  The millimeter-wave (mmW) communications is a key enabling technology in 5G
to provide ultra-high throughput. Current mmW technologies rely on analog
phased arrays to realize beamforming gain and overcome high path loss. However,
due to a limited number of simultaneous beams that can be created with
analog/hybrid phased antenna arrays, the overheads of beam training and beam
scheduling become a bottleneck for emerging networks that need to support a
large number of users and low latency applications. This paper introduces
rainbow-link, a novel multiple access protocol, that can achieve low latency
and massive connectivity by exploiting wide bandwidth at mmW frequencies and
novel analog true-time-delay array architecture with frequency dependent
beamforming capability. In the proposed design, the network infrastructure is
equipped with the true-time-delay array to simultaneously steer different
frequency resource blocks towards distinct directions covering the entire cell
sector. Users or devices, equipped with a narrowband receiver and either a
single antenna or small phased antenna array, connect to the network based on
their angular positions by selecting frequency resources within their rainbow
beam allocation. Rainbow-link is combined with a contention-based grant-free
access to eliminate the explicit beam training and user scheduling. The
proposed design and analysis show that rainbow-link grant-free access is a
potential candidate for latency-critical use cases within massive connectivity.
Our results show that, given less than 1e-5 probability of packet loss, a
rainbow-link cell, over 1 GHz bandwidth using 64 element antenna array, attains
sub-millisecond user-plane latency and Mbps user rates with an approximate 400m
line-of-sight coverage and a density of up to 5 active single antenna users per
second per meter square.
