Sectorization of UHF RFID Tags using a

Steerable Phased Antenna Array Siegfred Balon

1, Joseph Aurelio Buyco

2, and Joel Joseph Marciano Jr.

3

Wireless Communications Engineering Laboratory

Electrical and Electronics Engineering Institute

University of the Philippines-Diliman, Quezon City, Philippines

Email: 1sieg@eee.upd.edu.ph,

2jcbuyco@up.edu.ph,

3j.marciano@ieee.org

Abstract- The paper presents the development of an RFID

antenna array test bed for determining the sector location of a

passive UHF RFID tag. The sectorization is achieved by using

a 4-element uniform linear phased array of right-hand

circularly polarized (RHCP) patch antennas at the receiver,

which is also shown to improve the read range of the system.

The uniform linear phased antenna array is able to

electronically steer a beam with the use of a programmable

phase shift network. The phase shifters are able to provide 0°

to 180° shifts for each element, thus providing maximum

steering angles of +/-20° off broadside. The phase shift network

is controlled by a microcontroller with onboard digital-to-

analog converters (DAC). A 13-dB low-noise-amplifier stage

was also added to compensate for insertion losses in the

network. Experiments were performed to characterize the

sectorization performance of the system for fixed and moving

tags.

I. INTRODUCTION Radio Frequency Identification is a technology widely used in asset tracking and management systems [1]. Presently, there are several studies aimed at extending the capability of the technology that would enable applications such as location sensing [2-5]. There are numerous approaches to more sophisticated localization techniques. A basic approach is to use triangulation, which uses at least two readers (for 2D localization) having a known separation. The received signal strength indicator (RSSI) of a tag is translated by each reader to a measure of radial distance. The two radial distances (with respect to each reader) and the spacing of both readers form a triangle. Aside from using two readers which may be costly, the use of receive signal strength will not be useful for environments that are susceptible to multipath fading [2]. LANDMARC, on the other hand, use the k-nearest neighbor algorithm which utilizes position of fixed active (battery-operated) tags to provide reference landmarks [3]. The locations of tags are computed based on the signal strength of the four nearest reference tags. The accuracy of the algorithm, however, depends on the density of reference tags deployed on the field. Increasing the density of the deployment of the reference tags to improve accuracy also leads to an increase in computational complexity. In [4], physically rotating antennas are used to know which tags are present in an angular section. By using readers in multiple locations and interrogating tags with varying transmit power, an average error of 0.6 meters was achieved through a probabilistic algorithm. The aforementioned approaches in localization may be further enhanced if directive beamforming antenna arrays are used.

The directivity of the antenna pattern may reduce the deleterious effects of multipath and the ability of these arrays to perform electronic steering can be used to achieve localization without the need for manual antenna rotation. Appropriate signal processing may be performed on the array signals to extract information such as direction-of-arrival (DoA) as in [5], where an arbitrary array and in conjunction with MUSIC algorithm was explored in order to determine the location of a passive tag. In this paper, we describe the process of building the beamformer network with emphasis on integration of the different components. In particular, proper fabrication and interfacing of these parts is crucial to reduce system losses and to obtain the promised benefits of using directive antenna arrays. This steerable antenna array testbed is also used to determine the sector location of passive UHF RFID tags and to subsequently track their movement within the coverage area of the UHF RFID reader. The coverage area is divided into three sectors by using suitably directive beam patterns steered in two different directions. It should be noted that the implementation described in this paper utilizes the directivity of a beam-forming array for sector localization in the azimuth only – sectorization in elevation angle may be achieved by extending the array to a planar configuration. Section 2 discusses the components of the system and the individual tests associated with each stage. Section 3 presents the radiation pattern of the array at three steering angles at 915 MHz. and the sectorization experiment while Section 4 provides the conclusion.

II. SYSTEM DESIGN The block diagram of the system is shown in Figure 1. The phase shifters are programmable and control the beam steering angle of the system.

