We consider a communication scenario in which a message is received in the presence of partial-time Gaussian jamming and additive white Gaussian noise. We consider a quasi-static channel, in which the amplitude and phase are constant over each packet transmission. The receiver does not know the amplitude and phase of the incoming signal, which symbols are jammed, or even the statistics of the jammer, such as the jamming power and jamming probability. In this scenario, the receiver must accurately estimate the parameters of the channel and the jamming to achieve good performance. We apply the expectation-maximization (EM) algorithm to iteratively approximate the maximum-likelihood (ML) estimator for all of the parameters. We find that the overall performance of the EM algorithm is very sensitive to the initial estimates, so we propose a new initial estimator that offers good performance. The EM algorithm approach requires pilot symbols to resolve a phase ambiguity. Thus, we also present a blind estimation algorithm to avoid the reduction in overall code rate from the use of pilot symbols

We propose new collaborative reception techniques for use in the presence of a partial-time Gaussian jammer. Under the proposed techniques, a group of radios acts as a distributed antenna array by exchanging information that is then used to perform jamming mitigation. We propose two such jamming mitigation techniques that offer a tradeoff between performance and complexity. The results show that these techniques can allow communications in much more severe jamming conditions than when collaboration is not employed or when conventional collaboration techniques based on maximal-ratio combining are applied. Example scenarios with strong jamming show that three collaborating radios can reduce the frame error rate by more than two orders of magnitude over single-radio reception. In another scenario it is shown that a jammer must jam at least 75% of the transmitted symbols to produce an unacceptable frame error rate with three collaborating radios, but only 43% of the transmitted symbols if there is no collaboration.

Collaborative techniques have been proposed to improve communications in the presence of a hostile jammer. A distributed detection and decoding technique based on collaborative decoding has been shown to he effective for communications over non-fading channels for coherent communications. In this paper, we consider collaborative communications strategies for jamming channels in which the message and jamming signals experience independent block fading at each receiving node. Furthermore, we assume that the phase of the message signal is not known and must be estimated from the received signal. Joint decoding and estimation of all of the fading and jamming parameters is computationally prohibitive, so we develop iterative estimation and decoding algorithms. The fading coefficient and parameters of the jamming signal are estimated using the EM and BCJR algorithms. The results show that the proposed technique is an effective response to hostile jamming.

We consider the use of selective-retransmission automatic repeat request (ARQ) protocols to improve communication performance on partial-time jamming channels. The schemes that we propose are based on reliability-based hybrid ARQ (RB-HARQ), in which the set of bits to be retransmitted is adaptively selected based on the estimated a posteriori probabilities at the output of a soft-input, soft-output decoder. For channels with bursty partial-time jamming, the RB-HARQ techniques are particularly appropriate because these techniques can identify which symbols need to be retransmitted, and the bursty nature of the jamming reduces the overhead required to relay this information to the source. We extend our reliability measure to incorporate not only a posteriori probability information but also estimates of the probability that a bit was jammed. We compare the performance of the proposed scheme with that of conventional approaches, in which a predetermined set of bits is retransmitted in response to a packet failure. The results show that RB-HARQ schemes can achieve higher throughput than these conventional approaches in several scenarios.