Final Report
Free Space Optical Communication Link
Using LEDs
ECE 4007 Senior Design Project
Section L01, FSO Group
Adam Swett
Clayton Huff
Trang Thai
Nguyen Trinh
Submitted
May 1, 2008
FSO Group (ECE 4007 L01)
1
Table of Contents
Executive Summary………………………………………….. iii
1. Introduction………………………………………….…….. 1
1.1 Objective…………………………………………. 1
1.2 Motivation..………………………………………. 2
1.3 Background………………………………………. 2
2. Project Description and Goals……………………………. 3
3. Technical Specifications……………………………………. 5
4. Design Approach and Details………………………………6
4.1 Design Approach…………………………………. 6
4.1.1 Design Revision…………………………... 6
4.1.2 Shunt Peaking Circuit..…………………… 7
4.1.3 LED Modulation…………………………. 8
4.1.4 Free Space Optical Receiver Design………. 8
4.1.5 Free Space Optical Channel Setup……….…12
4.1.5.1 Intensity Modulation……………...13
4.1.5.2 Optical Channel Design…………..14
4.1.6 PCB Considerations and Revisions…………22
4.2 Codes and Standards…………………………….... 25
4.3 Constraints, Alternatives, Tradeoffs…………….... 28
5. Schedule, Tasks, and Milestones………………………….. 28
6. Project Demonstration…………………………………….. 29
7. Marketing………………………………………………….. 30
7.1 Marketing Analysis......…………………………… 31
7.2 Cost Analysis…………………………………...… 31
8. Summary……………………………………………….…... 32
9. References………………………………………………….. 33
Appendix 1 ………………………………………………….….34
Appendix 2 ……………………………………………………..35
Appendix 3 ...........................................................................…...36
FSO Group (ECE 4007 L01)
ii
Executive Summary
The Free Space Optical (FSO) LED link has the ability to connect two devices at
high-speed while taking advantage of the high bandwidth of optical communication and
the low cost and low power consumption of LEDs. This link will provide an alternative to
traditional RF wireless communication that is currently nearing its bandwidth limitations.
As the speed increases for data transmitted over a wire, it is necessary that wireless
communication continues unbounded. The FSO link also outperforms USB 1.1, USB 2.0,
and Bluetooth allowing for an additional market and perhaps a new standard for data
transmission. This will become necessary as file sizes increase and multimedia dominates
the business world.
A key advantage to the LED system will be cost. The FSO link will consist of a
transmitter and a receiver. The transmitter will have a target cost less than $50 and will
operate on two AA batteries. The receiver will also cost $50. Overall the cost of this
system is significantly less than a comparable LASER optical link and draws less power.
The FSO link will be built using top-of-the-line components and cost will continue to
decrease as the components become standard. LEDs are more directional than radio
waves, which prevents eaves-dropping.
The result of the project after testing, debugging, and data analysis was a working
link at one meter with a speed of 160 Mbps and a bit error rate 10-10. The target market
for this project is the average consumer who has a need for high-speed and low cost data
transmission.
FSO Group (ECE 4007 L01)
iii
Free Space Optical Communication Link
1. Introduction
There are various forms of wireless communication available such as
satellites/antennas, WiFi, and FSO communication with lasers or VCELs. Currently,
there are no forms of FSO communication that use a LED as the transmitting light source.
The FSO link using a LED will send data via free space between a transmitting unit and a
receiving unit.
1.1 Objective
The objective of the FSO link using a LED is to create a new form of optical
communication that can be marketed with small size and low cost. The link consists of a
transmitter and a receiver both of which are battery powered and portable.
The
transmitter circuitry takes in a data stream of "1s (+3.3V)" and "0s (0V)" and then
modulates the current through an LED.
The LED sends photons with intensities
proportional to the input data bits across the free space link to a PIN photo-diode. The
PIN diode is connected to the receiver which converts optical power into an output
voltage. The size of each module is approximately the same and should accommodate all
components needed to function. The LED, PIN diode, and other circuitry will be able to
operate off of a portable battery due to low power consumption. Inexpensive parts and
low power consumption present a lower marketing cost. The link was designed for two
different applications. The first is a short range application, such as, sending data from a
flash drive to a computer, and this application has a data transfer rate of 100 Mb/s at a
FSO Group (ECE 4007 L01)
1
range up to 10 cm. The second application sends data at 25 Mb/s at a range up to 1 m,
and this application could apply to a central classroom module sending data to laptops or
PDAs.
1.2 Motivation
The current form of free space optical communication using light emission resides
in the realm of lasers and VCELs. These devices produce optical intensities high above
any LED, and they can also switch faster from high intensity to low intensity and vice
versa. The drawback to using lasers and VCELs is the cost and power consumption.
Lasers cost much more than LEDs and require more expensive driver circuitry. They
also consume more power due to the stimulated emission process.
LED are both less expensive and require less power to operate than lasers. This
feature would allow for a low cost FSO link that operates at high-speeds. Another
important advantage of the FSO based on LEDs is that security of transmission can be
achieved. LEDs should be used in FSO communication so that the cost and size of the
final product will have numerous applications and will potentially replace the existing
indoor wireless networks..
1.3 Background
The current wireless technologies such as Bluetooth, WiFi, or WiMax have low
data transmission rate compared to that of the wired LAN systems. These networks
require licensing and pose security risks. Many efforts over the last decade have
developed an alternative for wireless networks. FSO networks can be set up within a day
with absolute security because the transmission is based on line-of-sight. The data rate for
optical systems is in the Gbps range. However, this technology is very expensive
FSO Group (ECE 4007 L01)
2
preventing it from replacing the current wireless networks such WiFi. LEDs were
proposed to replace the laser as the light sources in order to lower the cost while
maintaining the high-speed data rate and the security feature that FSO offers. There have
been experiments conducted using LEDs as the light source for a FSO link However;
there are no commercially available products that use LED FSO technology. A company
in Japan designed a LED driver that could modulate an LED at 400 MHz, but LEDs were
incapable of modulating at those speeds at the time [1].
2. Project Description and Goals
This project has two stages:


To create a link with a minimum data rate of 100 Mbps at a range of 10 cm in a
controlled environment.
To achieve a data rate of 25 Mbps at a range of 1 m under normal conditions
using the same link.
The controlled conditions are defined as the FSO link maintained within 10 cm
and operated in a noise free environment (i.e., in an enclosed pipe with no light source
other than the transmitting LED). The room conditions are defined as the FSO link at a
distance of 1 m being exposed to ambient light sources in a closed space. The test will be
conducted without excessive light such as sunlight. The LED link is designed to transmit
data bits from a voltage generator to an oscilloscope through free space using light from
an LED. The LED operates in the spectral range of infrared (above 700 nm in
wavelength). A PIN photodiode is employed on the receiver circuit to collect the light
sent from the transmitting LED. A system block diagram is shown in Figure 1.
FSO Group (ECE 4007 L01)
3
Figure 1. A block diagram of the FSO-LED system
The project is subjected to the minimum performance as described in the first step
of the approach. The second step is subjected to the minimum performance in which both
data transmission rate and range shall exceed that achieved in the first step. An ideal
target for the second step of the project within the time frame of the semester is to
achieve a minimum transmission rate of 25 Mbps at the range of 1 m. Due to the
theoretical limitation of the LED operation, the maximum data rate may not reach 1Gbps.
Nevertheless, at the transmission speed of 100 Mbps, the low cost LED link is fully
capable of replacing the wired LAN network and enables wireless communication
between numerous portable devices such as Mp3 players, digital cameras, portable
storages, and laptops.
FSO Group (ECE 4007 L01)
4
3. Technical Specifications
The transmitter/receiver design includes crucial specifications that must be met in
order to achieve a working FSO link. These proposed and actual specifications are listed
in Table 1.
Table 1. Proposed Versus Actual FSO Link Specifications
Specifications
Proposed
Actual
Tx Power Consumption
200 mW
690 mW
Tx Supply Voltage
+5 V
+3.3 V
Rx Power Consumption
200 mW
98 mW
Rx Supply Voltage
+5 V
-5 V, +5 V
Link Range
10cm  1m
10 cm  1.01m
Link Bandwidth
25  100 Mbps 25  160 Mbps
Transmitter Size
~ 4 x 4 inches
2 x 1.5 inches
Receiver Size
~ 4 x 4 inches
1.5 x 1.5 inches
The LED and PIN diode perform the photon transmission and receiving, and they
must be operated with the correct voltage and current levels to achieve the necessary
optical intensity. The input voltage to the transmitter will range from 0 –> +3.3 V, and
the power rail of the circuits will be at +3.3 V. The short range application for the FSO
link should send data at 100 Mbps at a distance of 10 cm, and the long range application
should have a data rate of 25 Mbps at a distance of 1 m. There should not be any rigorous
alignment required for the link set up. At the distance of 50 cm, the data rate should
remain constant within a 10o cone between the transmitter and receiver.
FSO Group (ECE 4007 L01)
5
4. Design Approach and Details
4.1 Design Approach
4.1.1 Design Revision
The Digital driver proved to be much more difficult to design than was originally
planned. The proposed circuit was experimental in nature meaning that strict simulations
could not accurately model the response of the circuit in real conditions. The spice
models for the LED did not exist and many of the parameters necessary were not
available. The alternative method to attaining the parameters was through a frequency
analyzer; this also was not a viable option. As such the proposed digital driver circuit that
was originally proposed could not be implemented in the final design.
Instead the MAXIM 3967 Driver was implemented. The suggested circuit for the
driver was given in the specification sheet.
Figure 2. Suggested Maxim circuit for MAX3967A LED driver.
FSO Group (ECE 4007 L01)
6
In the design implemented a shunt peaking circuit was added, the modulation current
resistor was set with a base of 700 ohms and a variable resistor following it in series, and
all pre-bias pins were tied to VEE.
4.1.2 Shunt Peaking Circuit
Figure 3. Schematic of shunt peaking circuit used in transmitter.
In an effort to lower power consumption in more typical LED applications
manufactures often will trade optical power and lower power consumption for speed.
Often times it is not necessary for an LED to switch quickly, however, in the case of data
transfer it becomes integral. Shunt peaking is a method that can be used to increase the
switching time of an LED using circuit components. The basic shunt peaking circuit is
shown in the figure. The two most important components are the inductor Lp and resistor
Rp. These provide the ratios that determine the new extinction ratio of the LED. The
FSO Group (ECE 4007 L01)
7
capacitors are used as a DC block to protect the LED. Finally the resistor Rs provides a
return path for current that would otherwise pass through the LED when OUT+ is turned
off.
Two equations govern the use of the shunt peaking circuit. The first thing that
must be determined is k which is the fraction of drive that will be used for shunt peaking.
This is a function of the total power needed to drive the link and the amount of power that
is currently being received. Once the value for the shunt peaking resistor is found the
desired fall time of the LED is entered into the first equation and a suitable value for the
inductor is found.
The values found in the shunt peaking equations are only theoretical. With typical
lab values of Inductors and resistors it is then necessary to experimentally determine what
combination of components produces the best signal.
4.1.3 LED Modulation
The LED used was the Optek OPF345a which has a rise and fall time of 4.5 ns
and was chosen because this time is relatively fast. In order to achieve the highest data
rate, the LED should not be completely turned off. This speeds the process in which
minority carries can effectively be created in the junction. A high enough bias level must
be chosen to turn on the LED while not turning on the LED all the way, this will avoid
damaging the LED.
4.1.4 Free Space Optic Receiver Design
A typical optical receiver consists of many stages as shown in Figure 4. Within
the scope of our project, the receiver board design is limited to the front-end components.
This includes a photodiode and a pre-amplifier (first block immediately following the
FSO Group (ECE 4007 L01)
8
photodiode in Figure 4). Therefore the data collected is raw data and is subjected to
further refinement of filtering and error corrections. Our receiver schematic is shown in
Figure 5 below where the p-i-n (PIN) photodiode OPF430 by Optek Inc. and the OPA657
transimpedance amplifier (TIA) chip by Texas Instruments Inc. were used.
Figure 4. Different stages in a typical optical full receiver.
Figure 5. Receiver board schematic.
The photodiode collects photons from the light source and converted it into
current. This current is extremely small and corresponds to the sensitivity of the
photodiode. Typically the photodiode generates about 0.55 uA for 1 uW optical power
received. Therefore it is followed by a pre-amplifier to amplify the electrical signal for
FSO Group (ECE 4007 L01)
9
further processing. A high-impedance technique is often used to develop a voltage
proportional to the light detector current. The impedance serves to reduce the thermal
noise and improve the receiver sensitivity. However, the leakage current could saturate
the PIN diode preventing the modulated signal from being detected. An improvement can
be obtained with the use of and LC feedback circuit. As the Q increases so does the
impedance of the LC circuit [5]. This alternative method is referred to as transimpedance
amplifier, which is the first stage of the receiver shown in Figure 4. Typical
transimpedance amplifier is shown in Figure 6. The bandwidth of the commercially
available transimpedance amplifier usually covers kHz to GHz range.
Figure 6. Tuned transimpednace amplifier with high Q [5]
In the receiver design, the TIA OPA657 was chosen because it has a wide
modulation bandwidth product (1.6 GHz) with a rise/fall time of 1ns. It has a JFET input
stage that allows low noise that is appropriate for low power optical system since the PIN
generated current is low, thus the noise should be kept significantly lower. The PIN
OPF430 was chosen because it has a small rise time of 2ns at low reverse bias voltage
(5V) and a good responsitivity (0.55A/W). The PIN together with the TIA make up the
front-end receiver with effective rise/fall time of about 3ns, which is sufficient to cover
our targeted modulation bandwidth of 150MHz.
FSO Group (ECE 4007 L01)
10
In Figure 5 the receiver schematic shows a feedback loop that sets the bandwidth
of the circuit. This feedback loop typically consists of a resistor and a capacitor that can
be determined according to the following relationship.
(Eq.1)
RF and CF are the feedback loop resistance and capacitance, CD is the diode capacitor,
and GBP is the gain bandwidth product [12]. The OPA430 PIN can be modeled in
simulation as shown in Figure 7. The PIN is reversed biased at -5V however; voltage
dropped across the PIN capacitor is about -3.5V to -4.5V. The decoupling circuit is to
eliminate the offset voltage at the output and to minimize the bias current going into the
TIA. The 100uA DC current included in the OPA430 equivalent circuit is to account for
the ambient light in the room and is discussed further in the next section.
Figure 7. The equivalent circuit model of the PIN OPA430.
The circuit was simulated in TINA TI simulation software [13]. Our circuit
theoretical response to a step input is a flat line, however a ringing is observed in real
circuit response (Figure 8) this is due to the parasitic capacitance that was introduced into
the circuit by defects in manufacturing such as intrinsic capacitors within discreet
FSO Group (ECE 4007 L01)
11
components and imperfect solder joints. The real circuit response can be simulated by
adding capacitance values into the theoretical circuit as illustrated in Figure 5. The Bode
plots of voltage gain magnitude and phase is plotted in Figure 9. The transfer function
shows that receiver has a bandwidth about 150MHz and relatively constant phase.
Figure 8. The receiver circuit step response.
Figure 9. The Bode plots of the receiver circuit.
4.1.5 Free Space Optic Channel Set Up
In order to construct a fast and reliable transmission link, the optical channel has
to ensure that sufficient optical power is collected at the photodiode. In addition, the
channel has to minimize transmission loss so that the power consumption may be kept
FSO Group (ECE 4007 L01)
12
low. The channel therefore can be characterized based on optical propagation, link power
budget, and bit-error-rate (BER) factors. These considerations also allow the LED link to
be evaluated with respect to its performance and operating conditions such as range and
alignment sensitivity between the transmitter and receiver.
4.1.5.1 Intensity Modulation and Direct Detection
The information that is transmitted in an optical link requires some form of
modulation in order to for the signal to be encoded into a physical parameter. This
physical parameter is usually the intensity, the frequency, or the polarization of the
emitted light. The modulating signal is then demodulated in the receiver to recover the
transmitted information. The optical link is based on low cost LEDs which emit
incoherent light and has a wide spectral width as well as a large angle spread. They are
almost perfect Lambertian sources. Therefore it is extremely difficult to collect
appreciable signal power in a single electromagnetic mode, thus the only practical
modulation is intensity modulation (IM). As a result, the most feasible down-conversion
technique is direct detection (DD) in which the current produced by the light detector is
directly proportional to the square of the received electric field [2]. Visible LEDs are
mostly manufactured for illumination instead of communication therefore their switching
speed is relatively low. In order to achieve transmission rate above 100Mbps,
commercially available infrared LED (850nm OPF345A by Optek Inc.) is employed in
our optical LED communication link. Correspondingly, the photodiode is required to
