Energy-Efficient Interactive Beam-Alignment for Millimeter-Wave Networks
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Millimeter-wave (mm-wave) will be the key enabling technology in the next generation wireless networks due to the abundant bandwidth availability. However, the high signal attenuation at these frequencies demands precise beam-alignment between transmitter and receiver, which may entail significant overhead in mobile environments. This paper proposes an optimal interactive beam-alignment and data communication protocol, with the goal of minimizing energy consumption under a minimum rate constraint. The base-station (BS) decides to perform beam-alignment or data communication and selects the beam parameters, based on feedback from the user-end (UE). Based on the sectored antenna model and uniform prior on the angles of departure and arrival (AoD/AoA), it is shown that a fixed-length two-phase protocol is optimal, with a fixed-length beam-alignment phase followed by a data-communication phase. Moreover, a decoupled fractional search method is shown to be optimal during beam-alignment, which decouples over time the alignment of AoD and AoA. Furthermore, a heuristic policy is proposed for non-uniform AoD/AoA prior with provable performance guarantees, and it is shown that the uniform prior is the worst-case scenario. The performance degradation due to detection errors is studied analytically and via simulation. The numerical results with analog beams depict the superior performance of our proposed scheme, showing up to 10 and 20 exhaustive search policies, respectively.
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