Technical
Article
How new ‘boostedNFC’ technology enables mobile
phones and wearable devices to emulate contactless
cards reliably
By Nicolas Cordier
Hundreds of millions of people have kept a contactless card in their wallet, purse or handbag during
the past few decades. The first contactless cards were cheap, credit card-sized pieces of plastic
which controlled access to trains, trams or buses. Quick and convenient, they deservedly became
popular with consumers. In the past decade, contactless technology has been extended to payment
cards (such as debit cards, credit cards and charge cards), helping to speed up transactions in busy
retail environments such as newsagents, coffee shops and convenience stores.
In fact, the conditions for widening the use of contactless technology are almost perfect:
•
consumers understand and accept the technology
•
a large installed base of contactless readers exists (in point-of-sale terminal equipment,
ticket barriers and elsewhere)
•
thousands of retailers, banks, transit network operators and others have developed payment and access-control application software that makes use of this contactless communications infrastructure
One big element, however, has been missing. Many consumers carry with them everywhere a device – the smartphone – that could in theory emulate contactless cards, enabling users to replace
multiple cards with a single device.
For merchants and transit operators, this is highly desirable: they can save the cost of supplying
cards, replacing their card with a downloadable app. And the increased convenience for the consumer promises to accelerate adoption of contactless technology, helping merchants and transit operators increase revenues and improve operating efficiency.
Mobile phone manufacturers are equally keen to implement contactless technology in their devices,
to add value and bolster their consumer appeal.
The first attempt was in the early 2000s, with the introduction of Near Field Communications (NFC),
a standard technology under the aegis of the NFC Forum. As well as card emulation, NFC technology supports a reader mode, permitting a device to read NFC tags, and a peer-to-peer mode.
But early implementations failed to make proper consideration of the electrical and mechanical constraints imposed by a mobile phone. As an NFC reader, in particular, the mobile phone has a far
smaller power budget and far less space available for an antenna than an application-specific NFC
reader such as a contactless payment terminal. Achieving even adequate reader mode performance with a mobile phone proved to be extremely difficult.
But even more damaging was the lack of a killer application. Put simply, phone users had no use for
an NFC reader in their pocket.
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If the phone can operate as an NFC tag, however, it can emulate multiple contactless cards. And
this is a killer app: not only can consumers use their favourite device to access buildings and board
trains and buses, they can also pay with it, use coupons and accumulate loyalty points at shops.
So how does a smartphone manufacturer go about implementing reliable NFC tag functionality? To
understand this, it is helpful first to describe the way that an NFC tag communicates with a reader.
Passive load modulation in contactless cards
NFC technology involves the inductive coupling of a pair of antennas. The coupling factor, k, represents how susceptible a reader/card antenna pair is to coupling, and is shown as a value between 0
and 1. Its value depends essentially on the geometric parameters of the antennas and the distance
between them.
In the normal case, when a card antenna is smaller than the reader antenna, the coupling factor is
proportional to the card antenna’s surface at any given distance from the reader antenna: the bigger
the surface, the higher the coupling factor.
The value of k is usually required to be between 0.03 and 0.3 when using the common ‘passive load
modulation’ (PLM) method of transmitting data.
A contactless NFC transaction is a sequence of reader commands, each followed by an answer
from the card. PLM, which is used in contactless cards and in all existing NFC phones, except the
Apple® iPhone® 6 and iPhone 6 Plus smartphones, operates by switching a passive (resistive or
capacitive) load that is connected to the card antenna (see Figure 1). A default load corresponds to
the non-modulated state and a switched load corresponds to the modulated state. When the reader
and card antennas are inductively coupled, the reader’s receiver can sense these load variations,
and decode them to extract the information from the signal.
Load s witching
transistor
Modulating signal (i.e 0: modulated
state, 1: non-mo dulated state)
k
Tuning
Capacitor
Card
Anten na
Reader
Antenna
Reader
Load
Fig. 1: a circuit implementing passive load modulation
When implementing an NFC system, it is important to take account of the load modulation amplitude: this is the difference between the modulated and the non-modulated voltage sensed by the
reader’s receiver (see Figure 3). If this amplitude falls below a certain minimum value, the receiver
will not be able to reliably sense the card’s modulations of the signal. All other things being equal,
the higher the value of k, the greater will be the load modulation amplitude.
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When used in a contactless card, PLM reliably produces sufficient load modulation amplitude. The
standard form factor of contactless cards, known as ID-1, is the size of a credit card. A large antenna is normally embedded in it, providing a range of as much as 6cm with today’s readers. Indeed, the performance of contactless card systems is so good that users are able to operate ticket
barriers in mass-transit systems, for instance, without removing their contactless card from their
wallet or handbag.
