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cc430modulator.m
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cc430modulator.m
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function baseBand = cc430modulator(bits,sps)
% This function replicates the (kind of screwy) MSK modulation used in the
% Texas Instruments CC430. Effectively, the input bits are flipped vs.
% standard MSK.
%First turn the bits into differentially encoded offset square pulses
%Flip bits (CC430 quirk)
bits = -bits;
%Initialization and one bit period offset between I and Q
iChan = ones(sps,1);
qChan = bits(1)*ones(2*sps,1);
diffs = abs(diff(bits));
for k = 1:length(diffs)
if mod(k,2) == 1 %odd - I channel
if diffs(k) > 0 % bit transition
iChan = [iChan; -1*iChan(end)*ones(2*sps,1)];
else % no bit transition
iChan = [iChan; iChan(end)*ones(2*sps,1)];
end
else %even - Q channel
if diffs(k) > 0 % bit transition
qChan = [qChan; -1*qChan(end)*ones(2*sps,1)];
else % no bit transition
qChan = [qChan; qChan(end)*ones(2*sps,1)];
end
end
end
%Now apply sinusoidal pulse shaping
for k = 1:length(iChan)
iChan(k) = iChan(k)*cos(pi/2*(k-1)/sps);
end
for k = 1:length(qChan)
qChan(k) = qChan(k)*sin(pi/2*(k-1)/sps);
end
baseBand = iChan(1:length(bits)*sps) + 1i*qChan(1:length(bits)*sps);
end