4 ms·

It doesn't sound like you're familiar with the Friis equation [1], which shows how carrier frequency is inversely related to transmission distance. The receive

by Dwolb 13y ago

It doesn't sound like you're familiar with the Friis equation [1], which shows how carrier frequency is inversely related to transmission distance. The receivers are probably around -90 dBm sensitivity, which would place the system's range around a kilometer and would improve depending on their PA and LNA characteristics. [1] http://en.m.wikipedia.org/wiki/Friis_transmission_equation http://en.m.wikipedia.org/wiki/Friis_transmission_equation

ohazi 13y ago
Please. This is actually a common misconception about the Friis equation. It assumes an isotropic antenna with an effective area that is dependent on lambda. But physical antennas have an effective area that varies with lambda squared (obvious on something like a horn or a dish with a real 2-D aperture, but even simpler antennas behave this way). It turns out that this perfectly cancels out the frequency dependent term in the equation when the transmitter and receiver use the same antenna, leaving you with only the reduction in power density as you move away from the transmitter. I had a professor give an epic lecture about this several years ago, but this is the best I could find on short notice [1]. As it turns out, there is a real source of frequency dependence in your path loss, which is the atmosphere. Your link budget calculation is really just an annoying geometry problem. Conservation of energy still holds, so you should be immediately suspicious when the naively applied Friis equation (which does not take atmospheric effects into account) makes received energy vanish just because you cranked up the frequency. Just as you should be suspicious when you receive more energy than you transmit when you're in the antenna's near field. Always know what your approximations imply. The receivers are actually a bit more sensitive than that (the TI chip is ~-120 dBm at the target frequency; the whole package could be close to -114 dBm if designed well). The claim I made was that you can't get half a km on 16 milliwatts. I cited 16 milliwatts because it's the max transmit power of the TI chip alone. You cannot make this go half a km with the antennas shown on the kickstarter page. You'd either need more power, or one of the antennas would need to be a dish. Even in an unreasonably quiet environment, you'd run out of energy at the receiver way before running into frequency or bitrate dependent effects. But it turns out that 16 milliwatts is not the output power used. TI makes an amplifier that increases the output power to half a watt. In this case you'll easily get the half-km range. [1]: http://www.dslreports.com/forum/r24210307-Reconciling-the-Friis-Equation http://www.dslreports.com/forum/r24210307-Reconciling-the-Fr...
- Dwolb 13y ago
  You're right we're speaking a bit tangentially. Hadn't heard of the Friis argument that frequency doesn't matter as long as antennas on RX and TX are the same. I'm not sure I understand it yet. Nevertheless, let's get some numbers on the page to see if I'm misunderstanding the subject. Here's a rough link budget with some estimates TX/RX antenna gain is around 0dBi (seems reasonable [1]) TX power = 16mW = 12dBm [2] RX sensitivity = -90dBm (personally, better than -100dBm seems aggressive) Roughly, our link budget formula can be... Path loss = TX power + TX gain + RX gain - Rx power [3] Path loss = 102dB Ideal free space loss over 1km at 900MHz is around 91dB [4]. Therefore, we have about 10dB margin in our link budget to transmit 1km. We also have a few dB margin in the antenna and receive sensitivity estimate. [1] http://store.rfdesign.com.au/antenna-900mhz-2dbi-monopole-rpsma/ http://store.rfdesign.com.au/antenna-900mhz-2dbi-monopole-rp... [2] http://www.cpcstech.com/dbm-to-watt-conversion-information.htm http://www.cpcstech.com/dbm-to-watt-conversion-information.h... [3] http://skypilot.trilliantinc.com/pdf/link_budget.pdf http://skypilot.trilliantinc.com/pdf/link_budget.pdf [4] http://en.wikipedia.org/wiki/Path_loss http://en.wikipedia.org/wiki/Path_loss
  - lpmay 13y ago
    Take this as the kind of half-cooked thought it is at this point (long day, about to go to bed and don't want to crack open the textbook), but the fact that your path loss leaves 10dBm of power above your Rx sensitivity doesn't mean you have 10dBm of link margin. I'm not completely sure, but I'd guess it means you have 10dB of SNR (theres a units from dBm to dB thing I haven't completely resolved in my head). Whether that makes your link work or not depends on the bitrate, modulation you choose and BER you can accept (and etc). If your modulation scheme needs 9dB of SNR (proportional to Eb/N0) to operate at an acceptable BER, your system may only have 1dB of actual link margin.
    - Dwolb 13y ago
      We're circling around some good stuff. Night though, thanks for the information to think about.
  - ohazi 13y ago

