Spectrum management in cognitive radio ad hoc networks

Abstract

urrent wireless networks are based on a fixed spectrum assignment policy that is regulated by governmental agencies. Although spectrum is licensed on a long-term basis over vast geographical regions, recent research has shown that significant portions of the assigned spectrum are utilized, leading to waste of valuable frequency resources [1]. To address this critical problem, the FCC recently approved the use of unlicensed devices in licensed bands. Toward this end, cognitive radio (CR) technology is envisaged that enables the identification and use of vacant spectrum, known as spectrum hole or white space [1]. In this article we focus on the challenges faced in CR ad hoc networks (CRAHNs), which do not have infrastructure support and must rely on local coordination for different CR functionalities.Since most of the spectrum is already assigned, a key challenge is to share the licensed spectrum without interfering with the transmission of other licensed users (also known as primary users or PUs). If this band is found to be occupied by a licensed user, the CR user moves to another spectrum hole to avoid interference. In CRAHNs the distributed multihop architecture, dynamic network topology, diverse quality of service (QoS) requirements, and time and location varying spectrum availability are some of the key factors that must be considered in network design. These challenges necessitate novel design techniques that simultaneously address a wide range of communication problems spanning several layers of the protocol stack.In CRAHNs CR users are mobile and can communicate with each other in a multihop manner on both licensed and unlicensed spectrum bands, as shown in Fig. 1a. Furthermore, due to the lack of central network entities, CRAHNs necessitate each CR user having all the spectrum related CR capabilities, and determining its actions based on local observation, leading to distributed operation [2]. In order to adapt to the dynamic spectrum environment, the CRAHN requires spectrum-aware operations, which form a cognitive cycle [1]. As shown in Fig. 1b, the steps of the cognitive cycle consist of four spectrum management functions: spectrum sensing, spectrum decision, spectrum sharing, and spectrum mobility. To implement CRAHNs, each function needs to be incorporated into the classical layering protocols, as shown in Fig. 2. The following are the main features of spectrum management functions:• Spectrum sensing: A CR user should monitor the available spectrum bands, capture their information, and then detect spectrum holes. Spectrum sensing is a basic functionality in CR networks, and hence closely related to other spectrum management functions as well as layering protocols to provide information on spectrum availability. • Spectrum decision: Once the available spectra are identified, it is essential that CR users select the best available band according to their QoS requirements [2]. Especially in CRAHNs, spectrum decision involves jointly undertaking spectrum selection and route formation.• Spectrum sharing: ...

A middleware framework for communication in cognitive radio networks

“…These are: Spectrum Sensing, Spectrum Modeling, Spectrum Sharing and Spectrum Mobility [1], [2], [5]. Basically, the management solution used in our system is an intelligent wireless communication system, defined to represent a set of algorithms for the management of CRN operations like:…”

“…Naturally, the diversity of network configurations and large variations in the frequency bands makes finding an appropriate e2e path between two CR users extremely challenging [7]. To achieve this, we have to consider three CRN routing categories [1], [2], [7]: static routing with variations in the order of hours or days, dynamic routing with temporal framings in the order of hours down to minutes and opportunistic routing (or highly dynamic) with very high variations in the order of minutes down to seconds. In short, we need to use a combined routing solution where the Decision Maker overlay uses a static routing model to locally compile the e2e path.…”

“…Taking into consideration the dynamic properties of CRNs an out-of-band frequency in the unlicensed spectrum, the socalled Common-Control Channel (CCC), can be used covering long distances though operating at low rates [1], [7]. Replacing the need for large-scale broadcasts over multiple channels, the CCC must be available to all nodes in the network at all times.…”

“…Hence, finding an appropriate path between a particular source node and a particular destination node becomes extremely complicated. We can partition the CRN routing in three categories by considering the activity and holding times of licensed users as a reference framing [1], [2], [7]:…”

“…Temporarily unused portions available in an n-dimensional space can be identified and used for communication and consequently exploiting under-utilized spectrum portions (socalled "spectrum holes"). Examples of such dimensions are frequency, power, time, geographical space, code and angle [1]- [3].…”