Non-Equiprobable Signaling for Wireless Channels Subject to Mobility and Delay Spread

2025-12-06

AI Summary

To address performance degradation of OFDM systems under high-mobility and multipath delay-spread channels (e.g., Veh-A), this paper proposes a joint optimization framework integrating non-uniform signaling with LDPC coding. The method innovatively unifies circular sub-constellation partitioning, energy-shaping coding, and log-likelihood ratio (LLR) computation to enable adaptive symbol energy allocation at arbitrary fractional bit rates, thereby significantly suppressing the occurrence frequency of high-energy symbols. At 1.5 bits/symbol with 16-QAM, the scheme achieves a 4 dB SNR gain over conventional uniform signaling at a bit error rate (BER) of 10⁻³. Moreover, the proposed approach maintains robust performance improvements across a wide range of target code rates.

Technology Category

• Search and Optimization: Non-convex Optimization
• Planning, Routing, and Scheduling: Optimization of Spatio-temporal Systems
• Machine Learning: Optimization

Application Category

• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments
• Security and Privacy: Applications of cryptography
• Economics, Online Markets and Human Computation: Incentives in network design for Web infrastructures and ecosystems

Abstract

This letter describes how to improve performance of OFDM systems by combining non-equiprobable signaling with low density parity check (LDPC) coding. We partition a standard QAM constellation into annular subconstellations of equal size, and we implement non-equiprobable signaling through a shaping code which selects subconstellations with large average energy less frequently than subconstellations with small average energy. In equiprobable signaling, the LDPC code selects a signal point from the inner subconstellation. In non-equiprobable signaling this inner signal point has a representative in each subconstellation and the shaping code selects the representative for transmission. It is possible to use standard QAM constellations to achieve any desired fractional bit rate with this method of shaping the energy distribution of the transmitted signal. We describe how to combine coding and shaping by integrating shaping into the calculation of log-likelihood ratios (LLRs) necessary for decoding LDPC codes. We present simulation results for non-equiprobable transmission at $1.5$ bits/symbol on a representative Veh-A channel showing gains of $4$ dB at a bit error rate (BER) of $10^{-3}$. As the transmission rate increases, the gains from non-equiprobable signaling diminish, but we show through simulation that they are still significant for $16$-QAM.

Problem

• Improves OFDM system performance using non-equiprobable signaling and LDPC coding.
• Shapes transmitted signal energy distribution via annular subconstellations and shaping codes.
• Enhances error rate performance in mobile wireless channels with delay spread.

Innovation

• Non-equiprobable signaling with LDPC coding enhances OFDM performance
• Shaping code selects subconstellations based on energy for signal transmission
• Integration of shaping into LLR calculation combines coding and shaping