Technnologies and Standards, London, UK, January
2001
).
Other methods based based on measuring the
signal strength or measuring the signal characteristic
patterns and multipath characteristics of radio
signals arriving at a cell site from a caller. For
measuring the signal strength, it employs multiple
cell sites to find the location. For measuring the
signal characteristic patterns, it identifies the unique
radio frequency pattern or "signature" of the call and
matches it to a similar pattern stored in its central
database (
Shu Wang, Jungwon Min, Byung K. Yi, 2008;
Y. Zhao, 2000).
TOA and TDOA are time-based measurement
technologies. They can be implemented either based
on the forward (down) link signal or reserved (up)
link signal. In addition, the location determination
capability can reside either at the network side or at
the mobile phone. In order to locate several base
stations or cell sites, the sensitivity of the mobile
phone may need to be increased. These methods also
require software modification on the mobile phone
and additional location determination units and
related software in the network. As discussed above,
the mobile phone needs to listen to the signals of at
least three base stations or cell sites. The visibility
and geographical locations of these base stations will
affect the availability and the accuracy of the
location determination (
Overview of 2G LCS
Technnologies and Standards, London, UK, January
2001).
The performance of the satellite-based GPS
receiver is getting better and better while the
receiver size and price keep going down. To develop
an assisted GPS (A-GPS) solution for the mobile
phone requires software and hardware modifications
of both the mobile phone and its communications
network.
The A-GPS use a GPS reference network (or a
wide-area DGPS network) whose receivers have
clear views of the sky and can operate continuously.
This reference network is also connected with the
cellular infrastructure, and continuously monitors
the real-time constellation status and provides
precise data such as satellite visibility, ephemeris
and clock correction, Doppler, and even the
pseudorandom noise code phase for each satellite at
a particular epoch time. At the request of the mobile
phone or location-based application, the assist data
derived from the GPS reference network are
transmitted to the mobile phone GPS receiver (or
sensor) to aid fast start-up and to increase the sensor
sensitivity. Acquisition time is reduced because the
Doppler versus code phase uncertainty space is
much smaller than in conventional GPS due to the
fact that the search space has been predicted by the
reference receiver and network. This allows for
rapid search speed and for a much narrower signal
search bandwidth which enhances sensitivity. Once
the embedded GPS receiver acquires the available
satellite signals, the pseudorange measurements can
be delivered to network-based position
determination entity (PDE) for position calculation
or used internally to compute position in the handset.
Additional assisted data, such as DGPS
corrections, approximate handset location or cell
base station (BS) location, and other information
such as the satellite almanac, ionospheric delay,
universal time coordinated (UTC) offset can be
transmitted to improve the location accuracy,
decrease acquisition time, and allow for handset-
based position computation. Several schemes have
been proposed in the standards which reduce the
number of bits necessary to be exchanged between
the handset and the network by using compression
techniques such as transmitting only the non-
redundant or the changes to parameters instead of
the raw parameters themselves. Other satellite
systems could be used, such as the Russian
GLONASS system, but none of the standards have
made provision for anything except GPS and the
GPS Wide Area Augmentation System (WAAS)
signals. Besides adding a GPS reference network
and additional location determination units in the
network, the mobile phone must embed, at a
minimum, a GPS antenna and RF down converter
circuits, as well as make provision for some form of
digital signal processing software or dedicated
hardware (
Overview of 2G LCS Technnologies and
Standards, London, UK, January 2001). Аll the radio-
based technologies discussed can be affected by
interference, blockage, and multipath.
3. GENERALIZED NET MODEL
In our model, we consider n patients. The i-th patient
has i
k
different sensors. The sensors for the i-th
patient are represented by the tokens δ
i,1
, δ
i,2
, ... ,δ
i,ik .
The tokens δ
i,1
, δ
i,2
, ... ,δ
i,ik
enter the net in place l
2
with initial characteristics:
"name of the patient; sensor's parameters"
The criterion for the correctness of the signal
detected by the sensors is represented by the token α
which stays permanently in place l
7
with initial
characteristic:
"criterion for the corectness of the signal"
The tokens d
i,1
, d
i,2
, ... ,d
i,ik
stay permanently in