[IP] more on "a bit outrageous" reply from Tropos on 1 more on Limits on wireless le ave U.S. at risk
Begin forwarded message:
From: Jim Thompson <jim@xxxxxxxxxxx>
Date: October 18, 2005 3:19:24 PM EDT
To: dave@xxxxxxxxxx
Cc: ron.sege@xxxxxxxxxxxxxxxxxx, "David P. Reed" <dpreed@xxxxxxxx>
Subject: Re: [IP] reply from Tropos on 1 more on Limits on wireless
le ave U.S. at risk
Dave,
Love the Tropos "ad", but the claims are a bit outrageous.
Whilte Tropos may claim EIRP as "power", (many WiFi vendors do), the
FCC part 15 rules used for WiFi limit non point-to-point applications
in the 2.4GHz ISM band to 30dBm of tx power into 6dBi of antenna
gain. Any additional antenna gain must be compensated by an equal
reduction in transmit power. It is also *highly* unlikely that this
"high power" is available at any data rate over, say, 24Mbps, given
the linearity (IP3) requirements due to PAPR issues at the higher
OFDM modulation rates, especially when Tropos claims 28W typical
power consumption.
The Tropos FCC filings tend to bear this out, as the maximum transmit
power (into the "7.4dBi" antenna) is specified as 28.54dBm (714mW)
for 802.11b modulation and 27.16dBm (520mW) for 802.11g modulation.
The Tropos test report to the FCC doesn't state which modulation rate
was used during the test, but they did have to drop 1.4dBm in order
to satisfy the FCC's out of band emissions requirement, presumably at
6Mbps. It would be interesting to know what Tropos claims for tx
power at 36, 48 and 54Mbps while keeping the 5210 in compliance with
FCC rules.
However, transmit power aside, what really bugs me is that I'm left
at ends trying to figure out where they managed to get a real world
radio technology with "-100dBm" receive sensitivity. A quick look
at the Tropos 5210 datasheet http://www.tropos.com/pdf/
5210_datasheet.pdf shows that this claim is for 1Mbps modulation, and
that what is claimed for rx sensitivity at 11Mbps is -92dBm. I'll
show that both of these values are improbable.
Theoretical receive sensitivity can be calculated as:
Receive sensitivity = Nt + Ns + SNRmin
where Nt is the thermal noise floor, Ns is the system noise
figure,and SNRmin is the minimum SNR required for a given bit-error
rate using the desired modulation.
802.11b's 11Mbps CCK requires a information theoretic SNR of about
7.8 dB to decode frames with a FER of 0.08. (We'll leave SIR and/or
SINR out of the discussion unless someone insists.) We'll call this
SNRmin.
All matter at temperatures above absolute zero (0K, about -460F)
radiates electromagnetic energy. The amount of energy is related to
temperature -- the hotter the matter, the more energy is radiated.
This energy is described by Boltzmann's Constant, 'k' (k =
-198.6dBm/degreesK-Hz). This constant, multiplied by the temperature
of the matter a receiver views and the system bandwidth, yields an
irreducible background noise against which a desired signal must
compete. This is thermal noise.
For the purpose of this exercise, lets use 80F, or about 300K
(T=300K). RF engineers typically use this number as a rule of thumb,
and its
roughly the year-round temp here in Hawaii. :-) Its probably not
far off the daytime temp in NOLA, either.
802.11b uses a channel 22Mhz wide.
Tn(dBm) = -198.6 + 10 * log10(300) + 10 * log10(22,000,000)
= -100.4dBm.
Receive sensitivity = Nt + Ns + 10log(BW) + SNRmin
-92dBm = -100.4dBm + Ns + 7.8 dB
You can see we're in-trouble already. (Or my math is out to lunch,
and someone will correct me.)
The thermal noise floor that the receiver 'sees' is at -100.4dBm, we
require 7.8dBm of SNR (for a perfect CCK receiver at 11Mbps in the
absence of any co or alternate channel interference), and Tropos has
promised us -92dBm rx sensitivity at 11Mbps. This leaves us with
0.6 dB for a total system noise figure for the LNA and the rest of
the receiver chain, which I find ... unlikely.
Dropping down to the BPSK modulation used for 802.11's 1Mbps DSSS,
the required SNR for a 0.08 PER is reduced to 0.2, but we still have
trouble meeting the Tropos spec, as
Receive sensitivity = Nt + Ns + 10log(BW) + SNRmin
-100dBm = -100.4dBm + Ns + 0.2 dB
and now we have a mere 0.2 dB for our system noise figure.
