DW1000 User Manual
© Decawave Ltd 2017
Version 2.12
Page 229 of 242
As the difference in reply time of the two devices increases there is a linear increase in the error in the
calculated time of flight which can approach 30 cm for a reply-time difference of 100 µs.
Advantages
Drawbacks
Requires only simple mathematical operations to
derive a result
Reply times must be the same – this is difficult to
achieve. If they are not the same then an error is
introduced into the result the magnitude of which
depends on the difference between the reply times
The ranging exchange is longer than necessary
because all reply times must be as long as the
longest reply time
12.3.4 Comparison between DS and SDS two-way ranging
12.3.4.1
Introduction
It is useful to compare the asymmetric and symmetric double-sided two-way ranging schemes in some real
world applications so the implications of each can be seen.
12.3.4.2
Infrastructure based asset tracking
One anchor device sends a packet, P1, to many, say 5 tags. Each tag responds with a packet to this anchor in
successive responses. Tag 1 responds with packet P2a after time t, Tag 2 responds with P2b after time 2t,
Tag 3 responds with P2c after time 3t and so on until all tags have responded. Finally the anchor closes off
the round with a final packet P3.
Each tag can now calculate its distance from the anchor after a sequence of just 7 messages. If the anchor
had used symmetric S-TWR it would be forced to have the same delay for each tag interaction and a
minimum of 3 messages per tag, or 15 messages would be required.
In the asymmetric case the number of packets required is N+2 whereas in the symmetric case it is 3N.
12.3.4.3
Infrastructure based asset tracking
In this scheme a mobile tag (on an asset say) ranges to three fixed anchors. Each anchor then calculates the
distance to the tag. These three distances are then combined in an infrastructure-based solver to locate the
tag.
With symmetric double-sided two-way ranging this needs 3 messages per distance measurement which is a
total of 6 transmissions and three receptions to derive the location.
In the asymmetric ranging scheme the tag sends a Poll message which is received by the three anchors in the
infrastructure who reply in successive responses with packets RespA, RespB & RespC after which the tag
sends the Final message received by all three anchors. This allows the tag to be located after sending only 2
messages and receiving 3. This scheme is illustrated in Figure 39.
This represents a substantial saving in message traffic thereby saving battery power and air-time.
