Sparse-DFT and WHT Precoding with Iterative Detection for Highly Frequency-Selective Channels

TL;DR

This paper introduces low-complexity iterative detection methods for sparse DFT and Walsh-Hadamard precoded channels, demonstrating improved performance and efficiency in highly frequency-selective wireless channels.

Contribution

It proposes feasible MAP detection algorithms for sparse Walsh-Hadamard precoding and compares various precoders with EP-based receivers, highlighting the advantages of SWH-Max-Log-MAP.

Findings

SWH-Max-Log-MAP outperforms other methods in performance and complexity for QPSK and 16-QAM.

Sparse precoding reduces transceiver complexity while maintaining performance.

The proposed methods are effective for highly frequency-selective channels.

Abstract

Various precoders have been recently studied by the wireless community to combat the channel fading effects. Two prominent precoders are implemented with the discrete Fourier transform (DFT) and Walsh-Hadamard transform (WHT). The WHT precoder is implemented with less complexity since it does not need complex multiplications. Also, spreading can be applied sparsely to decrease the transceiver complexity, leading to sparse DFT (SDFT) and sparse Walsh-Hadamard (SWH). Another relevant topic is the design of iterative receivers that deal with inter-symbol-interference (ISI). In particular, many detectors based on expectation propagation (EP) have been proposed recently for channels with high levels of ISI. An alternative is the maximum a-posterior (MAP) detector, although it leads to unfeasible high complexity in many cases. In this paper, we provide a relatively low-complexity \textcolor{black}{computation} of the MAP detector for the SWH. We also propose two \textcolor{black}{feasible methods} based on the Log-MAP and Max-Log-MAP. Additionally, the DFT, SDFT and SWH precoders are compared using an EP-based receiver with one-tap FD equalization. Lastly, SWH-Max-Log-MAP is compared to the (S)DFT with EP-based receiver in terms of performance and complexity. The results show that the proposed SWH-Max-Log-MAP has a better performance and complexity trade-off for QPSK and 16-QAM under highly selective channels, but has unfeasible complexity for higher QAM orders.