Why OQPSK matlab's function ber is differ?

I am testing the PSK series myself.

I check the BER curve, but only OQPSK is different from the theory. What is the problem?

Where does the 6dB difference from QPSK come from?

clc

clear all;

close all;

snr = -2:1:10;

numframes = 100;

ber = comm.ErrorRate; %bpsk

ber2 = comm.ErrorRate; %qpsk

ber3 = comm.ErrorRate('ReceiveDelay',2); %oqpsk

ber_grape = zeros(1,length(snr));

ber_grape2 = zeros(1,length(snr));

ber_grape3 = zeros(1,length(snr));

ebno = convertSNR(snr,'snr','ebno',BitsPerSymbol=1);

ebno2 = convertSNR(snr,'snr','ebno',BitsPerSymbol=2);

ebno3 = convertSNR(snr,'snr','ebno',BitsPerSymbol=2);

oqpskmod=comm.OQPSKModulator('BitInput',true);

oqpskdemod=comm.OQPSKDemodulator('BitOutput',true);

for ii = 1:length(snr)

for counter = 1:numframes

data = randi([0 1],10000,1,'int8');

%BPSK

modSignal = pskmod(data,2,InputType='bit');

[rxsig, noisevar] = awgn(modSignal,snr(ii));

demodSignal = pskdemod(rxsig,2, ...

OutputType='bit', ...

NoiseVariance=1);

errStats = ber(data,int8(demodSignal));

%QPSK

noCoding = pskmod(data,4,InputType='bit');

rxNoCoding = awgn(noCoding,snr(ii));

rxBitsNoCoding = pskdemod(rxNoCoding,4,OutputType='bit');

errStatsNoCoding = ber2(data,int8(rxBitsNoCoding));

%OQPSK

noCoding2 = oqpskmod(data);

rxNoCoding2 = awgn(noCoding2,snr(ii));

rxBitsNoCoding2 = oqpskdemod(rxNoCoding2);

errStatsNoCoding2 = ber3(data,int8(rxBitsNoCoding2));

end

ber_grape(ii) = errStats(1,1);

ber_grape2(ii) = errStatsNoCoding(1,1);

ber_grape3(ii) = errStatsNoCoding2(1,1);

reset(ber);

reset(ber2);

reset(ber3);

end

semilogy(ebno, ber_grape, ...

ebno2, ber_grape2, ...

ebno3, ber_grape3)

xlim([0 inf])

ylim([10^-5 inf])

Answer 1

[Gokul Nath S J]

Hi Seongjong Kim,

Based on my understanding it seems that the BER for QPSK and OQPSK is showing a difference of 6 dB in the graph. As an alternative if you use the following code, the BER will come out almost similar values.

EbNo = (0:10)';
M = 4; % Modulation order
berQ = berawgn(EbNo,'psk',M,'nondiff');
berOQpsk = berawgn(EbNo,'oqpsk','nondiff');     

You can also refer the following example to find more information on the computation,

https://www.mathworks.com/help/comm/ref/comm.errorrate-system-object.html

For further information on the code, kindly refer the following link.

https://www.mathworks.com/help/comm/ref/berawgn.html

Thanks,

Gokul Nath S J