CARRIER RECOVERY AND SYMBOL SYNCHRONIZATION IN SIGNAL DEMODULATION

Mukul Chauhan

National Institute of Technical Teachers’ Training & Reasearch

Chandigarh

Why we go for Carrier recovery and

synchronization???????????

CARRIER RECOVERY In a digital communication system, information can be sent

over a carrier through changes in its fundamental characteristics.

These characteristics phase, frequency, and amplitude, are modified by the transmitter and must be detected by the receiver.

Thus, it is absolutely essential for a receiver to recover the carrier.

Carrier recovery is required if the signal is detected coherently.

CARRIER RECOVERY(CONTN….)

Receiver is assumed to be able to generate a reference carrier whose frequency and phase are identical to those of the carriers at the transmitter.

When the exploits knowledge of carrier’s phase to detect the signals, the process is called coherent detector.

Carrier signal is generated by the local oscillator. LO at the receiver is generally not synchronous with that of

the transmitter. To match the receiver carrier oscillator frequency and phase to

match that of the transmitter signal, carrier recovery is done.

STEP FOR CARRIER RECOVERY

Two steps are there First step:-Carrier synchronization parameter

are estimated. Carrier synchronization Parameters

-carrier frequency offset

-carrier phase offset Second step:-receiving signal is corrected

according to the estimates made.

carrier frequency offset Due to the frequency instability in oscillator carrier phase offset

Due to the phase instability in oscillator

Assume that the frequency and phase of a carrier are fc

and φc ,and the frequency and phase of a LO at the

receiver are f0 and φ0.

Then after the received signal mixed with the LO,

CARRIER FREQUENCY OFFSET f= fc- f0

CARRIER PHASE OFFSET φ= φc- φ0

SYMBOL SYNCHRONIZATION Required in every digital communication system which

transmits information synchronously. Also known as timing recovery. In an practical system ,not only an isolated signal but a

sequence of symbols has to be transmitted. To perform demodulation ,the receiver has to know the

start and stop times of the individual signal. The output of the demodulator must be sampled

periodically at the symbol rate. So it can defined when to sample and when to reset the

correlators.

SYMBOL SYNCHRONIZATION(CONT..)

We can say the process of selecting instances is called synchronization.

To perform this periodic sampling ,we required a clock signal at the receiver.

Tx and Rx clocks are likely to have mismatch. Clock recovery tries to synchronize the receiver clock

with the symbol-rate transmitter clock to obtain samples at appropriate instances.

BINARY PSK RECEIVER In Binary phase shift keying the transmitted signal is a

sinusoid of fixed amplitude. In has one fixed phase when the data is at one level and when

the data is at other level the phase is different by 180 degree. A block diagram of a binary PSK signal demodulator and

detector shown in figure(1). The carrier phase estimate is used in generating the

reference signal for the correlator. System synchronizer controls the sampler and the output of

the signal pulse generator.

FIGURE 1(BINARY PSK RECEIVER)

M-ARY PSK RECEIVER Two correlators (matched filter) are required to correlate

the received signal with the two quadrature carrier signal

and ,where is the carrier phase estimation.

o Detector is now a phase detector.o Compares the received signal phases with the possible

transmitted signal phases.

FIGURE 2(M-ARY PSK RECEIVER)

M-ARY PAM RECEIVER A single correlator is required. Detector is an amplitude detector, which compares the

received signal amplitude with the possible transmitted signal amplitudes.

An AGC at the front end of the demodulator to eliminate channel gain vibration. which would affect the amplitude detector.

AGC has a relatively long time constant, so that it does not respond to the signal amplitudes vibrations that occur on a symbol-by-symbol basis.

AGC maintains a fixed average (signal plus noise) power at its output.

FIGURE 3(M-ARY PAM RECEIVER)

QAM RECEIVER Really Quadrature Phase Amplitude modulation. AGC is required to maintain a constant average power

signal. Similar to PSK receiver. Detector computes the Euclidean distance between the

received noise-corrupted signal point and the M possible transmitted points, and selects the signal closest to the received point.

Signal of similar energy and highly cross correlated have a small euclidean separation.

FIGURE 4(QAM RECEIVER)

Thank you