# Theoretical BER Evaluation of Passive RFID Tag-To-Tag Communications

Tarik Lassouaoui, Florin Hutu, Yvan Duroc, Guillaume Villemaud

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Tarik Lassouaoui, Florin Hutu, Yvan Duroc, Guillaume Villemaud. Theoretical BER Evaluation of Passive RFID Tag-To-Tag Communications. 2020 IEEE Radio and Wireless Symposium, Jan 2020, San Antonio, United States. pp.1-4. [hal-02458829](https://hal.science/hal-02458829)

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Distributed under a Creative Commons Attribution 4.0 International License
Theoretical BER Evaluation of Passive RFID Tag-To-Tag
Communications

Tarik Lassouaoui ¹, Florin Hutu ¹, Yvan Duroc ² and Guillaume Villemaud ¹

¹Univ. Lyon, INSA Lyon, Inria, CITI, F-69621 Villeurbanne, France

²Univ. Lyon, UCBL, Ampere Laboratory, F-69622 Villeurbanne, France

***Abstract***— The concept of UHF RFID passive tag-to-
tag communication has been introduced and opens new
promising perspectives in the field of Internet-of-Things. This
concept enables two powerless tags to exchange information
using an external source. Moreover, a simulation framework
has been proposed as a tool allowing the performance
evaluation of tag-to-tag radio links in terms of Bit Error
Rate (BER). The aim of this paper is to provide a theoretical
framework for BER estimation. The theoretical framework
takes into account various parameters such as modulation
rate, receivers topology and the electromagnetic coupling
of the tags. Furthermore, in order to validate the obtained
results, they are compared with the ones obtained by realistic
simulations.

***Index Terms***— RFID tags, UHF antennas, Bit error rate, Amplitude shift keying, Demodulation.

I. INTRODUCTION

Radio frequency identification (RFID) is still a fast
emerging technology with numberless applications espe-
cially in the areas of security, logistics or traceability [1].
RFID is also an evolving technology since new capabilities
such as sensing, energy harvesting, spatial identification
are added [2][3]. Another example of this tendency is
the development of inter-tag communication concept for
applications where data rates can be low and the receiver
tag uses passive RF part (i.e. envelope detector)[4].

The concept of tag-to-tag (T2T) communication be-
tween two passive UHF RFID tags has already been
proposed and experimentally validated in [5] for short
distances. The main idea is to establish communication
between two passive tags placed in close proximity. Energy
feeding of these tags is achieved by an external source
which can be a classical RFID reader or any other source
providing a power level large enough in order to activate
the tags. The communication between the tags is performed
by backscattering modulation of the incoming RF sources
such as WiFi signals, TV transmissions [4] and Bluetooth
4.0 Low Energy standard [6].

However, since the first studies few works have been
done about this topic. In [7], the design of a power efficient
demodulator is investigated to enable the tag-to-tag link
to operate at long distances with a modulation depth as
low as 75%. In [8], a novel multiphase backscattering
technique is proposed to overcome phase cancellation and

demonstrating the capability of tag-to-tag link to operate
at long distances (above 3m) under high excitation signal.

All things considered, these efforts mostly focus to deal
with the performance limitations of the passive UHF RFID
systems, like tag characteristics in terms of antenna gain
polarisation and impedance matching, sensitivity of the
receiver which defines the minimum level of the tag signal
and propagation environment [9].

This paper focuses on the evaluation of the passive
T2T communication in terms of bit error rate (BER)
as traditionally done for evaluating wireless transmission
systems. The performance depend on several parameters:
i) electromagnetic coupling (i.e., the mutual coupling
between the tags or the orientation of the tags with
respect to the source); ii) modulation depth; iii) receivers
topology (coherent or non-coherent demodulation). The
general objective is to achieve a global optimization of
the communication link between the tags. More precisely,
antenna or radio front-end considerations may be extracted
by such kind of analysis.

The proposed approach is based on the simulation
framework discussed in [10]. The main idea is to consider
the antennas of the two tags (LT -listener tag and RT -
reader tag) as an array of two mutually coupled radiating
elements. With respect of the orientation of the illum-
nating source relative to the tags plane, the far field gain
of the antenna array is varying. Moreover, the impedance
provided by the LT antenna varies with respect of the
distance between the two tags. The gain and impedance
modification has an impact on the modulation depth and
consequently on the quality of the communication. As
stated, the present work gives for ASK modulation the
analytical formula for the probability of error taking into
account the particular case.

