Dear Sayhia Tidjani, i think the article " Random, Persistent, and Adaptive Spectrum Sensing Strategies for Multiband Spectrum Sensing in Cognitive Radio Networks With Secondary User Hardware Limitation" and survey paper "Cooperative spectrum sensing in cognitive radio networks: A survey" will give you best idea to choose your thesis tittle.

you can also read the following document "http://www.hiit.fi/u/bayhan/cn_s2013/CN-S2013-SpectrumSensing.pdf" presented by Suzan Bayhan.

Dear Sayhia,

The most efficient algorithm can be one only. Without comparing on data, it is hard to say about any particular algorithm to be efficient.

you need to differentiate between analytical algorithms such as FFT and Wavelet and the global optimization methods such as genetic algorithm. Generally, the optimization methods are much better than analytical methods for finding the optimized solution.

Thank you all for the answers.