4 ms·

Just curious – why is range limited by a protocol? Why couldn't you send Bluetooth over hundreds of miles with a big enough antenna, for example?

by 0898 11y ago

blhack 11y ago
Not limited by the protocol per se, but limited by the design requirements of BLE. BLE; you're talking about power requirements of ~10mA transmit (current consumption), and averages in the <10μA range. That's the whole point of the tech. It's stuff that can run for years on a watch battery. Now: how LORA is accomplishing such crazy long ranges when supposedly consuming similar amounts of current goes a little beyond my understanding of RF. There are a few people commenting in this thread that seem to have the knowledge, though!
- makomk 11y ago
  They're achieving those ranges by being very, very slow. It's probably easiest to explain how this works with conventional narrowband digital modulation schemes. For a typical modulation scheme, the bandwidth of the modulated signal is proportional to the bitrate. There's also some minimum signal-to-noise ratio below which the receiver can't decode it. By sending more slowly, you get a narrower signal, which means that the receiver can listen to a narrower slice of spectrum. Even though the received signal's no more powerful than before, because the receiver is no longer hearing all the noise outside of that narrower band the SNR improves and it can decode much weaker signals. LORA appears to use direct-sequence spread spectrum, which basically means that the transmitter applies a spreading code to convert the narrow signal to a very wide one and the receiver uses the same code to despread it again and pick out the signal. Its range improvement is still based on the exact same principle of speaking very slowly to improve your SNR though.
slewis 11y ago
You can amplify a signal and send it far. But due to FCC regulation in the 900MHz band for example you can only instantaneously transmit at 30W. Another way to make a signal go further is to transmit at a lower data rate (thus spreading your signal out over time so that in effect you have more power in the signal). The FCC limits "dwell time" in a single channel to .4s in the 900MHz band. Yet another way to increase range is to increase your receive sensitivity. Lora's Chirp Spread Spectrum coding actually allows signals to be received below the noise floor. You can liken this to decrypting data: to an observer the signal looks like noise, but if you know how to look within the noise you can pull the actual coded information out. All of these LPWAN technologies use different forms of coding and signal spreading over time to get long range. Note how spreading your signal over time means your throughput goes down.
- Natanael_L 11y ago
  Any MU-MIMO technology in sight for these radio protocols? Would be a great way to potentially extend range and throughout if multiple spatial channels could be used on each frequency / radio channel for every pair of connected devices. If it could be done efficiently enough, it could also reduce congestion even in tight crowds for things like wearables by reducing interference.
  - slewis 11y ago
    I haven't seen anything using MIMO yet. Not sure how that would affect power usage. The closest thing now is that a key aspect of Ingenu's solution is receive diversity. All of their client radios require two antennas.
- makomk 11y ago
  It's important to realise that - at least if I'm understanding spread spectrum technology correctly - there's no such thing as a free lunch, or free receive sensitivity. If you take an ordinary narrowband signal and convert it to spread-spectrum, you can indeed make it disappear beneath the noise floor by spreading it out, and a receiver that knows where to look can despread it and pull a signal out from what looks like noise, but the effective received signal strength and signal to noise ratio after despreading is still exactly the same as it was with the narrowband signal. You haven't gained anything, you're just moving your signal around a bit. The only advantage of this spread-spectrum technique is that other signals that don't use the same spreading code are spread out by the despreading process and look like wideband noise to the receiver.
- ac29 11y ago
  > But due to FCC regulation in the 900MHz band for example you can only instantaneously transmit at 30W. The limit is actually 4W ERP (1 watt transmit + 6dB antenna gain) per CFR 15.247: https://www.law.cornell.edu/cfr/text/47/15.247 https://www.law.cornell.edu/cfr/text/47/15.247 If you are aware of an exception to this, I would be very interested.

Just curious – why is range limited by a protocol? Why couldn't you send Bluetooth over hundreds of miles with a big enough antenna, for example?

by 0898 11y ago

Just curious – why is range limited by a protocol? Why couldn't you send Bluetooth over hundreds of miles with a big enough antenna, for example?

blhack 11y ago

Not limited by the protocol per se, but limited by the design requirements of BLE. BLE; you're talking about power requirements of ~10mA transmit (current consumption), and averages in the <10μA range. That's the whole point of the tech. It's stuff that can run for years on a watch battery. Now: how LORA is accomplishing such crazy long ranges when supposedly consuming similar amounts of current goes a little beyond my understanding of RF. There are a few people commenting in this thread that seem to have the knowledge, though!

makomk 11y ago
They're achieving those ranges by being very, very slow. It's probably easiest to explain how this works with conventional narrowband digital modulation schemes. For a typical modulation scheme, the bandwidth of the modulated signal is proportional to the bitrate. There's also some minimum signal-to-noise ratio below which the receiver can't decode it. By sending more slowly, you get a narrower signal, which means that the receiver can listen to a narrower slice of spectrum. Even though the received signal's no more powerful than before, because the receiver is no longer hearing all the noise outside of that narrower band the SNR improves and it can decode much weaker signals. LORA appears to use direct-sequence spread spectrum, which basically means that the transmitter applies a spreading code to convert the narrow signal to a very wide one and the receiver uses the same code to despread it again and pick out the signal. Its range improvement is still based on the exact same principle of speaking very slowly to improve your SNR though.

slewis 11y ago

You can amplify a signal and send it far. But due to FCC regulation in the 900MHz band for example you can only instantaneously transmit at 30W. Another way to make a signal go further is to transmit at a lower data rate (thus spreading your signal out over time so that in effect you have more power in the signal). The FCC limits "dwell time" in a single channel to .4s in the 900MHz band. Yet another way to increase range is to increase your receive sensitivity. Lora's Chirp Spread Spectrum coding actually allows signals to be received below the noise floor. You can liken this to decrypting data: to an observer the signal looks like noise, but if you know how to look within the noise you can pull the actual coded information out. All of these LPWAN technologies use different forms of coding and signal spreading over time to get long range. Note how spreading your signal over time means your throughput goes down.

Natanael_L 11y ago
Any MU-MIMO technology in sight for these radio protocols? Would be a great way to potentially extend range and throughout if multiple spatial channels could be used on each frequency / radio channel for every pair of connected devices. If it could be done efficiently enough, it could also reduce congestion even in tight crowds for things like wearables by reducing interference.
- slewis 11y ago
  I haven't seen anything using MIMO yet. Not sure how that would affect power usage. The closest thing now is that a key aspect of Ingenu's solution is receive diversity. All of their client radios require two antennas.
makomk 11y ago
It's important to realise that - at least if I'm understanding spread spectrum technology correctly - there's no such thing as a free lunch, or free receive sensitivity. If you take an ordinary narrowband signal and convert it to spread-spectrum, you can indeed make it disappear beneath the noise floor by spreading it out, and a receiver that knows where to look can despread it and pull a signal out from what looks like noise, but the effective received signal strength and signal to noise ratio after despreading is still exactly the same as it was with the narrowband signal. You haven't gained anything, you're just moving your signal around a bit. The only advantage of this spread-spectrum technique is that other signals that don't use the same spreading code are spread out by the despreading process and look like wideband noise to the receiver.
ac29 11y ago
> But due to FCC regulation in the 900MHz band for example you can only instantaneously transmit at 30W. The limit is actually 4W ERP (1 watt transmit + 6dB antenna gain) per CFR 15.247: https://www.law.cornell.edu/cfr/text/47/15.247 https://www.law.cornell.edu/cfr/text/47/15.247 If you are aware of an exception to this, I would be very interested.