LoRa Wifi Ranging and SX1280 chip

The Semtech SX-1280 $3 three dollar ranging chip was announced summer 2016 with first mover’s advantage to market. Let’s work to understand what LoRa ranging is and isn’t.

In the limit 200 kHz RF bandwidth, 0.5 kbps data bandwidth and -130 dBm sensitivity set the corner for a remarkable tradespace of speed vs. range. With these specs and clear line of sight, well over 100 km range is possible for special configuration (not configurations for everyday use, more for radio amateurs). The narrow bandwidth choices also help range and reliability in the highly-congested 2.4 GHz band.

LoRa ranging performance

The Semtech SX-1280 2.4 GHz long-range radio transceiver in ranging mode may use 0.4, 0.8, or 1.6 MHz, relying on a slave unit to act as a sort of bent-pipe to relay the signal back with fixed device delay. Without using more advanced techniques to constrain the problem, by

ΔR = c2B

We would expect at best 3e8/(2*1.6e6) = 94 meters range resolution – about a football field or city block length. Assuming a sensor fusion application, this ranging would not replace GPS in and of itself, but would indeed provide an excellent supplement for dense urban areas, such as large malls, warehouse/factory, and parking garages. LTE location accuracy can do significantly better than this due to the typical 5-10 MHz or more bandwidth readily yielding sub-100 m location accuracy. One of the key well-known limits of such low RF-bandwidth wireless location systems as demonstrated by SciVision, Inc. via model and simulation to US Dept. of Transportation personnel is multipath.

Multipath refers to the cancellations and self-interference causes from slightly time-delayed, strong reflections reaching the receiver. Multipath is often worst in dense urban areas, likewise hindering accuracy of GPS (who hasn’t heard a “recalculating route” message from their GPS navigator in dense downtown areas).

LoRa ranging CONOPS

  • Exactly one LoRa ranging slave can be ranged by the (likely infrastructure) master at one time.
  • An ID number of 8 or more bits is used to uniquely identify slaves in range.
  • multiple master stations improve accuracy (with cost of time)

Only the master gets the ranging result (which is communicated to the slave).


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