Re: Differential Encoder/Decoder in GSM

richard_zhang wrote:

(snip)
If it is a typo error in 3GPP spec, then I can close my case here
immediately, but who can make sure that it is an error in 3GPP?

3GPP spec here:
http://www.3gpp.org/ftp/Specs/html-info/45004.htm

Do you have a reference for the decoder? That would
make it easier to verify.

-- glen



Hi Glen,

3GPP spec does not specify decoder. but we can derive it from the encoder
equation.
step 1: Y(i) = X(i) xor X(i-1)
step 2: Y(i) xor X(i-1) = X(i) xor X(i-1) xor X(i-1)
step 3: Y(i) xor X(i-1) = X(i)

Here step1 is the encoder in 3GPP; step2 is to xor both sides of encoder
by X(i-1); in step3 we get decoder, since X(i-1) xor X(i-1) always equals
0.

I am also thinking, maybe GSM receiver uses some demodulation algorithm
which can avoid using the differential decoder,then there will be no the
error propagation problem. But not sure whether this is true.

Any comments welcomed. Thank you.




.

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