Survey of Range Improvement of CommercialRFID Tags With Power Optimized Waveforms

Matthew S. TrotterEmail: mtrotter@gatech.edu

Gregory D. DurginEmail: durgin@gatech.edu

Abstract—The power sensitivity of passive Radio FrequencyIdentication (RFID) tags heavily affects the read reliability andrange. Inventory tracking systems rely heavily on strong readreliability while animal tracking in large elds rely heavily onlong read range. Power Optimized Waveforms (POWs) providea solution to improving both read reliability and read range byincreasing RFID tag RF to DC power conversion efciency. Thispaper presents a survey of the increases and decreases to readrange of common RFID tags from Alien and Impinj with Higgs,Higgs 2, Higgs 3, Monza 3, and Monza 4 RFICs. In addition,POWs are explained in detail with examples and methods ofintegration into a reader.

I. INTRODUCTION

In inventory tracking systems, tagged inventory passesthrough portals surrounding passive RFID tags with RFIDreader antennas in close proximity to the targeted tags. Rangeis not an issue here, but reliability of a tag response isdependent on multipath fading, tag orientation, and impedancemismatch among other electromagnetic propagation barriers,much of which is beyond the control of the system user.A solution to improving reliability in the presence of thesepropagation barriers is improving the power sensitivity of theRFID tag itself.

In toll collection systems, the automobile with an RFID tagpasses underneath the reader antenna located a long distanceabove the tag. Both range and reliability are issues in thiscase [1], especially if the tag collection system operates in thepassive UHF band, which only has ranges of several meters[2]. Again, a solution to improving range and reliability isimproving the power sensitivity of the RFID tag itself.

Power Optimized Waveforms (POWs) are an off-tag methodof improving the power sensitivity of RFID tags, thus im-proving range and reliability of the tag response. POWs aretransmitted by the reader and require no changes whatsoeverin the RFID tag. According to this work, most current passiveUHF RFID tags show greater power sensitivity when read withPOWs.This paper rst presents current research in range improve-

ment followed by an explanation of POWs and how theyt into the big picture. Then, POW implementation into anRFID system is discussed to give the reader an idea of howa system is built. Finally, a survey of the power sensitivity

This research was sponsored by NSF career grant #0546955.M. S. Trotter and G. D. Durgin are with the Department of Electrical and

Computer Engineering, Georgia Institute of Technology, Atlanta, GA, 30332USA e-mail: mtrotter@gatech.edu and durgin@gatech.edu.

Fig. 1. An RFID reader sends either CW or POW at the same transmitpower to power passive RFID tags.

improvements (or reductions) for common commercial passiveUHF tags are provided to show the practical results of usingPOWs.

II. PASSIVE TAG POWER SENSITIVITY RESEARCH

There are two main categories of research performed withthe intent to improve power sensitivity: on-tag and off-tagresearch. On-tag research focuses on reducing tag power con-sumption in the tag’s physical structure. For instance, inves-tigating low-parasitic components, low-power circuit design,duty-cycling, impedance matching, and tag antenna designare all considered on-tag research. Off-tag research focuseson reducing tag power consumption without changing the tag.Two specic examples of off-tag research are explained below:intermittent continuous wave (CW) transmission and auxiliaryCW transmission.Matsumoto and Takei found an efciency improvement of

charge pumps using intermittent CW transmissions switchedon and off periodically reduced the tag’s power consumptionby 25% [3]. In their experiments, a 900 MHz CW transmissionwas turned on and off at rates of 1 kHz, 100 kHz, and10 MHz. These intermittent CW waveforms are identical tosquare POWs discussed in Section V-C.

Park et. al. doubled the read range of passive RFID tagsusing an auxiliary CW transmitter in addition to the inter-rogating reader [4]. The auxiliary reader is triggered by aquery command from the interrogating reader, and the tagreceives the CW transmitted by both readers. This off-tagmethod increases the power available to the tag.

IEEE RFID 2010

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Fig. 2. Pulsed Interval Encoded (PIE) reader transmit signals demodulated in a typical envelope detector shows voltage ripple using POWs making symboldetection more difcult.