have comparable modulation bandwidth and an optical spectrum in the range of 800-900
nm of wavelength in order to collect the emitted light from LED. The PIN photodiode
(OPF430 by Optek Inc.) can satisfy these requirements and is employed in our system.
FSO Group (ECE 4007 L01)
13
In a DD system, the optical incident power is detected by a photodiode and
converted into an electrical current. In room condition of operation, there is always
ambient light that enters the photodiode along with the signal light. The power of the
ambient noise, Pamb is determined as
Pamb = Namb∙Bopt
[Eq.2]
,where Namb denotes the ambient radiation, and Bopt denotes the optical bandwidth of the
photodiode. Therefore, the resulting electrical current has a DC component (IDC) of,
IDC = R∙Pamb
[Eq.3]
and a signal current of,
is = R∙Ps
[Eq.4]
where R denotes the responsivity of the photodiode (unit of A/W), and Ps is the optical
power of the signal [3]. The photodiode also produce a dark current typically on the order
of nA and is negligible. Since a transimpedance is used in the receiver to convert the
current generated by the PIN into a voltage signal, the responsivity that characterizes the
receiver board can be calculated in terms of incident power and voltage generated as,
Vpp = R∙Ptot
[Eq. 5]
where Vpp denotes the peak-to-peak voltage generated, and Ptot is the total power that is
the sum of Pamb and Ps. Here the responsivity has unit of V/W.
4.1.5.2 Optical Channel Design
After investigations, it was recognized that radiated power emitted by the LED
cannot be coupled sufficiently into the PIN directly even with a head-to-head
arrangement (the PIN is placed immediately in front of the LED). This is due to a small
FSO Group (ECE 4007 L01)
14
detection area of the PIN (Optek OPF430) which is designed for optical fiber coupling
and the diverse radiation pattern of the LED. Therefore the use of lenses is essential to
enable a working and reliable data transmission. The lenses are set up and attached to
specific locations relative to the LED and PIN as illustrated in Figure 10.
Figure 10. Optical Link geometry
The lenses are plano-convex having 1 in. diameter and focal length of 50 mm.
The LED and PIN are placed at the focal point of each lens as shown in Figure 10 so that
they can couple the light into a collimated beam. The radiation pattern of the LED can be
considered as mostly confined in the cone of angle φ but only the portion of light within
the cone of angle θ is coupled into the lens channel. Parallel light rays traveling from lens
1 to lens 2 is non-divergent, thus there is no lost of power across the optical channel of
free space range R (Figure 10). The PIN is placed at the focal point of lens 2, therefore it
receives almost all the power that is coupled at lens 1 and effectively converted that into
electrical currents. In the final products, the lens can be packaged together with the LED
FSO Group (ECE 4007 L01)
15
and PIN to form a transmitter (Tx) and receiver (Rx) packages. The beam width may be
adjusted into a divergent beam by shortening the distance f1 (Figure 10) if a wider angle
of illumination is desired when power budget of the link allows. This feature addresses
how easy to aim the receiver in order to establish the transmission. The received optical
power P at the receiver is expressed as,
[Eq. 6]
where Pt is the transmitted power from an LED, ф is the angle of irradiance with respect
to the transmitter axis, is the angle of incidence with respect to the receiver axis, and d is
the distance between an LED and a detector’s surface [4]. Ts(ψ) is the filter transmission
g(ψ) is the concentrator gain. ΨC is the concentrator field of view. Semi-angle m is the
order of Lambertian emission, and is given by the transmitter half angle (at half power) as
[Eq. 7]
Here, m= 1 from Ψ1/2= 60o (Lambertian transmitter) [4]. From the geometry shown in
Fig. 2 below, we can set ф = ψ. The concentrator is referred to lenses employed to
enhance the power at the receiver.
Figure 11. Geometrical set up of the propagation light. [3]
FSO Group (ECE 4007 L01)
16
However, the FSO link within the scope of this project is designed to produce a
collimated beam. Therefore, the free space distance d is theoretically free of constraint
and unlimited in the system of Fig. 1. Since the focal length f1 (50 mm) and the lens
diameter (1 in. or 25.4 mm) are fixed parameters, angle θ is constant and is about 28.5o.
The optical power is measured at A (PLED) and at D (PPIN) and the coupling efficiency
is calculated as the percent ratio of the two values as shown in Table I, where Vp-p
denotes the voltage amplitude for LED square waves set at different frequencies. Data in
Table 2 was taken with our designed driver and receiver boards using link configuration
shown in Figure 10. The responsivity is calculated according to Eq. 5.
Table 2. Power versus frequency measurements on our designed Tx and Rx boards
Freq (MHz)
160
100
50
Vpp (mV) (1sf)
785
960
965
PLED (mW)
0.75
0.79
0.8
PPIN (mW)
0.49
0.53
0.55
Efficiency (%)
65
67
69
R (V/W)
1.6E+03
1.8E+03
1.8E+03
An evaluation of our optical channel is set up as illustrated in Figure 12. The Rx
is moved sideway relative to the Tx, i.e. it is offset by a distance s (Figure 12) from the
collimated beam. Thus the power collected at PIN is decreased accordingly, and is
recorded with respect to the operating frequency and BER.
FSO Group (ECE 4007 L01)
17
Figure 12. Power budget evaluation using alignment offset.
The responsitivity of our receiver board is evaluated in this procedure with respect
to frequency as shown in Table. II. The evaluation procedure was performed at 160Mbps
on two sets of boards: 1) the MAXIM evaluation board as the LED driver MAXIM
3967A and a 1GHz PIN with a separate post amplifier. 2) Our designed driver and
receiver as described in earlier sections. Measured parameters include received power,
the mean voltage of logic ‘1’ (I1) and logic ‘0’ (I0), and the RMS noise voltage for both
logic levels (σ1 and σ0) (Appendix 1 and 2). Q values and therefore BER can be
calculated according to Eq. 8-9 below. The received power is plotted against offset s and
BER is plotted against the received power for both sets of boards in Figure 13-15.
Q
BER 
I1  I 0
1   0
 Q2 
 2 


