RFID电子标签英文文献

understand RFID
By Darren McCarthy
Tektronix

RFID applications are rapidly
growing as equipment prices
drop and global markets expand.
Moreover, many short-range near
feld communications (NFC) links
based on RFID technology are
also seeing broad adoption. RFID
and NFC technologies share a
variety of uncommon engineer-
ing measurement challenges.
Transient signals, bandwidth
inefcient modulations, back-
scattered data and passive tags
all require special measurement
capabilities not commonly found
in traditional test instruments. In
this article we will examine RFID
and NFC technologies and their
associated test and measurement
requirements.
As the cost for passive tags
drops thanks to advances in sub-
micron complementary metal
oxide semiconductors (CMOS),
the use of RFID for inventory ap-
plications is becoming nearly
universal. Many experts believe
the 96-bit Electronic Product
Code (EPC), as shown in Figure 1,
will be the next generation of the
Universal Product Code, the fa-
miliar General Trade Identifcation
Number imprinted in the barcode
on a majority of products sold
today. The varying applications
of EPC RFID tags have moved the
industry to classify the basic types
of RFID devices, ranging from 1 to
5 according to the tag’s read/write
capability and passive or active
power source.
RFID overview, challenges
The passive class 1 tag in the 900
MHz and 2.45 GHz frequency
ranges is ideal for many high-
volume applications. The high
frequency allows the interrogator
to read the tag with a directional
antenna for a greater commu-
nication range. Passive tags at
higher frequencies also work with
smaller, less complicated anten-
nas making them more suitable
for consumer applications.
Reading passive tags is some-
what diferent than the traditional
full duplex data link. Unlike tradi-
tional active data links, the pas-
sive tag relies on the RF energy it
receives to power the tag. Passive
tags modulate some of the energy
being transmitted by the interro-
gator to the tag in a process known
as backscattering (Figure 2).
By changing the loading of the
antenna from absorptive to re-
fective, a Continuous Wave (CW)
signal from the interrogator can
be modulated.
Passive tag readers are typi-
cally confgured as a homodyne
Figure 1: The 96-bit EPC number identifes more than the UPC barcode.
Figure 2: The passive tag backscatters the interrogator’s CW carrier, modulating it by changing the absorption
characteristics of the antenna. The passive tag also rectifes the RF energy to create a small amount of power to run
the tag.
1 eetindia.com | EE Times-India
Understand RFID, Part 1:
Technology
RFor single frequency conversion
receiver (Figure 3). A precision
frequency source in the interroga-
tor generates both the transmitter
signal and the local oscillator for
the reader’s receiver. The unique
homodyne architecture of the
Class 1 RFID system presents
some unusual challenges for
the engineer. The backscattered
modulation is far weaker than the
CW signal from the reader’s trans-
mitter used to power the tag dur-
ing backscattering. At base band
in the reader’s receiver, the CW
leakage translates to a large DC
ofset that can saturate sensitive
amplifers and digitisers.
Another challenge with the
passive tag RFID system is the pow-
ering of the tag from received RF
energy. Even though sub-micron
CMOS requires very little power to
operate, at a range of only a few
meters very little power (-10 to -15
dBm) is available. Complicating
matters further, regulatory bodies
worldwide have diferent maxi-
mum Efective Isotropic Radiated
Power limits.
Since the uplink from the Tag
(T) to the Reader (R) (denoted
T=>R) is modulated from the inter-
rogator’s CW signal, it is possible to
use spread spectrum techniques
such as frequency hopping. Any
spreading on the interrogator’s
signal will automatically be re-
moved in the homodyne down
conversion of the receiver since
it shares the same Local Oscillator
(LO) signal. After down conver-
sion the interrogator’s homodyne
receiver has separated In phase (I)
and Quadrature phase (Q) signals.
The down-converted base-band
signal is then digitised with ana-
logue to digital converters (ADC)
and digitally processed to deter-
mine the tag’s ID.
Modulation, coding
RFID systems usually use simple-
to-produce modulation tech-
niques and coding schemes that
lead to some design trade-ofs. A
typical example is ISO 18000 Type
C (also known as EPC Gen2, Class
1) which calls for Double Side
Band-Amplitude Shift Keying
(DSB-ASK), Single Side Band-ASK
(SSB-ASK) and Phase Reversal-
ASK (PR-ASK). ASK and PR-ASK
Modulation are illustrated in
Figure 4.
Amplitude shift keyed digital
modulations are spectrally inef-
fcient, requiring substantial RF
bandwidth for a given data rate.
Bandwidth efciencies of 0.20
bits per Hertz of RF bandwidth
are not uncommon for DSB-ASK.
One approach to improving
bandwidth efciency is to use
SSB-ASK. This is particularly im-
portant in European countries
where bandwidth restrictions
may preclude DSB-ASK.
The power efciency of both
DSB-ASK and SSB-ASK is depen-
dent on the modulation index.
