An experimental hybrid FSO/RF communication system

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 This paper presents an experimental free space optical (FSO) communication system combined with a redundant radio frequency (RF) communication link that provides high availability and continuous communication even in adverse weather conditions. The experimental hybrid system provides a wireless connection between the two of the five campuses of Ankara University that are located at different locations in the city. In this paper, the necessity of such a system is explained, rationale behind the selection of the design parameters is given and the structure of the hybrid system is outlined. Some results regarding the link availability and the communication traffic of the system that has been operational for over 4 months are also presented1.

Key Words Hybrid FSO/RF, laser communication, link availability

1. Introduction With the recent developments in semiconductor technology, free space optical (FSO) or optical wireless communication has become an attractive alternative to optical fiber communications or radio frequency (RF) systems. An FSO system offers much higher data rates when compared with an RF system, also easier to deploy and much less expensive than underground fiber. Although, fiber-optic cabling is still the preferred media for long distance, high bandwidth communication, because of FSO’s lower cost and significantly shorter installation time, FSO is now considered a viable option to fiber for short distances of 4 km or less [1,2]. However, despite the advantages that an FSO system holds over underground fiber links and almost equally less costly (in terms of deploying a link) RF links, there is one big challenge for optical wireless and that is the 1

Research supported by Ankara University Scientific Research Projects, Project No: 2001-00-00-006.

atmospheric attenuation. The attenuation occurs due to many factors such as -

Absorption (caused primarily by the water vapor and carbon dioxide) Scattering (depends on the wavelength used and the number and size of scattering elements in the air, i.e., fog) Shimmer (caused by a combination of factors, including atmospheric turbulence, air density, light refraction, cloud cover and wind).

The effect of all these factors appears as an atmospheric attenuation constant in the formulation that produce the level of received power at the receiver (the link budget equation) and is uncontrollable in an outdoor environment. Thus, not surprisingly, in heavy attenuation conditions the operation of an FSO link cannot be always 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 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 to create a hybrid FSO/RF system would provide a cheap (almost 10% cheaper than fiber optic link) and easily deployable solution. In fact, having a radio and laser in tandem works particularly well since microwave transmission is affected more by rain (as the carrier wavelength is closer to the size of a rain drop) and laser transmission is affected more by fog. Hence, The only weather condition that could affect the transmission of a hybrid FSO/RF system is the condition of simultaneous heavy rain and thick fog. Qualitatively, it could be said that these conditions would not occur simultaneously, because as the rain falls, the rain droplets would absorb the suspended fog water droplets, thus diminishing the fog. Moreover, the fact that the simultaneous occurrence of heavy rain and thick fog does not happen has been quantitatively analyzed and the results are presented in [3]. Hence, the combination of a laser and a radio working in tandem renders a high available communication link where the availability reaches 99.999% [3].

automatically switches over to the RF link, which is waiting as a hot stand-by.

Ankara University, where the experimental system has been set up, is one of the biggest universities in Turkey and has five campuses each encompassing several faculties that are located at different locations around the city. Such a structure requires Ankara University to resort to commercially available communication infrastructure, as it is legally impossible to dig up public land and lay its own cables. Therefore, as a license free and legal option it has been decided that an FSO link would provide a good means to connect the two campuses that have the most data and voice communication. Moreover, in order to increase the availability of the link, an RF back-up link has been decided to be integrated with the FSO system.

Thus, despite the reduction in the bandwidth, the link remains available. The RF link operates at 2.4 GHz linking the two terminals at 11 Mbps. After the switchover has happened the RF link stays as the primary link until the received signal level goes over a higher threshold, indicating a solid connection on the laser link can be re-established. When the increased signal level is detected the switch switches back to the laser link. The calculation for the switchover threshold levels is given in the next section.

2. The Hybrid FSO/RF System

3. General Design Considerations

The experimental system is designed to make use of the best features of two transport mediums, laser light and radio waves to form a single, high available, seamless wireless communication link between the two campuses. As shown in Figure 1, the system comprises three subsystems namely, i) the laser link, ii) a switch and iii) the RF link at each end.

