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VISIBLE LIGHT COMMUNICATIONS

CHAPTER: 1

INTRODUCTION

1.1 OPTICAL WIRELESS COMMUNICATION

Optical wireless communication is a technology that uses light propagating in free space

to transmit data for telecommunications or computer networking. This contrasts with

using solids such as optical fiber cable or an optical transmission line. The technology isuseful where the physical connections are impractical due to high costs or other

considerations.

The first indoor optical wireless system was developed in 1979.This system used

the infrared radiation which was spread in all directions. Such systems are called

diffused infrared systems. Since then several products using I radiation have been

successfully commerciali!ed. The advancement of ine"pensive opto#electronic devices$

such as %&'s$ ( I) diodes and avalanche photo#diodes *+('s, and various optical

components$ has resulted in the improvement of these systems. Indoor optical wireless

system s have been used in many applications in the past few years ranging from simple

remote controls in home to more comple" wireless local area networks. -any other

applications are envisaged for the future$ including data networking in the indoor

environment and the delivery of broadband multimedia services to mobile users within

such an environment together with general connectivity to base networks.

'epartment of &&$ -ITS

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1.1.1 Accessing Techniques fo O!"ic#$ Wie$ess co%%unic#"ion

There are multiple access techni/ues which define the way that terminals can

share the infrared medium simultaneously. These techni/ues are performed in the

physical layer. If different users share the infrared medium optimally$ it means their

signals can occupy the same time slot$ code or carrier fre/uency. 0efore going through

the multiple access techni/ues$ it is good to mention some of the infrared media features

as follows

The short wavelength makes it possible to get 2high angular resolution

in an 2angle#diversity receiver3.

4sing a short duty cycle pulse with I- reduces the transmitted power$

which makes the time#division#multiple#access preferred over the other

1. O!"ic#$ Mu$"i!$e&ing Techniques

Optical multiple"ing techni/ues can be divided into two techni/ues

5avelength 'ivision -ultiple +ccess *5'-+, and Space#'ivision -ultiple +ccess

*S'-+,. *i, 5'-+ In this techni/ue$ each transmitter transmits at different

infrared wavelengths using narrow#band emitters such as %'s. The receiver has a band

pass optical filter that e"tracts the wanted infrared wavelengths before the detection

process. The transmitter may be tunable so that it can transmit at different wavelengths

*e.g. tunable %'s,6 however$ such transmitters are currently e"pensive and need comple"

techni/ues to accurately tune them to certain wavelength. +lso large #area tunable band

pass filters such as 2single or multiple#stage abry (erot filters3 are e"pensive and

difficult to manufacture with 2wide# O8 *field of view,3.


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*ii, S'-+

This techni/ue makes use of an angle diversity receiver *having multiple

receiving elements that are directed in different directions, to receive signals from

different directions. or e"ample$ a hub may be capable of establishing a direct line#of#

sight *%OS, with several portable transceivers. This hub can use an angle diversity

receiver to reduce co#channel interference between channels in the same cell.

. E$ec"ic#$ Mu$"i!$e&ing Techniques

5hen different users share the same optical channel$ multiple"ing can be used

to enable reliable transmission. There are three multiple"ing techni/ues vi!.$ Time

#'ivision -ultiple +ccess *T'-+,$ re/uency#'ivision -ultiple +ccess *'-+,

and ode# 'ivision multiple +ccess *'-+,.

*i, T'-+

In this$ the time access is divided into time slots and in each time slot the usertransmits his signal. T'-+ transmissions having low duty cycles reduce the

transmitted power but these re/uire synchroni!ation. The reduction in transmitted power

makes the T'-+ techni/ue popular in wireless infrared systems T'-+ is efficient

under steady flow of information. :owever$ it can be very inefficient for busty

transmission. :ere we will use 5'-+ technology to have efficient.


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*ii, '-+

In this case$ the fre/uency a"is is sliced into fre/uency bands where each

channel occupies a certain band. 4pon reception$ the receiver filters out the

desired signal. '-+ is efficient in steady information flow applications.

:owever$ the power efficiency achieved by '-+ gets poorer as the number of users

increases.

*iii, '-+

This techni/ue assumes that different users use different orthogonal code

se/uences in a way that no interference is encountered. '-+ can be considered as a

hybrid combination of '-+ and T'-+ where multiple users operate at the same

time over the entire bandwidth of the time#fre/uency signal domain. The user signals are

separable since the codes used to modulate their signals are re#generated by the receiver.

There are two common '-+ techni/ues6 'irect Se/uence '-+ *'S#'-+, and

re/uency :opping '-+ *:# '-+,.

