Design of UHF RFID Reader And the Solution of

Crosstalk Problems

Hong Xia, Jiwen Wu Bicheng Xiao, Xiaopeng Zan, Guangbin Wang

Science of Control and Computer Engineering Science of Control and Computer Engineering
North China Electric Power University North China Electric Power University
Beijing, 102206, China Beijing, 102206, China
summerday@ncepu.edu.cn, wujiwenak@126.com fore_finger@126.com

Abstract—This paper introduced the design and realization of The paper has designed an UHF RFID reader hardware
UHF RFID reader system based on ARM9 microprocessor as the system and software system according with ISO/IEC18000-6C
controller and austriamicrosystems’ UHF RFID reader IC protocol standard. The hardware system take Samsung ARM9
AS3992 for the RF front-end processing. With designing of the hardware microprocessor as the main controller and
external power amplifier circuit , the external RF power detector austriamicrosystems' UHF RFID Reader IC AS3992 for RF
circuit and the reader antenna impedance matching digital front end signal processing. Besides, in order to improve the
tuning circuit in the hardware system , the transmitter output transmitter output power level and receiver S/N ratio, and to
power level is up to +33dBm, and the receiver S/N ratio has been realize the tags' stable distance of identification to 10 m, it
improved. The reader software system implemented high-speed
designed the external power amplifier circuit, the external RF
identification of ISO/IEC18000-6C tags and multi-tags inventory
power detector circuit and the reader antenna impedance
anti-collision algorithms, it has also solved the existing crosstalk
problems between readers in the industrial site through the IP matching tuning circuit. The reader software system has
multicast technology based on embedded Linux operating realized the ISO/IEC18000-6C standard protocol and multi-tag
system. The design UHF RFID reader system has been already anti-collision algorithms. By using the IP multicast technology,
applied effectively in the RFID management system of a certain it solves the existing crosstalk problem in multi-reader
power plant vehicles, and the operation result shows that it has situation. The design UHF reader has successfully realized
successfully realized purposes to identify and process the purposes to identify vehicles' tags in the distance of 10 meters
information of vehicles' tag in the distance of 10 meters with with stable performance.
stable performance.
II. THE DESIGN OF SYSTEM SCHEME
Keywords- RFID; UHF Reader; RF Impedance Matching;
Power detector; IP Multicast The design architecture of RFID system consist of 4 layers
as shown in Figure 1, including the application program layer,
I. INTRODUCTION the operation system, the layer of hardware control and the RF
front-end.
RFID (Radio Frequency Identification, RFID) is a
technology based on the theory of spatial coupling, which Application layer UHF Reader Firmware
alternating magnetic field or electromagnetic field, to achieve
non-contact transmission of information and through the
information passed to achieve the identification of its Operat ion system Linux OS
objectives [1]. An RFID system mainly includes RFID readers,
RFID tags and the system software. According to the different
Hardware control layer S3C2440A Hardware
operating frequency band, it can be divided into low-frequency,
high-frequency, UHF (Ultra High Frequency, UHF) and SP I interface
microwave types [2]. Compared with low and high frequency RF front-end A S3992 RFID module
RFID technology, UHF technology has longer identified
distance, high-speed reading and other advantages. Along with Figure 1. The design architecture of UHF RFID reader system
the price of the UHF tag declining, which greatly reduces the
threshold for application of RFID technology, UHF-band long- The application layer is responsible for the initialization of
range automatic identification technology applications has been the reader IC chip AS3992, the implementing of ISO/IEC
gradually applied by the vehicle, to modern logistics, e- 18000-6 C standard protocol and multi-tags inventory anti-
commerce, traffic management and military management and collision algorithms. Besides, it also solved the crosstalk
other fields, therefore it become one of the hot and important problems through the use of IP multicast technology.
development of RFID technology nowadays.
The operation system is the core layer. Based on running
Linux2.6 operating system, it includes the device drivers for spi
and network devices which are integrated in micro-controller,

