Initial Access Design for Millimeter Wave IoT Networks Based on Rainbow Link Protocol

Recently, a new multiple access protocol, called Rainbow-Link, has been proposed as a promising candidate to support various Internet of Things (IoT) networks. In Rainbow-Link, the base station (BS) is equipped with an analog true-time-delay (TTD) array that can create frequency-dependent rainbow beams in the millimeter-wave (mmW) band. The rainbow beams enable fast beam training and can support a large number of IoT devices at the same time thus reducing the latency in channel access. However, link establishment through initial access (IA) procedure using rainbow beams is not straightforward due to asymmetry in radio front-end and processing capabilities between IoT BS and terminals. Also, the superposition of multiple delayed signals introduced by the TTD array needs to be handled carefully in the synchronization process. In this article, we propose a low latency Rainbow-Link IA procedure based on the new rainbow beam design suitable for narrowband IoT processing. We design joint beam training and timing synchronization algorithms for narrowband IoT transceivers that are robust to severe multipath delay spread introduced by the TTD array. We perform a comprehensive performance analysis under different system parameters, including the number of antennas, the number of subcarriers, and the bandwidth ratio between BS and IoT terminals. Simulation results show that the proposed Rainbow-Link IA procedure can achieve millisecond-level latency with highly reliable synchronization accuracy.