5 ms·

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

by Dwolb 13y ago

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.)
- 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...

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

by 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.)

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...