1. technology and computing

A Secure Miniaturized Wireless Sensor Node
for a Smart Home Demonstrator
Antonio Jonjic∗† , Jasmin Grosinger∗ , Thomas Herndl† , Gerald Holweg† , Gottfried Beer‡ , and Wolfgang B¨osch∗
∗ Graz
University of Technology, Institute of Microwave and Photonic Engineering, Inffeldgasse 12, 8010 Graz
Center Graz, Infineon Technologies Austria AG, Babenbergerstraße 10, 8010 Graz, Austria
‡ Infineon Technologies AG, Wernerwerksstraße 2, 93049 Regensburg, Germany
† Development
Abstract— Recently, the technology of wireless sensor networks
(WSNs) experiences a growing use in home automation or
advanced industry infrastructure applications. Despite a strong
interest of industries in this technology, key issues like miniaturization and security of WSN nodes has not been solved yet.
State-of-the-art WSN nodes do not provide credible security nor
satisfying configurability and miniaturized implementations. This
publication deals with these limitations and presents a WSN
node that provides security, configurability, and a miniaturized
design. To show the sensor node feasibility, the WSN node is
implemented within a smart home demonstrator. Additionally, a
miniaturized pre-study WSN node design is presented using the
novel embedded wafer level ball grid array (eWLB) packaging
technology. Furthermore, an eWLB based WSN node design is
proposed that further miniaturize the presented WSN node.
I. I NTRODUCTION
The technology of wireless sensor networks (WSNs) has
reached hype peak, under the more common term the internet
of things [1]. WSNs consist of wireless connected embedded
systems - WSN nodes - that are equipped with various sensors
and actuators. WSN nodes are typically composed of three
units a transceiver, a microcontroller, and a sensor/actuator.
Recently, a WSN node system architecture has been presented
that added a fourth additional unit, a so called secure WSN
node [2]. This additional unit is a security controller or rather
a secure element (SE) with a near field communication (NFC)
interface. This paper demonstrates the functionality of the
security enhanced WSN node by implementing it in a smart
home demonstrator.
The demonstrator clearly shows the advantages of the
secure WSN node in terms of security and configurability.
So far, state-of-the-art WSNs rely on software based
security implementations [3] [4]. These implementations are
cumbersome in terms of hardware resources such as memory
demands. Compared to software based implementations,
hardware based implementations offer a energy efficient
security and crypto-key management [2].
To further enhance the miniaturization, the paper also
presents a WSN node miniaturization approach using
This work was performed as part of the K-project “Secure Contactless
Sphere — Smart RFID Technologies for a Connected World” that is funded
by the Austrian Research Promotion Agency (FFG) and the ARTEMIS JU
Project ”Internet of Energy”, Project No. 269374.
the novel embedded wafer level ball grid array (eWLB)
packaging technology. So far, miniaturization has been
implemented by several research groups based on 3D printed
circuit board (PCB) stacking designs [5], or 3D integrated
circuit (IC) stacking with through silicon via (TSV) designs
[6]. Both approaches enable small form factors but use
discrete production methods with wire bonding. However,
a discrete assembly of WSN nodes is not suitable for high
volume low production cost manufacturing. In comparison
to the implementations used so far, this paper presents a
miniaturization based on the eWLB packaging technology.
The eWLB packaging technology is a wafer level based
technology that enables low production and test costs and
allows to realize a multi metal layer heterogeneous system
integration.
II. S MART H OME D EMONSTRATOR
We build a smart home demonstrator to simulate a home
automation scenario. The demonstrator is shown in Fig. 1 and
consists of off-the-shelf real world electrical appliances that
are equipped with the invented secure WSN node. In detail,
the demonstrator consists of a smart socket that is connected
to a table fan, a smart wireless switch, and an near field
communication (NFC) smart phone. Smart socket and switch
are equipped with the realized WSN nodes (see Fig. 2 and 3).
To power up the smart home demonstrator, the demonstrator
sockets are connected to a wall socket with a power cable.
The wireless switch controls the ON and OFF state of the
table fan as shown in Fig. 1. The wireless link between the
switch and the sockets operates in the ultra high frequency
(UHF) range and is strongly encrypted. The encryption process
of data packets and the crypto-key storage is handled by the
SE. To establish the secure wireless link, an NFC smart phone
is used. The NFC smart phone is used for:
• cryptographic key generation and distribution to the WSN
nodes,
• fast installation of WSNs by taping the NFC smart
phone to WSN node equipped appliances, as described
in section III, and
• configuration of radio frequency (RF) parameters, like
frequency band selection (multi band transceiver) for
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interoperability reasons.
