Design Of Free Space Optical Communication Link

ABSTRACT

This paper presents the design of a very high-speed data link between two buildings in a University campus that will operate at gigabit rates. The project uses a cutting edge technology of eye-safe laser communication through free space. This is an all-optical design is future-proof in regards to technological advancement in the rate of data transmission and introduction of newer protocols. The two buildings are approximately 500 meters apart. The free-space optical link uses 1550 nm wavelength in normal usage but has a wireless link operating at 2.4 GHz as the back-up. The line of site alignment will be achieved using telescopes initially but will have automatic tracking alignment system. The wireless back-up link is used only in very dense fog conditions. This paper presents the design of only the free-space optical connection, some parts of which are implemented in laboratory setup.

CHAPTER ONE

1.0                                                                              INTRODUCTION

The technology of establishing a high-speed networking between two buildings or campuses is one of the three: 1) copper wire, 2) wireless and 2) optical fiber technology. The copper technology is low-speed, labor-intensive and requires a regime of permissions. The advantages are high reliability and full availability. The wireless technology uses a few GHz carrier, is medium speed (up to few Gigabits per second), has small link span and requires a regime of licenses. Advantage is the ease of deployment. The disadvantages are low reliability (high bit error rate) and severe fading in rain.

Fiber optic technology poses no foreseeable limit in speed, enables large link span, and has high reliability (very low reliability) and full availability. Laying optical fiber in the ground requires huge expenses and a considerable time and efforts go in obtaining permissions from various agencies^[1]. Once laid, the fiber cannot be re-deployed easily. Capital expenditure (CapEx) is tied directly to the off –network customer. If the customer is lost, so is the CapEx. Furthermore, trenching may run into physical obstructions.

The free-space optical (FSO) technology using eye-safe infrared lasers ^[2]offers fiber-like speed, does not require permissions and can be deployed and reconfigured in a short time. Disadvantages are strict line-of-sight alignment and severe fading in fog and smog conditions.

Fortunately, the wireless technology is not affected in the fog and smog conditions and the FSO technology is not affected in rain^[3]. The rain particles are comparable to the wavelength of the wireless frequencies. The fog and smog particles sizes are much smaller than rain drops but are comparable to infra-red wavelengths used in FSO.

Fig. 1 FSO/Wireless link between two buildings/campuses

1.1                                  BACKGROUND OF THE PROJECT

Spectrum scarcity, coupled with bandwidth appetites in metropolitan area networks (MANs), is forcing wireless operators and service provides to look at new methods to connect cells. They are faced with many options in attempts to meet the high bandwidth demand. The first and often times most obvious choice is fiber-optic cable, but the associated delays and costs to lay fiber often make it economically prohibitive. The second alternative is radio frequency (RF) technology, which is a mature technology for longer  distance transmission than FSO, but RF technologies cannot scale to optical capacities of 2.5 Gbps. The current RF bandwidth ceiling is 622 Mbps. The third option is copper-based technologies such as T1, cable  modem,  or DSL. Although copper infrastructure is available almost everywhere, it is still not a viable alternative because the bandwidth limitation of 2 to 3 Mbps makes it a marginal solution. The fourth and most viable choice is FSO.

The technology is an optimal solution, given its optical base, bandwidth scalability, speed of deployment, portability, and cost-effectiveness.

Free-space optics (FSO), also known as fiber-free or fiberless photonics, refers to the transmission of modulated light pulses through free space (air or the atmosphere) to obtain broadband communications. Laser beams are generally used, although non-lasing sources such as light-emitting diodes (LEDs) or IR-emitting diodes (IREDs) will serve the same purpose. FSO can be the best wireless solution where fiber optical cable is not available, high bandwidth (anywhere from 1 Mbps up to 1.25 Gbps) is required, and line-of-sight can be obtained to a target within a couple of miles.

