AN EXPERIMENTAL HYBRID FSO/RF COMMUNICATION SYSTEM

Ahmet Akbulut, Murat Efe, A. Murat Ceylan, Fikret Ari, Ziya Telatar, H. Gokhan Ilk and Serdar Tugac
Ankara University
Faculty of Engineering
Electronics Engineering Department
06100, Tandogan, Ankara, Turkey
{aakbulut,efe,mceylan,fari,telatar,ilk,tugac}@eng.ankara.edu.tr

Abstract atmospheric attenuation. The attenuation occurs due to

many factors such as
This paper presents an experimental free space optical
(FSO) communication system combined with a redundant - Absorption (caused primarily by the water
radio frequency (RF) communication link that provides vapor and carbon dioxide)
high availability and continuous communication even in - Scattering (depends on the wavelength used and
adverse weather conditions. The experimental hybrid the number and size of scattering elements in the
system provides a wireless connection between the two of air, i.e., fog)
the five campuses of Ankara University that are located at - Shimmer (caused by a combination of factors,
different locations in the city. In this paper, the necessity including atmospheric turbulence, air density,
of such a system is explained, rationale behind the light refraction, cloud cover and wind).
selection of the design parameters is given and the
structure of the hybrid system is outlined. Some results The effect of all these factors appears as an atmospheric
regarding the link availability and the communication attenuation constant in the formulation that produce the
traffic of the system that has been operational for over 4 level of received power at the receiver (the link budget
months are also presented1. equation) and is uncontrollable in an outdoor
environment. Thus, not surprisingly, in heavy attenuation
Key Words conditions the operation of an FSO link cannot be always
Hybrid FSO/RF, laser communication, link availability maintained, which reduces the availability. This problem
should be addressed properly in order to achieve a high
available link. A practical solution to this problem would
1. Introduction be to back up the FSO link with a lower data rate RF link.
Moreover, employing an RF link as a back-up to the FSO
With the recent developments in semiconductor to create a hybrid FSO/RF system would provide a cheap
technology, free space optical (FSO) or optical wireless (almost 10% cheaper than fiber optic link) and easily
communication has become an attractive alternative to deployable solution. In fact, having a radio and laser in
optical fiber communications or radio frequency (RF) tandem works particularly well since microwave
systems. An FSO system offers much higher data rates transmission is affected more by rain (as the carrier
when compared with an RF system, also easier to deploy wavelength is closer to the size of a rain drop) and laser
and much less expensive than underground fiber. transmission is affected more by fog. Hence, The only
Although, fiber-optic cabling is still the preferred media weather condition that could affect the transmission of a
for long distance, high bandwidth communication, hybrid FSO/RF system is the condition of simultaneous
because of FSO’s lower cost and significantly shorter heavy rain and thick fog. Qualitatively, it could be said
installation time, FSO is now considered a viable option that these conditions would not occur simultaneously,
to fiber for short distances of 4 km or less [1,2]. because as the rain falls, the rain droplets would absorb
the suspended fog water droplets, thus diminishing the
However, despite the advantages that an FSO system fog. Moreover, the fact that the simultaneous occurrence
holds over underground fiber links and almost equally of heavy rain and thick fog does not happen has been
less costly (in terms of deploying a link) RF links, there is quantitatively analyzed and the results are presented in
one big challenge for optical wireless and that is the [3]. Hence, the combination of a laser and a radio working
in tandem renders a high available communication link
1 where the availability reaches 99.999% [3].
Research supported by Ankara University Scientific Research
Projects, Project No: 2001-00-00-006.
Ankara University, where the experimental system has automatically switches over to the RF link, which is
been set up, is one of the biggest universities in Turkey waiting as a hot stand-by.
and has five campuses each encompassing several
faculties that are located at different locations around the Thus, despite the reduction in the bandwidth, the link
city. Such a structure requires Ankara University to resort remains available. The RF link operates at 2.4 GHz
to commercially available communication infrastructure, linking the two terminals at 11 Mbps. After the
as it is legally impossible to dig up public land and lay its switchover has happened the RF link stays as the primary
own cables. Therefore, as a license free and legal option it link until the received signal level goes over a higher
has been decided that an FSO link would provide a good threshold, indicating a solid connection on the laser link
means to connect the two campuses that have the most can be re-established. When the increased signal level is
data and voice communication. Moreover, in order to detected the switch switches back to the laser link. The
increase the availability of the link, an RF back-up link calculation for the switchover threshold levels is given in
has been decided to be integrated with the FSO system. the next section.