1.1. '#sic S(s"e% Configu#"ion

There are two basic configurations communications channels either use 'iffuse !#"hs o

Line Of Sigh" )LOS* !#"hsbetween transmitter and receiver.


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igure 1.1 'iffuse System

igure 1. 5ide %OS System

In a diffuse system an undirected source *usually %ambertian, illuminates the coverage

space$ much as it would be illuminated with artificial lighting. The high reflectivity of

normal building surfaces then scatters the light to create optical ;ether


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igure 1.> ?uasi 'iffuse State

?uasi diffuse systems minimi!e the number of multipath by limiting the surface

reflections$ but allow robust coverage by directing radiation to a number of surfaces so

that a suitable receiver may select a path from one surface only.

The basic inference is done by calculating the path loss and loss at receiver. It only

considers %OS links. The diffuse link model is shown below. or simplicity$ we consider

the angle between receiver center line and source#receiver line$ @$ as !ero. That means the

receiver is always pointing vertically to the ceiling.

igure 1.> %OS 'iffuse %ink -odel for (ath %oss


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rom the figure we can see that that +r donates the eceiver area Ama" is angle of

receiving aperture of signals.

1.+ ,ISI'LE LI-HT COMMUNICATION

8isible %ight ommunication *also known as Optical 5ireless ommunication O ree#

Space Optics,is a technology$ which is a result of the union of Optical ommunication

and 5ireless ommunication. In this communication system$ %&'B%aser 'iodes acts as

the transmitter and (hotodiodes acts as the receiver$ which is same as that in optical

communication. 0ut instead of optical fibers$ natural atmosphere acts as the carrier.

1..1,LC CHARACTERSTICS

The merits and demerits of this technology become apparent once we go through the

characteristics of visible light communication technology#

Hu%#n S#fe"( 8% poses no health ha!ards to human body. Thus$

the transmission power can be kept high if needed.

'#n/i"h 8isible light communications e"ploits the visible

region of electromagnetic spectrum. Thus it offers much larger fre/uencyband *CDD T:!, compared to that available in communications *

CDDE:!,.


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U0iqui"ous N#"ue 5e have a well#established lighting infrastructure through out

the world. In addition to it$ %&' based lighting devices are getting widespread

acceptance round the globe. Since 8% uses the already available visible light sources

for wireless communications$ so it is e"pected to become a ubi/uitous.

Secui"( +s 8% involves line of sight communication$ so it is impossible to tap the

ommunication without breaking the link. So it offers a very secure communication

and can be used in high security military areas where communication is prone to

eavesdropping.

1.. ,LC LIN )2RAMEWOR*

+ 8% link consists of a transmitter$ the propagation channel and a receiver. &ach of

these is described in the following sections.

Souces

5hite#light %&'s either use red$ green and blue %&'s that mi" to provide the

desired color$ or a single %&' *usually blue, that e"cites a yellow phosphor to create an

overall white emission. The ;triplet< approach allows the color to be altered by varying

the color to the %&'s$ and also allows different data to be sent on each device.

:owever$ maintaining color balance can be challenging and the devices are comple".

The single %&' approach is simpler and is therefore more attractive for ;general nm. urves of this measure of sensitivity

are often plotted with the S normali!ed in order to show relative spectral

response.

esponsivit y # e

e is a measure of sensitivity which takes into account the active area of the photodiode

chip. This parameter is obtained by dividing the short#circuit light current *m+#

micro amps, by the energy of the light per unit area *R5Bcm,.

?uantum &fficiency # ?.&.


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If the photodiode operated at 1DD efficiency each photon of light striking the

detector would result in one electron being added to the photocurrent. The ?.&. relates$

as a percentage$ the energy per photon and the /uantum yield$ electrons per photon.

%inearity

Shown below is the e/uivalent circuit for a photodiode. 4nder!ero applied reverse bias

the photocurrent will divide between the internal unction or shunt resistance and the

e"ternal load resistance.

igure . &/uivalent circuit for photodiode


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+.+.+ A4#$#nche Pho"o Dioe

+valanche photodiode detectors have and will continue to be used in many

diverse applications such as laser range finders and photon correlation studies. This paper

discusses +(' structures$ critical performance parameters and the e"cess noise factor.

+.+.+.1 In"ouc"ion

or low#light detection in the DD to 11>D nm range$ the designer has three basic

detector choices # the silicon (I) detector$ the silicon avalanche photodiode *+(', and

the photomultiplier tube *(-T,. +('s are widely used in instrumentation and aerospace

applications$ offering a combination of high speed and high sensitivity unmatched by

(I) detectors$ and /uantum efficiencies at U =DD nm unmatched by (-Ts.