In fact, consumers have come to expect contactless systems to operate instantly and perfectly reliably, and to permit them to hold their contactless device close to the reader in any orientation, and
with their hand, wallet or bag completely covering the device.
This presents a considerable challenge to the smartphone manufacturer. This is because a mobile
phone contains multiple radios and antennas and is encased in metal. The phone’s circuit boards
are extremely densely populated with components. And more and more of the available space is
now being taken by the larger displays and batteries demanded by consumers.
This environment could hardly be less suitable for PLM. The cramped form factor provides space
only for a tiny antenna. The abundant metal and competing RF signals also seriously impair the
ability of a mobile phone to couple with a reader’s antenna.
This results in a woeful consumer experience: frequent failed transactions, transactions taking a
second or more to be completed, and inconvenience as users are forced to remove their phones
from bags or cases and present them carefully to the reader.
How to compensate for low coupling factor
The design constraints on the phone manufacturer hinder any attempt to increase the k value
which, as shown above, would require a huge increase in the surface area of the phone’s NFC antenna.
Efforts to improve the performance of phones in card-emulation mode have therefore concentrated
on increasing the load modulation amplitude. And now, a successful method for doing so has been
found, called Active Load Modulation (ALM).
ALM makes use of a mobile device’s battery power supply. In ALM, a carrier signal synchronous
with the reader’s magnetic field is sent during the modulated state, and turned off during the nonmodulated state (see Figure 2). This is called the AND mode of operation. In a more efficient version of the technique, called the XOR mode, a carrier synchronous to the reader’s magnetic field is
transmitted during the modulated state, and a 180° phase-shifted signal is transmitted during the
non-modulated state. This XOR mode doubles the amplitude of the load modulation sensed by the
reader compared to AND mode. This allows the use of an even smaller antenna.
This ALM signal is coupled to the reader’s antenna. Depending on its phase difference with the
reader’s magnetic field, the coupled ALM signal is either added to or subtracted from the reader signal to form the load modulation signal.
The main benefit of ALM is that the same load modulation amplitude can be achieved as a PLM device, at a coupling factor 100 times lower (see Figure 3).
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Modulating s ignal (i.e 1: mo dulated
state, 0: non-modulated state)
k
Carrier
Freq uency
CLK
synch ro nization
block
Card
Anten na
Reader
Anten na
Reader
Matching capacitors
Fig. 2: circuit implementing active load modulation
Fig. 3: ALM permits the use of an antenna 100 times smaller than a typical PLM system’s antenna
A typical contactless card’s antenna surface area is 4,000mm2, permitting the successful implementation of PLM. An ALM circuit can achieve the same load modulation amplitude at the reader’s receiver, resulting in an identical consumer experience, with an antenna area of just 40mm2.
An antenna of this size has a cost to the phone manufacturer of almost zero – it can even be realised as a PCB antenna on the mother board. It requires no ferrite shielding. For comparison, the
cost of a large antenna with ferrite and connector is approximately US$1. Also, the NFC antenna
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can be located in the best position for the user. When positioned around the rear-facing camera, for
instance, it allows the user to simply tap the top of the phone to the reader.
By contrast, a large antenna on the back cover of a smartphone or in its battery pack has been the
cause of frustration for users. Typically, they hold the device in their hand, so the antenna is not
aligned with the reader antenna. When they swipe their phone in front of a contactess reader, the
transaction fails. Contactless communication then only takes place once the user has learned that
the phone has to be carefully placed in very close proximity to the reader – an action quite different
from that required to use a contactless card.
Fig. 4: a small antenna can be conveniently located on the device to provide for the best possible
user experience
In Figure 4, the reader antenna is marked in yellow. The large antenna required by PLM based solutions is depicted in orange, where the hand grips the device. By contrast, the small antenna used
by ALM solution, marked in red, is ideally placed for successful NFC transactions.
Because it enables the use of very small antennas, ALM is also well suited to wearable devices.
The space constraints imposed by wearables only leave room for a small antenna, and a PLM solution with a small antenna provides an unsatisfactory communication range.
Improved interoperability with transit and payment systems’ readers
The operating principle of ALM has been adopted in a family of NFC tag emulation ICs developed
by ams, all using its ‘boostedNFC’ technology. When implemented in the latest smartphones, boostedNFC provides performance which matches that of contactless cards with an antenna just 40mm2
in size.
