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Re: [Discuss-gnuradio] Posted: Enhanced GMSK demodulator

From: Steven Clark
Subject: Re: [Discuss-gnuradio] Posted: Enhanced GMSK demodulator
Date: Thu, 5 Jun 2008 17:52:58 -0400

On Thu, Jun 5, 2008 at 5:28 PM, Gregory Maxwell <address@hidden> wrote:
> On Thu, Jun 5, 2008 at 3:25 PM, Steven Clark <address@hidden> wrote:
> [snip]
>> It should be a
>> drop-in replacement for the existing demodulator in designs, except
>> that the transmitted data needs to be differentially encoded (you will
>> NOT, however, need a differential decoder on the receive side).
> Why?
> All the cases I can think of would be most logically implimented with
> the same interface (encoded or non-encoded) on both the encoder and
> decoders.
> If it's just an arbitrary decision then perhaps it should be changed
> before it enters the tree and people build other radios expecting that
> particular interface.

It is not an arbitrary decision -- it is required. I do agree that it
seems weird, but it is just a quirk of the way the math works out for
a 2-bit differential detector that it removes a differential encoding
in the process.
The Simon & Wang paper explains it better than I can:
"The dashed lines around the differential encoder indicate that it is
present for a two-bit differential detector but absent for a one-bit
differential detector at the receiver. As discussed in [16], this
differential encoding operation is required for the two-bit detector
in order that hard decisions made on the detector output reflect
decisions on the true input data sequence and not a differentially
decoded version of it as would be the case without the differential
encoder at the transmitter input."

If you can't get your hands on the Simon & Wang paper, let me know and
I can send you a copy of it.


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