Reduced complexity detection for network-coded slotted ALOHA using sphere decoding

Network-coded slotted ALOHA (NCSA) is a refinement to the classic slotted ALOHA protocol which improves throughput by enabling multiple source transmissions per ALOHA slot using physical-layer network coding (PNC). The receiver detects the network-coded combination of bits during every slot and recovers information bits by solving a system of linear equations. This work develops a receiver capable of detecting the network-coded combination of bits during a slot considering an arbitrary number of sources, orthogonal modulation, and a block fading channel. Maximum-likelihood detection of the network-coded symbol at the receiver becomes complex as the number of sources is increased. To reduce this complexity, sphere decoding is applied at the receiver to limit the number of constellation symbols the receiver must consider for detection. The system is simulated for two modulation orders and two through five sources, and error-rate performance results are provided.