July/August 2013 471527-3342/13/$31.00©2013IEEE

Digital Object Identifier 10.1109/MMM.2013.2259394Date of publication: 11 July 2013

FOCUSED

ISSUE FEATU

RE

How Good Is Your Tag?

Passive ultra-high-fre-quency (UHF) radio-fre-quency identification (RFID) has developed rapidly over the past two decades. In 2012 alone, about 4 billion tags

were sold worldwide [1]. Manufacture and deployment at this volume has been made possible by minimizing tag cost, making a strong disincentive for the added expense of RF performance testing. As a result, performance

metrics and test methods are not yet unified or generally adopted.

Large customers in industry and gov-ernment, however, wish to compare products with

parameters like read range and inventory rate, so this problem is attracting more attention, and test standards are improving. This article discusses the history, state of the art, and some future challenges in this rapidly evolv-ing area of study. In particular, we consider two metrics

Daniel Kuester and Zoya Popovic

Daniel Kuester (dkuester@phaseivengr.com) is an RF engineer at Phase IV Engineering, Boulder, Colorado, United States. Zoya Popovic is a Distinguished Professor at the University of Colorado, United States.

© istockphoto.com/albert lozano

48 July/August 2013

KeyPositions

(a) (b)

(c)

Hollowed Area to GiveEffect of Free Space

Capacitance Couplingto Tuning Post

Nonconductive Ins.and Support

Soft Ins. andSupportingRing

Front ofSeal

Carved Wooden EagleDesign Raised on This Plate

Silver Plated CopperCylinder-Hi-O Cavity

3-mm MetalDiaphragm

3/4"

9"

1/4—Wave Whip at330 MHz (Copper Rod)

Back of Seal

Carved Wood

Figure 1. Historical backscatter modulation devices: (a) the first German IFF system, the FuG 25a Erstling [9], (b) Stockman’s mechanically modulated backscatter device [4], and (c) Léon Theremin’s covert listening device “The Thing” in the hands of the US ambassador, and a block diagram of the implementation [3].

July/August 2013 49

for digitally modulated backscatter of passive UHF RFID tags and how tests can be simplified for reduced costs. Examples for some 860–960 MHz commercial tags and applications are given to illustrate the method.

Historical precedent in scattered modulation from radar targets and loaded antennas dates back to World War II. A significant problem to be solved was “identify friend or foe” (IFF)—discriminating between friendly and enemy aircraft on radar [2]. The German Luftwaffe first developed a crude approach to IFF: multiple aircraft performed synchronous roll maneuvers, collectively reflecting signature modulation. By 1941, they replaced this method with an active transmitting IFF transpon-der on each aircraft, the FuG 25a Erstling [Figure 1(a)]. Wattson-Watt in Britain tried load modulation with a dipole antenna stretched across the wings of a fighter aircraft in the late 1930s. By mechanically or electroni-cally shorting and opening the antenna terminals over time, airmen would reflect signal codes to identify themselves to British radar operators. Received signals at radar stations were very weak, however, so (like the Germans) the British developed active transponders to transmit IFF codes. By the mid-1940s, Russian inventor Léon Theremin developed a covert passive spy device based on load modulation of acoustic audio [3], [4]. Soviet children presented the American ambassador in Moscow with a U.S. State Department seal, which he placed in his office at the embassy [Figure 1(c)]. Hidden inside the seal was an antenna loaded by a piezoelec-tric crystal. When illuminated by a powerful UHF radio source across the street, reflected signals from the antenna were modulated with the acoustic audio in the ambassador’s office. The listening device later became known in the American press as “The Thing.” Down-conversion to audio with a direct conversion receiver allowed Soviet agents to listen to conversations in the ambassador’s office. Theremin’s device was not discovered until the 1950s; even then, Britain had to reverse engineer it after the U.S. government failed.

Early literature on communication by backscatter was published by Harry Stockman in the late 1940s, working at what is now the U.S. Air Force Research Laboratories [4]. He discussed various approaches to load modulation and modulation by translating or rotating reflectors mechanically. Initial experi-ments demonstrated a mechanically rotated reflector approach [Figure 1(b)]. The work was not sanctioned by the laboratory, and Stockman lost his job because of improper use of Air Force property soon after publish-ing his paper.

