bit error rate calculation of qpsk signal in awgn channel ,getting ber of agc is more than ber of noise (Implementing QPSK Transmitter and Receiver in Simulink in matlab)

clc;

clear all;

close all;

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

data_NZR=2*data-1; % Data Represented at NZR form for QPSK modulation

s_p_data=reshape(data_NZR,2,length(data)/2);

br=10.^6; %Let us transmission bit rate 1000000

f=br; % minimum carrier frequency

T=1/br; % bit duration

t=T/99:T/99:T; % Time vector for one bit information

% XXXXXXXXXXXXXXXXXXXXXXX QPSK modulation XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

y=[];

y_in=[];

y_qd=[];

for(i=1:length(data)/2)

y1=s_p_data(1,i)*cos(2*pi*f*t); % inphase component

y2=s_p_data(2,i)*sin(2*pi*f*t) ;% Quadrature component

y_in=[y_in y1]; % inphase signal vector

y_qd=[y_qd y2]; %quadrature signal vector

y=[y y1+y2]; % modulated signal vector

end

Tx_sig=y;

rolloff = 0.5; % Filter rolloff

span = 10; % Filter span

sps = 2; % Samples per symbol

rrcFilter = rcosdesign(rolloff,span,sps);

txSig = upfirdn(Tx_sig,rrcFilter,sps);

%%%%%%%%%%%%%%Adding AWGN Noise to the modulated signal%%%%%%%%%%%%%

SNR_db=-10;

noise_sig=awgn(txSig,SNR_db);

data1=demodulation(noise_sig,data);

agc = comm.AGC('MaxPowerGain',30);

agc_op = agc(noise_sig.');

data2=demodulation(agc_op,data);

rxFilt = upfirdn(agc_op,rrcFilter,1,sps);

rxFilt = rxFilt(span+1:end-span);

data3=demodulation(rxFilt,data);

beragc=biterr(data,data2);

bernoise=biterr(data,data1);

berrrc=biterr(data,data3);

function [y] = demodulation(fdata, data)

br = 10^6; % Transmission bit rate = 1000000

f = br; % Minimum carrier frequency

T = 1 / br; % Bit duration

t = T / 99 : T / 99 : T; % Time vector for one bit information

y = [];

Rx_sig = fdata; % Received signal

for i = 1 : 1 : length(data) / 2

% In-phase coherent detector

Z_in = Rx_sig((i - 1) * length(t) + 1 : i * length(t)) .* cos(2 * pi * f * t);

Z_in_intg = trapz(t, Z_in) * (2 / T); % Integration using the trapezoidal rule

if (Z_in_intg > 0) % Decision Maker

Rx_in_data = 1;

else

Rx_in_data = 0;

end

% Quadrature coherent detector

Z_qd = Rx_sig((i - 1) * length(t) + 1 : i * length(t)) .* sin(2 * pi * f * t);

Z_qd_intg = trapz(t, Z_qd) * (2 / T); % Integration using the trapezoidal rule

if (Z_qd_intg > 0) % Decision Maker

Rx_qd_data = 1;

else

Rx_qd_data = 0;

end

y = [y, Rx_in_data, Rx_qd_data]; % Received Data vector

end

end

Answer 1

[Binaya]

Hi Pavan, 

Based on my understanding, you would like to know why the ber (bit error rate) value of agc is more than that of ber of noise while implementing the QPSK transmitter-reciever pair in MATLAB.

While executing the provided code, it can be observed that “beragc”, “bernoise” and “berrrc” have values around 0.5. This is a clear indicator that the decoding of the transmitted signal is not done correctly.

You can verify the decoding algorithm used in the provided code to resolve the underlying error.

I hope this helps.  

Regards ,

Binaya