Comparison of Two Low Complexity Multiple Access Schemes

This paper studies multiple access channels with additive Gaussian noise in the low signal to noise ratio (SNR) regime. We compare the spectral efficiencies of the optimal superposition channel sharing scheme and two simple alternatives: the time division multiple access (TDMA) scheme and the parallel multiple access scheme with single user decoding (PMAS). We consider the situation when the receiver has multiple receive antennas while each transmitter only has single antenna. We show that, due to TDMA´s inefficiency in exploiting the multiuser multiplex gain, the relative spectral efficiency of PMAS over TDMA grows drastically as the number of receive antennas increase. The relative spectral efficiency of PMAS over the optimal scheme is approximately 1/2, irrespective of the number of receive antennas. Since the simplicities of PMAS and TDMA are similar, our results suggest that PMAS is a better alternative to TDMA for multiple access system in the low SNR regime