Modeling and Mitigating Beam Squint in Millimeter Wave Wireless Communication

There has been increasing demand for accessible radio spectrum with the rapid development of mobile wireless devices and applications. For example, a GHz of spectrum is needed for fifth-generation (5G) cellular communication, but the available spectrum below 6 GHz cannot meet such requirements. Fortunately, spectrum at higher frequencies, in particular, millimeter-wave (mmWave) bands, can be utilized through phased-array analog beamforming to provide access to large amounts of spectrum. However, the gain provided by a phased array is frequency dependent in the wideband system, an effect called beam squint. We examine the nature of beam squint and develop convenient models with either a uniform linear array (ULA) or a uniform planar array (UPA). Analysis shows that beam squint decreases channel capacity, and therefore, path selection should take beam squint into consideration. Current channel estimation algorithms assume no beam squint, and channel estimation error is increased by the beam squint, further decreasing the channel capacity.

Three problems involving phased-array beamforming are studied to incorporate and compensate for beam squint. First, we show that carrier aggregation can be used to improve system throughput to a point. We study the optimal beam alignment to maximize channel capacity, and demonstrate that, with sufficient band separation, focusing on only one band outperforms carrier aggregation. Approximations are developed for a system with two symmetric bands to determine the critical values of system parameters like band separation, and angle of arrival beyond which it is preferable not to aggregate. Second, beamforming codebooks are designed to compensate for one-sided beam squint by imposing a channel capacity constraint. Analysis and numerical examples suggest that a denser codebook is required compared to the case without beam squint, and the codebook size increases as bandwidth or the number of antennas in the array increases and diverges as either of these parameters exceeds certain limits. Third, to decouple the transmitter and receiver arrays with two-sided beam squint, and to extend conventional codebook design algorithms, codebooks with a minimum array gain constraint for all frequencies and angles of arrival or departure are also designed to compensate for the beam squint. Again, either the bandwidth or the number of antennas in the array is limited by the effects of beam squint if the other one is fixed.