Frequently Asked Questions - Performance of DW1000 Based Systems

 

RSSI values can be calculated. See chapter 4.7 assessing the quality of reception and the RX timestamp in the DW1000 User manual (Received Power Level). The DW1000 User manual is available from http://www.decawave.com/support

 

In Decawave’s demonstration application the tags and anchors use Two-Way ranging protocol to exchange messages and calculate range/distance between them. To calculate a single range a minimum of 3 messages are needed (if tag needs to be told of the range result, then either this information can be sent via next Response message or an additional 4 th message can be used (e.g. ToF report)).

DW1000 supports various data rates and preamble combinations. Depending on the preamble length and data rate used, a single message can vary between 190 µs (6.81 Mbps, 27 bytes, 128 preamble) to 3.4 ms (110 kbps, 27 bytes, 1024 preamble). This means that time to calculate a single range can vary from couple of milliseconds to tens of milliseconds.

In TDoA systems the blink frame (with preamble length of 64-symbols) and 12 octets of message payload, is around 110 µs this means that RTLS system can support 1700 blinks per second for 1 device or 170 blinks per second for 10 devices etc.

For more information see the DW1000 IC user manual Chapter 9 Node density and air utilisation, available from http://www.decawave.com/support#term4

 

This depends on the RTLS scheme employed, the tag blink rate, the message duration per tag and a number of other factors: -

  •  Data rate;
  •  Number of data bytes;
  •  Preamble length;

For further information one should read the DW1000 IC user manual Chapter 9 Node density and air utilisation. TREK1000 Expansion Options Instructions available as parts of the TREK1000 ARM version 2.10 package available from http://www.decawave.com/support/software

 

It may not be necessary to take any avoiding action depending on the tag density and the tag update rate. If these are sufficiently low then the probability of collisions will be very low and ALOHA-type access rules can be employed.

If tag density is high and high update rates are required then you can avoid collisions between ranging exchanges by dividing time into slots (using TDMA) for each tag's activities. One of the anchors can act as a “controller” node monitoring on-air activity and assigning “allowed” transmission periods to each tag.

 

There are various factors which influence the power consumption of the DW1000 during transmission and reception such as preamble length, data rate, number of data bytes and so on.

Detailed information on power consumption in the various different DW1000 states is available in our DW1000 datasheet and application notes APS001 DW1000 Power Consumption and APH005 DW1000 Power Source Selection. Datasheet and application notes are available on http://www.decawave.com/support

 The human body introduces approximately 30 dB of insertion loss so the transmitted signal from the tag will be heavily attenuated. Depending on the proximity of the tag antenna to the body the level of attenuation may be such that: - 

 

  • no signal at all is received at the intended receiver or the direct path signal is heavily attenuated and only a reflected signal is received at the intended receiver thereby giving an incorrect distance measurement
  • the direct path signal is heavily attenuated but is still received correctly at the intended receiver in which case the distance measurement will be correct.

 Most monopole antennas are designed to operate in free space i.e. not in proximity to the body. Proximity to the body reduces antenna efficiency and fidelity factor. This could distort the UWB pulse and thereby give an incorrect range measurement. The solution here is to design an antenna which takes the body proximity in account. Consult Decawave for more information on this area For more information on non-line- of-sight propagation see the three application notes: APS006 part 1, APS006 part 2 and APS011 Sources of error in TWR available on http://www.decawave.com/support