A System Design for UHF RFID Reader

Chen YingCollege of Communication

Hangzhou Dianzi University Hangzhou, China

chenyinghziee@yahoo.com.cn

Zhang Fu-hong College of Communication

Hangzhou Dianzi University Hangzhou, China

fuhong@vip.sina.com

Abstract—This paper introduces a system design for RFID reader. The RFID reader is compatible with EPC Class-1, Generation-2 Standard, operating at the 915MHz band. The UHF RFID reader includes RF analog front end (AFE), the base band design and clock control. The RFID RF AFE contains transmitting circuitreceiving circuit frequency synthesize, circulator, etc. The base band contains the FPGA chip, 100M hardware resources of network supporting, DDR SDRAM, FLASH, A/D, D/A, etc. The FPGA chip inseted NiosII soft core. This architecture is an advantage for implementing various kinds of RFID standards by changing the soft of NiosII core in FPGA, and efficiently reduces the design and development time and cost.

Keywords-RFID reader;RF AFE; base band; Nio ;

I. INTRODUCTION

Radio frequency identification (RFID) is a non-contact technology that means to automatically identify people and objects on the basis of radio waves. In recent years, RFID has received much attention as it has rapidly expending with applications such as the building access control, toll collection, vehicle parking access control, animal tracking, inventory management and so on.

Several frequency ranges are used in RFID technology, such as LF (125KHz, 135 KHz), HF(13.56 MHz), UHF(860~960 MHz), and microwave(2.4 GHz). UHF band is used worldwide for its long read range and low manufacturing cost in the distribution field.

RFID systems consist of three components: an antenna, a reader and a tag. This paper presents a system design for UHF RFID reader which is compatible with EPC Class-1, Generation-2 Standard, operating at the 915MHz band.

II. UHF RFID PROTOCOL OVERVIEW

“EPC Radio-frequency Identification Protocols Class 1 Generation 2 UHF RFID Protocol for Communication at 860 MHz – 960 MHz”, in short EPC C1G2, is the standard protocol developed by EPCglobal for RFID devices. This protocol outlines the air interfaces and commands between an RFID reader and an RFID tag.

This protocol defines the physical and logical requirements for a passive-backscatter, Interrogator-talks-first (ITF), radio-frequency identification (RFID) system operating in the 860 MHz – 960 MHz frequency range. The system comprises

Interrogators, also known as Readers, and Tags, also known as Labels. An reader transmits information to a Tag by modulating an RF signal in the 860 MHz – 960 MHz frequency range. The Tag receives both information and operating energy from this RF signal. Tags are passive, meaning that they receive all of their operating energy from the reader’s RF waveform. An reader receives information from a Tag by transmitting a continuous-wave (CW) RF signal to the Tag; the Tag responds by modulating the reflection coefficient of its antenna, thereby backscattering an information signal to the reader. The system is ITF, meaning that a Tag modulates its antenna reflection coefficient with an information signal only after being directed to do so by an reader. Readers and Tags are not required to talk simultaneously; rather, communications are half-duplex, meaning that readers talk and Tags listen, or vice versa. An reader sends information to one or more Tags by modulating an RF carrier using double-sideband amplitude shift keying (DSB-ASK), single-sideband amplitude shift keying (SSB-ASK) or phase-reversal amplitude shift keying (PR-ASK) using a pulse-interval encoding (PIE) format. An reader receives information from a Tag by transmitting an unmodulated RF carrier and listening for a backscattered reply. Tags communicate information by backscatter-modulating the amplitude and/or phase of the RF carrier. The encoding format, selected in response to reader commands, is either FM0 or Miller-modulated sub-carrier.

III. DESIGN IMPLEMENTATION

The figure 1 shows the block diagram of the UHF RFID reader. The UHF RFID reader consist of RF analog front end (AFE), the base band and clock control.

Figure 1. Block diagram of the RFID reader.

