Wideband mm-Wave Beam Training with True-Time-Delay Array Architectures

mm-Wave communications rely on beamforming gain from both transmitters and receivers to compensate for severe propagation loss. To achieve adequate gain, beam training is required to identify propagation directions. The main challenge in beam training arises from maintaining low overhead with increased array size. In this talk, we present a novel fast beam training technique that utilizes the true-time-delay (TTD) arrays. We first show that TTD arrays facilitate frequency dependent beam steering which is particularly attractive for direction finding. The proposed training procedure with TTD arrays then exploits this fact by using a single radio-frequency (RF) chain to multiplex different subcarriers into different sounding directions. We derive conditions on the parameters of TTD array configuration and physical layer processing to achieve scanning of the entire angular domain with a single orthogonal frequency-division multiplexing (OFDM) training symbol. We compare and contrast the delay resolution and range requirements for linear and planar arrays and experimentally demonstrate designed frequency dependent beamforming patterns using a custom TTD testbed. The estimation of propagation directions with high resolution is achieved via low-complexity digital signal processing of spatially coded subcarriers which requires an order-of-magnitude fewer training symbols than conventional approaches using phased antenna arrays. In order to overcome a large delay range requirements when using a single RF chain analog TTD arrays, we extend the proposed approach to hybrid analog-digital TTD arrays. We analyze the implementations of the proposed fast beamtraining approach using analog, hybrid, and digital TTD arrays and compare their performance, robustness to hardware impairments, and power consumption. Our analysis suggests that the analog and hybrid TTD arrays achieve a sub-degree beam alignment precision with 66% and 25% lower power consumption than a fully digital array, respectively.