In a network it may be useful to assess the quality of message reception from a particular node in order to
change network routing or configurations related to that node to improve the reliability of the
communications. For example to improve communications reliability the frame length might be shortened,
or the data rate might be reduced, or the preamble length might be increased. In other situations with
consistently good communications the preamble length might be shortened to reduce the transmission
time, saving power and leaving more air-time free for other nodes to communicate.
In a TDOA RTLS system where a particular tag’s transmission is received at multiple anchors the quality of
reception and more particularly the quality of the RX timestamp information might be used to select which
anchors’ RX message timestamps to feed into the location engine.
The following details the elements of receive status reported by the DW1000 that may be used to assess the
quality of a received message and any related timestamp.
The Standard Deviation of Channel Impulse Response Estimate (CIRE) Noise value, reported in the
STD_NOISE field of Register file: 0x12 – Rx Frame Quality Information may be used to give a measure
of the noise associated with this and the received frame’s timestamp measurement. The STD_NOISE
can be used as an absolute value or it may be compared with the First Path Amplitude value – in this
latter case it is recommended that the amplitude value used for comparison is the value reported in
FP_AMPL2 field of Register file: 0x12 – Rx Frame Quality Information.
With a higher absolute CIRE noise figure it is more likely that the quality of receive timestamp will be
poorer. High noise may mean that the real first path is irretrievably buried in the noise. Comparing
the noise with the First Path Amplitude can give additional indication as to the quality of the first
path measurement. Where the First Path Amplitude has a large headroom over the noise, then the
received frame’s timestamp is likely to have been determined more precisely than when the First
Path Amplitude is closer to the noise level.
It is possible to compute an estimated receive power figure (using the equation and details given in
section 4.7.2 – Estimating the receive signal power) – for the purposes of this discussion this will be
called RX_POWER. It is also possible to compute an estimated power for just the first path signal
(using the equation and details given in section 4.7.1 – Estimating the signal power in the first path)
– for the purposes of this discussion this will be called FP_POWER. Using these two calculations it
may be possible to say whether the channel is line-of-sight (LOS) or non-line-of-sight signal (NLOS).
As a rule of thumb, if the difference between RX_POWER and FP_POWER, i.e. RX_POWER –
FP_POWER, is less than 6dB the channel is likely to be LOS, whilst if the difference is greater than
10dB the channel is likely to be NLOS.
Where the RX timestamp relates to a frame that is received in the presence of a higher CIRE noise power, or
relates to a non-line-of-sight path with attenuated first path, then that receive timestamp is naturally likely
to be of lower quality than that determined from a crisp line of sight first path signal that is well above the
noise floor.
Where a location system has an excess number receive timestamps to choose from then a quality estimate
relating to each timestamp may be used to weight the timestamps or to choose the highest quality set to
feed into the multilateration function of the location engine.
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