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True-Time Delay-Based Hybrid Precoding Under Time Delay Constraints in Wideband THz Massive MIMO Systems

Dang Qua NguyenTaejoon Kim
Dec 2022
The beam squint effect that arises in the wideband Terahertz (THz) massivemultiple-input multiple-output (MIMO) communication produces a serious arraygain loss. True-time delay (TTD)-based hybrid precoding has been considered tocompensate for the beam squint effect. By fixing the phase shifter (PS)precoder, a common strategy has been designing TTD precoder under theassumption of unbounded time delay values. In this paper, we present a newapproach to the problem of beam squint compensation, based on the jointoptimization of the TTD and PS precoders under per TTD device time delayconstraints. We first derive a lower bound of the achievable rate and show thatin the large system limit the ideal analog precoder that completely compensatesfor the beam squint is equivalent to the one that maximizes the achievable ratelower bound. Unlike the prior approaches, our approach does not require theunbounded time delay assumption; the range of time delay values that a TTD canproduce is strictly limited in our approach. Instead of focusing on the designof TTD values only, we jointly optimize both the TTD and PS values toeffectively cope with the practical time delay constraints. Taking theadvantage of the proposed joint TTD and PS precoder optimization approach, wequantify the minimum number of TTDs required to produce a predefined array gainperformance. The simulation results illustrate the substantially improvedperformance with the array gain performance guarantee of the proposed jointoptimization method.