A smart phone application was programmed, which allows
a user to choose the amount of electrical energy he wants to
buy. After selecting the amount of energy (kWh), the user taps
the smart phone to the smart socket and all necessary data is
transmitted over the NFC interface to the smart socket. When
the smart socket is turned on, the table fan is switched on. A
current sensor that is integrated in the WSN node (see Fig. 3)
measures the current flow, and an energy metering program,
running on the WSN node, calculates the energy consumed by
the table fan. When the energy consumed by the fan reaches
the paid value, the power is switched off.
table fan (OFF)
smart socket
power cable
with secure WSN nodes. The secure WSN node is a 3 V
system with an on board voltage regulator. The WSN node
inside the smart switch is powered by a 9 V battery, and
the smart socket is powered by a 5 V switched-mode power
supply.
The visible winded copper wire is an NFC loop antenna and
is mounted bellow the cases of socket and switch.
secure element
(SLE78xx)
NFC loop antenna solid state relay
current sensor
(TLI4970)
general-purpose
microcontroller (32Bit)
transceiver
(TDA5340)
868MHz UHF
ceramic antenna
Fig. 3. Secure WSN node. The back (right/left picture) and the front
(right/left picture) of the sensor node are shown.
III. S ECURE WSN N ODE
NFC smart phone
table fan (ON)
smart switch
Security in WSNs is achieved by encrypting messages sent
among WSN nodes. To encrypt messages, a crypto-key must
be agreed on in all WSN nodes before starting the wireless
communication between them. With common WSN node
architectures, this is a non-trivial task [3] [4] due to hardware
constrains. Therefore, a secure WSN node architecture was
conceived, which improves the crypto-key management and
configuration capabilities of the WSN node.
Fig. 1. Smart home demonstrator. The upper part of the image
shows the smart home demonstrator with the smart socket, smart
switch, NFC smart phone, power cable and table fan. The lower part
of the image shows the table fan in ON state, after the wireless switch
or more exactly the push button has been pressed once.
supply battery
secure WSN node
NFC loop antenna
secure
WSN node
NFC
loop antenna
voltage carrying conductor
Fig. 2. Smart socket (left picture) and smart switch (right picture).
The image shows the installation of the PCB based secure WSN node
into the smart home demonstrator appliances.
Figure 2 shows the installation of the secure WSN nodes
into the smart socket and the smart switch that are equipped
Fig. 4.
Block diagram of the secure WSN node: The upper
block diagram depicts for comparison reasons a common WSN
node architecture, while the lower block diagram depicts the system
architecture of the proposed secure WSN node.
By adding the secure element with the NFC interface to
the WSN node, security and configuration aspects are added
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to the node. A detailed explanation about the secure element
with NFC interface is shown in previous publication [2]. With
the secure WSN node it is possible to facilitate cryptographic
key distribution, pairing (peer-to-peer network installation) and
configuration. Figure 5 shows the crypto-key assignment and
pairing process of the smart socket and the smart switch of the
smart home demonstrator. Just by taping the NFC smart phone
to the secure WSN node equipped appliances, smart socket and
smart switch are paired and can securely communicate with
each other via the UHF link.
Step2 : tap switch
Step1 : tap socket
Fig. 5. Crypto-key assignment and pairing process: With the NFC
enabled secure WSN nodes, crypto-key assignment and pairing of
appliances is done just by taping the NFC smart phone to the specific
appliances, i.e., the sockets and the switch.
pre-study WSN node
eWLB wafer
8mm
8mm
Fig. 6. Pre-study WSN node using the eWLB packaging technology:
This design contains a 868 MHz antenna that is implemented as a
planar inverted-F antenna (PIFA) and a current sensor (TLI4970).
The copper traces (interconnect wires and metal layers) are
embedded in the dielectric layers (see Fig. 7), which have
a relative permittivity εr of 3.5 and a loss tangent tan δ
of 0.020 (at 1 GHz). The maximum thickness of the mold
compound is mainly limited by economic aspects, in our case
it is constrained to a size of 1.7 mm.
Since the first generation, the eWLB technology has significantly evolved and supports now:
•
•
•
IV. M INIATURIZATION
The realized WSN node is implemented on a PCB with a
size of 19.4 mm x 23 mm. For an easy and simple integration
of the WSN node, a further miniaturization of the node is
beneficial. Our secure WSN node is a heterogeneous system,
with two chips, passive components, an UHF antenna, and
an NFC loop antenna (see Fig. 3). This complexity requires a
heterogeneous multi layer integration approach. We found that
eWLB packaging technology is able to address our advanced
requirements for a heterogeneous system-in-package (SiP)
WSN node. Figure 6 shows the eWLB wafer containing a
pre-study WSN node design. The pre-study eWLB design has
a transparent dielectric layer, this enables to see the metal
layer of the redistribution layer 1 (RDL 1). The RDL 1 layer
contains the UHF antenna and the current trail and digital
interface wiring of the current sensor.
multi-layer redistribution layers (RDLs),
large package sizes of up to 12 mm x 12 mm, and
double side metal layers at the top and the bottom of
the package for surface mounted device (SMD) pads
implementation.
