Impact of primary users activity on achievable average spectral efficiency of CDMA-based cognitive radio networks

In this paper we consider a multiuser CDMA/CDMA cognitive radio system that involves joint optimization of outer loop power control (OLPC) SNR-target and variable spreading factor of the secondary service, based on the number of active primary and cognitive users (CUs). Optimality is in the sense of maximizing average spectral efficiency (ASE) of the reference CU in the presence of multiple access interference (MAI), whilst maintaining an average transmit power constraint during the time when the licensed spectrum is used and when the secondary frequency band is exploited. The cognitive service employs access-bounded opportunistic spectrum access (AB-OSA) technique to exploit the under-utilized primary bandwidth. In the AB-OSA scheme, CUs´ access to the primary user´s spectrum is limited based on activity of the primary users. Considering frequency-selective fading channels, we optimize ASE of the reference CU where a coherent RAKE receiver is used with maximum ratio combining (MRC). Numerical results confirm that by using our joint optimization scheme in the cognitive radio system, considerable gain is achieved compared to a system that does not exploit optimum OLPC and subsequently the optimal SNR-target.