Broad Beam Designs for Broadcast Channels

In a massive multi-input multi-output (MIMO) cellular communication system, the conventional beam-sweeping scheme for common message broadcasting provides high beamforming gain but requires too many time slots due to the narrowness of the beams. To reduce the beam sweeping time while maintaining a sufficient beamforming gain, this paper focuses on designing broad beams with tunable beamwidths. First, by over-sampling a step-chirp analog signal, we construct a novel sequence family termed the generalized step-chirp (GSC) sequence with a simple closed-form expression, in which some parameters can be tuned to flexibly adjust the beamwidth and coarsen the phase resolution. The beamforming matrix of a uniform rectangular array (URA) of single–polarized antennas can be taken as the outer product of two GSC sequences. Second, by exploiting the full degree of freedom of URA of dual-polarized antennas, we further reduce the power variations in the passband by using a numerical algorithm. The algorithm can be drastically accelerated by exploiting the specific structure of the problem. Both schemes can be implemented using an analog phase shifter network (PSN) with finite resolution.