Comment by slewis | Hacker News Reader

Comment by slewis | Hacker News Reader

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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.

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.

> 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

If you are aware of an exception to this, I would be very interested.

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.

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.

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