[opendtv] Re: SFN tower spacing and mobility
- From: Albert Manfredi <albert.e.manfredi@xxxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Sat, 9 May 2015 18:32:18 -0400
Craig wrote:
Why are you limiting the discussion to 6 or 8 MHz channels?
Makes no difference. No matter the width of the channel, what counts is the
duration of the guard interval. In a much wider channel than 6 MHz, you can of
course fit more subcarriers, but what matters to mobility is the distance
between them.
So for example, 2K mode in a 6 MHz channel is about the same as a potential 6K
mode in a 20 MHz channel, in terms of subcarrier spacing and consequent tower
spacing.
And the proof is in the pudding. As I showed you, the tower spacing for 2K mode
in a 6 MHz channel is, in fact, only slightly better than what LTE stipulates,
for approximately equal spectral efficiency. Channel width doesn't change the
speed of light.
This is the only thing I saw in the presentation that goes beyond existing
LTE
Advanced as it relates to larger cells and/or "big sticks."
“Roof-top” reception in RURAL areas
• Higher user antenna height enables coverage even with low site density
• Longer cyclic prefix needed to support large site radius (standard change1)
Right. And that means that in the 2011 paper you have, on use of LTE for TV
broadcast, they aren't even assuming these important adjustments.
Also, keep in mind that "longer CP" is easier said than done, if you are
optimized for mobility. With very short duration symbols, you don't have the
luxury of introducing long GIs, without really eating into spectral efficiency.
For example, the DVB-T1 SFNs in Rome, Berlin, and Paris, all use 1/8 GI in 8K
mode, to keep the towers reasonably far apart. In terms of GI duration (which
is all that matters for tower spacing), this is equivalent to a 1/2 GI in 2K
mode. Which says right there that a smallish hit in spectral efficiency to
support SFNs in 8K mode becomes a 50% reduction in spectral efficiency to
support the same SFN configuration, in 2K mode.
The bottom line is that you pay a real price to support good mobility. LTE is
optimized for mobility. Whereas DVB-T2 can be adjusted for more or less
mobility support.
Bert
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