+.+.+.+ APD S"uc"ues

In order to understand why more than one +(' structure e"ists$ it is important to

appreciate the design trade#offs that must be accommodated by the +(' designer. The

ideal +(' would have !ero dark noise$ no e"cess noise$ broad spectral and fre/uency

response$ a gain range from 1 to 1DF or more$ and low cost. -ore simply$ an ideal +('

would be a good (I) photodiode with gainV In reality however$ this is difficult to achieve

because of the need to trade#off conflicting design re/uirements. 5hat some of these

tradeoffs are$ and how they are optimi!ed in commercially available +('s$ are listed

below.


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igure .C &lectrical fields response of +valanche photo diode *+(',

+bove figure shows the reach#through structure patented by (erkin&lmer which offers

the best available combination of high speed$ low noise and capacitance and e"tended red

response.

OPTICAL 2ILTERS AND CONCENTRATERS

eceivers typically employ either long pass or band pass o!"ic#$ fi$"es to

attenuate ambient light. %ong pass filters can be thought of as essentially passing light atall wavelengths beyond the cutoff wavelength. They are usually constructed of colored

glass or plastic$ so that their transmission characteristics are substantially independent of

the angle of incidence. %ong pass filters are used in almost all present commercial

infrared systems.

5here is the signal transmission of the filter $ is the concentrator gain and is the

concentrator O8*semi angle,. 4sually$ )on imaging concentrators

e"hibit a trade#off between gain and O8. +n ideali!ed non imaging concentrator having

an internal refractive inde" achieves a gain


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we see that as the O8 is reduced$ the gain within the O8 is increased.

The hemispherical lens is an important non imaging concentrator and is widely employed

in commercial systems. It achieves a wide O8 and omnidirectional gain$ making itespecially suitable for use in non directed links. + hemisphere can achieve

over its entire O8. 5hile this is sometimes called 2omnidirectional gain$3 a

hemisphere#based receiver is not truly omnidirectional$ but has an effective area

. 5hen long pass filtering is employed$ a planar long pass filter can

be placed between the hemisphere and the detector$ as shown in figure .

igure.=)on imaging optical concentrators hemisphere with planar optical

filter

+.5 SI-NAL 2ADIN-

+.5.1 RA3LEI-H 2ADIN-

Often$ the gain and phase elements of a channelWs distortion are conveniently

represented as a comple" number. In this case$ ayleigh fading is e"hibited by the

assumption that the real and imaginary parts of the response are modeled by

independent and identically distributed !ero#mean Eaussian processes so that the

amplitude of the response is the sum of two such processes.

+.5.+ RICIAN 2ADIN-

ician fading is a stochastic model for radio propagation anomaly caused by

partial cancellation of a radio signal by itself X the signal arrives at the receiver by

several different paths *hence e"hibiting multipath interference,$ and at least one of the

paths is changing *lengthening or shortening,. ician fading occurs when one of


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the paths$ typically a line of sight signal$ is much stronger than the others. In ician

fading$ the amplitude gain is characteri!ed by a ician distribution.


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CHAPTER :5

CONCEPTS O2 ,LC

5.1 MULTIPLE INPUT MULTIPLE OUTPUT )MIMO*

The idea of using multiple receive and multiple transmit antennas has emerged as

one of the most significant technical breakthroughs in modern wireless communications.

-I-O is the use of multiple antennas at both the transmitter and receiver to improve

communication performance. It is one of several forms of smart antenna technology.

-I-O technology has attracted attention in wireless communications$ because it offers

significant increases in data throughput and link range without re/uiring additional

bandwidth or transmit power. This is achieved by higher spectral efficiency and link

reliability or diversity

ig C.1 'ifferent antenna system

5.1.1 E3 'ENE2ITS

'epartment of &&$ -ITS Page 25


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(1)+rr a y g a in+rray gain means a power gain of signals that is achieved by using multiple#

antennas at transmitter andBor receiver. It is the average increase in the S) at the

receiver that arises from the coherent combining effect of multiple antennas at the

receiver or transmitter or both. If the channel is known to the transmitter with multiple

antennas$ the transmitter can apply appropriate weight to the transmission$ so

that there is coherent combining at the receiver. The array gain in this case is called

transmitter array gain. &ssentially$ multiple antenna systems re/uire some level of

channel knowledge either at the transmitter or receiver or both to achieve this array gain.

