DW1000 User Manual
© Decawave Ltd 2017
Version 2.12
Page 213 of 242
In an RTLS the accuracy of the DW1000’s RX timestamps can give sub 10 cm resolution. Note, however that
the geometry of anchors with respect to the tag can smear the accuracy of the calculated location when
individual measurements are combined. Having additional anchors in range of the tag can offset this if it
allows the system to select anchors with best geometry and best receive signal quality with respect to the
tag being located.
9.8 General considerations
This chapter discussed the operational design choices with respect to selecting data rate, preamble length,
and PRF, and the trade-offs involved in selecting for high node density versus long operating range. The
discussion also indicated how these choices affect the power consumption.
In general infrequent transmission of short packets is best for a long battery life. When a large quantity of
data needs sending it is best to employ the longest frame length so that the overhead of the preamble and
other parts of the frame header represent a smaller proportion of the overall frame. Listening for and
receiving messages is relatively the most power expensive operation within the DW1000. It is
recommended that devices needing to listen all the time are connected to a mains derived power source.
This would include anchor nodes in an RTLS system, and those nodes of a wireless sensor network (WSN)
that are hubs nodes responsible for routing messages.
For mains powered nodes, the power consumption in the receiver is not an issue, but for battery powered
nodes where it is important to ensure a reasonable battery life, it will be necessary to include schemes to
reduce the time spent listening. Table 59 lists and briefly describes some techniques that can save power in
receiving.
Table 59: Techniques to save power in receiving
Name
Description
Low-Power
Listening
This is a technique based on the DW1000 preamble sampling function. The IC can
“sniff” the air for a short period looking for preamble, and if it does not see it return
to low power sleep mode. The transmit device has to send its message with long
preamble and repeat it for a period to guarantee that the preamble overlaps with the
period of RX sniffing. If the IC sees preamble it stays activated until it receives the
message after which it can interrupt the host microprocessor to process it. With
address filtering enabled the IC can return to sleep as soon as an address mismatch is
seen.
This technique really only suits cases where traffic volume is low so the predominant
activity of all nodes is occasionally sniffing the air for preamble. As every message will
wake up every node in range, at least until the address mismatch, the power drain to
all nodes can be quite high if there are frequent transmissions.
Poll for message
This is a technique where a battery powered node periodically sends a poll message
and listens thereafter for a response from the mains powered infrastructure. The
response is only sent if the infrastructure has a message for the polling node. The
node sending the poll can delay turning on the receiver to allow for the response time
of the infrastructure, and thereafter can use preamble detect timeout to turn of the
receiver if a response is not starting. The predominant activity is then periodically
sending the poll and listening for a short time for a response that does not come.
