  Massive MIMO is a promising technology in future wireless communication
networks. However, it raises a lot of implementation challenges, for example,
the huge pilot symbols and feedback overhead, requirement of real-time global
CSI, large number of RF chains needed and high computational complexity. We
consider a two-tier precoding strategy for multi-cell massive MIMO interference
networks, with an outer precoder for inter-cell/inter-cluster interference
cancellation, and an inner precoder for intra-cell multiplexing. In particular,
to combat with the computational complexity issue of the outer precoding, we
propose a low complexity online iterative algorithm to track the outer precoder
under time-varying channels. We follow an optimization technique and formulate
the problem on the Grassmann manifold. We develop a low complexity iterative
algorithm, which converges to the global optimal solution under static
channels. In time-varying channels, we propose a compensation technique to
offset the variation of the time-varying optimal solution. We show with our
theoretical result that, under some mild conditions, perfect tracking of the
target outer precoder using the proposed algorithm is possible. Numerical
results demonstrate that the two-tier precoding with the proposed iterative
compensation algorithm can achieve a good performance with a significant
complexity reduction compared with the conventional two-tier precoding
techniques in the literature.
