Compress-and-forward receiver cooperation for virtual MIMO with finite-alphabet modulation

In the downlink transmission system with a multi-antenna base station (BS) and a cluster of single-antenna mobile stations (MSs), the throughput can be greatly enhanced by exploiting the cooperating channel among MSs and forming virtual multiple input and multiple output (MIMO). With MSs close together, compress-and-forward is the most commonly used cooperation strategy. Considering the fact that in practical systems the transmit signal is usually finite-alphabet modulated symbols, e.g., quadrature amplitude modulation (QAM), the compress-and-forward cooperation is investigated correspondingly. Regarding the bit error rate (BER) performance of virtual MIMO systems, two regions are identified: the channel-noise-dominant region and the compression-noise-dominant region. In the channel-noise-dominant region, the cooperation system yields almost no loss compared to the BER lower bound. While in the compression-noise-dominant region, the system BER is limited, i.e., with an error floor. Through theoretical analysis, the minimum compression rate to guarantee that the system works in the channel-noise-dominant region is obtained. The minimum compression rate is determined mainly by three terms: the modulation alphabet size, the constellation position and the signal-to-noise ratio (SNR), whose explicit expression has also been derived.