1
 Q  e
erfc 

2
 2  Q 2
FSO Group (ECE 4007 L01)
[Eq. 8]
[Eq. 9]
18
Table 3. Responsivity evaluated at 160MHz for our receiver board
s (mm)
PPIN (mW)
Vpp (mV)
R (V/W)
0
0.49
785
1.6E+03
2
0.48
740
1.5E+03
4
0.45
640
1.4E+03
6
0.41
535
1.3E+03
8
0.33
405
1.2E+03
10
0.24
290
1.2E+03
12
0.16
174
1.1E+03
14
0.08
71
8.9E+02
16
0.04
12
3.0E+02
1.4
1.0E+00
1.0E-01
1.0
1.0E-02
0.8
BER
Received Power (mW)
1.2
0.6
1.0E-03
0.4
1.0E-04
0.2
1.0E-05
0.0
0
2
4
6
8 10 12
Offset s (mm)
14
16
(a)
0
18
0.2
0.4
0.6
0.8
1
1.2
1.4
Received Power (mW)
(b)
Figure 13. Measurements at 160 Mbps on set 1 boards for a) the received power vs offset
s and b) BER vs the received power.
FSO Group (ECE 4007 L01)
19
0.6
Received Power (mW)
0.5
0.4
0.3
0.2
0.1
0.0
0
2
4
6
8 10
Offset s (mm)
12
14
16
18
(a)
1.0E+00
1.0E-01
1.0E-02
BER
1.0E-03
1.0E-04
1.0E-05
1.0E-06
1.0E-07
1.0E-08
1.0E-09
1.0E-10
0
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Received Power(mW)
(b)
Figure 14. Measurements at 160Mbps on set 2 boards for a) the received power vs offset
s and b) BER vs the received power.
FSO Group (ECE 4007 L01)
20
The power is decreased almost linearly as the offset distance is increased (Figure
13a-14a) since the beam is collimated and therefore the coupled power is proportional to
the offset distance. BER is shown to increase as the received power is decreased in both
Figure 13b and 14b. From Figure 5b, it is concluded that we have achieved a BER on the
order of 10-10 at 160 Mbps significantly exceeding the data rate stated in the project
proposal. At the offset of 14 mm with corresponding received power of 0.08 mW, we
still achieve a BER of 10-8. The eye diagram for our FSO-LED link using our designed
driver and receiver boards at 160 Mbps is shown in Figure 15. All measurements were
independent of the range d. However, in the lab measurements were recorded at the range
d of about 37 mm between lenses for set 1 boards, and at d of 1.011 m for set 2 boards.
Note that this range of transmission is also exceeding the proposed value, and in theory it
can be extended further without power penalty. However, the aiming angle between the
receiver and the transmitter becomes narrower and thus more difficult to perform
alignment. In final products, it is suggested that more than one LEDs and PINs are used
so that the wider angle is covered over large distance of communication, which is highly
achievable as the proof-of-concept has been shown from our measurements. The scope
noise was recorded to be 829.5 uV, and the dark noise of receiver was recorded to be 1.34
mV both in RMS values.
FSO Group (ECE 4007 L01)
21
Figure 15. The eye diagram of FSO-LED link at 160 Mbps for 1ns division
Figure 16. The eye diagram of FSO-LED link at 160 Mbps for 2ns division
4.1.6 PCB Considerations and Revisions
The PCBs used in this project were two layered with the top layer containing signal
traces and the bottom layer being a ground plane. The boards were designed using a
software called "PCB Artist" and were fabricated by Advanced Circuits. The important
considerations to the design of these boards are listed below.
FSO Group (ECE 4007 L01)
22

The LED and PIN diode should be as close to the chips as possible to allow for
quick switching and power efficiency.

The polygon outlined in blue is a copper pour area and should take up as much of
the bottom layer as possible. Microstrip transmission lines are designed to run
above an infinite ground plane; this design combats the effects of magnetic and
electric fields.

The by-pass capacitors for the chip should be as close to the chip pin as possible.
These capacitors filter out noise generated on the microstrip for DC traces.

The transmission lines for +IN and –IN need to be almost identical in size and
length. It is recommended by the chip manufacturer to make the lines 50Ω, but
this proved to be difficult and the board still worked without 50Ω lines (higher
frequencies depend more on line impedance).

