IEICE TRANS. COMMUN., VOL.E97–B, NO.7 JULY 2014

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PAPER

Experimental Evaluation of Passive MIMO Transmission with
Load Modulation for RFID Application
Keisuke TERASAKI†a) , Student Member and Naoki HONMA† , Member

SUMMARY
This paper describes experiments on passive MultipleInput Multiple-Output (MIMO) transmission with load modulation. PIN
diodes are used as the variable impedance element at the tag side to realize multi-level modulation. The results indicate that the transmission
rate of passive MIMO is up to 2 times higher than that of Single-Input
Single-Output (SISO) with the same transmission power when the distance
between the reader and the tag is 0.5 m. Also, when the distance is 1 m,
MIMO oﬀers up to 1.7 times higher transmission rate than SISO. These results indicate that passive MIMO oﬀers high-speed data transmission evenwhen the distance is doubled.
key words: MIMO, RFID, load modulation, multilevel modulation

1.

Introduction

Radio Frequency Identification (RFID) has attracted muchattention as the alternative to bar code readers. RFID is avery practical technology for identifying and managing people and goods using small wireless chips called tags; current applications include travel cards, employee ID cards,
and electronic money. Passive RFID systems, the dominant
type, use load modulation [1] to establish a channel fromtag to reader. Load modulation enables the tag to transmit
its information to the reader by controlling the reflection ofwaves by altering the tag side’s load impedance. Passive
RFID has the advantage that it eliminates the transmitterfrom the tag side, but conventional load modulation schemes
employ single antennas and on-oﬀ-keying, and so cannot attain high-speed data transmission. Thus recent demands for
low power and high-speed data rates remain unsatisfied.
One solution is to set multiple antennas in the reader
and the tag, see Fig. 1(b) [2], [3]; Fig. 1(a) shows the conventional single antenna. By using multiple antennas, multiple
signals can be transmitted in parallel [4]–[6]. It oﬀers highspeed data transmission not by extending the bandwidth but
by spatial-multiplexing [7], [8]. But studies [2] and [3] use
only binary on/oﬀ keying.
For further increase of the transmission data rates, wehave proposed the passive MIMO technique, a high-speed
data transmission method that uses load modulation andmultiple antennas in both reader and tag, and multilevel
modulation that improves the bits carried per symbol. We
set the load impedance in stages to establish many reflection
coeﬃcients rather than binary on/oﬀ [9]–[11]. We proved
Manuscript received November 29, 2013.
Manuscript revised March 16, 2014.
†
The authors are with the Graduate School of Engineering,
Iwate University, Morioka-shi, 020-8551 Japan.
a) E-mail: t2312026@iwate-u.ac.jp
DOI: 10.1587/transcom.E97.B.1467

Fig. 1

RFID system.

the validity of this method mainly by simulation, but no experiments were conducted at that time.
2.

Experimental Setup

2.1 Passive MIMO Transmission with Load Modulation
In passive RFID, the reader transmits an unmodulated continuous wave. This wave is reflected at the tag side, and thencaptured by the reader. At the tag side antenna, the load
impedance is altered to control the amplitude and phase of
the reflected wave. The reader can detect the presence andstate of the tag from the reflected wave, and acquire the data
held by the tag.
In passive MIMO, multiple signals can be reflected inparallel by the multiple antennas. Figure 1(b) shows the system model of passive MIMO. In this figure, m refers to thereader-receiver and tag antennas. The configurations with
m = 1 and m = 2 mean SISO and MIMO, respectively. The
number of reader-transmitter antennas is 1 in this study.
In this paper, T, P, and R represent the transmitting, tag,and receiving ports, respectively. Figure 2 shows the scattering matrix model explaining this antenna system. Here, aTis the transmitted signal from T port, aP = [aP1 , · · · , aPm ]T
is the signals from P port, bR = [bR1 , · · · , bRm ]T is the signals to R port, and bP = [bP1 , · · · , bPm ]T is the signals to P
port, where [·]T is the matrix transposition. The relationshipbetween bR and bP can be expressed as

c 2014 The Institute of Electronics, Information and Communication Engineers
Copyright IEICE TRANS. COMMUN., VOL.E97–B, NO.7 JULY 2014

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Fig. 2

Scattering matrices equivalent model.

