GDMA, LDS-CDMA, and HDS-CDMA for Uplink Communication Systems.

Gain division multiple access (GDMA) and low-density spreading code division multiple access (LDS-CDMA) are non-orthogonal multiple access methods that can be applied to the uplink communication system, where each user transmits its message to a common base station through an independent fading channel. We may interpret the LDS-CDMA scheme as a combination of several dependent GDMA systems, where a GDMA system is applied to each resource. The original message passing for LDS-CDMA may also be interpreted as the message passing among various GDMA systems to employ the dependency. We extend the interpretation to construct high-density (HDS) CDMA schemes. The advantage of HDS-CDMA schemes can be the requirement of a smaller number of resources or better error performances.