Spectrum Handoff Management in Cognitive Radio Networks: Solutions, Modeling, and Future Directions

Introduction

The increasing demand for new wireless services and applications, as well as high capacity, the wireless networks become highly heterogeneous, with mobile devices consisting of multiple radio interfaces. In this context, it is essential to have updated information on radio environments to enhance the overall network performance. Most of the radio spectrum is predominantly assigned by the national authority to several service providers, companies or institutes for exclusive use over national areas on a long term basis. The outcomes of several investigations have shown that the lack of spectrum is not an issue, but the fact that radio resources are not being used efficiently. According to the Federal Communication Commission (FCC), up to 85% of the assigned spectrum is underutilized in major urban areas due to static spectrum allocation policy (FCC, 2002). On the other hand, the unlicensed portion of the 2.4 GHz ISM band has become overcrowded. To overcome spectrum scarcity, a promising functionality is required to be built terminals that have cognitive capability to assist with the Dynamic Spectrum Allocation (DSA) for efficient utilization of radio resources by changing the spectrum allocation on demand.

CR network architecture can be classified as ad hoc and infrastructure (Figure 1). In an infrastructure based network, CR node senses spectrum and pass this information to CR base station for spectrum decision to identify the unused spectrum. In Ad-Hoc networks, there is no base station so that intended transmitter and receiver nodes can exchange control information to make spectrum related functionalities such as spectrum decision, spectrum sharing as well as spectrum mobility. In dynamic radio environments, the unused spectrum or spectrum hole is varied in time and location over wide frequency range including both unlicensed and licensed bands. However, the most important challenge is to identify the spectrum holes and utilized them to maximize the throughput without interfering with licensed users. Therefore, CR nodes should immediately vacate the spectrum upon detection of licensed users on operating frequency band, known as spectrum mobility. In CR networks, spectrum mobility introduces a new type of handoff called spectrum handoff. Liodakis et al. (1994) defined handoff as a process of transferring the current communication channel to another associated with a call while maintaining it. In traditional cellular communication systems, handoff is occurred due to degradation of receive signal strength (RSS) when a mobile station (MS) moves from one cell to another cell through the cellular coverage area. Despite of degradation of RSS, spectrum handoff in CR networks can occur due to reappear of licensed users on serving channel leading to significant challenges such as licensed users detection and prediction, protocol adaptability with frequency changes, switching overheads, and most dominantly handoff latency. Research on CR networks mostly focused on spectrum sensing, spectrum decision and spectrum sharing whereas the spectrum handoff is less explored area in CR networks. In (CR) ad-hoc networks spectrum mobility is one of the main performance bottlenecks which include transmission delay, routing discovery as a consequence throughput degradation. This problem is related to multichannel MAC problem in traditional mobile ad hoc networks despite of fixed channel assignment. Thus, a novel spectrum assignment and management scheme is required to improve the handoff latency.

Figure 1.

Cognitive radio network architecture