In this paper we consider a wideband millimeterwave and terahertz (THz) beamforming to support simultaneous data communication with multiple spatially separated users. The beamforming objective is to orthogonalize users across subbands by deploying frequency-dependent beams with a subband-specific spatial response. Analog True-Time-Delay (TTD) antenna arrays are a promising architecture for generating subband beams using a single radio frequency RF chain. An analog TTD array can realize subband beams however the beams cannot achieve flat gain response in each subband and uniform gain distribution across subbands, limiting their utility in practical deployments. Hybrid TTD arrays offer additional degrees of freedom to improve the beamforming design. This work proposes the optimization of the Hybrid TTD codebooks to achieve the desired sub-band beams. We propose an alternating minimization algorithm and a low-complexity heuristic solution for subband beam design. Simulation results show beamforming performance improvement as a function of RF chains, and demonstrate that scaling RF chains proportional to half of the number of users is sufficient to achieve near-digital beamforming performance.

True-time-delay (TTD) arrays can implement frequency-dependent rainbow beams and enable fast beam alignment in wideband millimeter-wave (mmWave) systems. In this paper, we consider 3D rainbow beam training with planar TTD arrays. We first design a planar TTD-array-based codebook that can realize a partial or complete scan of the entire 3D sphere using a single Orthogonal Frequency Division Multiplexing (OFDM) symbol. We demonstrate that a rainbow-beam codebook with a proper configuration of delays and phase shifts, and a sufficient number of subcarriers, can simultaneously probe all angular directions in the desired coverage sector with the required beamforming gain. Leveraging this full coverage codebook, we propose two frequency-domain power-based 3D beam training algorithms for fast estimation of the azimuth and elevation angles corresponding to the dominant propagation direction. The proposed algorithms take into account practical hardware-imposed delay range constraints. We numerically evaluate the performance of the proposed algorithms in mmWave channels as a function of the TTD codebook and system parameters. We show that the proposed 3D beam training offers considerable overhead reduction relative to the state-of-the-art methods with planar phased antenna arrays (PAA).

Millimeter-wave (mmWave) systems require a large number of antennas, which makes the beam training challenging and time-consuming for conventional phased arrays. Recently, a true-time-delay (TTD) array-based beam training algorithm has been shown as an effective solution to overcome the training overhead in large arrays. In this paper, we present a custom-built over-the-air (OTA) testbed to study the effects of hardware impairments on the TTD-based beam training and verify its feasibility in a real system. We proposed an orthogonal matching pursuit (OMP) based reconstruction algorithm along with a phase calibration dictionary to combat nonidealities such as strong frequency selectivity and phase misalignment in the received raw IQ signal. Post-processing results showed that with the nonideality effects properly handled, the 3D TTD beam training algorithm can achieve high AOA estimation accuracy.

Wideband millimeter-wave and terahertz (THz) systems can facilitate simultaneous data communication with multiple spatially separated users. It is desirable to orthogonalize users across sub-bands by deploying frequency-dependent beams with a sub-band-specific spatial response. True-Time-Delay (TTD) antenna arrays are a promising wide band architecture to implement sub-band-specific dispersion of beams across space using a single radio frequency (RF) chain. This paper proposes a structured design of analog TTD codebooks to generate beams that exhibit quantized sub-band-to-angle mapping. We introduce a structured Staircase TTD codebook and analyze the frequency-spatial behaviour of the resulting beam patterns. We develop the closed-form two-stage design of the proposed codebook to achieve the desired sub-band-specific beams and evaluate their performance in multi-user communication networks.