Since FSO is a convergence of two disparate technologies, it is not clear whether it is a wireless or optical system. FSO is an optical technology and not a wireless technology for two basic reasons. First, FSO enables optical transmission at speeds of up to 2.5 Gbps and in the future 10 Gbps using WDM. This is not possible using any fixed wireless/RF technology existing today. Second, FSO technology requires no FCC licensing or municipal license approvals and thus obviates the need to buy expensive spectrum. This distinguishes it clearly from fixed wireless technologies.

FSO is not new. It was developed more than three decades ago.  Then, it was used by the military and space aviation pioneers to provide secure and rapidly deployable communications links. For example, it is  being used to carry data within digital computing systems (Gourlay et al., 1998), in cross-bar switching (Rajkumar et al., 1996), optical interconnections (Jahns, 1994), and optoelectronic sampling (Wu and Zhang, 1997). Recent developments in optical technology have advanced FSO to mainstream communications applications and make it an alternative to RF wireless.

1.2                                                 OBJECTIVE OF THE PROJECT

This work takes a closer look at FSO technology, its strengths and drawbacks. It examines how FSO is responding to high bandwidth communication needs in the metro area and how FSO beats competing local access alternatives such as DSL.

1.3                                  SIGNIFICANCE OF THE PROJECT

1. Free Space Optical Communication (FSO) offers highly directional, high bandwidth communication channels, especially in avionic application.
2. Its links can provide fiber-like data rates over short distances with low probability of interception. It also provides last mile solutions in many of today static communication system substituting wired or microwave system.
3. This FSO communication with low power consumption, light weight and low cost. The system is battery power.
4. Free space optics is a flexible network that delivers better speed than broadband [1].
5. Installation is very easy and it takes less than 30 minutes to install at normal locations [1].(c)It has very low initial investment [3].
6. It is a straight forward deployment system. There is no need for spectrum license or frequency coordination between users as it is required in radio and microwave systems previously [7].
7. It is a secure system because of line of sight operation and so no security system up gradation is needed [7].
8. High data rate can be obtained which is comparable to the optical fiber cable’s data rate but error rate is very low and the extremely narrow laser beam enables having unlimited number of FSO links which can be installed in a specific area [7].
9. There is immunity to radio frequency interference [7].
10. Electromagnetic and radio-magnetic interference cannot affect the transmission in FSO link [8].(i)FSO offers dense spatial reuse [8].
11. Low power usage per transmitted bit is merit of FSO system [8].
12. There is relatively high bandwidth [8].
13. It has flexible rollouts [9].
14. Transmission of optical beam is done in air. Hence, transmission is having speed of light [10].

1.4                              APPLICATION OF THE PROJECT

FSO communication link is currently in use for many services at many places. These are described below in detail:

(a) Outdoor wireless access: it can be used by wireless service providers for communication and it requires no license to use the FSO as it is required in case of microwave bands.

(b) Storage Area Network (SAN): FSO links can be used to form a SAN. It is a network which is known to provide access to consolidated, block level data storage [2].

(c) Last-mile access: to lay cables of users in the last mile is very costly for service providers as the cost of digging to lay fiber is so high and it would make sense to lay as much fiber as possible. FSO can be used to solve such problem by implementing it in the last mile along with other networks. It is a high speed link. It is also used to bypass local-loop systems of other kinds of networks [3].

(d) Enterprise connectivity: FSO systems are easily installable. This feature makes it applicable for interconnecting LAN segments to connect two buildings or other property [3].

(e) Fiber backup: FSO can also be applicable in providing a backup link in case of failure of transmission through fiber link [3].

(f) Metro-network extensions: it can be used in extending the fiber rings of an existing metropolitan area. FSO system can be deployed in lesser time and connection of the new networks and core infrastructure is easily done. It can also be used to complete SONET rings [3].

(g) Backhaul: it can be helpful in carrying the traffic of cellular telephone from antenna towers back to the PSTN with high speed and high data rate. The speed of transmission would increase [3].

(h) Service acceleration: it can also be used to provide instant service to customers when their fiber infrastructure is being deployed in the mean time [3].

(i) Bridging WAN Access: FSO is beneficial in WAN where it supports high speed data services for mobile users and small satellite terminals and acts as a backbone for high speed trunking network [4].

