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Another possibility is a patch antenna. It consists in a very low profile metal structure able to work very close to the ground plane. This type of antenna can be built in the PCB used and occupies part of the available space.
220.127.116.11. Ceramic antennas These types of antennas, also known as chip antennas, which are intended to minimise the space occupied by the antenna and they are usually mounted on the PCB. They use a ceramic material with a higher dielectric constant (compared to the one of the air) and lower losses.
They present usually low antenna gain (poor radiation efficiency) since they perform worse than an isotropic antenna in terms of gain. Also, they are very sensitive to surrounding components, so it is very important to follow the recommendations from the manufacturer for mounting it on a PCB.
18.104.22.168. Baluns The balun is an element used to convert a single-ended (unbalanced) input signal to a balanced output signal with impedance transformation.
Therefore, a balun solves the connection of lines with different impedances and provide a balanced output. Baluns can be built using the PCB itself or as a small ceramic component. Some SiP chips incorporate the balun reducing the number of external components.
22.214.171.124. Antenna connectors If the antenna is not made on the PCB or soldered directly, an antenna connector is required. If the antenna is separated from the board, an RF cable is needed.
The usage of a cable facilitates the location of the antenna in relation to the board. A special attention should be paid to the losses and cost of the cable. They are usually manufactured as small length cables (e.g. 5 to 10 cm long) with connectors and are referred to as “pigtail” cables.
The most common connectors are the SubMiniature version A (SMA) and the miniature RF connector called “U.FL”. See Fig. 5.7 for an example of both connectors.
5.1.5. Sensor and actuator devices The availability of sensor devices that can be used in a sensor node is increasing thanks to miniaturisation and price reduction. The main problem related to sensors is the different formats and interfaces required. Each sensor requires a supply voltage and delivers the data in analog or digital format. If analog measurement is provided, this can be in the form of voltage or current. The relevant data for the user (substance concentration, temperature, acceleration, etc.) should be derived thanks to a curve provided by the manufacturer. In some cases, this relation is linear and then the conversion is quite simple, but is quite common to find non-linear functions. It is also common to give this curve in relative changes, so the sensor node should be calibrated.
If the sensor has to be changed after a first calibration, a new curve should be adjusted with a new calibration. There are initiatives to avoid most of these problems, such as the IEEE 1451 standard  and the offering from manufacturers to give a wide set of sensors with a unique interface such as phidgets .
There is a large variety of sensors (and actuators). The following is a nonexhaustive list that aims at introducing the most common sensors.
• Temperature. This type of sensor can be based in a thermistor (a device able to change the resistance according to the temperature) or a thermocouple that is a junction of metals that produces a voltage when they have different temperature.
• Air humidity. Also known as hydrometer, it measures the relative humidity of the air. At present, it can be implemented using a resistive material that changes according to the humidity of the air.
• Light. It measures the amount of visible light. It can be based on a photodiode or a photo resistor.
Fig. 5.8. Boards for the Mica2, MicaZ and Iris with sensors
• Acceleration. It measures the acceleration produced by a movement. It is based on piezoelectric, magnetic induction or capacity effect to mention the most common ones. It can provide information from one dimension, two or three. It can be used to measure vibration or as mobility detector.
Also, as the gravity force appears as a type of acceleration it is possible to use this type of sensor to measure the inclination of the sensor.
• Presence. It is implemented mainly using passive infrared (PIR). It consists in an integrated circuit able to receive infrared light and to measure differences between them. If a person enters in the range of the sensor, the heat differences (that result in an infrared emission) will be detected indicating the presence of a person.
• Force. It measures the force applied to the sensor. It can be based on the variation of resistance when pressure is applied.
• Sound. This consists in a microphone able to transform an audible pressure signal into an electrical magnitude.
• Soil moisture. Used in gardening and agriculture, this type of device should provide the content of water in soil. One of the most common solutions for this type of sensor is based on the capacitance variation at different frequencies.
• Proximity. This is a sensor able to detect the presence of a close object without getting in touch with it. It is based on an electrostatic or magnetic field that is modified by the nearby object. It is clear that the last one is only affected by metallic objects.
Chapter 5. Sensor node platforms
• Magnetic field. Also called magnetometer, it measures the magnetic field strength. It can be used as a compass based on the magnetic field of the earth or to detect the proximity of a metallic object that modifies the magnetic field. This type of sensor can be scalar (measuring the total magnetic field) or vectorial (measuring values of each component of the field).
• Ultrasound receiver. Similar to a microphone, this device is able to convert an ultrasound (higher than 20 kHz) pressure wave into an electrical signal. Usually, this type of sensor uses the 40 kHz frequency, but it is also possible to find devices working at the frequency of 235 kHz.
They are used to receive reflections from pulses generated by an ultrasound transmitter and detecting the presence of obstacles.
• Infrared receiver. It produces a signal proportional to the amount of infrared radiation received. It is based on a photodiode.
• Gas sensors. They can be implemented in several ways, but the most attractive ones are the chemical ones, which require low power consumption and present small size. A drawback is that they require replacement after several years of usage. Common gas sensors are able to detect a variety of gases such as CO, CO2, NO2, CH4, NH3, O2 or smoke.
The following is a non-exhaustive list of actuators.
• Electric switch. It can be based on a simple on-off switch or a dimmer able to control the power supply to the controlled device.
The simplest controlled device can be a light bulb, but for example door locks, blinds or a heater can be controlled with this simple device.
• Ultrasound transmitter. This is a device able to send an ultrasound signal when commanded. This device is used in combination with an ultrasound sensor to detect obstacles or to perform distance measurements.