The rest of the paper is organized as follows. Section
II describes more in detail the concept of tag-to-tag (T2T)
communication with versatile modeling tools are required
to understand and master the design of interacting-oriented
tags. The theoretical investigation aimed to rewrite bit error
probability according to crucial parameters that improve
the range is detailed in section III. Finally conclusion and
perspectives are given in section IV.
## II. TAG-TO-TAG SIMULATION FRAMEWORK

A T2T communication requires an external electromagnetic source in the employed frequency band that delivers a signal with a power high enough to ensure the tags activation. The information exchange is performed based on the same source by using the retro-modulation principle.

<figure><img src="image_2.png" /><figcaption>Fig. 1. General principle of tag-to-tag communication.</figcaption></figure>

As mentioned previously, the RT will modulate the incoming signal to send a message towards the LT (Fig. 1). From the electromagnetic field perspective, the two tags may be described as an array of two mutually coupled antennas. The resulting antenna gains and the impedance variation of the LT antenna with respect of the RT variation are extracted from an electromagnetic field simulation performed with CST microwave studio software.

Then, system-level simulations are performed using Keysights Advanced Design System (ADS) software. The synoptic of the simulation framework is depicted in Fig. 2. As can be remarked, the external source (here a CW) is modulated in amplitude (ASK) by switching between the two gains and impedance of the antennas ($G_{on}$, $Z_{on}$ and $G_{off}$, $Z_{off}$ respectively). The switch is performed in the rhythm of the data sent from the RT towards the LT.

<figure><img src="image_3.png" /><figcaption>Fig. 2. Synoptic of the tag-to-tag system-level simulations.</figcaption></figure>

## III. ANALYTICAL FORMULATION OF BER

The variation in terms of antenna array gain and the impedance presented at the input of the LT, can be associated to a variation of the amplitude of the CW signal, at the rhythm of the data sent by the RT. In order to receive the data, on the LT, a demodulation stage is employed. The demodulation may be coherent or non-coherent. For the two cases, analytical expression of the probability of

error with respect of signal to noise ratio can be developed [11][12].

The probability of error $P_e$ in the case of a coherent and non-coherent reception is given below.

### A. Coherent detection

<figure><img src="image_4.png" /><figcaption>Fig. 3. Coherent ASK receiver.</figcaption></figure>

The coherent or synchronous demodulation requires a carrier recovery circuit to transpose the received signal to the low frequencies. A block diagram of coherent detection is shown in Fig. 3. In this circuit, a local oscillator is tuned to oscillate at the same frequency as the received signal, the received RF signal multiplies with the local oscillator through an RF mixer generating the sum and the difference frequencies between them. The lowpass filter is used to recover the bandpass signal by eliminating the sum frequency component.

$$P_{e,ASKC} = \frac{1}{2} \cdot \text{erfc} \left( \sqrt{\frac{d^2}{4N_0}} \right) \quad (1)$$

The erfc function is given as:

$$\text{erfc}(x) = \frac{2}{\sqrt{\pi}} \int_{0}^{x} e^{-t^2} dt \quad (2)$$

### B. Non coherent detection

<figure><img src="image_5.png" /><figcaption>Fig. 4. Non-coherent ASK receiver.</figcaption></figure>

The asynchronous ASK detector consists of a bandpass filter to reduce noise outside the frequency band, an envelope detector as a half-wave rectifier used to recover the baseband signal and a quantifier which delivers a digital output. The non-coherent detection does not require a synchronization mechanism, in the presence of noise that disturbs the signal, a system using non-coherent detection has a lower BER than coherent detection.

$$P_{e,ASKNC} = \frac{1}{2} \cdot \exp \left( -\frac{d^2}{4N_0} \right) \quad (3)$$
Where *d* corresponds to the difference between the amplitudes of the high and low bit states, and *N*₀ is the noise power spectral density.

$$E_b = \frac{1}{2} \cdot \left( \frac{A_1^2}{2} \cdot T_s + \frac{A_0^2}{2} \cdot T_s \right) \quad (4)$$