The system has been designed to communicate through the FSO link unless the performance of the FSO is degraded due to adverse weather conditions. When the degradation occurs and the quality of the communication reduces the system switches over to the RF link via a switch. A threshold level on the received laser power must be determined for a seamless switchover to the RF link. In threshold level calculations, the gains and losses of optical components have been neglected and a 3 dB margin has been added on top of the calculated level instead in order to account for then neglected optical component losses. Even when the system switches to the radio, the received power level is still monitored for a quick recovery of the laser link so that a much higher bandwidth could be utilized. However, the system does not switch back over to the laser link even when the FSOto-RF switchover threshold power level is achieved again. In fact, the power level to go back on the FSO is higher than the FSO-to-RF threshold. The reason for having two different switchover thresholds (a higher one for switching back to the laser from the radio than the one to switch from the laser to the radio) is to reduce the switchover frequency and prevent the system switching back and forth constantly. Next two subsections give the design parameters for both the laser and the radio and show how the FSO-to-RF switchover threshold has been calculated

Under normal operating conditions data is transmitted over the laser link where the laser link provides a 155 Mbps full duplex connection. The wavelength of the laser used is 1550 nm. Other available wavelengths for FSO applications are 780 nm and 850 nm near infrared spectrum. However, 1550 nm band, also the choice for the fiber optic telecommunication applications, is better suited for optical wireless. The main benefit of the 1550 nm band is the ability to transmit more power. The power density at 1550 nm band nearly 50 times that at 780 nm and is still safe for the human eye. Since, more power means better ability to overcome atmospheric attenuation, the 1550 nm band has been chosen. Despite the more power, in heavy attenuation the laser signal might still get degraded. The signal level at the receiver gets checked every 5 seconds and when the received signal level falls below a certain threshold, the laser stops transmitting data to the switch. As soon as the degradation at the signal level is detected, the switch Laser Transceiver

Data in/out

Switch

1550 nm FSO connectivity

Transmission path (2.9 km)

RF Transceiver

Laser Transceiver

Switch

RF Transceiver 2.4 GHz RF connectivity

Figure 1: Basic hybrid FSO/RF system architecture

Data in/out

The attenuation coefficient has contributions from the absorption and scattering of laser energy by different aerosols and molecule in the atmosphere. Since laser wavelength is chosen to fall inside transmission windows of the atmospheric absorption spectra, the contributions of absorption to the total attenuation coefficient are very small. The effects of scattering, therefore, dominate the total attenuation coefficient. The type of scattering is determined by the size of the particular atmospheric particle with respect to the transmission laser wavelength.

3.1 Laser Link The laser link refers to a pair of FSO transceivers each aiming a laser beam at the other, creating a full duplex communication link. A laser system requires careful planning and analysis prior to equipment installation. A poorly designed path may result in periods of system outages, increased system latency, decreased throughput or a complete failure of communication across the link. Hence, before determining the link feasibility, link’s atmospheric attenuation and geometrical spreading loss must be calculated.

The final atmospheric phenomenon that significantly impacts laser propagation is scintillation. At low altitudes, scintillation effects arise from the temperature differences between the ground and air and the resulting heat exchange. The index of refraction of air changes with temperature and the heat exchange causes local index variations that have different scale sizes that effect the laser propagation. The index changes result in lensing effects along the optical path. The dominant scintillation effect occurs when the scintillation scale is comparable to the beam size. This scale size event defocuses the beam and leads to significant intensity variations in the received amplitude of the laser signal. These “fades” in signal amplitude have nominally been in the - 7 to -10 dB range, although deeper events have been measured. To overcome these coherent events, systems have used multiple transmitting apertures of sufficient separation and temporally incoherent laser transmissions. As a result, the systems have regained some of the fade losses by making the individual beams uncorrelated to the atmospheric scale size.

The parameters taken into account when calculating the link equation of the transmitter and receiver used in the laser subsystem are given in Table 1. Transmission rate Range Laser output power

155 Mbps. 2.9 km 640 mW (4 transmitter at 160 mW) Transmitter beamwidth 2.8 mrad Wavelength 1550 nm Transmitter type Directly modulated laser diode Modulation format OOK (NRZ waveform) Receiver area 314 cm2 Detector PIN photodetector. Table 1. Transmitter and receiver parameters of the laser system Laser links can be affected by a variety of atmospheric phenomena, fog and scintillation being the most common by far. Free space, low altitude laser transmissions are range and bit error limited by the atmospheric energy losses resulting from scattering during haze, rain, snow, and fog conditions in the atmospheric channel. Most free space laser transmission wavelengths are primarily chosen for their very low absorption losses, so that molecular energy transitions do not absorb free space laser energy.

To determine the atmospheric attenuation of optical signals, the following equation is utilized to base the attenuation on the visibility and the incident wavelength λ [5].

3.91  λ  σ=   V  550nm 

P( R) = e −σR P( S )

(2)

where, σ is atmospheric attenuation (or scattering) coefficient, V is the visibility (in km), λ is the wavelength (in nm) and δ is the size distribution of the scattering particles. For the exponent δ, 1.6 indicates a good visibility (V>50 km), 1.3 a visibility V=6-50 km and 0.585xV1/3 for visibility less than 6 km.

