Channel Estimation During Data period

On 6 Aug, 10:41, "cpshah99" <cpsha...@rediffmail.com> wrote:
> >On 5 Aug, 19:52, Eric Jacobsen <eric.jacob...@ieee.org> wrote:
>
> >> When the channel coherence time gets so small that the channel changes
> >> substantially from symbol to symbol, generally all bets are off.
>
> >This is the reason why underwater communication is hard.
> >The channel characteristics can change substantially over
> >a small area, and over a short time.
>
> >Within a couple of meters, you can loose or gain yet another
> >multipath. Within minutes, the tide can change the depth
> >enough to signifcicantly change the multipath pattern.
>
> >>=A0That's
> >> a nearly impossible task unless you're willing to spend a LOT of power
> >> and bandwidth overhead on pilot tones and/or other mechanisms to
> >> facilitate faster adaptation.
>
> >Under water, you con't have much bandwidth. First of all,
> >absorption losses are proportional to frequency. Scattering
> >also becomes worse at high frequencies, so you want to
> >work in low frequency bands. But then geometric effects
> >come in to play, with normal mode interference from
> >both sea surface and the bottom.
>
> >And of course, the andwidth you do have, is contamicated
> >by more or less random interference caused by severe
> >multipath propagation.
>
> >Rune
>
> Thanks Rune
>
> I am glad somebody understands what I am doing... :-)

I didn't know you were into underwater comms?

Rune

>
>I didn't know you were into underwater comms?
>
>Rune
>

Since past two years I have been doing this. It is fascinating area
though

Chintan

On 6 Aug, 11:58, "cpshah99" <cpsha...@rediffmail.com> wrote:
> >I didn't know you were into underwater comms?
>
> >Rune
>
> Since past two years I have been doing this. It is fascinating area
> though

I don't know. I tend to like to work with solvable,
useful stuff. There is no doubt that a functioning
underwater comm system would be useful, but at sea
there is so much that varies both severly and fast.
I am not sure how solvable the task is, if the desired
ranges exceed a couple of water depths.

A couple of years ago I was at sea during a survey
of an underwater cable. The bathymetry was complicated,
so there were reference beacons deployed along this line.
The beacons were stands with transponder buoys. The
transponders emitted a response when they recieved a
query that contained some code individual to the
transponder. The response time and knowledge of
the beacon location was then used to improve the
nav data from the ROV.

There were a lot of such beacons but only so many
codes. So it was not uncommon to reuse the codes
from one end of a beacon array to the other. One
essentially relied on that the transmission channel
was so poor that only the closest beacon would
reply to any given query.

So one day at the daily meating the navigation guy
said they had to shut down transponders. All of a
sudden they had recieved responses from the far-end
beacons in addition to the close ones. This was a
bit puzzling, but no one could figure out why.

After the meating I went outside, only to find that
the sea was dead calm. And I mean *dead* calm. It's
probably the only time at sea that I have been able
to see the rings of insects landing on water from
50 m away.

The usual sea state includes *some* form of wave
ripple on the surface; if nothing else so at least
just enough to break a perfect reflection off the
surface. No such thing.

And that's the only explanation for the sudden
responses from the far transponders I can come up
with: The usual surface roughness, that messed up
the transmission path and scattered the signal
energy and broke up pulse coherence, was gone.

If a 2cm wave (or lack of same) is enough to perturb
the comm channel *that* much, the people who look at
these things are in for some serious problem-solving.

Rune

>On 6 Aug, 11:58, "cpshah99" <cpsha...@rediffmail.com> wrote:
>> >I didn't know you were into underwater comms?
>>
>> >Rune
>>
>> Since past two years I have been doing this. It is fascinating area
>> though
>
>I don't know. I tend to like to work with solvable,
>useful stuff. There is no doubt that a functioning
>underwater comm system would be useful, but at sea
>there is so much that varies both severly and fast.
>I am not sure how solvable the task is, if the desired
>ranges exceed a couple of water depths.
>
>A couple of years ago I was at sea during a survey
>of an underwater cable. The bathymetry was complicated,
>so there were reference beacons deployed along this line.
>The beacons were stands with transponder buoys. The
>transponders emitted a response when they recieved a
>query that contained some code individual to the
>transponder. The response time and knowledge of
>the beacon location was then used to improve the
>nav data from the ROV.
>
>There were a lot of such beacons but only so many
>codes. So it was not uncommon to reuse the codes
>from one end of a beacon array to the other. One
>essentially relied on that the transmission channel
>was so poor that only the closest beacon would
>reply to any given query.
>
>So one day at the daily meating the navigation guy
>said they had to shut down transponders. All of a
>sudden they had recieved responses from the far-end
>beacons in addition to the close ones. This was a
>bit puzzling, but no one could figure out why.
>
>After the meating I went outside, only to find that
>the sea was dead calm. And I mean *dead* calm. It's
>probably the only time at sea that I have been able
>to see the rings of insects landing on water from
>50 m away.
>
>The usual sea state includes *some* form of wave
>ripple on the surface; if nothing else so at least
>just enough to break a perfect reflection off the
>surface. No such thing.
>
>And that's the only explanation for the sudden
>responses from the far transponders I can come up
>with: The usual surface roughness, that messed up
>the transmission path and scattered the signal
>energy and broke up pulse coherence, was gone.
>
>If a 2cm wave (or lack of same) is enough to perturb
>the comm channel *that* much, the people who look at
>these things are in for some serious problem-solving.
>
>Rune
>

You are absolutely correct.

