A massive multiple-input multiple-output (MIMO) system using a couple of hundred antenna elements has been arising as an important technology to keep the communication range in higher frequency bands. A further enhancement to increase the number of both transmit and receive antennas will be the next step. It requires the complexity proportional to at least the third power of the number of antenna elements to detect spatially-multiplexed signals in general. However, the belief propagation (BP)-based detector is implementable with lower complexity, i.e., the order of square of the number of elements, and achieves very good BER performance in a QPSK modulated massive MIMO system. In this paper, we apply the BP algorithm to a 16-QAM modulated massive MIMO system and propose a sequential reliability update to exploit a difference in error tolerance. The BER evaluation results have shown that the BP works well for a 16-QAM signal detection and that the sequential update is effective to the M-QAM modulated systems.
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