Similar to the chamber experiments, the transmitter itera-tively sends Query commands to the tag using 1-W EIRP. Since there are copious propagation paths at the concrete hallway, the severe multipath fading results in unstable readability and a shorter read range of a C1G2 tag. A photograph of our measurement is shown in Fig. 21. Likewise, the measurements within 100 cm are skipped because there is no access problem.
The tags are measured every 5 cm thereafter. A read range comparison result of an experiment is shown in Fig. 22.
As shown in Fig. 22, the tested regular C1G2 tag can be read stably within 255 cm. Due to the multipath fading, the C1G2 tag can be read in some regions after 290 cm. The C1G2
Fig. 20. Read range comparison inside the NTUST microwave anechoic chamber, where the first bar denotes the read range of a regular C1G2 tag, and the second bar ( = 0:54 P = 0) denotes the read range of a powerless EPT.
Fig. 21. Photograph of the experiment at a hallway of the International Building, NTUST.
tag cannot be read after 400 cm due to its power requirement constraint. Under the same condition, the tested EPT with dBm can be read stably within 435 cm with two short gaps, where the multipath fading are severe as indicated in the bottom widow of Fig. 22. The EPT cannot be read after 460 cm using a regular Query command due to the reader command modulation depth constraint. Utilizing modified Query commands ( or ), the longest read range of the EPT can be extended to 650 cm or so, despite that there is a 1-m gap due to a deep multipath fading. It is also
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1396 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 57, NO. 5, MAY 2009
Fig. 22. Read range comparison at a hallway of the International Building, NTUST, where the first bar denotes the read range of a regular C1G2 tag, and the second bar ( = 0:54 P = 0) denotes the read range of a powerless EPT.
noteworthy that the multipath fading before 470 cm does not affect the readability of the EPT with battery power.
As illustrated in Fig. 22, the EPT with battery power shows an excellent overall performance improvement in terms of the maximum read range and fading resistant capability.
IV. CONCLUSIONS
The prototype EPT demonstrates significant performance im-provement in terms of read range extension and fading resis-tant capability. Due to utilizing a regular C1G2 tag IC, an EPT behaves as a regular C1G2 tag. When a modified reader com-mand (by changing the power distribution of a reader comcom-mand without reader average power increment) is used, the perfor-mance of an EPT can be further improved. However, the power cannot be too low; otherwise, the reverse-link-limited range can be shorter than the forward-link-limited range. Moreover, a very low can also result in a tag power outage when it backscatters its MBS. It is noteworthy that an EPT can be con-tinuously used as a regular C1G2 tag after it uses up its battery power.
Compared with a regular C1G2 tag in the same access range, an EPT requires less reader power. This advantage can be very attractive in the applications using low-power mobile readers.
The prototype EPT still needs some improvements. First, as seen in the prototype EPT, the transition with an SMA con-nector is convenient for many experiments; however, the transi-tion and other components are too big and costly compared with a C1G2 tag. Using a CMOS VCO IC and an antenna integrated
with a battery is necessary for producing a commercialized EPT.
Second, finding the optimal impedances of the BAC and antenna for an EPT is a complicated problem due to the nonlinearity of the IC impedance, which requires further investigation. Third, the size, cost, and life span of a battery are very challenging fac-tors in practical EPT usages. Since the power requirement of an EPT is very low (in the order of W), utilizing the technology of rechargeable paper battery could be a good solution for this problem.
APPENDIX
In order to derive the optimum value of , we use (2) to substitute in (9) and rewrite (9) as
(A1)
where .
Assuming that the tag is far away from the reader and , we can individually examine the inequalities in (A1), and have
(A2) and
(A3) When considering a free-space path loss only, is re-verse proportional to the square of , where is the distance between the tag and reader. Apparently, (A2) represents the tag minimum power constraint that limits the read range of an EPT.
On the other hand, (A3) represents the constraint of modulation depth of a received reader command, which also limits the read range of the EPT.
Let , where is a constant taking the
factors of reader signal wavelength and tag antenna gain into account, and denotes the EIRP of the reader signal. The distance square can be derived from (A2) as
(A4) Let , we can rewrite (A3) as
(A5)
From (A4) and (A5), the maximum read range in the for-ward link is obtained, when the denominators in both equations are equal, i.e.,
(A6) The optimum that results in the longest read range of the EPT can be derived from (A6) as
(A7)
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LIU et al.: NOVEL BATTERY-ASSISTED CLASS-1 GENERATION-2 RFID TAG DESIGN 1397
By substituting (A7) into either (A4) or (A5), the maximum read range of the EPT can be derived as
(A8)
ACKNOWLEDGMENT
The authors would like to thank Prof. C. Yang and Prof. T. Ma, both with National Taiwan University of Science and Technology, Taipei, Taiwan, for their valuable comments on this work. The authors also appreciate the valuable help from P.-C. Chen, D.-C. Hsin, and W.-C. Lin, all with National Taiwan University of Science and Technology, for their assis-tance with measurements and simulations.
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Available: http://www.impinj.com/
Hsin-Chin Liu (S’01–M’02) was born in Hualien, Taiwan, in 1967. He received the B.S. degree in communication engineering from National Chiao Tung University, Taiwan, in 1989, and the M.S.
and Ph.D. degree in electrical engineering from Pennsylvania State University, University Park, 1993 and 2003 respectively.
From 1993 to 1994, he was a Software Engineer with Alcatel, Taiwan. From 1994 to 1996, he was a Hardware Engineer with Siemens, Taiwan. From 1996 to 1998, he was a Member of Research Staff with the Computer Center, National Dong Hwa University, Taiwan. In 2003, he joined the Department of Electrical Engineering, National University of Kaoh-siung, Taiwan, as an Assistant Professor. Since 2004, he has been an Assistant Professor with the Department of Electrical Engineering, National Taiwan Uni-versity of Science and Technology, Taipei, Taiwan. His current research inter-ests include wireless communications, smart antenna technologies, and RFID systems.
Meng-Chang Hua was born in Changhua, Taiwan, in 1986. He received the B.S. degrees in electrical engineering from the National Taiwan University of Science Technology, Taipei, Taiwan, in 2006, and is currently working toward the Ph.D. degree in elec-trical engineering at the National Taiwan University of Science Technology.
His research interests include wireless communi-cations, array signal processing, smart antennas, and RFID systems.
Chih-Guo Peng was born in Kaohsiung, Taiwan, in 1982. He received the M.S. degrees in electrical engi-neering from the National Taiwan University of Sci-ence Technology, Taipei, Taiwan, in 2008.
From 2006 to 2008, he was a Research Assistant with the Communication Research Laboratory, National Taiwan University of Science Technology, where he participated in several cooperative projects.
In 2008, he joined one of the focus centers of the Industrial Technology Research Institute, Hsinchu, Taiwan, which develops technologies for image processing and RFID systems.
Jheng-Peng Ciou was born in Taoyuan, Taiwan, in 1984. He received the M.S. degrees in electrical engi-neering from the National Taiwan University of Sci-ence Technology, Taipei, Taiwan, in 2009.
From 2006 to 2008, he was a Research Assistant with the Communication Research Laboraotry, Na-tional Taiwan University of Science Technology. His research areas are RFID systems and anticollision methods.
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