The average energy per bit *E*<sub>b</sub> is defined by the amplitudes of the two states *A*<sub>0</sub>, *A*<sub>1</sub> of the binary flow and bit time *T*<sub>s</sub>.

$$A_{1/0} = A \cdot G_{on/off} \left( 1 - \left| \frac{Z_{on/off} - Z_0}{Z_{on/off} + Z_0} \right| \right) \quad (5)$$

It is worth noticing that the modulation depth *M* depends on the gain and switching impedance together with the received power coming from the external source.

$$M = \frac{A_1 - A_0}{A_1} \quad (6)$$

For binary modulation schemes, we have two signals *S*<sub>1</sub>(*t*) and *S*<sub>0</sub>(*t*).

$$S_1(t) = \sqrt{\frac{2E_b}{\beta}} \cdot \varphi(t) \quad (7)$$

$$S_0(t) = \alpha \cdot S_1(t) \quad (8)$$

With:

$$\beta = G_{on} \left( 1 - \left| \frac{Z_{on} - Z_0}{Z_{on} + Z_0} \right| \right) + G_{off} \left( 1 - \left| \frac{Z_{off} - Z_0}{Z_{off} + Z_0} \right| \right) \quad (9)$$

$$\varphi(t) = \sqrt{\frac{2}{T_s}} \cdot \cos(2\pi f_c t) \quad (10)$$

$$d = \sqrt{\frac{2E_b}{\beta}} \cdot (1 - \alpha) \quad (11)$$

The parameter *d* is strictly related to the average energy per bit *E*<sub>b</sub> which is defined according to the amplitudes of the two states *A*<sub>0</sub>, *A*<sub>1</sub> of the binary flow and modulation rate α=A₀/A₁.

By replacing the term *d* in (1) and (3) with (11), we obtain:

$$P_{e,ASKC} = \frac{1}{2} \cdot \text{erfc} \left( \sqrt{\frac{(1-\alpha)^2}{\beta} \cdot \frac{E_b}{2N_0}} \right) \quad (12)$$

The erfc function is defined in (2).

$$P_{e,ASKNC} = \frac{1}{2} \cdot \exp \left( - \frac{(1-\alpha)^2}{\beta} \cdot \frac{E_b}{2N_0} \right) \quad (13)$$

$E_b/N_0$ is the signal to noise ratio (SNR) for a digital signal, its a normalized version of SNR for a defined bandwidth and bit rate, used as a figure of merit.

## IV. SIMULATION RESULTS

By using the simulation framework previously described, BER variation has been calculated. The simulations were performed considering the LT and RT illuminated by a CW at 868 MHz and with a power level of -50 dBm.

In order to have good accuracy for the obtained BER values, the number of transmitted bits was 10<sup>6</sup>. The bit time was set to 50 nsec. The gain and the coupling impedance value are defined to have several modulation depths ranging from 10% to 100%. Without loss of generality, the simulation considered the impedance matching case (where Z<sub>on</sub> = Z<sub>off</sub> = 50 Ω) together with a mismatch case.

The BER value is estimated by comparing the base band signal of RT with the recovered signal of LT using synchronous or asynchronous demodulation, the digital version is recovered with a decision circuit after the demodulation.

As can be seen from Fig. 5 and Fig. 6, for a coherent and non-coherent detection, by assuming that the signal collected by the listener tag is degraded by AWGN. The performance of the T2T is proportional to the modulation depth that must be maximized with a proper impedance matching strategy accounting for the electromagnetic coupling among tags.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Coherent demodulation: BER variation for different modulation depths in matched case.</figcaption></figure>

We focus on the non coherent detection results that is used in passive T2T system for simplified circuitry and very low power cost.

In Fig. 6, for a bit error rate of 10<sup>-5</sup>, ASK<sub>NC100%</sub> needs a signal to noise ratio of approximately 13 dB while ASK<sub>NC50%</sub> is around 19 dB. The reason is ASK<sub>NC50%</sub> expends energy in both symbols used for transmission, but the symbols are alike and distance between them in signal
space is small called modulation depth, the receiver is not able to make the difference between two binary levels in low signal to noise ratio.