(j) It can be used to communicate between point-to-point links, for example, two buildings, two ships, and point-to-multipoint links, for example, from aircraft to ground or satellite to ground, for short and long reach communication [5].

(k) Military access: as it is a secure and undetectable system it can connect large areas safely with minimal planning and deployment time and is hence suitable for military applications [6].

1.5                                      PROJECT MOTIVATION

Our world is fast becoming a small village by adopting faster and faster communication and networking technology. The road to new economy goes through the educational institutions and research laboratories. The way we interact with each other, the way our students are educated today will be very different in the short future than we can imagine now. It is therefore, extremely important that the communities are ready with the communication and networking infrastructure to absorb and adapt the future high-tech life-style. This technology would enable fast dissemination of modern technological advances into the community This is an emerging technology and it will serve as the catalyst for innovation and fast lane transformation to new economy in the neighboring region.

1.6                                               LIMITATION OF THE PROJECT

The advantages of free space optics are easy to come. But as the medium of the transmission is air for FSO and the light passes through it, some environmental challenges are unavoidable. Troposphere regions are the region where most of the atmospheric phenomenon occurred [11]. The effect of these limitations over the atmosphere is shown in Figure 1. Some of these limitations are briefly described below:

(a) Physical obstructions: flying birds, trees, and tall buildings can temporarily block a single beam, when it appears in line of sight (LOS) of transmission of FSO system [1].

(b) Scintillation: there would be temperature variations among different air packets due to the heat rising from the earth and the man-made drives like heating ducts. These temperature variations can cause fluctuations in amplitude of the signal which causes “image dancing” at the FSO receiving end. The effect of scintillation is addressed by Light Pointe’s unique multibeam system [1].

(c) Geometric losses: geometric losses which can be called optical beam attenuation are induced due to the spreading of beam and reduced the power level of signal as it travelled from transmitted end to receiver end [7].

(d) Absorption: absorption is caused by the water molecules which are suspended in the terrestrial atmosphere. The photons power would be absorbed by these particles. The power density of the optical beam is decreased and the availability of the transmission in a FSO system is directly affected by absorption. Carbon dioxide can also cause the absorption of signal [9].

(e) Atmospheric turbulence: the atmospheric disturbance happens due to weather and environment structure. It is caused by wind and convection which mixed the air parcels at different temperatures. This causes fluctuations in the density of air and it leads to the change in the air refractive index. The scale size of turbulence cell can create different type of effects given below and which would be dominant:

(i)If size of turbulence cell is of larger diameter than optical beam then beam wander would be the dominant effect. Beam wander is explained as the displacement of the optical beam spot rapidly.

(ii)If size of turbulence cell is of smaller diameter than optical beam then the intensity fluctuation or scintillation of the optical beam is a dominant one. Turbulence can lead to degradation of the optical beam of transmission. Change in the refractive index causes refraction of beam at different angle and spreading of optical beam takes place [11].

(f)Atmospheric attenuation: atmospheric attenuation is the resultant of fog and haze normally. It also depends upon dust and rain. It is supposed that atmospheric attenuation is wavelength dependent but this is not true. Haze is wavelength dependent. Attenuation at 1550nm is less than other wavelengths in haze weather condition [11]. Attenuation in fog weather condition is wavelength independent.

(g)Scattering: scattering phenomena happen when the optical beam and scatterer collide. It is wavelength dependent phenomenon where energy of optical beam is not changed. But only directional redistribution of optical energy happens which leads to the reduction in the intensity of beam for longer distance. Atmospheric attenuation is divided into three types [12]:

(1). Rayleigh scattering which is known as molecule scattering.

(2). Mie scattering which is known as aerosol scattering.

(3). Nonselective scattering which is known as geometric scattering.

The type of scattering depends upon the physical size of the scatterer [1]:

• When it is smaller than the size of wavelength, Rayleigh scattering.
• When the size of the scatterer is comparable to the wavelength, Mie scattering.
• When it is much larger than the size of wavelength, nonselective scattering.

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