Energy-efficient delay-tolerant cognitive radio networks

Undoubtedly the upward growth trend in aggregate mobile Internet IP traffic is expected to continue steadily. In this emerging mobile environment with increased data traffic and always-on applications, the limitations of battery technologies lead to drastically shorten recharging cycles of mobile devices. For mobile applications that can tolerate a moderate delay which account for a high proportion of global mobile traffic, a technique that postpones the data transmission to high-rate hotspots could effectively provide significant energy gains, which can be translated to increased battery lifetime. In this thesis, potential gains are explored resulting from utilization of two important technologies for future and emerging wireless networks, namely Cognitive Radios and Delay Tolerant Networking. Both of them are in essence opportunistic in their operation and so far have been considered in isolation. Considering that an increased number of worldwide countries are permitting operation of cognitive radio systems in the spatially vacant licensed analog TV bands, this would enable new possibilities to provision further capacity increase for wireless broadband and multimedia services. Hierarchical CR networks improve spectrum efficiency by allowing the low-priority SUs to temporarily seek the wireless spectrum that is licensed to different organizations. Once mobile devices are equipped with multiple air-interfaces allowing them to connect to cellular networks, Wi-Fi and White-Fi, they could switch among these networks to seek and use any licensed spectrum bands as long as they avoid interference being caused to TV receivers. When wireless nodes are competing for secondary access to the medium, the estimation of probability of PU arrival rate and service time is important for mobile devices (SU) to effectively occupy the primary spectrum. The mobile nodes firstly contact a trusted database for historical information about PU traffic at a specific location and time duration so as to estimate the probability for the SU connections. Then, regarding the SU traffic, it is shown that it can be modeled as an M/M/K/L queuing system which allows to analyze the capability that the system can serve users simultaneously. As location of mobile users is the key to determine the capacity of accessible wireless service for themselves, stochastic characteristics of user mobility are studied in terms of user velocity, direction changes, and route selection distribution. Moreover, when mobile terminals are moving among different cells supporting different network technologies, the 3 performance of vertical handover and cell residence time in the coverage of Wi-Fi/WhiteFi hotspots would greatly affect the overall efficiency of wireless transmission. In this scenario, if the mobile applications could tolerate some delay, the proposed schemes can significantly avoid the drain of mobile device batteries by making selective use of the nearby high-speed hotspots. Nowadays, with the surge of the diverse and ubiquitous Internet applications, mobile users expect to enjoy wireless Internet connectivity anywhere and at any time. According to the inherent mobility of mobile users, optimal stopping problem is formulated for energy-delay trade-off, which the stopping decision would be made based on channel conditions, delay constraints, and energy cost. In addition, for popular video streaming applications on portable devices that could be watched several times by one user, there are trade-offs between storing video content locally at the DRAM of the device or allowing deleting the content from the local memory and relaying in wireless streaming in near-future requests of the same content. To this end, a scheme has been proposed where the mobility of the user is taken into account together with the probability of a user requesting the same content multiple times so that a decision is taken of whether or not the content should be stored locally. Finally, since the proliferation of always-on Internet applications has put significant strain on the battery capabilities, the problem of prolonging battery lifetime of mobile devices is introduced. Previous research has revealed that the data downloading via wireless radios is a dominant energy consumption factor in mobile devices. To avoid the drain of mobile device batteries, based on the delay tolerance of mobile Internet applications, the proposed strategies are designed in which mobile terminals can intelligently switch among cellular, Wi-Fi and White-Fi interfaces, by considering the energy cost, RF coverage, capabilities, and transmission algorithm. Numerical experiments on various mobility models reveal that the energy cost of wireless transmission closely relates to user locations, mobility pattern, spectrum availabilities as well as applications’ delay tolerance and available wireless access technologies.