Installation guide

12
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
Location Tracking Approaches
In close, confined indoor areas, both ToA and TDoA have traditionally suffered from less than optimal
performance, especially in situations where the mobile station is likely to be surrounded by objects that
promote multi-angular RF scattering and reflection. Interestingly, the effects experienced under such
conditions appear to worsen with narrow-band implementations of TDoA versus wider band
implementations such as WLANs. Capitalizing on this phenomena, alternative methods of implementing
TDoA such as the 2.4 GHz approach described in ANSI INCITS 371.1/ISO24370 have been developed.
ANSI INCITS 371.1 implements 2.4 GHz Binary Phase Shift Keying/Direct Sequence Spread Spectrum
(BPSK/DSSS) with an occupied bandwidth of 60 MHz, allowing for improved TDoA performance under
adverse multipath conditions.
Received Signal Strength (RSS)
This guide has now discussed two lateration techniques (ToA and TDoA) that use elapsed time to
measure distance. Lateration can also be performed by using received signal strength (RSS) in place of
time. With this approach, RSS is measured by either the mobile device or the receiving sensor.
Knowledge of the transmitter output power, cable losses, and antenna gains as well as the appropriate
path loss model allows you to solve for the distance between the two stations.
The following is an example of a common path loss model used for indoor propagation at 2.4 GHz:
PL = PL
1meter
+ 10log(D
n
) + S
In this model:
• PL represents the total path loss experienced between the receiver and sender in dB.
• PL
1meter
represents the reference path loss in dB when the receiver-to-transmitter distance is 1 meter.
• D represents the distance between the transmitter and receiver in meters.
• n represents the path loss exponent for the environment.
• S represents the degree of shadow fading present in the environment in dB.
Path loss (PL) is the difference between transmitted power and received power, and represents the level
of signal attenuation present because of the effects of free space propagation, reflection, diffraction, and
scattering. The path loss exponent (n) is a function of frequency, environment, and obstructions.
Commonly-used path loss exponents range from a value of 2 for open free space to values greater than
2 in environments where obstructions are present. At 2.4 GHz, for example, a typical path loss exponent
for an indoor office environment is 3.3, and for a more dense home environment is 4.5.
S represents the degree of shadow fading associated with the environment. Indoor shadow fading varies
depending on the number of obstructions present. In an environment with many partitions, walls, or other
obstructions interfering with line of sight between the mobile device and each receiver, S may be in the
range of ± 7dB and sometimes more.
Using the standard practice for calculating receiver signal strength given known quantities for transmit
power, path, antenna, and cable losses, you have the following:
RX
PWR
= TX
PWR
– Loss
TX
+ Gain
TX
– PL + Gain
RX
– Loss
RX
Directly substituting the path loss model for PL in the equation above allows you to solve for distance
D assuming all other variables are known:
where the meaning of the terms in the equation above are:
• Rx
PWR
represents the detected receive signal strength in dB.
n
GainLossSPLGainLossTXRX
invD
RXRXmeterTXTXPWRPWR
10
log
1
−
−+−+−+−
=