2. The Hybrid FSO/RF System 3. General Design Considerations

The experimental system is designed to make use of the The system has been designed to communicate through
best features of two transport mediums, laser light and the FSO link unless the performance of the FSO is
radio waves to form a single, high available, seamless degraded due to adverse weather conditions. When the
wireless communication link between the two campuses. degradation occurs and the quality of the communication
As shown in Figure 1, the system comprises three sub- reduces the system switches over to the RF link via a
systems namely, i) the laser link, ii) a switch and iii) the switch. A threshold level on the received laser power
RF link at each end. must be determined for a seamless switchover to the RF
link. In threshold level calculations, the gains and losses
Under normal operating conditions data is transmitted of optical components have been neglected and a 3 dB
over the laser link where the laser link provides a 155 margin has been added on top of the calculated level
Mbps full duplex connection. The wavelength of the laser instead in order to account for then neglected optical
used is 1550 nm. Other available wavelengths for FSO component losses. Even when the system switches to the
applications are 780 nm and 850 nm near infrared radio, the received power level is still monitored for a
spectrum. However, 1550 nm band, also the choice for the quick recovery of the laser link so that a much higher
fiber optic telecommunication applications, is better bandwidth could be utilized. However, the system does
suited for optical wireless. The main benefit of the 1550 not switch back over to the laser link even when the FSO-
nm band is the ability to transmit more power. The power to-RF switchover threshold power level is achieved again.
density at 1550 nm band nearly 50 times that at 780 nm In fact, the power level to go back on the FSO is higher
and is still safe for the human eye. Since, more power than the FSO-to-RF threshold. The reason for having two
means better ability to overcome atmospheric attenuation, different switchover thresholds (a higher one for
the 1550 nm band has been chosen. switching back to the laser from the radio than the one to
switch from the laser to the radio) is to reduce the
Despite the more power, in heavy attenuation the laser switchover frequency and prevent the system switching
signal might still get degraded. The signal level at the back and forth constantly. Next two subsections give the
receiver gets checked every 5 seconds and when the design parameters for both the laser and the radio and
received signal level falls below a certain threshold, the show how the FSO-to-RF switchover threshold has been
laser stops transmitting data to the switch. As soon as the calculated
degradation at the signal level is detected, the switch
1550 nm FSO connectivity
Laser Laser
Transceiver Transceiver

Data Data
Switch Switch
in/out Transmission path (2.9 km) in/out

RF RF
Transceiver Transceiver
2.4 GHz RF connectivity
Figure 1: Basic hybrid FSO/RF system architecture
3.1 Laser Link The attenuation coefficient has contributions from the
absorption and scattering of laser energy by different
The laser link refers to a pair of FSO transceivers each aerosols and molecule in the atmosphere. Since laser
aiming a laser beam at the other, creating a full duplex wavelength is chosen to fall inside transmission windows
communication link. A laser system requires careful of the atmospheric absorption spectra, the contributions of
planning and analysis prior to equipment installation. A absorption to the total attenuation coefficient are very
poorly designed path may result in periods of system small. The effects of scattering, therefore, dominate the
outages, increased system latency, decreased throughput total attenuation coefficient. The type of scattering is
or a complete failure of communication across the link. determined by the size of the particular atmospheric
Hence, before determining the link feasibility, link’s particle with respect to the transmission laser wavelength.
atmospheric attenuation and geometrical spreading loss
must be calculated. The final atmospheric phenomenon that significantly
impacts laser propagation is scintillation. At low altitudes,
The parameters taken into account when calculating the scintillation effects arise from the temperature differences
link equation of the transmitter and receiver used in the between the ground and air and the resulting heat
laser subsystem are given in Table 1. exchange. The index of refraction of air changes with
temperature and the heat exchange causes local index
Transmission rate 155 Mbps. variations that have different scale sizes that effect the
Range 2.9 km laser propagation. The index changes result in lensing
Laser output power 640 mW effects along the optical path. The dominant scintillation
(4 transmitter at 160 mW) effect occurs when the scintillation scale is comparable to
Transmitter beamwidth 2.8 mrad the beam size. This scale size event defocuses the beam
Wavelength 1550 nm and leads to significant intensity variations in the received
amplitude of the laser signal. These “fades” in signal
Transmitter type Directly modulated laser diode
amplitude have nominally been in the - 7 to -10 dB range,
Modulation format OOK (NRZ waveform)
although deeper events have been measured. To overcome
Receiver area 314 cm2
these coherent events, systems have used multiple
Detector PIN photodetector. transmitting apertures of sufficient separation and
Table 1. Transmitter and receiver parameters of the laser temporally incoherent laser transmissions. As a result, the
system systems have regained some of the fade losses by making
the individual beams uncorrelated to the atmospheric
Laser links can be affected by a variety of atmospheric scale size.
phenomena, fog and scintillation being the most common
by far. Free space, low altitude laser transmissions are To determine the atmospheric attenuation of optical
range and bit error limited by the atmospheric energy signals, the following equation is utilized to base the
losses resulting from scattering during haze, rain, snow, attenuation on the visibility and the incident wavelength λ
and fog conditions in the atmospheric channel. Most free [5].
space laser transmission wavelengths are primarily chosen −δ
for their very low absorption losses, so that molecular 3.91  λ 
energy transitions do not absorb free space laser energy. σ=   (2)
V  550nm 
The attenuation of laser power through the atmosphere is where, σ is atmospheric attenuation (or scattering)
described by the exponential Beers-Lambert Law [4]. coefficient, V is the visibility (in km), λ is the wavelength
(in nm) and δ is the size distribution of the scattering
P( R) particles. For the exponent δ, 1.6 indicates a good
τ ( R) = = e −σR (1)
P( S ) visibility (V>50 km), 1.3 a visibility V=6-50 km and
where τ(R) = transmittance at range R, P(R) = laser 0.585xV1/3 for visibility less than 6 km.
power at range R, P(S)= laser power at the source, and
σ = attenuation or total extinction coefficient (per unit Table 2 presents the atmospheric attenuation values at
length) 1550 nm band calculated using Eqn. 2.

