Re: [Discuss-gnuradio] Calculating additive noise power for known signal

[Andy Walls]

On Wed, 2016-08-24 at 12:00 -0400, address@hidden
wrote:
> Date: Tue, 23 Aug 2016 16:42:33 -0700 (MST)
> From: Paul Creaser
> To: address@hidden

> In a real system the signal would be a preamble, which would normally be used
> for synchronization purposes at the receiver end (For example PLC system).
> 
> "I'm not quite sure what you mean by "cyclic noise", but the example you
> give is 50 Hz (or 60 Hz) hum, so a narrowband interference."
> 
> Cyclic noise, perhaps my understanding is incorrect. In a PLC system, the
> noise rejection system uses a Zero Cross detection to detect the beginning
> of a power cycle (50Hz/60Hz). During the cycle at the same phase point,
> noise repeatedly occurs. So on a scope you would see a nice sine wave for
> the power and noise which repeats at the same phase point in the power
> cycle.

Ah, now I understand: a periodic noise impulse.  So definitely not an
AWGN.


> This is different from the 50 Hz noise I suggested previously. However the
> idea/hope is that this noise is narrow band noise and because of its
> repetitive behaviour,

Impulsive noise is usually broadband.  Signals that are concentrated in
the time domain are spread out in the frequency domain, and vice versa
(this falls out from the Fourier transform).

A periodic, broadband noise impulse is essentially going bump up the
noise floor for a short time every 50th (or 60th) of a second.



> .i.e. occurring in the same point in the cycle, and
> relatively stable amplitude, it should be possible to remove it from a known
> signal.
> 
> I will take on board all the useful advice and continue my studies.
> 

So I have a couple of thoughts:

1. The LMS adaptive filter I suggested earlier won't do much good in the
face of broadband periodic noise impulses.  It still may help with
equalizing a poor channel, since PLC transmission lines are usually of
uncontrolled and unknown quality.

2. Your communication system should still use appropriate channel
filters and pulse matched filters, to mitigate the effects of broadband
noise that shows up outside your channel of interest.

3. HF systems are the only other systems I'm familiar with that deal
with impulsive noise.  HF communications systems often employ FEC and
interleavers to combat impulsive noise.

4. Adaptive noise cancellation techniques may help here, but I'm no
expert on them.  A quick Google turns up some decent material.

5. If you track the timing of the impulsive noise bursts, is it feasible
to have your system schedule its communications between the bursts? (I
don't know your topology.)  Think of it as a TDMA timeslot clock. :)
1/60th of a second at 9600 symbols/second is 160 symbols.

-Andy