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Management of radio spectrum is of fundamental importance now days for wireless communication systems (WCS). Recent studies have shown that most of the available wireless spectrum has been allocated for different WCS and the assigned spectrum is significantly underutilized. For any new WCS, there is a problem of spectrum availability. For this reason dynamic spectrum access techniques are developed. Cognitive radio gives solution for dynamic access of spectrum. Cognitive radio aimed to improve spectrum utilization in which primary (licensed) and secondary (un-licensed) users co-exist simultaneously. Secondary users are allowed to opportunistically access the spectrum not used by primary users. Whenever primary users require to access the spectrum, secondary user leaves that spectrum and gets some other vacant spectrum. This allows both primary and secondary users to communicate seamlessly. Moreover cognitive radio has many smart features like quickly sensing the presence of primary users and utilizing several bands efficiently.
Unlicensed spectrum access is openly available but it has strict laws and regulations for utilizing the spectrum. The access to unlicensed spectrum is open but its utilization is strictly regulated. An unlimited number of users are sharing the same unlicensed spectrum. Spectrum usage is allowed for all devices that satisfy certain technical rules or standards in order to mitigate potential interference.
Of course, opening such a dramatic amount of spectrum is a very significant challenge. In most developed countries, the bulk of the available spectrum is already assigned. In addition, antenna size limitations and electromagnetic wave propagation characteristics limit useful spectrum for cellular handsets to the range of a few hundred megahertz to roughly 3 GHz. However, the FCCâ€™s Spectrum Policy Task Force Report identified that most spectrum goes unused majority of the time. Consequently, spectrum scarcity is mostly driven by archaic systems for spectrum allocation but not by a fundamental lack of spectrum.
Recent advances have enabled wireless handsets with data download capability of 110Mbps. In the next few years, this will be extended to roughly 100Mbps and then on to over 1Gbps in the next decade. This ability to handle data at very high speed will enable consumers to easily handle high quality audio, video, and high resolution images. However, it is often overlooked that as high performance wireless data services are widely deployed, lack of additional spectrum will become a serious limitation.
The cellular market now has new trends towards high data rate services (3G, 4G) like video conferencing, live audio and video streaming, Network gaming, etc. Now to provide different types of several data services, large quantity of wireless spectrum is required. New communication technologies are required to provide these several data services using the available scarce spectrum. Cognitive Radio is one of the main emerging technologies these days for fulfilling the new trends of 3G and 4G systems.
Current usable radio spectrum has almost been allocated to various spectrum- based services, which hinders the further advance and innovation of wireless communication. However, recent reports indicate unbalanced use in different spectrum bands: with a small portion of spectrum (e.g., cellular band, unlicensed band) increasingly crowded, most of the rest allocated spectrum is underutilized.
The demands of wireless services and applications are ever increasing. The existing regulatory system for license of wireless services is unable to meet the new requirements of the era and it has also created an artificial scarcity of spectrum. This resulted in increase of license fee for spectrum and many small and medium wireless service providers are unable to enter the market. Recent studies by FCC has shown that 70% of the assigned spectrum is underutilized. This huge difference of spectrum allocation and utilization provides the motivation to develop cognitive radio which can opportunistically use the spectrum and provides efficient utilization of spectrum.
Efficient spectrum management and utilization.
Cater Variant behavior of primary user (channel usage pattern).
Channel access time.
Resolve Channel contention.
Use the cognitive radio concept for best utilization of unused spectrum while avoiding collision among secondary user (unlicensed user) and between secondary user (unlicensed user) and primary user (licensed user).
How the secondary users decide when and which channel they should use to transmit/receive secondary user packets without affecting the communication among the primary users.
Cater the Synchronization between the secondary sender and receiver due to the difference of the channel availability between them.
Traditional hidden terminal problem.
Multichannel hidden terminal problem.
Design a MAC layer protocol for efficient spectrum utilization catering all these above mentioned problems, and simulate it in the OPNET v14.5 MODELER.
Thesis is organized into five chapters. Thesis organization is illustrated in the figure 1.1. Chapter 2 discusses the wireless spectrum and its assignment policies. Chapter 3 describes the cognitive radio and its physical and medium access layer mechanism. Chapter 4 gives the introduction of the OPNET and its module used. Chapter 5 describes the methodology and implementation of our purposed MAC protocol. Chapter 6 describes the results and conclusion we obtain.
Figure 1.1: Organization of thesis
To accommodate new WCS system or secondary users within available spectrum band, efficient utilization of Radio spectrum is very important. Since most of the spectrum has been allocated, this increases the demand to deploy dynamic spectrum access techniques. Cognitive Radio dynamically perform spectrum analysis and based on this analysis makes decisions for communication of both primary and secondary users simultaneously and seamlessly which ultimately increases the spectrum utilization. Cognitive radio offers a possible solution based on a more sophisticated system for allocating spectrum that can dramatically increase the amount of spectrum available to network operators and individual users. Our aim is to design an efficient MAC protocol that increases the wireless spectrum utilization.