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Re: [Discuss-gnuradio] The GMSK demodulation

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] The GMSK demodulation
Date: Fri, 28 Feb 2014 16:12:22 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.2.0

Hi Zhenhua,
as Aditya pointed out: Viterby is /not/ a demodulator.
You should read something on digital communication that explains the difference between channel coding and modulation, then everything will be clearer to you.

On 02/28/2014 04:04 PM, zhenhua han wrote:

What confused me is the Viterbi algorithm. I saw it can be used to demodulate GMSK in more than one place (such as here: http://www.mathworks.cn/cn/help/comm/ref/comm.gmskdemodulatorclass.html )

I think it's better for me to read some materials of Viterbi and find out what really it is.


2014-2-28 下午10:40于 "Aditya Dhananjay" <address@hidden> 写道:

As I have known, GSM uses GMSK modulation which BT = 0.3 and it uses Viterbi algorithm for demodulation. And I took a look at the code of GMSK demod code in GNU Radio, it use quadrature_demod but not Viterbi as demodulation method. So which one is better in doing demodulating GMSK?

Moreover, GSM uses Viterbi algorithm to decode the convolution encoding. Would it be possible to use quadrature_demod to demodulate GSM signal instead of Viterbi? What about the convolution decoding part (maybe by some other method)?

I think you are mixing up the modulation and coding components.

At the transmitter, you first code (convolutional code), and then you modulate (GMSK).

At the receiver, you first demodulate (quadrature demodulate) and then you decode (Viterbi).

Coding takes a stream of bits and converts it to another stream of bits. Modulation refers to the process of converting the post-coded bits into "symbols" that you will then send over-the-air.


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