Abstract
We focus on the design of vector perturbation (VP) precoding for multiuser multiple-input-single-output (MU-MISO) broadcast channel systems where the centralized transmitter equipped with multiple antennas and communicates simultaneously to multiple single-antenna receivers. While conventional VP requires the feedback of the channel matrix at the transmitter for precoding and the power scaling factor at the receivers for detection, VP precoding has so far been developed and analyzed under assumptions that the transmitter has perfect channel state information (CSI) or the receivers have perfect knowledge of the channel- and data-dependent power scaling factors. In practical limited feedback scenarios, wireless communication systems suffer from limited time and frequency resource for pilots to feed-forward information and only a quantized version of power scaling factors is available at the receivers; under such limitations, the performance of VP precoding will degrade significantly compared with ideal scenarios and would always encounter an error floor at mid-to-high signal-to-noise ratio (SNR) regions. Motivated by such observations, we propose a robust VP precoder design, which takes the imperfectness of CSI and power scaling factor jointly into account under the criterion of minimum mean-square error (MMSE). The closed-form expressions of the proposed precoder are then derived. As illustrated by the simulation results, the proposed VP precoder is less sensitive to CSI and power scaling factor imperfections compared with the classic VP precoder and other existing MMSE-based VP precoders, as it has a lower error floor when imperfectness is assumed to be fixed, and power scaling factor accuracy is shown to offer a non-linear performance gain compared with that of the linear gain CSI accuracy could offer.
Highlights
As is well known, multiple-input multiple output (MIMO) systems can provide higher sum rates compared with single antenna systems [1]
Zero-forcing (ZF) [4] precoding and minimum mean square error (MMSE) [5] are among the most popular linear precoding schemes, which enjoy a lower complexity but suffer a performance loss compared with non-linear precoding schemes when the number of users and base-station antennas are the same, the poor performance is due to the large spread in the singular values of the channel matrix [6]
Motivated by the fact that robust vector perturbation (VP) precoders can minimize inter-user interference and interference introduced by noise [13], imperfect channel state information (CSI) [15], [16], or inaccurate power scaling factors [19] under MMSE criterion, we propose a joint robust design of VP precoder
Summary
Multiple-input multiple output (MIMO) systems can provide higher sum rates compared with single antenna systems [1]. PRACTICAL OBSTACLES OF VP PRECODING Robust design for linear precoders have been widely studied as in [12], with regards to non-linear precoding scheme VP, the obstacles on the way to practical usage are mainly on two aspects: CSI imperfections and inaccurate power scaling factors It has been demonstrated in [7] and [13] that regularized VP and MMSE-VP outperform CVP in the entire range of SNR if perfect CSI is available at transmitter and power scaling factors is known in advance of transmission at the receivers ideally. As it is hard to obtain a practical VP precoding scheme without transmitting power scaling factors, this paper proposes a robust VP precoding scheme for a more general scenario where receivers obtain the power scaling factor through a limited feed-forward link In this scenario, only quantized version of the scaling factors is available, as averaged over the basic physical time-frequency resource block (RB) before, the inaccurate power factor would cause a performance floor as illustrated in [19]. En stands for taking expectation over n. (c), (c) denotes the real and imaginary part of c. denotes the floor operation
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