[opendtv] Re: Doug Lung series on OFDM

  • From: Craig Birkmaier <craig@xxxxxxxxx>
  • To: opendtv@xxxxxxxxxxxxx
  • Date: Wed, 14 Nov 2012 08:00:29 -0500

At 8:59 PM -0600 11/13/12, Manfredi, Albert E wrote:
BTW, certainly Doug has a point that use of COFDM would be better for mobile reception of TV than even ATSC-MH, but there is no reason at all to expect that the wireless ISPs would allow their blessed handsets to have the needed tuner anyway. And, users of handhelds, while they are on the run, don't really seem to want "by appointment" TV anyway.

Yes, the issue of compatibility with tables and smart phones is very real. But the reality is that broadcaster have little if anything to offer today. It is relatively easy now to download programs to these devices for consumption while mobile. In some cases a bit of planning is required, in others, desired content can be cached automatically. And for short clip based content the existing telco networks seem to be working. It is live content - especially news and sports - that present the greatest opportunities for broadcasters, and here, their spectrum, and an appropriate transmission infrastructure, are the key

Another point: did you notice how mobile reception suffers when the COFDM mode goes to fewer subcarriers? Which means, a scheme optimized for mobility will not be optimized for static indoor reception. You actually lose some resistance to multipath if you want to support reception at freeway speeds. Not good for indoor reception with handheld devices, in other words. We already discussed this some time ago. Not that conducive to a single RF infrastructure for all wireless TV.
 There were two paragraphs in Doug's series on COFDM that I found interesting:

The downside of the larger FFT is that as carrier spacing decreases, Doppler shift becomes more of a problem. The impact of Doppler shift is proportional to the carrier frequency, so VHF will be less impacted by it than UHF. Narrower channels (6 MHz in the United States versus 8 MHz in Europe) cram the carriers closer together, making the impact of Doppler worse.

Mike Simon of Sinclair Broadcast Group did an analysis of the impact of Doppler on different FFT sizes in a 6 MHz channel and found that at 600 MHz the Doppler shift became a problem over 95 mph with an 8K FFT and dropped to 47 mph with a 16K FFT. With 8 MHz of channel bandwidth, the speeds increase to 126 mph and 63 mph, respectively.

This makes me wonder if the VHF spectrum may be better for LTE based mobile applications, with the UHF spectrum used for COFDM one way broadcast?

And it is clear that broadcasters would benefit from pooling their remaining spectrum so that larger channels (8 MHz or more) could be used to improve the overall performance and efficiency of the system.

I would love to see a discussion about this with our expects who are actually working on the next TV standard.

Perhaps OTA station groups can seriously think about morphing into wireless ISPs, and also become TV OTT Internet sites.

The latter is already happening. I'm not sure that the FCC would allow the former.


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