2012 digital comm week 10
DESCRIPTION
dcomTRANSCRIPT
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EE4723 : Digital Communications II
Week 10:
Synchronization
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(Frequency, Phase, Symbol and Frame Synchronization)Carrier and Phase Recovery
Phase-Locked Loop
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Carrier Phase Recovery
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Carrier Phase Recovery
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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)
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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
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Pilot Tone: Transmitter
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Pilot Tone: Detector
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TTIB: Transmitter
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TTIB: Detector
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Extraction Methods
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Extraction Method: Feed-Forward Phase Estimator
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Extraction Method: Feedback Phase Estimator
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Decision-Directed Feedback Estimator
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Extraction Method: Removing Modulation
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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
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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
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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
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Phase Locked Loop
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Phase Locked Loop
Three main components: _ Phase detector (Multiplier) _ Loop filter
_ Voltage controlled oscillator (VCO) _ Voltage controlled oscillator (VCO)
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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.
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Bandpass Filter
BPSKInput
Squarer PLL FrequencyDivider
RecoveredCarrier
Squaring Loop
VCO out
Squaring Loop Carrier recovery circuit for a BPSK receiver
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Costas Loop
Received Signal
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Signal