Simulation and DSP Board Implementation of an Optical Transmission System using OFDM A Bachelor of Science thesis by Vinod Patmanathan [email protected]A thesis submitted in partial satisfaction of the requirements for the degree of Bachelor of Science (BSc.) in the School of Engineering & Science of the INTERNATIONAL UNIVERSITY BREMEN Supervisor: Harald Haas Spring Semester 2004
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Simulation and DSP Board Implementation of an Optical TransmissionSystem using OFDM
The future of lighting would be one using white LEDs. Noted for their high efficiency, LEDs are
much more efficient than their incandescent fluorescent lamps, and tungsten filament counter parts.
White LEDs have a high power output and are expected to serve in the next generation of lighting.
It is suggested that these devices would not just be used for the illumination of rooms, but also
for an optical wireless communications system. This system would be suitable for private networks
such as consumer communications networks. However it still remains necessary to investigate the
properties of white LEDs when they are used as optical transmitters. This is because LEDs have
an inherent non-linear transfer function, and might pose problems in specific systems. In this paper,
an OFDM (Orthogonal Frequency Division Multiplexing) based optical communications system is
proposed. The OFDM time signal is very susceptible to non-linearities. The non-linear effects of
the LEDs are simulated and the effect of this non-linearity is analyzed. The LEDs have an inherent
property that their forward voltage and their forward current are related in some nonlinear fashion.
These effects are simulated and it was confirmed that the proposed system could be used for free space
optical communication
keywords: Optical Wireless Communications, white LEDs, OFDM, Indoor visible light
communications
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Chapter 2
Introduction
In the 21st century, high speed data communication will play a greater role in people’s lives. A wide
variety of multimedia information will be available in any one place at any time. In the workplace,
collaborative work will be increasingly important, and thus the demand for office data communica-
tions systems, including accessibility of multimedia information will increase in the future. Electrical
appliances will be wirelessly linked with each other, and with the implementation of the Wireless
Home Link (WHL) [4] , people will be able to access the world wide web from anywhere in the home,
depicted in Figure 1.1. As bandwidth is a scarce resource, other methods should be taken into con-
sideration when designing an improved system. It is suggested that optical communications can meet
this demand. It provides a cheap, efficient, high data rate transmission scheme. Furthermore, LEDs
are much more efficient than other lighting means. It has an efficiency of 30-40 percent, compared
with tungsten filament lamps in the range of a few percent and fluorescent lamps that are slightly
higher.
2.1 Motivation and Proposed Research Questions
The optical wireless communication system is suitable for WHL which requires a high speed wireless
link. It is suitable for a non-public network because is does not require any license for use. Also, light
waves are only obstructed by physical obstacles and it is easy to prevent interference from adjacent
rooms. Furthermore, white LEDs do not occupy any radio frequency spectrum, and as such can be
used where electromagnetic interference is a main concern such as airplanes and hospitals. In this
paper an OFDM based optical communications scheme is proposed. The main reason for this proposal
is the fact that the OFDM time signal is is inherently an intensity modulated signal. The amplitude
of the signal varies with time and hence is an ideal input to an optical device. In this paper, a white
LED is proposed as a means to realize this. The system can thus serve the dual purpose of lighting as
well as a means for high data rate transmission. However, the LED has a non-linear transfer function.
This property can cause severe problems with the OFDM signal. This non-linearity, can be caused
2.2. OPTICAL DEVICES: THE WHITE LED CHAPTER 2. INTRODUCTION
by a time varying channel, or in this case a non-linear transmitter. The non-linearity would cause
the contents of one of the OFDM sub-carriers to spill into the next, and would result in errors at the
receiver. In the proposed system, the LED is driven in the most linear region and the results observed.
The effects of this non-linearity is investigated.
Figure 1.1: Wired Backbone and Wireless Access Networks [8]
2.2 Optical Devices: The White LED
The emergence of white LEDs has drawn much attention since they are considered the next generation
of lighting. However, it has been impossible to obtain white LEDs till recently because of the lack
of highly efficient blue and green LEDs. Now, InGaN based green and blue LEDs have now become
commercially available. By using an appropriate mixture it is possible to fabricate white LEDs by
mixing the three primary colors (red, green, blue). The resulting white LEDs have a high power output,
high efficiency and are long lasting. It is said, that they would replace incandescent fluorescent lights
in offices and homes [2]. The system proposed has the following advantages:
• Optical data transmission with very little shadowing throughout a room by high power and
distributed lighting equipment.
