Study on effect of training for downlink massive MIMO systems with outdated channel

In this paper, we analyze the effect of uplink and downlink training on the downlink achievable rate for massive MIMO systems with outdated channel. Even though both uplink and downlink training have the same training overhead, the achievable rate is more dominated by the uplink training than downlink training. As the number of antennas goes to infinity, the performance gain of the downlink training is negligible. This fact implies that non-coherent detection is more desirable for massive MIMO systems in terms of throughput. The rate loss caused by outdated channel is also dominated by the uplink training. Our analysis gives a good insight to implement practical massive MIMO systems.