Spread spectrum RF control and telemetry

Now that I have both sides of the nrf24l01+ link covered, it was time to design a protocol to take advantage of them.

Design space

To recap, what I needed was a reliable means of commanding the robot and receiving telemetry, even in congested radio environments.  At competitions or events like Maker Faire, Robogames and such, the wireless environment is often totally trashed.  Hundreds of devices are operating in close proximity, across all spectrum bands, including plenty of things that probably aren’t licensed to be transmitting in the first place.  When we first built Super Mega Microbot, we used a custom protocol with a 5 GHz wifi transmitter as the physical layer and selected USB based dongles which allowed control over the physical layer.  USB proved problematic, and with national RF regulations, it is extremely challenging to find wifi devices which provide that level of control at the RF layer.  Also, even with full physical layer control, wifi is difficult to make work in a reliable manner as there is so much congestion in both the 2.4GHz and 5GHz bands and the channels are so wide.

What does usually work at these events, despite the extreme congestion, are standard hobby RC transmitters.  DSMX from Spektrum, is one of the more popular varieties.  It uses an off the shelf 2.4GHz RF IC, and then hops between frequencies every transmission based on a pseudorandom key shared between transmitter and receiver.  This enables many transmitters to share the same RF environment, and renders them extremely resistant to interference.

The biggest problem with DSMX for this application (and basically every other RC protocol), is that they are unidirectional.  At best, bidirectional solutions involve sticking two independent transmitter and receiver pairs in opposite directions.  This is despite the fact that most of the low level RF ICs actually support bidirectional communications natively.  Even then, the only supported telemetry forms are things specific to RC models.  Joint position feedback would need to be encoded in propeller RPM for instance!

Proposed design goals

The target features I’m looking to achieve with this protocol are:

  • Resistant to heavy RF interference
  • Bidirectional communication of arbitrary data
  • 50Hz update rate or higher
  • Multiple transmitters and receivers can operate in the same area without interfering or explicitly coordinating
  • Support data that is transmitted at different rates (i.e. voltage telemetry update can be low rate, but movement commands are high rate)
  • Control over source to add new features over time

Things I’m not necessarily trying to accomplish (yet):

  • Long range
  • A “binding” mode that uses RF to share the common psuedorandom key
  • A “stable” over the air protocol

Next up, a design which hopefully achieves these goals!

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