Carrier Recovery and Symbol Synchronization in signal demodulation

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 identical to those of  o f  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 are fc and  φc ,and the frequency and phase of a LO at the

receiver are f 0 and φ0. Then after the received signal mixed with the th e LO, CARRIER FREQUENCY OFFSET f= fc- f 0 CARRIER PHASE OFFSET φ= φc- φ0

Symbol synchronization Required in every digital communication system which transmits information synchronously. 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 synchr synchronizatio onization(CONT n(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 generating the reference signal for the correlator.

is used in

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 Figure 1(Binary PSK Receiver)

M-ary psk receiver  Two correlators (matched filter) are required requ ired to correlate the received signal with the two quadrature carrier signal and carrier phase estimation. o

o

,where

is the

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

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 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. 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.

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 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. 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.

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 Figure 4(QAM RECEIVER)

Thank you