by Dwolb 13y ago

ohazi 13y ago

Please. This is actually a common misconception about the Friis equation. It assumes an isotropic antenna with an effective area that is dependent on lambda. But physical antennas have an effective area that varies with lambda squared (obvious on something like a horn or a dish with a real 2-D aperture, but even simpler antennas behave this way). It turns out that this perfectly cancels out the frequency dependent term in the equation when the transmitter and receiver use the same antenna, leaving you with only the reduction in power density as you move away from the transmitter. I had a professor give an epic lecture about this several years ago, but this is the best I could find on short notice [1]. As it turns out, there is a real source of frequency dependence in your path loss, which is the atmosphere. Your link budget calculation is really just an annoying geometry problem. Conservation of energy still holds, so you should be immediately suspicious when the naively applied Friis equation (which does not take atmospheric effects into account) makes received energy vanish just because you cranked up the frequency. Just as you should be suspicious when you receive more energy than you transmit when you're in the antenna's near field. Always know what your approximations imply. The receivers are actually a bit more sensitive than that (the TI chip is ~-120 dBm at the target frequency; the whole package could be close to -114 dBm if designed well). The claim I made was that you can't get half a km on 16 milliwatts. I cited 16 milliwatts because it's the max transmit power of the TI chip alone. You cannot make this go half a km with the antennas shown on the kickstarter page. You'd either need more power, or one of the antennas would need to be a dish. Even in an unreasonably quiet environment, you'd run out of energy at the receiver way before running into frequency or bitrate dependent effects. But it turns out that 16 milliwatts is not the output power used. TI makes an amplifier that increases the output power to half a watt. In this case you'll easily get the half-km range. [1]: http://www.dslreports.com/forum/r24210307-Reconciling-the-Friis-Equation http://www.dslreports.com/forum/r24210307-Reconciling-the-Fr...

Dwolb 13y ago
You're right we're speaking a bit tangentially. Hadn't heard of the Friis argument that frequency doesn't matter as long as antennas on RX and TX are the same. I'm not sure I understand it yet. Nevertheless, let's get some numbers on the page to see if I'm misunderstanding the subject. Here's a rough link budget with some estimates TX/RX antenna gain is around 0dBi (seems reasonable [1]) TX power = 16mW = 12dBm [2] RX sensitivity = -90dBm (personally, better than -100dBm seems aggressive) Roughly, our link budget formula can be... Path loss = TX power + TX gain + RX gain - Rx power [3] Path loss = 102dB Ideal free space loss over 1km at 900MHz is around 91dB [4]. Therefore, we have about 10dB margin in our link budget to transmit 1km. We also have a few dB margin in the antenna and receive sensitivity estimate. [1] http://store.rfdesign.com.au/antenna-900mhz-2dbi-monopole-rpsma/ http://store.rfdesign.com.au/antenna-900mhz-2dbi-monopole-rp... [2] http://www.cpcstech.com/dbm-to-watt-conversion-information.htm http://www.cpcstech.com/dbm-to-watt-conversion-information.h... [3] http://skypilot.trilliantinc.com/pdf/link_budget.pdf http://skypilot.trilliantinc.com/pdf/link_budget.pdf [4] http://en.wikipedia.org/wiki/Path_loss http://en.wikipedia.org/wiki/Path_loss
- lpmay 13y ago
  Take this as the kind of half-cooked thought it is at this point (long day, about to go to bed and don't want to crack open the textbook), but the fact that your path loss leaves 10dBm of power above your Rx sensitivity doesn't mean you have 10dBm of link margin. I'm not completely sure, but I'd guess it means you have 10dB of SNR (theres a units from dBm to dB thing I haven't completely resolved in my head). Whether that makes your link work or not depends on the bitrate, modulation you choose and BER you can accept (and etc). If your modulation scheme needs 9dB of SNR (proportional to Eb/N0) to operate at an acceptable BER, your system may only have 1dB of actual link margin.
  - Dwolb 13y ago
    We're circling around some good stuff. Night though, thanks for the information to think about.
- ohazi 13y ago