A MULTI_Gbps OPTICAL WIRELESS TRANSMISSION LINK
Wen Zu
Department of Electrical and Computer Engineering
Ryerson University
350 Victoria St · Toronto, Ontario
Canada · M5B 2K3
Email: wzu@ee.ryerson.ca
ABSTRACT
Optical wireless is increasingly becoming an attractive
option for multi-gigabit-per-second (multi-Gb/s) short
range (up to 3.5 km) links where laying optical fiber is too
expensive or impractical. This article gives a practical
example of these links – a 5-beam optical wireless
transmission system. This link is running at 1550nm. It
provides wireless communication of digital signals up to
280m at -220dB/km atmospheric attenuation. The
bandwidth ranges from 1Gb/s to 4Gb/s depending on
weather. This system includes the necessary alignment and
tracking subsystem, and an adjustment subsystem. It has
large transmit optics that enables to satisfy all those eye
safety limits. Also with large receive optics, the alignment
and tracking of the optical axes are very easy over these
link distances. The integrated five beam channels and the
adjustment system assure excellent system dynamics that
guarantees highest availability even when reaching the
maximum link range.
Keywords: free space optics, FSO, optical wireless,
atmospheric attenuation, availability, link range, link
distance, bandwidth, RF, last mile
1. INTRODUCTION
Free space optical transmission, also known as FSO, has
become established over the past 10 years for the ultimate
customer just as for the carrier market as one of the
leading technologies for bridging the “last mile” without
wiring. Apart from transmitting only data it is also
possible to combine the transmission of data and voice
signals via an optical communication system. An
advantage of this technology is the transmission of signals
in the infrared band range of frequencies. Here clearly
higher bandwidth can be realized in contrast to the by far
better known RF and microwave systems. RF and
microwave technologies allow rapid deployment of
wireless networks with data rates from tens of Mb/s (pointto-multipoint) up to several
Figure 1. Flexible topologies of FSO
hundred Mb/s (point-to-point) [1]. However, spectrum
licensing issues will limit their market penetration. Optical
wireless can augment RF and microwave links with very
high (>1 Gb/s) bandwidth. In fact, it is widely believed
that optical wireless is best suited for multi-Gb/s
communication at a proper link range (<500m). The
advantages of this link are the followings:
 Transmission of very fast digital signals (up
to 40Gbps) over the “last mile”. [2]
 Quick, easy and flexible to install making it
also very suitable for interim applications.
Figure 1 shows the flexible topologies of
FSO systems.
 No effort required to obtain operating
approval from authorities.
 Ideal for secondary path implementation in
high speed networks and particularly for
backup solutions.
 Highly cost-effective, joint transmission of
voice and data signals in connection with
multiplexing.
optics. For (2), we design an alignment & tracking
subsystem and a large aperture receive optic.
2. SELLECTION OF WAVELENGTH
Figure 2.Mie scattering attenuation in dB/km for the
various fog distribution models. From R.M. Pierce et al,
“Optical attenuation in fog and clouds,” in Optical Wireless
Communications IV, Eric J. Korevaar, Editor, Proceedings of
SPIE Vol. 4530, 68 (2001).
One disadvantage of FSO systems is that laser power
attenuation through the atmosphere is variable and
difficult to predict, since it is weather dependent. From
Figure 2, we can see the most significant loss mechanisms
among atmospheric attenuation- Mei scattering attenuation
@1550nm is about -220dB/km (Max.). Attenuation higher
than 250dB/km in very heavy fog is occasionally observed
around the world. This factor limits the distance at which
FSO should be deployed.
Another disadvantage of FSO system is the
consideration of the eye safety. Because it launch laser to
the public space, laser eye-safety is an important
responsibility of the equipment manufacturers and service
providers. Current safety standards address laser eyesafety issues and provide guidance to our design.
Depending on these standards, we can calculate the max.
power dense is 100mw/cm² at 1550nm. [3]
To reach longer distance under eye safety limit, the only
way is to increase the maximum power which receivers
can get in the FSO system. There are two ways to fulfill
this: (1) Increase the transmitter output power maximally,
but do not exceed the eye safety limit. (2) Optimize optic
system to guaranty all output power from transmitters can
reach the surface of receivers.