Load modulation also found use for field measure-ments, with the notable early example of Richmond’s 1955 paper [5]. Measuring transmission power loss between an antenna and a probe required feed cables to each, perturbing the measured field. Applying load modulation to the probe’s terminal with a compact battery powered device removes one of those cables

at the expense of dynamic range since the received modulation reflected from the modulation load is so weak. The concept has more recently been extended (especially by Bolomey) for near-field imaging of bio-logical tissues with arrays of modulated field probes or by mechanically scanning a single modulated field probe [6], [7]. The first commercial applications of back-scatter communication that are similar to RFID were patented  in the mid-1970s [8]. These were targeted at inventory management, making them true precursors to modern RFID.

Current Status of Passive UHF RFIDsPassive and battery-assisted UHF RFIDs, as standard-ized in electronic product code (EPC) Global Class 2 and 3 [10] and International Standards Organization (ISO) 18000-6C and D [11], are unusual in that they employ digitally modulated backscatter for communication, not transmission. Backscatter modulation received by read-ers is weak, requiring sensitive detection, and is accom-panied by a much stronger interfering carrier leaked from the transmitter. The benefit is extremely low power consumption for the backscatter modulator in the tag, which is essentially a shunted varactor field-effect tran-sistor (FET) toggled between open and short states.

In wireless backscatter communication, the local oscillator (LO) is broadcast over the air at the carrier

Tag

Tag

Reader

Reader

Digital Baseband

Digital Baseband

IF

IF

LO

LO LO

RF

RF RF

PA

PA

RFReceiver

Receiver

(a)

(b)

Figure 2. Circuit topologies of (a) active (transmitting) modulation and (b) passive (backscattering) modulation. The backscattering topology effectively moves the LO out of the tag into the reader. The LO and RF signals in the backscatter modulation are incident and reflected waves sharing the same port [13].

The key tag design goal in these backscatter links is to maximize the proportion of power available at the tag antenna input that is reflected back out of the antenna.

50 July/August 2013

frequency. Without any other RF signal source, the reflected modulation from the reflective mixing in the tag appears to the reader as shifted to the carrier fre-quency. Each tag illuminated by the reader antenna’s field of view that mixes another signal with the car-rier adds its own modulation to the backscatter signal received by the reader, causing interference. Backscatter tags, by receiving the LO over the air, do not need their own RF oscillator or phase-locked loop (PLL). Removing these circuits reduces power consumption and total area (and therefore cost) of a tag chip. The penalty is that backscatter received by readers from tags is weak, limiting communication range and increasing the com-plexity and cost of the reader. Thus, backscatter is well suited for short-range communication where hardware cost and complexity is concentrated in the reader and the tag operates at very low power.

Simplified schematics for circuits that realize com-munication by transmission and backscatter are com-pared in Figure  2. Like up- and down-conversion in digital communication transmitters, the mixing process in backscatter modulation is represented as a mixer. Instead of the usual three ports for LO, RF, and base-band, however, the LO and RF become incident and reflected waves of a single combined port, so the reflec-tive mixer has only two ports. The key tag design goal in these backscatter links is to maximize the propor-tion of power available at the tag antenna input that is reflected back out of the antenna.

A basic performance limit in an RFID system, as in other wireless communication, is the maximum toler-able loss in the communication channel. State-of-the-art passive and monostatic UHF RFID systems can operate (for read operations only) up to about 45–50 dB of trans-mission loss between a reader and a tag. If the reader radiates in free space at the maximum legal amount of power [specified at 4 W effective isotropic radiated power (EIRP) in the United States], then this maximum loss corresponds to a maximum operating distance,

which is often about 12  m for typical omnidirectional tag antennas.

Communication between the reader and the tag is half-duplex and includes hand-shaking and anticollision, as shown in Figure 3. The tag must harvest enough power from the reader to turn on and decode requests from the reader, and the reader must receive strong enough tag backscatter to operate. Signal power levels in these asymmetrical links are esti-mable for idealized opera-

tion in free space with the Friis transmission equation and the radar equation. Either of these links may limit total loss (and free space range), particularly in the longest-range systems. It may not be clear to the user which link limits performance, particularly with frequency-hopping readers, making diagnosis of communication failures complicated and challenging.

