Directional Beam Alignment for Millimeter Wave Cellular Systems

Transmission in millimeter wave (mmW) band has a big potential to provide orders of higher wireless bandwidth. To combat the high channel loss in high frequency band, beamforming is generally taken to transmit along the direction that provides the maximum transmission gain. This requires the MAC protocol to facilitate the finding of the optimal beamforming direction. Exiting protocol suggests the rotational channel measurement which may introduce high measurement cost, and compromise the transmission capacity. This paper presents a comprehensive design for more efficient directional beam alignment in mmW cellular networks. Instead of exhaustively searching all possible beamforming directions at the transmitter (TX) and the receiver (RX), our proposed scheme selects only a fairly small number of TX and RX beam pairs to facilitate effective beam alignment. To avoid long and resource-consuming exhaustive search, our scheme not only takes advantage of the low rank characteristics of the channel to estimate the full channel information with a small number of measurements, but also further exploits the channel estimation from initial measurements to guide the selection of future beam pairs for more effective measurements later. These strategies help to speed up the process of finding satisfactory beam pairs. We perform extensive simulations to evaluate the performance of our proposed schemes, and our results demonstrate our scheme can significantly outperform other schemes in terms of measurement effectiveness and cost efficiency.