demodmap.m

数字通信第四版原书的例程

function x = demodmap(y, Fd, Fs, method, M, opt2, opt3)
%DEMODMAP Demaps a modulation mapped signal to the closest digit number.
%       DEMODMAP(Y, Fd, Fs) plots eye-pattern diagram for the signal Y. The
%       sampling frequency for Y is Fs (Hz) and the digital sampling frequency 
%       is Fd (Hz).
%
%       Z = DEMODMAP(Y, Fd, Fs, METHOD, OPT1, OPT2, OPT3) is a reverse mapping
%       of function MODMAP. This function demaps a sampling frequency Fs (Hz)
%       analog signal into a sampling frequency Fd (Hz) digit signal Z. Y is
%       a two column matrix with the first column is an in-phase component and
%       second column is a quadrature component. When METHOD = 'ask', 'fsk',
%       or 'msk', Y is a one column vector instead of a matrix. When Fd is a 
%       two element vector, the second element is the offset value for the
%       decision point. The offset timing in the plot is offset/Fs. The
%       default offset is 0. The received digital signal is sampled at time
%       point i/Fd + offset/Fs. The sample value keeps until time 
%       (i+1)/Fd +offset/Fs when the next sample point is taken place. The
%       demapping process finds the distance from the sample value to
%       all of the possible digital symbols. The digital symbols with the 
%       shortest distance to the current sampling point is the demodulated
%       output. 
%
%       METHOD is a string, which can be one of the following:
%       'ask'       Amplitude shift keying modulation.
%       'psk'       Phase shift keying modulation.
%       'qask'      Quadrature amplitude shift keying modulation, with choice
%                   of 'qask/cir', 'qask/arb' for different mapping table.
%       'fsk'       Frequency shift keying modulation.
%       'msk'       Minimum shift keying modulation.
%       'sample'    Down sampling from a higher sampling frequency signal.
%       'eye'       Eye-pattern plot.
%       'scatter'   Scatter plot.
%
%       Use DEMODMAP(METHOD) to view the help for a specific method.
%
%       See also MODMAP, DMOD, DDEMOD, AMOD, ADEMOD, MODCE, DEMODCE.
%

%       Wes Wang 1/9/95, 10/3/95
%       Copyright (c) 1995-96 by The MathWorks, Inc.
%       $Revision: 1.1 $  $Date: 1996/04/01 17:56:38 $

%position for optional parameters.
opt_pos = 5;
plot_const = 0;
if nargin < 1
    feval('help','demodmap');
    return;
elseif isstr(y)
    method = lower(deblank(y));
    if length(method) < 3
        error('Invalid method option for demodmap.')
    else
        method = method(1:3);
    end
    if nargin == 1
        addition = 'See also MODMAP, AMOD, ADEMOD, MODCE, DEMODCE.';
        callhelp('demodmap.hlp', method, addition);
        return;
    else
        disp('Warning: Worng number of input variable, use MODMAP for plotting constellations.');
        return
    end;
end;
if length(Fd) > 1
    offset = Fd(2);
    Fd = Fd(1);
else
    offset = 0;
end;
if (Fs == 0) | (Fd == 0) | isempty(Fd) | isempty(Fs)
    FsDFd = 1;
else
    Fs = Fs(1);
    FsDFd = Fs / Fd;
    if (ceil(FsDFd) ~= FsDFd) | (FsDFd <= 0)
        error('Fs / Fd must be a positive integer.')
    end;
end;
offset = rem(offset, FsDFd);
if offset < 0
    offset = rem(offset + FsDFd, FsDFd);
end;
[r, c] = size(y);
if r * c == 0
    y = [];
    return;
end;
if r == 1
    y = y(:)';
    len_y = c;
else
    len_y = r;
end;

if nargin < 3
    disp('Usage: Y=DEMODMAP(X, Fd, Fs, METHOD, OPT1, OPT2, OPT3) for modulation mapping');
    return;
elseif nargin < opt_pos-1
    method = 'sample';
end;

method = lower(method);

if length(method) < 3
    method = [method '   '];
end;

