Experimental Evaluation on IEEE 802.15.4
Compatible Backscatter
Yohei Konishi∗ , Takayuki Ueda† , Kazuhiro Kizaki† , Takuya Fujihashi† ,Shunsuke Saruwatari† , and Takashi Watanabe†∗ School of Engineering, Osaka University , Osaka, Japan† Graduate School of Information and Science, Osaka University, Osaka, Japan

Abstract—Wireless communications for wireless devices require a significant power consumption for signal amplification.
Such a significant power consumption may induce short lifetimeof battery-operated wireless devices, especially, sensor devices
and Internet of Things (IoT) devices. Backscatter communications have been proposed in recent years for ultra-low-power
wireless communications, e.g., Wi-Fi. In this study, we propose
and implement a novel backscatter communication scheme forsensor devices, i.e., IEEE 802.15.4-compatible backscatter. The
proposed backscatter scheme consists of three devices: RF signal
generator, backscatter transmitter, and standard IEEE 802.15.4
receiver. Specifically, the backscatter transmitter receives theRF signals with/without frequency hopping emitted from theRF signal generator and modulates the received signals into
IEEE 802.15.4 packets by switching the impedance of thetransmission antenna. Since the backscatter transmitter does notneed signal amplification, instead, only needs to switch on/off
state of the transmission antenna for packet transmissions, thepower consumption of the proposed scheme is significantly lower
than the conventional IEEE 802.15.4-based schemes. From theexperimental evaluations, we demonstrated that the proposed
backscatter scheme can realize IEEE 802.15.4 packet transmissions with low power at a certain angle and communication
distance. In addition, we also clarified the frequency hopping at
the RF signal generator can increase the number of the received
packets since it may reduce the dead spots due to multi-pathfading.

