Performance of pilot reuse in multi-cell Massive MIMO

Pilot reuse in multiple cell Massive MIMO induces coherent inter-cell interference that does not disappear with large numbers of antennas. A simple way to mitigate pilot contamination is to use a pilot reuse factor that is greater than one, which surrounds the home cell by one or more rings of non-contaminating cells. The penalty for this measure is that training consumes an increasing fraction of the slot. We perform comprehensive multiple cell semi-analytical simulations for various scenarios in order to determine the optimum pilot reuse factor. We find that in dense urban deployments, the relatively small cell size combined with low user mobility dictates pilot reuse seven. Conversely in suburban deployments pilot reuse three is optimal. Our conclusions are at variance with others who advocate pilot reuse one, which we attribute to a great extent to the fact that our performance criterion is 95% likely per user throughput rather than sum throughput.