Optimum Combining for Coherent FFH/DS Spread Spectrum Receivers in the Presence of Multi-Tone Jammer

Fast Frequency Hopping/Direct Sequence (FFH/DS) hybrid spread spectrum is an effective technique of the communication network for the robot swarms operating in complex electromagnetic environment. It combines the advantages of both Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS), and also uses the diversity-combining in time and frequency domain to effectively reject unwanted jamming. Multi-Tone Interference (MTI) is one of the most deleterious jamming waveforms for frequency hopping systems, and consequently a severe threat for FFH/DS systems. In this article, we explore the effect of MTI on coherent FFH/DS, and we have shown that, if spectral sensing mechanism for MTI frequency and power monitoring is available, the MTI in FFH/DS reception is a set of complex sinusoid components with deterministic amplitudes and random phases, instead of the conventionally assumed Gaussian random variables. In light of this new finding, a novel combining method is proposed. It utilizes the spectral sensing information to adaptively compute the combining weights, following Maximum Signal to Interference plus Noise (MSINR) law. Its combining performance is analyzed and compared with other existing methods, and verified by simulations. It is shown that the proposed adaptive MSINR combining has satisfactory performance under both medium and strong MTI scenarios.