The previous mentioned properties make the eWLB packaging technology advantageous for RF applications like a
wireless sensor node.
Air
1.7 mm
0.008 mm
0.45 mm
0.8 mm
Air
epoxy
die, flip chip copper trace
dielectric
A. eWLB Packaging
The eWLB packaging technology was developed as an
enhancement to the existing wafer level packaging (WLP)
technology that is constrained in package size and in integration capability [7]. The new technology eWLB allows a
much higher ball count as the package size extends beyond
the size of the chip, which is known as fan-out area [7]. This
fan-out area (see Fig. 7) enables to incorporate radio frequency
(RF) components like antennas and high quality factor passive
components like inductors and capacitors. The package mold
compound is an epoxy material with a relative permittivity
εr of 3.4 and a loss tangent tan δ of 0.003 (at 1 GHz).
PCB
fan-out
SMD pads
Fig. 7. The upper part of the picture shows the ADS substrate
model for the layout design of the eWLB packaging technology based
miniaturized WSN node. The substrate model shows the different
layers of the design with their thickness. The lower part of the picture
shows the schematic cross-section of a eWLB device mounted on a
PCB.
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9.3 mm
cap hat
monopole antenna
loading coil
matching network pads
antenna feed
eWLB Balls
ASIG
11.4 mm
Z
SMD pads
TDA5340
Y
X
PCB GND
Fig. 8. The layout design of the miniaturized WSN node with
a base loaded atenna. The antenna ”cap hat” requires the most
package substrate area enabling a reasonable antenna gain of -5.6 dBi.
The picture shows the different eWLB model layers of the WSN
node, colored by different materials: RDL, silicon, and PCB metal
layer. Additional the chip-dies of the transceiver TDA5340 and the
ASIG microcontroller with a temperature sensor and coil-on-chip HF
antenna for NFC communication are placed on top of the dielectric
substrate. The metal pads for the SMD passive components of the
load network are embedded on the surface of the eWLB package
bottom.
B. Antenna Design
The eWLB layout of the miniaturized WSN node was
designed using the Agilent Advanced Design System (ADS)
program [13], using the microwave simulation Momentum.
The layout design in Fig. 8 shows the WSN node components:
transceiver chip (TDA5340), application specific integrated
grain (ASIG) microcontroller chip, the custom-build 868 MHz
miniaturized monopole antenna, and a PCB.
The eWLB package additionally comprises metal pads for
the mounting of passive components of the class-E amplifier
loading network and blocking caps (see Fig. 8), which additional decreases the antenna efficiency. The NFC loop antenna
(13.56 MHz) is implemented as an coil-on-chip antenna on the
ASIG microcontroller. This implementation is in contrast to
the PCB based WSN node where the NFC loop antenna is an
external component. The ASIG with its on chip antennas is
presented in [9].
An ADS substrate model of the eWLB based miniaturized
WSN node was realized (as seen in Fig. 7) to design the
868 MHz miniaturized monopole. A miniaturized monopole
antenna (dark orange) was designed (see Fig. 8). Our WSN
node transceiver (TDA5340) operates at the 868 MHz UHF
frequency band. The antenna dimension is about a factor
10 smaller than the desired physical length to realize an
efficient antenna at 868 MHz (86.4 mm for λ/4). The challenge
was to reach a reasonable antenna gain, under the condition
to implement a 86.4 mm (λ/4) side-length antenna into a
9.3 mm x 11.4 mm package as can be seen in Fig. 8.
The very small size of the antenna makes the antenna
impedance dominantly capacitive. To electrically lengthen
the antenna we use a loading coil (see Fig. 8). The eWLB
packaging technology offers a toolbox of passive components
with high Q values [8], which we beneficially exploit for a
loading coil. Then, the custom-build monopole provides an
input impedance of 50 Ω exploiting an additional matching
network (see Fig. 8).
To additionally increase the antenna gain the approach of a
“cap hat” was used [12]. The “cap hat” greatly increases
the antenna gain reaching a value of −5.6 dBi for a base
loaded antenna [12]. In comparison the antenna gain was
−9.28 dBi without a “cap hat”. An antenna gain of −5.6 dBi
is a reasonable value considering an antenna area of less than
λ/36 x λ/30 (9.3 mm x 11.4 mm).
V. C ONCLUSIONS
We present a smart home demonstrator which contains
security enhanced WSN nodes. This WSN node has security
and configuration advantages in comparison to state-of-theart implementations, shown by the demonstrator. For further
miniaturization, we present a miniaturized WSN node based
on the novel eWLB packaging technology. We show that using
a ”cap hat” for our small antenna greatly increases the antenna
gain reaching a reasonable gain of −5.6 dBi. The simulated
results show that eWLB packaging is a promising technology
for WSN node applications. These simulation results will be
validated by measurements. The prototype production of the
eWLB miniaturized WSN nodes is already in process and will
be ready till end of April 2015.
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