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and provides the I/O interface for applications accessing to B. The Design of RF Module Circuit
low-level hardware devices. 1) The Overview of Reader IC AS3992: The RF circuit
The hardware control layer takes ARM9 S3C2440 as the module takes AMS AS3992 as the core part for longer range
controller unit, and constructed the minimum hardware system. identification and higher power output applications. AS3992 is
The data communication with the RFID module is through the an integrated analog front-end chip which provides data
spi interface. processing and ISO18000-6C / B 900MHz UHF RFID protocol
The RF front-end takes AS3992 as the core module, its handling, its receiving sensitivity is up to-86dB. The on-chip
main features include receiving data bytes from hardware transmitter supports the low level data coding, automatic
control layer, data coding and modulated carrier signal. generation of FrameSync, Preamble and CRC, it is also capable
Besides, it receives the tag backscattering signal and then of ASK or PR-ASK modulation. The following two outputs is
demodulation, decoding and assembly into the framed data, available: low power high linear output (~ 0dBm ) and high
which are accessible to the host system through a 24 byte output power output(~ 20dBm) , the former can be used to
AS3992 FIFO register. The RF front-end circuit composed of drive an external power amplifier to achieve higher power
AS3992 module circuit and its surrounding matching circuit,
output and longer-range with nominal output impedance of 50
900MHz Balun circuit, external high-power amplifier
SKY65111-348LF circuit, 20dB directional coupler ohm, while the latter can be used for antenna driving in case of
RCP890A10 and the antenna impedance digital tuning circuit, short range applications. The on-chip receiver with AM and
its block diagram shown in Figure 2. PM demodulation supports comprising two input mixers
Antenna
followed by the gain and filter stages, the digital base-band
circuit (including decision circuitry, bit decoder and framer).
The clean framed data is accessible to the host system (MCU)
Impedance Matching

via 24 byte FIFO.

Parallel or SPI serial interface can be selected for
Balun
communication between S3C2440A and the reader IC AS3992.
Directional UHF Reader
Coupler IC AS3992 The S3C2440A includes 2-ch 8-bit SPI. Therefore, the design
Pow er
LPF
Amplifier Balun system use SPI mode and external interrupt request mode to
communicate between the "master" S3C2440A and AS3992
Pow er D etector which behave as the "slave". In SPI mode, the maximum data
 rate of AS3992's communication is 2Mbps.
Figure 2. The block diagram of reader RF front-end circuit
2) The key technology of remote RF identification
a) The design of RF power amplifier circuit:In order to
III. THE DESIGN OF READER HARDWARE SYSTEM achieve long-range (~ +10 m) RF identification, the transmitter
UHF reader hardware circuit is mainly composed of two of UHF RFID reader requires generating between 28dBm to
parts-- the S3C2440A controller module, which includes its 33dBm output power with 50ohm load typically, so the design
peripheral interface circuits, and the RF circuit module. reader IC AS3992'transmitter chooses low power high linear
differential output to drive external power amplifier circuit.
A. The Design of Host Hardware Control Module The external RF power amplifier used in this design is
S3C2440A is a highly integrated 16/32-bit RISC Skyworksinc's SKY65111-348LF, which is a high-power
microprocessor chip produced by Samsung company. The amplifier IC with output power greater than 33dBm
MCU can stably operate at a maximum clock of 400MHz, and @915MHz. It has an integrated 3-stage analog power control
provides a complete set of common system peripherals [3], the voltage for achieving the desired output power levels. Table 1
integrated on-chip functions including NAND Flash controller, shows its general RF transmitting electrical specifications
TFT / STN LCD controller, DM9000 Ethernet controller and 2- under given conditions [5].
ch SPI controller, for which the difficulty of design of the UHF
reader hardware system is lower threshold. The UHF reader TABLE I. GENERAL RF TRANSMITTING ELECTRICAL SPECIFICATIONS
host hardware system is composed by S3C2440A controller
module, the DM9000 Ethernet interfaces, SPI, JTAG, UART, VCC=5V, VREF=3.5V, VAPC=2.7V, PIN=-30dBm, TMP=25DŽC
Parameter/Symbol Min. Typ. Max. Unit
SDRAM, NAND Flash and other peripheral devices. When the
Frequency range 902 928 MHz
system power up, it start running the boot-loader program Output P1dB/P1dB 29.5 dBm
saved in NAND Flash and then booting embedded Linux Gain/S21 40 dB
operating system start-up, completing the hardware devices
drivers initialization. Because of the limited length of this 1dB compression point is defined as the dB amplitude level
article, the transplanting methods of the Boot Loader and the of the input signal, which makes the gain of linear amplifier
Linux operating system can be referenced to the literature [4]. declines 1dB. The formula (1) based on input/output dB level
of signal power and the power amplifier gain is as follow:


 Vom-1dB= Vim-1dB + (Gain -1) (1) c) The design of reader’s antenna impedance matching
digital tuning circuit: RF circuit modules are generally
Where Vim-1dB, Vom-1dB respectively represents for input and
connected with each other by microstrips with the
output signal power of 1dB compression point and Gain
represents for the linear power gain level of amplifier. characteristic impedance Z0 = 50ohm. In order to achieve
maximum output power and improve the noise figure, the
According to formula (1), when the amplifier’s Gain is impedance matching is needed among each module and the
40dB, Vom-1dB is 29.5dBm, the input power Vim-1dB is equal to - microstrips, etc [6]. Besides, UHF RFID reader systems use an
9.5dBm. Therefore, the low power high linear output (~ 0dBm) external antenna to communicate with passive tags, but
way of the reader IC AS3992 can be taken to drive the external because of the antenna fixture variability in shape and size, it
power amplifier SKY65111-348LF to achieve the transmit leads to that the antenna load impedance and VSWR are easily
output power requirements for long-range(~+10 m) tags influenced by the external environment changes, resulting in
identification.
the loss of transmission power reader and receiver sensitivity
b) The design of external RF power detection circuit: decreasing [7]. According to the maximum RF transmission
The system transmit RF output power levels require dynamic distance (R) Friis formula (2) [8, 9]as follow:
tuning between +10dBm and +33dBm. In order to achieve the λ Pt ⋅ Gt ⋅ Gr ⋅ Co (2)
R=
desired output power, the system designs the external power 4π Pth
detector module, Maxim MAX2206EBS+, which is used to
measure and tune output power level. The Maxim Where Pt is the reader transmission output power level, Gt is
the gain of the external antenna, Gr, Co , Pth respectively
MAX2206EBS+ wideband (800MHz to 2GHz) power
represents for the gain of tag antenna, the transmission
detector's maximum RF input power is 20dBm and its output coefficient, and the threshold power to activate tag antenna
voltage increases monotonically with increasing input power chip’s operation. According to the formula (2), by improving
level. Pt, the transmission power of the reader' antenna end, the RFID
The power detector RF input pin connected to the system can effectively increase the RF maximum transmission
directional coupler coupled port P3 to measure its RF output distance. Therefore, in order to obtain the maximum transmit
power level, as shown in Figure3. The analog voltage output output power in the reader's antenna end, it has designed the
pin from the power detector is connected to the AS3992 ADC impedance matching circuit between the microstrip and the
pin for conversion, and the AS3992 DA converter output pin reader antenna as shown in Figure 4.
DAC is connected to SKY65111-348LF analog power control
C90 L9 C91 C106 Antenna
voltage input pin. When measured, the AD conversion result is RF_TX 1p5F 10nH 2p7F 100pF
available in the "ADC readout register" for the host system to SCLK SDAT
SDAT
read out through the SPI interface. The host system controls TUNE1 L11 TUNE3
voltage output voltage (0 ~ 3.2V) of AS3992 DA converter 6n8H

SCLK
SDAT

SDAT
RF+
RF+

RF+
RF+
SEN

SDAT
RF+
SEN

RF+

SEN
L10 RF+ R F-
output pin DAC, according to AD conversion result, to adjust 4n3H SCLK RF+ R F-
GND SCLK GND SCLK GND SCLK
the external power amplifier gain level, therefore it can achieve SDAT SDAT DGND SCLK
DGND

