SCMA detection with channel estimation error and resource block diversity

Future Fifth generation wireless networks should support, among other things, very large number of simultaneous connections. To address this requirement, various schemes of non-orthogonal multiple access (NOMA) were proposed that allow to increase the number of simultaneously active users. One of NOMA schemes is sparse code multiple access (SCMA), where sparse multidimensional codewords allow to use iterative detecting algorithms with reasonable complexity. In the paper, SCMA detection at different allocation of codeword symbols in resource blocks is investigated in the presence of channel estimation error. Uncoded and turbo coded SCMA is analyzed. Uplink channel with Rayleigh flat block fading is assumed. The effect of diversity at spreading codewords over the resource blocks is considered. The allocation of uncoded SCMA codeword symbols in four resource blocks decreases bit error rate, consequently permissible variance of channel estimation error can be increased. Simulation results show that required accuracy of the channel estimation depends on the turbo code block length. For maximum utilization of turbo code error-correction capability with short blocks (40 bits) normalized variance of channel estimation error should be less than 10−3. For turbo code with long blocks (1024 bits), estimation can be less accurate, with normalized variance up to 10−2. With such channel estimation accuracy, power loss is about 0.6–0.7 dB compared with the case of perfect estimation. The spreading of SCMA codewords over four resource blocks decreases the number of channel coefficients for estimation and increases bit error rate for coded system. SCMA with long blocks of turbo code and channel interleaving is also analyzed. The channel interleaving improves BER performance, but the scheme with spread codewords still does not outperform the scheme with allocation of codeword symbols into one resource block.