The transmission lines should never make 90º turns and should have proper
widths depending on the current flowing through them. This will prevent the
lines from bottle necking the current and acting like fuses.
The PCB layouts for the transmitter and the receiver are shown in Figure 17b and the
actual PCBs are shown in 17a. There are some necessary revisions for the next boards.
Tx revisions:

The top of L2 (connected to the LED) should also connect to VCC. Connecting
to the bypass capacitor C2 is a good choice since they are so close to each other.

Optional: the board size can easily be reduced by using 0402 or 0603 SMD
components. Components this size must be manually created in PCB Artist.
FSO Group (ECE 4007 L01)
23
Rx revisions:

An AC coupling circuit must be implemented between the PIN diode and the opamp. This takes the form of a shunt resistor connected in series with a capacitor.
The resistor drains the leakage current coming from the diode and the capacitor
blocks any DC component of the signal. The capacitor should be fairly large so
that most lower frequencies are passed (if it is to high, the lower frequency
content in a PRBS signal will be filtered out thus causing a substantial amount of
bit error).

The –Vb connection to the PIN diode should be switched with the case ground
connection (make the top thru hole of the diode footprint ground and make the
bottom thru hole –Vb).

Optional: as mentioned with the Tx, the board size can be reduced by using
smaller components.
Figure 17a. Actual Prototype PCB, Receiver is on the left Transmitter is on the right.
FSO Group (ECE 4007 L01)
24
Figure 17b. PCB layouts for the transmitter (left) and the receiver (right).
The blue parts of the boards indicate ground, the red indicates signal traces, the
grey indicate thru holes not connected to ground, and the peach colored circles are holes
used for mounting the boards. Values for each component on the Tx board are found in
Figure A3 in Appendix 3 and for the Rx board in Figure 5.
4.2 Codes and Standards
At this point, there are no standards for FSO communication. The Link that we
will create will exceed the speeds of common wireless transmission which are included
below. All of the following is referenced in [11].
Bluetooth Wireless Technology


Geared towards voice and data applications
Operates in the unlicensed 2.4 GHz spectrum
FSO Group (ECE 4007 L01)
25



Can operate over a distance of 10 meters or 100 meters depending on the
Bluetooth device class. The peak data rate with EDR is 3 Mbps
Able to penetrate solid objects
Is omni-directional and does not require line-of-sight positioning of connected
devices
Certified Wireless USB



Speed: Wireless USB is projected to be 480 Mbps up to 2 meters and 110 Mbps
for up to 10 meters. Wireless USB hub can host up to 127 wireless USB devices
Wireless USB will be based on and run over the UWB radio promoted by the
WiMedia Alliance.
Allows point-to-point connectivity between devices and the Wireless USB hub
Wi-Fi (IEEE 802.11)












Bluetooth technology costs a third of Wi-Fi to implement
Bluetooth technology uses a fifth of the power of Wi-Fi
The Wi-Fi Alliance tests and certifies 802.11 based wireless equipment
802.11a: This uses OFDM, operates in the 5 GHz range, and has a maximum data
rate of 54 Mbps
802.11b: Operates in the 2.4 GHz range, has a maximum data rate of 11 Mbps and
uses DSSS. 802.11b is the original Wi-Fi standard
802.11g: Operates in the 2.4 GHz range, uses OFDM and has a maximum data
rate of 54 Mbps. This is backwards compatible with 802.11b
802.11e: This standard will improve quality of service
802.11h: This standard is a supplement to 802.11a in Europe and will provide
spectrum and power control management. Under this standard, dynamic
frequency selection (FS) and transmit power control (TPC) are added to the
802.11a specification
802.11i: This standard is for enhanced security. It includes the advanced
encryption standard (AES). This standard is not completely backwards compatible
and some users will have to upgrade their hardware. The full 802.11i support is
also referred to as WPA2
802.11k: Under development, this amendment to the standard should allow for
increased radio resource management on 802.11 networks
802.11n: This standard is expected to operate in the 5 GHz range and offer a
maximum data rate of over 100 Mbps (though some proposals are seeking
upwards of 500 Mbps). 802.11n will handle wireless multimedia applications
better than the other 802.11 standards
802.11p: This standard will operate in the automotive-allocated 5.9 GHz
spectrum. It will be the basis for the dedicated short range communications
(DSRC) in North America. The DSRC will allow vehicle to vehicle and vehicle to
roadside infrastructure communication
FSO Group (ECE 4007 L01)
26


802.11r: This amendment to the standard will improve users’ ability to roam
between access points or base stations. The task group developing this form in
spring/summer 2004
802.11s: Under development, this amendment to the standard will allow for mesh
networking on 802.11 networks. The task group developing this formed in
spring/summer 2004
WiMAX (Worldwide Interoperability for Microwave Access and IEEE 802.16)








WiMax is a wireless metropolitan area network (MAN) technology
WiMax has a range of 50 km with data rates of 70 Mbps. Typical cell has a
shorter range
The original 802.16 standard operated in the 10-66 GHz frequency bands with
line of sight environments
The newly completed 802.16a standard operates between 2 and 11 GHz and does
not need line of sight
Delays in regulatory approval in Europe due to issues regarding the use of the
spectrums in the 2.8 GHz and 3.4 GHz range
Supports vehicle mobility for between 20 to 100+ km/hr. The 802.16e standard
will allow nomadic portability
The IEEE 802.16a and the ETSI HIPERMAN (High Performance Radio
Metropolitan Area Network) share the same PHY and MAC. 802.16 has been
designed from the beginning to be compatible with the European standard
Created to compete with DSL and cable modem access, the technology is
considered ideal for rural, hard to wire areas
Infrared (IrDA)



