Abstract
In this paper, we investigate the achievable rate and the average sum capacity of an underlay cognitive radio (CR) (also known as a secondary) system. We consider single user (SU) and multiple users (MU) secondary systems that deploy spread spectrum as the signaling technique which utilizes the whole available spectrum. The objective is to maximize the achievable rate and the average sum capacity of the SU and MU secondary systems, respectively, by allocating the optimum power to the secondary transmitter(s) (STs), such that the instantaneous (aggregate) interference power from STs is below a certain threshold at the primary receiver (PR), to guarantee the quality of service (QoS) of the primary system. Numerical and Monte-Carlo Simulation results show that the achievable rate of the SU system and the average sum capacity of the MU system can be enhanced significantly compared to other signaling techniques, because of the fact that in spread spectrum the signal's power is spread over a wider bandwidth, which gives the secondary system more freedom to allocate power to enhance the data rate.
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