CC2431_07 Datasheet, PDF(7/14 Page) Texas Instruments – System-on-Chip for 2.4 GHz ZigBee/ IEEE 802.15.4 with Location Engine

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CC2431_07 Datasheet, PDF (7/14 Pages) Texas Instruments – System-on-Chip for 2.4 GHz ZigBee/ IEEE 802.15.4 with Location Engine

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CC2431

2.1.1 Reference Coordinates

The Location Engine requires a set of

between three and eight reference

coordinates [x0, y0, x1, y1, â¦ x7, y7] 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

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.

Reference coordinates are loaded into the

RF register REFCOORD. Before writing to

REFCOORD, a 1 must be written to the

that a set of reference coordinates are

being written. Once the coordinate load

process commences (LOCENG.REFLD

=1), eight coordinate pairs must always be

written. However, it is possible for the

Location Engine to use less than eight

reference coordinates, by marking certain

reference coordinates as unused. Zeros

can 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, â¦, x7, y7] to the

have been written, a 0 is written to the

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 and eight RSSI values. The

radio parameters are the values A and n.

These radio parameters are used in the

Engineâs 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.

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.

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.

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.

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 PRELIMINARY Datasheet (Rev. 1.01) SWRS034A

Page 7 of 13

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