DGND

DGND
TUNE2
GND
VDD

VDD
the control and adjust of the output transmit power level until it SEN VDD

VDD
RF-
RF-

RF-
RF-

RF-
RF-
reaches the expected output power level. PE64904 PE64904 PE64904
C94 C96 C95
The design of RF front-end circuit including AS3992 3V3
C115 C113 C112
output matching circuit, the external power amplifier, the 100pF 10nF 2u2F
directional coupler circuit and the external power detector
shows in Figure 3.
Figure 4. The reader antenna impedance matching digital tuning circuit
C22 C23 C15 L13 C25 C26 C27
As3992 different outputs C30 4V7 10uF 1nF 100pF 1nH 100pF 1nF 10nF 4V7

VDD _B
RF O_P
12pF
Balun
C28
10uF The impedance matching digital tuning circuit of the reader
C30 L5 C32 16 15 14 13 L15
C35
47pF
L4
39nH
4
B2 GND 1 12pF 1nH 27pF 1
2
12 10nH L14
1n8H
C33 antenna composed of inductors, capacitors and digitally tunable
UB 2 11 100pF
C34
47pF
L6
39nH
6 B1
GND 3
C36
0p5F
3
SKY65111
10 C37 C38
capacitor (DTC) PE64904. The inductors use Coil-craft’s high-
4 9 15pF
RF O_N C39 4V7
C53 C54 C55
10uF 1nF 100pF
5 6 7 8 17 Pow er Amplifier
6p8F
4V7
Q series and the capacitors are selected Murata's GRM series
12pF
C60
2u2F
L7
1n2H C2
Vapc1
C58 C59
1nF
C62
2n2F
and GJM series, respectively for blocking and bypass for DC
100pF
4V7
R10 R19
100pF Vapc2 and high frequency applications. The PE64904 is a 5-bit 32-
C3 C24 C119
910R 2K4 1nF 100pF 1nF state DTC offering high RF power handling and can be used in
RCP890A10
LFCN-1000D+ 4 series (C=0.7-4.6pF) or shunt configurations (C=1.12-5.18pF)
MAX 2206EBS+ R10
910R RF_TX
2
P2 GND P1
1 3
GND
RFOUT RFIN
1 to support a wide variety of tuning circuit topologies. The
3V3 C88 R21 7 GND 8 C122 GND
2 1
47pF 17R8
3
P3 GND
GND
P4
4 100pF 2 device is controlled through the widely supported 3-wire (SPI
VCC RFIN
ADC_IN 4
OUT GND 3
R49
294R
R48
294R RF_RX Low Pass Filter compatible) interface [10].
C89
Directional Coupler
27pF Power D etector The operation principle of the reader antenna impedance
matching circuit is described as follow. The AS3992's receiver
Figure 3. The design circuit principle diagram of external power amplifier,
comprises the input RF level indicator which is used for
the directional coupler circuit and the external power detector
diagnostic of circuitry or environmental difficulties including
the reflection of poor antenna, reflection of reflective antenna's


environment, or directional device leakage. The measurement the system inventory procedure, the maximum tag inventory
of the level of the reflected transmitting carrier, can be efficiency can reach[12, 13]. Therefore, according to the
converted by measuring the DC levels on the receiver's mixers relationship between the current Q value and the detected
outputs which are proportional to the RF input level via collisions counts, the design of ISO/IEC 18000-6C tags
AS3992's internal AD converter, the result can be accessible to inventory flow chart with active tuning of Q numerical value is
the host system. The host system S3C2440A communicates shown in Figure 6.
with DTC PE64904 through the SPI interface, and dynamically
tunes the capacitance value until obtained the minimum Start inventory
reflected input RF level. In other words, the minimum level of
the reflected transmitting carrier reached, and it obtained the Send Select and
maximum transmit output power. PE64904 functional block Q uery Commands
diagram [10] shows in Figure 5.
Initialize anti-
collis ion algorithm

