[opendtv] IEEE 802.22

IEEE 802.22 is the new standard for using TV channels for broadband, over a 
wide area. It is now freely available at

http://standards.ieee.org/about/get/802/802.22.html

You can't fool mother nature, is the underlying message.

In the US, transmitter power for the base station and the consumer premises 
equipment (CPE) is limited to 4 W each. Which makes it a cellular system, not 
reaching anywhere close to the theoretical 100 Km (60 mile) max extended range 
limit of the MAC layer, clearly. In Canada, they allow up to 500 W for the base 
station. The document makes it obvious that what they are really targeting is a 
cellular system.

Up front, it says that a single base station can support up to 512 CPE devices. 
Figure 1 shows a use case with longer range transmissions, in low population 
density areas. With 64-QAM, it shows distances up to 15 Km (9.4 miles), but it 
isn't clear at what ERP. QPSK and 16-QAM are also supported. Quoting:

"A typical application can be the coverage of the rural area around a village, 
as illustrated in Figure 1, within a radius of 10 km to 30 km from the base 
station depending on its EIRP and antenna height."

Downstream transmissions can be unicast, multicast, or broadcast. TDM is used 
by the base station. Upstream has four optional contention control techniques. 
Clause 7 discusses the MAC.

The highest gross data rate, available with 64-QAM and 5/6 FEC, is 22.69 Mb/s. 
The options go from 4.54 Mb/s to 22.69 Mb/s, in a 6 MHz channel (Table 202).

Table 228 provides the C/N ratios available in different circumstances. In 
short, in a Gaussian channel, 64 QAM at 5/6 FEC is possible with 20.9 dB of 
C/N. In a "multipath channel," that increases to 40.4 dB C/N.

So, comparing this with 8T-VSB, Gaussian channel, they show that for 15.6 dB of 
C/N (close to the 15.1 or 15.2 often quoted for 8T-VSB), you need a 1/2 rate 
FEC, at 64-QAM. But that brings the gross data rate down to 13.61 Mb/s.

So I'll repeat what I thought previously. In an urban area, where you need to 
support high density wireless broadband, it's not at all clear to me that using 
the TV band 6 MHz channels buys you much. The data rates quoted above are 
aggregate, and they ain't all that impressive (how could they be?). In rural 
areas, sure. A few farmhouses miles apart, why not? And you also have more 
unused TV channels available.

Bert

 
 
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  • » [opendtv] IEEE 802.22 - Manfredi, Albert E