Low rate repeat codes are useful for satellite communication in severely contested environments. Frequency hopping is used in systems for protection. In response, MITRE developed a new signal-combining algorithm based on the maximum likelihood approach.
Maximum Likelihood Detection of Low Rate Repeat Codes in Frequency Hopped Systems
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In time diversity wireless and satellite communication, multiple copies of the same signal segment are transmitted during, different time intervals to improve signal detection. If the communication system is frequency hopped, coherent detection is infeasible. In the traditional approach to deal with this problem, the receiver uses only the reference signals for phase shift correction and combines soft symbol decisions obtained independently fromeach copy. In this paper, we develop the corresponding theoretical maximum likelihood (ML) detection problem, its solution, and a computationally efficient algorithm that is an approximate ML solution. We present several simulation experiments and results. The experiments include phase drifts allowable in practical systems and randomized variations in the locations of reference symbols. Results indicate power savings of up to 2.0 decibel (dB) over the traditional method for different system configurations. They also show that for short data segments used in systems over severely degraded channels, the results from the theoretical solution and our algorithm are virtually indistinguishable. The main impact of this research is that survivable and protected communication systems can take advantage of our new signal combining algorithm that offers considerable power savings.