School of Electrical and Computer Engineering Georgia Institute of Technology

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Cognitive Radio Ad Hoc Networks

Project Overview

Recent technological advances have resulted in the development of wireless ad hoc networks composed of devices that are self-organizing and can be deployed without infrastructure support. While ad hoc networks may support different wireless standards, the current state-of-the-art has been mostly limited to their operations in the 900 MHz and the 2.4 GHz industrial, scientific and medical (ISM) bands. Recent experiments have revealed that the portions of the spectrum licensed to operators in the television broadcast bands, and other governmental agencies have sparse utilization. This spectrum, if used by the ad hoc networks, could lead to reduced mutual interference, congestion, and energy savings by incurring fewer packet errors. As an example, sensor networks have nearly 90% packet collision rate in the presence of WLAN interferers in the ISM band, which may be avoided by intelligently identifying the vacant spectrum resource. The newly emerging Cognitive radio (CR) technology is envisaged to solve the problems in wireless networks resulting from the limited available spectrum and the inefficiency in the spectrum usage. Specifically in cognitive radio ad hoc networks (CRAHNs), the distributed multi-hop architecture, the dynamic network topology, and the time and location varying spectrum availability are some of the key distinguishing factors. In this project, the current research challenges of the CRAHNs are addressed from the following viewpoints:

• Spectrum Sensing: This is a primary function in the CR network, in which the CR users find available spectrum holes over a wide frequency range for their transmission, and monitor the currently used spectrum band to detect the presence of primary networks to avoid interference.
  
  
• Spectrum Decision: This helps the CR users to choose the best spectrum band among the available options. This decision may rely on past activity of the primary users (PUs) of the band, the channel conditions, among other factors. This brings about the need to establish node-level cooperation in CRAHNs.
  
  
• Spectrum Sharing: Since there may be multiple CR users trying to access the spectrum, the cumulative effect of their transmissions should avoid interference caused to the primary network. This process also incorporates the MAC layer spectrum access mechanisms.
  
  
• Spectrum Mobility: If the spectrum in use is reclaimed by a PU, the communication needs to be continued in another vacant portion of the spectrum. Spectrum mobility gives rise to a new type of handoff in CR networks, the so-called spectrum handoff, in which, the users transfer their connections to an unused spectrum band.
  
  
• Higher Layer Protocols: The higher layer protocols between a given source-destination pair must account for the spectrum sensing and switching functions undertaken locally by a node. The transport layer, in particular, must ensure reliable end-to-end delivery when periodic disruptions occur in the path due to the spectrum related functions.