Simulation and DSP Board Implementation of an Optical TransmissionSystem using OFDM

A Bachelor of Science thesis

by

Vinod Patmanathan

v.patmanathan@iu-bremen.de

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

2

Contents

1 Abstract 3

2 Introduction 4

2.1 Motivation and Proposed Research Questions . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Optical Devices: The White LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2.1 White LED as lighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Applications of LED Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.3.1 Traffic Information Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 System Description 8

3.1 Proposed System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Wireless Optical Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3.3 Noise Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.4 Communications Using White LEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4 OFDM Simulation 12

4.1 OFDM Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.2 Opto-Coupler . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.3 Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

4.4 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.4.1 Linear Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.4.2 Non Linear Region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

5 Conclusion 21

Bibliography 22

3

Chapter 1

Abstract

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

4

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]

6

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].

7

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].

9

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

10

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.

13

4.1. OFDM BLOCK DIAGRAM CHAPTER 4. OFDM SIMULATION

4.1 OFDM Block Diagram

Figure 1.3: Block diagram for optical OFDM using LEDs

• Random Bit Generator:This serves as the message source. It creates a random row vector

with the length specified. Symbol duration = Ts.

• BPSK Modulator: Modulates the input data using Binary Phase Shift Keying (BPSK)

• Serial-Parallel Converter: Splits the incoming data for separation into m sub-carriers. The

symbols have a duration of m · Ts, thereby expanding the symbols in time.

• Inverse FFT: Performs the IFFT operation on the incoming sub-carriers. Since the incoming

wave form is a series of rectangular pulses, this results in orthogonal sinc functions.

• Parallel to Serial Converter: This block multiplexes the modulated data for transmission

over the channel.

• Split real and imaginary parts: The real and imaginary parts are split, to be modulated on

each LED

• LED: A constant DC component K, is added on top of the signal, and this is used to drive the

LEDs. The non-linear transfer characteristics were modeled.

14

4.1. OFDM BLOCK DIAGRAM CHAPTER 4. OFDM SIMULATION

• AWGN: This block simulates the ambient noise in the system. It first measures the power of

the noise in the signal and adjusts the variance of the noise to meet the specified SNR.

• Photo Diode: The photo diode acts as a current source. It converts the incoming optical

luminosity into current. This is then converted into a voltage with the use of a trans impedance

amplifier.

• Constant, K: The constant component K is subtracted.

• FFT, BPSK Demodulator: These blocks work inversely to the FFT and BPSK modulator

to reconstruct the signal

• BER Calculator: This block compares the received data to the transmitted data and calculates

the number of bits that were in error.

The simple set-up was simulated with MATLAB to determine the bit error performance over a SNR

range of 1dB to 15db. This result was compared to the case without the use of LEDs (without the

consideration of the non linear effects). Next, the constant offset used to drive the LED is varied and

the corresponding BER is calculated.

In this simulation the effect of ambient light is ignored for simplicity. Also, the effect of varying the

distance between the transmitter and receiver (Optical Intensity) is not considered. Multipath effects

are not considered based on the assumptions above. The main purpose was to simulate the non linear

effects of the LED.

A similar investigation was conducted that examined the effect of the optical path difference between

the light sources [12]. It was found that a delay effects the performance when the data rate is high.

This problem is significant as high data rates are expected in the future. The delay however, is

minimized when using an OFDM system with a guard interval.

Having said that, there has not been any research into the non-linear effects of the LED on the OFDM

signal, and this is the purpose of this paper.

15

4.2. OPTO-COUPLER CHAPTER 4. OFDM SIMULATION

4.2 Opto-Coupler

The opto-coupler consists of the white LED and the photo-diode used in the simulation. The first step

in modeling the opto-coupler was to model the LED device used. The forward voltage of the LED

corresponded to some forward current. The forward current would then be translated to some optical

intensity. The optical intensity in the data sheet was normalized to a forward current of 20mA. This

value could be qualified with the knowledge of the external efficiency of the photo diode. However, in

this simulation so such attempt was made. Hence, an optical intensity vs. forward voltage curve can

be obtained.

On the receiver side, the incident optical intensity was converted into an electrical signal. The photo

diode effectively acts as a current source. This current source was then converted into a voltage with

the use of a trans impedance amplifier. The circuit used to drive the LED at the transmitter and the

receiver is shown in Figure 3.5.

Figure 2.4: Transfer Function of the Optocoupler

16

4.3. CIRCUIT CHAPTER 4. OFDM SIMULATION

4.3 Circuit

Figure 3.5: Circuit diagram of transmitter and receiver.

The top circuit in Figure 3.5 describes the transmitter. The input voltage is connected to the capacitor

on the left, which just permits the AC component of the OFDM signal through. Then a constant DC

component of 3.5V is added on top of the OFDM signal to drive the LED. This driving voltage, Vin

is directly proportional to the current through the LED. The resistor R7, controls the amplification.

On the receiver side, the photo diode acts as a current source, converting the optical signal into an

electrical signal. The current is converted into a voltage by a trans impedance amplifier. The gain

of this amplifier is controlled by the variable resistor. This resistor is carefully chosen such that with

an input of 3.5V, the desired output would also be 3.5V. In this case, given the characteristics of

the opto-coupler a value of 102 ohms was chosen. It should be noted that this value is crucial and

can severely alter the results if chosen differently. It can also be used to vary the gain depending

on the lighting conditions. For example in a darker room a higher gain would be required. In this

simulation, none of these effects were taken into consideration. It was assumed that the optical power

was constant.

