Optimum Reception of Digital Data Signals in the Presence of Timing-Phase Hits

Optimum Reception of Digital Data Signals in the Presence of Timing-Phase Hits By D. D. FALCONER and R. D. GITLIN (Manuscript received March 29, 1978) Protection switching of digital radio channels results in a timingphase discontinuity and occasional long error bursts in the demodulated data signal. Using an idealized mathematical model, we derive maximum likelihood receivers which rapidly track such delay hits, whether or not a timing-pilot tone is used. When the receiver is at a different physical location from the switch, the tracking algorithm must also sense the occurrence of a switch. A dual-mode, data-directed structure is revealed as being optimum; a narrowband tracking loop is used for steady-state operation, while a wideband tracking loop provides rapid recovery from the timing transient. An error-sensing nonlinearity, which incorporates hysteresis, inhibits erroneous noiseinduced mode transitions. Oversampling of the demodulated data signal rapidly establishes a coarsely quantized, optimum sampling phase and permits the dual-mode tracking loop to operate in a datadirected manner. Data-directed operation permits greater loop bandwidths, since the data energy is not perceived as noise. Simulation of a digital data transmission system employing a dual-mode, datadirected, and coarse-quantized timing loop has demonstrated dramatic reduction in the length of error bursts following a protection switch. For example, at the data rate of 1.544 Mb/s, a conventional phaselocked loop with a 100-Hz bandwidth, when displaced a half-symbol interval by a delay hit, would typically sustain an error burst 15,000 bits in duration.