DW1000 User Manual
© Decawave Ltd 2017
Version 2.12
Page 210 of 242
channel impulse response between the remote transmitter and itself. This is what gives the DW1000 its
good long-range performance and allows its accurate determination of the first path arrival time for the RX
timestamp.
The preamble sequence used at all data rates is the same, i.e. it does not depend on the chosen data rate.
The preamble sequence length, (i.e. the number of symbol intervals for which it is repeated), has a
significant effect on the operational range and the accuracy of timestamps. Table 57 gives some
recommended preamble sequence lengths to use depending on the data rate. In general, a longer preamble
gives improved range performance and better first path time of arrival information while a shorter preamble
gives a shorter air time and saves power. When operating a low data rate for long range, then a long
preamble is needed to achieve that range. At a high data rate the operating range is naturally short so there
is no point in sending a very long preamble as it wastes time and power for no added range advantage.
Table 57: Recommended preamble lengths
Data Rate
Recommended preamble
sequence length
6.8 Mbps
64 or 128 or 256
850 kbps
256 or 512 or 1024
110kbps
2048 or 4096
There are two choices of mean pulse repetition frequency (PRF) within the DW1000. These are 16 MHz PRF
or 64 MHz PRF. The 16 and 64 are “nominal” since the actual frequencies are related to the 499.2 MHz basic
time unit used and differ slightly between preamble and payload parts of the frame. The higher PRF gives
more accuracy on the first path timestamp and perhaps slightly improved operating range, however this
comes at the price of additional power consumption.
9.4 Power consumption
As described above, frames with lower data rate have the largest operating range. Such messages by their
nature also take longer to send and, as this means that the DW1000’s transmitter needs to be on for a
longer period, the result is that more power is consumed than for faster messages. The quantity of data
being transferred also has a bearing on this, i.e. higher data rates give more benefit when frame data sizes
are larger.
9.5 Node density and air utilisation
The ALOHA mechanism is the favoured channel access method in the IEEE 802.15.4 UWB PHY standard and
this is the channel access method employed by the DW1000. Essentially this involves assuming the air is idle
and transmitting whenever you want to. For ALOHA to work successfully total air utilisation has to be <18%
across all nodes in range of each other. With air utilisation above 18% collision probability is high and system
performance degrades quickly. Below the 18% value 97% of transmissions are likely to succeed without
collisions.