The attenuation of laser power through the atmosphere is described by the exponential Beers-Lambert Law [4].

τ ( R) =

−δ

(1)

where τ(R) = transmittance at range R, P(R) = laser power at range R, P(S)= laser power at the source, and σ = attenuation or total extinction coefficient (per unit length)

Table 2 presents the atmospheric attenuation values at 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 into the atmosphere experiences is geometrical spreading loss. Geometrical spreading loss is the expected attenuation of a signal as it travels away from the transmitting device. When a signal radiates from the transmitter, it spreads out over an increasingly larger distance. As the area covered increases, the power density (or the amount of power per unit area) decreases. This effectively weakens the laser signal. Link equation for laser link system is given by Eqn. 3 shows that the amount of received power is proportional to the amount of power transmitted and the receiver collecting area. It is inversely proportional to the square of the beam divergence and the square of the link range. It is also inversely proportional to the exponential of the product of the atmospheric attenuation coefficient and link range.

Prec = Ptr

Arec (−σR ) e (θR )2

(3)

where Prec is the received power, Ptr is the transmitted power, Arec is the receiver area, θ is the beam divergence (in radians) and R is the link range. The probability of bit error for OOK modulation is given as

PE =

 SNR  1  erfc  2 2  

(4)

For optical detection processes using photodetectors, the SNR can be written as [6]

SNR =

(Ps Rd )2 N0 B

(5)

where Ps is the required signal in watts, peak, Rd is the responsivity of the detector in amps per watt, N0 is the noise density in amps squared per hertz, and B is the receiver bandwidth in hertz. The responsivity of the PIN detector employed in the system is 0.9 A/W, which has the dark current level of 0.3 nA (max) and for PE = 10-9, SNR = 15.6 dB, the required power to achieve this probability is calculated as Preq = 3.5 µW. With a 3 dB link margin, the required power becomes Preq = 7 µW for a good quality transmission through laser. Therefore, 7 µW has been chosen as the threshold on received laser power level to switch to the radio. Every time the received power goes below the 7 µW threshold a switchover occurs and the RF link becomes operational. The power level to be reached again in order to resume transmission through laser again has set at 10 µW for the reasons defined above.

3.1 RF Link The RF link is a very good complement to FSO as RF link has very little problem penetrating fog, which poses a big problem for FSO. FSO on the other hand has little problem with rain, microwave radio’s biggest adversary. Thus, an FSO/RF hybrid system provides almost an allweather communication link. Since, some of the RF

frequency bands require licensing, an unlicensed radio band would be more suitable when considering a backup for FSO in order to keep the package as a whole unlicensed. Therefore, the RF part of the system has been chosen operate at 2400 MHz-2483.5 MHz (13 channels) ISM frequency band and use 802.11b DSSS technology that operates either at 11 Mbps or 5 Mbps data rate with CCK modulation. The RF link can also operate at even lower data rates, namely 1 Mpbs using a BPSK or 2 Mbps using QPSK modulations.

4 Some Results This section presents some results that have been collected from the experimental system. The hybrid FSO/RF communication system has been operational for over 4 months. It has been set up in a way that it logs all the information such as the communication traffic, received power level, time and date for later use. All this information can be downloaded remotely for analysis2. Also, the daily weather data for Ankara is, in hourly divisions, also collected from an international weather site [7] and stored in order to cross-check with the switchover times and dates. Figure 2 depicts the variation of the received power level with range (up to the experimental system communication range) at different visibility values. The first horizontal line from bottom indicates the calculated 3.5 µW power level. The next horizontal line up from it is the threshold power level, that is 7 µW, thus, the region below that threshold level can be considered as the RF region, i.e., the region where the radio is active. The top line in Figure 2 is the RF-to-FSO switchover threshold (10 µW) and the region above the threshold line shows the region and visibility levels where the FSO is active. As can be seen from the figure for a switchover from radio to laser, the visibility must increase to 1.6 km for the experimental system. Figure 3 shows the received power level over the time period where the system has been operational, i.e., from 14 January to 28 April 2003. The power threshold levels for switchovers are marked on the figure and starting from the bottom each line indicates the 3.5 µW, 7 µW and 10 µW levels respectively. The figure gives a rough idea of how much use of the RF link has been made and how much communication time would have been lost had it not been for the RF back up. In fact, Figure 4 illustrates the importance of having a radio working as a back to the laser. The figure shows how much the RF link was active during the 20 February-21 March period during which the weather conditions were particularly bad. During that 2

