In a digital communication system, the output of thedemodulator must be sampled periodically at the symbolrate, at the precise sampling time instants, tm = mT + τWhere,T is the symbol intervalτ is a nominal time delay that accounts for thepropagationtime of the signal from the transmitter to the receiver.
To perform periodic sampling we require a clocksignal at the receiver.The process of extracting such a clock signal at thereceiver is called symbol synchronization or timingrecovery.
Timing recovery is the most critical functionsthat is performed at the receiver of asynchronous digital communication system.The receiver must know the frequency (1/ T) atwhich the outputs of the matched filters orcorrelators are sampled,And where the samples to be taken within eachsymbol interval.
The choice of sampling instant within thesymbol interval of duration T is called the timingphase.Symbol synchronization can be accomplished inseveral ways.
In communication systems, the transmitter andreceiver clocks are synchronized to a masterclock, which provides a very precise timing signal.In this case, the receiver must estimate andcompensate for the relative time delay betweenthe transmitted and received signals.
In this method of achieving symbolsynchronization the transmitter simultaneouslytransmit the clock frequency 1/ T or a multiple of1/ T along with the information signal.The receiver may simply employ a narrowbandfilter tuned to the transmitted clock frequency and,thus, extract the clock signal for sampling.
ADVANTAGEThis approach has the advantage of beingsimple to implement.
1. The transmitter must allocate some of its available power to the transmission of the clock signal.2. Some small fraction of the available channel bandwidth must be allocated for the transmission of the clock signal.
This method is used in telephone transmissionsystems that employ large bandwidths totransmit the signals of many users.Here the transmission of a clock signal isshared in the demodulation of the signalsamong many users.