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ABSTRACT

Dragging the world towards wireless galaxy various sensors are already in a

broad use Today as part of different devices or as standalone devices connected to a

network usually toMonitor industrial processes, equipments or installations.

The advancements in technology, wireless communications have

enhanced development of small, low power and low cost devices. Such devices when

organized into a network, present a powerful platform that can be used in many

interesting applications.

Bluetooth is a low cost, short-range, wireless technology with small

footprint, low power consumption and reasonable throughput. Bluetooth wireless

technology has become global technology specification for always on wireless

communication not just as point to point but was a network technology as well.

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List Of Figures

NAME Pg.No.

1. Piconet.. 112. A Scatter net. 123. Wireless sensor network Architecture.. 144. A Bluetooth based smart pressure sensor node 165. Software Architecture of Gateway 186. Bluetooth hardware architecture 21

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SPREAD SPECTRUM

Basic principle:

Spread Spectrum modulation techniques are defined as being those techniques

in which The bandwidth of the transmitted signal is much greater than the bandwidth

of the original message, and the bandwidth of the transmitted signal is determined by

the message to be transmitted and by an additional signal known as the Spreading

Code.

TYPES OF SPREAD SPECTRUM

1. Frequency hopping Spread Spectrum2. Direct sequence Spread Spectrum

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FREQUENCY HOPPING SPREAD SPECTRUM

FHSS is a very robust technology, with little influence from noises, reflections, other radio

stations or other environment factors. In addition, the number of simultaneously active systems in

the same geographic area (collocated systems) is significantly higher than the

equivalent number for DSSS systems.

All these features make the FHSS technology the one to be selected for installations

designed to cover big areas where a big number of collocated systems is required and where the

use of directional antennas in order to minimize environment factors influence is impossible.

Typical applications for FHSS include cellular deployments for fixed Broadband Wireless

Access (BWA), where the use of DSSS is virtually impossible because of its limitations.

Basic Principle

The frequency of the carrier is periodically modified (hopped) following a specific

sequence of frequencies.

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FHSS main parameters

FHSS is defined (in IEEE 802.11) in the 2.4 GHz band as operating over 79 frequencies

ranging from 2.402 GHz to 2.480 GHz (country specific bands have different frequencies, defined

in IEEE 802.11 and IEEE 802.11.d).

Each of the frequencies is GFSK modulated, with a channel width of 1 MHz. The rates

defined are 1 Mbps and 2 Mbps (there are products in the market operating at 3 Mbps,

too)

System Behavior using FHSS

1.- Systems Collocation

2.- Noise and Interference Immunity

3.- The Near / Far problem

4.- Throughput

5.- Multipath Immunity

6.- Time and frequency diversity

7.- Security

8.- Bluetooth interference

Systems Collocation

The issue: How many independent systems may operate simultaneously without

interference?

For FHSS systems, IEEE 802.11 defines 79 different hops for the carrier

frequency. Using these 79 frequencies, IEEE 802.11 defines 78 hopping sequences (each with 79

hops) grouped in three sets of 26 sequences each. Sequences from same set encounter minimum

collisions and therefore may be allocated to collocated systems.

Theoretically, 26 FHSS systems may be collocated, but collisions will still occur

in significant amounts. To lower the amount of collisions to acceptable

levels, the actual number of FHSS collocated systems should be around 15.

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All the above is correct for the case in which the FHSS collocated systems operate independently,

without any synchronization among their hopping sequences.

If synchronization is allowed, 79 systems could be collocated (theoretically), each one of

themusing at any moment in time, one of the 79 available frequencies. However, this would

require expensive filters in the radio circuitry.

Actual products require about 6 MHz separation allowing the collocation of about 12

systems, without any collision! While such synchronization is not always allowed in the unlicensed

band of 2.4 GHz, it is common practice in the licensed bands.

The possibility of having collocated systems without collisions, has a tremendous impact

on the aggregate capacity / throughput of the installation as well as its efficiency in terms of bps

per Hz (see Throughput section, later in this paper).

For installations requiring big coverage and multiple collocated cells, it would

be much easier to use FHSS. DSSS could be used, too, but then, mechanically collocated cells

(antennas installed on same pole) should be made non overlapping cells at the radio level

through the use of directional antennas But directional antennas means limited coverage

requiring more systems to be installed which are difficult to design because of the collocation

issueThis severe limitation of DSSS is in effect for the 2Mbps flavor of DSSS as well as for the

11Mbpsone.

2. Noise and Interference Immunity

The issue: Capability to operate when other radio signals are present in the same band.

