In an optical wireless scattering communication network, the receiver needs to periodically listen to the channel to detect whether the channel is occupied by other transceiver pairs. It falls into the spectrum sensing category of the cognitive radio. In this work, we adopt sequential detection for the spectrum sensing, for both a photon-counting receiver and a photomultiplier tube (PMT) receiver. For the PMT receiver, to reduce the computational complexity, we propose a one-term approximation for the log-likelihood ratio (LLR) computation. We also extend the sequential detection to cooperative sensing with multiple photon-counting and PMT receivers. From the simulations, it is seen that the one-term approximation can show a comparable number of samples as that for the exact LLR computation to achieve certain miss detection and false alarm probabilities, significantly fewer than those required by the Gaussian approximation for a low PMT amplification factor. The numbers of samples for both the Gaussian and one-term approximations are close to those for the exact LLR computation for a large PMT amplification factor.
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