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Re: [Discuss-gnuradio] constellation object

From: Ben Reynwar
Subject: Re: [Discuss-gnuradio] constellation object
Date: Thu, 20 Jan 2011 13:36:46 -0700

On Thu, Jan 20, 2011 at 1:46 AM, Martin Braun <address@hidden> wrote:
> On Wed, Jan 19, 2011 at 11:55:30AM -0700, Ben Reynwar wrote:
>> I'm new to all this, so I don't have a good handle on soft versus hard
>> decision making.  My understanding is that a hard decision maker
>> simply returns the symbol value, and a soft decision maker would
>> return probabilities for various symbols values.  Then this
>> probabilistic information would be interpreted by the decoder which
>> would make the hard decision for sets of symbols simultaneously.  I
>> don't understand what receiver_cf is doing if it returning a stream of
>> floats.
> You've got that right: a soft decider doesn't really decide, but rather
> gives a value how good the estimate is. Say you have a binary output,
> 1 and -1. A soft decider can also give any value in between. If you get
> a 0, then the soft decider really has no clue what was actually
> transmitted and instead of guessing a binary value, it relays this
> uncertainty.
> One place this is really important is the channel decoding.

That makes sense.  What kind of values would you output when you have more
than 2 symbols?  Would you just give the distances to the closest n points?

>> On a related note, I've read that the minimum euclidean distance
>> minimizes the chance of error if you have white gaussian noise.  Is
>> this always a sensible assumption or are there practical situations in
>> which a different measure would be better?  If not, then the distance
>> could be used as a proxy for probability.  If others measures are
>> sometimes better, then it would be nice to keep things more general.
> Just a couple of euro-cents I'd like to add:
> - White noise is a perfectly valid assumption. In most cases, your noise
>  is WGN-ish anyway. If it isn't, then the constellation demodulator is
>  not the right place to handle it, you'd use some kind of pre-whitening
>  filter a priori.

Good to know.

> - I'd say if you have to implement a soft- and hard-decider for each
>  constellation anyway, you're general enough. With the aforementioned
>  point, this means the hard decider is sіmply always the minimum
>  euclidean distance, as you said already.
> - After having a quick peek at your code, I wasn't quite sure if you've
>  thought of differential PSKs?

I've just implemented it for PSK but have to tidy it up a bit before
pushing it to my github repo.  Works fine for differential PSK,
although I had to put map_bb blocks into the generic blocks to get
gray coding working.


> All that aside, I think this is a good approach. GNU Radio currently has
> a lot of fantastic DЅP code; what I miss is more structure, and I'm glad
> to hear about the plans to refactor the digital stuff.
> Cheers,
> MB
> --
> Karlsruhe Institute of Technology (KIT)
> Communications Engineering Lab (CEL)
> Dipl.-Ing. Martin Braun
> Research Associate
> Kaiserstraße 12
> Building 05.01
> 76131 Karlsruhe
> Phone: +49 721 608-43790
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> KIT -- University of the State of Baden-Württemberg and
> National Laboratory of the Helmholtz Association
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