Visibility (km) 0.1 0.2 0.3 0.5 1 2 3 5 10 20 50

dB/km (1550 nm) 128.2 59.6 37.7 20.9 9.2 3.9 2.3 1.2 0.44 0.22 0.06

Weather Fog Haze Clear

Table 2. Atmospheric loss (in dB/km) with respect to visibility at 1550 nm.
Another loss that the laser beam launched by transmitter frequency bands require licensing, an unlicensed radio
into the atmosphere experiences is geometrical spreading band would be more suitable when considering a backup
loss. Geometrical spreading loss is the expected for FSO in order to keep the package as a whole
attenuation of a signal as it travels away from the unlicensed. Therefore, the RF part of the system has been
transmitting device. When a signal radiates from the chosen operate at 2400 MHz-2483.5 MHz (13 channels)
transmitter, it spreads out over an increasingly larger ISM frequency band and use 802.11b DSSS technology
distance. As the area covered increases, the power density that operates either at 11 Mbps or 5 Mbps data rate with
(or the amount of power per unit area) decreases. This CCK modulation. The RF link can also operate at even
effectively weakens the laser signal. Link equation for lower data rates, namely 1 Mpbs using a BPSK or 2 Mbps
laser link system is given by Eqn. 3 shows that the using QPSK modulations.
amount of received power is proportional to the amount
of power transmitted and the receiver collecting area. It is 4 Some Results
inversely proportional to the square of the beam
divergence and the square of the link range. It is also This section presents some results that have been
inversely proportional to the exponential of the product of collected from the experimental system. The hybrid
the atmospheric attenuation coefficient and link range. FSO/RF communication system has been operational for
Arec (−σR ) over 4 months. It has been set up in a way that it logs all
Prec = Ptr e (3)
(θR )2 the information such as the communication traffic,
received power level, time and date for later use. All this
where Prec is the received power, Ptr is the transmitted information can be downloaded remotely for analysis2.
power, Arec is the receiver area, θ is the beam divergence Also, the daily weather data for Ankara is, in hourly
(in radians) and R is the link range. The probability of bit divisions, also collected from an international weather site
error for OOK modulation is given as [7] and stored in order to cross-check with the switchover
1  SNR  times and dates.
PE = erfc 
 (4)
2  2  Figure 2 depicts the variation of the received power level
For optical detection processes using photodetectors, the with range (up to the experimental system communication
SNR can be written as [6] range) at different visibility values. The first horizontal

SNR =
(Ps Rd )2 (5)
line from bottom indicates the calculated 3.5 µW power
level. The next horizontal line up from it is the threshold
N0 B power level, that is 7 µW, thus, the region below that
threshold level can be considered as the RF region, i.e.,
where Ps is the required signal in watts, peak, Rd is the
the region where the radio is active. The top line in Figure
responsivity of the detector in amps per watt, N0 is the
2 is the RF-to-FSO switchover threshold (10 µW) and the
noise density in amps squared per hertz, and B is the
region above the threshold line shows the region and
receiver bandwidth in hertz.
visibility levels where the FSO is active. As can be seen
from the figure for a switchover from radio to laser, the
The responsivity of the PIN detector employed in the
visibility must increase to 1.6 km for the experimental
system is 0.9 A/W, which has the dark current level of 0.3
system.
nA (max) and for PE = 10-9, SNR = 15.6 dB, the required
power to achieve this probability is calculated as Preq =
Figure 3 shows the received power level over the time
3.5 µW. With a 3 dB link margin, the required power
period where the system has been operational, i.e., from
becomes Preq = 7 µW for a good quality transmission
14 January to 28 April 2003. The power threshold levels
through laser. Therefore, 7 µW has been chosen as the
for switchovers are marked on the figure and starting
threshold on received laser power level to switch to the
from the bottom each line indicates the 3.5 µW, 7 µW and
radio. Every time the received power goes below the 7
10 µW levels respectively. The figure gives a rough idea
µW threshold a switchover occurs and the RF link
of how much use of the RF link has been made and how
becomes operational. The power level to be reached again
much communication time would have been lost had it
in order to resume transmission through laser again has
not been for the RF back up. In fact, Figure 4 illustrates
set at 10 µW for the reasons defined above.
the importance of having a radio working as a back to the
laser. The figure shows how much the RF link was active
3.1 RF Link during the 20 February-21 March period during which the
weather conditions were particularly bad. During that
The RF link is a very good complement to FSO as RF link
has very little problem penetrating fog, which poses a big
2
problem for FSO. FSO on the other hand has little The performance of the system can be monitored by public too
problem with rain, microwave radio’s biggest adversary. at http://fso-rf.eng.ankara.edu.tr . The web page allows both the
Thus, an FSO/RF hybrid system provides almost an all- ordinary public and the research group members access the
weather communication link. Since, some of the RF system remotely. However, the data can only be downloaded by
research group members and other authorized persons.
 period, Ankara suffered from severe snowstorms and fog,
which reduced the visibility forcing FSO to stop
transmission and switch to the RF. 1200
6
10 1000

