Spectrum Sensing of OFDM Waveforms Using Embedded Pilots in the Presence of Impairments

Given the widespread deployment of orthogonal frequency-division multiplexing (OFDM)-based wireless systems, reliable spectrum sensing of OFDM waveforms is of great interest for future cognitive radio systems. Use of the embedded pilots of OFDM signals has been extensively studied in the literature. Such techniques rely on having correlated pilots to improve detection sensitivity. However, the impact of synchronization in accuracy and implementation impairments has not been thoroughly studied in the literature. This paper discusses the pilot-aided cyclostationary detection (PACSD) method as a practical structure to detect OFDM signals under asynchronous conditions. The presented analysis quantifies the performance of a detection method without exhaustive searching, compared with an ideal system. This paper goes on to investigate the impact of radio-frequency (RF) front-end nonidealities on the overall performance of the PACSD. We develop closed-form expressions for performance loss in the presence of in-phase/quadrature (I/Q) imbalance, carrier frequency offset, phase noise, and sampling clock frequency offset (SCFO). These results show that the most detrimental impairment is that of SCFO, where a 20-ppm offset can result in a 9-dB performance loss. We then present a modified detector structure to compensate for this huge loss. Analysis and simulation show that, using the modified technique, we can get within 2 dB of the ideal no-offset case.