function mod_bits = tx_QPSK(in_bits)

[row,col] = size(in_bits);
mod_bits = zeros(row,col/2);
table = exp(i*[-3/4*pi 3/4*pi 1/4*pi -1/4*pi]);  % generates QPSK symbols
table = table([0 1 3 2]+1);  % Gray code mapping pattern for QPSK symbols
for n = 1:row
    inp = reshape(in_bits(n,:),2,col/2);
    mod_bits(n,:) = table([2,1]*inp+1);
end