Efficient Beam Sweeping Algorithms and Initial Access Protocols for Millimeter-Wave Networks

5G millimeter-wave (mmW) systems rely on electronically steerable antenna arrays to support directional communications. Directionality complicates the initial access (IA) process, whereby a base station (BS) announces itself to nearby user equipments (UEs), giving them the opportunity to associate with this BS. Existing approaches for IA suffer from long discovery time and/or nonnegligable probability of missing UEs. In this paper, we propose FastLink, an efficient IA protocol for mmW systems, in which discovery beacons are transmitted/received using the narrowest possible beams, allowing for high beamforming gains and low misdetection rate, while maintaining low discovery time. Fastlink executes a unique algorithm, called 3-dimensional peak finding (3DPF), to find the best beam in logarithmic time. We formulate the beam-finding process as a sparse problem and use compressive sensing to determine the minimum number of measurements needed for this process. We first study FastLink for the discovery of a single UE and then extend our analysis to a multi-user scenario. Both simulations and over-the-air experiments based on a custom mmW testbed are used to evaluate FastLink. Our results verify its efficiency, and show that it can reduce the search time by 90% compared to the scanning approach used in 802.11ad systems.