CARRIER RECOVERY BY RE-MODULATION IN QPSK

Transcription

1 CARRIER RECOVERY BY RE-MODULATION IN QPSK PROJECT INDEX : 093 BY: YEGO KIPLETING KENNETH REG. NO. F17/1783/2006 SUPERVISOR: DR. V.K. ODUOL EXAMINER: PROF. ELIJAH MWANGI 24 TH MAY 2011

2 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.

3 INTRODUCTION In many communication receivers it is necessary to regenerate at the receiver a waveform synchronous with the transmitter carrier. Carrier recovery circuit is used to estimate and compensate for frequency and phase differences between a received signal's carrier wave and the receiver's local oscillator. The frequency and phase variations must be estimated using information in the received signal to recover the carrier signal at the receiver and permit coherent demodulation.

4 CARRIER OFFSET ERRORS QPSK constellation illustrating the effect of phase/frequency offsets

5 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

6 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

7 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.

8 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.

9 THE SQUARING LOOP s 2 (t) =m 2 (t)cos 2 (ω c t+ф) = ½m 2 (t) + ½m 2 (t) cos (2ω c t+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.

10 THE 4 TH 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.

11 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.

12 COSTAS LOOP Used for carrier phase recovery from suppressedcarrier modulation signals. Two parallel tracking loops operating simultaneously from the same VCO The in-phase loop, uses the VCO, and the second, called the quadrature 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 product is scaled and filtered to produce the loop error used to control the VCO.

13 BPSK COSTAS LOOP The input to the Costas loop is the waveform written as y(t) = m(t)*sin (ω c t + ψ (t)) + n(t) The in-phase multiplier generates I(t) = m(t)* cosψ e + n mc (t) while the quadrature multiplier generates Q(t) = m(t) *sinψ e + n ms (t) The output of the multiplier is then I(t) Q(t) = m 2 (t) *sin(2ψ e )/ 2 + n sq (t)

14 QPSK COSTAS LOOP

15 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.

16 BPSK RE-MODULATOR LOOP

17 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 the Squaring 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.

18 SYSTEM DESIGN OF THE RE-MODULATOR

19 SIMULINK MODEL OF THE DESIGN CARRIER RECOVERY BY RE-MODULATION Random Integer Generator Random Integer scope1 pi /2 Constant 1 In Complex PhPhase Shift Phase Shifter scope2 In1 Out1 Lowpass MUX 2 QPSK QPSK Modulator Baseband AWGN AWGN Channel MUX 1 Complex to real LPF 1 Sign 1 Summer pi /2 Constant In Complex PhPhase Shift Phase Shifter MUX 3 In1Out1 Complex to real 1 Lowpass LPF 2 Sign 2 MUX4 Real to complex 2 Out1 VCO Discrete -Time VCO Complex to real 2 Out1In 1 Loop Filter FDATool PD PLL

20 INPUT QPSK CONSTELLATION

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

22 QPSK COSTAS LOOP OUTPUT

23 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.

24 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.

25 THANK YOU