Today's wireless networks are characterized by a fixed spectrum assignment policy. However, a large portion of the assigned spectrum is used sporadically and geographical variations in the utilization of assigned spectrum ranges from 15% to 85% with a high variance in time. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. This new networking paradigm is referred to as NeXt Generation (xG) networks as well as Dynamic Spectrum Access (DSA) and cognitive radio (CR) networks. Figure 1 shows a general architecture of CR networks.



Figure 1. Cognitive radio network architecture.

The key enabling technology of dynamic spectrum access networks is the cognitive radio. Cognitive radio techniques provide the capability to use or share the spectrum in an opportunistic manner. Dynamic spectrum access techniques allow the cognitive radio to operate in the best available channel. More specifically, the cognitive radio technology will enable the users to (1) determine which portions of the spectrum is available and detect the presence of licensed users when a user operates in a licensed band (spectrum sensing), (2) select the best available channel (spectrum decision), (3) coordinate access to this channel with other users (spectrum sharing), and (4) vacate the channel when a licensed user is detected (spectrum mobility). Once a cognitive radio supports the capability to select the best available channel, the next challenge is to make the network protocols adaptive to the available spectrum. These functionalities of CR networks enable spectrum-aware communication protocols. However, the dynamic use of the spectrum causes adverse effects on the performance of conventional communication protocols, which were developed considering a fixed frequency band for communication. So far, networking in CR networks is an unexplored topic.

In this project, we address the intrinsic challenges for networking in CR based architectures and lay out guidelines for further research in this area. More specifically, we will develop a novel spectrum sensing framework to maximize the transmission efficiency by satifying the interference constaint. Furthermore, a QoS aware spectrum decision framework is proposed to find the best available spectrum band by considering QoS requirements for both real-time and best effort applications. Also we account joint spectrum and power allocation for a distributed inter-cell spectrum sharing. To allow smooth transition into the detected portions of the spectrum, mobility management scheme for spectrum handoff in infrastructure based CR networks is proposed. Also, a new routing paradigm for the CR ad hoc network will be proposed that considers re-routing as well as spectrum handoff for seamless operation with primary users. To exploit the available but non-continuous wireless spectrum for high quality communication, we shall also explore multispectrum transport-layer techniques. We extend this approach for general purpose mesh-based architectures through our COginitive Mesh NETworks (COMNETs) framework. This framework allows identification of the primary user's operational channel and undertakes load sharing between the licensed and unlicensed bands. While mesh routers have higher computational ability the considerations of wireless sensor networks necessitate a different approach. For this, we propose a clustering based technique that allows determination of the primary user's location apart from its frequency. The computation is carried out at the destination after obtaining limited information from the sensor nodes. Finally, a testbed will be developed to comprehensively evaluate the developed protocols on a state-of-the-art CR transceiver and demonstrate the effectiveness of the cross-layer design techniques that are extremely crucial for CR networks.