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This is a question which I have been wondering ever since I started exploring LPWAN technologies.

All the LPWAN technologies I have found (like SigFox, LoRa, Ingenu etc.) work in UHF band (300 - 3,000 MHz). E.g. LoRa and SigFox work in 868 MHz while Ingenu in 2.4 GHz band.

Using lower frequencies like HF (3–30 MHz) and VHF (30–300 MHz) can provide much longer range at the cost of data rate. Now, LPWAN technologies serve applications requiring very low data-rate. Also, in many countries, there are bands in HF and VHF which have been de-licensed (e.g. 27 MHz and 168 MHz).

Why then do we not have any LPWAN technology in HF and VHF?

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This is rather a simplification, but the basic answer is channel capacity. The whole HF band is the width of a single GHz channel, and the VHF band is the width of the 2.4 GHz band. This means bandwidth at the HF/VHF bands is in very short supply.

Granted, there are some available bands, but they are so narrow as to only suit very low bandwidth. They tend to be used for short range (low power) applications, where cost is also a critical factor (or used to be).

The increased range is not actually an advantage, it makes for reduced re-use. If you consider the capacity of a channel across a city this becomes a bit easier to visualise. Range of a WiFi signal means that from one building to the next, it's possible to re-use most of the channels without getting an unusable interference scenario. If your WiFi had a 5km range (in the night when everyone else had turned their router off), everything would grind to a halt in the day.

Achieving optimum band capacity requires several channels (so a wide band), and a range which is no more than necessary (so neighbouring nodes can re-use channels more efficiently).

Although higher frequency signals tend to require more infrastructure, the cost of infrastructure is dropping, and the cost of mass produced generic RF is also now cheaper at high frequencies where entire circuits can be integrated. Contrast this to 20 years ago, when GHz design was still very niche - now it comes ready to use, off the shelf.

  • Thank you for a thorough answer and 2 very good points. Channel re-use was an interesting point which I didn't think about. However, regarding channel bandwidth, there are certain bands which offer comparable bandwidths. E.g. in India, 25 MHz to 27 MHz is delicensed as well as 865 MHz to 867 MHz. Both offer 2 MHz bandwidth. So, while we use the latter for LoRa, the former is used for legacy communication. – shivams Nov 14 '18 at 15:33
  • Its a long time since I looked, but I think the ~27 MHz band is very inconsistent globally. I have a nagging feeling there are some filter %bandwidth issues too. – Sean Houlihane Nov 14 '18 at 15:53

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