FIGURE 1. SYSTEM SET-UP

The antenna elements are probe-fed square patch antennas fabricated on epoxy boards. The phase shift values are controlled

Antenna Array

RFID Reader/ Middleware

Host Computer

To Reader Rx Antenna Port

Serial or LAN connection

Microcontroller Phase Shift Network

Proceedings of 2010 IEEE Student Conference on Research and Development (SCOReD 2010), 13 - 14 Dec 2010, Putrajaya, Malaysia

978-1-4244-8648-9/10/$26.00 ©2010 IEEE 16

through voltage sources to give phase shifts from 0° to 180°, which provides a maximum steering angle of 20° off broadside. The control circuit uses digital-to-analog converters (DACs) and a microcontroller to control the phase shift values. An 11dB low-noise-amplifier (LNA) stage was also cascaded after each antenna element to provide gain that compensates for the microstrip and cable line losses.

A. Antenna Array Elements

The square patch antenna element was designed to have a center frequency of 915 MHz. The two feed points are connected to a 90º hybrid coupler to provide dual polarization. The substrate used is inexpensive epoxy boards with approximate relative permittivity of 4.4. The schematic of the antenna element and measurement results are shown in Figure 2 and 3 below.

`

The value for the patch width (W) and length (L) and the feed point locations (xfeed and yfeed) were obtained using the design guidelines in [6]. The design was simulated using Ansoft Designer SV and tested using a microwave vector network analyzer (VNA). Measurements for S11 showed at least 10 db return loss between 905.5MHz to 920MHz and a resonant frequency of 912.7 MHz, which is equivalent to approximately 1.6% fractional impedance bandwidth. The S22 measurements yielded 1.8% fractional impedance bandwidth at 10dB return loss (906MHz to 922.5MHz with 914.5MHz resonance). The isolation of the two ports is at

least 23dB. To produce a right handed circular polarization, the two feed ports are connected to the through and coupled terminals of a 90° hybrid coupler.

B. Phase Shift Network

A Low Noise Amplifier (LNA) is cascaded after each antenna element. The LNA blocks have been biased to provide a +13dB gain. Apart from providing low noise figure, the LNA compensates for the losses on the transmission line and cables. The fabricated LNA blocks are shown in Figure 5.

The phase shifting elements used are analog phase shifters that operate at radio frequencies (RF). The phase shifter is capable of providing shifts from 0º to 180º through a DC voltage input from 0V to 15V. This DC control voltage is obtained from the output of a dual 10-bit digital-to-analog converter (DAC) that is further controlled by a microcontroller. One bit of the DAC is used for chip enabling, another bit is used for DAC selection and the remaining 8 bits are for voltage output control. The control therefore has 8-bit resolution and will be used to deliver a DC voltage output from 0V to 15V. Figure 6 shows the diagram, of the phase shifter elements and associated control blocks, while the actual fabricated phase shifter board is shown in Figure 7.

Feed with 90°

delay

In phase

feed

FIGURE 4. 4-ELEMENT UNIFORM LINEAR ARRAY

22

LW=

22

LW=

yfeed

xfeed

where:

78.28mm== LW

mmxy feedfeed 5.17==

FIGURE 2. RHCP ANTENNA CONFIGURATION

Phase Shifter DAC

Control Voltage

Combined output to reader antenna port

FIGURE 6. THE PHASE SHIFTER AND CONTROL BOARD BLOCK

DIAGRAM.

FIGURE 5 FABRICATED LOW NOISE AMPLIFIER (LNA) BOARDS

RF Out

RF In

Power Supply

Connections

Microcontroller

GPIO

LNA

Four-way Wilkinson combiner

FIGURE 3. SIMULATED S-PARAMETERS OF THE SQUARE

PATCH ANTENNA ELEMENT

Input from hybrid coupler output

+ +

+

+

+

+ * *

* *

* *

o o

o

o o o

∆ ∆

∆

∆ ∆ ∆

*

o

+

∆

17

Figure 8 shows the characteristic curve of the phase shifter measured using a microwave vector network analyzer. The results show that 0º, 60º, 120º, and 180º phase shifts, for example, may be obtained by setting the control voltages to 0V, 4.86V, 7.11V, and 12.33V respectively.

Figure 9 shows the integrated LNA and phase shifter network boards. The cascade of the LNA with the phase shifter did not preserve the expected 13dB gain – the insertion loss of the phase shifter varies relative to the phase shift, as seen in Figure 10. Figure 11 shows the complete block diagram of the beamforming network.

