A Proposal for Dynamic Power Control in RFID and Passive Sensor Systems Based on RSSI

Alírio Soares Boaventura Dep. Electrónica, Telecomunicações e Informática,

Instituto de Telecomunicações Universidade de Aveiro

Aveiro, Portugal a34422@ua.pt

Nuno Borges Carvalho Dep. Electrónica, Telecomunicações e Informática,

Instituto de Telecomunicações Universidade de Aveiro

Aveiro, Portugal nbcarvalho@ua.pt

Abstract—In this paper a proposal for a dynamic power control in RFID and Passive Sensor Systems based on received signal strength (RRSI) is presented. The main goal of these new proposals is the reduction of the power being transmitted by the RFID reader and thus reduction of power consumption and interference between the reader and other neighborhood systems. It is expected that using a protocol based on this scheme the overall system reader and mobile tag’s could operate with lower values of power sources.

Keywords-RFID, RSSI, Low Power, Passive Wireless Sensors

I.INTRODUCTION The transmitted power level of Radio Frequency

Identification (RFID) readers and passive sensor interrogators is critical. While the minimum power level is imposed by tag/sensor activation level and sensitivity, the maximum transmitted power is also important for several reasons: first of all, the maximum transmitted power is limited by regulations; second, RFID reader emissions can cause interference in neighboring systems and, finally, the optimization of the power level is important for energy saving.

Dynamic power control is a well known strategy implemented in cellular mobile systems to save energy both in the mobile station (MS) and in the base station (BS). Recently, some works have addressed power control algorithms in dense RFID networks (where several RFID readers operate in adjacent cellules) [1][2]. The objective is to reduce interference caused by reader emissions and to improve the network performance as well as the overall coverage range. This is achieved by dynamically adjust the RFID reader output power according to the interference level detected during tag interrogation and acceptable signal-to-noise ratio (SNR).

In the reader-tag communication level, conventional single RFID reader systems do not employ power control, the reader transmits a fixed power level independently of number of tags in the field and distance from tag to reader. In reference [3], a method for managing the power consumption of an RFID reader is presented, that includes, for instance, the use of proximity and motion sensors to detect an RFID tagged object in the field of the reader.

The work conducted in [4] presents an energy-efficient RFID inventory algorithm, which automatically estimates the number of tags to be read and incrementally adjusts the power level. This way no excessive power is transmitted and energy is saved (authors claimed more than 60% energy reduction).

In this paper we propose a dynamic power control architecture and algorithm based on the Received Signal Strength Indication (RSSI) received from the tag. Such a system has to employ an RSSI receiver in order to measure the strength of the received signal. According to the tag RSSI an estimate of the distance from the reader can also be obtained, opening other areas of interest.

II.PROPOSED ARCHITECTURE AND ALGORITHM This scheme is intended to be applied in passive RFID

UHF systems (ISO18000-6C, EPCGlobal Class 1 Gen2 [5]). The basic protocol interaction between an EPCGlobal compliant interrogator and a tag is depicted in Fig.1 [5].

Fig.1 Example of tag inventory and access after ISO18000-6C protocol

1 Reader sends a Query 2 Tag responds with

16 bits random number RN16

3 Reader sends ACK with the same RN16

4 Tag responds with its EPC

5 Reader issues Req_RN with the

same RN16 6 Tag responds with {handle}

7 Interrogator accesses tag. Each access

command uses handle as parameter

8 Tag verifies handle and ignores

command if handle does not match

Query(Q)

RN16

{PC,EPC}

Req_RN(EPC)

handle

Command(handle)

6th European Conference on Antennas and Propagation (EUCAP)

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After the first tag response (RN16) to the first reader command (Query), the reader measures the RSSI from the tag response and readjust its output power according to the estimated distance from the tag (received RSSI provides information about distance between reader and tag). An adequate algorithm to estimate distance from the RSSI should be implemented. Fig.2 shows the simplified flowchart of the proposed power control algorithm. The architecture of the proposed hardware is presented in Fig.3.

