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Homework Help: BER output problem for Radio over Fiber system/Optical Comm

  1. Apr 22, 2010 #1
    Hi guys, this is my first work in optical and first simulation in matlab. I am sorry if there is mistake and im really new to this.

    Here is what I did, im simulation an entire Radio Over Fiber system with Matlab, linear system.

    Let me insert my code first,
    Code (Text):
    %*****************************Defining constant parameter****************
    %Rb = 1e+6; %bit rate
    %Tb = 1/Rb; %bit duration / period
    Num_bits = 1e4; %number of bits

    nsamp = 20; %samples per symbols
    %Tsamp = Tb/nsamp; %sampling rate, fsamp = 20MHz.
    a = 0.2; %channel attenuation, dB/km
    L = 20; % Length of fiber
    Gain = 10; %Gain in dB

    %*****************************Filter definations************************

    snr_dB=0:20; %SNR in dB from 0 to 20

    SNR=10.^(snr_dB./10); %SNR in linear scale

    Tx_filter = ones(1,nsamp); %transmitter filter, in terms of
    % h(t),match filter
    Rx_filter = fliplr(Tx_filter/nsamp); % receiver filter (matched filter),
    % flip from left to right to make
    % it h(T-t), match filter

    Tx_OOK = randint(1,Num_bits); % randomly generating 1 and 0 in a
    % row of matrix for 1000
    % column,basically ON-OFF keying
    % with random bits

    %*****************************Channel input*****************************
    Tx_OOKin = 0+Tx_OOK; % Tx+with power, 0dBm,ideal.

    %*****************************Channel properties************************

    Txch = Tx_OOKin -(a*L); %Signal in channel, deducting with
    %channel attenuation=4dB

    Txout = 10.^(Txch./10); %Changing back the signal to linear

    Tx_signal = rectpulse(Txout,nsamp); % generate rectpulse for nsamp time
    % per symbol

    for i=1:length(snr_dB)
    Rx_signal = awgn(Tx_signal,snr_dB(i)+3-10*log10(nsamp),'measured');

    %*****************************Match filtering***************************
    MF_out = conv(Rx_signal,Rx_filter); %equivalent to y(t)=x(t)*h(t)

    MF_out_downsamp = MF_out(nsamp:nsamp:end); %sampling and hold

    %try without truncation


    Rxth = zeros(1,Num_bits); %Reset receiver before thresholding

    Rxth(find(MF_out_downsamp>0.45))=1; %value above 0.45 equivalent to one
    %at the receiver, thresholding.

    %*****************************BER CALC**********************************
    ber(i) = biterr(Tx_OOK,Rxth); %Matlab function for ber
    %******************Theoritical BER CALC**********************************
    ber_pr(i) = Qfunct(sqrt(SNR(i))); %theoritical for ber

    semilogy(snr_dB,ber,'b'); %Simulation
    hold on;

    Above is my entire code, the part i bold, i dun really get the idea, i took it from my friend cause at the receiver part, i dont really have an idea of how to receive the signal for at least with 10 different SNR rate and plot BER. My friend used the Match filter, but i dont really have the exact idea how match filter works.

    The Tx_filter and Rx_filter part i got it where we flip the entire input from transmitter and at the receiver. which is h(t) from transmitter becomes h(-t).

    I have uploaded the output and the blue line on the plot is the output that i got

    Attached Files:

  2. jcsd
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