Cognitive Radio Transmissions Exploiting Multi-User Diversity under Channel and Sensing Uncertainty

In this paper, a cognitive multiple access channel (MAC) in which the secondary transmitters opportunistically communicate with a secondary receiver (i.e., a secondary base station) is considered. It is assumed that the base station first determines the channel availability through channel sensing and then broadcasts a pilot symbol with energy depending on the channel sensing decision in order to facilitate the linear minimum mean-square estimation (LMMSE) of the channel fading coefficient between itself and each secondary user. The user with the largest estimated channel gain is selected opportunistically for data transmission during each frame. In this setting, achievable rates and energy efficiency are studied by exploiting the multi-user diversity in the presence of channel sensing and channel estimation errors. The interactions between number of users, channel sensing parameters and performance (e.g., detection and false-alarm probabilities), achievable rates, and energy efficiency are analyzed. Performance gains due to collaborative sensing and multi-user diversity are investigated.