Analysis of free space optical communication using LEDs
Chirag Gupta
Nitin Garg
Department of ece
ITM University
Department of ece
ITM University
chirag26121992@gmail.com
er_nitingarg@yahoo.com
ABSTRACT
The Free Space Optical (FSO) LED link has an ability to
connect two devices at high-speed, FSO takes advantage of
the high bandwidth of optical communication and uses low
cost and low power consumption of LEDs. This link provides
an alternative to traditional RF wireless communication that is
currently have bandwidth limitations. As the speed increases
for data transmitted over a wire, it is needed that wireless
communication continues unbounded. The FSO link also
outperforms USB 1.1, USB 2.0, and Bluetooth allowing for an
additional market and could be a new standard for data
transmission. This will moreover become essential as file size
is aggrandizing and multimedia is dominating the business
world.
Cost will be the key advantage of LED system. The FSO
link consists of a transmitter and a receiver. Transmitter has a
target cost less than $55 and operates on two AA batteries.
The receiver’s cost is $55. Overall the system’s cost is
significantly less than a comparable LASER optical link and
draws very less power. The FSO link is to be built using topof-the-line components and cost continues to decrease as the
components become standard. Again, LEDs are more
directional than radio waves, which prevents eaves-dropping.
Project results were tested, debugged, and analyzed at a
working link of one meter with a speed of 180 Mbps and a bit
error rate 10-10. Average consumer who has a need for highspeed and low cost data transmission are the target markets of
this project.
Keywords
LED, FSO, PIN photodiode
INTRODUCTION
There are diverse forms of wireless communication that are
available such as satellites/antennas, WiFi, and FSO
communication with lasers or VCELs.
Recently, no
application of FSO communication uses a LED as
transmitter light source. LED will send data via free space
between a transmitter and a receiver in FSO link.
The main goal of the FSO link using a LED is to create a new
form of optical communication that can has small size and
low cost utility. Transmitter and receiver used in the link are
portable and powered by battery. Data stream of "(+3.6V)1s "
and "0s (0V)" is taken by transmitter circuitry and then the
current is modulated through an LED.
Photons with
intensities proportional to the input data bits across the free
space link to a PIN photo-diode are send by LED. Optical
power is converted into an output voltage by receiver which is
connected to PIN diode. Each module is approximately of the
same size and should accommodate all components needed to
function. The LED, PIN diode, and other circuitry would be
able to operate on a portable battery due to low power
consumption characteristics. As a result lower marketing cost
is provided by inexpensive parts and low power consumption.
The link is designed for two different applications. First is
short range application, such as, sending data from a flash
drive to a computer, has a data transfer rate of 90-100 Mb/s at
a range up to 12 cm. Second application can send data at
30Mb/s at a range up to 1 m, and this application could apply
to a send data between a central classroom computer and
students laptop.
PROJECT DESCRIPTION
This project has two important tasks:


Link with data rate of 90-100 Mbps at a range of 12
cm in a controlled environment is to be created,
(short range application).
To achieve a data rate of 30 Mbps at a range of 1.2
m under normal conditions using the same link,
(long range application)
The controlled conditions for a FSO are defined as link
maintained within 12 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 1.4 m being exposed
to ambient light sources in a closed space. The test will be
conducted in the dim sunlight. Transmitted data bits from a
voltage generator to an oscilloscope through free space using
light from an LED are used to design LED link. The operation
range of LED is the spectral range of infrared (above 700 nm
in wavelength). A PIN photodiode is used on the receiver
circuit to collect the light sent from the transmitting LED. A
system block diagram is shown in Figure 1.
Figure 1
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 30 Mbps at the range of 1.2 m. Due to the
theoretical limitation of the LED operation, the maximum
data rate may not give 1Gbps. Nonetheless, at the
transmission speed of 100 Mbps, the low cost LED link can
fully replace a wired LAN network and can enable wireless
communication between many portable devices such as ipots,
digital cameras, portable storages, and laptops, mobiles.
Theoretically, response of the circuit should be flat line to step
input, but ringing response is observed. Response is ringing
due to the imperfections in parasitic capacitance inserted and
improper soldering.
Tx and rx pcb design
The PCBs which are used in this project were two layered.
The top layer contains signal traces and the bottom layer is a
ground plane. Software called "PCB Artist" was used to
design the boards and the boards were fabricated by
Advanced Circuits. Following fig. 2 shows the PCB layout for
transmitter and receiver. Left layout represents the transmitter
and right represents receive respectively.
Figure 3
Following figure 4 shows the Bode plots of voltage gain
magnitude and phase. The transfer function calculation shows
that receiver has a bandwidth of about 180MHz and a constant
phase.
Figure 2
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.
If these specification are not meant link might not work.
Table 1. Proposed Versus Actual FSO Link Specifications
Specifications
Tx Power
Consumption
Tx Voltage
Rx Power
Consumption
Rx Voltage
Link Range
Link Bandwidth
RESULT
Proposed
250 mW
Actual
690 mW
+5.2 V
210 Mw
+3.6 V
94 mW
+5.2 V
11cm  1m
30  100 Mbps
-5 V, +5 V
10 cm  1.05m
25  130 Mbps
Figure 4
Here, BER is taken on the order of 10-12 at 160 Mbps
significantly exceeding the data rate stated in the project
proposal. The eye diagram for our FSO-LED link using
designed driver and receiver boards at 180 Mbps is shown in
Figure 3. All measurements were independent of the range d.
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.
CONCLUSION
The project was successful according to the initial goals of the
project. A link was achieved at 1.2 meter with a speed of 180
Mbps and a BER of 10-12. 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 amendments of both the transmitter and receiver
boards. Link alignment plays an important in achieving a
more reliable data transfer at wider angles. It is also possible
to generate better results by using greater number of LEDs.
It will also be essential to interface both transmitter and
receiver with a data source and memory such as USB. The
market of FSO is serendipitous due to its low cost.
Figure 5
Following is the eye diagram os FSO-LED observed for 2ns
division at 180Mbps. It give a wider picture of scenario
observed.
REFERENCES
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Figure 6
MARKET AND COST ANAYLSIS
The main contestants of LED FSO Link would be Bluetooth
and IrDA products that are currently on the market. LED FSO
will have a much greater speed compared to its contestants.
Cost of Bluetooth adapters might be from $20 to $120
dependent on connection types (socket serial, USB,
CompactFlash, or PC card). If used with integrated system
operating speed of these devices can be up to 3 Mbps. With
PC Cards speed maxes out at 1 Mbps. IrDA operated products
are less in market. The price ranges from $5 to $65,
depending on the type of connection. IrDA speed connection
ranges from 9.8 kbps to 15 Mbps. The speed of the LED link
achieved up to 180 Mbps as tested. This speed can further be
increased in the future with the selection of better LED and
improvement in receiver. The cost of the prototype for both
receiver and transmitter is under $170. For mass production
this cost will significantly be decreased due to bulk sale of
components.
[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, High-speed FSO Communication
Link,” Senior Design project, Georgia Institute of
Technology, April, 2007.
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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/c
ompare, [Accessed Feb. 2, 2008].
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Mar, 2006.