978-1-4244-2251-7/08/$25.00 ©2008 IEEE

A. AFE design The RFID RF AFE contains transmitting circuit,receiving

circuit frequency synthesize, circulator, etc. The block diagram of the RFID reader AFE is shown in figure 2. The circulator, which determines the performance of the reader system. The signal transmitting to the tag and receiving from the tag use one antenna, at same frequency and at same time. Because the transmitting signal need to activate the tags in distance, transmitter power is far greater than receiver power. Because of the same antenna, it will leakage transmitting signal to the receiving path, so that all devices in receiving circuit will unable to work. We need a good performance circulator to make transceivers isolation. Because the receiver power is 0dBm and the transmitter power is 30dBm. we have chosen the circulator which have 30dBm isolation. Transmitting circuit received 70M IF signals from base band, and LC filter filtering the 70M IF signals quantization noise and spurious. Then these 70M IF signals enter mixer frequency to UHF signals. These UHF signals including various stray such as mirror image frequency signals, local oscillator leakage signals need to RF filter filtering. And then, these signals need to arrive at 30dBm power by the power amplifier. Finally, these signals can go out by the circulator and the antenna. Receiving circuit receive backscatter signals from tag, these backscatter signals mingled on the continuous launching carrier wave. These signals are divided into I and Q subchains. Then these signals pass the low noise amplifier to amplify, pass filter to filtering. Finally, these signals enter mixer frequency to 70M IF signals. Unlike an HF transponder, in UHF transponder the mixed frequency signal cannot be extracted directly from the carrier. The mixed frequency signal is created by frequency synthesize.

Figure 2. Block diagram of the RFID reader AFE.

The realization of RF AFE contains HYH504BZ circulator, MAMXSS0011 mixer, DF915S25A and LB070DS16 filters, RF5110 RF power amplifier, ATF54143 low noise amplifier. Figure 3 is a photograph of reader RF AFE.

Figure 3. The photograph of RF AFE.

B. Baseband design The block diagram of the reader base band is shown in

figure 4. The base band contains the FPGA chip which inserts the NIOS II soft core system to carry on protocol processor, 100M hardware resources of network supporting, DDR SDRAM, FLASH, A/D, D/A, etc. The main task of the base band section is to treat the communication protocol. The reader is compatible with EPC Class-1, Generation-2 Standard. The standard defines the protocol for a UHF passive backscatter RFID system, featuring the following capabilities. An reader sends information to one or more tags by modulating an RF carrier using double-sideband amplitude shift keying (DSB-ASK), single-sideband amplitude shift keying (SSB-ASK) or phase-reversal amplitude shift keying (PR-ASK) using a pulse-interval encoding (PIE) format. Tags receive their operating energy from this same modulated RF carrier. A reader receives information from a tag by transmitting an unmodulated RF carrier and listening for a backscattered reply. Tags communicate information by backscatter-modulating the amplitude and/or phase of the RF carrier. The encoding format, selected in response to reader commands, is either FM0 or Miller-modulated sub-carrier. The communications link between readers and tags is half-duplex, meaning that tags shall not be required to demodulate reader commands while backscattering. A tag shall not respond using full-duplex communications to a mandatory or optional command. An reader manages tag populations using three basic operations: a) Select. The operation of choosing a tag population for inventory and access. A select command may be applied successively to select a particular tag population based on user-specified criteria. This operation is analogous to selecting records from a database. b) Inventory. The operation of identifying tags. A reader begins an inventory round by transmitting a Query command in one of four sessions. One or more tags may reply. The reader detects a single tag reply and requests the PC, EPC, and CRC-16 from the tag. Inventory comprises multiple commands. An inventory round operates in one and only one session at a time. c) Access. The operation of communicating with (reading from and/or writing to) a tag. An individual tag must be uniquely identified prior to access. Access comprises multiple commands, some of which employ

one-time-pad based cover-coding of the R=>T link.

