Comment by lambdaone

Comment by lambdaone 21 hours ago

Mesh radio bandwidth is pretty poor. Firstly, you have to compete with many interferers (albeit this might get better if the power goes down), including other LoRa radios, but more to the point, long-distance connections consume bandwidth and aquire delay and delay variation at every intermediate hop. It might be reasonable to use it for text messaging, but with per-hop bandwidth ranging from 0.3 kbps to 27 kbps, which will get divided down further over shared multi-hop links it will be impractical to use it for anything else except perhaps very-low-bandwidth telephony over short distances or visiting minimalist text-only websites.

It might make more sense if augmented by fixed multi-megabit point-to-point microwave radio links to act as a backbone, with LoRa only functioning as an access network.

I'd be interested to hear what experiences people have had with doing this for real.

bkummel 20 hours ago

I think the point of the article is not to use that mesh network as a replacement for internet. I think the author's idea is that the mesh network would provide the "resilience club" a communication channel while they work on recovering the regular internet.

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lambdaone 21 hours ago

I've just realised I've talked my way into the idea of creating per-city club-operated backbone networks based on something like 100 Mbit point-to-point Ethernet-over-microwave links. With tall buildings as hubs, you might actually be able to build a decent mesh, with WiFi, LoRa or both acting as access networks. You'd definitely want to throttle per-client bandwidth to prevent people from abusing your very limited long-range mesh bandwidth. None of this would be cheap; decent microwave links cost thousands, and you'd need backup solar and battery power for every part of the network.

I'd also consider thinking about using the "big ears, small mouth" technique to push up bandwidth; if a fixed link using a technology such as LoRa could transmit at a legal EIRP level, but coupled this with a really high gain parabolic dish (I'm thinking re-purposed satellite dishes) and low-noise amplifier at each end on the receive side, you could get substantially higher end-to-end Eb/No, and thus much higher bandwidth and range than would otherwise be legally possible. At first glance, the necessary hardware to do this looks quite doable, either by active RF switching between antennae, or the use of a hybrid/circulator to do the necessary duplexing. I'd be interested to see if anyone has already built, or even manufactures, something like this, and what the practical and regulatory barriers are to implementation.

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myself248 20 hours ago

"Big ears, small mouth" is exactly what the regulations are designed to encourage, so I don't foresee regulatory issues.
You don't even need extra hardware for the duplexing; the common SX1276 chip has separate Tx and Rx pins which are typically combined on the PCB. All you need is to route a PCB that brings 'em out separately, if that's what you want to do.
In practice it's tricky to aim two dishes the exact same place, so using a single dish with a single antenna at its focus is probably quite a bit more practical. The SX1276 also has a PA control pin, invert that and you've got your LNA control signal. Or don't bother with the LNA, and simply mount the transceiver at the focus to minimize RF feedline losses. You'd give up a smidgen of performance but gain a lot of simplicity. (There would still be coax running down the boom, but it would be carrying the wifi/bluetooth signal outside the dish's aperture!)

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- lambdaone 18 hours ago
  
  I didn't know about the chip having separate TX and RX pins. There must be a gap in the market for rooftop-to-rooftop LoRa transceivers that don't cost a fortune. Even using something like a 24 dB gain antenna would push range up by a factor of 10 relative to a simple 4 dB antenna, or get a substantial improvement in bandwidth/reliability at the same range. For an even simpler design, you could just put a 20 dB attenuator between the transmit port and the antenna, reducing the effective forward gain of 4 dB, while getting the full 24 dB in the opposite direction. Proper RF engineering details an exercise for the student etc.
  
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  hadlock 14 hours ago
  
  Google "cantenna" made from a pringles can.
  
  Reply View | 1 reply
  
  myself248 12 hours ago
  
  Please don't. We have better antennas than a 2001 article of really really dubious RF engineering.
  
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eimrine 17 hours ago

That's true and proprietary modulation makes the situation worse.

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