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But antenna size reduction is not the only benefit that boostedNFC offers. It also permits configuration of various electrical parameters of the load modulation signal. These include:
•
an Automatic Power Control feature, which reduces the output voltage when the phone is
close to the reader (in other words, when the coupling factor is high). This prevents saturation of the reader’s receiver.
•
configuration of the phase difference of the Active Load Modulation signal with respect to
the reader’s magnetic field, improving the reliability of transactions when the phone is almost out of range of the reader.
•
configurable sensitivity, allowing the support of various antenna form factors. The extremely
low sensitivity that can be achieved ensures that the reader-to-card link will never limit the
transaction range.
•
an accurate timing mechanism which ensures that the delay between the reader command
and the card answer complies with the very tight limits set by the contactless standards.
This mechanism compensates for the variations occurring on the NFC controller side.
•
Automatic Gain Control, providing for correct demodulation of the reader’s signal, and accommodating wide variations in terminal design particularly in mass-transit systems.
All these features, which can be dynamically configured, result in much higher interoperability pass
rates compared to existing NFC solutions. When it comes to payment applications, a pass rate of
100% can easily be achieved.
In addition, boostedNFC technology gives OEMs a way to accommodate the inconsistency in the
installed base of readers, which results from the continual revisions of the EMVCo
(http://www.emvco.com/) contactless standard for payment card transactions. (It is not unusual for a
point-of-sale terminal to remain in operation for ten years.) The reader’s performance can also
change due to ageing, and variations in reader performance can also arise from manufacturing and
installation inconsistencies.
The very small antenna also avoids the problem of detuning of the reader antenna, which can occur
with larger antennas. The EMVCo requirement for contactless communication range has been relaxed to a minimum of 2cm, down from 4cm, for NFC phones to allow for the poor performance of
PLM solutions. This is hardly in tune with the wishes of the user. But fortunately, this relaxed requirement will not be necessary in future because the boostedNFC technology allows NFC phones
to perform as well as, or better than, legacy contactless cards.
The ams ALM technology is available in the AS3922, an NFC booster IC for use in UICC and microSD NFC adapter cards. These enable phones and small accessories with no built-in NFC functionality to emulate a contactless card.
The AS3923 and AS39230 are for integration into electronics devices as a companion to the NFC
controller, essentially replacing the controller’s analogue front end (see Figure 4). The AS39230
also supports the active peer-to-peer mode of NFC as well as card emulation.
In the ams implementation, ALM has almost no impact on battery run-time, since the boostedNFC
device stays in a power-down mode until it detects that a contactless transaction is to take place.
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Digital IC manufacturers have also attempted to implement ALM, but the analogue circuitry required
in an ALM system is poorly suited to the ultra-small circuit features found in advanced digital ICs. As
a result, the load modulation performance of digital Secure Elements or NFC controllers with integrated ALM is poor. In contrast, the pure analogue implementation found in the ams devices provides the best-in-class performance required to match the expectations of consumers who are used
to the effortless convenience of contactless cards, and as a result has already been selected for
use in leading smartphones on the market today.
VBAT
Miniature NFC
antenna
NFCC
AS3923
Digital
Interface
Fig. 5: boostedNFC implemented in a companion chip (the AS3923) to a host NFC controller
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Summary
Contactless cards are today widely used in mass-transit systems, and for contactless payments in
shops. Hundreds of millions of consumers carry with them everywhere a device – the smartphone –
that could in theory emulate smart cards, enabling users to replace multiple cards with a single device that they carry anyway.
Implementing contactless card functionality in the environment of a smartphone is immensely challenging. The passive load modulation method that contactless cards use to communicate with a
contactless reader requires a large antenna and a benign environment for RF signals. The
smartphone’s case is small, cramped, surrounded by metal objects and subject to RF interference.
As a result, manufacturers of smartphones and wearable devices are turning to an alternative
method of implementing NFC contactless communication: active load modulation. This article describes the operation of active load modulation, and illustrates its benefits when implemented in
smartphones.
It also describes the analogue boostedNFC technology developed by ams, which provides for superior modulation performance and a more reliable, convenient experience for the user of contactless
card emulation.
Biography
Nicolas Cordier is a wireless engineer at ams, based in Cupertino, California. In 2002, he graduated
from INSA Lyon in France and was awarded a Diploma of Engineering, specializing in telecommunications. He started his carrier in the terminal industry as an RF engineer designing mobile phones,
and then as a hardware engineering technical lead, designing contactless point-of-sale terminals. In
2008, he moved to the semiconductor industry, and has held various postions such as NFC system
validation leader and customer support specialist.
For further information
ams AG
Nicolas Cordier
Application Engineer
Tel: +1 408 454 2995
[email protected]
www.ams.com
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