Channel Feedback Based on AoD-Adaptive Subspace Codebook in FDD Massive MIMO Systems

Abstract

Channel feedback is essential in frequency division duplexing (FDD) massive\nmultiple-input multiple-output (MIMO) systems. Unfortunately, previous work on\nmultiuser MIMO has shown that the codebook size for channel feedback should\nscale exponentially with the number of base station (BS) antennas, which is\ngreatly increased in massive MIMO systems. To reduce the codebook size and\nfeedback overhead, we propose an angle-of-departure (AoD)-adaptive subspace\ncodebook for channel feedback in FDD massive MIMO systems. Our key insight is\nto leverage the observation that path AoDs vary more slowly than the path\ngains. Within the angle coherence time, by utilizing the constant AoD\ninformation, the proposed AoD-adaptive subspace codebook is able to quantize\nthe channel vector in a more accurate way. We also provide performance analysis\nof the proposed codebook in the large-dimensional regime, where we prove that\nto limit the capacity degradation within an acceptable level, the required\nnumber of feedback bits only scales linearly with the number of resolvable\n(path) AoDs, which is much smaller than the number of BS antennas. Moreover, we\ncompare quantized channel feedback using the proposed AoD-adaptive subspace\ncodebook with analog channel feedback. Extensive simulations that verify the\nanalytical results are provided.\n