Fig.2 Flowchart of the proposed power control algorithm

A) Signal Strength Measurement

Typical RFID receivers use In Phase and Quadrature (IQ) demodulators (Fig.4) to demodulate the signal reflected back from the tag [6]. The IQ configuration is used to overcome the problems associated to the simple ASK/PSK demodulator [7]: the signal reflected back from the tag is a phase-shifted version of the carrier transmitted by the reader and the phase shift depends on the distance between the reader and the tag. If the distance is such that the local oscillator and the received signal is in quadrature (90º phase shift) the output of an ASK/PSK demodulator is zero and the information from the tag is lost. By using the IQ architecture with two complementary branches the information loss is prevented. The same IQ receiver hardware can be used to measure the RSSI, an indication of the signal strength can be given by:

S2=I2+Q2 (1

90º

LPF

LPF

LO

I

QI In Phase

Quadracture

Q

Fig.4 IQ demodulator typically used in RFID receivers (left) and received constellation diagram (right)

UHF RFID Reader

PA

Transmitter

LNA

RSSIMeasure

RFID Tag

IQ - Demodulator

PowerControl

Receiver

Reader Command / Interrogation

Tag Response(Backscattered signal)

Modulator

Fig.3 Hardware implementation including an RSSI receiver and output power control

Pout=Pmax

Send QUERY(Q)

Receive Tag response

Re-adjust Power Pout=f(RSSI)

Measure RSSI

Complete protocol � Receive RN16 � Send ACK(RN16) � Receive EPC

…

S

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III.PRELIMINARY MEASUREMENTS In this work the first part of the power control scheme (basically, the first four steps of the flowchart of Fig.2) will be implemented with a dedicated measurement setup (similar to the one used in [8]), that is composed by an RFID reader, some external components (e.g. mixers), an oscilloscope (to capture the signals) and Matlab scripts (to analyze the waveforms and compute the RSSI). In this section we intend to obtain an indication of the signal strength received from the tag as a function of the distance between the reader and the tag and to evaluate whether this indication can or cannot provide useful information of the distance. To do so we will interrogate the tag with a commercial RFID reader and use a

dedicated measurement setup to measure the air interface waveforms which will be stored for further analysis and calculation of a signal strength indication using Matlab. The baseband waveform of several reader commands and tag responses were captured for the same transmitted power and for different distances between the reader and tag. As an example, Fig.5 depicts the baseband captured at the reader location for the tag at 2 meters away from the reader. Fig.6 is a zoom of the tag response received at the reader location. Table 1 shows the amplitudes of tag response, A. Based on these amplitudes it is possible to calculate an RSSI metric and have an estimation of the distance.

Fig.5 Baseband waveform of reader commands followed by tag responses for a reader-tag distance of 2 metres

Fig.6 Zoomed version of tag response

Table 1 Amplitude of the tag baseband signal at the receiver for

different distances between reader and tag

IV.CONCLUSIONS AND FUTURE WORK In this paper a dynamic power control scheme for application in RFID has been presented and, as a first step, a setup has been implemented to study the suitability of such scheme. A set of static measurements has been done to evaluate the amplitude of the tag response according to the distance from the reader. Based on the received amplitudes an RSSI metric and an estimation of distance can be calculated in order to dynamically re-adjust the transmitted power level. In the future the hardware needed for the proposed power control scheme should be implemented as well the algorithm presented in Fig.2. The estimation of energy saving using this proposal will also be done and the scheme will be validated in real scenarios.

REFERENCES [1] Cha, K., Ramachandran, A. and Jagannathan, S., “Adaptive

and Probabilistic Power Control Algorithms for Dense RFID Reader Network”, Networking, Sensing and Control, 2006. ICNSC '06. Proceedings of the 2006 IEEE International Conference on

[2] Kainan Cha, Jagannathan, S. and Pommerenke, D., “Adaptive Power Control Protocol With Hardware Implementation for Wireless Sensor and RFID Reader Networks”, System Journal, IEEE, 2007

[3] Tong-Hsiao Chang, Karl Edward Keppeler, Charles Rinkes, Patent: Methods and Systems for RFID Reader Power Management, 2009

[4] Xunteng Xu, Lin Guy, Jianping Wang and Guoliang Xing, “Negotiate Power and Performance in the Reality of RFID Systems”, Pervasive Computing and Communications (PerCom), 2010 IEEE International Conference on

[5] EPC Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID, Protocol for Communications at 860 MHz – 960 MHz, Version 1.2.0.

[6] Daniel M. Dobkin, The RF in RFID: Passive UHF in Practice [7] Abreu, R., Almeida, N., Matos, J.N., Carvalho,

N.B.; Gomes, J.S., “A Homodyne Low Cost Uplink Receiver for Digital Short Range Communication Systems”, Vehicular Technology Conference, 2007. VTC2007-Spring. IEEE 65th

[8] M. S. Trotter, J. D. Griffin and G. D. Durgin “Power-Optimized Waveforms for Improving the Range and Reliability of RFID Systems”, 2009 IEEE International Conference on RFID

Distance (m) 0.9 1.3 1.75 2 2.9 4.4

A (mV) 1.6 0.63 0.53 0.50 0.44 0.28

READER COMMAND

TAG

AA

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