Re: [Discuss-gnuradio] gsm gmsk demodulation

[Ben Wojtowicz]

It is defintely do-able in real time software.  I have done some work in the past that continuosly decoded all 8 timeslots in real time on a 1GHz PPC.  Also, the processor usage goes way down when you only look at 1 of the 8 timeslots, which is usually all that is needed.

Ben

 

On 6/6/08, Long, Jeffrey P. <address@hidden> wrote:

I agree with Steven, while there are definitely more "optimal"
solutions like MLSE they don't always make sense for every application.
GSM phones and the like have dedicated DSP resources to run a Viterbi
algorithm but is this doable on the typical gnu users computer? I will
defer to the experts on this but from my own low power(< 20 mA)
portable wireless experience the ad-hoc techniques sometimes make more
sense.

-Jeff

-----Original Message-----
From: discuss-gnuradio-bounces+jplong=mitre.org@gnu.org
[mailto:discuss-gnuradio-bounces+jplong=mitre.org@gnu.org] On Behalf Of
Steven Clark
Sent: Friday, June 06, 2008 10:58 AM
To: Ben Wojtowicz
Cc: address@hidden
Subject: Re: [Discuss-gnuradio] gsm gmsk demodulation

> On 6/6/08, Bob McGwier <address@hidden> wrote:
>>
>> This is not my professional experience.  The sounding data is used
to find
>> the channel and then the data symbols are soft detected through a
"viterbi
>> equalizer" in every implementation I am aware of that is any good at
all
>> with the exception of one I wrote several years ago which estimates
the data
>> given the channel and then restimates the channel and then the data
and then
>> the channel and then the data, etc.  MMSE and not MLE is the goal
and this
>> was a suboptimal implementation of the EM algorithm.  It was
suboptimal
>> since it did not estimate the data bauds using ALL observations but
only
>> those between sounding data.  Further,  assumptions that the
conditional
>> distributions of the data given the observations could be described
in 1st
>> and 2nd product moments (not Gaussian but having similar
properties).  This
>> has been published by many.  The computational complexity is on a
par with
>> the viterbi equalizer and it outperforms it.
>>
>> Most of the cell phones I know use the Viterbi equalizer.
>>
>> Bob

On Fri, Jun 6, 2008 at 9:41 AM, Ben Wojtowicz <address@hidden>
wrote:
> I agree with Bob, most gsm demodulators I have seen use a viterbi
equalizer
> (sometimes called MLSE equalization).
> Ben
>
>

Ok, good to hear from guys with more experience. So you would have a
viterbi equalizer to mitigate ISI, and then wrap that in a layer of
forward error correction? Is this computationally feasible for
cpu-based software radio? Sounds like it could get computationally
expensive pretty quick...

-Steven


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