Optimizing these links in systems that mix hard-ware from different manufacturers requires test data of transmit power and sensitivity performance of both the reader and tag. Testing the transmit power and sensitivity of a reader requires power measurement and reference tag modulation at its coaxial input ports. Tag tests are more complicated because a tag’s chip and antenna are integrated, requiring “black box” charac-terization of its sensitivity and the strength of reflected modulation. This kind of data is not given by current tag data sheets. Our focus here is on “black box” test and characterization of tag’s backscatter performance.

Black Box Metrics for Tag Backscattering

Radar Metric, Radar Cross SectionThis often-cited metric was adapted by [12] from the radar community for use as a tag backscatter metric. The concept is that some fraction of an incident carrier wave is reflected by the tag device under test (DUT) as modulated power. A tag can maximize the modulated radar cross section (RCS) and the power received by a reader by maximizing its antenna gain and circuit modulation efficiency. If received modulation power is defined as binary phase shift keying (BPSK), then arbitrary passive tag chip loads can reflect all of the incident power as modulation [13]. If power harvest-ing chip impedance is conjugate matched to that of its antenna, then the chip modulation efficiency is limited to 25%. The modulation RCS is a concise and effective application of the mature and proven RCS concept, but some challenges arise in its use:

Local Band Within860–960 MHz

International Operation860–960 MHz

40–640 kbps

40–640 kbpsRx DataVdc+

Vdc-

Dickson ChargePump

Tag RF Front End

Tx Data

BPSK Loss < 90 dBCW Loss ~ 20 dB

Simple Reader RF Front End40–640kbps

Tx Data 40–160 kbps

Rx DataI Q

IF

LO

LO

LORF

3 dB

3 dB90°

3 dB0°

IFIF

RF

30 dBm

PA

Figure 3. Circuits for simple UHF reader and tag front ends.

July/August 2013 51

• The use of modulation RCS in typical indoor deployment environments is complicated by fading that is not studied in the mature radar literature.

• The tag’s backscatter modulation efficiency of the tag chip is also very nonlinear, tending to fall sharply with increasing incident power, so the RCS must also be a function of the incident field strength. An example is shown in Figure 4. This linearity can be characterized at an operating point defined relative to the minimum turn-on level of the tag, but this limits use of the modu-lation RCS exclusively to passive [not battery-assisted passive (BAP)] tags.

Network Backscatter Metric, BBy assuming reciprocity in the channel losses between the reader and tag, [14] related power absorbed by a tag to backscattered power received by a reader. This idea has recently been leveraged to conceive a figure of merit, B, for tag backscattering power that applies in arbitrary flat-fading communication channels: first in [15] and [16] for use in self-sensing tag surroundings and self-sens-ing and, independently, in [17] for bounding received power by readers. Mathematically, the idea is simple:

dBm dBm ,B P Ptx BS0= +^ ^h h

where PTX0 is the threshold available transmit power from the reader to turn on the tag, and PBS is the received backscattered power. Both B and the received backscatter power vary with 1) frequency, 2) the power consumption in the tag, and 3) the power transmit to the reader relative to the turn-on threshold of the tag in its environment. Thus, like most definitions of mod-ulation RCS, B applies only to passive tags.

The metric B and modulation RCS metrics are fun-damentally different. Like modulation RCS, B values are proportional to received backscatter. However, while modulation RCS characterizes a tag in complete isolation, B describes the tag under the condition that the channel losses still allow turn-on.

Measurements of RCS and B Useful tag backscatter power performance data needs to be traceable to a national metrology institute like the National Institute of Standards and Technology (NIST) so that it can be comparable directly against transmit power in physical models (like the radar equation) and to give the same result in different labo-ratories. It should also have small uncertainty, when practical, for accurate link analysis [18]. This can be achieved by careful calibration of backscattered power [17], [19].

Measurements of Backscatter RCSBackscattered power measurements in UHF RFID standards are referenced in performance test standard

ISO 18046-3 [20] and detailed in ISO 18047-6 [21]. There are two versions of this standard: 1) ISO 18047-6 (2006 version) and 2) ISO 18047-6 (2011 version).

Antenna and Structural Mode Scattering Loss

902 MHz

0.6

-25

-30

-35

-40

-45

|Dt

1|2

(dB

)

-50

-55

-600.8 1.0 1.2

r (m)

1.4 1.6

928 MHzFitMeas.

FitMeas.

