5 ms·

From the datasheet of the TI radio they're using[0], the maximum programmable output power is +12 dBm, which is about 16 milliwatts. There's no way you're goin

by ohazi 13y ago

From the datasheet of the TI radio they're using[0], the maximum programmable output power is +12 dBm, which is about 16 milliwatts. There's no way you're going to see the advertised range with the antenna pictured. For comparison, the wifi hardware in your laptop typically outputs around 100-200 milliwatts. Edit: TI appears to have a "range extender" part (i.e. a power amp) that can kick the output power up to +27 dBm (500 mW) at the band that they're using. I don't see any mention of this part on the kickstarter page, but this is the only plausible way I can see for them to get anywhere near the range advertised. Half a watt seems pretty high to me too, but apparently it's comfortably within FCC limits [1][2], and since ISM devices are designed for consumer use (as opposed to licensed use by a competent radio operator), I don't think the same rules about being clever with output power would still apply here. [0]: http://www.ti.com/lit/ds/symlink/cc1101.pdf http://www.ti.com/lit/ds/symlink/cc1101.pdf [1]: http://www.afar.net/tutorials/fcc-rules/ http://www.afar.net/tutorials/fcc-rules/ [2]: http://www.beagle-ears.com/lars/engineer/wireless/fccrules.htm http://www.beagle-ears.com/lars/engineer/wireless/fccrules.h...

TaylorAlexander 13y ago
Hah, thanks for the edit. Yeah we've already gotten the range. I've even checked the output of our boards with a radio expert (google Earl McCune, he's an advisor), and using his many expensive Agilent scopes we've experimentally verified that performance of my design is pretty much exactly on par with the reference designs, taking into account that we are using a custom designed RF balun [1] instead of the discretes they use on the reference design. Our RF Expert confirmed I had made a good choice with that, as the losses are tiny compared to ease of manufacturing it provides. [1] (I think it's this one, I'd have to check the part number, they have a couple similar ones). http://www.johansontechnology.com/datasheets/chipset-specific/0915BM15A0001.pdf http://www.johansontechnology.com/datasheets/chipset-specifi...
- Dwolb 13y ago
  Are you sure you're able to legally transmit at 1km. Had thought antenna gain + transmit power had to be around 0 dBm, at least for 434MHz. Hadn't seen the rules for 900MHz though.
  - smandou 13y ago
    First thing that crossed my mind too. It's definitely forbidden in Europe.
    - TaylorAlexander 13y ago
      Yup, but we can 868MHz in Europe, and with all the support from international backers I think we may end up doing that as a stretch goal! Just have to talk it our with our RF expert, which we're doing!
  - lutorm 13y ago
    The Xbee Pro 900 is 50mW, for reference.
  - ohazi 13y ago
    This is what I initially thought as well, having built an ISM band transceiver around the 350/400 MHz bands a few years ago. I recalled much lower power limits. (I had posted something along the lines of "there's no way in hell the FCC would let you transmit at half a watt" but have since edited). Apparently the 900 MHz bands are different, and the FCC has rules governing output power going into the antenna (1 watt), as well as an effective radiated power (which essentially places limits on your antenna gain) (4 watts). [1]: http://www.afar.net/tutorials/fcc-rules/ http://www.afar.net/tutorials/fcc-rules/
    - Dwolb 13y ago
      Wow that is very much radiated power! I remember range testing a 434MHz product and being hundreds of meters away (line-of-site). The product passed FCC testing. So a watt going into a 6dBi antenna... Flutter's measurements make sense now!
  - kbaker 13y ago
    I'm pretty sure that to get 1 Watt you have to use direct sequence or frequency-hopping spread spectrum under FCC part 15.247... Just sitting on a frequency as a 15.249 device or general ISM device you aren't allowed to use much power at all. See FCC part 15.247: http://www.gpo.gov/fdsys/pkg/CFR-2001-title47-vol1/pdf/CFR-2001-title47-vol1-sec15-247.pdf http://www.gpo.gov/fdsys/pkg/CFR-2001-title47-vol1/pdf/CFR-2... I work with UHF Gen2 RFID readers all day which do 1 Watt/30 dBm into 6 dBi = 4 Watts EIRP, but they are definitely required to do hopping. - EDIT: and after looking at the 50 mW 900 MHz XBee units, they are doing hopping as well according to their datasheet. So it looks like if you want to keep the range, you may want to start looking at hopping algorithms...
Dwolb 13y ago
Wifi operates at a higher frequency and data rate, which both reduce range.
- TaylorAlexander 13y ago
  exactly this ^
- ohazi 13y ago
  Sigh I was wondering whether I'd get called out for omitting this. You're right, of course. :-) Well, you're partially right (and it's a bit of a mess, which is why I left it out in the first place). Yes, increasing the data rate reduces range when all else is equal. The carrier frequency doesn't actually matter. The bandwidth does matter, but not in the way that you'd expect. Increasing your channel bandwidth actually increases your range when your receiver is noise limited (which is necessarily the case when you're making the range/bitrate trade-off). But the "power" term in the standard formulas assumes you're measuring the total power in your band (not carrier power). So you can't reap the benefit of doubling your bandwidth without also doubling your transmit power (unless you're using a true spread spectrum transceiver, which is a different beast). There are plenty of other factors, but when you're thinking about these things, you ultimately want to form an intuition for "what is the minimum energy per bit needed to make my receiver happy?" and "how much transmit power do I need to deliver that energy, given a specific antenna and desired range?" [1]. This analysis lets you ignore the specifics (modulation technique, multiple carriers, multiple antennas, spread spectrum, etc) that make it difficult to compare this TI radio and your 802.11n card, but still gives you a very accurate result. My original point (which still stands) is that you can't feasibly build a 1 kbps transceiver with 16 mW transmit power and a two inch monopole antenna and expect to get anywhere near half a km of range with the TI radio. It simply isn't sensitive enough. [1] http://en.wikipedia.org/wiki/Eb/N0 http://en.wikipedia.org/wiki/Eb/N0 (As an aside, Eb/N0 was one of those things that I found incredibly confusing when I first learned about it in a communication theory class - I recall several homework assignments where we had to solve for Eb/N0 symbolically for various theoretical transceivers. I only realized how useful it was after building real radios and plugging in actual numbers. Once you get the hang of it, it becomes a very good bullshit detector for "overly optimistic" range, bitrate, and power trade-offs that doesn't require spending hours in front of a simulator.)

by ohazi 13y ago

TaylorAlexander 13y ago

Hah, thanks for the edit. Yeah we've already gotten the range. I've even checked the output of our boards with a radio expert (google Earl McCune, he's an advisor), and using his many expensive Agilent scopes we've experimentally verified that performance of my design is pretty much exactly on par with the reference designs, taking into account that we are using a custom designed RF balun [1] instead of the discretes they use on the reference design. Our RF Expert confirmed I had made a good choice with that, as the losses are tiny compared to ease of manufacturing it provides. [1] (I think it's this one, I'd have to check the part number, they have a couple similar ones). http://www.johansontechnology.com/datasheets/chipset-specific/0915BM15A0001.pdf http://www.johansontechnology.com/datasheets/chipset-specifi...

Dwolb 13y ago
Are you sure you're able to legally transmit at 1km. Had thought antenna gain + transmit power had to be around 0 dBm, at least for 434MHz. Hadn't seen the rules for 900MHz though.
- smandou 13y ago
  First thing that crossed my mind too. It's definitely forbidden in Europe.
  - TaylorAlexander 13y ago
    Yup, but we can 868MHz in Europe, and with all the support from international backers I think we may end up doing that as a stretch goal! Just have to talk it our with our RF expert, which we're doing!
- lutorm 13y ago
  The Xbee Pro 900 is 50mW, for reference.
- ohazi 13y ago
  This is what I initially thought as well, having built an ISM band transceiver around the 350/400 MHz bands a few years ago. I recalled much lower power limits. (I had posted something along the lines of "there's no way in hell the FCC would let you transmit at half a watt" but have since edited). Apparently the 900 MHz bands are different, and the FCC has rules governing output power going into the antenna (1 watt), as well as an effective radiated power (which essentially places limits on your antenna gain) (4 watts). [1]: http://www.afar.net/tutorials/fcc-rules/ http://www.afar.net/tutorials/fcc-rules/
  - Dwolb 13y ago
    Wow that is very much radiated power! I remember range testing a 434MHz product and being hundreds of meters away (line-of-site). The product passed FCC testing. So a watt going into a 6dBi antenna... Flutter's measurements make sense now!
- kbaker 13y ago
  I'm pretty sure that to get 1 Watt you have to use direct sequence or frequency-hopping spread spectrum under FCC part 15.247... Just sitting on a frequency as a 15.249 device or general ISM device you aren't allowed to use much power at all. See FCC part 15.247: http://www.gpo.gov/fdsys/pkg/CFR-2001-title47-vol1/pdf/CFR-2001-title47-vol1-sec15-247.pdf http://www.gpo.gov/fdsys/pkg/CFR-2001-title47-vol1/pdf/CFR-2... I work with UHF Gen2 RFID readers all day which do 1 Watt/30 dBm into 6 dBi = 4 Watts EIRP, but they are definitely required to do hopping. - EDIT: and after looking at the 50 mW 900 MHz XBee units, they are doing hopping as well according to their datasheet. So it looks like if you want to keep the range, you may want to start looking at hopping algorithms...

Dwolb 13y ago

Wifi operates at a higher frequency and data rate, which both reduce range.

TaylorAlexander 13y ago
exactly this ^
ohazi 13y ago
Sigh I was wondering whether I'd get called out for omitting this. You're right, of course. :-) Well, you're partially right (and it's a bit of a mess, which is why I left it out in the first place). Yes, increasing the data rate reduces range when all else is equal. The carrier frequency doesn't actually matter. The bandwidth does matter, but not in the way that you'd expect. Increasing your channel bandwidth actually increases your range when your receiver is noise limited (which is necessarily the case when you're making the range/bitrate trade-off). But the "power" term in the standard formulas assumes you're measuring the total power in your band (not carrier power). So you can't reap the benefit of doubling your bandwidth without also doubling your transmit power (unless you're using a true spread spectrum transceiver, which is a different beast). There are plenty of other factors, but when you're thinking about these things, you ultimately want to form an intuition for "what is the minimum energy per bit needed to make my receiver happy?" and "how much transmit power do I need to deliver that energy, given a specific antenna and desired range?" [1]. This analysis lets you ignore the specifics (modulation technique, multiple carriers, multiple antennas, spread spectrum, etc) that make it difficult to compare this TI radio and your 802.11n card, but still gives you a very accurate result. My original point (which still stands) is that you can't feasibly build a 1 kbps transceiver with 16 mW transmit power and a two inch monopole antenna and expect to get anywhere near half a km of range with the TI radio. It simply isn't sensitive enough. [1] http://en.wikipedia.org/wiki/Eb/N0 http://en.wikipedia.org/wiki/Eb/N0 (As an aside, Eb/N0 was one of those things that I found incredibly confusing when I first learned about it in a communication theory class - I recall several homework assignments where we had to solve for Eb/N0 symbolically for various theoretical transceivers. I only realized how useful it was after building real radios and plugging in actual numbers. Once you get the hang of it, it becomes a very good bullshit detector for "overly optimistic" range, bitrate, and power trade-offs that doesn't require spending hours in front of a simulator.)