systems can handle much more RFID transponders with
a lower overall throughput. CDMA-based system, on the
other hand, are able to handle a limited amount of RFID
tags with higher overall throughput. For instance, assuming
a limited amount of RFID transponders for a traffic rate
G = 0.73. The throughput of unslotted ALOHA would be
S
ALOHA
= 16.95% and the throughput of slotted ALOHA
S
unslotted ALOHA
= 35.18%. A CDMA-based system with a
spreading factor C of 10 would have there its maximum
throughput of S
CDMA,C=10
= 58.31%. This scenario is
shown in Figure 2.
Finally, it can be stated that CDMA-based RFID systems
may be better for particular applications, in which the
number of transponders is limited and the inventory process
has to be made very fast, e.g., fast production lines and
automation processes.
Particular slotted ALOHA CDMA systems and corre-
sponding performances may be found in [30], [31]. Other
works describe certain CDMA systems with error correc-
tion which really outperform the TDMA-based systems.
Examples can be found in [32]–[36]. Also this list is not
complete it gives a short overview of CDMA-based system
performances.
IV. BASIC PRINCIPLES OF CDMA FOR RFID
This section shows some basic work regarding UHF RFID
systems in conjunction with CDMA.
An IC design for an experimental transponder is offered
in [37]. The transponder uses time-hopping DS-CDMA, but
operates within the RFID HF region (13.56MHz). There-
fore, the transponder cannot be compared directly with the
proposed approach, but shows, that a fully integrated circuit
for future releases of CDMA-based RFID UHF transponders
is possible. Wang et al. [38] describes an anti-collision
method based on CDMA. Gold codes are used as spreading
sequences. Also, a first design of a transponder is outlined.
Within this design, a field effect transistor (FET) is used as
source for backscattering. A new transmission scheme for
TTF transponders is presented in [39]. The uplink is based
on asynchronous DS-CDMA. It is shown, by simulation,
that the proposed CDMA method outperforms the classical
RFID transmission in terms of channel capacity. Also, Gold
codes are used to separate the various RFID transponders.
Mutti and Floerkemeier [40] explore the chances how
CDMA methods can help to inventory large buildups of
transponders. A combination of slotted ALOHA and CDMA
is proposed to receive better inventory results. Therefore,
the CDMA method is only used at the moments when
the channel collides. Also, they show that Gold codes are
very suitable for the usage within RFID systems. Simulation
results show that Gold codes outperform Kasami codes.
A new anti-collision method is presented in [9] with the
goal to achieve a fast inventory process. A so called spread
partial-Q slot count algorithm which is based on slotted
ALOHA CDMA increases the overall throughput at the cost
of bandwidth and complexity.
Other algorithms are presented in [41], [42], where the
throughput is increased by using dynamic slotted ALOHA
CDMA algorithms with orthogonal variable spreading fac-
tors. Theoretic analysis and simulations show an increasing
performance regarding the identification process. However,
the proposed algorithms perform different, when the condi-
tions of the RFID system changes. A better gain may only
be achieved, if the system itself is involved into the design
of the algorithms.
Another approach is described by Liu and Guo [4], that
uses Huffman spreading sequences to improve the process of
inventory. Therefore several performances have been com-
pared. Simulation results show the increase in performance
if proposed Huffman spreading sequences are used. Anyway,
a direct hardware implementation has to consider the various
states within the IQ constellation diagram to be imaged to
the backscatter modulator.
A minimum mean-squared error single user adaptive
receiver for the asynchronous DS-CDMA system, based on
the least-mean-square (LMS) algorithm is presented in [43].
The article states that the proposed algorithm achieves a
faster convergence rate than the transversal LMS algorithm.
Wuu et al. [44] presents a zero-collision scheme, that is
based on CDMA and hash-chain mechanisms. The results
of the paper show that with the applied techniques, not only
a zero-collision scheme, but also a secure channel may be
realized to outperform standard technologies. Anyway, the
authors assume, that CDMA can be implemented into RFID
systems, particularly RFID transponders.
Summing up, it can be stated, that the usage of CDMA
for RFID not only provides better system performance, but
also offers advanced security issues. Furthermore, very low
effort was put into the realization of such a CDMA-based
RFID system as such. Therefore, this missing piece is one
of the subjects of this work.
TX path
Control
RX path
TX
RX
Transponder 1
Code 1
Transponder 2
Code 2
Transponder n
Code n
RFID reader
Figure 3. Basic architecture of RFID system
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