An Improved Dropping Algorithm for Line-of-Sight Massive MIMO With Tomlinson–Harashima Precoding

Abstract

One of the problems in line-of-sight massive MIMO is that a few users can have correlated channel vectors. To alleviate this problem, a dropping algorithm has been proposed in the literature, which drops some of the correlated users to make the spatial correlation among the remaining users be less than a certain threshold. Thresholds were found by running a large number of simulations. In this paper, the same dropping algorithm is analyzed for a known nonlinear precoder: Tomlinson-Harashima precoder. Instead of simulationbased thresholds, closed-form analytical expressions are derived in this paper for two power allocation schemes: max-min and equal received power control schemes. It is shown that the derived thresholds are optimal in terms of achievable sum-rate when there is only one correlated pair of users. For channels with multiple pairs of correlated users, simulation results show that using the derived thresholds improves the 5th percentile sumrate. Due to the fairness criterion of max-min, the improvement for max-min power control is much higher than equal received power control.Index Terms-Correlated users, line-of-sight, massive MIMO, max-min power control, Tomlinson-Harashima precoding.1 Lowercase, bold lowercase, and bold uppercase letters denote scalars, column vectors, and matrices, respectively. | · | and · denote the absolute value and l 2 -norm operators. The superscripts * , T , and H denote complex conjugate, un-conjugated transpose, and conjugated transpose, respectively. diag(p) denotes a diagonal matrix with diagonal entries taken from p. This is the author's version of an article that has been published in this journal. Changes were made to this version by the publisher prior to publication.The final version of record is available at http://dx.

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“…5]. The reduction of the sum-rate due to the high correlation is considerable for LOS environments with max-min power control as reported in [5], [6] (max-min power control is used to provide uniformly good service for the users as reported in [3]). In addition, it is shown in [5,Fig.…”

“…2] that when there is only one pair of highly correlated users, the signal to noise ratio with max-min power control will drop significantly. To deal with highly correlated scenarios in LOS environments with max-min power control, [3], [5], [6] studied dropping algorithms. However, dropping users may not be desirable in the case of latency-sensitive communication.…”

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