Symbol-timing recovery for M-PSK modulation schemes using the signum function

Abstract

Introduces new symbol timing recovery schemes which have performance intermediate between fully decision-directed (DD) and non-data aided (NDA). The authors refer to these schemes as being amplitude-directed (AD) and consider their use with M-PSK type modulations. These new schemes arise quite naturally when one realises that the optimal extraction of timing information from a complex PAM data stream should be considered to be a complex operation and not, as has been done to date, simply a real operation applied independently to the in-phase and quadrature components of the complex baseband signal. The new AD schemes, operating at 1 or 2 samples per symbol, can be used in place of conventional NDA schemes for M-PSK modulations as they are invariant to carrier phase error, have lower self-noise and are not prone to hangup. The paper begins with a general discussion of the structure of asynchronous timing recovery loops, including the various loop components of timing-error detector (TED), loop filter, interpolator and number-controlled oscillator (NCO). The authors then concentrate on the various timing-error detector schemes comparing their open and closed-loop performance and introduce the new AD schemes. Finally, an application of AD timing recovery in a joint frequency and timing recovery loop is suggested.