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TRANSCRIPT
A true full-duplex communication between HFcontactless reader and card
Florian PEBAY-PEYROULACEA-LETI
MINATEC CampusF-38054 GRENOBLE
Jacques REVERDYCEA-LETI
MINATEC CampusF-38054 GRENOBLE
Abstract—This paper presents a full-duplex commu-nication for high-frequency RFID at 13.56 MHz. Thework presented in this study is based on the ISO 14443very high bitrate (VHBR) modulations, which are definedfor a half-duplex communication scheme. Moreover, thesemodulations appear to be orthogonal when transmittedsimultaneously over the same carrier. The aim of thissystem is to use the VHBR modulations in a full-duplexscheme to double the global bitrate.
The innovation presented in this paper improves thesystem performances without decreasing the transmissionrobustness, and without modifying neither the modulatorsnor the demodulation blocks.
Index Terms—Contactless, RFID, full-Duplex, half-duplex, Very High Bitrate (VHBR), phase modulation, loadmodulation.
I. INTRODUCTION
Nowadays contactless system use-cases and applica-tions require the transfer of even larger amount of data ina faster way. An instance of such a requirement for theend user is the fast transfer of biometric data stored in ae-passport, or media files between NFC devices. Anotherexample, for card suppliers, is the time to download theoperating system into the card (personalization) whichbecomes today part of the critical path in the productionprocess.
For a couple of years, some research centers and majorcompanies have been involved in the development of newtechnologies to improve the bitrate of communicationbetween contactless cards and readers [1]. These researchand development works are conducted for both UHF andHF systems.
This article focuses, more precisely, on the physicallayer defined in ISO/IEC 14443 [2] and its very high bi-trate extension. This new technology extension provides
Reader
Card
r2r1
a1 a2
Fig. 1. Half-duplex example
communication between reader and card with bitratetoward one megabit per second keeping a 13.56 MHzcarrier and uses a half-duplex scheme. By multiplexingreader and card emitted data (full-duplex scheme) amuch higher global bitrate may be reached.
After a state of the art overview, this article willexplain how a full-duplex communication can be estab-lished between card and reader taking benefits of thevery high bitrate modulations. Then solutions for animplementation will be given, and system performanceswill be discussed.
II. STATE OF THE ART OF FULL-DUPLEX
A. Full-duplex description
The major parts of contactless systems provides a half-duplex communication [3]. It means the reader and thetransponder do not transmit information simultaneously,but sequentially.
Figure 1 illustrates a typical half duplex transmission,composed of successive reader requests and card an-swers, each separated by a frame delay time (ie. guardtime). The requests and answers could be multiplexed toobtain a full-duplex communication.
With such a full-duplex communication, the globalsystem bitrate could be significantly increased. In thecase of two data streams symmetrically transmitted be-tween reader and card (illustrated in figure 2), bitratecould be doubled.
2011 IEEE International Conference on RFID-Technologies and Applications
978-1-4577-0027-9/11/$26.00 ©2011 IEEE 473
Reader
Card
r3r1
a1 a3
r2 r4
a2
Fig. 2. Full-duplex example
B. Full-duplex implementations
Today low frequency (LF) full-duplex tags are widelyaddressed; such tags are available on market for supplychain by OregonRFID for instance [4], livestock trackingby Y-Tex [5]. . . The benefit of full duplex for LF RFIDis mainly to increase the number of UID scan in a giventime, and as a consequence, increase the infrastructuretracking reliability.
Another research field is the full-duplex for UHFRFID, first demonstrated in 2009 by Ashri and Sharafaat 900 MHz [8], then at 7,9 GHz by Pelissier andGomez in 2010 with a 112 Mbit/s bitrate [6]. TheSkywork company now provides full-duplex UHF RFIDchips [7] by separating frequency band for transmittingand receiving the information at respectively 5.8 GHzand 2.4 GHz.
