Performance analysis on precoding and pilot scheduling in very large MIMO multi-cell systems

We investigate pilot contamination problem for very large MIMO multi-cell TDD system. The asymptotic sum rate of two typical precoding schemes, i.e., the single-cell zero forcing (ZF) precoding and the multi-cell minimum mean square error (MMSE) based coordinated precoding are first derived. Results show that these two schemes have the same asymptotic sum rate expression as the number of the base station antenna going to infinity, revealing that coordinated precoding based on local channel state information (CSI) only provides marginal gain in the presence of pilot contamination. Based on our derivations, a pilot scheduling scheme is further proposed to mitigate pilot contamination, which could provide much better performance. Numerical results finally verify our derivations and the proposed scheme.