Study of Free Space Optical System Using Multi-Hop Technique

Volume 2, Spl. Issue 2 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

Study of Free Space Optical System Using Multi-Hop Technique Charu Sharma1, Amit Kapoor2 Assistant Professor, Department of Electronics and Communication Engineering, BUEST, Baddi, India Charu.sharma@baddiuniv.ac.in1, amit.kapoor@baddiuniv.ac.in2

Abstract- Multi hoping with EDFA is one of technologies that are used to eliminate the effect of weather conditions such as dense fog and rain on free space optical communication link.

This technology is used to improve the quality of Free Space Optics (FSO) communication system and increase the maximum transmission range.

In this paper decode and forward multi-hoping with EDFA as preamplifier is used to enhance the link quality and link range.

Keywords: Free Space Optics, EDFA, Multi-hop

I. INTRODUCTION Optical fiber till date is the highest data carrying capacity cables. The data carrying capacity of optical fibers is sufficient to fulfill the present need of information transfer but a few problems with optical fiber communication have noticed as the cost of employing fiber cable is very high and fiber cable cannot reach up to the user premises therefore the high data rate connections are not delivered up to user end also in some of the areas laying fibers are not allowed like in national parks or sometimes laying fibers is not a convenient option like on road crossing, housing complexes. All these difficulties can be resolved by Free Space Optics (FSO) communication networks wirelessly with same data rate as supported by optical fibers.

Free-space optical communication (FSO) is an optical communication technology that uses LOS (line of sight) communication system uses carrier frequency (in 20 THz 375 THz range). In “free space optics”, free space means air, outer space, vacuum, or something similar. FSO is an emerging broadband wireless access candidate also known as light wave communication .This is a complementary technology to the radio frequency technology. The range of FSO is 1-4 Km. Free Space Optics (FSO) technology has seen multiple utilization both for military and commercial purposes and is often employed to solve the “last mile” access problem.

FSO Consume a relatively low power, offer a high security due to beam confinement within a very narrow area and are less sensitive to the electromagnetic interference [1]. FSO is an emerging last mile technology which can provide high data rate and license free spectrum.

FSO is a communication process that uses light containing information to travel in free space to exchange data between two or more points [2].

FSO utilizes a highly directed beam of light radiation between two end points to transfer information (data, sound or video). This is parallel to OFC (optical fiber cable) networks, apart from that light pulses are send through free air as a substitute of OFC cores. An FSO unit comprises of an optical transceiver with a laser transmitter and a receiver to have bi-directional capability [3].

BUEST, Baddi

RIEECE-2015

A. A few points of FSO 1. Consume a relatively low power, offer a high security due to beam confinement within a very narrow area and are less sensitive to the electromagnetic interference 2. FSO links are low cost, simple and easy to install, flexible, and license-free 3. Avoids electromagnetic pollution and wiretapping safety.

4. FSO links are highly directional. Making FSO links are less dependent on telecom operator making operation cost effective.

5. It is protocol independent hence it can support multiple platform and Interfaces.

6. Transmission using FSO is very secure due to narrow optical beams.

The major design problem in FSO is the weather conditions which limit its link length and degrade the performance of the system. Atmospheric turbulence can degrade the performance of free-space optical links, particularly over ranges of the order of 1 km or longer. In homogeneities in the temperature and pressure of the atmosphere leads to variations of the refractive index along the transmission path.

These indexes in homogeneities can degrade the quality of the received signal and can cause fluctuations in received signal. These fluctuations can lead to an increase in the link error probability, limiting the performance [4].

In order to increase the reliability of the system one major technique is to scale down the distance in multiple hops. Major multiple hop techniques are decode and forward and amplify and forward [5].

Use of relay methods reduces path loss compared to direct communication link also improve small scale fading [6].

In this paper simulation of a direct FSO link of length 4 Kms is performed, also simulation of FSO link of dual hop is performed with EDFA as preamplifier and comparison is made.

II. RESULTS AND DISCUSSION Results are simulated on a single channel FSO system.

A comparison is made for a link length of 4 Kms with and without multi-hop technique with EDFA as preamplifier.

In multi-hop system the signal transmitted in the first hop is received after 2 Kms of link length and decoded, amplified and then forwarded. The results are analyzed on the basis of parameter such as BER and Q factor.

A. Effect of multi-hop network on BER and Q factor In multi-hop system, at each hop data is first decoded amplified and then forwarded. This enhances the system

288

Volume 2, Spl. Issue 2 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

performance; this is evaluated here by considering two factors BER and Q factor.