With a modulation index of one
or On and Of Keying (OOK) of the
carrier, the lowest Carrier to Noise
required to achieve a given Bit
Error Rate is obtained for DSB-ASK
and SSB-ASK. Unfortunately, this
also provides the least amount
of RF power transport on the
downlink to supply the tag with
energy. Ideally, the of time of
the carrier should be minimised
so that the tag doesn’t run out
of power. The carrier to noise re-
quirements should also be mini-
mised to maximise ID read range.
For many modulations these are
conficting goals.
PR-ASK is a modulation that can
minimise the carrier to noise re-
quirement in a narrowband while
maximising the power transport to
the tag. This modulation has carrier
to noise and bandwidth require-
ments more closely matching PSK
than DSB-ASK, making it attractive
for narrowband and longer-range
applications. DSB-ASK is the least
bandwidth efcient modulation,
but the easiest to produce by OOK
of the carrier signal.
Data encoding
Before modulation, the data must
be encoded into a serial informa-
tion stream. There are many types
of bit encoding schemes available
as shown in Figure 5, each with
diferent strengths. Data encod-
ing is critical for RFID applications
due to such factors as the lack
of precision timing sources on
board the passive tag, challeng-
ing bandwidth requirements and
the need for maximum RF power
transport to energise the tag.
Manchester-L (Bi-Phase-L) and
Pulse Interval Encoding (PIE) are
popular for interrogator to tag
(R=>T) communications. These
coding schemes are based on
transitions and are self-clocking,
greatly reducing the complexity
of the synchronisation circuitry re-
quired in the power-starved tag.
PIE encoding is based on a
given minimum pulse duration or
Figure 3: A typical homodyne interrogator or tag reader simplifed block diagram.
Figure 4: ASK modulation depth, rise time and fall time are typically specifed to ensure readers can properly power
tags and interpret data symbols.
2 eetindia.com | EE Times-Indiainterval such as 20 s. This period,
called a Tari, is named after the ISO
18000-6 Type A Reference Interval.
One and zero bits as well as spe-
cial symbols like Start Of Frame
(SOF) and End Of Frame (EOF) are
composed of difering numbers
of Tari periods. This makes the
transmission length for a given
number of bits variable, but since
PIE encoding is self-clocking the
variable length has little efect.
The Tari length is also the mini-
mum pulse width for the modu-
lated signal, an important factor in
determining the bandwidth of the
transmitted signal. The shorter the
Tari length, the greater the band-
width requirement for the signal.
More recent standards such as the
ISO 18000-6, Type C allow for sev-
eral Tari lengths (6.25, 12 and 25 s)
to accommodate difering world-
wide regulatory spectral emission
requirements.
Another important property for
RFID Pulse Code Modulation (PCM)
coding schemes is the DC spectral
component. Backscattering tags
modulate a carrier signal. The car-
rier signal is then fltered out as a
base band DC level back in the
tag reader, leaving only the much
weaker uplink modulation from
the tag. Coding schemes in the
tag require the uplink to the read-
er to have little or no DC energy to
confict with the carrier signal.
Miller and FM0 encoding share
this property of little or no DC
energy in their spectrums. ISO
18000-6 Type C further enhances
the Miller encoding by ofering
diferent sub-carrier rates. One,
two, four and eight times the sub-
carrier frequency enable adjust-
ment of the modulation encoding
to optimise read range, speed or
bandwidth.
Amplitude-based modula-
tions used in many RFID systems
are susceptible to rapid signal
fading conditions. Pallets of tags
traveling on forklifts past readers
located between metal trucks and
warehouses can undergo dev-
astating multi-path conditions.
Rapid Rayleigh fading or shadow-
ing can be indistinguishable from
amplitude modulation, leading to
bit errors.
Another RFID consideration is
some form of anti-collision proto-
col to enable reading all the tags
in the interrogator’s feld of view.
There are two basic types of anti-
collision protocols, deterministic
and probabilistic. Popular RFID
protocols are the deterministic
binary tree and the probabilistic
ALOHA and slotted ALOHA ap-
proaches.
The binary tree method search-
es for tag IDs that ft a specifc
binary number while the proba-
bilistic ALOHA protocol allows the
tag to send its message and if the
message doesn’t get through, it
simply tries again later until it does.
The slotted ALOHA approach uses
synchronisation between all the
tags, so communications packets
are not interrupted mid-stream in
the transmission. Additional ef-
ciency gains are possible by using
Listen Before Talk (LBT) schemes.
Figure 5: RFID systems use a variety of PCM bit coding schemes.
Figure 6: Popular protocols that arbitrate collisions between simultaneous
transmissions back from multiple tags include ALOHA, slotted ALOHA and
the Binary Decision Tree.
3 eetindia.com | EE Times-India

这个里面资料多可以看看有没有你要的

请留下联系方式，我做过类似的毕设，是关于读写器reader的simulink仿真的，我电脑里有个文献。 可以发给你。本回答被提问者采纳

我也同求