Differential 16-QAM and 16-APSK for Uplink Massive MIMO Systems

A differential quadrature amplitude modulation (QAM) scheme for uplink massive multiple-input-multiple-output systems was recently proposed; this scheme outperformed the conventional differential amplitude-phase shift keying (APSK) scheme for 1000 receiver antennas. However, the detectors of these schemes are different, and the QAM detector suffers from error floors if the number of antennas is not sufficiently large. In this letter, we generalize the QAM detector and propose a new detector that presents better error performance at low bit error rates when the number of available antennas is not extremely large. We find that, using the same detector, 16-APSK outperforms 16-QAM for any number of antennas.