The performance of the system can be monitored by public too at http://fso-rf.eng.ankara.edu.tr . The web page allows both the ordinary public and the research group members access the 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

Laser link active

4

Logarithmic Received Power (uW)

D u ratio n (m in )

1000

10

600 400

2

10

200 0

0

10

V= V= V= V= V= V= V=

-2

10

-4

10

0.5

0.5 km 1 km 1.6 km 2 km 3 km 10 km 50 km

RF link active

1

1.5 Range (km)

2

2.5

100 90 80 70 60 50 40 30 20 10 7 3.5 0 0

1

2 Time (month)

3

Figure 3: The received power level during the time the system has been operational As Figure 4 indicates, the RF link was active for almost 10 hours on 21 February 2003 and it was on for even longer, 16 hours, starting on the 24th of February until the next day. Table 3 gives the RF link-active durations with cross reference to the weather conditions recorded on the day for the entire period that the system has been operational. Please note that the durations where the radio was active for less than 2 minutes are not shown in this table.

Feb 20

Feb 21

Feb 24-25

Mar 21

Figure 4: The duration that the RF link was active between 20 February to 21 March

RF-to-Laser transition margin

Figure 2: Received power vs. range for different visibility levels.

Avarage Received Power

800

Date

2.9

Feb 2 12:25:57- Feb 12:27:48 2003 Feb 18 03:04:23-Feb 03:05:54 2003 Feb 18 03:07:03-Feb 03:09:54 2003 Feb 18 03:20:34-Feb 03:23:11 2003 Feb 20 01:49:41-Feb 02:17:26 2003 Feb 20 02:51:51-Feb 03:09:30 2003 Feb 20 14:21:48-Feb 16:54:14 2003 Feb 20 17:19:43-Feb 17:50:49 2003 Feb 20 17:50:54-Feb 18:06:42 2003 Feb 20 19:18:07-Feb 20:11:00 2003 Feb 20 20:38:24-Feb 21:01:59 2003 Feb 21 05:11:48-Feb 05:20:55 2003 Feb 21 05:38:59-Feb 16:21:34 2003 Feb 21 17:02:07-Feb 17:14:25 2003 Feb 24 20:33:03-Feb 12:58:38 2003 Feb 26 18:11:27-Feb 18:18:05 2003 Mar 17 01:41:20-Mar 01:48:58 2003 Mar 21 20:05:04-Mar 20:10:45 2003 Mar 21 20:29:46-Mar 21:04:54 2003 Mar 21 21:10:36-Mar 21:35:33 2003 Mar 21 21:38:31-Mar 21:54:38 2003

2

Duration (min) 2

Weather

18

2.5

H. Cloudy Snowy

18

2.9

Snowy

18

2.6

Snowy

20

27.7

Snowy

20

17.7

Snowy

20

142.4

Snowy

20

31

Snowy

20

15.8

Snowy

20

53

Snowy

20

23.5

Snowy

21

9

Hazy

21

631.8

Foggy

21

12.3

Snowy

25

990

26

6.7

Snowstor m Snowy

17

7.5

Hazy

21

5.6

Snowy

21

35

Snowy

21

25

Snowy

21

16

Snowy

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.

References [1] T.H. Carbonneau and D.R. Wisely, Opportunities and Challenges For Optical Wireless; The Competitive Advantage of Free Space Telecommunication Links in Today’s Crowded Marketplace, Wireless Technologies and Systems: Milimeter Wave and Optical, Proceedings of SPIE, 3232, 1997, 119-128. [2] D.R. Wisely, M.J. McCullagh, P.L.Eardley, P.P. Smyth, D. Luthra, E.C. De Miranda and R.S. Cole, 4-km Terrestrial Line-of-Sight Optical Free-Space Link Operating at 155 Mbit/s, Free Space Laser Communication Technologies VI, Proceedings of SPIE, 2123, 1994, 108-119. [3] I. I. Kim and E. Korevaar, Availability of Free Space Optics (FSO) and Hybrid FSO/RF Systems, Available at http://www.freespaceoptics.com/White_Papers.htm [4] H. Weichel, Laser Beam Propagation in the Atmosphere, Proc. SPIE, Bellingham WA, 1990. [5] P. Kruse et al., Elements of Infrared Technology (John Wiley & Sons, 1962). [6] S.G. Lambert, W.L. Casey, Laser Communications in Space, (Boston: Artech House, 1995). [7]http://english.wunderground.com/global/stations/17129 .html