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Communication at millimeter wave (mmWave) bands is expected to become a keyingredient of next generation (5G) wireless networks. Effective mmWavecommunications require fast and reliable methods for beamforming at both theUser Equipment (UE) and the Base Station (BS) sides, in order to achieve asufficiently large Signal-to-Noise Ratio (SNR) after beamforming. We refer tothe problem of finding a pair of strongly coupled narrow beams at thetransmitter and receiver as the Beam Alignment (BA) problem. In this paper, wepropose an efficient BA scheme for single-carrier mmWave communications. In theproposed scheme, the BS periodically probes the channel in the downlink via apre-specified pseudo-random beamforming codebook and pseudo-random spreadingcodes, letting each UE estimate the Angle-of-Arrival / Angle-of-Departure(AoA-AoD) pair of the multipath channel for which the energy transfer ismaximum. We leverage the sparse nature of mmWave channels in the AoA-AoD domainto formulate the BA problem as the estimation of a sparse non-negative vector.Based on the recently developed Non-Negative Least Squares (NNLS) technique, weefficiently find the strongest AoA-AoD pair connecting each UE to the BS. Weevaluate the performance of the proposed scheme under a realistic channelmodel, where the propagation channel consists of a few multipath scatteringcomponents each having different delays, AoAs-AoDs, and Doppler shifts.Thechannel model parameters are consistent with experimental channel measurements.Simulation results indicate that the proposed method is highly robust to fastchannel variations caused by the large Doppler spread between the multipathcomponents. Furthermore, we also show that after achieving BA the beamformedchannel is essentially frequency-flat, such that single-carrier communicationneeds no equalization in the time domain.

Efficient Beam Alignment for mmWave Single-Carrier Systems with Hybrid MIMO Transceivers