  Millimeter-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. This paper presents a novel one-shot beam
training technique that utilizes the emerging architecture of true-time-delay
(TTD) arrays. We first show that TTD arrays facilitate frequency dependent beam
steering. The proposed training procedure with TTD arrays then exploits this
fact by using a single radio-frequency-chain to multiplex different subcarriers
into different sounding directions. We derive conditions on the parameters of
TTD array configuration and physical layer to achieve scanning of the entire
angular domain with a single orthogonal frequency-division multiplexing (OFDM)
training symbol. The estimation of propagation directions with high resolution
is achieved via low-complexity digital signal processing of spatially coded
subcarriers. Simulation results show that this TTD array based approach
requires an order-of-magnitude fewer training symbols than those of phased
arrays.
