Asynchronous timing recovery in DSP based PSK modems

Fully digital asynchronous timing recovery has been demonstrated to give excellent results in laboratory experiments on speech rate (2400 and 4800 b/s) modems with both rectangular and RC pulse shaping. F.M. Gardner´s (1986) zero crossing tracker timing error detector has been applied to rectangular and RC-pulse-shaped speech-rate modems with good results on the additive noise channel. The nondata-aided (NDA) form of the zero crossing tracker has been found to be immune to pattern noise for any data phase transition, making it ideal for timing error detection in M-PSK and M-DPSK systems with M>4. In RC-pulse-shaped systems with α down to 60%, intersymbol interference (ISI) induced self noise is sufficiently controlled by loop filtering. For systems with a limited number of samples per symbol, polyphase filter interpolation can be used to improve performance