III. POWER OPTIMIZED WAVEFORM THEORY OFOPERATION

Early generation RFICs were power-limited; the internalcircuitry consumed so much power that the charge pumpwas guaranteed to be operating efciently. Current RFICs areapproaching a point where they are now voltage-limited, sothere actually has not been a big push recently to reduce RFICpower consumption [5]. Adoption of POW transmit signals infuture RFID readers and standards could provide much biggergains than the current study demonstrates.

The conventional method for powering RFID tags wirelesslycomes from the EPCglobal, class 1, generation 2 (“gen2”)standard, which species a Continuous Wave (CW) powertransmission from the reader having a constant envelope. Thisprovides a steady source of power for the tag to harvest, albeitvery inefciently. The power harvesting circuitry in commonRFID tags is based on the Dickson charge pump topology[6] and operates with efciencies of less than 60% [7] involtage-limited tags. This efciency is improved by usingPower Optimized Waveforms (POWs).

A POW focuses the steady wireless power of the CW intoshort, repeated, and impactful bursts of power that improvethe efciency of the tag’s charge pump without increasing thereader’s transmit power. Figure 1 shows a reader using CWand then using a POW. In the gure, the average transmitpower remains the same in both cases, but the POW producesa short time window of large voltage peaks.The POW’s large voltage peaks improve the efciency of

the tag’s charge pump [8]. An RFID tag’s charge pump isa combination of a rectier and a voltage booster. It rstrecties the tag’s received RF signal into a DC current, whichis then fed into a series of voltage boosting stages. This circuitconsists of diodes and capacitors and behaves nonlinearly. Thepower efciency increases nonlinearly with increases in themaximum of the input voltage [9], meaning the efciency will

be large if the input is a large-amplitude RF waveform. Anincreased efciency for the same input power yields a morepower-sensitive tag, thus improving range and reliability.

The frequency spectrum of a POW is wider than a CW’sspectrum of the same signal power. A POW is made witha non-constant envelope modulated on the carrier frequency.The envelope is a periodic baseband signal that is upconvertedto the carrier frequency to form the POW. In the passband,the POW has discrete subcarrier frequency components thatare each lower-powered than a comparable CW spectrum.Power is divided among these subcarriers instead of beingconcentrated at a single frequency as in CW transmission. Thesum of the subcarrier powers is equal to the comparable CWpower, thus the POW has its spectral power spread out overa larger bandwidth. The subcarrier frequency spacing is theinverse of the POW period. A POW with many subcarriersor large subcarrier spacing cannot conform to the “gen2”standard because of its inability to contain its spectrum withinthe allotted 500 kHz reader channels [10]. However, manypassive “gen2” tags are still able to demodulate, power up,and backscatter using POWs as shown by the results of thiswork.A limitation for POW performance is more difcult enve-

lope detection within the RFID tag. An RFID tag’s typicalenvelope detector consists of a rectifying diode connected toa capacitor in parallel with a voltage comparator as shown inFigure 2. The capacitor voltage closely follows the envelopeof the tag’s received waveform while the comparator makessymbol decisions. Typical RFID tag capacitors are speciedlarge enough so that the envelope detection follows UHFoscillations accurately as shown. The POW introduces lowfrequency oscillations at the rate of the POW frequency,adding signicant ripple to the capacitor voltage. A largeripple induced by a slow-frequency POW and a small envelopecapacitor will introduce symbol detection errors. The tag

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Fig. 3. A POW source is created from the output of a Digital to Analog(D/A) converter while the POW samples are stored in memory.

will not respond if symbol errors are made, even if there issufcient power available. Ripple can be mitigated if a fast-frequency POW is implemented.

