How to convert a bitstream into groups of m-bits ?

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Afaq Ahmad
Afaq Ahmad 2021-1-11
评论： Walter Roberson 2021-1-14
I am trying to implement an M-ary system. I have a stream of bits which I want to group into m-bits. For example, for an 8-ARY system, I want to group it into 3 bits. For an 16-ARY system, i want to group it into 4-bits.
How could the bits be grouped in a generalized manner?

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Walter Roberson
Walter Roberson 2021-1-11
编辑：Walter Roberson 2021-1-11
M2 = ceil(log2(M));
L = length(StreamOfBits);
eL = ceil(L/M2)*M2;
if L ~= eL
StreamOfBits(end+1:eL) = 0; %pad if need be
end
M_ary = reshape(StreamOfBits, M2, []).';
Leave off the .' if you want columns instead of rows.
This algorithm does not work in its present form if M is not a power of 2.
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Afaq Ahmad
Afaq Ahmad 2021-1-13
Yes thanks! but i cross checked it and it didn't make any difference. Maybe there is a difference you could point out?
Walter Roberson
Walter Roberson 2021-1-14
eL = ceil(L/M2)*M2;
vs
eL = floor(L/M2)*M2;
makes a difference on the case where the length of the bit stream is not an exact multiple of the number of bits at a time you want. In that case, you want to pad with extra bits, and the version with ceil() gives you the extended length whereas the version with floor() would give you the length up to the last full group.

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