I. I NTRODUCTION
Nowadays, every device around us can easily to be connected to the Internet via wired/wireless networks as an
Internet-of-Things (IoT) device. The number of IoT devices
globally might become 39.4 billion by the year 2020 becauseof device miniaturization and lower prices [1]. Each IoT
device is often battery-powered and connected to the Internet
via wireless networks to send the collected data to the sink
node, e.g., server. For wireless communications in IoT devices,
ZigBee [2], which is a communication protocol based on IEEE
802.15.4, is widely used to realize low-power and reliablecommunications.
However, each IoT device still suffers from powerconsumption problems, as it consumes considerable power due
to signal amplification in wireless communications, even in
the case of ZigBee. For example, each IoT device may use
tens or hundreds of milliwatt for signal amplification [3]. A
considerable power consumption by each battery-powered IoT
device reduces its lifetime. Although the battery of each IoT
device can be replaced if the battery is dead, the replacement
requires additional cost, and the cost increases with the numberof IoT devices in sensor networks. To reduce the batteryreplacement cost in IoT devices, we must realize a further lowpower communication technique for IoT devices. To reducethe battery-replacement cost in IoT devices, some studies
considered energy harvesting [4], [5] via solar cells instead ofbattery. However, the power supply from solar cells is small
and unstable. Notably, an unstable power supply results inunstable wireless communications.
In recent years, backscatter communications [6]–[8] havebeen proposed for realizing low-power wireless communications. They can significantly reduce the power of wirelesscommunications, as transmission signals are not amplified at
the transmitter. Instead, the backscatter transmitter (hereinafterreferred to as backscatter device) receives RF signals fromanother RF device (hereinafter referred to as RF signal generator) and transmits packets by quickly switching the impedanceof the antenna. In this case, such high-speed switching canmodulate the received RF signals into packets. Because thesignal amplification is not required on the backscatter device,
the backscatter-based wireless communications can be realizedwith ultra-low power consumption.
The existing backscatter schemes have been designed forvarious wireless techniques such as Wi-Fi [9], [10], BLE [11],and visible light [3]. Our study focuses on backscatter communication, which is compatible with IEEE 802.15.4, to realize ultra-low-power wireless communications for the sensor
and IoT nodes. Some studies [12]–[14] aimed at the same
purpose to realize ultra-low-power IEEE 802.15.4-compatible
communications. Although they have demonstrated that IEEE
802.15.4-compatible backscatter can be realized by using thebackscatter device with a simple switch of the transmission
antenna, the existing studies still have issues related to deadspots due to multi-path fading between the RF signal generator
and backscatter device. Although the RF signal generator
sends unmodulated and continuous RF signals to the backscatter device for modulations, the unmodulated signals may
suffer from environmental multi-path fading at the backscatterdevice. The multi-path fading causes dead spots, which meana significantly low received signal strength at certain receiver’s
position, and brings a small number of the received packets
at the receiver because the backscatter communications do notuse signal amplification before transmissions.
In this paper, we propose a novel backscattercommunication scheme to realize ultra-low-power wirelesscommunications for sensor and IoT devices. Specifically,
we implement an end-to-end IEEE 802.15.4-compatible
backscatter scheme. The proposed scheme comprises thefollowing three devices: an RF signal generator, a backscatterdevice, and a standard IEEE 802.15.4 receiver. The RF
signal generator sends unmodulated and continuous RF
signals as a power supply. Here, the RF signal generator
can adopt Frequency-Hopping Spread Spectrum (FHSS) [15]to the generated RF signals to eliminate the effect of deadspots due to environmental multi-path fading [16]. The
backscatter device modulates the RF signals into IEEE
802.15.4-compatible packets by switching the impedance ofthe transmission antenna, and the standard IEEE 802.15.4
receiver can receive the packets without any modification.
From evaluation results, the proposed IEEE 802.15.4compatible backscatter scheme realizes IEEE 802.15.4 packet
transmissions in certain received angle and distance evenwith low-power consumption. In addition, we demonstrated
an impact of FHSS on the communication reliability of the
proposed backscatter scheme.
The major contributions of our study is four-fold:• We implement a testbed of backscatter communications
compatible with IEEE 802.15.4 to demonstrate the feasibility of the ultra-low-power wireless communications
for sensor and IoT devices.
• We also implement FHSS on the proposed backscatter
scheme to eliminate the dead spots due to multi-path fading between the RF signal generator and the backscatterdevice.
• The proposed IEEE 802.15.4-compatible backscatter device reduces the power consumption to less than 1/1000
compared with the conventional IEEE 802.15.4 transmitter since the backscatter device mainly consists of one
simple switch for packet transmissions.
• From the experimental evaluations, we have demonstrated
that FHSS can increase the number of the received packets in the proposed IEEE 802.15.4-compatible backscatter
since it may reduce the effect of the dead spots.
II. BACKSCATTER C OMMUNICATION
In wireless communications, signal amplification resultsin considerable power consumption, thereby shortening the
lifetime of each wireless device, especially an IoT device,ultimately decreasing the lifetime of sensor networks. In
sensor networks, IEEE standard 802.15.4 is generally used for
low-power and reliable wireless communications [17], [18].
However, the power consumption required for wireless signaltransmission is still high because each wireless transmitter
performs signal amplification for realizing transmissions. General wireless communication circuits mainly comprise digital
and analog circuits. Specifically, the digital circuits perform
modulation, and the analog ones perform signal amplification. Although the power consumption of digital circuits
was significantly reduced using the scaling law of metal–
oxide–semiconductor field-effect transistor [19], the power
consumption of analog circuits is still high. Therefore, the'5)6LJQDO*HQHUDWRU)UHTXHQF\
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Fig. 1. Proposed IEEE 802.15.4-Compatible Backscatter.
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power consumption required for performing wireless signaltransmission is still high.
A backscatter communication system realizes ultra-low
power wireless communication by not opting for an analog
circuit at the transmitter [6]–[8]. The communication is realized by reflecting and absorbing the RF signals transmitted
by RF signal generators. Specifically, the backscatter device
transmits data by switching the state of the antenna, i.e., on/off,at high speed for modulating the received RF signals. Notethat to prevent the interference between the signals transmitted
by the backscatter device and ɹ the RF signal generators,
the backscatter device must change the frequency of thetransmission signals via frequency shift [10]. For example,
when the signals from the other devices use the frequencyof f (GHz) and the backscatter device switches the state of
the transmission antenna at the frequency of ∆f , then thefrequencies of the signals transmitted from the backscatterdevice are f − ∆f and f + ∆f .
Passive Wi-Fi [9] was proposed to realize Wi-Fi Backscatter
communications. The backscatter device reflects and absorbs
RF signals transmitted by RF signal generators to performdifferential phase shift keying modulation. The modulated
signal is received and demodulated using a receiver equippedwith a commercially available Wi-Fi chip. Because the datatransmission device need not perform signal amplification for
performing signal transmission, Wi-Fi communication realizedwith only 1/1,000 to 1/10,000 of the power consumption ofthat of the standard Wi-Fi communication at the same data
rate.
III. P ROPOSED IEEE 802.15.4-C OMPATIBLEBACKSCATTER
A. Overview
The overview of the proposed IEEE 802.15.4-compatible
backscatter communication scheme is depicted in Figure 1.
The proposed scheme comprises an RF signal generator, abackscatter device, and the standard IEEE 802.15.4 receiver.
The RF signal generator transmits unmodulated and continuous signals of multiple frequencies to support frequency hopping. The backscatter device transmits IEEE 802.15.4 packets
by modulating the unmodulated and continuous signals that aretransmitted by the RF signal generator. The standard 802.15.4
receiver receives and decodes the IEEE 802.15.4 packets fromthe backscatter device. Each device is connected to a PC via
wire, and each device controlled using the PC.

