Research On OFDM-based Cognitive Radios

With the great improvements of the wireless communication techniques, much higher data transmitting rates are needed by peoples. Based on Shannon's Information Theory, the resources of the wireless communication are much more required correspondingly. Currently, The scarcity of frequency resources has become one of the biggest obstacles in wireless communication. On the other hand, large amounts of the wireless resources that have been regulated by many wireless Standards are partly idle on the dimensions of time and frequency. In the year 1999, Dr. Joe Mitola proposed the concept of cognitive radio. Connecting the cognitive science with the software defined radio, this type of agile radio could detect the idle resources in the dimensions of time, frequency or space, and reuse them in high efficiency. As a result, cognitive radio techniques have been becoming the new hot points in both the research and the developing fileds in wireless communication.At present, multicarrier modulation techniques ,such as OFDM, have been adopted by many wireless protocols. Non-Contiguous OFDM(NC-OFDM),with the characteristics of flexible frequency selection and adaptive power loading, is more appropriate for cognitive radio systems as the physical layer access technique. Meanwhile, many shortcomings of OFDM, such as high Carrier frequency offset sensitivity and Peak-to-Average Power Ratio (PAPR), are also some obstacles in OFDM based cognitive radios. Besides, the out-of-band radiation derived from the side-lobe leakage is considered as the one of the biggest difficulties in OFDM based cognitive radios.This thesis will talk about the out-of-band radiation problem in OFDM based cognitive radio systems, and try to find out some feasible methods to overcome this shortage. The main work includes:1. Set up the mathematical model of the out-of-band radiation problem in OFDM based cognitive radios, and explain why the larger side-lobes will increase the inference from the secondary users to the primary users.2. Based on the model proposed afore, the out-of-band radiation power will be evaluated by both theory deduction and computer simulation. Meanwhile, the optimal parameter of the SOCW window in the spectrum pooling system will also be worked out.3. Perform the adaptive power control method to find out the maximal channel capacity of the OFDM based cognitive radio system under the interference restriction of the primary users. In this part, based on the traditional water-filling power loading theory. We will propose several optimal and sub-optimal algorithms under the two different interference scenarios in spectrum pooling. Whereafter, we combine the optimal SOCW window and the adaptive power control algorithms and demonstrate that these two methods will restrain effectively the out-of-band interference from the secondary users to the primary users in OFDM based cognitive radio systems.