• Lighting equipment using white LEDs are easy to install and aesthetically pleasing.
• Compared to infrared wireless communications, visible LED light has higher power, therefore
receiver terminal does not need to narrow the angle.
• They are not subjected to, or cause radio/electromagnetic interference.
• Devices for an optical wireless transmission system are low in cost, thus optical wireless com-
munications systems are suited for consumer communication network.
White LEDs are considered the future of indoor lighting. They can be used not only for lighting
but for wireless data transmission as well[7]. In general, the home would be installed with many
5
2.2. OPTICAL DEVICES: THE WHITE LED CHAPTER 2. INTRODUCTION
lighting sources in such a way that there are no dark areas. When looked at from a communications
engineer point of view, this translates into LOS links without shadowing because of the sheer number
of transmitters evenly distributed within a room. This is one of the assumptions made in the model.
Also, since this system is using white LEDs, it can attain high luminosity as a lighting source, and
thus high quality transmission for an optical wireless system.
2.2.1 White LED as lighting
Figure 2.2: Two types of white LEDs [13]
Figure 2.3: Emmision Spectra of One Chip and Multichip LEDs [13]
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2.3. APPLICATIONS OF LED COMMUNICATION CHAPTER 2. INTRODUCTION
White LEDs have been classified into two types [5, 3], as illustrated in Figure 2.2. The first, is
fabricated using a blue LED chip and phosphor. In this type, there is a YAG(Yttrium Aluminum
Garnet) phosphor layer on top of the blue LED chip. This is illustrated in Figure 2.2(a). With the
supply of electric current, blue light is emitted from the chip. This excites the phosphor which emits
a yellow fluorescence. The mixture of this yellow and blue emission is a white emission.
The second type of LED, known as “multi-chip-type white LEDs,” is fabricated by mixing light from
LEDs composed of the three primary colors, i.e red,green and blue. In this case, the LED would emit
each color simultaneously as shown in Figure 2.2(b).
Both these types have their drawbacks and advantages. The single chip type blue LEDs have a much
lower cost, bright output, but difficulty in high data rate transmission. This is due to the fact that
the phosphor layer emits light after the blue emission of the LED chip. The resulting response time
is lower than the multi-chip type white LEDs. Multi-chip LEDs have much better color rendering
and hence, more suitable for visible light communications. The frequency response of the two types of
white LEDs is shown in Figure 2.3. With the use of multi-chip LEDs, the red, green and blue LEDs
can be modulated simultaneously.
2.3 Applications of LED Communication
2.3.1 Traffic Information Systems
Figure 3.4: Applications of LED Communication System [8]
A practical use of LED Communications would as optical beacons that can be used to gather traffic
information. It can be used to control traffic and to supply real time traffic information. Thus
far, optical beacons would have to be implemented via a separate infrastructure. With this system,
existing LED traffic light can be used as a means for traffic control [9].
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8
Chapter 3
System Description
3.1 Proposed System
We have proposed a system that utilizes white LEDs. The proposed system is illustrated in Figure 1.1.
Data within the home LAN is transmitted through the optical access points in each room and these
access points are composed of white LEDs that light simultaneously providing high optical power.
These LEDs not only perform the function of lighting up the room, but also to convert electrical
signals into visible light signals. The blinking modulated light is of high enough frequency that it is
imperceivable to the naked eye.
Hence, the traditional function of the lamp is not forsaken when it is used as a tool for wireless optical
communications. In the system proposed, a directed LOS (line of sight) link is proposed. Amplitude
(intensity) modulation and direct detection is used. The OFDM time signal has a peak voltage of
0.1V and as such a DC component is added on top of it to drive the LED and to make this changing
forward voltage imperceivable to the observer. In this model, the LOS link is not obstructed. The
user terminal serves the function of receiving the optical pulses. It is composed of photo diodes that
serve the purpose of converting the incident optical signal into electric signals.