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C ONVERSION TABLE OF SYMBOL TO CHIP
SymbolChip (C0∼C31)
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The proposed IEEE 802.15.4-compatible backscatter device
comprises only one single-pole double-throw (SPDT) switch
and a microcomputer. The power consumption of the proposed
backscatter device is only 21 µW with 3.0 V voltage supply.
Note that the transmission circuit in the conventional IEEE
802.15.4 standard comprises a D/A converter, local oscillator,
quadrature converter, and power amplifier. The power consumptions are 28.05 mW at -25 dBm and 57.42 mW at 0 dBm,
respectively, with 3.3 V voltage supply.
B. RF Signal Generator
The RF signal generator comprises an IEEE 802.15.4compatible system-on-chip (Texas Instruments CC1352 P-2)
and a monopole antenna (Daiichi Denki Kogyo H2401SBII).
It is controlled using a PC connected with a universal serial
bus interface. It can change the frequency of the RF signals
every 254 ms to realize frequency hopping. Specifically, thefrequency of the RF signal can be set at the interval of
0.01 MHz in the range from 2.400 to 2.500 GHz, and theoutput power can be set at the interval of 0.5 dB in the range
from -22.5 to -10 dBm.
C. Backscatter Device
The backscatter device comprises a microcomputer (STMicroelectronics STM32F446RE), SPDT RF Switch, andmonopole antenna (D240DSI H2401SBII). It modulates thesignals received from the RF signal generator by using
the offset-quaternary phase shift keying (O-QPSK) modulation format. Specifically, the backscatter device continuously
switches the SPDT RF Switch that is connected to the antenna
to GND or shorting or opening. Simultaneously, the frequency∆f of the modulation signal is determined using the speed of
the switching between short and open in the SPDT RF Switch.
In this case, the frequency of the modulated signal is f − ∆fand f + ∆f .

The flow of the modulation signal generation is depicted
in Figure 2. The backscatter device generates an on/off signal
for SPDT RF switch on the basis of the transmission data.
It turns on/off its own antenna according to the generated
on/off signal. The flow from “Bit to Symbol” to “O-QPSK
Modulator” depicted in Fig. 2 follows the IEEE 802.15.4
standard [17], [18]. The ʠ Bit to Symbol ʡ part divides thetransmission bits into four bits to make one symbol. A symbol
is a value from 0 to 15. Theʠ Symbol to Chip ʡpart converts
each symbol into 32 chips according to the symbol value
in the conversion table as shown in Table I. A total of 32
chips, i.e., C0 ∼ C31 are placed in the I-Phase and Q-Phase
according to the O-QPSK modulation as Figure 3. Figure 4
shows an example when the backscatter sends a symbol of 0.
As shown in Figure 5, when the value of chip is 1, the value
converts to a positive sine wave quarter cycle in the case ofstandard IEEE 802.15.4. In the Pulse Shape part, when thevalue of chip is 1, the backscatter device regards the value as“+.” However, when the value of chip is 0, the backscatterdevice regards the value as “−.” In addition, the backscatterdevice inserts “0” between “+” and “−” to change the phaseof the clock signal by 90 degrees. In the backscatter device, thephase angle is expressed using a combination of the I-Phase
and Q-Phase shapes. Figure 6 also shows an example of thepulse shape part when the backscatter device sends Symbol-0.
The Backscatter modulator in Fig. 7 creates four-phase clock
signals (0, 90, 180, and 270 degrees) with phases shifted by
90 degrees. Each clock signal constitutes one cycle of eightclocks. The backscatter device selects a clock signal with aphase that matches the phase (0, 90, 180, 270 degrees) createdby the Pulse Shape part, every the number of clocks shown asMod. 5 types of Mod values shown in Table II are available.
In this study, Mod value is fixed at 12 because Mod 12 has thebest communication performance. On the basis of the clock
signal selected, the backscatter device turns on/off the SPDT
RF Switch.
D. Standard IEEE 802.15.4 Receiver
The standard IEEE 802.15.4 receiver comprises an IEEE
802.15.4-compatible system-on-chip (CC1352 P-2, manufactured by Texas Instruments) and a monopole antenna(H2401SBII, manufactured by Dai-ichi Denki Kogyo). The
receiver receives the IEEE 802.15.4 packet generated from thebackscatter device. Simultaneously, the frequency for receiving
the IEEE 802.15.4 packets is controlled using the PC and determined at the same timing as the RF signal generator. Since

