Installation guide

11
Wi-Fi Location-Based Services—Design and Deployment Considerations
OL-11612-01
Location Tracking Approaches
Figure 4 Time Difference of Arrival (TDoA)
A fourth receiving sensor and third hyperbola may be added as an enhancement to perform TDoA
hyperbolic multi-lateration. This may be required to solve for cases where there may be more than one
solution when using TDoA hyperbolic tri-lateration.
Modern TDoA system designers have derived methods of coping with local clock oscillator drift that are
intended to avoid the strict requirement for precision time synchronization of TDoA receivers. For
example, a calibration time source can be used periodically to calculate time adjustments from a
reference clock source. These clock adjustments can then be used to correct for reference clock offsets
elsewhere in the system. In the case of TDoA receivers that are capable of transmitting packets as well
(such as 802.11 WLAN access points), another innovative approach involves the periodic exchange of
“timing” packets between receivers. In this approach, time offsets between each receiver and a
“reference receiver” can be quantized, with the resulting time adjustment then applied accordingly.
Airport ranging systems are a well-known example of TDoA systems in use today. In the world of
cellular telephony, TDoA is also referred to as Enhanced Observed Time Difference (E-OTD), and offers
an outdoor accuracy in that application of about 60 meters in rural areas and 200 meters in RF-heavy
urban areas.
In terms of both advantages and shortcomings, both ToA and TDoA have several similarities. Both have
proven very suitable for large- and very large-scale outdoor positioning systems. In addition, good
results have been obtained from ToA and TDoA systems in semi-outdoor environments such as
amphitheaters and stadiums, and contained outdoor environments such as car rental and new car lots or
ports of entry. Indoors, TDoA systems exhibit their best performance in buildings that are large and
relatively open, with low levels of overall obstruction and high ceilings that afford large areas of
clearance between building contents and the interior ceiling.
In many cases, however, both ToA and TDoA systems have typically required specialized infrastructure
installed alongside that required conducting normal day-to-day 802.11 WLAN data exchange. In some
cases, this is masked by common external housings designed to accommodate both a standalone TDoA
receiver as well as an 802.11 access point. This is expected to change as increased focus is placed on
integrated 802.11/TDoA infrastructure silicon, with the culmination of such efforts being a
fully-integrated 802.11/TDoA access point.
190537
A
B
C
X
TDOA
C_A
TDOA
B_A