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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.
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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
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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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