-B is illustrated in Fig. 13.6. MIMO-B uses spatial
multiplexing with vertical encoding. As shown in Fig. 13.6, all the symbols in
vertical encoding are from ONE FEC coded block. The modulated symbols are divided
sequentially into two paths. Then the symbols of each path are mapped onto
the sub-channels of the OFDMA down link. The symbols from the two paths are
passed through the IFFT and transmitted using two separate antennas.
Fig. 13.6 System Architecture for MIMO-B (Spatial multiplexing) in WiMAX
Given the same path loss and BTS output power, the signal quality that can be
achieved with MIMO-A is at least 3dB higher than that of MIMO-B. The reason for
this difference is because MIMO-A is a rank-1 scheme in which the same symbol