Faster-Than-Nyquist Signaling Via Spatiotemporal Symbol-Level Precoding for Multi-User MISO Redundant Transmissions

Abstract

This work addresses the issue of interference generated by co-channel users in downlink multiantenna multicarrier systems with frequency-packed faster-than-Nyquist (FTN) signaling. The resulting interference stems from an aggressive strategy for enhancing the throughput via frequency reuse across different users and the squeezing of signals in the time-frequency plane beyond the Nyquist limit. The error-free spectral efficiency is proved to be increasing with the frequency packing and FTN acceleration factors. The lower bound for the FTN sampling period that guarantees information losslesness is derived as a function of the transmitting-filter roll-off factor, the frequency-packing factor, and the number of subcarriers. Space-time-frequency symbol-level precoders (SLPs) that trade off constructive and destructive interblock interference (IBI) at the single-antenna user terminals are proposed. Redundant elements are added as guard interval to cope with vestigial destructive IBI effects. The proposals can handle channels with delay spread longer than the multicarrier-symbol duration. The receiver architecture is simple, for it does not require digital multicarrier demodulation. Simulations indicate that the proposed SLP outperforms zero-forcing precoding and achieves a target balance between spectral and energy efficiencies by controlling the amount of added redundancy from zero (full IBI) to half (destructive IBI-free) the group delay of the equivalent channel.

Symbol-Level Precoding With Constellation Rotation in the Finite Block Length Regime

“…SLP is a non-linear technique that uses CSI along with users' data symbols to form the precoder. Several SLP schemes exploiting different properties of the communication setup have been proposed [5]- [11]. In [5], the authors have shown how to split the interference into constructive and destructive parts, and how to eliminate the destructive part via a selective channel inversion.…”

“…Those results have been extended to multi-level modulations in [9]. Other aspects related to, for instance, non-linear channels [10] or faster-than-Nyquist redundant signaling [11] have also been addressed. We refer the reader to the comprehensive survey in [12] for further details.…”