Nonlinear Quasi-Synchronous Multi User Chirp Spread Spectrum Signaling

TL;DR

This paper introduces two new nonlinear chirp designs for multi-user chirp spread spectrum systems, significantly improving performance under quasi-synchronous conditions with channel dispersion and Doppler shifts.

Contribution

The paper proposes novel nonlinear chirp sets, derives analytical bit error probabilities, and demonstrates their superiority over classical and existing nonlinear chirps in realistic dispersive channels.

Findings

New nonlinear chirps outperform classical linear chirps.

Analytical BER expressions validated by simulations.

Enhanced robustness in dispersive air-ground channels.

Abstract

Multi user orthogonal chirp spread spectrum (OCSS) can improve the spectral inefficiency of chirp spread spectrum (CSS) but is only feasible with perfect synchronism and without any channel dispersion. Asynchronism, channel dispersion, or unexpectedly large Doppler shifts can cause multiple access interference (MAI), which degrades performance. Conditions with small timing offsets we term quasi-synchronous (QS). In this paper, we propose two new sets of nonlinear chirps to improve CSS system performance in QS conditions. We analytically and numerically evaluate cross-correlation distributions. We also derive the bit error probability for Binary CSS analytically and validate our theoretical result with both numerical and simulation results; our error probability expression is applicable to any binary time-frequency (TF) chirp waveform. Finally, we show that in QS conditions our two new nonlinear chirp designs outperform the classical linear chirp and all existing nonlinear chirps from the literature. To complete our analysis, we demonstrate that our nonlinear CSS designs outperform existing chirps in two realistic (empirically modeled) dispersive air to ground channels.