We consider an underlay cognitive radio system consisting of a secondary transmitter (ST), K secondary receivers (SRs) and a primary receiver (PR). Data transmission is granted to the ST under the constraint that the generated peak interference to the PR is below a predetermined interference temperature level. Assuming that the ST can adaptively adjust its transmit power according to the fading state of the channel, our objective is to analyze the link reliability in terms of the symbol error rate (SER). In particular, we derive the exact SER expressions under the assumption that opportunistic scheduling is applied at the ST, where the SR with the highest channel gain is exclusively selected for data transmission. Numerical simulations are performed to verify the obtained SER expressions. Our results show that there exists an error floor region due to the interference constraint, i.e., the SER approaches to a certain constant as the allowed peak transmit power threshold increases. Moreover, we show that opportunistic scheduling can improve the SER performance under a fixed interference constraint.

We consider the overlay cognitive radio channel where the cognitive user is admitted to transmit simultaneously with the primary user provided that the instantaneous rate of primary link is not degraded. Assuming causal knowledge of the primary user's message at the cognitive transmitter (CT), we analyze the transmission rates of the cognitive user in the single-input multiple-output (SIMO) and the multiple-input single-output (MISO) configurations. In particular, the CT uses a part of its transmit power to assist the primary user in delivering the primary user's message and the other part of its power is used to deliver its own message. The transmit power and the beamformers are designed to maximize the rate of the cognitive user while fixing the the rate of the primary user. Our simulation results show that the proposed scheme yields improved system performance compared to the interweave scheme where any interference to the primary user is prohibited.

We study the fundamental capacity limits of the underlay cognitive broadcast (BC) channel under average transmit power and average interference power constraints. In a fading environment, the sum capacity is found from a constrained water-filling solution given in. This solution is investigated for different average transmit power budgets and average interference thresholds, and its specific regions are characterized. Further, capacity expressions are derived for the Rayleigh fading case. In a system with K users and Rayleigh fading channels, it is shown that the capacity scales like log(log(K)) for large K. This result indicates that the same multiuser diversity gain can be achieved in a cognitive BC system as in the conventional BC system without spectrum sharing.

In this paper, the downlink opportunistic scheduling approach is studied in a multiuser environment with single-antenna transmitter and users. Exact bit error rate (BER) expressions are derived under the assumptions of full and quantized channel state information (CSI) at the transmitter. These expressions are then used to optimize the channel state feedback parameters. Moreover, asymptotic BERs are investigated in the limiting cases of a high signal-to-noise ratio (SNR) and a large number of users K. It is shown that both under the full and quantized CSI assumptions, the achieved BER is proportional to SNR-Kand K-SNRin these two asymptotic cases, respectively. This means that the diversity order is equal to K, whereas the multiuser diversity gain is equal to SNR. In the case when the CSI feedback is quantized, the impact of feedback errors on the achieved BER is studied. It is shown that the opportunistic scheduling can greatly improve the BER performance even if the feedback is quite low-rate and erroneous.

The performance of multiple-antenna communication systems is known to critically depend on the amount of channel state information (CSI) available at the transmitter. In the low-rate CSI feedback case, an important problem is what kind of information should be submitted to the transmitter in each feedback cycle and what is the optimal transmission strategy in this case. In this paper, we address this problem in the multiple-input single-output (MISO) case by analytically comparing the bit error rate (BER) performance of different low-rate feedback based transmitter strategies involving various combinations of transmit antenna selection, Alamouti's spacetime coding, and adaptive power allocation.