Joint Gray-Mapping for Multilayer Multicast NOMA With Arbitrary Modulation Orders

This work considers the design of a generalized Gray-mapping process to multilayer multicast non-orthogonal multiple access (NOMA) transmission with arbitrary modulation orders. Unlike orthogonal multiple access, joint-multilayer Gray-mapping can provide significant energy savings and bit error rate (BER) improvements, which can be used to alleviate the degradation caused by the inherent multilayer interference in NOMA. The obtained improvement is due to the increased Euclidean distance that Gray-mapping provides for certain layers/symbols. To evaluate the impact of Gray-mapping, closed-form expressions are derived for the exact BER with imperfect successive interference cancellation. The obtained analytical and simulation results demonstrate that the proposed scheme can offer up to 10 dB gain and 94% energy saving compared to conventional NOMA in certain scenarios, where such a performance gain can be shared between the layers by selecting an appropriate power assignment. Moreover, feasibility maps are generated to demonstrate the additional flexibility that Gray-mapping can offer in terms of quality of service satisfaction for the various layers at lower signal to noise ratios.