ΓbP = aP
 

SRT
bR
=
bP
SPT

SRP
SPP



aT
aP



Fig. 3

System model.

(1)
(2)

where SRT , SRP and SPT are the scattering matrices that denote the channels among T, P, and R ports. SPP are the scattering matrices that denote the reflection and coupling coefficients at the tag antenna. From the relationship shown in(1) and (2), the observed signal at port R is described as,



−1
bR = SRT − SRP SPP − Γ−1 SPT aT= SRT aT(3)

Fig. 4

Reader receiver system.

where SRT (m × 1) is the channel between T and R. The
reflection coeﬃcient of the termination loads is given as
Z1 − Z0
Zm − Z0,...,(4)
Γ = diag
Z1 + Z0
Zm + Z0
where Z1 , . . . , Zm correspond to the load impedance valuesof tag antennas 1, . . ., m, respectively, and Z0 is the reference
impedance. In this paper, PIN diodes are used as the variable
impedance elements at the tag side; their impedance is set bychanging the applied current i.e., Z1 , . . ., Zm depend on thebias. Therefore, received signal vector y can be expressedasy = SRT x + n(5)where x is the signal emitted by the reader transmitter; it istaken to be a constant scalar. n is the noise vector. In thisexperiment, only y can be observed, not SRT . The above expressions indicate that altering the load impedances changes
the reflected signals. We realize multi-level modulation by
setting several diﬀerent impedance values.
2.2 Experimental System
Figure 3 shows the experimental system model of passive
MIMO. In this paper, the transmitted signal is unmodulated continuous wave from a signal generator (SMB 100A
SIGNAL GENERATOR). The noise is added in a postprocessing step after the experiment, and this noise power isassumed to be suﬃciently larger than that observed during
the experiment. Figure 4 shows the reader-receiver system.
The receiving antennas are connected to the down converter
through the low noise amplifier. The down-converter transforms the received RF signals into the baseband I (real) and

Fig. 5Experimental setup.

Q (imaginary) part voltages. This system has four baseband
ports (I×2 + Q×2) in total since it has two RF receiving
ports. The output baseband signals are measured by thedata acquisition device (NI PCIe-6363). A computer adds
noise to this received signal and calculates the received signal vector, y. At the tag side, the bias voltages for the PIN
diodes are controlled by the waveform generator (YOKOGAWA VB8000 Arbitrary waveform generator). The bias
voltages are determined so as that the constellation of reflection coeﬃcients of the PIN diodes are distant from eachother.
Figure 5 shows a photo of the experimental setup. In
this figure, we conducted the indoor experiment, and useda patch antenna array with m = 2. The distance between
the reader and the tag is 1 m, and the antenna height was
1.2 m for both the reader and tag. The element spacing was
1λ for both the reader and tag, where λ is wavelength ina vacuum. Figures 6(a), (b) shows close up of the tag an-

TERASAKI and HONMA: EXPERIMENTAL EVALUATION OF PASSIVE MIMO TRANSMISSION WITH LOAD MODULATION FOR RFID APPLICATION

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Fig. 6

Tag antenna.

