Exhaustive, Iterative and Hybrid Initial Access Techniques in mmWave Communications

In millimeter wave (mmWave) communications with possible gigabit-per-second data rate, the severe path loss can effectively be alleviated by adaptive beamforming using antenna arrays. However, this complicates the initial access (IA), by which a mobile user equipment (UE) establishes a physical link connection with a mmWave base station (BS). The total duration of IA directional search can be very long since multiple preambles should be repeatedly transmitted for all transmit and receive beam pairs. In this work, we focus on IA techniques based on analog beamforming with uniform planar array antennas without context information from microwave BSs. We revisit current techniques of exhaustive search and iterative search based on BS training, and propose a novel hybrid training method, where in the first stage BS performs wide beam search, and in the second stage, the UE will perform reverse training according to the best wide beam decided in the first stage. For the three IA techniques, we derive the total consumed time slots and the received SNR expressions in each stage. Through simulations and comparisons, we demonstrate that the proposed hybrid training method shortens the searching delay and gives the same access error probability as iterative search.