LoRa Wifi Ranging and SX1280 chip

The Semtech SX-1280 $3 three dollar ranging chip was evaluated for LoRa ranging. In general, 200 kHz RF bandwidth, 0.5 kbps data bandwidth and -130 dBm sensitivity trade lower speed for maximum range. The narrow RF channel bandwidth helps range and reliability in the highly-congested 2.4 GHz band.

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 subordinate 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 = c⁄2B

We would expect at best 3e8 / (2*1.6e6) = 94 meters range resolution. 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.

CONOPS:

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

Only the LoRa controller gets the ranging result (which is communicated to the node).

Notes: