Iterative-Detection-Aided Tomlinson-Harashima Precoding for Faster-Than-Nyquist Signaling

Heng Wang,Hao Liang,Biao Cai,Zhibin Feng,Xiaohu Liang,Aijun Liu

doi:10.1109/access.2019.2962069

Abstract

Faster-than-Nyquist(FTN) is a promising technique due to higher spectral efficiency, but at a cost of introducing the inter-symbol interference (ISI) which usually requires the computationally demanding detection algorithms. In order to reduce the detection complexity at the receiver, in this paper, a novel Tomlinson-Harashima precoding (THP) scheme is proposed for FTN system. In the conventional THP (CTHP) scheme, the ISI introduced by time-packing can only be diminished from several preceding information symbols, which results in large residual ISI and an unattractive bit-error-rate (BER) performance. Moreover, due to the suboptimal receiver - a simple modulo operation which ignores the correlation between received symbols, CTHP faces obvious capacity loss and is ineffective to execute interference cancellation for FTNS. In this paper, residual ISI of FTNS in CTHP is formulated in detail and modeled mathematically. Further more, an improved THP method with an optimized receiver based on the soft interference cancelation (SIC) algorithm and iterative turbo process is proposed to combat residual ISI for FTNS. Computational complexity analysis and numerical simulation results show that the proposed scheme not only has inexpensive computational cost but also greatly outperforms CTHP and other cited schemes. Moreover, for moderate time-packing, it can approach the ISI-free BER performance boundary and is also competitive to the MAP equalization technique.

Highlights

In recent years, to meet the ever increasing demand of high spectral efficiency, faster-than-Nyquist (FTN) technique has received considerable attention and has been regarded as one of the candidate modulation schemes for 5G communications [1]–[5]
For the purpose of comparison, we present the performance of conventional THP (CTHP), soft interference cancelation (SIC) equalization, bidirectional interference cancellation technique (BICT) and MAP equalization which considers the BCJR algorithm based on Ungerboeck Model
When inter-symbol interference (ISI) becomes serious with τ = 0.8, as shown in Fig.11, the BER performance of all schemes has been degraded in a certain extent, but the IDA-TomlinsonHarashima precoding (THP) still shows better performance than BICT, CTHP and SIC, while it is slightly inferior to the MAP

Summary

INTRODUCTION

To meet the ever increasing demand of high spectral efficiency, faster-than-Nyquist (FTN) technique has received considerable attention and has been regarded as one of the candidate modulation schemes for 5G communications [1]–[5]. (2) At the receiver, a soft interference cancelation method with turbo equalization is proposed to replace original modulo operation to compensate the BER performance loss and the transmitted signal is estimated and reconstructed to obtain better performance of ISI cancelation. Since the CTHP method was originally proposed to combat ISI of orthogonal Nyquist signaling, ISI from following symbols isn’t processed at all by TH precoder resulting in a large amount of residual ISI at the receiver. THE PROPOSED ITERATIVE-DETECTION-AIDED TOMLINSON-HARASHIMA PRECODING (IDA-THP) SCHEME According to the analysis above, since noncausality and symmetry of ISI matrix in FTNS isn’t taken into account in the CTHP method, there’s a lot of residual ISI. In this paper, residual interference is modeled as gaussian noise and a symbol-bysymbol detection method based on SIC equalization is employed to replace the original modulo operation at the receiver.

RESIDUAL ISI MODEL

COMPLEXITY ANALYSIS AND SIMULATION RESULTS

CONCLUSION