Over the past decade, Rapid growth in the field of wireless communication has increased demand for spectrum resources and created a shortage of available radio spectrum. The cognitive radio technology is one of the promising technology to address the scarcity of spectrum resources, from the introduction of cognitive technology there is a lot of research is going on its spectrum sensing techniques, this paper aimed to provide an in-depth review on recent advances in spectrum sensing techniques and thoroughly presents the merit, demerits, and scope of further research of each spectrum sensing techniques. Finally, this review paper is presented to help new researchers and to make them familiar with concepts of spectrum sensing.

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Pilots are inserted with the OFDM symbol in the transmitter in the pilot based channel estimate and it is detected on the receiver. Three approaches are known as random, orthogonal, and Whale Swarm optimization are included in the pilots. The optimized pilot delivers better performance than the orthogonally and randomly positioned pilots do. Random, orthogonal, and whale swarm methods are supposed to utilized to examine the pilot carrier position. It is calculated that three separate algorithms are used: Least Square (LS), Least Square modified (LS-Mod), and MMSE. Several optimization algorithms have been introduced in MIMO-OFDM device channels to increase data quality, reduce BER, MSE, and SER. The performance of both of the above-proposed optimization algorithms is contrasted with the current simple algorithm and have shown improved outcomes in the MIMO-OFDM networks. The proposed optimization algorithms are implemented with the software MATLAB. To optimize the channel, the Meta Heuristic algorithm such as the whale swarm algorithm (WSA) is proposed.

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