رسائل دكتوراة
English
2017
Cooperation and Coordination Strategies in Cognitive Radio Networks
Imen Sahnoun, Ines Kammoun Jemal , Mohamed Siala
المدرسة العليا للمواصلات - تونس-جامعة قرطاج · تونس
Cognitive radio is an exciting emerging technology that has the potential of dealing with the stringent requirement and scarcity of the radio spectrum and offers the ability of radio sensing, self adaptation anddynamic spectrum sharing. In this thesis, we are interested in the case where secondary users are allowed to communicate concurrently with primary users provided that they do not create harmful interference tothe licensed users. More precisely, the secondary transmission depends on a variable cost that reliably quantifies the interference power generated by the secondary user at the primary base stations. Here, weaim to improve the cognitive system performance in terms of throughput. For this aim, we first propose touse an adaptive modulation technique at the unlicensed user in order to maximize its data rate. We study the system behavior as a function of several parameters such as the maximum allowed cost, the number of basestations in the primary network, the secondary user location and its transmission power. Then, we proposeto use cooperative techniques only if the resulting average interference power at the primary receiver isabove a certain threshold. For relaying scenario, we adopt a space-time coding protocol based on Alamouti Space Time (ST) code as well as an Amplify and Forward (AF) protocol. Through simulations, we showthat, these adaptive cooperative protocols have better performance compared with the classical cognitive system. To further enhance the performance of the cognitive radio system, we propose, next, novel energy alloca-tion schemes for the secondary users, considering the cooperative multi-hop transmission. Our optimiztion problem is formulated as a maximization of the instantaneous received signal to noise ratio, under interfer-ence power constraints that are imposed to protect the primary network. In other words, we must maintain the interferences generated by the secondary transmissions to the primary network below an acceptable threshold level. We start by resolving geometrically our optimization problem for two and three hops. A variety of simulation results shows that our proposed energy allocation approach, combined with adaptive modulation, gives a better performance compared to the classical cooperative scheme with uniform energy distribution. Then, we propose an analytical optimal solution to the problem for the 2-hop case. We, also, show that the analytical resolution leads to the same results as the geometrical one. Afterwards, we consider the case of multi-antenna secondary base station (BS-S) that does not have anyprior information about the channel from the primary users. Hence, in order to maintain a limited interfer-ence during the downlink transmissions to the secondary users, the BS-S needs to get a perfect or partial knowledge of the primary users channels. For this aim, we investigate the identification problem of the primary active users and propose two solutions. We start by proposing an efficient and reliable method fora blind active primary users identification, without any energy or radio resource losses, based on randomly rotated constellations for each transmitted symbol at the primary users. Then, we consider the realisticnon-line-of-sight (NLOS) environment, and propose a novel and efficient primary users identification, us-ing compressive sensing (CS). Our focus is on the angular sparsity of the received signal at the BS-S, givenan unknown number of primary user source signals impinging upon the antenna array from different di-rections of arrival (DOA). Given multiple snapshots, multiple measurement vectors (MMV) are available at the secondary base station and considered for primary channel detection over the angular domain using the regularized MMV-FOCal Under determined System Solver (M-FOCUSS) algorithm. Next, we develop novel methods for paths separation and primary channels estimation based on their auto correlation matrix properties. Finally, we focus on the down link transmissions and propose to design a beamforming vectorat the BS-S, based on the estimated channels, that maximizes the desired signal power to each secondary receiver while minimizing the total interference towards all primary receivers.
