A frequency diversity spread spectrum system for communication in the presence of in-band interference

Protection against in-band interference is required in antijam systems and when low level spread spectrum signals are overlaid on top of existing narrow-band users. We propose a secure spread-spectrum system whose optimum receiver is easy to build. We use frequency diversity which allows the receiver to distinguish unjammed signal replicas from their jammed versions. The system can also resist bandlimited partial-time jamming. The only choice left to a wise jammer to maximize our error probability is to spread his signal like ours. We derive the optimum receiver which jointly performs symbol detection and interference rejection. Side information, needed by this receiver, can easily be estimated. However, if the interference bandwidth is narrow enough compared to the signal bandwidth, side information is not needed by a simpler and near-optimum receiver. The bit error probability is evaluated for QPSK modulation and compared with that of direct sequence spread-spectrum