till now i am able to find bessel function in FM only. your answer is increasing my curiosity. ie HOW it is spreading its tentacles to such extent , for QAM, PSK, DQPSK..?
So.explain in detail
bessel function can be found in any mathematical text eg.krizic.this is a generalized orthogonal function .this is used in fm because there is no direct expansion in trigo. identity which is used to expand cos of sine or sine of cos. thats why it is useful.