In the system proposed, a consideration for the optical luminance of the LED light is necessary. The
ISO (International Organization for Standardization) generally standardizes the luminance of lights.
For normal office operations, it is states that an luminance of 200-1000[lx] is required. From the results
of numerical analysis [10, 11], sufficient luminance can be achieved with 600-1000 LEDs. This figure
will reduce in the future as more powerful LEDs are developed.
3.2. WIRELESS OPTICAL CHANNEL CHAPTER 3. SYSTEM DESCRIPTION
Figure 1.1: Wireless optical data transmission also working as indoor lighting [12]
3.2 Wireless Optical Channel
A wireless optical channel is assumed and this is applied to the computer simulation. In the optical
link the DC gain is given by:
H(0) =
(m+1)A
2πd2 cosm(φ)Ts(ψ)g(ψ) cos(ψ), 0 ≤ ψ ≤ Ψc
0, 0 > Ψc
(3.1)
where A is the physical are of the photo diode, ψ is the angle of incidence, φ is the angle or irradiance,
Ts(ψ) is the gain of an optical filter, g(ψ) is the gain of the optical concentrator, and d is the distance
between transmitter and receiver. Ψc denotes the width of the field of vision at the receiver. The
optical concentrator g(ψ) can be given as:
g(ψ) =
n2
sin2 Ψc, 0 ≤ ψ ≤ Ψc,
0, ψ > Ψc
(3.2)
where n denotes the refractive index.
m is the directivity of the emision pattern, and is related to the general Lambertian radiant intensity.
It is given by:
m =ln2
ln(cos Φ1/2)(3.3)
where Φ1/2 is the semi angle at half power. For example, Φ1/2 = 60 degrees (Lambertian transmitter)
corresponds to m = 1. However, Φ1/2 = 15 degrees, corresponds to m = 20.
If φ is kept small, we can increase H(0) by narrowing the transmitter semiangle Φ1/2, thereby increas-
ing m [2].
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3.3. NOISE MODEL CHAPTER 3. SYSTEM DESCRIPTION
3.3 Noise Model
The channel can be modeled with an Additive White Gaussian Noise (AWGN) model. Shot noise
dominates the quality of transmission in an optical channel [2]. The desired signal contains a time
varying shot noise process which has an average rate of 104 to 105 photons per bit. In the model
that is used , however, intense ambient light striking the detector has a steady shot noise having a
rate that is in the order of 107 to 108 photons/bit, even if a narrow-band optical filter is used at the
receiver. We can thus ignore the shot noise caused by the signals and use a Gaussian process to model
the ambient induced shot noise [6].
In the case that there is little or no ambient light present, the dominant noise source is pre-amplifier
noise, that is signal independent and Gaussian (although often non-white). As such the optical wireless
channel can be modeled as follows:
y(t) = Rx(t)⊗ h(t) + n(t) (3.4)
y(t) represent the signal current at the receiver, x(t) is the transmitted optical pulse, n(t) is the noise
modeled as an AWGN process and ⊗ denotes convolution. The noise, n(t) is observed in the electrical
to optical converted signal x(t). R represents the optical to electrical conversion efficiency at the user’s
terminal photodiodes.
In this paper, it is assumed that a non directed, LOS path exists. The transmitted signal is not
blocked and the relation h(t) = H(0) holds [1]. Therefore, the received optical power can be derived
from the transmitted optical power as follows:
Pr = H(0)Pt (3.5)
In the model used multipath fading can be ignored. This is because we use an information carrier in
the order of 1014 Hz. Detector dimensions are in the order of thousands of wavelengths, which leads
to efficient spectral diversity that minimizes the effects of multipath fading.
The received electrical SNR is given by [5],
SNR =R2P 2
r
N0B(3.6)
This is assuming the fact that h(t) is dominated by a Gaussian component that has a double sided
power spectral density N0 over the desired bandwidth B.
