Cognitive Radio Networks: Issues and Solutions

Introduction

Traditional wireless networks are running with fixed spectrum assignment policies regulated by government agencies. Spectrum is assigned to service providers on a long term basis for large geographical regions. The spectrum is allowed to be used by licensed users. Federal Communications Commission (FCC) measurements have indicated that 15-85% of the time, many licensed frequency bands remain unused while some other bands are highly over-crowded. These overcrowded bands face the issue of spectrum scarcity. In order to better utilize the licensed spectrum, FCC has launched a secondary markets initiative, whose goal is to remove regulatory barriers and facilitate the development of secondary markets in spectrum usage rights among the wireless radio services. The inefficient usage of the existing spectrum can be improved through opportunistic access to the licensed bands without interfering with the primary users. This introduces the concept of Dynamic Spectrum Allocation (DSA), which implicitly requires the use of cognitive radios to improve spectral efficiency.

Cognitive radio is an intelligent radio that is aware of its surrounding environment and dynamically adapts to the transmission or reception parameters to achieve efficient communication without interfering with primary users. Thus, a Cognitive Radio Network (CRN) (Akyildiz, 2006) consists of primary and secondary users. The primary users are the licensed users and hence have exclusive rights to access the radio spectrum, whereas the cognitive users are the unlicensed users that can opportunistically access the free spectrum bands, without causing harmful interference to primary users. This introduces cognition and adaptation to physical layer of the network. These networks work on multi-channel environment with dynamic availability of channels based on primary user activity. These channels can be of different frequencies and/or different bandwidth. The operation of these networks is very different from simple multi-channel environments as different frequencies have different characteristics, like transmit power levels, transmission distance and multipath effects.

The first step to accommodate cognitive radio and dynamic spectrum sharing is to open the TV whitespaces for other users. Best practices for sharing these bands are currently being studied by International Telecommunication Union (ITU) Radio-communication Study Groups 1, 5 and 6. ITU-R Study Group 1, working on Spectrum Management, is expected to provide a Report on best practices in Spectrum management for cognitive radio systems by mid-2014.

CRNs generate new challenges to a variety of stakeholders. For RF equipment vendors, the challenge is to build equipment which can be operated on any spectrum band and capable of doing on the fly changes in the radio transmission parameters. For wireless service providers, the challenge is to use their own resource efficiently and opportunistically use the other bands while maximizing their profit. As other providers can use their band opportunistically, security is also a major concern from user’s perspective. Ensuring the licensing policies which enable the secondary usage of the spectrum is a challenge for regulatory bodies. Licensing policies should provide benefit to licensed users as well as cognitive users.

To deal with the cognition in radio (i.e. physical) layer, upper layers should also be modified to adapt to the reconfiguration done at physical layer. Thus, adaptation and cognition should also be included at upper layers to achieve the benefits of cognitive radio technology. This makes a network as cognitive network (Thomas, 2006). According to (Thomas, 2005) cognitive network is defined as:

As cognition should be incorporated in upper layers to effectively utilize the benefits of cognition at physical layer cognitive networks and CRNs are used interchangeably in literature. In this book chapter also we use these term to refer to network that includes cognition at upper layers also.