tennas. The PIN diodes are biased by way of the antenna
conductor. The bias port is located at the center point of thepatch antenna, through which the RF current does not flow.
The other side of the PIN diode is connected to the substrate
ground through the pin.
Figure 7 shows the sketches of the complex plane that
explain the method to determine the reflection coeﬃcients;
the continuous line shows the range of reflection coeﬃcients given by the diode. In this case, the number of thebits/symbol is 2, and 4 reflection coeﬃcients are needed.
The map of the reflection coeﬃcients can be recognized as
the constellation diagram. Note that the locations of thecoeﬃcient need to be distant from each other as shown in
Fig. 7(a). When the reflection coeﬃcients are determinedat random like Fig. 7(b), the distances among the reflection
coeﬃcients are closer than that in Fig. 7(a). This yields low
noise tolerance since there are only small changes in the reflected signals.
Figure 8 shows the method used to determine thebias voltages; the continuous line shows the coeﬃcient trajectory when the applied voltage to the diode is varied
from 0 V to 1.9 V. In Fig. 8(a), ‘+’ represents the determined reflection coeﬃcients, and 4 reflection coeﬃcients(2 bits/symbol/tag-antenna) correspond to 4 bias voltages (0,0.6, 0.68, 1.9 V). In Fig. 8(b), ‘+’ represents 16 reflection
coeﬃcients used when transmission uses 4 bits/symbol/tagantenna. It can be seen that the distances among the chosenreflection coeﬃcients are much closer than this true for the
2 bits/symbol/tag-antenna case. This suggests that the noise
tolerance is lowered if the transmission rate is increased.

Fig. 7

Determination of reflection coeﬃcients: tag side.

Fig. 8Determination of diode bias voltages: tag side.

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Fig. 9Time change in bias value.

Figure 9 shows temporal transition in bias voltage,
where the bias voltage output by the VB8000 is directly
measured by the NI PCIe-6363, and the transmission rateis 2 bits/symbol/tag-antenna with m = 2. In this figure, thecontinuous and dashed lines show the biases for tag antennas 1 and 2, respectively. The several diﬀerent signals areachieved by applying independent voltages to tag antenna 1
and 2. Symbol length is defined as a period wherein the certain termination combination of the biases is given and kept.
The symbol rate is 40 kHz. Zero-mean Gaussian white noise
was added to the received signal. Maximum likelihood detection (MLD) [12] was used as the decoding algorithm at
the reader.
3.

Fig. 10

Constellation of received signal in SISO.

Measured Results

This section discusses the measured results.
We compare the received signals at the reader receiverfor SISO (conventional) and 2 × 2 MIMO. Since the distribution and amplitude range of the received signals varywith changes in antenna position and environment, the distribution of the received signal points shown here is only
an example. Figure 10(a) shows the example of the constellation of the received signals when m = 1 and the termination impedances are switched among 4 values. The
circle is the ideal received signal point obtained by averaging the received signal points in each symbol. The crossis the received signal point that includes noise. This figure indicates that the number of the ideal received signal
points is 4; that is, the transmission rate of 2 bits/symbol
can be achieved. Figure 10(b) shows the example of theconstellation of the received signals when the termination
impedances are switched among 16 values. From this figure, the number of ideal received signal points are 16, thatis, the transmission rate of 4 bits/symbol can be achieved.
The results in Fig. 10(a) show that the signal points are sufficiently separated, so it is easy to demodulate. However,the signal points are close set in Fig. 10(b), which means
low noise tolerance.
Next, the results when the number of antennas is increased to m = 2 are shown. Figure 11 shows the example of the constellation of the received signals when the termination impedances are switched among 2 values, where

Fig. 11 Constellation of received signal in 2 × 2 MIMO (transmission
rate of 2 bits/symbol).

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Fig. 12 Constellation of received signal in 2 × 2 MIMO (transmission
rate of 4 bits/symbol).

