Spectral efficiency of uplink SCMA system with CSI estimation

Sparse code multiple access (SCMA) is a non-orthogonal multiple access scheme. This scheme allows to increase the number of active users inside a given time-frequency resource. The paper considers the spectral efficiency of coded uplink SCMA system in block fading channel with account for channel state information estimation quality. Two channel estimation schemes are investigated, one is based solely on pilot signals, another uses hard decisions after SCMA decoding to improve estimation quality. Orthogonal and non-orthogonal pilot signals are considered. Increasing the number of pilot signals improves channel state information, but, on the other hand, it reduces spectral efficiency of the communication system, so the number of pilots should be minimized. The simulation results are provided for different pilot signal lengths. Spectral efficiency obtained with channel estimation based on orthogonal pilot sequences is equivalent to it in the case of data-aided estimation with SCMA non-orthogonal pilots. For small number of pilot symbols, the energy gain of data-aided algorithm is approximately 1 dB as compared with an estimate based on pilot signals only. For the investigated SNR range (SNR <; 10 dB) and short non-orthogonal pilot signals, four iterations of Message Passing Algorithm are required for data-aided estimation and detection without decreasing spectral efficiency