Backscatter Control Logic FPGA Article

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Backscattering control logic component using
FPGA device
Article · January 2011
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Backscattering Control Logic Component Using
FPGA Device
Silmina Ulfah1, Fiftatianti Hendajani2, and Sunny Arief Sudiro2
1MIS Magister Program, Gunadarma University
2Computer Science Study Program, STMIK Jakarta STI&K
E-mail: silmina.ulfah@live.com, fivtatianti@jak-stik.ac.id, sunny@jak-stik.ac.id
Abstract—The work presented in this paper contributes to
the research of UHF RFID systems especially on backscattering
control logic. We present backscattering control logic concept,
designing backscattering control logic component based on FPGA
using VHDL, and simulate the component design in the form of
behavioral simulation and post-route simulation.
I. INTRODUCTION
THE world becomes wireless. Radio Frequency Identifica-
tion (RFID) is the hottest technology in wireless appli-
cations area. Its unique advantages such as data transmission
with extreme low power or even without power in tag can be
the biggest beneficial for goods management. In the coming
years, RFID technology can be the perfect replacement option
of bar code which is widely used in supermarkets for many
decades.
A backscatter RFID system is basically a radar system
in which the reader (radar transceiver) provides the radio
frequency signal for communications in both directions. The
tag has no transmitter power generating source, but uses
the impinging (incident) power from the reader on which to
modulate its response. In passive systems the reader power
field is also used to provide the necessary operating voltages
for the tag circuits. The tag may change either the amplitude
or phase of the re-radiated signal depending on whether
the real or reactive part of the impedance is changes. In
passive systems the reader power field is also used to provide
thenecessary operating voltages for the tag circuits, concerning
the design of RFID UHF tag, including backscattering control
logic module using Mentor Graphics, has been presented by
Ahmed Mohamed Ashry [1].
There have been numerous publications on antennas for
RFID tags but only few works have analyzed tags backscat-
tering control logic. This paper presents a design and a sim-
ulation backscatter control logic component on the RFID tag
using VHDL (Very High Speed Integrated Circuit Hardware
Description Language) based on FPGA.
This article is decomposed as follows. In the second section
we describe the concept of RFID design. In the section 3
we present an approach and the hardware implementation
consideration including the simulation result. We show the
conclusion at the end of this article.
II. CONCEPT OF RFID
Since Michael Faraday identified the field of electromag-
netism about the relationship between light and radio waves in
1845, people have pursued convenient and rapid technologies
using electromagnetic properties. Radio Frequency Identifica-
tion (RFID) technology is one of these. These technologies
have been in existence since World War II. The precursory
device of RFID technology was Leo Theremin radio wave
decoder that the Soviet government used for reconnaissance in
1940s. Related transponder technology was originated from a
discrimination system used to distinguish friendly and enemy
aircraft. Allied aircraft sent out a signal while passing near
friendly forces, and the Identification: Friend or Foe (IFF)
system identified the signal. [2]
Radio Frequency Identification is automatic identification
method, used for remotely storing and retrieving data. RFID
can be used to transmit contactless small amounts of data
over a distance. Or can be explain as processing identification
someone or something using radio frequency transmission.
Radio frequency used to read information from small device
called tag or transponder (transmitter and responder). Radio
frequency waves are electromagnet waves with a wavelength
between 0.1 cm - 1000 km. Radio frequency has a frequency
value between 30 Hz to 300 GHz. Another type of electro-
magnet waves are like infrared, visible light waves, ultraviolet,
gamma rays, X-rays and cosmic rays. RFID uses radio waves
with a frequency of 30 KHz - 5.8 GHz.
The RFID system is very similar to Smart Card technolo-
gies, the information is stored in the memory chip (transponder
card). But unlike in the smartcard, the power supply to the chip
is achieved without the use of batteries. The power supply is
generated from an RFID reader. RFID combined eminence that
not available on another identification technology. RFID do not
need line of sight to operated, can operated to environment
condition variety, and also serve high integrity level. RFID
systems are operated in various frequency bands: In the low
frequency (LF) domain at around 125 kHz and in the high
frequency (HF) domain at 13.56MHz passive tags are coupled
with the reader via a dominantly magnetic field, and transmit
information to the reader by a load modulation technique. The
ultra high frequency (UHF) domain operates in the band of
860-960 MHz. Here, the RFID tags utilise backscatter mod-
ulation for communication. Finally, RFID has been extended
ICACSIS 2011 ISBN: 978-979-1421-11-9
51

to the microwave band at 2.4 and 5.8 GHz. [3]
The RFID reader transmits a signal in the form of EM
(Electromagnetic) waves. An RFID tag within the field of the
RFID reader receives the waves and converts the EM waves
into voltage, to power the chip and electronic circuit in the tag.
