Blind Bit-Rate Detectors for variable-gain multiple-access systems in unknown Gaussian channel

We propose a robust maximum a posteriori probability (MAP) blind bit-rate detector (BBRD) for a fixed frame-length multiple access system which employs variable-gain receiver power and repetition encoding. this detector considers the rate detection (RD) as a multihypothesis test and maximizes the likelihood functions (LF)s to find the true bit-rate. Assuming that we have no knowledge about the receiver gain and the noise variance and using the maximum likelihood (ML) estimates of the unknown parameters in the LFs, the resulting GLR test fails, since a possible transmitted sequence of one hypothesis is also a possible transmitted sequence for another hypothesis. To overcome this problem, we propose a hybrid likelihood ratio test (HLRT) by assuming the information sequence as independent uniformly distributed random variables, averaging the LFs over them and then substitution of the ML estimates of the receiver gain and the noise variance in the resulting LFs. In addition, we propose a quasi-HLR detector, that substitutes the ML estimates of the unknown gain and noise variance from the original pdf in the resulting LFs after averaging over the information sequence. Simulation results compare the performances of the new BBRDs.