Re: [Discuss-gnuradio] constellation object

[Martin Braun]

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.

> 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.
- 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?

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)
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