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IV. S IMULATION R ESULTS
In this section, we report the simulations that were performed to verify the effect of the fading in the IEEE 802.15.4compatible backscatter. In this evaluation, we assumed a twowave model while considering both the direct and reflected
waves from the ground. We further assume that the antennaheight is 1 m from the ground. In addition, we assume thatthe reflected wave is delayed on the basis of the sum of the
distance between the RF signal generator and ground andthe distance between ground and the receiver. At this time,the direct and reflected waves are attenuated because of the
propagation distance.
We first evaluate the received signal strength indicator(RSSI) on the backscatter device when the RF signal generator
transmits un-modulated and continuous signals using a single
frequency and frequency hopping, respectively. The frequencyof the unmodulated and continuous signals transmitted using
a single frequency was set to 2.4 GHz. On the other hand, theunmodulated and continuous signals transmitted via frequency
hopping used the frequencies between 2.402 and 2.480 GHz at
the interval of 1 MHz, for a total of 79 channels. In frequency
hopping, the channels are switched 1600 times per second. In
this case, the hopping channel is determined using a uniform
distribution from all channels.
In Fig. 8, we depict the RSSI performance as a functionof the communication distance between the RF signal generator and backscatter device. It can be seen that the RSSI
performance for the single-frequency scheme is significantly
low at a certain communication distance due to dead spots,
where the path difference between the direct and reflectedsignals becomes half-wavelength period [20]. On the other
hand, the frequency hopping scheme does not fluctuate the
RSSI performance even if the phase difference between the
direct and reflected signals changes with the propagation
distance. Improving the RSSI of the backscatter device mightenhance the communication reliability of the IEEE 802.15.4
packets transmitted from the backscatter device without signal
amplification.
We then evaluated the end-to-end communication performance of the proposed IEEE 802.15.4-compatible backscatter

Fig. 8. RSSI performance as a func- Fig. 9. BER performance with respecttion of communication distance.
to noise variance.

with single-frequency/frequency-hopping. The RF signal generator transmits unmodulated and continuous signals using asingle frequency and frequency hopping, respectively, to thebackscatter device. The backscatter device sends 64 bits as
O-QPSK-modulated signals using the direct-sequence spreadspectrum technique, which follows the IEEE 802.15.4 modulation format, and the standard IEEE 802.15.4 receiver thendemodulates the signals received. Here, we assumed that themodulated signal from the backscatter device used the center
frequency of 2.4 GHz and bandwidth of 2 MHz. The spreadingcode sequence used for the direct spreading was of 32 bits. We
also assumed that the distance between the backscatter device
and receiver was 1000 mm. In addition, the transmission powerof the backscatter device in each scheme is based on the
RSSI performance at the backscatter device. The transmitted
O-QPSK symbols are impaired by additive white Gaussian
noise (AWGN) with mean zero and variance of σ 2 (dBm).
In Fig. 9, we depict the average bit error rate (BER) of
each scheme as a function of the noise variances of AWGN.
Here, we consider two cases of the communication distance theRF signal generator and the backscatter device. The first casefollows uniform distribution between 0 mm and 3,000 mm todiscuss the average BER performance. The second case is in
the dead spot of 1,850 mm to discuss the impact of multi-pathfading. From the simulation results in Fig. 9, we can see thefollowing observations:•

Even in the dead spot, IEEE 802.15.4-compatible
backscatter using frequency hopping maintains a low
BER since it may eliminate the effect of multi-path fadingat the backscatter device.

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• The average BER performance of both schemes are
similar irrespective of the noise variances.
•

V. E XPERIMENTAL E VALUATION
A. Equipment
We then experimentally investigated the communication
performance of the proposed IEEE 802.15.4 compatible
backscatter scheme. Our experiment was conducted using an
RF signal generator, a backscatter device, and the standard
IEEE 802.15.4 receiver deployed in a shield tent. In Fig. 10,
Rx denotes the standard IEEE 802.15.4 receiver, BS thebackscatter device, and Tx the RF signal generator. The
backscatter device was installed at the center of the turntable,
and the receiver was installed at the edge of the turntable.
The term D1 represents the communication distance between
the backscatter device and receiver, and D2 represents thecommunication distance between the backscatter device andRF signal generator. Notably, D1 was fixed at 250 mm. We
varied D2 within the range from 1000 mm. In addition, we
rotated the turntable from 30 degrees to 330 degrees in thesteps of 5 degrees. The backscatter device sends 1000 packets
at each angle. The RF signal generator transmits unmodulated
and continuous signals of 10 dBm. The bit rate is 250 kbps.
B. Baseline Performance
In Fig. 11, we depict the number of the IEEE 802.15.4
packets received at the receiver as a function of the turntable
angles at the communication distance D2 of 1000 mm. Here,in Fig. 11 (a), the RF signal of 2.405 GHz is transmitted, andin Fig. 11 (b), the RF signal of 2.475 GHz is transmitted fromthe RF signal generator. From the experimental results, it isobserved that the angle of the turntable may affect the numberof the received IEEE 802.15.4 packets. For example, when thefrequency of the RF signal was set to 2.405 GHz, many packets
could be received correctly when the angle of the turntable was

