| Cognitive cooperative relay networks based on cognitive radio and cooperative communication technology can improve the utilization efficiency of spectrum resources as well as the ability to combat channel fading and expand coverage. In this thesis, performance of underlay cognitive relay networks is studied in three specific scenarios. The main contributions of this thesis are as follows:1. Study of cognitive relay networks with imperfect channel state information (CSI):In an underlay spectrum sharing scenario, the cognitive (secondary) system should have knowledge of the CSI of the interference links between secondary users (SU) and primary users (PU), in order to limit the interference induced by SU to PU. However, perfect CSI is actually often unavailable. For this reason, this thesis investigates the effect of imperfect CSI on the performance of both the primary and the secondary system. Exact expressions for the interference probability of PU and the outage probability of the secondary system are derived over Rayleigh and Nakagami-m fading channels, respectively. The results show that the interference probability of the primary user is always0.5under the condition of imperfect CSI when no power control mechanism is adopted, so power control is necessary for SU. Imperfect CSI will cause a great degradation in the outage performance of the secondary system, although it has no impact on the diversity gain. Moreover, the impact of imperfect CSI of the first-hop interference link is much greater than that of the second-hop interference link. In addition, we can compensate for the outage performance loss due to imperfect CSI by increasing the number of relays.2. Study of multi-antenna cognitive relay networks with orthogonal space-time block coding (OSTBC) transmission:OSTBC is a simple scheme to achieve multi-antenna diversity. This thesis analyzes the outage performance of a cognitive relay network with OSTBC transmission, in which the source node and the relay node are equipped with multiple antennas. Both the exact expression of the outage probability and its high-SNR approximation are derived. It is shown that the cognitive relay system can obtain full diversity gain. Like the single-antenna cognitive system, there is an outage saturation phenomenon in the multi-antenna cognitive system due to the peak interference power constraint and the peak transmit power constraint. Besides, the results show that increasing the number of antennas may not improve the outage performance when the peak interference power constraint or the peak transmit power constraint is small, but will significantly improve the outage performance when the peak interference power constraint and the peak transmit power constraint are large.3. Study of cluster-based multi-hop cognitive relay networks:Multi-hop relaying can expand the coverage of wireless systems. In this thesis, performance of a cluster-based multi-hop cognitive relay network with partial decode-and-forward (DF) relay selection scheme is studied over Nakagami-m fading channels. Considering the effect of imperfect CSI of the secondary transmission links, we derive the exact expressions for the outage probability and bit error rate (BER). Asymptotic outage analysis in high SNR regime reveals that the diversity order is determined by the minimum fading severity parameter of all the secondary transmission links, irrespective of the CSI imperfection. It is shown that the fading severity of the secondary transmission links has more influence on the outage performance than that of the interference links. We also conclude that, for secondary nodes whose transmit power is restricted by the interference constraint of the primary user, increasing the number of relaying hops is an effective way to improve their transmission performance. Besides, increasing the number of available relays in each relay cluster can mitigate the performance degradation caused by CSI imperfection. |
PDF Full Text Request