17

4.4. SIMULATION RESULTS CHAPTER 4. OFDM SIMULATION

4.4 Simulation Results

The model of the opto-coupler has inherent non-linearities in certain regions. This is apparent in the

regions around 3V and 4V. Driving the LED in these regions would cause a high BER rate. However,

when driven in the most linear region, in this case with a forward voltage of 3.5V, it is found that the

non linearities do not effect the OFDM signal significantly. As can be seen from the simulation, the

resulting BER performance is very similar to that of the standard benchmark. The system achieves

an SNR of 10−3 with 7dB SNR, Figure 4.6.

4.4.1 Linear Region

Figure 4.6: BER vs. SNR for LED driven system (blue) againts the benchmark results (red)

Figure 4.6 described the bit error performance of an OFDM signal. A random 4096 bit stream is

modulated on 2048 sub-carriers. The signal is transmitted in parallel over two LEDs transmitting

the real and imaginary parts. Additive White Gaussian noise is added to the individual parts, and

the optical signal is received at the photo diode. It is assumed that there is no loss in optical power.

The current induced in the photo diode, in the range of a few micro amps, is converted into a voltage

and the appropriate signal processing, as described in Figure 1.3 is performed, and the bit error rate

calculated. The bit error performance, is almost the same as that of the standard curve. This implies

that the non-linearities of the opto-coupler do not corrupt the signal significantly, when driven with a

forward voltage of 3.5V. Next, the effect of changing the bias point of the LED is simulated. That is,

by varying the input voltage such that the LED is driven in the non-linear region. This is important

18

4.4. SIMULATION RESULTS CHAPTER 4. OFDM SIMULATION

in modeling a real OFDM signal that has a high peak to average ratio.

4.4.2 Non Linear Region

Figure 4.7: BER performance of system with varying offsets

Figure 4.7 illustrates the effect of changing the offset voltage used to drive the LED on the OFDM

time signal. The offset voltage on the side of the receiver is varied with respect of the driving offset,

K on the transmitter. The change in offset, ∆K = Krcv −Ktrx. Krcv is fixed at 3.5V. and Ktrx is

varied. This can seen as the effect of changing the bias point of the LED, i.e. driving it with different

forward voltages. Referring to the graph it can be seen that changing the bias voltage can have severe

effects on the OFDM signal. The blue curve is the standard curve when ∆K = 0, in the subsequent

curves, the ∆K is increased and this leads to higher bit error rates. This confirms the hypothesis that

the point at which the LED is driven significantly effects the performance of the OFDM type system.

This might lead to problems when the OFDM signal has a high amplitude. It seems apparent that

this amplitude must be minimized and this can be controlled by varying the number of sub-carriers,

and hence the expansion of the symbol duration before the IFFT.

19

4.5. SUMMARY CHAPTER 4. OFDM SIMULATION

4.5 Summary

The purpose of this project was to study the feasibility of having an OFDM based optical transmission

system using white LEDs. An OFDM block was simulated, illustrated in Figure 1.3. A simple

LOS(Line of Sight) path was assumed between the transmitter and receiver. Multipath effects were

not considered, therefore the channel was modeled as having a constant DC gain. White Gaussian

Noise was added to this signal, and the BER rate calculated at the receiver. The non linear effects of

the LED were taken into account, with respect to the OFDM signal having a high crest factor. The

forward voltage at which the LED was driven was varied the effect of which on the OFDM signal was

simulated.

There are various areas that are left to be investigated. For one, the effects of a varying amplitude

of the OFDM signal should be simulated. With a larger amplitude, the probability of some parts

of the signal that is driven into the non-linear region is higher.

Next, the effects of ambient light is not simulated. There is bound to be high amount of interference

since the signal is transmitted with visible light. These effects should be taken into consideration. A

method to minimize this might be to use a CDMA based system coupled with an OFDM one. This

will also enable the system to distinguish one user from the next.

The use of multi-chip type LEDs would be a viable method of transmitting the real and imaginary

parts of the OFDM signal using different colors on a single LED. In addition the LED is more

responsive, therefore higher data rates are possible.

The effect of varying the gain in the trans impedance amplifier should be studied. This would

be of use when the incident optical power is not constant. The gain can compensate for lower or

higher power. A feedback system with could be developed to vary the resistance according to the

input power to maintain a constant output voltage.

Also, the up-link in the communication system should be considered. So far the probably

application of this system would be for broadcasting purposes, but an efficient scheme to enable the

up-link should be investigated further, with the possibility of using a corner cube modulator [7].

20

21

Chapter 5

Conclusion

In this paper I have proposed an OFDM based optical communication system using white LEDs.

White colored LEDs are thought of as the lighting of the future, and it is proposed here as part of the

Wireless Home Link. The non-linear effects of the LED is discussed in relation to the OFDM signal.

The non-linear effects of the LED are simulated on the computer, and leads to the conclusion that this

type of transmission scheme is possible, but with several limitations. The non-linearities of the LED

does effect the OFDM signal, however this effect can be minimized. Still, it is important to consider

this effect when implementing such a system. Small variations in the driving voltage of the LED has

a significant effect on the BER performance.

The voltage at which the LED is driven has a major influence on the OFDM signal. Driving it in

the range of 3.5V ensures optimum results as this is the most linear region. In conclusion, an optical

OFDM transmission scheme is realizable, when the optical device is driven in its linear region. Further

research into this field would make and OFDM based LED system feasible in the areas mentioned

above.

22

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