I'll leave it as an exercise for Tropos to fill in the noise figure
for the LNA and receiver on their "5210" designs. They'll be
listed on the datasheet(s) for the part(s) used. I find it
difficult to believe that they can meet their advertised spec.
Note that any additional noise or interference (i.e. "real world
conditions" will significantly impact the SNR required for successful
demodulation.
I have a suspicion about which 802.11 chipset is used in the Tropos
5210. If my suspicions are correct, the figures Tropos quotes are the
chipset vendor's figures *at the chipset inputs*. Now, these are
excellent specs (for the chipset), but any *system* design will have
additional sources of noise, and these appear to have been discounted
away by the Tropos marketing department.
I could continue about the throughput achievable with a 10 hop
network diameter using the 802.11 MAC, but Dr. Reed is more qualified
than I to discuss that and similar points.
jim
On Oct 18, 2005, at 6:07 AM, David Farber wrote:
_______________ Forward Header _______________
Subject: RE: [IP] more on Limits on wireless leave U.S. at risk
Author: ron.sege@xxxxxxxxxxxxxxxxxx
Date: 18th October 2005 6:09:16 am
Dave,
Tropos has shipped a couple of hundred of our Tropos 5210 mesh
routers into
MS and LA in the days following the storm, and had a few hundred
installed
in the stricken area previously. These are high-power (36 dBm),
high rx
sensitivity (-100 dBm), outdoor-constructed 802.11b/g access points
with
embedded mesh routers so they can backhaul wirelessly amongst each
other to
a source of Internet connectivity. Each has a 1,000 ft plus range
to an
outdoor Wi-Fi device, emergency vehicle with external antenna or
building
with a window-mounted CPE. So, a couple of hundred nodes
represents 10-15
sq mi or so of contiguous coverage in typical configuration. Every
10 nodes
or so are fed with a Motorola Canopy "WiMAX" link, typically shot
from the
roof of an MCI PoP, or from city backhaul locations. These devices,
at these
densities, are non line of sight so can be installed by city
workers with
bucket trucks on street lamps, with power taken from street-light
photo
cells. They will self-configure, find their backhaul, optimize
throughput
and route around problems. They can be battery and solar-powered
due to
their low wattage (28 watts or so).
Last I have heard, we were in 25 or so FEMA and Red Cross shelters
in NO,
Biloxi, Lamar-Dixon and Baton Rouge. We are around the NO airport
and on a
couple of cruise ships off the gulf that are housing FEMA workers.
We had
200 nodes previously installed in high-crime areas of NO doing video
surveillance. As the power has been restored to the street lights,
these
nodes have come back up on their own and are performing their
functions
again. We are now in the process of expanding that network as a
"force
multiplier" for the police. Data applications as well as Vonage
phones and
Skype are active on the networks.
The CIO of NO is actually in DC today testifying about the benefits
of Wi-Fi
mesh.
Hope that helps. You can see more on our technology at www.tropos.com
Ron Sege
President and CEO
Tropos Networks
555 Del Rey Ave
Sunnyvale, CA 94085
www.tropos.com
408-331-6810 office
650-861-7564 cell
617-407-5000 international cell
408-331-6530 fax
The leading supplier of products for building true metro-scale Wi-
Fi mesh
networks.
-----Original Message-----
From: David P. Reed [mailto:dpreed@xxxxxxxx]
Sent: Monday, October 17, 2005 5:09 PM
To: dave@xxxxxxxxxx
Cc: Ip Ip; ron.sege@xxxxxxxxxxxxxxxxxx
Subject: Re: [IP] more on Limits on wireless leave U.S. at risk
Gerry Faulhaber wrote:
Reed claims firms were offering WiMax and WiFi mesh networks for
first responders in the wake of Katrina and Rita. He also mentions
the role of municipal WiFi in this effort. Coulda happened, but it
seems wildly unlikely. Is there any proof of this?
I'm a bit skeptical about Reed Hundt's broad claims, too. However, I
do know that Tropos and others who have such technology were
attempting
to demonstrate the value of their systems post-Katrina, so there
almost
certainly was some deployment, given the value to the companies of the
opportunity to show their stuff.
I've cc'ed Ron Sege of Tropos, who may have more direct knowledge
and data.
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