Antijamming Capacity and Performance Analysis of Multiple Access Spread Spectrum Systems in AWGN and Fading Environments

A unified multiple access channel model based on the time-bandwidth dimensionality is considered for the user capacity analysis of a wide variety of multiuser spread spectrum systems contaminated by smart jamming. Different users are distinguished by signatures of either direct sequence or hopping type. It is found that the jammer should spread its energy evenly over all degrees of freedom in order to minimize the average capacity or maximize the outage probability. Also, for the communicator to best make use of the channel resources, it is necessary to avoid hopping type signatures. As a practical realization, we consider the performance analysis of a synchronous frequently cited multicarrier frequency hopping CDMA system in AWGN and Rayleigh fading jammed uplink channels. We show that the best counter-jamming performance is attained when the number of subcarriers is maximal. It is demonstrated that when the receiver does not know the jammer state, concentrating instead of spreading the jamming power over the channel degrees of freedom will give rise to the worst performance for the communicators. Optimal weights for the receiver soft outputs in different channels are also obtained, for practical purposes.