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EE4723 : Digital Communications II
Week 10:
Synchronization
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 1
(Frequency, Phase, Symbol and Frame Synchronization)Carrier and Phase Recovery
Phase-Locked Loop
Carrier Phase Recovery
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 2
Carrier Phase Recovery
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 3
Carrier Phase Synchronization Techniques Reference methods
out of band pilot tone transparent tone-in-band (TTIB)
Extraction methods feed-forward (with or without timing synchronization)
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 4
feed-forward (with or without timing synchronization) data aided and non-data aided Phase-Locked Loop
feed-back (with or without timing synchronization) data aided and non-data aided
Pilot Tone: Transmitter
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 5
Pilot Tone: Detector
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 6
TTIB: Transmitter
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TTIB: Detector
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 8
Extraction Methods
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 9
Extraction Method: Feed-Forward Phase Estimator
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 10
Extraction Method: Feedback Phase Estimator
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 11
Decision-Directed Feedback Estimator
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 12
Extraction Method: Removing Modulation
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 13
Carrier Recovery Carrier Recovery is the process of extracting a phase-coherent
reference carrier form the received signal .This is sometimescalled phase referencing
In phase modulation techniques, the binary data is encoded asa precise phase of the transmitted carrier (referred to asabsolute phase encoding)absolute phase encoding)
Depending upon the encoding method ,the angular separationbetween adjacent phasors may vary between 30 and 180 .
To correctly demodulate the data, a phase-coherent carrier isrecovered and compared with the receiver carrier in a productdetector
Carrier Recovery To determine the absolute phase of the received carrier, it is
necessary to produce a carrier at the receiver that is phasecoherent with the transmit reference oscillator. This is the mainfunction of a carrier recovery circuit.
With PSK and QAM, the carrier is suppressed in the balancedmodulator and therefore , not transmitted specifically.
Consequently, at the receiver, the carrier cannot simply betracked with a standard Phase Locked Loop (PLL).
With suppressed carrier systems, such as PSK and QAM,sophisticated methods of carrier recovery are required , such asSquaring loop, a Costas loop, or a Remodulator
Phase Locked Loop
_ Invented in 1932
_ Based on feedback control theory
_ Has many applications in communications:
_ Carrier synchronization: Costas receiver _ Carrier synchronization: Costas receiver
_ Demodulation: e.g., DSB, FM
_ Frequency multiplication and division
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 16
Phase Locked Loop
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 17
Phase Locked Loop
Three main components: _ Phase detector (Multiplier) _ Loop filter
_ Voltage controlled oscillator (VCO) _ Voltage controlled oscillator (VCO)
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 18
Squaring Loop Squaring loop is a common method of achieving carrier
recovery for BPSK
The received BPSK waveform is filtered and then squared.Filtering reduces the spectral width of the received noise
The squaring circuit removes the modulation and generates the The squaring circuit removes the modulation and generates thesecond harmonic of the carrier frequency. This harmonic isphase-tracked by the PLL.
Output frequency from the VCO of the PLL is then divided by2 and used as the phase reference for the product detectors.
Bandpass Filter
BPSKInput
Squarer PLL FrequencyDivider
RecoveredCarrier
Squaring Loop
VCO out
Squaring Loop Carrier recovery circuit for a BPSK receiver
Costas Loop
Received Signal
6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 21
Signal