Rapid development of devices and applications results in dramatic growth of wireless traffic, which leads to increasing demand on wireless spectrum resources. Current spectrum resource allocation policy causes low efficiency in licensed spectrum bands. Cognitive Radio techniques are a promising solution to the problem of spectrum scarcity and low spectrum utilisation. Especially, OFDM based Cognitive Radio has received much research interest due to its flexibility in enabling dynamic resource allocation. Extensive research has shown how to optimise Cognitive Radio networks in many ways, but there has been little consideration of the real-time packet level performance of the network. In such a situation, the Quality of Service metrics of the Secondary Network are difficult to guarantee due to fluctuating resource availability; nevertheless QoS metric evaluation is actually a very important factor for the success of Cognitive Radio. Quality of Experience is also gaining interest due to its focus on the users' perceived quality, and this opens up a new perspective on evaluating and improving wireless networks performance. The main contributions of this thesis include: it focuses on the real-time packet level QoS (packet delay and loss) performance of Cognitive Radio networks, and evaluates the effects on QoS of several typical non-configurable factors including secondary user service types, primary user activity patterns and user distance from base station. Furthermore, the evaluation results are unified and represented using QoE through existing mapping techniques. Based on the QoE evaluation, a novel cross layer RA scheme is proposed to dynamically compensate user experience, and this is shown to significantly improve QoE in scenarios where traditional RA schemes fail to provide good user experience.