FT8 over LoRa

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Posting on “off-topic”, as previous LoRa-related posts appeared here.

I put together a simple firmware and QSO controller for a Heltec LoRa board, which can be controlled by one’s phone and allows for simple FT8-style QSOs over the LoRa physical layer. The idea is the same as the APRS-over-LoRa devices many of us are using: implement a well-known amateur radio protocol over a different modulation/physical layer.

Here’s a video with a simple test (https://www.youtube.com/watch?v=3_DpiqInHS8), and here’s the code on github: GitHub - indrusiak/ft8-lora: FT8/LoRa (FT8 over LoRa) · GitHub

And here comes a question. The LoRa board I’m using communicates over the ITU region 1 ISM 433 MHz band, which overlaps with part of the UK 432MHz band. So if I make QSOs with other operators using similar boards, while following the criteria in subsection 3.7.1 of the SOTA General Rules, would I be able to qualify a summit activation? Or in other words, is there any intrinsic limitation preventing us from using LoRa as a valid amateur radio mode?

73 Leandro G5LSI

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You will not be flavour of the month with Ofcom. The desire is for ham LoRa to be up at 439MHz so the higher output powers and much increased duty cycle of Amateur LoRa does not spoil the ISM LoRa users of 433MHz.

You can put it on 433. But you shouldn’t. Put it up at 439 and play nice with the other band users.

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Thanks for the info, I was not aware of that. I’ll try to find more details in the Ofcom website.

439 MHz sounds good to me, I’ll change the code and do some tests.

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For years I have been fascinated by spread-spectrum modulation and its capabilities for below-the-noise reception. But it wasn’t until I and others set up LoRa iGates in G/LD that I discovered – walking around my local area as well as going up SOTA / WOTA summits - how amazing the Chirp version of SS is at making tracker-to-iGate/digipeater contacts (for APRS) in seemingly impossible terrain with such low power and small antennas.

So, adapting the existing ‘tracker’ firmware for FT8-like texting between mobile units would seem an ideal way for local / regional mobile-to-mobile comms in mountainous terrain (more effective than VHF analog modes and more convenient than via NVIS on 60m). There was some discussion on this reflector on how to set up trackers for text messaging but I don’t recall seeing anything on how it worked in practise.

It would be interesting to see some field trials to find out how practical it is without iGates for mobiles to receive each other’s signal. This fits very well with one of A.R.s prime directives: experimentation.

Re the 433MHz vs 439MHz discussion, it seems to me (although I’m far from being an expert) that – apart from avoiding potential interference to ISM users, the UK 439MHz configuration has several important advantages:

  1. The much higher RF output allowed by Ofcom
  2. Brilliant SNR (decodes up to -20 dB below noise) using SF=12 c.f. at least -7.5 dB SNR needed for SF=7 (as used by “433” configuration)
  3. leading to better range

The disadvantages are:

  1. Lower data rate (~293 bps at 125 kHz) compared to SF=7 (~5.47 kbps at 125 kHz)
  2. which leads to longer Time-on-Air: ~1.3s/packet vs 30-50ms/packet
  3. which leads to higher battery consumption

In practise they shouldn’t be a problem.
Re 2) the longer T-o-A times are only a problem if it were to lead to multiple packet collisions, which with small numbers of mobile units operating at the same time in the same area is very unlikely.
Re 3) I find I can do many activations using my tracker before any significant drop in battery charge. Unless you are texting like a teenager in love, the text-over-LoRa ought not to be worst than using APRS-over-LoRa.

Of course, if it proves practical and used for SOTA, it might be a contender for the “Ultimate SOTA Minority Sport” prize [MT, make a note to create one] simply trying to find a chaser with the same setup.

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