Figs. 11(a) and (b) show the signals at receiving antenna 1
and 2, respectively. From this figure, the number of ideal
received signal points is 4, that is, the transmission rate of
2 bits/symbol can be achieved.
Figure 12 shows the example of the constellation of
the received signals when the termination impedances are
switched among 4 values. From this figure, the number ofideal received signal points is 16, that is, the transmission
rate of 4 bits/symbol can be achieved. Like the result in
Fig. 12, the received signal points are likely to overlap eachother. Although bit errors may occur if only one received
antenna is used in this case, MLD with multiple receiving
antennas can distinguish these signals much better than the
single antenna receiver.
Figure 13 shows the transmission power versus Bit Error Rate (BER) characteristics for SISO and 2 × 2 MIMO,
where the noise power is −60 dBm. Figure 13(a) shows theresult when the distance between the reader and the tag is
0.5 m and 13(b) shows the result when the distance is 1 m.
In these figures, the number in parentheses, such as (2 bits×1) means (bits per symbol × the number of tag antenna),
Therefore, in both cases of SISO (2 bits ×1) and 2×2 MIMO
(1 bit ×2), transmission rate is 80 kbits/s. From these results, the transmission power required to achieve the transmission rate of 80 kbits/s with BER=10−2 is decreased by
8.4 dB, and that for 160 kbits/s is decreased by 14.9 dB whenusing MIMO with reader-tag separation distance of 0.5 m.
When the distance is 1 m, the transmission power requiredfor 80 kbits/s is decreased by 6.2 dB, and that for 160 kbits/s

Fig. 13

BER characteristic.

is decreased by 12.3 dB when using MIMO. These results
indicate that MIMO has superior BER characteristics compared to SISO regardless of the distance between the reader
and the tag, and MIMO is more eﬀective if the required
transmission rate is high.
Figure 14 shows the transmission power with
BER=10−2 versus the transmission rate for SISO and 2 × 2
MIMO, where the noise power is −60 dBm. Figures 14(a)and (b) show the results when the distances between thereader and the tag are 0.5 m and 1 m, respectively. These
results indicate that MIMO is more eﬀective if the required
transmission rate is high, and that the transmission rate ofpassive MIMO is up to 2 times higher than that of SISO forthe same transmission power when the distance between thereader and the tag is 0.5 m. When the distance is 1 m, thetransmission rate of MIMO is up to 1.7 times higher thanthat of SISO. These results indicate that passive MIMO offers high-speed data transmission.
Figure 15 shows the BER characteristics when the distance between the reader and the tag is varied; the transmission power is 10 dBm, the noise power is −60 dBm, and thetransmission rate is 160 kbits/s. The results show that theavailable distances in MIMO and SISO are 1.7 m and 1.1 m,
respectively. This means that passive MIMO can improve
the transmission distance because it realizes spatial diversity gain through the use of multiple antennas.

IEICE TRANS. COMMUN., VOL.E97–B, NO.7 JULY 20141472

is 1 m, MIMO is up to 70% faster than SISO. These results
indicate that passive MIMO is superior to the conventionalload modulation scheme for realizing high-speed and longdistance RFID systems.
Acknowledgements
This research was partially supported by JSPS KAKENHI(25709030).
References

Fig. 14

Transmission power versus transmission rate.

Fig. 15

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Distance characteristic.

Conclusion

In this paper, we presented the results of experiments on our
passive MIMO transmission proposal, which uses load modulation and MIMO techniques, and proved its validity. PIN
diodes are used as the variable impedance elements at the
tag side to achieve multi-level modulation. The measured
transmission rate of passive MIMO is up to 2 times that of
SISO at the same transmission power when the distance between the reader and the tag is 0.5 m. When the distance

Keisuke Terasaki
received the B.E. degree
in electrical and electronic engineering from
Iwate University, Morioka, Japan in 2012. He iscurrently in the master program in Iwate University. His research interest is feasible load modulation techniques for multiple antenna systems.

TERASAKI and HONMA: EXPERIMENTAL EVALUATION OF PASSIVE MIMO TRANSMISSION WITH LOAD MODULATION FOR RFID APPLICATION

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Naoki Honma
received the B.E., M.E., andPh.D. degrees in electrical engineering from Tohoku University, Sendai, Japan in 1996, 1998,
and 2005, respectively. In 1998, he joined the
NTT Radio Communication Systems Laboratories, Nippon Telegraph and Telephone Corporation (NTT), in Japan. He is now working for
Iwate University. He received the Young Engineers Award from the IEICE of Japan in 2003,
the APMC Best Paper Award in 2003, and theBest Paper Award of IEICE Communication Society in 2006, respectively. His current research interest is planar antennas
for high-speed wireless communication systems. He is a member of IEEE.