Recently we did some trials and I am trying to analyse the data. As I
managed to decode some packets with zero error, and some packets failed due
to poor synchronization and equalization.

And then I did channel estimation, so I found out that the real villain
was channel. I was present while we were recording the data and I knew that
this will be almost impossible task to decode. 

The thing about underwater comms is that all the worst case scenarios of
RF comms are common in underwater comms. Everything comes together because
the signal model is not narrowband anymore.

No wonder there are very few people working in this area.

Chintan

On 6 Aug, 12:43, "cpshah99" <cpsha...@rediffmail.com> wrote:

> And then I did channel estimation, so I found out that the real villain
> was channel. I was present while we were recording the data and I knew that
> this will be almost impossible task to decode.

The only chance I can see that *might* work is what you do:
Start with a good estimate for the channel, and then keep
updating it as the platforms move and time passes by.

Of course, this means that the Rx and the Tx need to be
very close at the start, so this is not a good way to
establish commuunications between arbitrary users.

And once the channel estimate deviates off track, you are
screwed.

> The thing about underwater comms is that all the worst case scenarios of
> RF comms are common in underwater comms.

If you are lucky, yes. Most of the time, conditions under
water are far worse at sea than anything EM comms will ever
expertience outside... I don't know what would come remotely
close. A turbulent plasma field?

> Everything comes together because
> the signal model is not narrowband anymore.
>
> No wonder there are very few people working in this area.

Food for thought...?

Rune

On 6 Aug, 14:58, Rune Allnor <all...@tele.ntnu.no> wrote:
> On 6 Aug, 12:43, "cpshah99" <cpsha...@rediffmail.com> wrote:

> > The thing about underwater comms is that all the worst case scenarios of
> > RF comms are common in underwater comms.
>
> If you are lucky, yes. Most of the time, conditions under
> water are far worse at sea than anything EM comms will ever
> expertience outside... I don't know what would come remotely
> close. A turbulent plasma field?

The best EM equivalent to the underwater comms problem
I can come up with, is:

A turbulent dielectric medium, filled with clouds of
point scatterers (as well random individual scatteres)
between two rough metal sheets that both move on a time
scale similar to the EM wave period and are less than
10 wavelengths apart.

Something like that.

Ah, yes, I almost forgot: Every mobile Tx or Rx can be
expected to move at speeds > 0.001 c.

Rune

>On 6 Aug, 14:58, Rune Allnor <all...@tele.ntnu.no> wrote:
>> On 6 Aug, 12:43, "cpshah99" <cpsha...@rediffmail.com> wrote:
>
>> > The thing about underwater comms is that all the worst case scenarios
of
>> > RF comms are common in underwater comms.
>>
>> If you are lucky, yes. Most of the time, conditions under
>> water are far worse at sea than anything EM comms will ever
>> expertience outside... I don't know what would come remotely
>> close. A turbulent plasma field?
>
>The best EM equivalent to the underwater comms problem
>I can come up with, is:
>
>A turbulent dielectric medium, filled with clouds of
>point scatterers (as well random individual scatteres)
>between two rough metal sheets that both move on a time
>scale similar to the EM wave period and are less than
>10 wavelengths apart.
>
>Something like that.
>
>Ah, yes, I almost forgot: Every mobile Tx or Rx can be
>expected to move at speeds > 0.001 c.
>
>Rune
>

I was thinking about supersonic comms....

And yes, the relative motion is another pain....

Chintan

Rune Allnor wrote:


> The best EM equivalent to the underwater comms problem
> I can come up with, is:
> 
> A turbulent dielectric medium, filled with clouds of
> point scatterers (as well random individual scatteres)
> between two rough metal sheets that both move on a time
> scale similar to the EM wave period and are less than
> 10 wavelengths apart.
> 
> Something like that.
> 
> Ah, yes, I almost forgot: Every mobile Tx or Rx can be
> expected to move at speeds > 0.001 c.

You forgot about the main problem: the unexperienced DSP programmer :-)


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

On 6 Aug, 16:52, Vladimir Vassilevsky <nos...@nowhere.com> wrote:
> Rune Allnor wrote:
> > The best EM equivalent to the underwater comms problem
> > I can come up with, is:
>
> > A turbulent dielectric medium, filled with clouds of
> > point scatterers (as well random individual scatteres)
> > between two rough metal sheets that both move on a time
> > scale similar to the EM wave period and are less than
> > 10 wavelengths apart.
>
> > Something like that.
>
> > Ah, yes, I almost forgot: Every mobile Tx or Rx can be
> > expected to move at speeds > 0.001 c.
>
> You forgot about the main problem: the unexperienced DSP programmer :-)

I would never say that's *not* a problem, but it is
hardly the main obstacle in this particular application.

Rune

>
>
>Rune Allnor wrote:
>
>
>> The best EM equivalent to the underwater comms problem
>> I can come up with, is:
>> 
>> A turbulent dielectric medium, filled with clouds of
>> point scatterers (as well random individual scatteres)
>> between two rough metal sheets that both move on a time
>> scale similar to the EM wave period and are less than
>> 10 wavelengths apart.
>> 
>> Something like that.
>> 
>> Ah, yes, I almost forgot: Every mobile Tx or Rx can be
>> expected to move at speeds > 0.001 c.
>
>You forgot about the main problem: the unexperienced DSP programmer :-)
>

HI Vlad

I assume you had a very bad day when u wrote this comment :-). 

Nevertheless, I take it as encouraging remark.

Chintan