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30 to 120 degrees. However, when the frequency of the RF
signal was set to 2.475 GHz, many packets were correctly
received when the angle of the turntable was 190 degrees
to 290 degrees. When the angle of the turntable changes,
the frequency at which the radio-wave-reception intensity
decreases due to fading on the data receiver also changes;
therefore, the number of received packets changes for eachset RF signal frequency.
We evaluated the number of received packets dependingon the frequency of the RF signal at the fixed angle of theangle turntable. Here, the angles of the turntable were fixed at
150 and 200 degrees, respectively. In Fig. 12, we depict the
number of the packets received at the receiver as a functionof the RF signal frequency for the communication distanceD2 of 1000 mm. Here, the frequency of the RF signal ischanged in the range from 2.405 to 2.480 GHz in the steps of
5 MHz. From the experimental results, it was observed thatthe number of received packets significantly varied according
to the RF signal frequency. For example, in Fig. 12 (b),when the RF signal frequency was set to 2.415, 2.430, 2.460,
and 2.480 GHz, the receiver correctly received many packets.
However, IEEE 802.15.4 packets could not be received atcertain frequencies such as 2.405 and 2.410 GHz.

C. Effect of Frequency Hopping
We now discuss the impact of frequency hopping on thereliable communications of the IEEE 802.15.4-compatible
backscatter communications. The number of channels used by
the RF signal generator and the receiver ranged from 2.405 to
2.480 GHz in the intervals of 5 MHz, i.e., 16 channels. The
RF signal generator changed the frequency of the RF signals
every 254 ms. The hopping order of the available channels is
8, 15, 1, 2, 4, 9, 3, 10, 11, 16, 7, 13, 6, 12, 9, 14, and 5.
In Fig. 13 (a), we depict the number of packets received
at the receiver as a function of the received angles for thecommunication distance of 1000 mm and 16 channels. Fromthe experimental results, it can be seen that compared withthe case of communication using a single frequency, thecommunication can be correctly received at a wider angle byusing frequency hopping.
To verify the effect of frequency hopping, the number ofreceived packets was measured by varying the number ofchannels used for frequency hopping. In Fig. 13 (b), we
depict the total number of packets received over the entire
angle of the turntable as the function of number of channels
used for frequency hopping between the RF signal generator
and backscatter device. The experimental results show thatthe number of packets increases with the number of hopping channels. This means that the IEEE 802.15.4-compatible
backscatter communication using multiple frequencies canreceive many packets to reduce the effect of fading.
Finally, we evaluated the performance of the proposed
scheme in terms of the fairness across the received angles.
For this purpose, we used Jain’s fairness index [21] as themetric of fairness. Jain’s fairness index takes a value from n1
to 1. A closer value to 1 represents high fairness. Notably,1
n is the worst case. Since n is the number of received angle
samples of 61, the fairness index ranges from 0.016 to 1.
In Fig. 13 (c), we depict the fairness index as a functionof the number of hopping channels. The fairness improved as
the number of hopping channels increased. For example, when
RF signal generator used a single frequency for backscatter
communications, the fairness index was 0.124. On the other
hand, the fairness index increased to 0.562 upon using 16
channels for frequency hopping. These results assert that thereceiver can receive IEEE 802.15.4 packets from many angles
by using frequency hopping at the RF signal generator.
VI. C ONCLUSION
In this paper, we proposed and implemented IEEE 802.15.4
compatible backscatter communication scheme to realize ultralow-power communications for sensor and IoT devices. Since
the backscatter device can send IEEE 802.15.4 packets fromthe received RF signals emitted from the RF signal generator,
it only uses 21 µW for packet transmissions. From the experimental evaluations, the proposed IEEE 802.15.4-compatible
backscatter communications realize packet transmissions at a
communication distance of 1 m with a slight power consumption. In addition, we demonstrated the frequency hopping canincrease the number of the successfully received packets since
it may reduce the effects of multi-path fading.
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