iqam.m

linear time－frequency toolbox

function xo=iqam(xi,order)
%IQAM  Inverse QAM modulation of signal.
%
%    order must be a power of 2.
%    xo is a vector of 0's and 1's. 

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);

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

% We round the argument of the complex numbers to the closest root of
% unity of order "order"
work=round(angle(xi)/(2*pi)*order);

% work now contains negative numbers. Get rid of these
work=mod(work,order);

% Reverse the optimal bit ordering.
reversebitorder=zeros(order,1);
reversebitorder(bitorder+1)=(0:order-1).';

% Apply the reverse ordering
work=reversebitorder(work+1);

xo=zeros(nbits,symbols);
% Reconstruct the bits
for bit=1:nbits
  xo(bit,:)=bitget(work,bit).';
end;

xo=xo(:);