<figure><img src="image_7.png" /><figcaption>Fig. 6. BER variation for non-coherent demodulation for different modulation depths in matched case.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 7. BER variation for non-coherent demodulation in the miss-match case.</figcaption></figure>

The difference between the theoretical results and the simulation in Fig. 7 comes from the way in which the BER and SNR is determined in simulation (statistical computation), while in MATLAB we define an interval for the SNR in which the theoretical BER is plotted.

## V. CONCLUSION

This paper presents a theoretical background for the analysis of a passive tag-to-tag communication. BER evaluation demonstrates the importance of modulation depth, which is defined by the gain, coupling impedance and

received power from the external source. Concluding that, the distance between the tags, their orientation and the position of the source play a crucial role in performance limitations of passive tag-to-tag systems and that the theoretical formulation will help to design an efficient T2T system.

A further study will be performed by completing the theoretical study, the aim is to make simulation framework as realistic as possible by considering several parameters like electromagnetic coupling (i.e. complex coupling impedance) and different tags configuration where the antennas are spaced, staggered and arbitrarily oriented. Also, by adding scenarios where the external source is a complex RF waveform from, using a more realistic radio channel and receivers with passive circuits. This will lead us to measure the maximum RT-to-LT tag communication distance in a variety of controlled scenarios.

## VI. ACKNOWLEDGEMENT

This work was supported by the SPIE ICS-INSA Lyon IoT chair.

## REFERENCES

[1] K. Finkenzeller, *RFID handbook: fundamentals and applications in contact-less smart cards and identification*. New York, NY, USA: John Wiley and Sons, Inc., 2003, p. 427, (in English).

[2] P. V. Nikitin, R. Martinez, S. Ramamurthy, H. Leland, G. Spiess, and K. V. S. Rao, "Phase based spatial identification of uhf rfid tags," in *2010 IEEE International Conference on RFID (IEEE RFID 2010)*, April 2010, pp. 102–109.

[3] S. Tedjini, G. Andia-Vera, M. Zurita, R. C. S. Freire, and Y. Duroc, "Augmented rfid tags," in *2016 IEEE Topical Conference on Wireless Sensors and Sensor Networks (WiSNet)*, Jan 2016, pp. 67–70.

[4] V. Liu, A. Parks, V. Talla, S. Gollakota, D. Wetherall, and J. R. Smith, "Ambient backscatter: Wireless communication out of thin air," *SIGCOMM Comput. Commun. Rev.*, vol. 43, no. 4, pp. 39–50, Aug. 2013. [Online]. Available: http://doi.acm.org/10.1145/2534169.2486015

[5] P. V. Nikitin, S. Ramamurthy, R. Martinez, and K. V. S. Rao, "Passive tag-to-tag communication," in *2012 IEEE International Conference on RFID (RFID)*, April 2012, pp. 177–184.

[6] J. F. Ensworth and M. S. Reynolds, "Every smart phone is a backscatter reader: Modulated backscatter compatibility with blue-tooth 4.0 low energy (ble) devices," in *2015 IEEE International Conference on RFID (RFID)*, April 2015, pp. 78–85.

[7] Y. Karimi, A. Athalye, S. R. Das, P. M. Djuri, and M. Stanaevi, "Design of a backscatter-based tag-to-tag system," in *2017 IEEE International Conference on RFID (RFID)*, May 2017, pp. 6–12.

[8] J. Ryoo, J. Jian, A. Athalye, S. R. Das, and M. Stanaevi, "Design and evaluation of bttn: A backscattering tag-to-tag network," *IEEE Internet of Things Journal*, vol. 5, no. 4, pp. 2844–2855, Aug 2018.

[9] P. V. Nikitin and K. V. S. Rao, "Performance limitations of passive uhf rfid systems," in *2006 IEEE Antennas and Propagation Society International Symposium*, July 2006, pp. 1011–1014.

[10] L. Zhou, F. Hutu, G. Villemaud, and Y. Duroc, "Simulation framework for performance evaluation of passive rfid tag-to-tag communications," in *2017 11th European Conference on Antennas and Propagation (EUCAP)*, March 2017, pp. 500–504.

[11] J. Kabara, P. Krishnamurthy, R. A. Thompson, and D. Tipper, *The Physical Layer of Communications Systems*. Boston, USA: Artech House., 2006, p. 888, (in English).

[12] P. Z. Peebles, *Digital Communication Systems*. Harlow, United Kingdom: Prentice Hall., 1986, p. 432, (in English).