This presentation is helpful for digital communication subject. This ppt is in a very simple manner.

1. Seminar Topic
on
Symbol Timing Estimation
Presented to: Presented By:
Dr. Garima Saini Sachin Maithani
ME( Regular )

2. Table of Content
• 1.Symbol Timing Estimation
• 2.Symbol Synchronization
• 3.Advantage and Disadvantage
• 4.Self synchronization
• 5. Maximum likelihood Timing Estimation
• 6.References

3. SYMBOL TIMING ESTIMATION
• In a digital communication system, the o/p of the demodulator must be sampled periodically
at the symbol rate at the precise sampling time instants
tm = mT +τ, where T: symbol interval
τ: time delay, which accounts for the propagation time of the signal from the transmitter to the
receiver.
To perform this periodic sampling, we require a clock signal at the receiver.
The process of extracting such a clock signal at the receiver is usually called symbol
synchronization or timing recovery.

4. SYMBOL TIMING ESTIMATION
• Two important considerations are there as far as receiver is concerned
1.The receiver must know the freq. at which the o/p of the matched filter are sampled.
2.Where to take the samples within each symbol interval.

5. SYMBOL TIMING ESTIMATION
• Symbol sync. Can be achieved in many ways as follows.
• In some communication systems, the transmitter and receiver clocks are synchronized to
a master clock, which provides a very precise timing signal.
• In this case, the receiver must estimate and compensate for the relative time delay
between the transmitted and received signals.
• For e.g. Radio communication system in VLF band (below 30 kHz) where precise clock
signals are transmitted from a master radio station.

6. SYMBOL SYNCHRONIZATION
• In another method we simultaneously transmit the clock frequency 1/T or a multiple of
1/T along with the information signal.
• The receiver may simply employ a narrowband filter tuned to the transmitted clock
frequency, for extracting the clock signal for sampling.

7. SYMBOL SYNCHRONIZATION
• This method is frequently used in telephone transmission systems that
employ large bandwidths to transmit the signals of many users.
• In such a case, the transmission of a clock signal is shared in the
demodulation of the signals among the many users.
• Through this shared use of the clock signal, the penalty in the transmitter
power and in bandwidth allocation is reduced proportionally by the number
of users.

8. Advantages and Disadvantages
1.Advantages:
• Simple to implement.
2. Disadvantages:
• The transmitter must allocate some of its available power to the
transmission of the clock signal.
• Some small fraction of the available channel bandwidth must be allocated
for the transmission of the clock signal.

9. Self synchronization
• In another method clock signal can also be obtained from the received data signal.
This method is called self synchronization.
Self synchronization is classified into two categories namely-:
1.Decision Directed Timing Estimation.
Also known as Maximum Likelihood Timing Estimation.
2.Non-Decision Directed Timing Estimation.

10. Maximum likelihood Timing Estimation
• Our aim is to obtain the ML estimate of time delay(τ).
• Let us start with the baseband PAM waveform, it is represented as
……………….1
where
……
…………….2

11. Maximum likelihood Timing Estimation
• In Decision-directed timing estimators the information symbols from the output of the
demodulator are treated as the known transmitted sequence.
• In this case, the log-likelihood function has the form as
Where
…………………..3
is the coefficient of Log -Likelihood Function.

12. Maximum likelihood Timing Estimation
• By putting the value of eq. (2) into eq. (3) we get
• …………….4
• where
Where Eq.(5) represents the output of Matched Filter.
= ………………5

13. Maximum likelihood Timing Estimation
From calculus we know that for to be the ML estimate of τ:
………………6
So if the first derivative of Log-Likelihood Function w.r.t. time delay is equal to zero then
ML of time delay is achieved.

14. Maximum likelihood Timing Estimation
• Implementation of Eq.(6) is given by fig below.
r(t)
Matched
Filter g(-t)
Sampler
VCC
Sampling
&
Threshold
Decision

15. Maximum likelihood Timing Estimation
• We should observed that the summation in the loop serves as the loop filter whose
bandwidth is controlled by the length of the sliding window in the summation.
• The output of the loop filter drives the voltage-controlled clock (VCC), or voltage-controlled
oscillator, which controls the sampling times for the input of the loop.
• Since the detected information sequence {In} is used in the estimation of τ, the estimate is
decision-directed.

16. References
• John G. Proakis , Masoud Salehi, Digital Communications.
• www.google.com