RF + RF-
G et Tags
[slot=0]
Serial
Interface
CMOS Control One tag None Tags multi-Tags
Dri ver [slot=0] [slot=0] [slot=0],colliding
Figure 5. The PE64904 functional block diagram
Save tag data slot_count-- collis ions++
(EP C,PC...)
IV. THE DESIGN OF READER SOFTWARE SYSTEM
Send N Slot_count Y Collisions Y
In UHF RFID reader systems, the reader IC AS3992 QueryRep = =0? = =0?
provides integrated analog RF front-end, and handles ISO/IEC N
18000-6C 900MHz RFID protocol internally. However, the N Collisions N Collisions
procedure realization of EPC C1G2 protocol, the tags anti- <=(1<<4)/8? > =(1<< 4)/4?
collision algorithms, and the application of IP multicast Y Y
technology to solve existing crosstalk problem in multi reader
Send Send Send
situation are all implemented based on S3C2440A Linux Q ueryAdjustU p QueryAdjustNIC QueryAdjus tDown
software system.

A. THE IMPLEMENTATION of ISO/IEC 18000-6C Y Continue

PROTOCOL AND ANTI-COLLISION ALGORITHM inventory?
The reader IC AS3992 supports full EPC C1G2 digital N
protocol, and according to EPC C1G2 protocol, the readers Return
manage tag populations using the three basic operations-
Select, Inventory, and Access. Each of these operations Figure 6. The ISO/IEC 18000-6C tags inventory flow chart
comprises one or more commands.
The ISO/IEC 18000-6 protocol includes three types A, B, B. THE SOLUTION of MULTI-READERS CROSSTALK
C, and the difference with each other is that the tags anti- PROBLEMS
collision algorithms and data coding in different ways [11]. For
ISO 18000-6C standard protocol, it uses of slotted random anti- The transmission of UHF frequency signal has
collision algorithm (also known as Q anti-collision algorithm) characteristics such as high data rate, covering large area, and
[12]. The working principle of Q anti-collision algorithm is that avoiding or even passing through encountered obstacles. There
the inventory command Query contains a slot-count parameter may exist crosstalk problems in the multiple UHF readers
Q. Upon receiving a Query participating Tags pick a random environment [14]. The actual system test shows that when
value in the range (0, 2^Q-1), inclusive, and shall load this multiple UHF reader equipment were deployed in a site, and
value into their slot counter. Tags that pick a zero shall transit were required that each tag must be identified by each reader
to the reply state and reply immediately. Tags that pick a according to the readers number orders, there’s crosstalk
nonzero value shall transit to the arbitrate state and await a problem between the current reader and other subsequent
QueryAdjust or a QueryRep command. Q is an integer in the reader. As shown in Figure7 is the case that a certain power
range (0,15); the corresponding Tag-response probabilities plant's UHF readers exists crosstalk problem when identifying
range from 2^0=0 to 2^-15=0.000031. If multiple tags reply in tags on the coal vehicles. Despite the distance between the K1
slot (a) due to slot counter values are equal to 0, then the reader reader’s antenna and the K2 reader’s antenna reach 40m, there
should detect the collision occurred. Besides, when numerical still existing crosstalk problem sometimes that the rear reader
value of 2^Q and the number of the unidentified tags get about #K2 identifies the tag located in the front reader #K1, resulting
equal for taking fine adjustment algorithm of Q value during in the recognition sequence error occurs.