The Federal Communications Commission (FCC) Part 15,Section 247 rules govern digitally-modulated intentional radia-tors as well as frequency-hopping systems. The gen2 standardadopts a frequency-hopping scheme, and only a POW with asignal bandwidth less than 500 kHz conforms. However, theFCC states for digitally-modulated, non-frequency-hoppingsystems, the transmitted waveform’s “minimum 6 dB band-width shall be at least 500 kHz” [11]. So if an RFID readerchooses not to conform to the gen2 standard, a POW musthave a 6-dB bandwidth greater than 500 kHz, thus making anyPOW compliant that stays within 902 - 928 MHz. Furthermore,a digitally-modulated RFID reader using POW can transmit amaximum of 1 W of average conducted power [11]. This doesnot include periods of “off” signal period like the intermittentCW transmissions by Matsui and Takei [3]. The POW mustalso stay in the allotted 902 - 928 MHz Industrial Scienticand Medical (ISM) band.

IV. POW IMPLEMENTATION

A POW is implemented only in the reader hardware; the tagdoes not need to be redesigned in order to use POWs becausePOWs are an off-tag method for improving range. Within thereader, there are two implementation issues: POW sourcing,amplication, and mixing, and POW demodulation. These twoissues are addressed below.

A. POW Sourcing, Amplication, and MixingThe most cost-effective, simple, and efcient way of cre-

ating a POW source at baseband is a digital implementationshown in Figure 3. One or two periods of the POW is storedin memory and sent to a Digital-to-Analog (D/A) converter.This output is low-pass ltered to remove quantization noise,and the result is the POW signal at baseband. Once thePOW baseband signal is sourced with a D/A converter, itmust be amplied and upconverted to passband. A criticalissue in amplifying POWs is avoiding voltage clipping at theamplier output. POWs have an intentionally high Peak-to-Average Power Ratio (PAPR), which increases charge pumpefciency. Thus, it is imperative that the POW peaks do notget clipped.

Figure 4a shows how a POW can be modulated onto a localoscillator (LO) at the passband center frequency premixedwith reader data. There is one class A amplier that takesthe passband reader data as an input and amplies it with

Fig. 4. The reader transmit signal consisting of data and POW can be mixedand amplied by either (a) a class A amplier with POW providing the powersupply or (b) distributed amplication of equally-power signals into classAB or class C ampliers. Alternatively, the POW can be (c) sourced andamplied at passband with class AB or class C ampliers.

a gain linearly proportional to the POW voltage waveform.The amplier uses the POW as its power input rather than itssignal input. Therefore, there will not be any voltage clippingat the signal output due to the POW’s voltage peaks. Anothermodication to this method is premixing the data with thePOW instead of with the LO, ensuring minimal distortionat the amplier output. A Class A amplier is used here toavoid any distortion of the output signal. This method hasthe least number of necessary components to fully implementPOW. However, Class A ampliers only operate with up to25% power efciency and are more costly than other types ofampliers [12].Figure 4b shows the distributed amplication method. The

reader signal, consisting of POW mixed with the LO, andreader data is split into equal-power signals, ampliedin parallel, and nally combined to form the transmittedsignal. This method avoids voltage clipping since the signalvoltage input into each amplier is smaller than the pre-

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Fig. 5. Matched lter demodulator for demodulating passive RFID tags’backscatter with a POW carrier.

split voltage. This method may use Class AB or Class Campliers to get power efciencies up to 78.5% or 90%,respectively [12]. These ampliers are cheaper on average thanclass A ampliers but also add more distortion.

In Figure 4c, discrete frequency subcarrier sources andampliers are combined to form a POW at passband. Thefrequency sources are individually tuned to the frequencies ofthe desired POW passband spectrum, and the ampliers’ gainsare individually tuned to t the POW passband spectrum asin the gure. Class AB or class C ampliers are used in thisconguration since they operate with high power efciency.This method works best for POWs with many subcarriers sincethere is no need for upconversion, which adds intermodulationdistortion. There will be much less distortion from the ampli-ers in this method as opposed to the other methods as well.Its disadvantage is the large cost of implementing so manycomponents. The reader hardware will need much more boardspace as well.

