A modified gardner detector for multilevel PAM/QAM system

In multilevel pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM) systems, there is a strong demand for a reliable and precise timing phase provided by the timing recovery circuit. However, reducing timing phase jitter just by narrowing the bandwidth of the timing recovery loop will decrease the capability of tracking the time variant channel. To avoid this drawback, the only way to improve pattern-dependent timing jitter is to utilize some signal processing means. In this paper, we propose a modified Gardner detection scheme for symbol timing recovery of multilevel PAM/QAM signals, which offers a theoretically jitter-free phase detector even in the case of non-coherent demodulation. Also in practical applications, such detector with reduced complexity can provide a remarkable amount of jitter reduction. Simulation results show that the proposed modification leads to significant performance improvement for multilevel PAM/QAM signals.