![](bg2.png)
Normally the reader is connected to a computer and
used to read and write data. The RFID transponder is
programmed with a unique code and located on
object to be identified. The RFID transponder can be
active or passive (Weinstein, 2005). The built-in
battery of the active transponder increases its size
and limits its applications. In this paper, when we
say RFID transponder, it is a passive transponder.
Passive transponders operate at different
frequencies. As mentioned before, we chose a low
frequency transponder in our system. The size of the
passive transponder can be as small as several
millimetres, such as TK5552 (Atmel, 2003) which is
only 12 mm long with plastic housing. The length of
TK5552 is almost the coil’s length used in (Bladen,
1993). The power in the electromagnetic field
received from the reader is the only power used for
the data transmission between reader and
transponder (Finkenzeller, 2003).
The operating principle of the RFID transponder
is transformer-type coupling between transponder
and reader. When the transponder is in a magnetic
field, the alternating current in the reader coil
induces a current in the transponder’s antenna coil
and this current is used to power the transponder
electronics. ID infromation stored in the transponder
is sent back to the reader by loading the
transponder’s coil in a changing pattern over time,
which affects the induced e.m.f. in the reader (Want,
2004). This process is called load modulation.
2.2 CDMA Technology
As described in Part 1, we find the image of the
endoscope by locating transponders installed along
the shaft of the endoscope. Our system is a wireless
system, and all transponders share the same
frequency resource, so the received signal is a
combination signal from all transponders. To
separate each transponder’s signal at the receiver,
we program each transponder with a different
spread-spectrum code. When the received signal
goes through a corresponding correlator, each signal
is separated individually. But this does not mean we
can use an unlimited number of transponders in our
system. MAI (Multiple Access Interference) is a
factor which limits our system performance.
The other reason that we use spread-spectrum
code is to increase SNR. We know the modulation
method for RFID system is load modulation. Since
the RFID transponder we used is very small
(diameter of TK5552 is less than 3 mm) (Atmel,
2003), and it works at a long distance (up to 35 cm),
the coupling of transponder coil and reader coil is
very weak. This results in the voltage variation on
the reader coil being very small. The location and
orientation of each transponder are calculated by this
voltage. To get accurate location information, we
need to increase SNR. The definition of processing
gain of a spread spectrum system tells us if the
spread-spectrum code is N bits long, the processing
gain is N (Proakis, 2001) and SNR in dB increases
by
NSNR
improved 10
log20=
(1)
Now we can see, by combinating RFID and
CDMA techonology, it is possible to replace wired
coils in previous systems with RFID transponders.
3 LOCATION THEORY
To estimate each transponder’s location (x-y-z),
three pairs of circular coils are used in this system.
Each axis has a pair of parallel co-axial reader coils
like Figure (3). We use two parallel reader coils
instead of one reader coil on each axis for two
reasons. One reason is to get powerful enough fields
to activate the RFID transponder, and the other is to
cancel the effects due to modulation index. Part
4.1.2 gives the explanations in details.
We start from a simple case: the transponder
moves along the x direction. If we can locate a
transponder which moves along the x-axis or moves
along a trail parallel to the x-axis, it is possible for
us to extend it to three dimensions by replication.
The detected voltage signal at the reader coil is
an induced voltage which depends on the mutual
inductance of the reader coil and transponder coil.
To locate a transponder is to find out the
mathematical relation of this induced voltage and
distance between reader coils and transponder coil.
This part aims to find out this relation.
3.1 Mutual Inductance of Coils
The Biot-Savart law tells us that the magnetic field
flux density B at any point P of a circular coil can be
calculated by (Stratton, 1941):
)](
)(
)(
)([
2
22
222
0
kE
xra
xra
kK
I
B
axis
+−
−−
+=
η
π
μ
(2)
)](
)(
)(
)([
2
22
222
0
kE
xra
xra
kK
r
x
I
B
radius
+−
++
+−=
η
π
μ
(3)
BIODEVICES 2008 - International Conference on Biomedical Electronics and Devices
250