(2)'iv e rsi t y g a inIn a wireless channel$ signals can e"perience fadings. 5hen the signal power

drops significantly$ the channel is said to be in a fade and this gives rise to high 0&.

'iversity is a powerful techni/ue to mitigate fading in wireless links$ so diversity is

often used to combat fading. 'iversity techni/ues rely on transmitting the signal over

multiple *ideally,

(3)-ultipl e " ing g a inSpatial multiple"ing gain is achieved when a system is transmitting different

streams of data from the same radio resource in separate spatial dimensions. 'ata is

hence sent and received over multiple channels # linked to different pilot signals$

over multiple antennas. This results in capacity gain at no additional power or

bandwidth.

5.1.+ MIMO s(s"e%

*i, MIMO ch#nne$ %oe$

5e consider -I-O channels with NT transmit and NR receive antennas. The

block diagram of such a -I-O channel model is shown in .The channel matri" H is

acomple" matri" with

jth transmit antenna to the ith receive antenna. 5e suppose that the power of the

received signal for each receive antennas is e/ual to the sum of transmit power Es.



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onse/uently$ we ac/uire the normali!ation value of the channel matri" H$ for a

deterministic channel condition as follow$

If the channel coefficients are random$ the normali!ation value will apply to the e"pected

value. The received signal at ith receive antenna is given by

.

mutual informationI(s;() in e/uation is given by$

where$ INRis an identity matri"$ ES is the power across the transmitter

irrespective of the number of antennas NT $ RSS is the covariance matri" for

transmit signal and the superscript : stands for conugate transposition. rom e/uation$

the general capacity of the -I-O channel is



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ig C.C eceiver structure for space time block coding

5.1.5 A!!$ic#"ions of MIMO

Spatial multiple"ing techni/ues make the receivers very comple"$ and thereforethey are typically combined with O'- where the problems created by a multi#

path channel are handled efficiently. The I&&& HD.1Fe standard incorporates

-I-O#O'-+.

-I-O is also used in -obile radio telephone standards such recent CE((

and CE((.In CE(($ :igh#Speed (acket +ccess plus *:S(+Y, and %ong

Term &volution *%T&,.

5.1.6 MIMO #n ,LC

In the application considered here each individual %&' has very limited

bandwidth$ but there are many available for data transmission$ but it is not possible to

precisely align a detector and receiver array as the receiver moves around the coverage



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area. -I-O provides an opportunity to do this$ we consider two 0& room distribution for

)+* IMA-IN- DI,ERSIT3 OPTICAL MIMO S3STEM

igure shows a = channel visible#light indoor optical wireless system. +n

imaging receiver Z1F[ is used in place of the non imaging devices. %ight propagates from

the four transmitter %&' arrays to the receiver as before$ and each %&' array is imaged

onto a detector array$ where images may strike any pi"els or group of pi"els on the

array$ and be in arbitrary alignment with them.

RE2ERENCES



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Z1[ Toshihiko omine and -asao )akagawa 2undamental +nalysis for8isible#

%ight ommunication System using %&' %ights3 I&&& Transactions on onsumer

&lectronics$ 8ol. >D$ )o. 1$ &04+\ DD=.

Z[ \. Tanaka$ S. :aruyama and -. )akagawa$ 25ireless optical transmission with

the white colored %&' for the wireless home links$3(roc. of the 11th Int.

Symposium on (ersonal$ Indoor and -obile adio ommunications *(I-

DDD,$ %ondon$ 4S$ pp.1C>#1C9$DDD.

ZC[ . . Efeller and 4. 0apst$ 25ireless in#house data communication via

diffuse

infrared radiation$3 (roc. I&&&$ vol. F7$ no. 11$ pp. 1=7=#1=HF$1979.

Z=[ +. (. Tang$ ]. -. han and . (. :o$ 25ireless Infrared ommunication %inks

4sing -ulti#0eam Transmitters and Imaging eceivers$3 (roc. I&&& Int. onf. on

ommunications$ pp. 1HD#1HF$ 'allas$ T^$ ]une 199F.

Z>[ Talha +. han$ -uhammad Tahir and +hmad 4sman 28isible %ight

ommunication using 5avelength'ivision -ultiple"ing for Smart Spaces3$

Science$ vol. CDH$ no. >7F$ p. 17=$ DD>.

ZF[ T. omine and -. )akagawa$ 2undamental analysis for visible#light

communication

system using %&' lights$3 I&&& transactions on onsumer&lectronics$ vol. >D$ no.

1$ p.

1DD$ DD=.


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