• Infrared transmitter. This is a device able to send an infrared signal when commanded. This device is used in combination with an infrared sensor to detect obstacle or to perform distance measurements.
As it has been mentioned, there exists a variety of interfaces for sensors.
One of the most used is the 4-20 mA loop for analog signals. It is particularly interesting to transport signals through long lines. The 0 value from the analog signal corresponds to a current of 4 mA. This trick allows verifying the loop and detecting when it breaks.
Another analog interface used is the 0-1 Volt one. The voltage level represents the measured magnitude.
5.2. Sensor node router and gateway The sensor nodes platforms are able to be adapted to offer either routing within the WSN or gateway functionality towards networks outside the WSN.
The common gateway modules available support: USB, WiFi, serial port, GSM/GPRS, Bluetooth connectivity, etc.
5.3. Sensor node platforms To facilitate size and price reduction, sensor nodes should be based on integrated circuits (ICs), reducing the additional components needed (which is also known as lowering the system BOM15). With this aim, some manufacturers offer one single IC with the processor, memory and the radio transceiver. This approach is named System on Chip (SoC) and allows very compact designs, but combining radio elements with processing elements results in suboptimal designs. Fig. 5.9 is an example of a SoC from Chipcon (now Texas Instruments) .
BOM stands for Bill Of Materials and refers to the component count and cost of the whole system.
Another approach that allows optimising the IC technologies for the specific purpose is the System in Package (SiP). Different ICs are mounted on the same substrate and encapsulated as a single IC. An example of this solution is offered by Freescale with the MC13224V IC.
Other wireless sensor providers, which are not IC manufacturers, mount the different elements on a Printed Circuit Board (PCB) to build a module. The module has a set of connectors to facilitate the integration to a main PCB and requires no additional components to work. Some of these modules offer a simple communications serial interface based on AT commands. Fig. 5.10 shows a module made by MeshNetics (now Atmel) . This module uses one processor and a transceiver from Atmel. Other modules are the Xbee family from Digi  and the modules from Jennic , Radiocraft , Telegesis , or from RF Monolitics  to mention some of the currently available ones.
There exist other manufacturers in the market that offer a fully operational sensor node including batteries, antenna and some sensor devices.
The most well known family of systems is the one developed by University of California (UC) Berkeley such as Rene, Mica, MicaDot, Mica2 and MicaZ  (see Fig. 5.11). Some of them are commercialised by Crossbow. Another evolution from the designs of UC Berkeley is the MoteIV, known as TelosB.
 In fact some of these designs, such as TelosB, are an open hardware platform and there exist some similar platforms from several manufacturers such as Zolertia  or MAXFOR . These platforms are intended mainly as a platform for educational purposes and development. Another quite popular platform is iMote2 , developed initially by Intel and commercialised by Crossbow. This platform represents an evolution on performance offering more processing power than the previously mentioned nodes.
Fig. 5.11. Images of different full operational platforms. From left to right:
the MicaDot, MicaZ and Iris sensor nodes
Other families of sensor nodes worth to mention are:
• Arduino platform. This is also an open hardware platform with multiple designs  and a large developers community.
• SunSpot is a platform used to develop research projects .
• WaspMote is a general platform with multiple available sensors and communications facilities .
• Sensinode  and Arch Rock  are companies offering a complete solution including sensor nodes and gateways. The most relevant aspect of these companies is the support of IP connectivity using 6LoWPAN.
REFERENCES Meshnetics modules: http://www.meshnetics.com/zigbee-modules/  Crossbow sensor nodes:
http://www.xbow.com/home/wHomePage.aspx  Zolertia website: http://www.zolertia.com/  MAXFOR website: http://www.maxfor.co.kr/eng/index_en.html  Modules from Digi Networks: http://www.digi.com/products/wireless/  Modules from Jennic: http://www.jennic.com/products/modules/  Modules from Radiocraft: http://www.radiocrafts.com/  Modules from Telegesis: http://www.telegesis.com/  Modules from RF Monolitics:
http://www.rfm.com/products/oem_standalone.php  Arduino project website: http://www.arduino.cc/  WaspMote from Libelium: http://www.libelium.com/products/waspmote  Smart Dust project:
http://robotics.eecs.berkeley.edu/~pister/SmartDust/  Atmel MCUs: http://support.atmel.no/bin/customer  Chipcon MCUs:
http://focus.ti.com/mcu/docs/mcuhome.tsp?sectionId=101  Freescale MCUs: http://www.freescale.com  RF circuits from TI.
http://focus.ti.com/paramsearch/docs/parametricsearch.tsp?family=a nalog&familyId=368&uiTemplateId=NODE_STRY_PGE_T  RF circuits from Nordic.
 SiP from Freescale.
http://www.freescale.com/files/rf_if/doc/data_sheet/MC1322x.pdf?p spll=1  SoC from Chipcon. http://focus.ti.com/lit/ds/symlink/cc2430.pdf  RF transceiver from Radiopulse.
http://www.radiopulse.co.kr/eng/main.html?mode=02_05  Sensor node antennas.
www.freescale.com/files/rf_if/doc/app_note/AN2731.pdf  IEEE 1451 web page. ieee1451.nist.gov  Phidgets. http://www.phidgets.com/  SunSpot platform. http://www.sunspotworld.com/  Sensinode platform. http://www.sensinode.com/EN/company.html  Arch Rock platform. http://www.archrock.com/
6. Topology control Topology control is an important technique used in WSNs for optimizing key performance parameters, such as network lifetime or throughput, while maintaining other performance parameters such as network connectivity.
The main topology control approaches can be classified into two categories: