Research On Efficient Cooperative Technologies For Cognitive Relay Networks

Driven by the demand for wireless high-speed service, wireless communication technologies have been changing daily, and a variety of wireless applications and wireless devices are continuously being developed. However, limited available spectrum resources and the reduced reliability of wireless links have become the main factors hindering the further development of wireless communications. In order to overcome these problems, a series of new technologies have been proposed, of which cognitive radio and cooperative relaying are considered the two most representative technologies. Although cooperative diversity can increase the ability to counter channel fading, and cognitive radio can enhance the utilization of the licensed spectrum in wireless communication systems, they are not sufficient to solve the above problems. Therefore, this dissertation considers a combination of cognitive radio and cooperative relay technologies by introducing cooperative relay technology into cognitive radio networks to constitute a cognitive relay network. The proposed cognitive relay network integrates the efficient spectrum utilization rate of cognitive radio technology and the high-diversity gain of cooperative relay technology, thus providing a feasible solution that enables us to achieve the target of simultaneously improving the spectrum utilization and wireless link reliability. This dissertation studies the efficient cooperative technologies that exist for cognitive relay networks, and focuses mainly on how to improve cooperative diversity and efficiency in cognitive relay networks. This study aims to provide theoretical guidance for the design of future cognitive relay networks. It also provides theoretical and algorithmic support for the design of radio-resource management protocols such as relay selection and power allocation. In general, the main contributions of our works can be summarized as follows:1) Research on the energy-efficient technologies of cognitive two-way relaying networksExisting studies into cooperative technologies in cognitive relay networks have focused mainly on capacity and outage probability, and studies on energy efficiency have been relatively few. However, energy efficiency is important for cognitive relay networks in which both the spectrum and power are limited. Thus, we investigate energy-efficient cooperative technology in a cognitive two-way relaying network having a typical decode-and-forward and amplify-and-forward protocol with underlay spectrum sharing. First, for the single-user scenario, we derive the closed-form expression of the optimal power allocation (OPA) using a high signal-to-noise ratio (SNR) approximation scheme. Based on these results, we propose an energy-efficient joint-relay selection and power allocation scheme. Simulation results show that the proposed scheme has an obvious energy-efficiency performance gain when compared to the capacity maximization with traditional one-way relaying or random relay selection schemes. Moreover, for the multi-user scenario, we present a joint bandwidth and power allocation to maximize the energy efficiency of the cognitive relay network. It is shown that the corresponding resource allocation problems are equivalently reformulated as convex optimization problems, and can therefore be solved efficiently. Based on these results, we propose an energy-efficient joint bandwidth and power allocation scheme. Simulation results show that the proposed scheme can further improve the energy efficiency of the cognitive relay network when compared to the capacity maximization or equal bandwidth and equal power allocation schemes.2) Research on the performance of cognitive two-way relaying networksPerformance analysis is a basic issue and is a current topic in research focusing on cognitive radio networks, although physical network-coding cooperative has attracted a lot of attention when applied to cognitive cooperative networks. However, there is still a lack of research on its performance. In this study, we investigate the performance of cognitive two-way relaying networks using physical network coding in an underlay spectrum sharing system with two different cooperative approaches. For the single relay decode-and-forward cooperation, we derive the closed-form expression of the outage probability, as well as the asymptotically tight upper and lower bounds of the average bit-error probability with BPSK modulation. Further, for the multi-relays selection cooperation, we also derive the closed-form expression of the outage probability, which is asymptotically tight in the high-SNR region. The simulation results further validate the theoretical analysis, and confirm that cooperative networks using physical network coding has a better performance with respect to the outage probability compared to the traditional network coding method, especially in conditions where the interference temperature threshold is large.3) Research on outage performance of cognitive cooperative technologies based on enhanced MIMO techniquesConsidering that MIMO techniques have significant diversity gain, cognitive relay networks are presently more likely to incorporate MIMO technology. It should be noted that we can obtain full diversity gain by using the orthogonal space-time coded technique. Therefore, we study the outage performance of cognitive relay networks using the orthogonal space-time code technique with underlay spectrum sharing and the decode-and-forward method. In addition, we study two different operating modes. In the half-duplex relay mode, existing studies for the performance have focused mainly on the Rayleigh fading channel, which is not sufficient to reflect the actual channel fading effect. Therefore, we analyze the outage performance of cognitive relay networks in the Nakagami-m fading channel. We derive the closed-form expression and study the effects of the data transmission channel and interference channel. These results further extend the related results of existing works. In the full-duplex relay mode, we present a performance analysis model that is suitable for cognitive full-duplex relay networks. In this model, we consider the transmission interference, echo interference, interference by the primary user, as well as the interference temperatures. Based on this model, we propose an OPA scheme to minimize the outage probability subject to the interference constraints in the cognitive full-duplex relay network. We also derive the outage probability of cognitive networks in noise-limited and interference-limited environments. Simulation results show that the cognitive full-duplex relay network has better performance in the condition that the interference channel is worse or there are strict interference temperature constraints when compared to cognitive half-duplex relay networks. They also indicate that the proposed OPA scheme achieves a performance improvement in terms of the outage probability when compared to the equal-power allocation scheme.From the dissertation, it can be found that, in cognitive relay network, selecting suitable cooperative strategy and resource allocation (e.g.relay selection, power control and bandwidth allocation) can effectively use the time domain, power domain, space domain and other multi-dimensional characteristics, which can significantly improve the performance and reduce the energy cost of systems.