FHSS systems operate with SNR (Signal to Noise Ratio) of about 18 dB.DSSS systems,

because of the more efficient modulation technique used (PSK), can operate with SNR as low as 12

dB.

3- Near / Far problem

The issue: The problems generated to a receiver by other active transmitters

located in its proximity, are known as Near / Far problems.

The interfering signals described above may be generated for example by a foreign radio

transmitter located close to my receiver. The signals generated by the foreign transmitter,

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being received by my receiver at higher power levels, could blind it , making it unable to hear its

partner.

While the problem is critical in DSSS systems, if the receiver is FHSS, the worst

case will be that the foreign transmitter will block SOME hops, forcing my FHSS system to

work in less than optimum conditions, but allowing it to work!

4- Throughput

The issue: What amount of data is actually carried by the system (measured in bps).

The RATE of a system is defined as the amount of data (per second) carried by a system

WHEN IT IS ACTIVE. As most communications systems are not able to carry data 100% of the

time, an additional parameter - the THROUGHPUT

It is defined, as the AVERAGE amount of data (per second) carried by the system. The

average is calculated over long periods of time. Obviously, the throughput of a system is lower than

its rate.

In addition, when looking for the amount of data carried, the overhead introduced by the

communication protocol should be considered.

5- Multipath

The issue: Environments with reflective surfaces (such as buildings, office walls, etc.)

generate multiple possible propagation paths between transmitter and receiver and therefore the

receiver receives multiple copies of the original (transmitted) signal, shifted in time.

The multiple copies of the original signal arrive at the receiver with different instantaneous

amplitudes and phases. The mixing of these copies at the receiver results in having some

frequencies canceling one another, while other frequencies will sum up. The result is a process of

selective fading of frequencies in the spectrum of the received signal.

FHSS systems operate with narrow band signals located around different carrier

frequencies.Ifat a specific moment, the FHSS system is using a carrier frequency significantly

faded as a result of multipath, the FHSS receiver could not get enough energy to detect the radiosignal. (narrow rectangle in fig.3b). The resultant loss of information is corrected by re-

transmitting the lost packets.

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6- Time and frequency diversity

Both DSSS and FHSS retransmit lost packets, until the receiving part acknowledges correct

reception. A packet could be lost because of noises or multipath effects.

This capability of a system to repeat unsuccessful transmissions at later moments in time is

known as time diversity.

DSSS systems use time diversity, but the problem is that they retransmit on the

same 22 MHz sub band! If the noise is still there or if the topography of the site did not change,

and as a result the multipath effects will be again present, the transmission could be again

unsuccessful! The multipath effects are a function of frequency.

For same topography, some frequencies encounter multipath effects, while others do not.

FHSS systems use time diversity (they retransmit lost packets at later moments in time)

but they also use frequency diversity (packets may be retransmitted on different frequencies /

hops). Even if some hops (frequencies) encounter multipath effects or noises, others will not, and

the FHSS system will succeed in executing its transmission.

7.- SecurityThe issue: Protecting the transmission against eavesdropping IEEE 802.11 compliant DSSS

systems use one well known spreading sequence of 11 chips, andcan modulate one of the 14

channel defined in the standard. As the sequence used is known, the carrier frequency is fixed for a

given system, and the number of possible frequencies is limited, it would be quite easy for a

listener to tune in on the DSSS transmission.

Message protection should be achieved by encrypting the data. This option increases the

price of the product, while lowering its performance, because of the processing power needed for

the encryption process.

In FHSS, the frequencies to be used in the hopping sequence may be selected by the

user. In the unlicensed band, any group of 26 frequencies or more (out of the 79 available) is

legal. To tune in, a listener should know the number of frequencies selected in the system,

the actual

frequencies, the hopping sequence, as well as the dwell time! The FHSS modulation acts as a

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layer 1 encryption process. There could be no need for application level encryption!

8.- Bluetooth / IEEE 802.15 WPAN interference

The issue: System behavior in the presence of Bluetooth / IEEE 802.15 collocated systemsBluetooth radio is a FHSS operating in the 2.4 GHz band, with a dwell time of 0.625ms (1,600

hops per second).

When DSSS executes a transmission, it is using 22 MHz for the duration of the transmitted

frame. When FHSS executes a transmission, it is using 1 MHz for the duration of the transmitted

frame. The chances of having Bluetooth hitting the 22 MHz band used by DSSS are higher than

the chances of it hitting the 1 MHz band used by FHSS.

