On 9/19/2013 9:09 AM, Andy Lee wrote:
The math bears that out, but nonetheless OFCOM are still
effectively proposing that *no* in-block transmissions are allowed
in that channel.
Same is true for the FCC. The FCC also assumes that "off" channels
are not used by WSDs.
And that means NO intentional radiation in the channel.
Specifying a low power level like -33 dBm is effectively the same
thing as specifying no in-band emissions there.
If you are reading this as "transmitted emissions in band should be
lower than -33dBm" then, no, this is not the same as "no emissions" per
FCC rules. If one were to intentionally transmit at -33dBm where no
intentional emissions are allowed, that would not be the same as "no"
under the rules.
Even if someone did try to put an in-band emission there (regardless
of how impractical that actually is), it would be too low to cause
interference to anyone and no harm has been done to the "RF environment".
Should be a little careful here for a couple reasons. I've designed and
shipped communication systems that operate quite well with transmit
power levels intentionally below -33dBm and do very useful things. It's
pretty easy to design a receive with sensitivity better than -96dBm, so
an intentional radiator at -33dBm near by will be noticed - at 900 MHz
your 10dB above sensitivity at 10m and at 54MHz you'd be 10 dB above
sensitivity at 100m, possibly. And when it comes to protecting their
spectrum, people always use the best propagation model to calculate
interfence impact on them :-). I've probably misunderstood something
badly, but -33dBm is certainly enough power to be noticed.
I have tried the "too low to be noticed" argument a few times in the
past in regulatory discussions, and incumbent and/or protected users
usually set the threshold closer to -120dBm and usually argue that even
at really low TX power, the "aggregate impact" of multiple devices in
the "RF Environment" is an unacceptable impact. We had trouble with
this argument when the intentional radiator TX limit was below the
maximum unintentional emission limits for most non-transmitting devices
(-41.3dBm).
So it would be fairly important to ensure a device does not interpret
-33dBm as it's ok to use that channel so long as it's TX power is below
-33dBm.
I may be off base here because of how y'all see the DB providing and
what I am expecting to support future needs. "protected users" includes
more than TV stations in the US.
Not sure that helps, but hope so.
The advantage of having an "well behaved" spectrum profile (contiguous
and without special values like -inf) is that the channel selection
algorithm on the WSDs becomes much simpler and logical. There's no
need to have lots of special boundary condition checks and code paths
that only get triggered under special circumstances. I'd like to
avoid ambiguous interpretations on the device side as much as possible.
Certainly ambiguous interpretations on the device side are a bad thing.
Some apps are aggregating multiple TV channels to get a logical channel
that that can carry a 'wide' data pipe. Other uses can fit many useful
channels in one TV channel. In 802.15.4m we divide the TV channel up
into multiple physical channels carrying low data rate signals. Right
now we assume a peak power level for the TV channel, and we get that
value with the start frequency and width (or end frequency) from the
database. However, I've heard it proposed to the FCC that we may get
different power limits different parts of the TV channel, changing over
time, as a protection mechanism for deal with narrow band protected
users while allowing effective use of the rest of the TV channel. For
what we do, this is a good thing - our physical channel may be less than
200kHz wide. There are a LOT of applications in IoT that need < 200kHz
channels, and blocking an entire TV channel to protect a wireless
microphone using < 200kHz of it means there are 28 other usable
channels not being used - not efficient use of the spectrum.
Hope this helps.
-Ben
Andy Lee | Google Inc. | [email protected]
<mailto:[email protected]> | 408-230-0522
On Thu, Sep 19, 2013 at 8:30 AM, Ray Bellis <[email protected]
<mailto:[email protected]>> wrote:
On 19 Sep 2013, at 16:21, Andy Lee <[email protected]
<mailto:[email protected]>>
wrote:
> This actually shows that Ofcom has picked a power threshold on
channel 38 of -33 dBm. If you look at the ACLR numbers for class
3, 4, and 5 devices, you'll see they are 10 dB apart from each
other and that they all work out to be -33 dBm on channel 38.
>
> The reason there are no limits for class 1 and 2 devices is
because the in-band power on channels 37 and 39 cannot exceed +36
dBm/8MHz, and therefore the power on channel 38 will naturally be
less than -33 dBm.
The math bears that out, but nonetheless OFCOM are still
effectively proposing that *no* in-block transmissions are allowed
in that channel.
Per OFCOM / ETSI device interface requirements it would be
incorrect for us to actually send that value of -33 dBm to a device.
Ray
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