B. POW Demodulation

The backscattered signal received by the reader consistsof the tag’s backscatter modulated on the passband POW.Figure 5 shows a receiver chain that demodulates the tag’sbackscatter. The receiver rst downconverts the received signalto a baseband signal, which is sent through a matched lterwith an impulse response of the time-reversed POW. Thematched lter output is then sampled and sent to the symboldecision device. An alternative implementation is a correlationdemodulator where a POW is correlated with the basebandreceived signal, and sent to the same symbol decision device.The correlation demodulation method is more difcult withPOWs that have a long time period. It is important in this caseto match up the received POW’s phase with the correlatingPOW’s phase. The matched lter is preferred for POWs withlong time periods, while correlation demodulation is preferredfor POWs with short periods. It is more cost-effective tocorrelate a POW than design and build a matched lter.

V. EXAMPLE POWS

The N-POW, Square POW, and Gaussian POW are pre-sented in this section along with the relationships betweenthe POW parameters of bandwidth, POW period, subcarrierfrequency spacing, peak power, average power, and Peak-to-Average-Power Ratio (PAPR). Figure 6 illustrates these param-eters and the differences between each POW. These POWs

are used in the tag survey to demonstrate the performancedifferences when switching from one POW to another.

A. N-POWThe N-POW is created with baseband subcarriers equally

spaced apart by frequency spacing . The time-domain andpower-spectrum equations for the N-POW at baseband are:

pow (1)

POW

Hz (2)

The time-domain equation 1 is the summation of cosinewaves, each at integer multiples of the fundamental frequency,

. The POW period of the N-POW is the reciprocal ofits frequency spacing, . The baseband powerspectrum of the N-POW in equation 2 has a bandwidthof (Hz). When modulated at the UHF passband, thebandwidth becomes (Hz).

The peak power, average power, and PAPR of the N-POWis

peak power (3)

average power (4)

PAPR (5)

The peak power depends only on the number of subcarriers,and the average power is the same for any N-POW. Theserelationships are simple due to the normalization in thetime-domain denition, equation 1. The PAPR scales linearlywith the number of subcarriers.

The N-POWs chosen for testing in the tag survey are:1) 1-POW with ns and PAPR = 22) 2-POW with ns and PAPR = 43) 3-POW with ns and PAPR = 64) 4-POW with ns and PAPR = 8

The time period is the same for all the tested N-POWs, 333.3ns, corresponding to a frequency spacing of MHz.

B. Gaussian POWA Gaussian pulse train is an effective POW for maximizing

PAPR. The pulse is approximately zero for a large portionof the POW period and reaches its maximum quickly beforefalling just as quickly. This is especially true for GaussianPOWs with low variances. The parameters for a GaussianPOW are amplitude ( ), POW period ( ),

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and variance ( ). The baseband time-domain and powerspectrum equations for the Gaussian POW are:

pow (6)

POW

Hz (7)

The time-domain signal is a pulse train of standard normalpulses, , where is the pulse location in timeand is the variance. The effect of neighboring Gaussianpulses’ tails is small compared to the current Gaussian pulse.The power spectrum is just a scaled standard normal sampledat subcarrier frequency spacing (Hz).

Using the small Gaussian tail approximation, the peakpower, average power, and PAPR of the Gaussian POW are:

peak power (8)

average power erf (9)

PAPRerf

(10)

Here, erf , is the standard error function and is approx-imately one for the large argument wherethe POW period, , is longer than the standard deviation,

. It has a negligible effect on the average power and PAPRequations. The Gaussian POW is the only POW with PAPRdependent on POW period. Increasing the POW period pullsthe Gaussian pulses farther apart reducing the average powerand increasing PAPR.

The Gaussian POW has the most facilitating spectrum sincemost of its power is concentrated at the center of the POWspectrum. The shape of the Gaussian POW, along with itsvoltage peak, will be maintained even if its spectrum is cutoffat the edges of the passband. The Gaussian POW has a nar-rower spectrum than an equivalent N-POW for the same PAPR.There is a trade off between PAPR and envelope detectionperformance of the Gaussian POW. Envelope detection sufferswith the high-PAPR Gaussian POW because of its staccatolarge peak voltage much like the N-POW’s peak voltage. Awide Gaussian POW in the time domain with a large standarddeviation will have a low PAPR but perform well in envelopedetection.