Based on the above observation, we can conclude that, basically, DSSS is more sensitive to

Bluetooth interference than FHSS.

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BLUETOOTH

Bluetooth operates in the unlicensed ISM band at 2.4 GHz frequency band and use

frequency hopping spread spectrum technique. A typical blue tooth device has a range of about 10meters and can be extended to 100 meters. A Communication channels supports total bandwidth of

1 Mb/sec. A single connection supports a maximum asymmetric data transfer rate of 721 Kbps

maximum of three channels.

BLUETOOTH-NETWORKS

In Bluetooth, a piconet is a collection of up to 8 devices that frequencies hop together. Each

piconet has one master usually a device that initiated establishment of the piconet, and up to 7 slave

devices. Masters blue tooth address is used for definition of the frequency hopping sequence. Slave

devices use the masters clock to synchronize their clocks to be able to hop simultaneously.

APICONET

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When a device wants to establishment a piconet it has to perform inquiry to discover other

blue tooth devices in the range. Inquiry procedure is defined in such a way to ensure that two

devices will after some time, visit the same frequency same time when that happens, required

information is exchanged and devices can use paging procedure to establish connection.

When more than 7 devices need to communicate, there are two options. The first one is to

put one or more devices into the park state. Blue tooth defines three low power modes sniff, hold,

and park. When a device is in the park mode then it disassociates from and piconet, but still

maintains timing synchronization with it. Master of the piconet periodically broadcasts beacons

(warning) to invite the slave to rejoin the piconet or to allow the slave to rejoin. The slave can rejoin

the piconet only if there are less than seven slaves already in the piconet. If not so, the master has to

park one of the active slaves first. All these actions cause delay and for some applications it can be

unacceptable for eg : process control applications, that requires response from the command center

(central control room).

Scatter net consists of several piconets connected by devices participating in multiple

piconet. These devices can be slaves in all piconet or master in one piconet and slave in other

piconet. Using scatter net higher throughput is available and multi-hop connections between devices

in different piconets are possible. i.e. theunit can communicate in one piconet at time so they jump

from pioneer to another depending upon the channel parameter.

A SCATTER NET

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BLUETOOTH BASED SMART SENSOR NETWORKS

The main challenge in front of Bluetooth developers now is to prove interoperability

between different manufactures devices and to provide numerous interesting applications. One of

such applications is wireless sensor networks.

Wireless sensor networks comprise number of small devices equipped with a sensing unit,

microprocessor, and wireless communication interface and power source.

1. An important feature of wireless sensor networks is collaboration of network nodes duringthe task execution.

2. Another specific characteristics of wireless sensor network is Data-centric natureAs deployment of smart sensor nodes is not planned in advanced and positions of nodes in

the field are not determined, it could happen that some sensor nodes end in such positions that they

either cannot perform required measurement or the error probability is high. For that a redundant

number of smart nodes are deployed in this field. These nodes then communicate, collaborate and

share data, thus ensuring better results.

Smart sensor nodes scattered in the field, collect data and send it to users via

gateway using multiple hop routes.

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A WIRELESS SENSOR NETWORK

The main functions of a gateway are

1. Communication with sensor networks Shortage wireless communication is used. It provides functions like discovery of smart sensor nodes, generic methods of

sending and receiving data to and from sensors, routing.

2. Gateway logic It controls gateway interfaces and data flow and from sensor network. It provides an abstraction level that describes the existing sensors and their

characteristics.

It provides functions for uniform access to sensors regardless of their type,

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location or network topology, inject queries and tasks and collect replies.

3. Communication with users Gateway communication with users or other sensor networks over the internet,

WAN, satellite or some shortage communication technology.

From the user point of view, querying and tasking are two main services provided bywireless sensor networks.

Queries are used when user requires only the current value of the observedphenomenon.

Tasking is a more complex operation and is used when a phenomenon has to be observe

Over a large period of time. Both queries and tasks of time to the network by the gateway which

also collects replies and forwards them to users.

SENSOR NETWORK IMPLEMENTATION

The main goal of our implementation was to build a hardware platform and generic software

solutions that can serve as the basis and a test bench for the research of wireless sensor network

protocols.

Implemented sensor network consists of several nodes and a gateway. Each smart node can

have several sensors and is equipped with a micro controlled and blue tooth radio module.

Gateway and smart nodes are members of the piconet and hence maximum seven smartnodes can exist simultaneously in the network.For example, a pressure sensor is implemented, as

Bluetooth node in a following way.

