qam.m

linear time－frequency toolbox

function xo=qam(xi,order)
%QAM  QAM modulation of input signal.
%    Usage:  xo=qam(xi,order);
%
%    xi must be a vector of 0's and 1's. 
%    order must be a power of 2.

% Verify input

if ~all((xi==0) + (xi==1))
  error('Input vector must consist of only 0s and 1s');
end;

if mod(log2(order),1)~=0
  error('order must be a power of 2.');
end;

% Get the optimal ordering of bits
bitorder=circbitflip(order);

% nbits is number of bits used. Everything will be ordered
% in groups of this size.
nbits=log2(order);

L=length(xi);
symbols=L/nbits;

% nbits must divide L
if rem(symbols,1)~=0
  error('Length of input must be a multiple of log2(order)');
end;

xi=reshape(xi,nbits,symbols);

two_power=(2.^(0:nbits-1)).';

% This could be vectorized by a repmat.
xo=zeros(symbols,1);
for ii=1:symbols
  xo(ii)=sum(xi(:,ii).*two_power);
end;

% xo now consist of numbers in the range 0:order-1
% Convert to the corresponding complex root of unity.
xo=exp(2*pi*i*bitorder(xo+1)/order);