There are two Gaussian POWs chosen for testing in the tagsurvey:

1) Sharp Gaussian POW with V s, ns,ns, and PAPR = 4.24

2) Wide Gaussian POW with V s, ns,ns, and PAPR = 2.12

The PAPR for the these Gaussian POWs are similar to the 1-and 2-POWs’ PAPR, but these Gaussian POWs have a largerpeak voltage. It is expected that these Gaussian POWs willprovide more range and reliability than the 1- and 2-POWs.

C. Square POWA square wave pulse train can be used as a POW as well.

It mitigates ripple voltage in envelope detection since its peakvoltage lasts longer than N-POW’s peak voltage pulses. Thesquare POW has four parameters: high voltage ( ), lowvoltage ( ), duty cycle ( , unitless between ), andPOW period ( ). The baseband time-domain andpower-spectrum equations are:

pow rect (11)

POWsinc

Hz (12)

The Square POW time domain equation is created with aninnite summation of rectangle pulses located everyseconds and dilated by seconds. The POW spectrumis a sinc function multiplied by a pulse train with subcarrierfrequency spacing (Hz). The Square POW hasa DC component separate from the sinc function due to thelow voltage, in the time-domain equation.

The peak power, average power, and PAPR of the squarePOW are:

peak power (13)

average power (14)

PAPR (15)

The peak power of the square POW is the instantaneouspower of the high pulse of its square wave, which is sustainedfor seconds during the POW period. A square POWwith a high duty cycle will have approximately the samepeak power and average power, resulting in a low PAPR andineffective POW. The PAPR is more dependent on the ratio

when the duty cycle is small.The Square POW is also more exible in terms of PAPR

than the N-POW since there are more parameters and the N-POW’s PAPR can only take integer values. The Square POW

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Fig. 6. Time- and frequency-domain plots of one time period of the 4-POW, Gaussian POW, and Square POW all upconverted to a carrier frequency, .Each plot consists of one POW period, , of the POW voltage waveform (top), signal power waveform (middle), and power spectral density (bottom)showing the peak power, average power, and frequency spacing. The 4-POW is efciently implemented at passband in a discrete subcarrier implementation(Section IV-A). The Gaussian POW is best suited for maximizing Peak-to-Average-Power Ratio (PAPR). The Square POW’s envelope is more easily detectableby passive RFID tags’ envelope detectors (Section III) than the other POWs.

TABLE IRFID TAGS TESTED IN THE POW SURVEY

RFID TagMake and Model RFIC type Antenna Type

Alien 9440-02 Higgs Meandering DipoleAlien 9529 Squiggle-SQ Higgs 2 Square Near/Far Field

Alien 9540 Squiggle Higgs 2 Meandering DipoleAlien 9662 Higgs 3 Short Dipole with Patches

Impinj Thin Propeller Monza 3 Meandering DipoleImpinj Pre-Release Sample Monza 4 Crossing Dipoles

has a large bandwidth due to its sharp corners and at edgesin the time-domain, which may alter its FCC compliance.Envelope detection is made easier with the Square POW dueto its long periods of high voltage. The peak of the envelopedetector will hold its voltage for longer amounts of time thanother POWs and will not have enough time to discharge greatlyand develop a large ripple.

The Square POW chosen for testing in the tag survey hasa high voltage V, low voltage V, and dutycycle , or 40% of the POW period, which is

ns. These parameters give the square POW a PAPR.

VI. TAG SURVEY

The wide sampling of tested RFID tags covers multipleantenna designs and ve different RFICs. The purpose is toassess the amount of tag sensitivity improvement using POWsover a wide range of design implementations. Table I lists the

make and model, RFIC type, and antenna type for each tagtested. Many other tags not listed on Table I were attempted,but no backscattered response was found. The untested tags arethe UPM Raatec Trap, Impinj Paperclip, Impinj Blade, andImpinj Satellite. Each have a Monza 3 RFIC, yet the systemcould not illicit a backscatter response with POW at any powerlevel.

