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CC2431-RTR1 数据表(PDF) 7 Page - Texas Instruments |
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CC2431-RTR1 数据表(HTML) 7 Page - Texas Instruments |
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7 / 15 page  CC2431 SWRS034 Page 7 of 14 2.1.1 Reference Coordinates The Location Engine requires a set of between three and eight reference coordinates [x0, y0, x1, y1, … 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 register bit LOCENG.REFLD to indicate 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, …, x7, y7] to the register REFCOORD. After all coordinates have been written, a 0 is written to the register bit LOCENG.REFLD. 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 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. 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. |
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