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more on "a bit outrageous" reply from Tropos on 1 more on Limits on wireless le ave U.S. at risk
From: David Farber <dave () farber net>
Date: Tue, 18 Oct 2005 18:51:52 -0400
Begin forwarded message: From: Jim Thompson <jim () netgate com> Date: October 18, 2005 3:19:24 PM EDT To: dave () farber net Cc: ron.sege () troposnetworks com, "David P. Reed" <dpreed () reed com>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 () troposnetworks com 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 totheir 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 had200 nodes previously installed in high-crime areas of NO doing videosurveillance. 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 andSkype are active on the networks.The CIO of NO is actually in DC today testifying about the benefits of Wi-Fimesh. 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 faxThe leading supplier of products for building true metro-scale Wi- Fi meshnetworks. -----Original Message----- From: David P. Reed [mailto:dpreed () reed com] Sent: Monday, October 17, 2005 5:09 PM To: dave () farber net Cc: Ip Ip; ron.sege () troposnetworks com 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, Ido know that Tropos and others who have such technology were attempting to demonstrate the value of their systems post-Katrina, so there almostcertainly 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.------------------------------------- You are subscribed as jim () netgate com To manage your subscription, go to http://v2.listbox.com/member/?listname=ipArchives at: http://www.interesting-people.org/archives/interesting- people/
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