The tag thus transmits back a modulated signal containing the
RFID code.
A. Basic Component of RFID System
RFID Reader included antenna, connects with the tag and
the host computer. The reader receives the tags information
and sends it through standard interface to the host computer.
It creates a read area between tags and readers. The tags emit
identifiable radio waves and te readers receive this information
through their internal antennas. The range of the read area
depends on both the readers power and the frequency used to
communicate as well as the tag used. Lower-frequency tags
can be read from shorter distances and higher-frequency tags
from longer distances.[4]
Fig. 1. Working of a typical RFID system.[4]
RFID tags, also known as RFID transponders, comprise a
memory chip and an antenna. They can differ in many respects.
A tags performance parameters are its read range, transmission
speed (data rate), bulk-read capability, and the impact caused
by surrounding objects. The frequency, the orientation to the
reader field, and the design and size of the antenna determine
an RFID tags read range, its resilience to environmental fac-
tors, and its bulk capability. The frequency and the associated
transmission protocol (anti-collision algorithm) determine the
basic rate of data transmission or the bulk read-speed.
RFID tags are categorized as either passive or active de-
pending on whether they have an on-board power source or
not, as follows :[5]
a) Passive tags: do not have an integrated power source
and are powered from the signal carried by the RFID reader.
Generally, these tags are powered by the reader antenna
through an antenna located on the tag. The readers transmis-
sion is coupled to the specially designed antenna through in-
duction or E-field capacitance which generates a small voltage
potential. This power is then used by the IC to transmit a
signal back to the reader or reflect back a modulated, encoded
identification.
b) Semi-passive tag: Tags have an on-board power
source, such as a battery, which is used to run the microchip
circuitry. However these tags utilize a battery but still operate
using backscatter techniques. Tags of this type have tags were
studied, while As was estimated using graphical methods. In
this study, we provide closed-form As calculation, while we
study both passive, as well as semipassive, tags within more
generalized context; our approach provides tag load selection
constraints and rules without restricting discussion to specific
tag/reader circuitry or minimum scattering antennas.
c) Active tags: Incorporate a battery to transmit a signal
to a reader antenna. These tags either emit a signal at a
predefined interval or transmit only when addressed by a
reader. Either way, the battery provides the power for RF
transmissions, not an inductive or capacitive coupling. As a
result of the built-in battery, active tags can operate at a greater
distance and at higher data rates, in return for limited life,
driven by the longevity of the built in battery, and higher costs.
For a lower cost of implementation, passive tags are a more
attractive solution.
Table I gives another comparison of various tags.
TABLE I
RFID TAGS COMPARISON.[6]
Passive Semi-Passive Active
Battery Needed Needed Not Needed
Life time Long Medium Short
Communication Distance Short (1m) Medium (100m) Long(1km)
Reliability Reliability Less Reliable Less Reliable
Cost Cheap Expensive Expensive
B. Tag Architecture
In this article, the design of a UHF RFID Tag is present.
The following is an explanation based on Fig. 2.[7]
Fig. 2. Block Diagram of RFID Tag.[7]
Rectifier, as the RFID tag is a passive system, a
DC voltage must be generated to bias the circuits of
the tag. The rectifier is the main block in the RFID
tag as it provides the needed DC voltage to the other
blocks of the system. The DC voltage is generated by
converting the received RF signal into a DC power.
The main challenge in designing the RFID rectifier
is to generate the required DC power using the low
voltage amplitude of the RF signal with acceptable
conversion efficiency.
Demodulator, the demodulator is the block which is
responsible for detecting the data sent by the reader
to the tag. In this system, the reader sends the data
as short gaps in the RF signal. So a simple envelope
detector is used to detect these gaps. The width of
ICACSIS 2011 ISBN: 978-979-1421-11-9
52

the gaps is chosen for optimum operation as was
discussed in system design. The envelope detector
used should be fast enough to detect short gaps. But
in the same time, it should consume minimum power
to avoid degrading the overall system performance.