 d<6m, 40m>
identified current tag data among the previous readers nodes
....... ....... read[0 ... N-1] .
UHF UHF UHF
Reader#K1 Reader#K2 Reader#N The new allocated tag node will be dynamically inserted
into each reader's tag tree through the way of IP multicast
transmission only happened when number 0 reader correctly
identify tag data, and only when the last number X reader
...... Car_tag#x ....... Car_tag#y ....... Car_tag#z ...... identify and process the tag node data correctly, the tag node
can be removed from the tag tree.
Coal vehicles driver direction
V. PERFORMANCE ANALYZING
Figure 7. The crosstalk problem existing in multi-readers inventorying tags
In China, the RF specifications testing for UHF band RFID
In order to solve the crosstalk problems between the UHF reader facilities use the "Provision for 800/900MHz Radio
readers, The paper has realized the reader's tag inventory Frequency Identification (RFID) technology applications
operation synchronous through the IP multicast technology and (Tentative)” as the reference standard [17]. In this document, it
the design tag balance binary tree. gives out some RF specifications that a RFID wireless
transmitting device should meet up with. It includes: carrier
1) The design of IP multicast application:IP multicast is a frequency tolerance: 20e-6; Channel bandwidth and the
method of sending Internet Protocol (IP) datagrams to a group occupied channel bandwidth (at 99% capability): 250KHz˗
of interested receivers in a single transmission, only after transmission power limit: 33dBm; Adjacent Channel Power
joining a particular IP multicast group, can a host receive data leakage Ratio(ACPR):40dB(the first adjacent channel),
packets that sent to that group[15]. 60dB(the second adjacent channel), the spurious emission
In order to use multicast technology to solve the crosstalk limit; When working at the operating mode of frequency-
problem, it require that all UHF readers are serialized, and join hopping spread spectrum, each channel's maximum dwell time
a IP multicast group after power up, when a tag is correctly should below 2s. And so on.
identified, each reader can not only send tag node data packets
to, but also can receive update tag data from the multicast To analyze the design UHF RFID reader's RF performance,
group. Besides, to quickly lookup and update a tag node data, it the Tektronix's real-time spectrum analyzer RSA3408B was
designs a balanced binary tree to build the tag tree to organize used. It works at the frequency-hopping spread spectrum mode,
identified tag data. Therefore, the average lookup time and the maximum dwell time was set up as 2s. The testing
complexity is O(logN) [16]. The design data structure of a tag result is as follows: carrier frequency tolerance: 10e-6; channel
node in the tag tree contains the tag information and the bandwidth: 180 KHz; the maximum transmitting power could
identification state of arrays corresponding to each reader, the reach 33dBm; ACPR: 45.97dB (the first adjacent channel),
tag node data structure is described as below. 69.54dB (the second adjacent channel). The measure result
shows that the design RFID reader's performance meets up
struct tag_inode{ //the tag data node with the standard described in "Provision for 800/900MHz
char *tagData; //the tag data content Radio Frequency Identification (RFID) technology applications
(Tentative)".
unsigned char numOfReaders; //the number of readers
……….. VI. CONCLUSION
//tag states corresponding to each reader In this paper, it has designed a UHF RFID reader system
according with ISO/IEC18000-6C protocol standard. The
unsigned char statInReaders[numOfReaders]; design RF front-end circuit module has solved the problem that
the reader antenna load impedance and VSWR are easily
unsigned char delete; // whether to delete the tag node
influenced by the external environment changes, due to the
}; reader's antenna fixture variability in shape and size, which
results in the reflecting of reader's transmitter power and the
2) The tag tree dynamic insert, update and delete decreasing of receiver sensitivity. It has realized the
node:When the reader numbered N (N>=0) identifies a tag data transmitting output power level up to +33dBm and effectively
successfully, it find out the node data structure from the tag improved the receiver S/N ratio and the tag identify distance.
tree, according to tag data , then get the state array As to the reader software module, on the one hand, by
statReaders[] of the node data structure. if statInReaders [0...N- implementing anti-collision algorithms, Random Slotted, it has
1] are true, the reader's current tag identification process is solved the multi-tags inventory collision problem, improved
viewed as valid, and then update the array element the tag’s inventory efficiency. On the other hand, it has
statReaders[N] as true , send the updated tag node data to IP effectively solved the UHF reader’s crosstalk problem by using
multicast group to update the tag node date in other readers' tag the IP Multicast technology and the design of balanced binary
tags tree. The operation results show that the design UHF
tree. Otherwise, the tag-reading operation is viewed as invalid RFID reader system has successfully realized purposes to
because there are at least a reader that has not yet been identify and process the information of tags on vehicles in the
distance of 10 meters with stable performance.


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