Patch Antenna

Reference ScattererDipole

1.2 m

Port 2 Calibration Plane

Port 1 Calibration Plane

1.0 m

1.0 m

Figure 5. Backscattered power for a calibration target in the NIST RFID anechoic chamber, fit to the expected /r1 4 . The mean-squared error of the fit (computed in linear units) is 0.45 dB [31].

0

Rec

eive

d B

acks

catte

r P

ower

Rel

ativ

e to

Tag

Tur

n-on

(dB

)

-5

-10

-15

-200 5 10

Transmit Power Above Tag Turn-on (dB)15 20

910 MHz

Figure 4. Normalized RCS of 20 different commercial RFID tags from 0 to 20 dB above turn-on power at 910 MHz. Traditional linear radar targets would exhibit constant RCS (flat 0 dB).

By the mid-1940s, Russian inventor Léon Theremin developed a covert passive spy device based on load modulation of acoustic audio [3], [4].

52 July/August 2013

RCS Measurement ApproachISO 18047-6 (2006 version) contains test recommenda-tions for RCS that calibrate receive tag signals against carrier scattering measurements off of a thin /2m rod [21]. This rod is the continuous-wave (CW) calibration target. This test approach is similar to many RCS mea-surements for which the DUT is a passive structure that reflects only CW. Reference signals with either calibration target create two discrete states. For /2m rod targets, the detector measures the magnitude of

the vector change in reflected vector states looking into an antenna between measurements 1) with the calibration target and 2) without the calibration tar-get. This is the calibration reference target with some known CW RCS; for the thin /2m rod, various authors give this RCS as in the range RCS ( . . )0 75 0 15cal

2! m= [22], [23]. For modulated calibration targets, the same measurement is performed between the vector states. The modulated target’s RCS can be computed accord-ing to [24].

We summarize here measurement with either the thin half-wavelength rod, or a modulating calibration target as described in [24].

1) The operator places the DUT in the test zone and adjusts the transmit power to a desired level rela-tive to the tag’s turn-on threshold.

2) Backscattered power from the DUT, ,Pdut is mea-sured with a quadrature demodulator detector. The operator leaves the transmitter at this power level.

3) The operator leaves the transmitter on, places the calibration target in the test zone and measures received modulation power, .Pcal For calibration against CW targets, this means the vector differ-ence in scattering with and without the /2m tar-get in the test zone. For modulated targets, this involves measurement of the vector difference of each modulation state.

The modulation RCS of the DUT is then simply

RCS (in m ) RCS (in m ) /P Pdut cal dut cal2 2 #=

in linear units.Care must be taken to avoid disturbing the test

zone between measurements of the calibration target and the DUT. Displacing walkway absorber, antenna positions or orientations, or receiver cables adds error to the measurement. Some examples of mea-sured changes in reflection power seen in the NIST RFID test chamber are listed in Table 1, shown for comparison with the measured value of the calibra-tion target.

If the transmission and detection hardware is very stable and the environment is close to an ideal anechoic environment, the dominant source of abso-lute uncertainty is the RCS of the calibration standard.

RCS Measurement ApproachThe 2011 update to ISO18047-6 adopts methods pro-posed in [25]. It is almost the opposite of the 2006 ver-sion: instead of calibration against a known scattering target, each term in the radar equation is computed or measured: transmit power, reader to tag antenna sepa-ration, backscattered power, reader antenna gain(s), wavelength, and polarization losses. The reader antenna gain contributes the most uncertainty [26] of any single source in this measurement.

Table 1. example measurements of errors from unintended events in the test zone.

Error SourceRelative Error Power

Thin /2m -long rod RCS target –40 dB

Engineer in the chamber door –49 dB

Pen left in test zone (r = 1 m) –45 dB

Rotate reference antenna 15° –41 dB

Small RFID tag left inchamber (r = 1 m)

–49 dB

890

840

-4-8

900 910Frequency (MHz)

r = 0.6 m

r = 2.0 m

Loss

Nor

mal

ized

to A

nech

oic

Mea

sure

men

t (dB

)

920 930 940

Patch Antenna

Reference ScattererDipole

1.2 m

Port 2 Calibration Plane

Port 1 CalibrationPlane (Behind Post)

1.0 m

1.0 m

Antenna and Structural Mode ScatteringAntenna Mode Scattering Only

840

-4-8

Figure 6. Line-of-sight multipath errors in calibration target measurements in a nonanechoic lab. The solid and dashed curves are multipath errors in CW and modulated calibration targets [31].