Digital Control, the digital control block is the block
responsible for generating all the control signals
needed in the RFID tag system. The design of the
digital control is based on understanding the system
operation as was explained in the system design
section. To simplify the design of the digital control,
it is divided to two sub-blocks (this article is focused
on second sub-blocks):
• Mode selector: This sub-block is responsible for
determining the mode of ope-ration of the RFID
tag.
• Backscattering control: This sub-block is respon-
sible for controlling the period at which the
backscattering is active.
Oscillator, the clock used in the system is the
extracted clock from the incoming RF signal. But for
an efficient backscattering, a faster clock is needed to
modulate the scattered signal. This section discusses
the design of this internal clock generator.
Modulator, the modulator is simply a switch that
either shorts the input impedance of the chip, or
leaves it matched with the antenna. The switch is
implemented as a single NMOS transistor. The size
of the transistor should be large enough to give a low
on-resistance, but it should not be too large to avoid
loading the control logic circuit which drives it.
The RF signal supplied by the antenna is modeled as RF
source in series with the antenna impedance. The clock and
the data are extracted, and the system jumps to the Read mode,
when the pattern is recognized. The RFID tag sends its output
as a backscattered sequence, which can be noticed as notches
in the RF signal.
C. Communication mechanism
To avoid complicated synchronization circuits, the reader
fully controls the communication between the reader and the
tag,i.e. the RFID tag cannot send data unless triggered by the
RFID reader. The system clock is extracted directly from the
received signal from the RFID reader as shown in Fig. 3. The
communication between the RFID reader and the RFID tag
can be divided according to communication direction into two
links:[1]
Forward link: This is the communication link from the RFID
reader to the RFID tag. In power-up mode, a continuous RF
wave is transmitted from the RFID reader to the RFID tag,
which is used to power the tag. After entering the addressing
mode, the data is sent from the RFID reader to the RFID tag
as short gaps in this continuous wave.
Reverse link : This is the communication link from the
RFID tag to the RFID reader. This link is active only in
reading mode, where the RFID tag needs to send its data to
the RFID reader. The communication in the reverse link is
achieved using backscattering. The reflected wave should be
detected by the RFID reader.
Fig. 3. Communication Mechanism. [1]
D. Modulation Scheme
The modulation type should be choosen carefully to be sim-
ple and power efficient. In the forward link, the most suitable
modulation type is ASK-PWM (amplitude shift keying-pulse-
width modulation). In ASK the bits are sent as short gaps in the
RF Signal. This modulation type has the advantage of simple
and low power detection circuits. Pulse-width modulation is
used to differentiate between logic 1 and logic 0. PWM is
chosen in this system as it has the advantage of simple clock
extraction and detection circuits. In the reverse link, the most
suitable modulation scheme is backscattering. Backscattering
is a low-power modulation scheme in which the RFID tag acts
as a reflector that reflects a part of the incident RF wave back
to the RFID reader.[1]
Backscattering can be either ASK or PSK backscattering.
In ASK-backscattering modulation, the chip impedance is
varied between perfect match (Rin = Rant) and complete
mismatch (Rin = 0) as shown in figure 4, where Rin is the
input resistance of the chip and Rant is the impedance of the
antenna. In PSK-backscattering modulation,the real part of the
chip impedance is kept in match with the antenna, whereas the
imaginary part is varied between two capacitive and inductive
values ass shown in part (b). ASK backscattering is much
simpler and more efficient than PSK backscattering. The only
advantage of PSK is the possibility of full-duplex operation,
which is not needed for low-rate applications of RFID. PSK
backscattering is better than ASK backscattering in terms of
power, but this comparison was concerned only with available
power to the tag. The complexity and process dependence of
the PSK backscattering makes it less attractive for low-cost
applications. [1]
Fig. 4. Backscattering Types : (a) ASK and (b) PSK.[1]
E. Backscattering
The communication between a tag and a reader is achieved
by two basic methods, namely, inductive or near-field coupling
ICACSIS 2011 ISBN: 978-979-1421-11-9
53

and backscatter or far field coupling. When the tag is located
at a very close distance from the base station antenna, the data
exchange from the tag to the antenna occurs due to the voltage
induced in the tag coil through the antenna coil. This system
behaves like a transformer type coupling, wherein the reader
antenna acts as a primary coil and the tag coil as a secondary
coil of the transformer.
