WHTDM: Walsh-Hadamard Transform Division Multiplexing for Doubly-Selective Channels

TL;DR

WHTDM introduces a Walsh-Hadamard transform-based multicarrier waveform that reduces complexity and enhances performance in doubly-selective channels, especially at high mobility.

Contribution

It replaces the FFT in OFDM with a real-valued Walsh-Hadamard transform, eliminating complex multipliers and improving BER performance with lower complexity.

Findings

WHTDM outperforms conventional OFDM at high mobility in simulations.

WHTDM achieves over an order of magnitude lower BER at 120 km/h.

WHTDM has 2.5 times lower transmitter complexity than OFDM.

Abstract

We propose Walsh-Hadamard Transform Division Multiplexing (WHTDM), a multicarrier waveform that replaces the conventional IFFT/FFT pair in OFDM with a real-valued, unitary Walsh-Hadamard transform (WHT). WHTDM inherits the CP-OFDM transceiver structure while eliminating all complex multiplications from the transform stage, yielding a transmitter with zero real multipliers in the core modulation block. For detection under doubly-selective channels, we adopt a cross-domain memory approximate message passing (CD-MAMP) equalizer that operates on the banded structure of the equivalent WHT-domain channel matrix. Simulation results under the 3GPP TDL-C channel model at 28 GHz demonstrate that WHTDM with CD-MAMP significantly outperforms conventional OFDM 1-tap MMSE at high mobility, achieving over an order of magnitude lower BER at 120 km/h. Among the compared CD-MAMP-equalized new waveforms, WHTDM achieves the best BER performance while maintaining a transmitter complexity 2.5 $\times$ lower than OFDM and completely eliminating complex multipliers from the transform stage, making it well-suited for low-power IoT terminals.