July/August 2013 53

RCS Measurement: Practical ChallengesMany organizations lack the resources for a full anechoic environment and instead perform range mea-surements in offices or labs without an absorber. How significant are multipath errors in these more reflective environments? Figures 5 and 6 show contrasting exam-ples of measurements in these environments.

While every fading environment differs, the curves in Figure 6 give some idea of the phenomena testers might see in a lab, significantly greater than the 0.45 dB mean square error (MSE) of the fit to /r1 4 in Figure 5. Thus, an anechoic environment is necessary for RFID tag backscatter measurements unless errors beyond several dB are acceptable.

Measurements of BB can be measured to within 0.5 dB of absolute uncer-tainty with calibrated modulation power measurements [17]. This propagates to about the same uncertainty in estimates of the minimum bound for backscattered power. The theory leads us to expect we can measure the figure of merit with much better stability than RCS in reflective environments (Table 2).

Measurements of the effects of detun-ing of B of a commercial tag near a large aluminum plate in an anechoic chamber are shown in Figure 7 and compared against the effect on the /P Pbs tx (mea-sured in RCS tests) that is predicted by the radar equation. The results are shown relative to the same measurement in an anechoic environment (0 dB represents the same value as the anechoic measure-ment). These effects would be impossible to measure directly in RCS tests, because

/P Pbs tx combines both detuning effects and multiple reflection path losses.

Measurements under stronger non-line-of-sight (NLOS) fading are presented for the same tag in Figure  8. Again, as in Figure 7, measurements are relative to the anechoic measurement. The two tagged dielectrics (foam with permittiv-ity near one and wood with permittivity

near three) separate the tag from the metallic scatterers by /2m at 915 MHz. Like the detuning tests in Figure 8, the measured B of the tag is stable to within 1 dB. In con-trast, measurements of Ptx0 and Pbs (not shown) differ from the anechoic measurements by up to 15 dB. Thus, the figure of merit B is defined and measurable even in harsh NLOS propagation environments.

Use of B in Link AnalysisConsider a passive monostatic reader-tag system that tolerates dBL 45= of transmission loss in turning on the tag chip. By reciprocity, the backscatter link suffers

3.0 10

5

0

-5

-10

B R

el. t

o Fr

ee S

pace

(dB

)

1.5

0.0

-1.5

-3.00 10 20 30

Height Above Aluminum Plate (cm)40 50

Pbs /P

tx Rel. to Free S

pace (dB)

860 MHz 910 MHz 960 MHz

Figure 7. Measurements of B and backscatter loss for commercial tag detuned by an aluminum plate. Within /2m above the plate, B is stable to within less than 1 dB [17].

1 2 3 4

5

6

78

9

10

0.0

-0.4

B N

orm

aliz

edto

Fre

e S

pace

(dB

)

-0.8

0.0

-0.4

B N

orm

aliz

edto

Fre

e S

pace

(dB

)

-0.8

860

910

960

Frequency (MHz)86

091

096

0

Frequency (MHz)

Figure 8. Measurements of B of a commercial tag in ten positions in NLOS propagation compared to anechoic measurements on two different dielectric objects. Tests were performed 0.8 dB above the tag turn-on power [17].

Table 2. estimated uncertainty in RCS and B in the anechoic, lOS fading, and NlOS fading tests.

Minimum Measurement Uncertainty (dB)

environment RCS b

Anechoic 0.9 0.5

LoS + fading 6 0.5

NLoS + fading 15 0.8

Many organizations lack the resources for a full anechoic environment and instead perform range measurements in offices or labs without an absorber.

54 July/August 2013

this loss out to the tag and then back to the reader again, so if the tag is on, the backscatter link may have up to dBL2 90= of loss. If the tag is not on, the back-scatter link will not exist—the tag will not be able to modulate any response back to the reader, and the backscatter link does not exist.

This also means there is a maximum operating loss in the backscatter link and therefore a minimum backscattered power received by the reader. In a sys-tem transmitting dBmP 30tx = with that has the back-scatter modulation efficiency at 25% ( dB),e 6=- we expect that the minimum backscattered BPSK power received from the tag is simply

[ ] (in dBm) (in dBm) (in dB) (in dB),min P P L e2bs tx= - +

where the backscatter modulation efficiency, e, is the proportion of available power out of the tag antenna that reabsorbed as backscatter modulation. Under the BPSK definition of backscattered signaling, the maximum value of e is -6 dB for well-matched power harvesting. For example, in a typical system with dBmP 30tx =

( dB)e 6=- and maximum operating transmission loss of 45 dB, the minimum power bound on received back-scattered power is -66 dBm.

