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The authors assume a linear equidistance antenna array as the receiver for a fixed framelength multipleaccess system, which employs variablegain receiver power and repetition encoding. They propose a robust maximum a posteriori probability (MAP) blind bitrate detector (BBRD). This detector considers the rate detection (RD) as a multihypothesis test and maximises the likelihood functions (LFs) to find the true bitrate, whereas the complex amplitude of the received signal, the noise variance and the direction of arrival are unknown parameters. First, assuming that the location parameter is known and the information sequence are independent and uniformly distributed random variables, the authors propose a hybrid likelihood ratio test (HLRT). The proposed HLRT requires to solve a set of nonlinear equations that have no closedform solution. Thus, an iterative numerical algorithm is proposed. In addition, a quasiHLR detector which has a significantly lower computational complexity is also proposed. In the case of unknown location parameter, the authors develop two quasiHLR methods. They use fast Fourier transform and search to estimate the unknown location. In QHLRTMethodI, the nonlinear equation similar to the one in known location parameter is iteratively solved. In QHLRTMethodII, a low complexity solution is proposed. Simulation examples evaluate and compare the performances of the proposed BBRDs.
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