Sphere Decoder for Massive MIMO Systems

The increasing demand for higher data rates and for more connected devices has led to Massive MIMO (MMIMO) Technology. The large number of antennas makes the Maximum-Likelihood (ML) detector infeasible to be implemented due to high complexity, despite its optimal performance. Sphere Decoder (SD) has a bit error rate (BER) performance similar to ML detector, therefore making it more efficient (0.5-1.25 dB gain) than Linear Detectors (LD), proposed in the literature. However, the low complexity of LD and the non-deterministic behavior of SD are the main reasons that prohibit the use of sphere decoding methods in MMIMO systems. The results of this paper disrupt conventional thinking and show that there may be a future for SD in certain MMIMO system. The number of visited nodes during detection and the Initial Radius (IR) method are crucial for the computational complexity of SD. In this paper, an effective IR method, decreasing significantly the complexity and the number of visited nodes is proposed. Furthermore an optimization at tree searching further reduces the number of visited nodes, where in combination with an implementation featured with one-node-per-cycle architecture minimize the latency and make the SD attainable to large-scale systems for E-b/N-0 >= 4 dB. Hardware aspects are investigated for both a Virtex-7 FPGA and a 28 nm ASIC technology.