We focus on the following steps to meet (1): (a)
Selecting proper wavelength. (b) Designing a 5- beam
channels and matched adjustment subsystem system. (c)
Using high output power lasers and large aperture transmit
Historically, most FSO systems have been designed to
use wavelengths in the near-visible infrared spectral region
(~ 780 nm to ~ 850 nm), principally because of the
availability of efficient and reliable semiconductor diodebased sources at those wavelengths, and, for the 780 nm
devices, the cost advantages of utilizing the same
wavelength as is used in CD writers. In the past few years,
systems operating at 1550 nm have been developed. Also,
some researchers developed FSO systems based on 9~10
micron laser. However, with further analysis and research,
those ideas were withdrawn.
Despite the fact that the most of the early optical
wireless systems operated in the near infrared at
wavelengths of 785 nm or 850 nm, for the following
reasons, a better choice is to use wavelengths near 1550
nm.
 50 times more transmitted power at 1550 nm
than 800 nm considering the eye safety limit.
Food and Drug Administration (FDA) at Unite
State considers power density of about 100
mW/cm² at 1550 nm (or 2 mW/cm² at 780
nm) safe to the unaided eye [3].
 Receivers have nearly 3 dB better receiver
sensitivity at 1550nm than 850nm. Because
the 1550 nm photon has half the energy of a
780 nm photon, for the same (electronic)
preamplifier noise, an optical pulse at 1550
nm can be detected with ~ 3 dB less optical
power.
 1550nm is the most commonly specified
wavelength range for fiber-based optical
communication. The supporting technical base
for this wavelength range is vast and growing
rapidly every year. Therefore, it will be easy
to access new cost-effective technologies to
update our design, and to keep this design on
the top performance.
.
3. DESIGN OF MAIN STUCTURE
TRANSCEIVER
DATA SEPARATOR
Data in
Data out
Adjustment system
MODULATOR 1
LASER 1
Transmit optic 1
MODULATOR 2
LASER 2
Transmit optic 2
MODULATOR 3
LASER 3
Transmit optic 3
MODULATOR 5
LASER 4
Transmit optic 4
Demoulator 5
Alignment and Tracking
system
Pre-Amplifier and filter 5
InGaAs APD Detector 5
Receive optic 5
LASER 5
Transmit optic 5
2D SERVO SYSTEM
Figure 4.Strcture of Transmit Head.
TRANSCEIVER
Data out
Data in
DATA COMBINER
MODULATOR 5
Demoulator 1
Pre-Amplifier and filter 1
InGaAs APD Detector 1
Receive optic 1
Demoulator 2
Pre-Amplifier and filter 2
InGaAs APD Detector 2
Receive optic 2
Demoulator 3
Pre-Amplifier and filter 3
InGaAs APD Detector 3
Receive optic 3
InGaAs APD Detector 4
Receive optic 4
Demoulator 4
Alignment and Tracking
system
Pre-Amplifier and filter 4
2D SERVO SYSTEM
Figure 5.Strcture of Receive Head.
Our system is a five-beam FSO system. That mains it
has five laser transmitters, four in Transmit Head, one in
Receive Head; five APD receivers, one in Transmit Head,
four in Receive Head. Also, it has five modulators, five
demodulators, five receive optics, five transmit optics, and
five pre-amplifier & filters. Figure 4 and Figure 5 show
this completed structure. Figure 6 shows the surface of
Transmit Head – Tx Matrix, and surface of Receive Head
Tx Matrix
vertically until the Rx1 gets the maximum
power.
Rx Matrix
Tx1
Rx1
44
44
Tx2
Rx2
Ø1
0
5.0
Ø1
Tx3
44
5.0
Ø1
44
Rx5
Ø1
Tx5
0
Rx3
Rx4
Tx4
Figure 6.Tx Matrix and Rx Matrix
– Rx Matrix. We designed an adjustment subsystem to
turn the angles of these transmit optics for higher data rate
in clear air, or better BER in fog or rain. Two alignment
and tracking subsystem and matched 2D Servo systems
were used in our system to maintain a top performance in
all condition. We employed excellent transmit optics to
collimate the output beams, and to minimize the spread of
these beams for a long distance. Through receive optics,
those beams totally focus on the surface of receivers.