The figure of merit B serves as the remotely measur-able black box parameter that we can use to easily calcu-late the minimum BPSK received by a reader under the condition that the tag is turned on. The calculation is:

min[ ]Pbs (in dBm) = B (measured at tag turn-on) - Ptx0 (in dBm).

Unlike the radar model, this bound is general in any flat-fading environment. Passive UHF RFID modulation is slow at 640 kb/s bit rate, so most envi-ronments are flat-fading environments. Unlike RCS, neither B nor the minimum power bound depend on the antenna gains or orientations in the system, leav-ing fewer variables and keeping the analysis simple.

Examples of many measured B performed in the NIST antenna lab are given in Figure 9. For each circle, the area is proportional to that of the tested tag, the bar shows the range of values across the band, and the color indicates a unique manufacturer. The data show a clear trend downward in B and therefore with the minimum backscattered power. One tested tag (not shown here) has

,B 36=- corresponding to minimum backscatter power of [ ] dBmmin P 66bs =- (as in the example above). We can conclude here that a reader with sensitivity to BPSK modulation of around -70  dBm in a low-interference environment can be relied on to detect backscatter for tags made through 2012 but not future tags that turn on with less power (and thus operate with greater link loss).

ConclusionsThe most common application of RFID is identification for inventory, but use as a communication back end for sensing is becoming more common. The greatest ben-efits for wireless RFID-enabled sensing arise in indus-trial applications where manual or wired measurement is impractical or unsafe in real time. Important sensing parameters can include temperature, gas or liquid pres-sure, fluid level, acceleration, and mechanical strain.

(a) (b) (c)

Figure 10. (a) Power generation, (b) rail transport, and (c) oil refining all benefit from wireless sensing of mechanical wear and maintenance status data [27]–[29], but present harsh mechanical and RF environments. RFID is finding increased use in enabling communication in these applications despite traditional use in ultra-low cost identification.

B (

dBm

)2

-10

-15

-20

-25

-30

-35

2006

2007

2008

2009

2010

2011

2012

Estimated Tag Manufacture Year

p– = 0.8 dB, 860–960 MHz

Figure 9. Measured trends in B (and thus minimum BPSK power received by a reader) over time. Colors represent chip manufacturers, the solid line represents variation in performance across the band, and the size of the circle represents the size of the inlay [17].

July/August 2013 55

Application often involves deep oil wells, large moving machinery, busy airports, rail yards, or isolated areas where human labor is scarce (Figure 10). These introduce diverse challenges to passive wireless communication, like rapid motion, interference, deep fading, lossy mate-rials, and balancing power consumption of communi-cation against sensing circuitry. For example, Phase IV Engineering in Boulder, Colorado, United States, devel-oped the first passive wireless sensing chip in the 1990s and currently provides passive and BAP sensors for monitoring critical physical parameters in real time. Some UHF sensing tags have been developed for indus-trial environments pictured in Figure 10, which comple-ment the well-proven inductive tags used to monitor internal temperature of cattle, tire pressure, etc. [30]. Backscattered telemetry needs to be received reliably, even in harsh fading and interference channels. During the design phase, this can only be ensured with repeat-able and broadly applicable hardware performance tests and metrics such as the ones described in this article.

The UHF RFID protocols incorporate error correc-tion coding, but bit errors force repeated transactions, slowing inventory and communication rates. Slower speed is a more significant problem in sensing, since the backscatter link must convey sensor data in addi-tion to identification codes. Ensuring fast and consis-tent sensor backscatter links in these environments is therefore important, particularly with BAP sensors that allow greater link loss. Analysis of this behav-ior requires an estimate for backscattered power. Compared to test and characterization with RCS, the B test has several advantages:

1) trivial estimation of the lower bound for back-scatter received by readers

2) test and application in realistic slow-fading envi-ronments

3) reduced dependence on parameters that are redundant with power harvesting losses

4) remove requirement for anechoic test chamber.Analysis of minimum backscatter power via the B

parameter lets engineers quickly and accurately mea-sure tag performance to bound received backscatter power and thus gauge the need for more costly diver-sity schemes.

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