NIOS II

PCEthernetPHY&MAC

JTAG

DDR SDRAM Memory

FLASH Memory

Button,

LED

modulator

demodulator

AD

DA

Rx

Tx

clock

Figure 4. Block diagram of the RFID reader baseband.

The architecture is not only good for being implemented in EPC C1G2 protocol, but also for other RFID standards. Depending on their different logic function, the base band is divided into several modules. The A/D changes the 70M IF Rx signal into digital signal. The Rx signal is demodulate and decoder by the demodulate and decoder module. The Tx signal, which is performed by modulated and encoder with the PIE, is modulate and coder by the modulate and coder module. The DA changes digital signal into the 70M analog Tx signal. The control unit, which use the NiosII soft core inserted in FPGA, performs all commands and controls. This architecture is an advantage for implementing various kinds of RFID standards by changing the soft of NiosII core. The module of DDR SDRAM, FLASH implement the control of the memory. The ethernet module performs commands and controls with PC.

The realization of base band contains AD9248, DA5674, Cyclone EP2C35F672I FPGA, MT46V16M16TG DDR SDRAM and S29GL256N FLASH .Figure 5 is a photograph of an assenmbled testing PCB of base band.

Figure 5. The photograph of base band.

C. Clock design The clock module produces 1 group 55MHz clock, which

actuates by the electric circuit in the base band module. The clock divide into 5 groups, which uses to make the ADC sampling clock (1 group), the PLL clock in FPGA works (3 groups), as well as disposition chip clock (1group). Moreover,

the clock produces 1 group 220MHz clock, which takes the DAC sampling clock. In this clock module, we select the 55MHz clock as ADC and the FPGA clock, which facilitates the synchronous processing in ADC and the FPGA logarithm. Moreover the DAC uses 4 time of interpolation filters, after interpolates filter, the data rate become 220MHz from 55MHz, therefore the sampling frequency of DAC is 220MHz.

IV. EXPERIMENTAL RESULTS

We divided into three parts to verify the results of this reader: The waveform of transmitting signal from reader, the waveform of backscatter signal from tag and the signal quality indicators. Signals launching from antenna to tags acquisition by the spectrum analyzer show in figure 6. These signals use PIE coding, signal power at 30dBm, and low signal power 4.4dBm. When the distance is 1.5m between antenna and tag, the tag was activated. Information by backscatter-modulating the amplitude and phase of the RF carrier acquisition by the spectrum analyzer show in figure 7.

Figure 6. Waveform of transmitting signal from reader .

Figure 7. Waveform of backscatter signal from tag.

The tag backscatter signal quality indicators show in figure 8. The backscatter signal modulation depth is 15.859%, the duty cycle is 49.041%, rise times are 1.713us, fall times are 3.8934us, receiving link frequency is 64.20 kHz, which is compatible with the EPC Class-1, Generation-2 Standard.

Figure 8. Backscatter signal quality indicators.

V. CONCLUSIONS

In this paper, we presents a system design for UHF RFID reader. The UHF RFID reader is compatible with EPC Class-1, Generation-2 Standard, operating at the 915MHz band. The UHF RFID reader includes RF AFE, the base band and clock control. The base band, which inserted the NiosII soft core in FPGA, performs all commands and controls. This NIOSII based baseband structure is an advantage for implementing

various kinds of RFID standards, and efficiently reduces the design and development time and cost.

REFERENCES

[1] EPCglobal, “EPC radio-frequency identity protocols class-1 generation-2 UHF RFID protocol for communications at 860 MHz - 960 MHz version 1.0.9”, EPCglobal Standard Specification, 2004.

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[4] Chen Linying, Hou Chunping,Mao Luhong , Wu Shunhua, Xu Zhenmei, and Wang Zhenxing, “A Verification Development Platform forPassive UHF RFID Reader”, Chiese Journal of Emiconductors, Beijing, 2007, pp.1696-1700.

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