D u ratio n (m in )
800
4
10
Laser link
600
active
Logarithmic Received Power (uW)

400
2
10
200

0
Feb 20 Feb 21 Feb 24-25 Mar 21
0
10
RF-to-Laser Figure 4: The duration that the RF link was active
V= 0.5 km between 20 February to 21 March
V= 1 km transition
-2 V= 1.6 km margin
10 V= 2 km RF link Date Duration Weather
V= 3 km active (min)
V= 10 km Feb 2 12:25:57- Feb 2 2 H.
V= 50 km 12:27:48 2003 Cloudy
-4
10
0.5 1 1.5 2 2.5 Feb 18 03:04:23-Feb 18 2.5 Snowy
2.9
Range (km) 03:05:54 2003
Feb 18 03:07:03-Feb 18 2.9 Snowy
Figure 2: Received power vs. range for different 03:09:54 2003
visibility levels. Feb 18 03:20:34-Feb 18 2.6 Snowy
03:23:11 2003
100
Feb 20 01:49:41-Feb 20 27.7 Snowy
02:17:26 2003
90 Feb 20 02:51:51-Feb 20 17.7 Snowy
03:09:30 2003
80
Feb 20 14:21:48-Feb 20 142.4 Snowy
70 16:54:14 2003
Avarage Received Power

Feb 20 17:19:43-Feb 20 31 Snowy

60 17:50:49 2003
50
Feb 20 17:50:54-Feb 20 15.8 Snowy
18:06:42 2003
40 Feb 20 19:18:07-Feb 20 53 Snowy
20:11:00 2003
30
Feb 20 20:38:24-Feb 20 23.5 Snowy
20 21:01:59 2003
Feb 21 05:11:48-Feb 21 9 Hazy
10 05:20:55 2003
7
3.5 Feb 21 05:38:59-Feb 21 631.8 Foggy
0
0 1 2 3 16:21:34 2003
Time (month)
Feb 21 17:02:07-Feb 21 12.3 Snowy
Figure 3: The received power level during the time the 17:14:25 2003
system has been operational Feb 24 20:33:03-Feb 25 990 Snowstor
12:58:38 2003 m
As Figure 4 indicates, the RF link was active for almost Feb 26 18:11:27-Feb 26 6.7 Snowy
18:18:05 2003
10 hours on 21 February 2003 and it was on for even
Mar 17 01:41:20-Mar 17 7.5 Hazy
longer, 16 hours, starting on the 24th of February until the 01:48:58 2003
next day. Table 3 gives the RF link-active durations with Mar 21 20:05:04-Mar 21 5.6 Snowy
cross reference to the weather conditions recorded on the 20:10:45 2003
day for the entire period that the system has been Mar 21 20:29:46-Mar 21 35 Snowy
operational. Please note that the durations where the radio 21:04:54 2003
was active for less than 2 minutes are not shown in this Mar 21 21:10:36-Mar 21 25 Snowy
table. 21:35:33 2003
Mar 21 21:38:31-Mar 21 16 Snowy
21:54:38 2003
Table 3. RF link-active durations with cross reference to
the weather conditions
5. Conclusion
This paper presented an experimental hybrid FSO/RF
communication system. The system connects two of the
five campuses of Ankara University. The need for such a
system was explained and selection and calculation of
certain design parameters of the system are given. The
system has been in operation since 14 January 2003 and
certain communication data have been collected. These
data and the importance of having an RF back up to the
FSO are also illustrated. The collected data indicate that
the laser link has been up for 99.986% of the operation
time.

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