Keeping distance fixed the BER value observed is 2.06e10 whereas with using hoping for same distance range, the BER reduces to 0. Similarly the Q factor without using hoping technique is 6.18 and with hoping it enhances to 69.80. Results clearly indicate the effect of hopping to enhance the system performance.

B. Effect of channel attenuation on link range Attenuation due to adverse weather conditions is one major barrier in FSO communication link. Considering this simulation is done on both direct and multi-hop communication link for varying attenuation values. The results are given in the table below.

Fig.1. Attenuation Vs. Distance for multi-hop link

Table 1. Attenuation Vs. Distance and BER For Direct communication link

For multi- hop link

Attenuation

Distance (Kms)

BER

Distance (Kms)

BER

1 dB

4

2.06e-10

12

1.3e-9

2 dB

3.2

7.8e-9

9

4.5e-9

3 dB

2.6

4.2e-10

8

5.8e-9

5 dB

2

2.7e-10

6.5

7.7e-10

7 dB

1.7

1.3e-9

5.5

9.8e-10

It can be observed from the table that as the attenuation limit of the channel is increasing the link distance goes on decreasing for both direct communication as well as for multi-hop communication link. From the results it is clear that by using hoping system the link range achieved is greater. The link range achieved is almost 3 times that of direct communication system for all the values of attenuation.

The graphs are plotted for direct communication link and multi-hop communication link (Fig 1 and Fig 2 respectively). Graphs clearly indicates that more the attenuation less is the link range

Fig.2. Attenuation Vs. Distance for direct communication link Table 2. Receiver aperture Vs. BER Receiver aperture (cm)

BER for Direct communication link

BER for Dual hop link

20

1.3e-13

9.8e-19

25

5.5e-29

7.19e-36

30

2.3e-54

1.7e-55

35

1.2e-91

2.3e-100

40

1.12e-141

1.12e-160

c. Effect of receiver aperture on BER in rain environment Simulation is performed to find the effect of receiver aperture on BER in rain conditions from light to medium rain whose values lie in the range of 2.8142 dB/Km to 7.7573 dB/Km according to ITU recommendation[7]. The simulation is performed for direct and dual hop link.

From the results in table 2, it is observed that increasing receiver aperture increases the signal strength. At 20 cm receiver aperture the BER for direct communication link is 1.3e-13 and with multi-hop it is 9.8e-19. Increasing receiver aperture by 5 cm the BER reduces to 5.5e-29 and 7.19e-36 for direct and dual hop communication link. It is clear that increasing receiver aperture reduces the BER. It is also clear from the results that less BER is achieved with multi-hop communication system than direct communication system.

289

III. CONCLUSION Effect of multi-hop on BER and Q factor is studied and it is found that multi-hop reduces the BER and increases the Q factor of the system. Effect of attenuation on the link range is analyzed and it is found that more attenuation in the channel will reduce the link length. The effect of receiver aperture on BER is examined and it is concluded that more receiver aperture helps to reduce BER of the system and increases the performance.

REFERENCES [1]Charu Sharma, Sukhbir singh, Bhubneshwar Sharma, “Investigations on Bit Error Rate Performance of DWDM Free Space Optics System Using Semiconductor Optical Amplifier in Intersatellite Communication”, IJERT, Vol. 2, Issue 8 Aug 2013, ISSN: 2278-0181 [2]http://en.wikipedia.org/wiki/Freespace_optical_communication [3]https://www.academia.edu/3409528/Free_Space_Optics [4]Xiaoming Zhu and Joseph M. Kahn, “Free-Space optical communication through atmospheric turbulence channels”, IEEE transactions on communications, Vol. 50, No. 8, August 2002

BUEST, Baddi

RIEECE -2015

Volume 2, Spl. Issue 2 (2015)

e-ISSN: 1694-2310 | p-ISSN: 1694-2426

[5]Theodoros A. Tsiftsis, Harilaos G. Sandalidis, George K. Karagiannidis, and Nikos C. Sagias, “Multihop Free-Space optical communications over strong turbulence channels”, Communications, 2006. ICC '06, IEEE international conference on, Volume: 6, Page(s): 2755 – 2759, ISSN-8164-9547

BUEST, Baddi

RIEECE-2015

[6] Ehsan Bayaki, Diomidis S. Michalopoulos, and Robert Schober, “EDFA-Based All-Optical relaying in free-space optical systems”, Vehicular Technology Conference (VTC Spring), 2011 IEEE 73rd, Page1 – 5, ISSN: 1550-2252.

[7] ITU-R recommendation P.184, prediction methods required for the design of terrestrial free-space optical link.

290