[opendtv] Re: Seeing Ghosts on a Single Frequency Network

From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Date: Tue, 15 Feb 2011 14:20:35 -0500
This is the killer reception condition, which is less likely to obtain if
the two signals experience considerable multipath.  Strong multipath thatis
different in the two signals may allow COFDM reception.

Consider now how to cope with this killer reception condition.  If the
receiver is not moving, one carefully locates the simple reception antenna
where received signal strength is close to maximum.  If the receiver is in a
vehicle, a complex antenna system can be used to secure adequate received
signal strength whether or not the receiver is moving.

Receivers hand-held by a pedestrian walking in a direction away from one
transmitter and towards the other fare very badly in an SFN, however.  An
MFN is better for these receivers, as well as making antenna systems for
vehicles simpler.  The cost for the better reception the MFN can provide is
a more complex receiver, of course, and the antenna has to be broadband.

Al

----- Original Message ----- 
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Friday, February 11, 2011 5:36 PM
Subject: [opendtv] Re: Seeing Ghosts on a Single Frequency Network


> John Shutt wrote:
>
> > Charlie's tests (as described in the article referred to in
> > the subject line,) was for a single 10 uS echo 0.5 dB down
> > from the main signal.
> >
> > According to Charlie, a 10 uS echo equates to a 6 mile
> > difference in signal path length between the two sources in
> > an SFN.  So you have to be 6 miles closer to one transmitter
> > than the other transmitter to see a 10 uS echo. If you are
> > equidistant to both towers, the echo delay would be 0 uS.
> > Since from that equidistant point you could go 6 miles
> > closer to antenna A, or you could go 6 miles closer to
> > antenna B, a 10 uS echo tolerance gives you 12 miles of
> > overlap between transmitters in your SFN to play with.
>
> All due respect to Charles Rhodes, when I read that, I got a really
uncomfortable feeling that this was way too optimistic and that readers
would misinterpret what it means about the viability of SFNs. But in the
end, his essential message was, even with these optimistic models, it still
won't work reliably. Of course, now the dangling bit left out is, is there a
magic bullet?
>
> Here's a simpler way to look at it, and way more real-world.
>
> The speed of light is 3E8 meters/sec in free space, slightly slower
through the atmosphere. Therefore, light requires 3.33 usec to travel 1 Km,
or 5.36 usec to travel one mile.
>
> Therefore, if two towers are 12 miles apart, *and* you time-align the
transmission of symbols from the two towers (which would not be required in
a MFN, btw), the signal from tower A reaches tower B 64.3 usec delayed,
compared with the symbol just now being transmitted by Tower B.
>
> If you DO NOT time-align the transmission of symbols from the towers, and
Tower B is a passive DOCR, then the symbol from Tower B will reach back to
Tower A with a delay of 128.6 usec, + any additonal delay in the DOCR
electronics, compared to the time when Tower A originally transmitted its
symbol.
>
> What does this mean in practice? It means that in an urban area, where
buildings exist and signals to the closest tower may very well be
attenuated, if you are hoping for reliable reception *at all*, you have to
have receivers that can accommodate those echo delays.
>
> Same applies to more rural areas, by the way, if there are hills or other
obstructions that may attenuate signals from the closest tower, while not
attenuating as much the signal from a more distant tower. And the insistence
that these SFN towers be small and low (e.g. low on the rooftop of downtown
high rise buildings, or low towers similar to cellular towers in rural
settings) only makes the problem that much more acute. The closest tower
would then be much more likely to be attenuated.
>
> COFDM receivers have low tech equalizers, so you have to rely on the GI to
prevent inter-symbol interference. In COFDM 8K mode, 6 MHz channels, this
means that towers 12 miles apart can be accommodated safely with a 1/16 GI
if time-aligned, or 1/8 GI if not time aligned. Check out the Euro SFNs, and
you will see that this is how they are set up. That's why Berlin uses 1/8
GI, and Rome uses 1/4 GI.
>
> With 8-VSB, and towers 12 miles apart, you MUST time-align the towers, and
even then, the Samsung Gemini is the only receiver that can hope for decent
reception, assuming only these two towers exist and that the closest tower
is not attenuated. (No 8-VSB receiver I've seen measured to date works well
with 60 usec of stong pre-echo, not even the Gemini.)
>
> > With two big stick transmitters of equal power, spaced 50
> > miles apart, the overlap area would be from 19 miles away
> > from transmitter A to 31 miles away from transmitter A,
> > which would correspond to the inverse distances from
> > transmitter B.
>
> Now, surely you know that is unrealistic, right? The overlap area will
change according to obstructions, according also to weather conditions,
according to tree limbs swaying in the breeze, and so forth. That's why the
model Rhodes used was unrealistically optimistic.
>
> > Oh, and Charlie conveniently left out multiple echoes and
> > complex echoes in his ATSC testing.  The more complex the
> > echo ensemble, the better a multi-carrier scheme with long
> > guard interval performs compared to a single carrier scheme
> > with a complex receiver equalizer.
>
> As I expressed to Mark Aitken, I think, the article may have been
interpreted by readers that there's a magic bullet, aka COFDM, that would
solve the problems. There isn't. COFDM does not solve the problem, as proven
conclusively by Qualcomm MediFLO, and also by the Ibiquity testing of the
two-tower radio SFN in Baltimore.
>
> > And if the ultimate goal of this entire exercise was not to
> > provide seamless mobile service, but instead to simply provide
> > uniform coverage to fixed receivers, then I agree with you
> > that an SFN isn't necessary.
>
> John, a MFN avoids all of the problems we discussed here, even for mobile
service. And there's another aspect that hasn't been mentioned, and is long
overdue. You CANNOT keep increasing the COFDM subcarriers, in the hope of
creating longer GIs, if you want to optimize this for mobile service. Why?
Because you cannot slow down the symbols indefinitely. A moving vehicle
cannot see these slow symbols becoming warped while they are being received,
and expect good reception. So in fact, your idea of 12 MHz is a good idea,
because it would allow doubling the number of subcarriers WITHOUT slowing
down the symbols, compared to 6 MHz, so you'd see longer duration GIs and
decent mobile reception. But, of course, now you have to ask yourself, why
not just go MFN.
>
> Bert
>
>
> ----------------------------------------------------------------------
> You can UNSUBSCRIBE from the OpenDTV list in two ways:
>
> - Using the UNSUBSCRIBE command in your user configuration settings at
FreeLists.org
>
> - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word
unsubscribe in the subject line.
>

 
 
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at 
FreeLists.org 

- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.
References:
- [opendtv] Re: Seeing Ghosts on a Single Frequency Network
  - From: Manfredi, Albert E
[opendtv] Re: Seeing Ghosts on a Single Frequency Network

Other related posts:
