IEICE Communications Express, Vol.4, No.8, 258–263

Fast demodulation methodfor passive MIMO
communication by using
beam-forming at receiver
Ryota Takahashi1, Keisuke Terasaki1, Kentaro Murata2,Naoki Honma1a), and Yoshitaka Tsunekawa1
1

Graduate School of Engineering, Iwate University,

4–3–5 Ueda, Morioka, 020–8551, Japan
2

Graduate School of Engineering, National Defense Academy,1–10–20, Hashirimizu, Yokosuka, Kanagawa, 239–8686, Japana) honma@iwate-u.ac.jp

Abstract: In this letter, we propose the demodulation method to improvethe complexity of demodulation in passive MIMO (Multiple-Input MultipleOutput) scheme. This scheme uses the multiple antennas at both reader andRFID (Radio Frequency Identiﬁcation) tag to increase the data-rate, but thecomplexity of MLD (Maximum Likelihood Detection), which is used as
demodulation algorithm, increases exponentially with the number of antennas. The proposed method divides tag antennas into two groups that do notinterfere each other by decoupling tag antenna groups using 180-degreehybrids at tag side and beam-forming at receiver side. Individual demodulation for each group improves the complexity of MLD signiﬁcantly.
Simulation results show the proposed method can reduce the complexityof MLD with degrading slight BER (Bit-Error-Rate) performance. The
results reveal the proposed method is effective in reducing demodulation
complexity of passive MIMO transmission even when the number of tag
antennas is increased.
Keywords: MIMO, RFID, load modulation, beam-forming, MLD
Classiﬁcation: Antennas and Propagation
References© IEICE 2015
DOI: 10.1587/comex.4.258
Received July 6, 2015
Accepted July 13, 2015
Published August 10, 2015

[1] A. Juels, “RFID security and privacy: a research survey,” IEEE J. Sel. AreasCommun., vol. 24, no. 2, pp. 381–394, Feb. 2006. DOI:10.1109/JSAC.2005.
861395[2] V. Stanford, “Pervasive computing goes the last hundred feet with RFID
systems,” IEEE Pervasive Comput., vol. 2, no. 2, pp. 9–14, Feb. 2003. DOI:10.
1109/MPRV.2003.1203746[3] V. Chawla and D. S. Ha, “An overview of passive RFID,” IEEE Commun. Mag.,
vol. 45, no. 9, pp. 11–17, Sep. 2007. DOI:10.1109/MCOM.2007.4342873[4] K. Terasaki, K. Kinami, and N. Honma, “Passive MIMO transmission using loadmodulation,” 2012 Int. Symp. Antennas Propagat. (ISAP 2012), Electric Proc. ofISAP 2012, POS2-27, Oct. 2012.

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IEICE Communications Express, Vol.4, No.8, 258–263[5] K. Terasaki and N. Honma, “Feasible load modulation technique using multiple
antenna systems,” Electron. Lett., vol. 48, no. 18, pp. 1090–1091, Aug. 2012.
DOI:10.1049/el.2012.1817[6] A. van Zelst, R. van Nee, and G. A. Awater, “Space division multiplexing(SDM) for OFDM systems,” IEEE VTC 2000-Spring, Japan, vol. 2, pp. 1070–1074, May 2000. DOI:10.1109/VETECS.2000.851289[7] J. C. Coetzee and Y. Yu, “Port decoupling for small arrays by means of aneigenmode feed network,” IEEE Trans. Antennas Propag., vol. 56, no. 6,
pp. 1587–1593, Jun. 2008. DOI:10.1109/TAP.2008.923301[8] K. Murata, H. Morishita, N. Honma, K. Nishimori, and D. M. Klymyshyn,“Analog-digital hybrid eigenmode transmission for massive short-range
MIMO,” 2014 IEEE International Workshop on Electromagnetics, vol. 2,
pp. 223–224, Aug. 2014. DOI:10.1109/iWEM.2014.6963717

1© IEICE 2015
DOI: 10.1587/comex.4.258
Received July 6, 2015
Accepted July 13, 2015
Published August 10, 2015

Introduction

RFID (Radio Frequency Identiﬁcation) is the technology to identify human andmaterials by using radio frequency tags [1]. For example, it is used electronicmoney, travel card, employee ID card, and so on [2]. Especially, passive RFID getsa lot of attention in the RFID scheme, because passive RFID tags do not need thepower supply [3]. Due to its convenience, the passive RFID is expected to be used
for the logistic management, and so on. However, passive RFID has a problem thatthe transmission speed between reader and tags is not sufﬁciently high if they needto transfer a large data, such as photo images, video, and so on [4].
To increase the data rate in passive RFID communication, the passive MIMO(Multiple-Input Multiple-Output) technique has been proposed [4, 5]. Passive
MIMO uses the multiple antennas at both reader and tag sides, and also uses
multi-level load modulation by variable impedance. The tag data can be parallellytransmitted to the reader antennas. At the reader side, the multiple load modulated
signals are observed, and they are decoded by using MLD (Maximum LikelihoodDetection) [6]. In this scheme, parallel data transmission is realized without
extending frequency resources. When mutual coupling at the tag side is existed,
the load modulated signals are modulated again by other tag. Therefore, the otherdemodulation scheme, such as eigenmode beam-forming, cannot realize the parallel
data transmission. MLD can decode the multiple signals from the tag antennas evenin this case. But, the complexity of MLD exponentially increases with the numberof antennas.
In this letter, the demodulation method that signiﬁcantly decreases the complexity of MLD by introducing analog signal processing is proposed. The analog signal
processing at the tag side is realized by using 180-degree hybrid and this decouples
the mutual coupling between two antenna groups that are conﬁgured symmetricallyto each other [7, 8]. This scheme enables the reader receiver to demodulate thesignal sets corresponding two antenna groups individually by using the digital
beam-forming. Therefore, the complexity of MLD is signiﬁcantly reduced since thetotal number of the signals for demodulation is reduced. The effectiveness ofproposed method is evaluated numerically.

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2Group demodulation method by using beam-forming

Fig. 1.

Proposed system modelFig. 1 shows the proposed system model of group demodulation method. HRT
represents the propagation channel from reader-transmitter (Tx) to reader-receiver(Rx), and HPT , HRP are respectively represented the propagation channel from Tx
to Px and from tags (Px) to Rx. SPP is the scattering matrix representing theS-parameter of tag antenna. S0PP is the scattering matrix including 180-degreehybrids. 180-degree hybrids can decouple two groups of the tag antennas, when
the structure is symmetric and the number of antennas is even [7, 8]. In case ofconﬁguring tag antennas symmetrically, S0PP is represented as!
SPP1MP =2
O0
ð1Þ
SPP ¼
O
SPPMP =2þ1MP© IEICE 2015
DOI: 10.1587/comex.4.258
Received July 6, 2015Accepted July 13, 2015
Published August 10, 2015

where SPP1MP =2 is the scattering matrix representing the S-parameter which iscomposed of tag1; . . . ; MP =2, and SPPMP =2þ1MP is composed of tagMP =2 þ 1; . . . ;MP . O is zero matrix. From (1), two antenna groups are isolated completely. Reader
can discretely demodulate the signal sets corresponding to two groups by beamforming. Since the number of the antennas in each group can be reduced by half,
the complexity of MLD is reduced signiﬁcantly compared with that of the conventional method.
Fig. 2(a) shows an estimation method of the channel via Px, Hk . MP is thenumber of tag antennas, MR is the number of receiver antennas. Proposed methodneeds the channel via Px for beam-forming at reader, but Rx can only observe thesum of the direct signal and reﬂected signal via Px. Therefore, the reader needs toestimate the channel from Px to Rx by excluding the direct path. Fig. 2(a) shows
the way to estimate the direct path channel. Reference impedance terminates eachtag antennas to suppress the reﬂection from tag, that is, the reﬂection coefﬁcient is
0. Therefore, only the direct path channel, HRT , can be observed at the reader.
Fig. 2(b) shows estimation of the sum of direct and reﬂected path channel. In orderto observe the channel components, one of the reﬂection coefﬁcients of the tag
antenna terminations is set to 1 (open) and the rest of the other terminations are set

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IEICE Communications Express, Vol.4, No.8, 258–263(a) Estimation of the direct path channel(b) Estimation of the sum of direct and reflected path

Fig. 2.

Channel estimation

to 0. Hk is the reﬂected path channel when the termination condition of the tags isrepresented by k , which is given as
k ¼ diagð0 . . . 0; open k ; 0 . . . 0Þð2Þ

where open k is the reﬂection coefﬁcient when the load impedance of k-th port isopened. The sum of the direct and reﬂected channels, H, which is observed atreceiver, is represented by
H ¼ HRT þ HRP ðS0PP 1 1k Þ HPT

ð3Þ

where HPT , HRP is the channels from Tx to Px and from Px to Rx, respectively.
Since HRT has been estimated, Hk via tagk can be calculated by eliminating HRTfrom (3) and represented as
Hk ¼ HRP ðS0PP 1 1k Þ HPT :ð4Þ

The reader can divide and demodulate the signal sets corresponding twoantenna groups by beam-forming. For example, when the reader demodulates thesignals from Group A, which is composed of tag1; . . . ; MP =2, null beam-formingthat directs null directivity to Group B, which is composed of tagMP =2 þ 1; . . . ;MP . The beam-forming weight can be calculated from the singular value decomposition (SVD) of the channel between the reader and Group B tags as,
eB ½SB ; 0V H
HMP =2þ1 ; . . . ; HMP ¼ ½U B ; Uð5ÞB© IEICE 2015
DOI: 10.1587/comex.4.258
Received July 6, 2015
Accepted July 13, 2015
Published August 10, 2015

where U B and e
UB represent signal and noise space eigenvectors, respectively, andU B is used for receiving signals from Group A since no interference from Group B
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eB at the reader. Similarly, the signal sets of Group B can beis received by using U
demodulated in the same manner. Therefore, the proposed method can demodulate
the signal of each group discretely.
3Simulation

3.1 Simulation setup
All the antenna elements in the simulation are half-wave dipoles for simplicity. The
frequency is 2.47 GHz, the distance between reader and Px is 1 m, and the distance
between Tx and Rx is 1 m too. The tag element spacing is 0.5 wavelength, and thereceiver element spacing is 1.0 wavelength. The transmission power is 20 dBm, andthe noise is assumed to be zero-mean Gaussian white. The propagation channel iscalculated by Clarke’s model.
3.2 Simulation results
Fig. 3(a) shows BER (Bit-Error-Rate) versus noise power, where the MP and MR
are 8. From this ﬁgure, it is clear that proposed method needs 180-degree hybrid todemodulate the received signals accurately. The deterioration in the curve, ‘w/o
hybrid’, is caused by the interference between two antenna groups, and thecombination of the beam-forming and 180-degree hybrid successfully suppresses(a) BER versus noise power© IEICE 2015
DOI: 10.1587/comex.4.258
Received July 6, 2015
Accepted July 13, 2015
Published August 10, 2015(b) Complexity of MLD versus number of tag antennas

Fig. 3.

Simulation results

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IEICE Communications Express, Vol.4, No.8, 258–263

this interference. Also, proposed method causes degradation in BER performancecompared with the conventional method. Since the proposed method uses nullbeam-forming, the SNR is slightly degraded.
Fig. 3(b) shows the complexity of MLD versus the number of tag antennas incase of QPSK modulation. The result shows proposed method signiﬁcantly
improves the complexity of MLD. For example, when the number of tag antennas
is 8, the search number of conventional method is 65536 per symbol to demodulate
all signals from the tags, whereas the search number of proposed method is 256 persymbol. The proposed method becomes more effective when the number of theantennas is increased because the complexity of MLD increases exponentially with
the number of tag antennas.
4Conclusion

This letter has proposed the method to reduce the complexity of MLD in passive
MIMO by combining the analog signal processing and digital beam-forming. Whenthe number of the antennas at both reader and tag sides is 8, proposed method
can reduce the complexity of MLD to 1/128 of conventional method, and theeffectiveness increases exponentially with the number of antennas. These results
show that proposed method is effective in reducing the complexity of MLD inpassive MIMO communication.
Acknowledgments
This research was partially supported by JSPS KAKENHI (25709030).

© IEICE 2015
DOI: 10.1587/comex.4.258
Received July 6, 2015
Accepted July 13, 2015
Published August 10, 2015

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