The sensor is connected to the Bluetooth node consists of the pressure sensing element,

smart signal-conditioning circuitry including calibration and temperature compensation, and the

Transducer Electronic Data Sheet (TEDS). These features are built directly into the sensor

microcontroller used for node communication control plus memory for TEDS configuration

information.

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SMART SENSOR NODE ARCHITECTURE

The architecture shown in figure can easily be developed for specific sensor configurations

such as thermocouples, strain gauges, and other sensor technologies and can include sensor signal

conditioning as well as communications functions.

Conditioned along sensor signal is digitized and digital data is then processed using stored

TEDS data. The pressure sensor node collects data from multiple sensors and transmits the data via

Bluetooth wireless communications in the 2.4GHz base band to a network hub or other internet

appliance such as a computer.

The node can supply excitation to each sensor, or external sensor power can be supplied. Up

to eight channels are available on each node for analog inputs as well as digital output. The sensor

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Implementation Of Gateways

Complete software functionality is developed in java

It can be implemented by the use of either bluetooth PCMCIA CARD or using infraredconnection

Java community undertook the first effort towards bluetooth stack API application Ex-JSR-82 JABWT

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Applications of Smart Sensor Networks

Military applications Monitoring friendly forces, equipment and communition Reconnaissance of opposing forces and terrain Battlefield surveillance Battle damage assessment Nuclear, biological and chemical attack detectionEnvironmental applications Forest fire detection Biocomplexity mapping of the environment Flood detection and Precision agricultureHealth applications Tele-monitoring of human physiological data Tracking and monitoring patients and doctors inside a hospital Drug administration in hospitalsIndutrial safety Monitoring building and vehicle Managing inventory control Monitoring the status of different machines in factories,along with the air pllution or fire

monitoring.

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Bluetooth module Hardware Architecture

The CPU core allows the blue tooth module to handle inquiries and filter page request

without involving the host device. The host controller can be programmed to answer certain page

messages and authenticate remote links. The link manager(LM) software runs on the CPU core.

The LM discovers other remote LMs and communicates with them via the link manager protocol

(LMP) to perform its service provider role using the services of the underlying LC. The link

manager is a software function that uses the services of the link controller to perform link setup,

authentication, link configuration, and other protocols. Depending on the implementation, the

link controller and link manager functions may not reside in the same processor.

Another function component is of course, the antenna, which may be integrated on

thePCB or come as a standalone item. A fully implemented blue tooth module also

incorporateshigher-level software protocols, which govern the functionality and interoperability

with othermodules.

Gate way plays the role of the Piconets master in the sensor network. It controls

establishments of the network, gathers information about the existing smart sensor nodes and

sensor attached to them and provides access to them.

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Discovery of the smart sensor nodes

Smart sensor node discovery is the first procedure that is executed upon the gateway

installation. It goals to discover all sensor nodes in the area and to build a list of sensors

characteristics and network topology. Afterwards, it is executed periodically to facilitate

addition of new or removal of the existing sensors. The following algorithm is proposed.

When the gateway is initialized, it performs bluetooth inquiry procedure. When the blue

tooth device is discovered, the major and minor device classes are checked. These parameters are

set by each smart node to define type of the device and type of the attached sensors. Service

class field can be used to give some additional description of offered services. If discovered

device is not smart node it is discarded. Otherwise service database of the discovered smart node

is searched for sensor services. As currently there is no specific sensor profile, then database is

searched for the serial port profile connection parameters. Once connection strings is obtained from

the device. Blue tooth link is established and data exchange with smart mode can start.

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CONCLUSION

Blue tooth represents a great chance for sensor-networked architecture. This

architecture heralds wireless future for home and also for industrial implementation.

With a blue tooth RF link, users only need to bring the devices within range, and the

devices will automatically link up and exchange information.

Thus implementation of blue tooth technology for sensor networks not only cuts wiring

cost but also integrates the industrial environment to smarter environment.

Today, with a broader specifications and a renewed concentration on

interoperability, manufacturers are ready to forge ahead and take blue toothproducts tothe

market place. Embedded design can incorporate the blue tooth wireless technology into a

range of new products to meet the growing demand for connected information appliances.

FUTURE TASKS

-enabled data concentrator

for data acquisition and analysis.

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REFERENCES

G.I.Pottie, W.J.KaiserWireless Integrated network sensors,Communications of the

ACM, May 2002.

C.Shen, C.Srisathapomphatsensor networking architecture and application, IEEC

personal communication. Aug,2001.

r networks, RTCBPA, June 2003.

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