CC2431 Datasheet, PDF (7/16 Pages) Texas Instruments – System-on-Chip for 2.4 GHz ZigBee-TM / IEEE 802.15.4 with Location Engine

2.1.1 Reference Coordinates
The Location Engine requires a set of between
three and 16 reference coordinates [x0, y0, x1,
y1, …, x15, y15] to be input. The reference
coordinates express each reference nodes
position in meters, as unsigned values in the
interval [0, 63.75] meters. The finest possible
readout resolution is 0.25 meter. The format
used is fixed-point data with the two LSBs
representing the fractional part and the
remaining six bits representing the integer
part, thus e.g. 63.75 is represented as 0xFF.
Reference coordinates are loaded into the RF
register REFCOORD. Before writing to
REFCOORD, a 1 must be written to the register
bit LOCENG.REFLD to indicate that a set of
2.1.2 Measured Parameters
After the reference coordinates have been
written, a set of measured parameters must be
input to the Location Engine. These
parameters consist of two radio parameters:
Four search boundary coordinates and 16
2.1.2.1 Parameter Definitions
The measured parameters are described in
this section together with how these should be
estimated.
2.1.2.1.1 Parameter A
The radio parameter A is defined as the
absolute value of the average power in dBm
received at a close-in reference distance of
one meter from the transmitter, assuming an
omni-directional radiation pattern. For
example, if the mean received power at one
meter is -40 dBm, the parameter A is specified
as 40.
2.1.2.1.2 Parameter n
The radio parameter n is defined as the path
loss exponent that describes the rate at which
the signal power decays with increasing
distance from the transmitter. This decay is
proportional to d^-n where d is the distance
between transmitter and receiver.
The actual parameter n value written to the
Location Engine is an integer index value
selected from a lookup table shown in Table 2.
CC2431
reference coordinates are being written. Once
the coordinate load process commences
(LOCENG.REFLD =1), 16 coordinate pairs
must always be written. However, it is possible
for the Location Engine to use less than 16
reference coordinates, by marking certain
reference coordinates as unused. Zeros shall
be used to fill the unused reference coordinate
slots, and they will be interpreted as unused
when 0.0 is loaded as the RSSI value for
those reference coordinates.
The reference coordinates are written in the
order [x0, y0, x1, y1, …, x15, y15] to the
register REFCOORD. After all coordinates have
been written, a 0 is written to the register bit
LOCENG.REFLD.
RSSI values. The radio parameters are the
values A and n. These radio parameters are
used in the Engine’s algorithm used to find the
estimated location. The parameters A and n
can be adjusted to describe the propagation
environment in which a network of devices will
operate.
The Engine expects the parameter A to be in
the range [30.0, 50.0] with precision 0.5. The
parameter A is given as an unsigned fixed-
point value where the LSB bit is the fractional
bit and the remaining bits are the integer part.
A typical value for A is 40.0.
As an example, in the case when the value
n=2.98 is found from measurements, the
closest available value of n in the lookup table
is 3.00, corresponding to index 13. Therefore,
the integer value 13 is used for the parameter
n written to the Location Engine.
Refer to section 2.1.2.1.3 in order to find the
value for n to be used.
CC2431 Data Sheet (Rev. 2.01) SWRS034B
Page 7 of 15