Today existing systems targeting high frequency (HF)RFID use a dual band scheme to perform a full-duplextransmission. [9] gives a good overview of this kind ofsystems: a UHF carrier is used for the transmission fromreader to card, and a HF carrier for the transmissionfrom card to reader. Still no full-duplex communicationis specified in the Near Field Communication (NFC)standards neither ECMA 340 [10] nor ISO 18092 [11].
The novel approach described in this paper is a oneband full-duplex transmission: the 13.56 MHz carrier ismodulated at the same time by the reader and the cardto transmit their data.
III. SYSTEM DESCRIPTION AND OBJECTIVES
The aim is to develop a prototype which performs atrue full-duplex contactless communication for HF RFIDas a proof of concept. True full-duplex in this contextmeans simultaneous transmission of data symbols fromreader to card and from card to reader on the 13.56 MHzcarrier.
In order to demonstrate that full-duplex could be easilyintegrated in a standard product, the system uses a stan-dardized air interface, the work presented in this study isbased on the modulations defined in the ISO 14443 veryhigh bitrate proposal. This choice is discussed in part IV.These very high bitrate modulations are composed of:
• a carrier phase modulation from reader to card,
TX
RXReader
IC
Card
Fig. 3. System overview
• a load modulation from card to reader.The full-duplex system described in this article con-
tains (figure 3) a reader prototype, an antenna designedfor very high bitrate with a quality factor Q of 10, anda passive card prototype.
The reader embeds a transmission block for the phasemodulation and a reception block for the subcarrier loaddemodulation. On the card side, the hardware is inte-grated into an ASIC, and contains the phase demodulatorand the subcarrier load emission block.
The reader transfers the energy required to powerthe card through the antenna. The electromagnetic fieldemitted by the reader also contains the clock for the carddigital block, and the data exchanged between reader andcard.
IV. BENEFITS OF VERY HIGH BITRATE MODULATION
In order to transmit data in both link from reader tocard and card to reader simultaneously, the informationshall be transmitted using an orthogonal way. The full-duplex transmission described herein takes benefits ofthe phase and load orthogonal modulations, defined forvery high bitrate systems [1], [12], [13], to transmitsymbol simultaneously in both ways.
A. Modulation from reader to card
From the reader to the card, ISO 14443 very highbitrate proposal defines a fc = 13.56 MHz carrier phasemodulation. Data to be transmitted are packed intomultiple-bit symbols and mapped to phases shifts. Sym-bols are defined by duration of n/fc. Figure 4 illustratesa modulation example at 6.8 Mbit/s with four 4/fc longsymbols (each of them represents two data bit).
In order to provide an efficient energy transfer to thecard, such a reader must use a high Q factor antenna. Themain drawback of this kind of antenna is their limitedbandwidth, which has a negative impact on the cardreceived signal, and that could be considered as a band-pass filter. This negative impact is commonly namedchannel effect.
The signal illustrated in figure 4 is transmitted overa Q = 10 antenna. Due to the band-pass filtering,
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Data (bit)
Symbol 01 = π/2 00 = π/4
RF
Fig. 4. Phase modulation example
Symbol pi/2 pi/4
T T − δt
RF∆H
Fig. 5. Channel effect
the received signal shows (in figure 5) attenuationsfor high frequencies occurring at phase changes. Theseattenuations are characterized with the field amplitudevariations (∆H) that could be observed.
B. Modulation from card to reader
The very high bitrate proposal defines the same kind ofload modulation as defined for ISO 14443 type B. Databits are coded into subcarrier phase shifts (0 or π) andmodulated by connecting a load to the card antenna. Bitduration is defined by duration of n/fc, this modulationinvolves only two symbols (ie. two bits).
To perform a higher bit rate transmission, the sub-carrier frequency has to be increased. As an example,the figure 6 illustrates a 3.4 Mbit/s transmission witha 3.4 MHz subcarrier, a bit duration of 4/fc, and twosubcarrier phase shifts.
Mostly, on reader point of view, received signal couldbe considered as an amplitude modulated signal over the13.56 MHz carrier. The received signal quality suffersfrom the antenna restricted bandwidth too.
Another concern that should be discussed is the influ-ence of the card power consumption in the backscatteredfield. Depending on card design and technology, the vari-ation of the consumed power in the card chip – typicallywhen processing data – may impact the field amplitude.The chip has to be designed not to interfere with thetransmitted signal with its own power consumption.
Subcarrier
RF
Data
Fig. 6. Load modulation example
Reader→Card Card→Reader
0
1
-1
0 1 0 1
0
1
-1
Fig. 7. Half-duplex constellations
C. Modulations orthogonality
Symbols transmitted by the reader shall not collidewith symbols transmitted simultaneously by the card.With the orthogonal phase modulation (from the readerto the card) and the load modulation (from the card tothe reader), symbol collisions are avoided.
In order to caracterize the orthogonality of phaseand load modulation, constellation diagrams and mod-ulation error rate (MER) [14] are measured. Symbolsare emitted from the reader and the card through theirantennas, field signal is measured with a wideband sense-coil. Modulation orthogonality measurements take intoaccount the channel effect.
First, the measures for the phase and load modulationare done separately. Then, the two modulations are eval-uated simultaneously. Modulation error rate are reportedin table I and constellation diagrams in figures 7 and 8.
Comparing constellations for the two unidirectionaltransmissions and the bidirectional one, makes appearslightly wider spots. Nevertheless, observed differencecould not be considered as significant.
Moreover, the MER measurements report more clearlythat the bidirectional transmission does not impact nei-ther the signals nor the symbols demodulation.
Phase and load modulations could be considered asorthogonal in this system, and so be used for a full-duplex transmission.
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Reader→Card Card→Reader
0
1
-1
0 1 0 1
0
1
-1
Fig. 8. Full-duplex constellations
Reader→Card Card→ReaderHalf-duplex 44 dB 45.7 dBFull-duplex 42.3 dB 41.4 dB
TABLE IFULL AND HALF-DUPLEX MER COMPARISSON
V. DATA FRAMING
As defined in ISO 14443, the communication schemeis a reader-talk-first one, and data symbols are transmit-ted into frames. The use of frames guarantee:
• the synchronization of the transmitter and receiver,• the localization of data symbols,• the detection of transmission errors with a CRC.Frames are delimited with a start-of-frame and a end-
of-frame; each of them are defined in the very high bitrate proposal and won’t be discussed in this article.
Contrary to half-duplex, in a full-duplex system aslightly different mechanism is defined for communica-tion establishment; principle is illustrated in figure 9.
The reader starts the communication, ¬ a start-of-frame is emitted, the card detects the start-of-frame,® after a fixed time the reader and the card start to emitdata symbols, ¯ the reader emits an end-of-frame.
This study only focuses on the ideal case where thereader and the card have the same amount of data totransmit, reader can send the end-of-frame without a cardframe length management.
The demonstrator allows two full-duplex communica-tion modes:
• infinite frames with pseudo random data for biterror ratio (BER) measurement purpose,
• finite frames for frame error rate measurement andreal data transmission.
VI. DEMODULATION
We demonstrated in part IV-C that the chosen mod-ulations could be considered as orthogonal. This part
Reader
Card
¬
®
¯
Fig. 9. Data framing
÷n . . .RF ∆ϕ
DQ
DLL
phase
+–
chargepump
DFF
Fig. 10. Phase demodulator architecture
will focus, independently, on how the card and readerdemodulators are implemented. Modulation blocks arequite trivial and won’t be discussed here.
A. Card demodulator
When the carrier phase changes, its instantaneousperiod is slightly different from 1/fc. By measuring thisinstantaneous period, it is possible to know the phasevariation. This principle is illustrated in figure 4, wherethe first symbol has a duration of T and the second onea duration of T − δt.
As defined in part IV-B, symbols are modulated withina n/fc duration. By measuring the duration of n signalinstantaneous periods, the demodulator directly obtainsthe phase variation from the previous symbol to thecurrent one. To perform the phase demodulation, thetime occurred between n edges of the carrier is measuredpermanently.
The period measurement is provided by a delay lockedloop (DLL) composed of 150 delay cells (fig. 10). Thereceived carrier clock is divided by n and injected intothe DLL. Each output of the DLL cells are latched andencoded into a 8 bits word. This byte contains the phasevariation between the current symbol and the previousone. A charge pump is used to control the cells voltageand adapt their delay to an entire division of the 13.56MHz period when the carrier isn’t modulated.
With such a demodulator architecture, the use of aphase locked loop (PLL) or an IQ demodulator, whichmay drastically increase the chip surface, the power
476
RF
sampling clock
sampled signal
Fig. 11. Carrier sub-sampling
consumption and the cost, is avoided. This card phasedemodulator is part of the card ASIC and is producedwith the AMS 0.35µm technology.
B. Reader demodulation
The card load modulated carrier appears to be an am-plitude modulated signal, and the carrier is still emittedby the reader. So the receiver (ie. the reader) is alreadysynchronized with the incoming signal.
After a gain control stage, the high-band signal is con-verted by an analog to digital converter. This converteris clocked by the same 13.56 Mhz signal which is usedfor the carrier generation. As illustrated in figure 11, theconverter output is the baseband signal.
Moreover, in a full-duplex transmission, during readerreception, carrier phase is modulated (by reader TX).Using the phase modulated signal (generated by readerTX) to clock the converter, allows to sample the signalalways on carrier arches and obtain an accurate loaddemodulation.
VII. MEASUREMENTS AND PERFORMANCES
The laboratory setup is illustrated in figure 12. TheTX and RX on reader side are performed by an AlteraCyclone II FPGA ¬. The reader antenna has a Qfactor of Q = 10. The card prototype ® embeds theanalog front-end ASIC and an FPGA for the basebandprocessing.
To evaluate the full-duplex contactless communicationperformances, a bit error ratio (BER) is measured.Frames containing 107 pseudo random data bits aresimultaneously transmitted by the reader and the card.Received data are compared bit to bit to compute theBER. Transmission is evaluated with different distancesfrom card to reader’s antenna.
The transmission bitrate is fixed at 1.7 Mbit/s. Thereader and the card both transmit symbols of 8/fc long.The symbol set has a cardinality of two (bits are mappeddirectly to symbols).
Fig. 12. Measurement setup
10−7
0 20
10−6
10−5
10−4
40 60 80 100 120
distance (mm)
BE
R
Reader→CardCard→Reader
Fig. 13. Half-duplex bit error ratio
In a first mode, BER is measured for two unidirec-tional transmissions (from reader to card and card toreader). Results are summarized in the figure 13. Incontactless application it is commonly admitted [15]that for a reliable communication the BER shall beinferior to 10−4. Yet the half-duplex transmission couldbe considered as reliable from zero to 80 mm.
Then, in a second mode, the BER is measured for thefull-duplex transmission and is compared to the referencemeasurement done previously for half-duplex. Resultare plotted in figure 14. The full-duplex transmissionis reliable from zero to 60 mm (BER inferior to 10−4).Robustness is slightly decreased for the full-duplex trans-mission compared to the half-duplex one. Yet the full-duplex system prototype presented here provides a robustcommunication.
477
10−7
0 20
10−6
10−5
10−4
40 60 80 100 120
distance (mm)
BE
R
Reader→CardCard→Reader
Fig. 14. Half-duplex bit error ratio
VIII. CONCLUSION
The contactless physical layer for a full-duplex com-munication presented in this article uses the phase andload modulations defined for ISO 14443 very high bitrateproposal. No additional hardware developments on thetransmission and reception parts are required.
First, we demonstrated that these two modulationscould be considered as orthogonal and that symbolstransmitted simultaneously in both ways do not generatecollisions. Then we evaluated the system performancesby measuring the bit error rate. Results shows thatthe full-duplex communication does not decrease thetransmission robustness compared to half-duplex.
Moreover benefiting from very high bitrate modu-lation, the global system bitrate could be relevantlyincreased with a full-duplex transmission.
REFERENCES
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