Abstract
With the rapid development of wireless services and applications, the currently radio spectrum is becoming more crowded. How to accommodate more wireless services and applications within the limited radio spectrum becomes a big challenge faced by modern society. Cognitive radio (CR) is proposed as a promising technology to tackle this challenge by introducing secondary users (SUs) to opportunistically or concurrently access the spectrum allocated to primary users (PUs). Currently, there are two prevalent CR models: the spectrum sharing model and the opportunistic spectrum access model. In the spectrum sharing model, the SUs are allowed to coexist with the PUs as long as the interferences from SUs do not degrade the quality of service (QoS) of PUs to an unacceptable level. In the opportunistic spectrum access model, SUs are allowed to access the spectrum only if the PUs are detected to be inactive. These two models known as underlay and overlay schemes, respectively. This thesis studies a number of topics in CR networks under the framework of these two schemes. First, studied cognitive radio transmissions under QoS delay constraints. Initially, we focused on the concept: effective capacity for cognitive radio channels in order to identify the performance in the presence of QoS constraints. Both underlay and overlay schemes are studied taking into consideration the activity of primary users, and assuming the general case of channel fading as Gamma distribution. For this setting, we further proposed a selection criterion by which the cognitive radio network can choose the adequate mode of operation. Then, we studied the cognitive radio transmissions focusing on Rayleigh fading channel and assumed that no prior channel knowledge is available at the transmitter and the receiver. We investigated the performance of pilot-assisted transmission strategies. In particular, we analyzed the channel estimation using minimum mean-square-error (MMSE) estimation, and analyzed efficient resource allocation strategies. In both cases, power allocations and effective capacity optimization were obtained. Effective capacity and interference constraint were analyzed in both single-band and multi-band spectrum sensing settings. Finally, we studied optimal access probabilities for cognitive radio network using Markov model to achieve maximum throughput for both CR schemes.
Highlights
Wireless communication has become an integral part of human life
Cognitive user is assumed to perform sensing in multiple channels and select a single channel for transmission with rate and power that depend on both sensing outcomes and fading distribution
In order to identify the presence of primary user with unknown frequency locations, energy detector serves as the optimal sensing scheme since it only needs to measure the power of the received signal
Summary
As it is well known, that in spectrum sharing systems, the secondary user can adopt two types of access schemes: overlay scheme and underlay scheme. An optimal access probability with different criteria is given in [35] for pure underlay scheme. Based on [35], this chapter proposes a mixed overlay and underlay access scheme. The secondary users access the channel with an optimal probability in an underlay scheme when the spectrum is occupied by the primary user. While, when the spectrum is idle, the secondary users access the channel in an overlay manner. This approach can maximize the total average throughput for the secondary users and limit the interference on the primary user. The optimized access strategy proposed in this chapter shares some similarities to the work done in [35].
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