if strcmp(method(1:3), 'ask')
    if offset == 0
        offset = FsDFd;
    end;
    yy = y([offset : FsDFd : len_y], :);
    if nargin < opt_pos
        error('Not enough input varibale for DEMODMAP.')
    end;
    index = ([0 : M - 1] - (M - 1) / 2) * 2 / (M - 1);
    x = [];
    for i = 1 : size(y, 2)
        [tmp, x_p] = min(abs(yy(:, i*ones(1, M)) - index(ones(1, size(yy, 1)), :))');
        x = [x x_p'-1];
    end;
elseif findstr(method, 'fsk')
    if nargin < opt_pos + 1
        Tone = 2 * Fd / M;
    else
        Tone = opt2;
    end;
    if offset == 0
        offset = FsDFd;
    end;
    yy = y([offset : FsDFd : len_y], :);
    if findstr(method, 'max')
        % in case yy is the correlation output.
        [tmp, x] = max(yy');
        x = (x-1)';
    else
        index = [0:M-1] * Tone;
        x = [];
        for i = 1 : size(y, 2)
            [tmp, x_p] = min(abs(yy(:, i*ones(1, M)) - index(ones(1, size(yy, 1)), :))');
            x = [x x_p'-1];
        end;        
    end;
%psk has combined with qask.
elseif findstr(method, 'msk')
    %This is a special case of fsk call back to get fsk
    method(1) = 'f';
    M = 2;
    Tone = Fd;
    x = demodmap(y, Fd, Fs, method, M, Tone);
elseif ~isempty(findstr(method, 'qask')) |...
         ~isempty(findstr(method, 'qam')) |...
         ~isempty(findstr(method, 'qsk')) |...
         ~isempty(findstr(method, 'psk'))
    if findstr(method, '/ar')
        % arbitraryly defined I, Q. 
        if nargin < opt_pos + 1
            error('In correct format for METHOD=''qask/arbitrary''.');
        end;
        I = M;
        Q = opt2;
        M = length(I);
    elseif ~isempty(findstr(method, '/ci')) | ~isempty(findstr(method, 'psk'))
        % circle defined NIC, AIC, PIC.
        if nargin < opt_pos
            if findstr(method, 'psk')
                error('M-ary number must be specified for psk demap.')
            else
                error('Incorrect format for METHOD=''qask/arbitrary''.');
            end;
        end;
        NIC = M;
        M = length(NIC);
        if nargin < opt_pos+1
            AIC = [1 : M];
        else
            AIC = opt2;
        end;
        if nargin < opt_pos + 2
            PIC = NIC * 0;
        else
            PIC = opt3;
        end;
        inx = apkconst(NIC, AIC, PIC);
        I = real(inx);
        Q = imag(inx);
        M = sum(NIC);
    else
        %consider as square style.
        [I, Q] = qaskenco(M);
    end;
    if offset <= 0 
        offset = FsDFd;
    end;
    yy = y([offset : FsDFd : len_y], :);
    [len_y, wid_y] = size(yy);
    if (ceil(wid_y/2) ~= wid_y/2)
        error('qask demap requires input is a matrix with even number of columns.');
    end;
    x = []; I = I(:)'; Q = Q(:)';
    for i = 1 : 2 : wid_y
        [tmp, x_p] = min(...
            ((yy(:, i*ones(1, M)) - I(ones(1, len_y), :)).^2 + ...
            (yy(:, (i+1)*ones(1, M)) - Q(ones(1, len_y), :)).^2)');
        x = [x x_p'-1];
    end;
elseif findstr(method, 'samp')
    %This is made possible to convert an input signal from sampling frequency Fd
    %to sampling frequency Fs.
    if offset <= 0 
        offset = FsDFd;
    end;
    yy = y([offset : FsDFd : len_y], :);
elseif findstr(method, 'eye')
    % plot eye-pattern plot
    eyescat(y, Fd, Fs, offset, offset);
elseif findstr(method, 'scat')
    if nargin >= opt_pos
        if isstr(M)
            M = M(1);
        else
            M = '.';
        end;
    else
        M = '.';
    end;
    eyescat(y, Fd, Fs, offset, M);
else
    %The choice is not a valid one.
    disp('You have used an invalid method. The method should be one of the following string:')
    disp('  ''ask'' Amplitude shift keying modulation;')
    disp('  ''psk'' Phase shift keying modulation;')
    disp('  ''qask'' Quadrature amplitude shift-keying modulation, square constellation;')
    disp('  ''qask/cir'' Quadrature amplitude shift-keying modulation, circle constellation;')
    disp('  ''qask/arb'' Quadrature amplitude shift-keying modulation, user defined constellation;')
    disp('  ''fsk'' Frequency shift keying modulation;')
    disp('  ''msk'' Minimum shift keying modulation;')
    disp('  ''sample'' Convert sample frequency Fd input to sample frequency Fs output.')
end;