Study of various out-of-band radiation suppression methods

The radio frequency (RF) spectrum is a vital and limited resource that underpins modern wireless communication systems. Its management, governed by regulatory bodies such as the Federal Communications Commission (FCC) in the United States and the Finnish Communications Regulatory Authority (FICORA) in Finland, allocates specific frequency bands for both licensed and unlicensed users. Due to the increasing demand for wireless communication, the RF spectrum is becoming congested, leading to the underutilization of certain frequency bands. One promising solution is Cognitive Radio (CR), which enables opportunistic spectrum usage by detecting and utilizing idle spectrum while avoiding interference with licensed users. Orthogonal Frequency Division Multiplexing (OFDM) has emerged as a robust technology for CR systems, offering flexibility and adaptability to dynamic spectrum environments. However, OFDM systems face challenges, particularly in managing out-of-band (OOB) emissions. This chapter reviews various methods for suppressing out-of-band radiation in OFDM-based CR systems, focusing on sidelobe suppression techniques that enhance the coexistence of secondary users (SUs) with primary users (PUs). A novel modification to the constellation expansion method is proposed, achieving a 35 dB reduction in OOB radiation. The comparative analysis highlights the trade-offs between computational complexity and suppression efficiency, providing insights for future research on algorithmic optimization aimed at achieving higher spectral efficiency. Furthermore, the benefits and trade-offs of these methods in terms of system complexity, spectral efficiency, and overall performance are examined.