SYNCHRONIZATION TECHNIQUES IN SPECTRUM POOLING OFDM

Abstract

With ever increasing devices and applications, and burgeoning user base, wireless communication is creating an endlessly increasing demand for more spectrum access. But since the basic requirement for wireless communications, the amount of available radio frequency spectrum, is fixed, various regulatory authorities around the world are engaged in licensing this natural resource which tends to create a scarcity of usable spectrum. The spectrum utilization surveys show that large part of this licensed spectra remains unused for a large amount of time, indicating that there is no actual shortage of usable free frequency bands. To overcome this issue, Dynamic Spectrum Access (Cognitive Radio) systems have been suggested, which are computationally intelligent and can dynamically adjust their parameters such that the unused frequency bands, in the licensed spectra, can be used by other Non-license holder Cognitive radio permits, in principle, a more efficient use of the radio spectrum. This could lead to a multidimensional reuse of the spectrum in space and time, overcoming spectrum scarcity, which has been an obstacle for broadband wireless communication development. OFDM with its natural ability to allow transmission at required sub-carriers presents itself as the perfect choice for such a system In this dissertation, we consider synchronization problem in a fully centralized DSA architecture based approach called Spectrum Pooling. Synchronization, especially for cognitive radios, though recognized as a challenging problem, is largely an unexplored area. The thesis gives a brief overview of various synchronization schemes available in literature. Further, An OFDM system is presented which adapts itself to the changing sub-carrier occupation of the licensed users. The application and performance of various timing and frequency synchronizatiorillechniques, in such a system, is considered. An adaptive filtering based scheme is alg'o' proposed for suppression of the licensed user interference signal, and its performance evaluated for various synchronization schemes.