3.4 Communications Using White LEDs
In the proposed system, the LEDs transmit the same data. The blinking rate of the LEDs are
sufficiently high to be imperceivable to the human eye, and thus cannot detect it. Therefore, the
primary role of the LEDs (lighting) is not disrupted by optical wireless communications. The effect
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3.4. COMMUNICATIONS USING WHITE LEDS CHAPTER 3. SYSTEM DESCRIPTION
Figure 4.2: Emmision Spectra of multi-chip-type white LED and PD responsitivity; Black lines illus-trate emission spectra, and dotted lines the responsitivity of three PDs [13]
of shadowing is also avoided by using distributed lighting sources. In the case the simulation with
OFDM, a directed LOS path is chosen as proof of concept. In addition intensity modulation and direct
detection is used as a means of optical pulse modulation. The optical paths are seldom obstructed,
and the optical signals are received at the user terminal with the use of photo diodes (PD). The
ambient light is filtered with the use of an optical bandpass filter.
Only the desired wavelength will be passed through the bandpass filter. The spectral response of
photodiode is quite wide an as such we need to select the wavelength that maximizes the response
in a PD. The optical filters are constructed with the use of multiple thin dielectric layers, and rely
on optical interference [5],[11]. In the simulation, the real and imaginary parts of the OFDM time
signal are split up and transmitted through separate LEDs. However with the multi-chip-type LED,
it is possible to modulate the real and imaginary parts simultaneously on a single device [13]. With
the use of very narrow filters, illustrated in 4.2, this then can be recovered at the receiver. A block
digram for a system that utilizes this type parallel tranmission is illustrated in Figure 4.3. However,
in this simulation the two parts were transmitted separately on two LEDs.
Figure 4.3: Simulation model for parallel transmission system using multi-chip-type white LEDs [13]
11
12
Chapter 4
OFDM Simulation
In conventional multi-carrier transmission systems, symbols are modulated on sub-carriers separated in
frequency by a certain guard band. These guard bands are sufficiently large to allow for orthogonality
to be maintained at the receiver. After multiplying the waveform by a carrier, these signals and
multiplexed together and transmitted over the channel.
Figure 0.1: Orthogonal OFDM subcarriers (red), and the signal without time expansion before theIFFT (blue)
In an OFDM based signal, the orthogonality property of the signal is maintained with the use of the
IFFT. A serial bit stream is separated into m parallel sub-carriers. This effectively lengthens the
duration of each bit by a factor of m, the number of sub-carriers. The IFFT is performed on the sub-
carriers, and what results is a series of overlapping sinc functions. These functions overlap significantly
but yet are orthogonal. This is because at the sampling instant of each individual sub-carrier, all
other sub-carriers are zero. The lengthening of the symbol duration allows for the compression of the
individual sinc functions after the IFFT. This allows for significantly more symbols to be modulated in
parallel, resulting in an efficient use of bandwidth resource, as illustrated in Figure 0.1. The resulting
CHAPTER 4. OFDM SIMULATION
time signal is amplitude modulated. This is precisely the reason why it is suitable for use with the
LED. The LED cannot transmit phase information, but only intensity, which makes it a suitable
candidate for OFDM transmission. Also, the OFDM signal is robust against multi-path effects that
result in frequency selective fading. The number of bit errors can be reduced by using an OFDM
signal due to the fact that only a certain sub-carriers will be attenuated, but the majority can still be
decoded without error.
Figure 0.2: OFDM time signal
The simulation for the optical OFDM chain and the modeling of the analog parts were performed in
MATLAB. The main concern was the inherent structure of the OFDM signal. It has the property of
having a large crest factor i.e a high peak to average ratio as illustrated in Figure 0.2. This is the
inherent obstacle in using an OFDM based system.
For example, power amplifiers used are not linear and as such can distort the signal and result in high
bit errors. Also, when used in a time varying channel, a doppler shift can cause non-linear effects.
This again might pose a problem with LEDs because of the non linear transfer function of the LED.
This would cause the content of one of the OFDM sub-carriers to spill into the next, and result in
errors.
The transfer characteristics of the LED and the Photo Diode were modeled initially, and the OFDM
signal that was transmitted through the LED, with an appropriate voltage offset to drive the LED. This
optical signal was used as input to the system embedded in additive white Gaussian noise. Finally,
the optical signal was converted into a current by the photo diode, and then into a corresponding
voltage with the use of a trans impedance amplifier, and the constant voltage subtracted.