Beam broadening for phased antenna arrays using multi-beam subarrays

Consideration of mmWave frequencies with large phased antenna arrays has become increasingly important for providing multi-Gb/s wireless data communication. However, large phased antenna arrays tradeoff gain for very narrow beam widths, which may not always be desirable for outdoor, mobile communication. This paper provides a systematic approach for beam broadening for phased antenna arrays, with unit amplitude constraint and without turning off antennas, using multi-beam subarrays and without any increase in hardware complexity. We first show that the beam resulting from the full array lies in the region defined by the sum of the individual subarrays and beams can be shaped within the region using RF or baseband phase precoding. While both conjugating and flipping the weights for subarrays generate similar subarray responses, only flipping the weights guarantees a symmetric response of the full array about boresight. We develop expressions for the resultant array factor, the locations for the beam directions of the subarrays and the half power beam width of resulting beam. We show that the broadening factor is proportional to the square of number of subarrays and the final array can be designed to have less than a 3 dB ripple in the passband.