Random Guessing for Beam Alignment: A Low Complexity Strategy for Reducing Blockage in mmWave Communications

Abstract

High frequency (millimeter wave and higher) systems are being used for curb-to-home services and indoor networks with fixed transmitter and receivers. However, the environment between them can shift due to moving flora and fauna causing sudden blockages. To mitigate outages due to blocking, we investigate the use of a two-beam system rather than a conventional single line-of-sight (LOS) beam. A two-beam system requires a design that can adjust both the transmit phase and delay on one of the beams to ensure a strong signal when both beams are not blocked, and the ability to adapt the transmission rate when the receiver power drops due to intermittent blockages on either beam. We propose a low-complexity co-phasing strategy in which the transmitter guesses the phase and delay offset between the two beams until the receiver indicates a satisfactory channel has been established. Exact co-phasing of the beams is not required and the average number of guesses needed to find an appropriate delay and phase is relatively small. Once the link has been established, the transmitted signal power is split between the two beams to achieve maximum throughput for a fixed total power budget limit. The resulting scheme is not only computationally efficient, but is also robust to channel estimation errors that typically plague transmit adaptation strategies. Numerical results demonstrate the effectiveness of the approach, showing that a capacity within 0.1 dB of perfect co-phasing can be achieved with fewer than 100 guesses. In 90 % of the cases, 25 guesses were required on average to achieve a power that is within 0.1 dB of the optimum.