We propose a physical-layer technique for checking the integrity of information provided by an intermediate node (relay) in cooperative relay communication systems. The proposed approach, built on the theory of likelihood ratio test, exploits the physical-layer signal from the source in detecting any data modification (false data injection attack) made by the relay. We present the optimum detection method that minimizes the end-to-end outage probability and the minmax detection method that minimizes the maximum possible outage probability under unknown attack probability. We show that the optimum detection technique can provide almost the same end-to-end outage probability that can be provided by the ideal cryptographic technique that perfectly detects the false injection at the expense of high computational cost and bandwidth overhead. The proposed physical integrity check can be used as an additional layer of protection or can complement to the conventional cryptographic techniques.Index Terms -Physical integrity check, false data injection attack, likelihood ratio test, optimum detection, minmax detection, cooperative relay communications.

We study coherent binary direct-sequence code division multiple access (DS-CDMA) systems operating over frequencyselective Nakagami fading channels. In particular a performance comparison between maximal-ratio combining (MRC) and equal-gain combining (EGC) RAKE receivers is considered. For interference limited systems, perfect MRC RAKE receivers can accommodate about 5m/(5m -1) more users than EGC RAKE receivers, where m is the Nakagami fading parameter. MRC has a higher rate of improvement than EGC as the number of combined paths increases. However, imperfect or inaccurate channel fading estimation leads to serious degradation in the performance of MRC receivers and in that case EGC becomes superior to MRC.

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