FastLink: An Efficient Initial Access Protocol for Millimeter Wave Systems

To overcome the high propagation loss and satisfy a given link budget, millimeter wave (mmW) communication systems rely on highly directional antennas, both at the base station (BS) and the user equipment (UE). Due to this directionality, initial access (IA) and association can be particularly challenging. Existing approaches for IA in directional networks suffer from long discovery time and/or high misdetection probability of the UE. In this paper, we propose FastLink, an efficient IA protocol for mmW systems with electronically steerable antennas. FastLink always transmits/receives using the narrowest possible beam, allowing high beamforming gains and low misdetection rate. It uses a unique binary-search-based algorithm, called 3DPF, to scan only a small subset of the angular space and find in logarithmic time the best transmit-receive beam pair. We formulate the beam-finding process as a sparse problem, exploiting the poor scattering nature of mmW channels. Compressive sensing is then used to determine the minimum number of measurements needed to reconstruct the sparse channel. 3DPF is incorporated into FastLink to establish the directional link, and the required messaging between the BS and the UE is explained in detail. For performance evaluation purposes, we first conduct simulations based on NYU mmW channel model and then experiment with a custom mmW testbed utilizing uniform planar arrays and operating at 29 GHz frequency. Our extensive simulations and hardware experiments verify the efficiency of FastLink, and show that 3DPF can reduce the search time by 65 - 99% compared to 802.11ad-like beam finding scheme.