Summary 154

In Chapter 1, a detailed introduction of CR technology is presented and many definitions for CR are proposed. The industry standards and regularization of CR also form part of this chapter. We have explained how the unoccupied spectrum band could be utilized by cognitive radio and be applied in existing mobile networks, industry, disaster management, and public safety. We continue our discussion and highlight the issues and limitations of CR that are acting as a red flag in this technology. We explain why this technology, even after having been extensively researched, has not yet been widely accepted and implemented. In Chapter 2, we reviewed opportunistic MAC protocols for cognitive radio networks, which integrate the cooperation and scheduling amongst CR nodes.

Our findings in the literature motivated us to design a hybrid CR MAC protocol that accumulates the advantages of existing CR MAC protocols of both GCCC and non-GCCC families and is able to efficiently discover, recover and converge on a common control channel. In Chapter 3, we provide detailed architecture and description of our scheme, named as Dynamic, Decentralized and Hybrid MAC (DDH- MAC) protocol. Our protocol is dynamic because whenever there is a PU claim on a

- 155 - control channel, nodes adapt according to the environment and switch to another backup channel.

Defining cognitive radio, exploring its limitations and providing solutions to the faced challenges, have been our major achievement in this Chapter 1. Our contributions in Chapter 2 are the identification and the classification of existing CR MAC protocols. Our achievement in this chapter was providing a new feature to classify the existing CR MAC protocols and developing a model for classification of different MAC protocols for cognitive radio networks.

Using mathematical modelling, we analyse the performance of our proposed DDH-MAC protocol, we used the Markov chain model in Chapter 4 for two different approaches and determined the desirability of the CR network deploying our protocol. Each SU joins the network by receiving the BF and learning the network status through overhearing the control information over the control information. Our first approach evaluates the system performance with coalition amongst PUs and SUs. This means that both the PU and the SU can simultaneously access the spectrum subject to SUs‟ constraints. We extend our model in the second approach by considering a network scenario where SUs could not transmit until the PU vacated the occupied spectrum. Using these modelling techniques, we developed our protocol which enabled us to envision the performance of our MAC protocol.

Our main contributions in Chapter 5 are two-fold: first, implementation and simulation of our proposed DDH-MAC protocol; and second, combining two modelling techniques (analytical evaluation plus simulation) to observe and verify its correctness. We built our own simulation model for DDH-MAC. In this model, we enabled a SU to create a new CR network (if it was the first SU in the vicinity), and allowed other SUs to join an existing CR network after receiving the BF which is periodically broadcasted in the 2.4GHz spectrum band. Our protocol particularly addressed the unavailability or the saturation problem of the control channel. The suitability and correctness of our framework were further revealed after obtaining the global and local (object) statistics for parameters such as throughput, traffic sent, collision on control channel, queuing delay, signal-to-noise ratio, and network performance with and without ideal channel conditions etc. For our comparative performance evaluation with other CR MAC protocols, the aggregated throughput of DDH-MAC protocol was demonstrated to be better than CREAM-MAC protocol.

- 156 - In Chapter 6, our main contribution was to enhance the performance of the DDH-MAC protocol. We optimized the DDH-MAC performance in two aspects: firstly, we incorporated a multi-fold security in DDH-MAC; and secondly, we made the DDH-MAC protocol energy efficient. We have presented a 4-tier security model and have incorporated security in DDH-MAC in all possible ways, which is the first part of our contributions in this chapter. At first level of security, we avoid using the GCCC and secretly transmit the FCL on a local control channel so that only the SUs deploying DDH-MAC can retrieve the FCL. At second level, we encrypt the BF and then launch it in the GCCC. The third level of security is achieved by inclusion of a time-stamp in outgoing data frames. The dynamicity of the control channel serves as the fourth level of security in DDH-MAC.

Our second contribution in Chapter 6 is making DDH-MAC protocol energy efficient. Note that the vital reason for a high amount of energy consumption is the frame re-transmission after any PU claim. DDH-MAC saves a significant amount of energy by avoiding frame re-transmission. Secondary users always have access to a backup channel, so SUs simply switch to the backup channel and avoid the transmission of those frames for rediscovering and recovering the control channel. Switching to a backup control channel not only saves time but also enables nodes with delay-sensitive data to quickly utilize the unoccupied spectrum band, which is another remedy for achieving a better energy efficiency in DDH-MAC.