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Wednesday, February 23, 2011

OFDM-Based Cognitive Radio

If we could define COGNITIVE RADIO as an intelligent wireless system that is aware of its surrounding environment through sensing and measurements; a system that uses its gained experience to plan future actions and adapt to improve the overall communication quality and meet user needs. Then One main aspect of cognitive radio is its ability to exploit unused spectrum to provide new ways of communication.

Hence, cognitive radio should have the ability to sense and be aware of its operational environment, and dynamically adjust its radio operating parameters accordingly. For cognitive radio to achieve this objective, the Physical Layer (PHY) needs to be highly flexible and adaptable. A special case of multicarrier transmission known as OFDM is one of the most widely used technologies in current wireless communications systems and it has the potential of fulfilling the aforementioned requirements of cognitive radios inherently or with minor changes. By dividing the spectrum into sub-bands that are modulated with orthogonal subcarriers

OFDM removes the need for equalizers and thus reduces the complexity of the receiver. Because of its attractive features, OFDM has been successfully used in numerous wireless technologies including Wireless Local Area Network (WLAN), Wireless Metropolitan Area Network (WMAN), and the European Digital Video Broadcasting (DVB). It is believed that OFDM will also play an important role in realizing cognitive radio concept by providing a proven, scalable, and adaptive technology for the air interface.

In this article discuss the cognitive OFDM Conceptual model. Below is an illustration of this model;

The cognitive engine is responsible for making the intelligent decisions and configuring the radio and PHY parameters. The spectral opportunities are identified by the decision unit based on the information from policy engine as well as local and network spectrum sensing data.

The policy engine provides information to the cognitive engine concerning the current policies to be considered depending on the system location. This will ensure that the cognitive radio will not use illegal waveforms or breach any policies. On the other hand, the local spectrum sensing unit process the spectrum information and identify licensed users accessing the spectrum, their signal specifications such as the their bandwidth and power level, and detect spectrum opportunities that can be exploited by cognitive radio. Once the required information is available, the decision unit can make a conclusion on the best course of action for the system. The decision includes choosing the appropriate channel coding, modulation, operation frequencies, and bandwidth. At this stage, OFDM technology gets the upper hand over other similar transmission technologies with its adaptive features and great flexibility. By only changing the configuration parameters of OFDM and radio, the cognitive system can communicate with various radio access technologies in the environment, or it can optimize the transmission depending on the environmental characteristics. The radio circuit is divided into a digital part (digital IF, ADC, and DAC) and an analog part (software tunable analog radio). Both parts are reconfigurable by the cognitive engine to increase the flexibility of the system. This includes controlling the operating frequency, bandwidth, filters, and mixers. Even antenna parameters (e.g. number of antennas, beam forming) can be configured to improve the system performance.

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