III. EXPERIMENTAL RESULTS

A. Antenna Array Characterization A linear phased array forms a beam by introducing a uniform inter-element phase shift at each antenna element. In this implementation, up to 60° relative phase shift could be produced which could only steer beams at 19.47° (or approximately 20°) off array broadside. The range of beam steering angles is therefore between 70° to 110° when measured from the axis formed by the antenna array elements. To characterize the array, the radiation pattern was obtained at broadside direction and at ±20°off broadside at an operating frequency of 915MHz. The power radiation patterns obtained are shown in Figure 12. These are merely approximations since the test environment is an open area test site. With experimental measurements, the measured gain of each beam (referenced to Cushcraft® 8dBiC RCHP antenna) is shown in Table 1.

From Figure 11, the total gain at broadside (no phase shifting required) is expected to be approximately 15.2dB plus the gain of the isotropic, circular polarization antenna. Testing of each antenna element yielded 1dBic gain at 915MHz. When the losses of the cables and connectors are taken into account, this is reduced by 2dB since the 2m RG174 cable used has a loss of approximately 1dB/meter. The connectors used in the interfaces, unequal antenna and amplifier gain and inexact phase shift add further losses.

FIGURE 7. THE ACTUAL PHASE SHIFTER BOARD.

Phase Shifter DAC circuitry

Input Bits

FIGURE 8. MEASURED CHARACTERISTIC CURVE OF THE PHASE SHIFTER

Phase Shift

(deg)

Control Voltage (V)

Phase Shifter

Board

Signal

Path

Signal

Path Signal

Path

Signal

Path

Wilkinson Power Combiner

Signal to the Reader

FIGURE 11. THE PHASE SHIFTER NETWORK BLOCK DIAGRAM.

LNA Board

Phase Shift (degrees)

FIGURE 10. INTEGRATED LNA AND PHASE SHIFT BOARD GAIN

RELATIVE TO THE PHASE SHIFT

FIGURE 9. INTEGRATED LNA AND PHASE SHIFTER BOARDS

10.7 to

12.2 dB

+4.5dB

15.2 to 16.7 dB

Gain

(dB)

LNA Board LNA Board LNA Board

Phase Shifter

Board

Phase Shifter

Board

Phase Shifter

Board

18

TABLE 1. EFFECTIVE GAIN OF THE ARRAY

Cushcraft 8dBiC RHCP (reference) -34.53 dBm received power

Array Broadside -30.4 dBm received power

( ~12.13dBiC)

Array at 110° steering angle

-32.86 dBm received power (~9.67 dBiC)

Array at 70° steering angle

-31.33 dBm received power (~11.2dBiC)

B. Sectorization Experiment for Fixed Tags The measured radiation pattern of the array showed that the directive beam can be steered to different directions. We set up a test bed to characterize the ability of the system to localize RFID tags. Figure 13 shows two tags placed at angles of 70˚ and 110˚ at a distance of 4m away from the antenna array and RFID reader.

The RFID interrogator uses a Cushcraft patch antenna with a gain of 8 dBiC. This antenna is used at transmit and is placed on top of the antenna array on one end of the setup. The tags face the reader antennas directly and are at the same height as the transmitter antenna. The RFID tag is therefore expected to be able to receive as much power as possible from the transmit antenna using this configuration. The experiments showed that tags located in different directions are sufficiently distinguished by using the RSSI values – and the combination of the beam steering array and RSSI values constitute a form of “spatial filter”. When the beam is switched in the direction of Tag 1 (at 70º), the average RSSI value for the desired tag (Tag 1) is 1183. When it is switched to the direction of Tag 2 (110º), the average RSSI value for the desired tag (Tag 2) is 946. A “hard” decision threshold is therefore empirically set at an RSSI level of 1000 for 70 º and 850 for 110 º.

TABLE 2. TAG RSSI VALUES

Steer Direction 70 degrees

Tag 1 RSSI Average 1183

Steer Direction 110 degrees

Tag 2 RSSI Average 946

After setting the threshold, only the tags in the correct sector steer was reported by the reader to the host computer.

