[opendtv] Re: FCC chairman offers plan to save broadcasters - CNET
- From: Craig Birkmaier <craig@xxxxxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Mon, 14 Apr 2014 08:37:47 -0400
On Apr 14, 2014, at 4:35 AM, Albert Manfredi <albert.e.manfredi@xxxxxxxxxxx>
wrote:
>
> So, you think Harris folk are totally clueless? The simple fact is, a
> backhaul network for SFN transmitters CANNOT be as sloppy as IP. Or if it is,
> then it requires very careful GPS timing at each transmitter, to time each
> transmission precisely. We have been over all of this countless times, Craig,
> and you're still lost. SFNs are not like just having street lamps light up a
> parking lot, Craig.
The ATSC 3.0 companies are not clueless. They are behaving exactly as one would
expect, trying to create a standard that they can milk for royalties. There is
no money to be made -for them - with Broadcast LTE. Qualcomm is in a good
position to capitalize on this, as are the companies that are in the LTE patent
pool.
And to be fair, in many markets a single big stick with a few gap fillers is
probably the best solution. And then there is the minor issue of propagation
delay that you are failing to consider. They can't just take the signal from a
big stick and rebroadcast it via the SFN sites, as this will introduce a
significant delay that will look like multipath to receivers that can see both
the big stick and one or more SFN cells. I'm not familiar with how they are
dealing with this issue, but my guess is that they use a back haul network to
keep everything in sync.
>
> A clever person would have wondered, by now, why it is that all practical
> SFNs out there are basically built like this.
>
The folks at DVB are reasonably clever. Here is what they say on Wiki:
In order to achieve the same transmission time on all transmitters, the
transmission delay in the network providing the transport to the transmitters
needs to be considered. Since the delay from the originating site to the
transmitter varies, a system is needed to add delay on the output side such
that the signal reaches the transmitters at the same time. This is achieved by
the use of a special information inserted into the data stream called the
Mega-frame Initialization Packet (MIP) which is inserted using a special marker
in the MPEG-2 Transport Stream forming a mega-frame. The MIP is time-stamped in
the SFN adapter, as measured relative the PPS signal and counted in 100 ns
steps (period time of 10 MHz) with the maximum delay (programmed into the SFN
adapter) alongside. The SYNC adapter measures the MIP packet against its local
variant of PPS using the 10 MHz to measure the actual network delay and then
withholding the packets until the maximum delay is achieved.
The article goes on to say that there is about a five microsecond window with
DVB-T2 to work with, before the reception of multiple signals is beyond the
ability of the guard bands to compensate.
If you look at actual and proposed SFN deployments in Europe the SFN vary from
2-3 transmitters to dozens to cover a country the size of the UK. This is
nothing like Broadcast LTE; it is much more like what the Harris engineer was
proposing.
>> And the former Harris engineers did not say anything about hundreds of
>> small sticks.
>
> Inform yourself, Craig, and then do the freakin' math! Ask someone who knows
> these things, if you are unable to look it up. In SFN mode, to achieve x
> b/s/Hz, WHAT TOWER SPACING ARE WE TALKING ABOUT WITH LTE?
Tell us Bert, since you seem to know the Answer. Then explain why tower spacing
is so variable in telco LTE networks. Could it have something to do with the
population density and need for greater spectral reuse?
>
> Arguing from a point of ignorance is never convincing!
>
> Once you have the tower spacing, do the trig and figure out how many towers,
> to cover the market area. I spoonfed all of this to you at least 2 years ago.
> Now it's time you did this on your own, hoping perhaps that way it will
> stick. Otherwise, it's like a broken record. Can you not see why this is so
> aggravating?
There is no correct answer Bert. The area covered by a cell is proportional to
the power level and tower height; this is variable, not fixed, at least until
the propagation delay exceeds the slop provided by the guard bands, which can
also be variable.
This is going to be different in every market because of the size of the
market, the need to protect adjacent markets, the desire to create sub markets
with different content, and the degree of terrain blocking and multipath that
the designers are trying to deal with.
Regards
Craig
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