CARRIER RECOVERY BY RE-MODULATION IN QPSK

PROJECT INDEX : 093BY:

YEGO KIPLETING KENNETHREG. NO. F17/1783/2006

SUPERVISOR: DR. V.K. ODUOLEXAMINER: PROF. ELIJAH MWANGI

24TH MAY 2011

OBJECTIVES• Study carrier recovery in digital communication systems(BPSK and QPSK).• Compare re-modulation and Costas-based carrier recovery methods.• Design and demonstrate carrier recovery by re-modulation for use in QPSK demodulation.

INTRODUCTION• In many communication receivers it is necessary toregenerate at the receiver a waveform synchronous withthe transmitter carrier.• Carrier recovery circuit is used to estimate andcompensate for frequency and phase differences between areceived signal's carrier wave and the receiver's localoscillator.• The frequency and phase variations must be estimatedusing information in the received signal to recover thecarrier signal at the receiver and permit coherentdemodulation.

CARRIER OFFSET ERRORS

• QPSK constellation illustrating the effect of phase/frequency offsets

TYPES OF CARRIER OFFSETS

• Two types of Carrier offsets:• Frequency offsets• phase offset

• Causes :• Instability in transmitter or receiver oscillator• Thermal noise such as AWGN • Transmission delay• Doppler Effect

CARRIER SYNCHRONIZATION• Synchronization parameters include carrier frequency offset and carrier phase offset.Occurs in two subsequent phases• Carrier acquisition: transition from a large initial uncertainty about the synchronization parameters to a small steady-state estimation of error variance.• Carrier tracking: The estimates per burst so that the variations of the carrier phase over the burst can be tracked

CARRIER RECOVERY METHODS• For a signal containing a dominant carrier spectral

line, carrier recovery can be accomplished with a simple BPF at fc or a PLL.

• However, many modulation schemes most signal power is devoted to modulation where the information is present and not the carrier.

• Reducing the carrier power results in greater transmitter efficiency.

• Different methods must be employed to recover the carrier in these conditions.

THE PLL

• PLL enables receiver to adaptively track and remove phase/frequency offsets.• Loop filter filters the phase error signal to provide control signal to VCO.• Loop bandwidth determine range of error signal frequencies to be passed.

THE SQUARING LOOP

• s2(t) =m2(t)cos2(ωct+Ф)= ½m2 (t) + ½m2 (t) cos (2ωct+2Ф)

• Control signal measures phase /frequency mismatch between VCO and input signal.• Second harmonic of carrier divided by 2 to get phase coherent carrier.• Pre-squaring filter used to minimize squaring loss.

THE 4TH POWER LOOP

• Loop operates at 4fc• Frequency of signal divided by 4 to give recovered carrier frequency fc• Narrow loop bandwidth implies longer synchronization time and reduced capture range due to multiple phase slipping.

M-TH POWER LOOP FOR MPSK SIGNALS

• The output is the input raised to the M-th power and contains many spectral components.

• A sinusoid of frequency Mfc is passed though a narrow BPF to isolate it and remove more of noise.

• Dividing by M yields carrier at fc.

• Used for carrier phase recovery from suppressed-carrier modulation signals. • Two parallel tracking loops operating simultaneously from the same VCO

• The in-phase loop , uses the VCO, and the second, called thequadrature loop uses a 90 degree shifted VCO.• The I and Q multiplier outputs are filtered by the low pass filters.• The I and Q arm filter outputs are multiplied together and the productis scaled and filtered to produce the loop error used to control theVCO.

COSTAS LOOP

BPSK COSTAS LOOP• The input to the Costas loop is the waveform written as y(t) = m(t)*sin (ωct + ψ (t)) + n(t)• The in-phase multiplier generatesI(t) = m(t)* cosψe + nmc(t)• while the quadrature multiplier generatesQ(t) = m(t) *sinψe + nms(t)• The output of the multiplier is thenI(t) Q(t) = m2(t) *sin(2ψe)/ 2 + nsq(t)

QPSK COSTAS LOOP

RE-MODULATION • Carrier recovered through a combination of a

phase locked loop, a remodulator and a demodulator.

• Basic operation is the multiplication of the input signal by its demodulated baseband to retrieve the carrier.

• Exhibits performance limitation in high speed acquisition /synchronization.

BPSK RE-MODULATOR LOOP

COMPARISON OF THE CARRIER RECOVERY METHODS

Recovery Method Advantages Disadvantages

Squaring Loop • Squaring function easy to implement. • Loop offset cause phase offset at the recovered carrier.• Limited acquisition time due to loop bandwidth.• Requires extra 6 dB S/N at input due to doubled RMS jitter.• Prone to false lock.

Costas Loop • Easy to implement .• Better performance than theSquaring loop. • Requires careful I & Q arm matching.

• Limited acquisition time due to loop bandwidth.Re-modulator • Better tracking capability than Squaring loop.

• Faster Acquisition time than a Costas loop.• Requires precise delay matching in the remodulator path due to LPF group delay.• Limited acquisition time due to loop bandwidth.

SYSTEM DESIGN OF THE RE-MODULATOR

SIMULINK MODEL OF THE DESIGNCARRIER RECOVERY BY RE-MODULATION

PLL

scope2

scope1

Summer

Sign 2

Sign 1

Real to complex 2

Out1

Random IntegerGenerator

RandomInteger

QPSKModulatorBaseband

QPSK

PhaseShifter

In

Ph

ComplexPhase Shift

PD

MUX4MUX 3

MUX 2

MUX 1

LoopFilter

FDATool

LPF 2

Lowpass

LPF 1

Lowpass

Constant 1

pi /2

Constant

pi /2

Complex to real 2

In1Out1

Complex to real 1

In1Out1

Complex to real

In1Out1

AWGNChannel

AWGN

Phase Shifter

In

Ph

ComplexPhase Shift

VCO

Discrete -TimeVCO

INPUT QPSK CONSTELLATION

RE-MODULATOR OUTPUT

• QPSK Constellation at re-modulator output• No frequency / phase offsets as this has been corrected by the carrier recovery circuit.

QPSK COSTAS LOOP OUTPUT

ANALYSIS• The two schemes recovered the frequency and phase offsets. • The recovered symbols by the Costas loop were more scattered around the ideal symbol positions. • From the re-modulator the symbols were more concentrated around their ideal positions.• QPSK re-modulator gave better performance than the Costas loop method.

CONCLUSION

• Carrier recovery methods studied in this project included the squaring loop, the Costas loop and the re-modulator.

• Comparison of the performances of these loops was also done.

• The re-modulation approach was found to be better than the Costas loop.

THANK YOU