IrDA is used to provide wireless connectivity for devices that would normally use
cables to connect. IrDA is a point-to-point, narrow angle (30° cone), ad-hoc data
transmission standard designed to operate over a distance of 0 to 1 meter and at
speeds of 9600 bps to 16 Mbps
IrDA is not able to penetrate solid objects and has limited data exchange
applications compared to other wireless technologies
IrDA is mainly used in payment systems, in remote control scenarios or when
synchronizing two PDAs with each other
Radio Frequency Identification (RFID)


There are over 140 different ISO standards for RFID for a broad range of
applications
With RFID, a passive or unpowered tag can be powered at a distance by a reader
device. The receiver, which must be within a few feet, pulls information off the
FSO Group (ECE 4007 L01)
27


‘tag,’ and then looks up more information from a database. Alternatively, some
tags are self-powered, ‘active’ tags that can be read from a greater distance
RFID can operate in low frequency (less than 100 MHz), high frequency (more
than 100 MHz), and UHF (868 to 954 MHz)
Uses include tracking inventory both in shipment and on retail shelves
802.20



Considered to be mobile wireless broadband wireless access.
Maximum data rate expected to be 1 Mbps, operating in licensed bands below 3.5
GHz
Supports vehicle mobility up to 250 km/hr
4.3 Constraints, Alternatives, Tradeoffs
The main constraint for the system is the modulating frequency of the LED. LEDs
operating at speeds over 250 MHz are expensive. In order for the FSO link to achieve
speeds above 250 Mbps, additional logic must be employed.
A major trade-off for the FSO link is speed vs. range. As the range of the link
increases, the data rate decreases and vice versa. Due to this trade-off, FSO links must be
designed for either short or long range applications. A primary goal of this project s is to
design a suitable compromise between data rate and transmission range.
5. Schedule, Tasks, Milestones
The project tasks are divided between the transmitting end and the receiving end
of the FSO link. A. Swett and C. Huff are in charge of researching, designing, and
building the FSO transmitter. Likewise, T. Thai and N. Trinh will be researching,
designing, and building the FSO receiver. T. Thai is also responsible for setting up and
evaluating the free space optical channel performance. Deliverables, demonstrations, and
testing will be accomplished as a team. The Gantt chart is shown in Figure 18.
FSO Group (ECE 4007 L01)
28
Figure 18. Gantt chart for FSO project (Green = low risk, Yellow = medium risk, Red =
high risk).
The milestones of the project are shown as blue tick marks in Figure 18. Early
milestones include finishing designs and assembly of the transmitter/receiver. Most of
the team's milestones will be accomplished in the testing phase where link bandwidth and
range should keep approaching the desired goals.
6. Project Demonstration
Each part of the project will be extensively tested in the lab by two groups during
the semester. A. Swett and C. Huff will test the transmitter end while N. Trinh and T.
Thai will test the receiver end. In the end, the two groups will test the two ends and
demonstrate that the connection works according to the requirement set forth by the
project. The final demonstration of the project was held April 23rd. The link was
FSO Group (ECE 4007 L01)
29
demonstrated in the High Speed Optics Laboratory in the Tech Square Research
Building.
There were 2 demonstrations that exhibited connections between the transmitter
and receiver with 1) a minimum speed of 150 Mbps and a distance of 30 cm 2) a speed of
160 Mbps at a distance of 1 m with normal room condition, i.e., normal ambient lighting.
The group set up the two ends of the system with the function generator at the sending
side and an oscilloscope at the receiver side as illustrated in Figure 1. The demonstration
was considered successful with a BER of 10-10.
7. Marketing
7.1 Marketing Analysis
The LED FSO Link will have to compete against Bluetooth and IrDA products
that are currently on the market. Compared to Bluetooth and IrDA, LED FSO will have a
much greater speed. Bluetooth adapters can cost from $20 to $120 depending on
connection types (socket serial, USB, CompactFlash, or PC card). Their operating speed
can achieve up to 3 Mbps but only with integrated system. The speed maxes out at 1
Mbps for adapters with the PC Card connection type. There are fewer products on the
market for IrDA. The prices can range from $6 to $60, depending on the connection
type. The speed of the IrDA connection ranges from 9.6 kbps to 16 Mbps. The speed of
the LED link achieved up to 170 Mbps as tested. This speed can be increase in the future
with better LED selection and improved receiver. The cost of the prototype for both
receiver and transmitter is under $150. In production this cost will significantly decrease
due to mass production and bulk sale of components.
FSO Group (ECE 4007 L01)
30
7.2 Cost Analysis
The total cost of building the prototype is estimated by assuming all four group
members are receiving a typical entry-level engineering’s salary of $60,000 per year and
working 15 hours per week on the project for 16 weeks. The $60,000 salary is for 48