C. Sectorization Experiment for Moving Tags The same setup is used for tracking moving tags, except that the tags are placed closer to the RFID reader to help ensure more reliable measurements. The microcontroller was programmed to change the phase shifter bias and hence, switch the beams between 70° and 100° every 400 milliseconds. The RFID reader continuously polls for tags within this time span using the continuous mode feature of the reader. When a tag is read, the reader sends the tag ID to the host software. The software can track the change in the beam steering direction by monitoring the states of the microcontroller data ports.

For the moving tag experiment, only two angle steers were made. The tag was made to move at a speed of around 2.5 m/s on the average. The system successfully achieved coarse localization of the moving tags and determined whether it was on the left or the right side of the reader for all of the trials made. However, there were instances of missed reading when the tag changed sectors.

FIGURE 13. TEST BED FOR THE SECTORIZATION EXPERIMENT

LAYOUT AT EEEI ROOFDECK

Reader System

2.5m

2.7m 2.7m

70° steer 110° steer

FIGURE 15. MOVING TAG TEST SETUP DISTANCES

Track Tag

Rx Antenna Array

Tx Antenna

Tag 2

Tag 1

95cm

75cm

95cm

Antenna stand

Open area

3.8m

4m

4m Covered

Tag 1

Tag 2

EEEI Roof deck

Reader system

90o

-90o

0o

180o

φ

60o

-60o

30o

-30o

120o

-120o

150o

-150o

0.5 1

90o

-90o

0o

180o

φ

60o

-60o

30o

-30o

120o

-120o

150o

-150o

0.5 1

FIGURE 12. RADIATION PATTERN FOR (A) 70°, (B) 90° AND (C) 110°

STEERING DIRECTIONS

(a) (b)

(c)

90o

-90o

0o

180o

φ

60o

-60o

30o

-30o

120o

-120o

150o

-150o

0.5 1

19

TABLE 3. SUMMARY OF THE MOVING TAG EXPERIMENT

IV. CONCLUSION

A 4-element antenna array and the phase shift network was designed and implemented for sectorization application of passive UHF RFID tags. The results show that this testbed was able to steer beams at +/- 20° off broadside. Broader steering angles are possible if a phase shifter with wider phase shift will be used. The directive beams from the antenna array system was also able to localize tag placed in two different locations coinciding with the extreme angle beams. Furthermore, the setup was able to read stationary tags at distances of extended distances of 4.3 meters at broadside and 4 meters off-broadside. The technique can be further improved to cover elevation using a planar array.

V. ACKNOWLEDGEMENT The authors would like to acknowledge the support of

the Department of Science and Technology - Engineering Research and Development for Technology (DOST-ERDT) through its research dissemination grant.

VI. REFERENCES

[1] “RFID Business Applications”. RFIDJournal.com [Online]. Available: http://www.rfidjournal.com/article/articleview/1334/1/129/ [2] Bouet, M., A.L. dos Santos, RFID Tags: Positioning Principles and Localization Techniques Wireless Days 2008. 1st IFIP , pp.1-5, 24-27, Nov. 2008 [3] Ni, L.M., Yunhao Liu, Yiu Cho Lau, A.P. Patil, “LANDMARC: Indoor Location Sensing Using Active RFID,” Proceedings of the First IEEE International Conference

on Pervasive Computing and Communications, 2003. (PerCom 2003), pp 407 – 415, 2003. [4] Alippi, C., D. Cogliati, G. Vanini, “A Statistical Approach to Localize Passive RFIDs,” Circuits and Systems, 2006. ISCAS 2006. Proceedings. IEEE International Symposium on Digital Object, 2006. [5] Abedin, M.J., A.S. Mohan, "RFID Localization Using Planar Antenna Arrays With Arbitrary Geometry," Microwave

Conference, 2008. APMC 2008. Asia-Pacific , vol., no., pp.1-4, 16-20 Dec. 2008 [6] Milligan, Thomas A. Modern Antenna Design, 2nd Edition. IEEE Press, John Wiley and Sons, Inc., Publication, 2005.

Number of trials 15

Detected change of sector

9

Tag detected in only one sector

6

20