OFDM Carrier Frequency Recovery

I used your example " syncCarrierFreqMSKSignalRecovery" to construct my model and apply it for QPSK system,it works . but when i applied it to OFDM-QPSK , it doesn't work.

Your matlab code for Single carrier system is

freqOffset = 0; 

phaseOffset = 0;

hPFO = comm.PhaseFrequencyOffset('FrequencyOffset', freqOffset, ... 'PhaseOffset', phaseOffset, ... 'SampleRate', samplesPerSymbol/Ts);

rx_signal1l = step(hPFO,tx_signal);

   rx_signal = rx_signal1l.';

hDelayInstFreq = dsp.Delay;

hMovAve = dsp.DigitalFilter('TransferFunction','FIR (all zeros)', ... 'Numerator', ones(1,100)/100);

hCumSum = dsp.CumulativeSum('Dimension','Channels (running sum)', ... 'FrameBasedProcessing', true);

%%Calculate the frequency shift estimate

P = 2;

currInstFreq = rx_signal.^(2*P);

prevInstFreq = step(hDelayInstFreq, currInstFreq);

freqShiftEst1 = currInstFreq .* conj(prevInstFreq);

% Average over 1 frame

freqShiftEst = mean(freqShiftEst1);

% Moving average over 100 frames

freqShiftEst = step(hMovAve, freqShiftEst);

freqShiftEst = angle(freqShiftEst) / (2*P);

% Convert digital frequency to Hertz

freqShiftEst = freqShiftEst / (2*pi*Ts);

%%Apply frequency correction

freqCorr = repmat(-freqShiftEst*Ts, samplesPerFrame, 1);

% Calculate cumulative phase shift

freqCorr = step(hCumSum, freqCorr);

% Apply frequency shift correction

rxSym = rx_signal .* exp(1i*2*pi*freqCorr);

Kindly advice , how to modify the previous code to use with OFDM System

[re ib]

Any help :(

[Walter Roberson]

Who is "your" in "your example" ?

Please expand on "does not work" ?

[re ib]

There is an example in Matlab help examples named "syncCarrierFreqMSKSignalRecovery" that deals with frequency offset , AWGN for QPSK. i want to used it to make OFDM- QPSK based. how can i modify this example?

Answer 1

[ofdma_matlab]

hPFO = comm.PhaseFrequencyOffset('FrequencyOffset', freqOffset, ... 'PhaseOffset', phaseOffset, ... 'SampleRate', samplesPerSymbol/Ts);

can you please explain the line above !!!!! what do you mean by 'comm.PhaseFrequencyOffset' ? lemme understand it first .

thanks in advance

[re ib]

In Matlab R2011a , it uses this formula to represent Frequency offset behaves like:

yt = xt.* exp(2*j*pi*F_Offset(x)/fs*[0:length(xt)-1]);

with reference:

http://www.mathworks.com/help/comm/ref/comm.phasefrequencyoffsetclass.html

and it works probably for QPSK not for OFDM QPSK