Iterative Near-Maximum-Likelihood Detection in Rank-Deficient Downlink SDMA Systems

In this paper, a precoded and iteratively detected downlink multiuser system is proposed, which is capable of operating in rank-deficient scenarios, when the number of transmitters exceeds the number of receivers. The literature of uplink space division multiple access (SDMA) systems is rich, but at the time of writing there is a paucity of information on the employment of SDMA techniques in the downlink. Hence, we propose a novel precoded downlink SDMA (DL-SDMA) multiuser communication system, which invokes a low-complexity near-maximum-likelihood sphere decoder and is particularly suitable for the aforementioned rank-deficient scenario. Powerful iterative decoding is carried out by exchanging extrinsic information between the precoder's decoder and the outer channel decoder. Furthermore, we demonstrate with the aid of extrinsic information transfer charts that our proposed precoded DL-SDMA system has a better convergence behavior than its nonprecoded DL-SDMA counterpart. Quantitatively, the proposed system having a normalized system load of , i.e., 1.333 times higher effective throughput facilitated by having 1.333 times more DL-SDMA transmitters than receivers, exhibits a ldquoturbo cliffrdquo at an of 5 dB and hence results in an infinitesimally low bit error rate (BER). By contrast, at , the equivalent system dispensing with precoding exhibits a BER in excess of 10%.