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

6/5/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications II EE4723 7

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