weeks per year and 40 hours per week and therefore the salary is $31.25 hourly. The cost
of labor is shown in Table 2.
Table 2. Labor Cost
Salary
Hourly wage for each member
Total hour per week of all members
Total hour of all members
Total labor cost
$60,000.00
$31.25
60 hours
960 hours
$30,000.00
The cost of the prototype is shown in table 3. The amounts shown are what was proposed
and what actually was spent. Also, there is a bill of materials included in Appendix 2.
Table 3. Prototype Design Parts Cost
Parts Type
EVKit for Tx
Led and PIN-diode
PCBs and Other Parts Used
Total
Targeted Cost
$300
$25
$100
$425
Actual Cost
$300
$12
$130
$442
The total cost for the design and construction of the prototype is estimated to be
$30,442. An estimated target price for the receiver based on parts ordered in large
quantities is approximately $15.
A target price for the final product should be
approximately $50 each after factoring in production cost, marketing cost, and
distribution cost. This price will allow the FSO link to compete with other products on
the market.
FSO Group (ECE 4007 L01)
31
8. Summary
The project was successful according to the initial goals of the project. A link was
achieved at 1 meter with a speed of 160 Mbps and a BER of 10-10. Initially the proposal
included a link with a slower data rate and a longer range and a link with a faster data rate
and a shorter range. In the final product it was discovered that the link range had little
effect on the overall performance. It is possible to achieve greater speeds with future
revisions of both the driver and receiver boards. Link alignment should be studied further
to achieve a more reliable data transfer at wider angles. It is also possible to achieve
better results by using more than one LED and PIN. Also, the pre-bias and modulation
currents should be studied using the evaluation board and the custom receiver to
determine if it is possible to further reduce thermal noise created by the LED. A package
should be created in which an internal power supply (battery), casing, and lens are
integrated. It will also be necessary to interface the transmitter and the receiver with a
data source and memory such as USB and a hard drive. The receiver prototype should be
developed into a commercial receiver with more filtering and error correction stages
along with interfacing.
FSO Group (ECE 4007 L01)
32
9. References
[1]
Suzuki, Tomihiro (July 1986). High-Speed 1.3-um LED Transmitter Using GaAs
Driver IC. Journal of Lightwave Technology, vol. lt-4, no. 7.
[2]
M. D. Kotzin, “Short-range communications using diffusely scattered infrared
radiation,” Ph.D. dissertation, Northwestern Univ., Evanston, IL, June 1981.
[3]
R. Otte, “Low-Power Wireless Optical Transmission,” Delft University Press,
Delft, Netherlands, 1998.
[4]
C. G. Lee, C. S. Park, J. H. Kim, and D. H. Kim, "Experimental verification of
optical wireless communication link using high-brightness illumination light-emitting
diodes," Opt. Eng. 46, 125005 (2007).
[5]
D. Johnson “Handbook of Optical Through the Air Communications,” [Available
Online]: http://www.imagineeringezine.com/ttaoc/lightpro.html, [Accessed Feb. 3, 2008].
[6]
M. D. Kotzin, “Short-range communications using diffusely scattered infrared
radiation,” Ph.D. dissertation, Northwestern Univ., Evanston, IL, June 1981.
[7]
J. M. Kahn and J. R. Barry, “Wireless infrared communications,” Proc. IEEE,
vol. 85, no. 2, pp. 265–298, 1997.
[8]
C. G. Lee, C. S. Park, J. H. Kim, and D. H. Kim, "Experimental verification of
optical wireless communication link using high-brightness illumination light-emitting
diodes," Opt. Eng. 46, 125005 (2007).
[9]
B. Clarke, K. Hamilton, D. Hembree, T. Marsh, and C. Young, “Low-cost, Highspeed FSO Communication Link,” Senior Design project, Georgia Institute of
Technology, April, 2007.
[10] D. Johnson “Handbook of Optical Through the Air Communications,” [Available
Online]: http://www.imagineeringezine.com/ttaoc/lightpro.html, [Accessed Feb. 3, 2008].
[11] Bluetooth Corporation, [Online], [Available Online]:
http://www.bluetooth.com/bluetooth/technology/works/compare, [Accessed Feb. 2,
2008].
[12]
Application Note OPA657, “1.6GHz, Low-Noise, FET-Input Operational
Amplifier”, Texas Instruments Inc., Mar, 2006.
[13]
Available online. http://focus.ti.com/docs/toolsw/folders/print/tina-ti.html.
Accessed May 1, 2008.
FSO Group (ECE 4007 L01)
33
Appendix 1
Table 3. Received power and BER measurements with evaluation Maxim 3967A and the
1GHz-post Amp (using Tektronix CSA907R Error Detector, lowest capable BER
reading is 10-8)
s (mm)
Po (mW)
BER (10^-5)
2
1.2
1.6
4
1.1
1.2
6
0.95
1.8
8
0.8
2.8
10
0.63
3.4
12
0.45
11
14
0.26
110
16
0.08
N/A
Table 4. Received power and BER measurements with our designed boards
(using Tektronix 11801B Digital Sampling Oscilloscope)
mean-V1
mean-V0
RMS
RMS
s (mm)
(mV)
(mV)
Noise 1
Noise 0
Q (calc)
(calc)
0
308
-302
52
48
6.1
5.4E-10
2
305.9
-298
52
47
6.1
5.4E-10
4
268.2
-261
47
40
6.1
4.9E-10
6
221.6
-218
38
35
6.0
7.6E-10
8
174.5
-172
30
28
6.0
7.6E-10
10
125.1
-126
21
21
6.0
1.2E-09
12
74.3
-78.5
14
14
5.6
1.1E-08
14
29.3
-33.7
6
6
5.4
2.9E-08
16
2.4
-8.2
2
2
3.2
7.1E-04
FSO Group (ECE 4007 L01)
BER
34
Appendix 2
Table 5. Bill of Materials
FSO Group (ECE 4007 L01)
35
Appendix 3
Figure A3. Driver Schematic
FSO Group (ECE 4007 L01)
36