Three-Stage Serially Concatenated Codes and Iterative Center-Shifting K-Best Sphere Detection for SDM-OFDM: An EXIT Chart Aided Perspective

Iterative if-best sphere detection (SD) and channel decoding is appealing, since it is capable of achieving a near- maximum-a-posteriori (MAP) performance at a low complexity. However, a potentially excessive computational cost is imposed, especially in a high-throughput Spatial Division Multiplexing (SDM) aided OFDM system employing a large number of transmit antennas and/or high-order modulation schemes. This problem is further aggravated, when the number of transmit antennas exceeds that of the receive antennas, namely in the challenging scenario of rank-deficient systems. In order to further reduce the complexity imposed, we propose a unity-rate-code-aided (URC) three-stage concatenated transceiver constituted by an inner and outer channel encoder/decoder pair as well as the SDM aided OFDM transmitter and K-best SD. We have demonstrated that our proposed three-stage scheme is capable of achieving a substantial performance gain over the classic two-stage scheme. For example, given a target Bit Error Ratio (BER) of 10 5, the three-stage SD-aided receiver is capable of achieving a performance gain of 4 dB over its two-stage counterpart in an (8 x 4)-element SDM/OFDM system. Furthermore, we have also investigated the achievable performance of our novel center- shifting aided SD in the context of this three-stage scheme. Consequently, an additional 0.5 dB performance gain can be attained. Finally, the convergence behavior of the proposed schemes were studied with the aid of 3D Extrinsic Information Transfer (EXIT) charts and their 2D projections.