Using backscatter technology, interference from nearby
transmitters can be avoided, since the reader controls the
frequency of operation and can shift it if nearby transmitters
are operating at the same frequency. Also, the reflected signal
strength from the tag is proportional to the incident interroga-
tor signal, so tags outside the incident beam focus area will
reflect a weaker signal that the reader antenna can reject.
Whenever the tag receives a signal from the base station,
a voltage is sensed by a chip embedded in the tag. The DC
voltage helps to charge a capacitor in the same circuit. This
capacitor in turn operates a diode, which causes the chip
information to be sent in the form of an electrical signal to
the tag antenna. The antenna transmits the chip information in
the form of modulated RF waves. This response of the tag is
determined by an induced voltage, which can be computed by
using the radar equation. The induced voltage depends on the
field produced by the antenna (both transmitter and receiver)
and the effective antenna height (form factor) of the tag. The
chip in the tag helps in responding to the commands sent by
the reader through the antenna obeying a definite protocol. The
induced voltage causes a change in the RF impedance in the
tag which causes a production of a backscatter signal that is
detected by the base station antenna.[5]
The backscatter technique employs the principle of scatter-
ing of the incoming RF signal in a controlled manner. As
shown in figure, for example, the tag antenna reflects the
incident RF signal when the control data bit is ”1” and absorbs
RF energy when it is ”0”. In general, the scattered RF signal
can be modulated by any means that alters the RF current
flowing in the antenna, i.e. by changing the impedance with
which the antenna is terminated. Any method that dynamically
varies the resistance or reactance placed across the antenna
terminals in accordance with the desired modulation pattern
can be used. [7]
III. DESIGN AND SIMULATION
A. Design of Backscattering Control Logic Component
The digital control block is the block responsible for gen-
erating all the control signals needed in the RFID tag system.
The design of the digital control is based on understanding the
system operation as was explained in the theory section. This
part is active in reading mode. It controls the backscattering
operation to avoid re-injection of the backscattered signal. The
backscattering control logic that will be design is shown in
figure 5. And the entity of backscattering control logic based
on FPGA is shown in Fig. 6 .
B. Implementation Issue
The performance of the digital control may be different
after implementation due to some issues that will be discussed
Fig. 5. Backscattering control logic design.[7]
Fig. 6. Backscattering control logic entity using VHDL.
in this sub-section. Some of these issues can be solved by
modifying the implemented logic, while others need a change
in the transistor level implementation as will be explained.
1) Scattering Glitch Problem: The glitch problem can be
eliminated by forcing the scattering control signal to be low
when the external clock is high. This can be done using two
NOR-gates as shown in Fig. 7.
Fig. 7. Scattering glitch solution.[7]
2) Dead Lock Problem: The external clock that is used
to control the control logic is extracted by the demodulator.
But the demodulator is controlled by the control logic, i.e. the
control logic determines when the demodulator is on and when
it is off. This makes a closed loop containing the demodulator
and the control logic. This loop should be studied carefully to
avoid the instability of the RFID tag. The dead lock problem
can be eliminated by ensuring that the demodulator is always
active as long as the RFID tag is in address mode (READ=0).
This can be achieved by adding two AND-gates as shown in
Fig. 8
Fig. 8. Dead lock solution.[7]
C. VHDL Design of The Component
Nand gate, is part of the design backcattering control logic.
There are three nand gates that will be used, as can be seen in
Fig. 5. D-Flip-Flop Gate, dff gate is arranged according to the
figure 5. Can be seen that each gate has input dff different.
ICACSIS 2011 ISBN: 978-979-1421-11-9
54

dff on the second block, the input comes from dff first output
block, and so on until the output is obtained Q4. Inverter gate is
designed in accordance with the design backscattering control
logic. As seen in the Fig. 5, the incoming clock input on a
series of D-flip-flop negated first.
After the declaration of input and output (see Fig. 6), the
next step is to create a package of each component of the
previous block has been created, namely nand, dff and inverter.
