On two-user degraded GBC with finite input constellations

Two-user Single-Input Single-Output degraded Gaussian broadcast channel (GBC) with finite input constellations is addressed in this paper. Multiple downlink multiple access schemes including orthogonal multiple access (OMA) schemes and non-orthogonal multiple access (NOMA) schemes are compared from several aspects. For OMA schemes, power control is not considered and constant power for each symbol is assumed. For NOMA schemes, two typical schemes are considered which are traditional linear superposition coding (SC) and recently proposed bit division multiplexing (BDM). BDM is essentially an instantiation of non-linear SC. Capacity region analysis and simulation results are performed to give an insight into the downlink multiple access schemes. The results show that BDM can increase the system throughput compared with OMA schemes and has lower implementation complexity as well as better practical performance compared with traditional linear SC. Given typical transmission rate requirements of two users, the gaps to the union theoretical SNR thresholds for two users by BDM are both less than 2.5dB with independent demapping employed at receiver, which are far better than those achieved by OMA schemes and traditional linear SC.