Investigation On Optimal Link Control And Cooperative Relay Scheme In Wireless Cognitive Radio Networks

With the development of cognitive radio technology, the current state of spectrum scarcity and spectrum under-utilization can be greatly alleviated, by allowing the unlicensed users to opportunistically access to the licensed spectrum without interrupting the transmissions of the licensed users.Thanks to the cognitive radio technology, dynamic spectrum access is proposed to improve the spectrum utilization, in which optimal link control of the unlicensed cognitive user is extremely important. In addition, cognitive radio based spectrum sharing is able to re-alize heterogeneous network coexisting, while incorporating with cooperative relay may further enhance the system performance. In this dissertation, the optimal link control and cooperative relay mechanism in wireless cognitive networks are intensively investigated, and the contents of this work are listed as follows:For delay insensitive service, the optimal link control mainly focuses on how to reduce the interference introduced to the primary link and how to combat the fading channel of the secondary link. In the context of cognitive OFDM system, we investigate the bit and power allocation problem while guaranteeing the primary link against information rate loss. We find the conversion relationship between the unmatched transmit power and bit rate, and then propose a greedy bit allocation algorithm and prove its optimality. In the context of the cognitive radio network where both the primary and secondary users can operate with Adaptive Modulation and Coding (AMC) technology, we aim to maximize the average spectral efficiency of secondary link under the constraint of average spectrum efficiency and packet error rate of the primary link. For different cases of availability of primary link information to the secondary user, i.e.,1)it has no any primary link information,2) it knows the transmission mode of primary transmitter, and 3) it knows the SINR at primary receiver, three AMC-based adaptive link control policies for the secondary user are proposed.For delay sensitive service, the optimal link control in wireless cognitive networks becomes even more challenging.Considering an ON/OFF primary channel, the arrival rate of secondary user with delay related QoS requirement, is limited by both the effective capacity provided by primary channel,and the packet collision probability constraint of primary link. Depending on whether the sensing parameters could be adjusted or not, two spectrum utilization approaches, i.e.,η-probability random access and sensing parameter adjustment are proposed to fully utilize the transmission opportunities.Considering the Rayleigh fading channel,we investigate the optimal power control to improve the effective capacity of the secondary user. When the primary user adopts fixed transmit power, the restricted transmission outage probability is relaxed to an average interference constraint for the secondary user. On the contrary, if the primary user adaptively adjusts its power according to the water-filling policy, then the transmission outage probability is relaxed to an instantaneous interference constraint. Finally, we find the optimal power control policies for both cases based on convex optimization, and investigate the impact of QoS exponent on them.Finally, a cognitive relay assisted spectrum sharing architecture is proposed to enable the legacy non-cognitive secondary system to coexist with the primary system. Based on that, an opportunistic cognitive relaying (OCR) scheme is specially devised, in which the cognitive re-lay opportunistically switches among three different working modes, i.e.,relay for primary link (RPL),relay for secondary link (RSL) or relay for neither of the links (RNL), respectively, based on the channel-dependent observation of both systems. In addition, the transmit power for cog-nitive relay and secondary transmitter in each mode are optimally determined by maximizing the transmission rate of secondary system while keeping or even reducing the outage probability of primary system. In the case of no direct links, the cognitive relay has to simultaneously assisting the transmissions over both primary and secondary links.Two decode-and-forward schemes with different levels of complexity, i.e.,SD-SC (successive decoding then superposition coding) and OIC-DPC(opportunistic interference cancellation then dirty paper coding) are presented respec-tively. In order to maximize the transmission rate of secondary link while providing adequate primary link protection, the transmit power for the secondary transmitter and the decoding order for the relay, as well as the power division for the regenerated signals are optimally determined in both schemes.