Figure 7. Use of Tracking system. From CANON,
“Introduction of an Optical Wireless communication
system, 2002”
4. ALIGNMENT ANDTRACKING SUBSYSTEM
When FSO systems are settled on the building’s roof or
any other place, we need to use Alignment and Tracking
system (A&T system) to co-align the axes of transmit
optics and receive optics. This process consists of the
following steps:
A. Roughly aligning Transmit Head and Transmit
Head manually. Finely align two head’s central
line parallel to the ground.
B. A&T system at Transmit Head measures the
response Rx5 (Figure 6), give instruction to the
2D servo system. The servo system turns the
Transmit Head’s angle horizontally until the Rx
5 gets the maximum power. Then, A&T system
repeats this process, except the 2D servo system
turn the system angle vertically until the Rx 5
gets the maximum power. After this step, these
five beams should reach the center of those five
receive optics independently. A&T system at
Transmit Head gives signal to A&T system at
Receive Head through channel Tx3 and Rx3
C. A&T system at Receive Head controls 2D its
servo system to turn Receive Head’s angle
horizontally until the Rx1 gets the maximum
power. A&T system repeats this process, except
the 2D servo system turn the system angle
Figure 8. Performance of Tracking system. From
CANON, “Introduction of an Optical Wireless
communication system, 2002”
After this step, the axes of the transmit optics and
receive optics should be conjugated.
For FSO systems, a tracking scheme is essential to
maintain proper pointing of the transceivers at each other
to establish error-free communication. For transmitters, the
tracking ensures that narrow beams is pointed at receivers
with minimal residual jitter in the presence of atmospheric
beam-wander, building sway, and wind or temperature
loading effects. For receivers, the tracking ensures a tight
focus on a relatively small detector in the presence of
atmospheric induced angle-of-arrival fluctuations, roof
vibration caused by air-conditioning units, wind loading
effects, or uneven thermal loading, e.g. sunshine on one
side. Tracking system can keep the pointing jitter or drift
to less than 100 µrad.[4] Tracking subsystem in our
systems allows us to use sub-milliradian (<0.5mrad)
beams for communication. Figure 7 and figure 8 present
samples of the tracking system working and performance.
Tx Matrix
Rx Matrix
T1
R2
R1
T5
T2
T3
T4
R5
T1
R3
(a)
R4
R2
R1
another part is sent to the channel 3, 4, as shown in Figure
9 (b). Data parts will reach Receive optic 1&2 and 3&4. In
heavy fog, Adjustment system will turn all the three angles
of Transmit optic 1, 2, 4, making beams reach Receive
optic 3. Data separator will keep input data, and send it to
all four channels. Finally, receivers get maximum power,
as shown in Figure 9(c). In all these processes, Adjustment
system controls the data combiner at Transmit Head
through Tx1-Rx1 channel synchronously.
T5
T2
R3
T3
T4
R5
T1
(b)
R4
R2
R1
T5
T2
R3
T3
T4
R5
(c)
R4
Figure 9. Five-beam structure and Adjustment
subsystem
Tracking system uses the same mechanical structure
with Alignment system, but different working ways. The
following is its three working processes:
· Tracking system operates every 1 hour
automatically, and repeats A step in the
beginning of this section.
·Tracking system operates monthly, and repeats
A+B.
· Tracking system operates yearly, and repeats
A+B+C.
·Yearly check those two head’s central lines, be
sure they are parallel to the ground.
5. ADJUSTMENT SUBSYSTEM
The main feature of our design is the five-beam structure.
Its working status is controlled by the adjustment system.
According to weather/BER, Adjustment system turns
angles of Transmit optics. Figure 9 shows this process.
When the air is clear, Adjustment system control the data
separator to divide input data to four parts. Transmit Head
uses four individual channels to transmit every data part.
Receive Head also uses four channels to receive data parts,
and send them to the data combiner to recover data, as
shown in Figure 9(a). If the air is not clear, or the BER
decrease to a threshold, Adjustment system turn the angle
of Transmit optic 1 and 4, inducing their beams reach the
surface of Receive optic 2, 3 individually. In the meantime,
Adjustment system controls the data separator to divide
input data to two parts. One part is sent to the channel 1, 2,
The angles that Adjustment system turns can be
initialized at the settlement of the system depending on the
link range, and will not be changed in the future, except
re-settle system.
