  Network-coded slotted ALOHA (NCSA) is a re- finement to the classic slotted
ALOHA protocol which im- proves 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.