The test system shown in Figure 7 consists of a customreader, the RFID tag under test, and a receiver. The reader usesMATLAB to produce the baseband POW mixed with querybits or just the query bits. Then, it is mixed up to passbandand sent through an amplier and adjustable attenuator. Theadjustable attenuator is used to back down the transmit poweruntil the RFID tag no longer backscatters a response. Thereceiver is a separate direct down-conversion system thatfeeds the received query and backscatter response into anoscilloscope. The presence of backscatter was observed on theoscilloscope screen.

The physical environment is relatively sparse with nometallic reectors near the measurement area. A qualitativemeasurement of multipath fading, where the receiver antennais moved around and the query waveform is measured, showsmultipath is present but minimal. The presence of multipathdoes not affect POW gain signicantly.

The test method is to measure the minimum transmit powerrequired to illicit a backscatter response from each RFID tagat a xed range. The owchart in Figure 8 shows the completeprocedure. There are eight different test cases for each tag inTable I: seven POWs and one CW base case.The minimum transmit power is tested using the owchart

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Fig. 7. An RFID reader sends either CW or POW modulated with the query command to power passive RFID tags. The attenuator was increased to backdown the transmit power until the tag’s backscatter was no longer detected by the oscilloscope in the direct-conversion receiver.

TABLE IISURVEY OF POW GAINS AND READ RANGE GAINS W.R.T 20 m BASELINE ON COMMON RFID TAGS

Gen2 CW Sharp Wide SquareTag type, measured range Min. TX Power 1-POW 2-POW 3-POW 4-POW Gauss. POW Gauss. POW POW

Alien 9440-02, 6.2 dBm -0.2 dB -0.3 dB -0.2 dB -0.1 dB +0.1 dB +0.3 dB +0.4 dBmeasured @ 34 cm (-0.5 m) (-0.7 m) (-0.5 m) (-0.2 m) (+0.2 m) (+0.7 m) (+0.9 m)Alien 9529 Squiggle-SQ, 11 dBm – – – – – – +0.1 dBmeasured @ 18.5 cm – – – – – – (+0.2 m)Alien 9540 Squiggle, 3.5 dBm +1.2 dB +1.5 dB +1.6 dB +1.8 dB +1.9 dB +1.3 dB +1.5 dBmeasured @ 31 cm (+3.0 m) (+3.8 m) (+4.0 m) (+4.6 m) (+4.9 m) (+3.2 m) (+3.8 m)Alien 9662, 5.6 dBm +1.5 dB +1.6 dB +1.6 dB +1.6 dB +2.2 dB +2.1 dB +1.5 dBmeasured @ 34.5 cm (+3.8 m) (+4.0 m) (+4.0 m) (+4.0 m) (+5.8 m) (+5.5 m) (+3.8 m)Impinj Thin Propeller, 3.8 dBm +0.2 dB -0.6 dB -1.3 dB -1.4 dB -0.9 dB -0.4 dB -0.6 dBmeasured @ 44.5 cm (+0.5 m) (-1.3 m) (-2.8 m) (-3.0 m) (-2.0 m) (-0.9 m) (-1.3 m)Impinj Pre-Release Sample, 4.2 dBm +1.0 dB +1.9 dB +1.6 dB +2.3 dB +2.5 dB +1.5 dB +1.4 dBmeasured @ 35.5 cm (+2.4 m) (+4.9 m) (+4.0 m) (+6.1 m) (+6.7 m) (+3.8 m) (+3.5 m)

in Figure 8 for each of the eight test cases. Then, each of theseven POW’s minimum transmit powers are compared to theCW base-case transmit power. The difference (in dB) is thePOW gain:

POW Gain (dB) CW min TX Power (dBm)POW min TX Power (dBm)

(16)

A positive POW gain represents an improvement in tag sen-sitivity while a negative POW gain represents a reduction intag sensitivity.

The tag sensitivity improvements are tabulated in Table II.The rst data column is the minimum transmit power requiredto illicit a backscatter response using CW in the “gen2”standard (in dBm). The following data columns are POW gains(in dB) and their corresponding read range gains (italic entriesin m) with respect to 20 m. The formula for calculating readrange gains is:

Read Range Gain (m) (17)

where is the POW gain quoted in the table (in dB).This formula is derived from the Friis free space equation;details are found in [8].