After that, the identity of the signal from each port in the
whole system. After the signal is declared, the next stage is
the mapping of each of the signals to the VHDL modules that
already exist in a sequential manner, as described in the script.
library IEEE; use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
entity project is
Port ( d : in STD_LOGIC;
clk_ext : in STD_LOGIC;
clk_int : in STD_LOGIC;
read : in STD_LOGIC;
det_en : out STD_LOGIC;
scatter_en : out STD_LOGIC);
end project;
architecture Structural of project is
--------------------------------------
component nand2 is
port(a, b: in STD_LOGIC ; c : out STD_LOGIC);
end component;
--------------------------------------
component dff is
port(d, clk, rst: IN STD_LOGIC;
q, qb: OUT STD_LOGIC);
end component;
--------------------------------------
component inverter is
port(a : in STD_LOGIC ; b : out STD_LOGIC);
end component;
--------------------------------------
component gnor is
end component;
--------------------------------------
component gand is
end component;
signal clk, clk_n, q0, q1, q2, q3 :STD_LOGIC;
signal q3b, q4, q4b, out1, out2, out3,out4 :STD_LOGIC;
begin
u1 : nand2 port map (clk_int, read, clk);
u2 : dff port map (d, clk, clk_ext, q0);
u3 : dff port map (q0, clk, clk_ext, q1);
u4 : dff port map (q1, clk, clk_ext, q2);
u5 : inverter port map (clk, clk_n);
u6 : dff port map (q2, clk_n, clk_ext, q3, q3b);
u7 : dff port map (q3, clk, clk_ext, q4, q4b);
u8 : nand2 port map (q0, q4b, out1);
u9 : nand2 port map (q1, q3b, out2);
u10: gnor port map (out1, clk_ext, out3);
u11: gnor port map (out2, clk_ext, out4);
u12: gand port map (out3, read, det_en);
u13: gand port map (out4, read, scatter_en);
end Structural;
D. Simulation
As has been described previously, the simulation results of
this work will be compared with simulation results that have
been done by [7]. Figure 9 showing the results of simulations
of the backscattering control logic in [7].
This first pass simulation is typically performed to verify
RTL (behavioral) code and to confirm that the design is
functioning as intended. (see Fig. 10) It can be deduced
from the simulation results that the RFID tag responds to
the received RF signal as expected. The clock and the data
Fig. 9. Backscattering simulation control by Ahmed Ashry [7]
are extracted, and the system jumps to the Read mode, when
the pattern is recognized. The RFID tag sends its output as
a backscattered sequence, which can be noticed as notches in
the RF signal.
Fig. 10. Behavioral Simulation Result
The second simulation is placed and routed si-mulation
of the design on the chip, also known as timing simulation.
Simulation is performed and the results are displayed in the
simulator. If no stimulus is available, the design is simply
compiled and loaded in the simulator. Then, a stimulus file
and perform a simulation on the design in the simulator must
be created see the result in Fig. 11.
Fig. 11. Post-Route Simulation Result
IV. CONCLUSION
This paper mainly describes the design and simulation of
Backscattering Control Logic which is based on FPGA, XIL-
INX ISE (Project Navigator). Using these tools time required
to get desired results has become less. VHDL has been used to
enter hardware description. VHDL codes have been written,
synthesized, mapped then successfully simulated by Behav-
ioral Simulation and Post-Route Simulation. Backscattering
control logic design that is made has the same result with
a design that was made using Mentor Graphics by Ahmed
Ashry [7].
ICACSIS 2011 ISBN: 978-979-1421-11-9
55

ACKNOWLEDGMENT
We like to thank you to STMIK Jakarta STI&K for the
support to this research and publication.
REFERENCES
[1] Ahmed Ashry, CMOS RFID Design, Master of Science in Electrical
Engineering, Ain Shams University, Cairo, 2007.
[2] P. Pursula, Analysis and design of uhf and millimetre wave radio fre-
quency identification, Masters thesis, Helsinki University of Technology,
Espoo, Finland, January 2009.
[3] S. Garfinkel and H. Holtzman,Understanding RFID Technology,
Garfinkel.Book, June 2005, p. 15.
[4] P. V. Nikitin and K. V. S. Rao, Measurement of backscattering from rfid
tags, Intermec Technologies Corporation, 2006.
[5] J. Griffin, The fundamentals of backscatter radio and rfid systems, Disney
Research, 4615 Forbes Ave. Pittsburgh, PA 15213, Tech. Rep., 2009.
[6] D. Henrici, Security and privacy in large-scale rfid systems challenges
and solutions, Dissertation, University of Kaiserslautern, Kaiserslautern,
April 2008.
[7] Ahmed Ashry, KhaledSharaf, MagdiIbrahim, A Compact low-power UHF
RFID Tag, Microelectronics Journal, vol. 40, no. 11, pp. 1504-1513, 2009.
ICACSIS 2011 ISBN: 978-979-1421-11-9
56

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