Group-orthogonal MAC for cooperative spectrum sensing in cognitive radios

In cognitive radio systems, secondary users (SUs) detect and utilize spectrum holes that are not being used by the licensed primary user (PU). The detection performance can be improved if SUs cooperate among each other or via a fusion center (FC). In this work, we propose group-orthogonal multiple-access channel (MAC) in which several SUs belonging to the same group send their test statistics through an orthogonal channel. The fusion center (FC) uses a simple reconstruction method by linearly combining the outputs from multiple orthogonal groups. We study the optimal weights at the FC and the optimal number of groups (D) as a function of the system parameters such as channel gains, target false alarm probability, and noise statistics. When D = M (number of secondary users), which is also known as orthogonal signaling, the noise and signal contributions from each user are optimally combined at the fusion center. When D= 1, which is also known as non-orthogonal signaling, the signals are mixed in the air; hence no need for combining at the FC. In this case, at the FC, noise contribution is reduced. However, the performance gains of optimal weighting is lost. The group-orthogonal MAC model allows us to quantify this trade-off. For equal channel gain case, we derive the optimal number of groups and weighting coefficients. For non-equal channel gain case, we provide optimal solutions for special cases with limiting noise and signal powers in addition to the simulation results for general cases.