VII. ANALYSIS

For the Alien tags, the Sharp Gaussian POW provided themost sensitivity improvement on average, and the 1-POW pro-vided the least improvement on average. This shows that POWgain and PAPR are directly related and implies POWs withhigh voltage peaks improve tags’ sensitivity more than POWswith low voltage peaks. The high voltage peaks of the SharpGaussian POW drove the charge pump efciencies higher thanthe low voltage peaks of the 1-POW, thus producing more tagsensitivity. The Higgs 2, Higgs 3, and Monza 4 RFICs showedthe most sensitivity improvement using POWs.

The Alien and Impinj Pre-Release Sample tags showed sen-sitivity improvement while the Impinj Thin Propeller showedsensitivity reduction. The Alien 9440 tag with a Higgs RFICshowed sensitivity reduction for some POWs and improvementwith the Gaussian and Square POWs. The Impinj tag with aMonza 3 RFIC, however, showed no improvement for anyPOW. This is due to the power management unit architecturethe Monza 3 RFIC uses. The Monza 3 uses a small chargepump driven by the antenna to drive oscillators, which drivea large charge pump that powers the rest of the RFIC [13].The POWs have no effect on the large charge pump but shouldimprove the efciency of the small charge pump, which shouldproduce a minute POW gain. Since the table shows a negativePOW gain, this means the POW either reduced the efciency

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Fig. 8. Procedure owchart for the tag survey. The difference in TX powerbetween the CW and POW transmissions is the sensitivity improvement orreduction.

of the small charge pump or reduced the effectiveness ofenvelope detection. The actual reason may be a combinationof both effects.

The Monza 4 RFIC experienced the largest range improve-ment overall with POWs. This is due to the change from asmall and large charge pump, as in the Monza 3, to a simplerarchitecture with one charge pump. The POW is more effectiveon the Monza 4 charge pumps than any other.

VIII. CONCLUSION

Improving range and reliability is benecial for RFID tagsystems in many situations including inventory tracking andautomatic toll collection. This paper presented Power Opti-mized Waveforms (POWs), a reader carrier wave that improvestag power sensitivity without making any changes to the tag.POWs improve the RF to DC power conversion efciency inthe tag charge pump. Many tags with different antenna designsand RFICs were tested with N-POWs, Gaussian POWs, anda Square POW to measure tag sensitivity improvements orreductions. Higgs 2, Higgs 3, and Monza 4 tags experiencedthe most improvement while Higgs 1 and Monza 3 tagsexperienced the most reduction. The Sharp Gaussian POWswere the most effective while the 1-POW was the leasteffective. Gaussian POWs have the best combination of largepeak power and reliable envelope detection in the tag.

It is still untested how POW period plays a role in POWgain. POW period mainly affects envelope detection, and

testing should verify this. Also, it was recently discoveredthat POWs provide the ability to spatially address tags anddetermine their range. This is an inexpensive and simplemethod for adding accurate ranging onto an RFID tag network.Lastly, multiple access schemes and multiple reader modeswith POWs are still undeveloped.

Overall, POWs provide a way to increase range and reli-ability of passive RFID tags. No changes are needed on thetag, and implementing POWs in the reader requires just a fewdesign changes.

ACKNOWLEDGMENT

A special thank you to Rafael Martinez for providingbackground in RFID inventory tracking and logistics.

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[11] FCC Rules and Regulations Part 15 Section 247 (15.247), Operationwithin the bands 902 - 928 MHz, 2400 - 2483.5 MHz, and 5725 - 5850MHz, FCC.

[12] G. Gonzalez, Microwave Transistor Ampliers, E. Svendsen and R. Ker-nan, Eds. Prentice-Hall, Inc., 1997.

[13] R. A. Oliver and C. J. Diorio, “Rd tags with power rectiers that havebias,” U.S. Patent 7 561 866, September 26 2005.

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