6. TRANSMIT OPTIC AND RECEIVE OPTIC
Transmit optics in our system have two functions. One
is to collimate the output beams. This keeps the beams
reach the surface of Receive optic with a very small spread.
Another function is that choosing a large transmit aperture,
we can use high output power lasers and still maintain eyesafe levels. Assuming Dense of power on Transmit optic
equals Eye safety limit (100mw/cm²@1550nm), from
laser power is 1w, and transmit area is πr², we can get
r:1.78cm. So, we choose Transmit aperture 4cm.
Receive optic focus the beams on the surface of APD
receivers. Choosing large Receive Aperture could
simplify the design of Alignment and Tracking. Assuming
Transmit Divergence equals 0.1 mrad(1/e² ), beams spread
at 280m equals 7.9cm. We choose Receive Aperture:
15cm.
7. LINK MARGIN AND RANGE
7.1. Assumptions:
Transmitter: Laser @1550nm x 4
Average Laser Power: 1000mw/30dBm
Transmit Divergence: 0.1 mrad(1/e² )
Transmit aperture: 4cm
Receiver: InGaAs APD @1550nm x 4
Receiver Sensitivity: 1000nw/-30 dBm
Receive Aperture: 15cm
Max Data Rate: 4Gbps
BER: 1.00E-12
Transmit Optics Degradation: -1dB
Receive Optics Attenuation: -1dB
7.2. Calculation
Link margin= Transmit Power x 4 (36dBm)-Receiver
sensitivity (-30dBm)-Geometric Range Loss(1dB)Transmit Optics Degradation(1dB)-Receive Optics
Attenuation(1dB)-Filter Loss(1dB)= 62dB
From Figure 2, we can see the maximum atmosphere
attenuation at 1550nm is -220dB/km. So, the link range is
281m @ 1G/bs or 254m @.4G/bs.
8. CONCLUSION
From this design, we can see that using 5-beam
structure will not increase the link range greatly.
Comparing two-beam structure (one for upload, one for
download), the link rang difference between these two
systems is 27m. However, multi-beat will greatly increase
the data rate of FSO system. For some areas the clear air is
the majority weather condition or heavy fog (atmosphere
attenuation >-150 dB/km, visibility range of about 113 m
[6]) is very rare, the performance of this system is
significant (347m@4G/bs, atmosphere attenuation:
-150dB/km). The better way to increase link range is to
choose high performance APD receivers with extra-lower
sensitivity (1nw).
The design of Transmit optics and Receive optics is one
of the most difficult, but also most important part in our
system. If we can work out Transmit divergence small
than 0.05 mrad(1/e²), and increase the aperture of Receive
optics to 40cm, we can cut the Alignment and Tracking
system. The system could still keep high performance in
all weather. What we need to add is only an accessorial
tool for fast alignment used when we settle the system at
the beginning. Although Alignment and Tracking system
is essential at current systems, it is complex, expensive,
and it lowers the liability of whole system.
9. REFERENCES
[1] J.R. Barry, “Wireless Infrared Communication”,
Kluwer Academic Press, Boston, 1994, 1st edn.
[2] P.L. Eardley and D.R Wiseley, “IEE Proc.Optoelectron”, 143, 330 (1996) A.B. Smith, C.D. Jones,
and E.F. Roberts, “Article Title,” Journal, Publisher,
Location, pp. 1-10, Date.
[3] Class 1M limit under International Electrotechnical
Commission (IEC) 60825 which has been temporarily
adopted by FDA’s Center for Devices and Radiological
Health (CDRH)
[4] Dr. Muthu Jeganathan and Dr. Pavel Iono, “MultiGigabits-per-second Optical Wireless Communications v”
[5] Jim Alwan, Ph.D. “EYE SAFETY AND WIRELESS
OPTICAL NETWORKS”
[6] Scott Bloom, PhD, “THE PHYSICS OF FREE-SPACE
OPTICS”