Multi-Branch Binary Modulation Sequences For Interferer Rejection

Abstract

When the techniques of random modulation are used in wideband communication receivers, one can design spectrally shaped sequences that mitigate interferers while preserving messages to reduce distortion caused by amplifier nonlinearity and noise. For sampling rates that are too high for standard modulation, one can instead rely on multi-branch architectures involving multiple modulators working at reduced sampling rates. In this paper, we propose an algorithm to design a set of binary sequences to be used in multi-branch modulation to mitigate a strong interferer while allowing for stable message recovery. The implementation consists of a quadratic program that is relaxed into a semidefinite program combined with a randomized projection. While interferer signals are often modeled as a subspace under the discrete Fourier transform, spectrum leakage occurs when the signal contains so-called off-grid frequencies. The Slepian basis provides a much better-suited representation for such bandlimited signals that mitigates spectrum leakage. We use both representations during the evaluation of our design algorithm, where numerical simulations show the advantages of our sequence designs versus the state of the art.