Massive MIMO is a promising technology in future wireless communicationnetworks. However, it raises a lot of implementation challenges, for example,the huge pilot symbols and feedback overhead, requirement of real-time globalCSI, large number of RF chains needed and high computational complexity. Weconsider a two-tier precoding strategy for multi-cell massive MIMO interferencenetworks, with an outer precoder for inter-cell/inter-cluster interferencecancellation, and an inner precoder for intra-cell multiplexing. In particular,to combat with the computational complexity issue of the outer precoding, wepropose a low complexity online iterative algorithm to track the outer precoderunder time-varying channels. We follow an optimization technique and formulatethe problem on the Grassmann manifold. We develop a low complexity iterativealgorithm, which converges to the global optimal solution under staticchannels. In time-varying channels, we propose a compensation technique tooffset the variation of the time-varying optimal solution. We show with ourtheoretical result that, under some mild conditions, perfect tracking of thetarget outer precoder using the proposed algorithm is possible. Numericalresults demonstrate that the two-tier precoding with the proposed iterativecompensation algorithm can achieve a good performance with a significantcomplexity reduction compared with the conventional two-tier precodingtechniques in the literature.
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