1 Arajind

Ground To Space Optical Communication Characterization Essay

University Institute of Engineering & Technology, Panjab University, Chandigarh, India

Copyright © 2015 Aditi Malik and Preeti Singh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

FSO is a communication system where free space acts as medium between transceivers and they should be in LOS for successful transmission of optical signal. Medium can be air, outer space, or vacuum. This system can be used for communication purpose in hours and in lesser economy. There are many advantages of FSO like high bandwidth and no spectrum license. The transmission in FSO is dependent on the medium because the presence of foreign elements like rain, fog, and haze, physical obstruction, scattering, and atmospheric turbulence are some of these factors. Different studies on weather conditions and techniques employed to mitigate their effect are discussed in this paper.

1. Introduction

FSO (free space optics) is an optical communication technology in which data is transmitted by propagation of light in free space allowing optical connectivity. There is no requirement of the optical fiber cable. Working of FSO is similar to OFC (optical fiber cable) networks but the only difference is that the optical beams are sent through free air instead of OFC cores that is glass fiber. FSO system consists of an optical transceiver at both ends to provide full duplex (bidirectional) capability. FSO communication is not a new technology. It has been in existence from 8th century but now is more evolved. FSO is a LOS (line of sight) technology, where data, voice, and video communication is achieved with maximum 10Gbps of data rate by full duplex (bidirectional) connectivity [1].

An effective FSO system should have the following characteristics [1]:(a)FSO systems should have the ability to operate at higher power levels for longer distance.(b)For high speed FSO systems, high speed modulation is important.(c)An overall system design should have small footprint and low power consumption because of its maintenance.(d)FSO system should have the ability to operate over wide temperature range and the performance degradation would be less for outdoor systems.(e)Mean time between failures (MTBF) of system should be more than 10 years.

2. Applications

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].

3. Merits

(a)Free space optics is a flexible network that delivers better speed than broadband [1].(b)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].(d)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].(e)It is a secure system because of line of sight operation and so no security system upgradation is needed [7].(f)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].(g)There is immunity to radio frequency interference [7].(h)Electromagnetic and radio-magnetic interference cannot affect the transmission in FSO link [8].(i)FSO offers dense spatial reuse [8].(j)Low power usage per transmitted bit is merit of FSO system [8].(k)There is relatively high bandwidth [8].(l)It has flexible rollouts [9].(m)Transmission of optical beam is done in air. Hence, transmission is having speed of light [10].

These merits indicate the significance of FSO system over different communication systems. Comparison of different systems based on various parameters is mentioned in Table 1.

Table 1: Comparison of FSO with different communication system.

4. Limitations

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.

Figure 1: Atmospheric effects on FSO system [1].

The type of scattering depends upon the physical size of the scatterer [1]: (i)When it is smaller than the size of wavelength, Rayleigh scattering.(ii)When the size of the scatterer is comparable to the wavelength, Mie scattering.(iii)When it is much larger than the size of wavelength, nonselective scattering.

Atmospheric Weather Conditions. Atmosphere is the medium of transmission for a FSO link. Attenuation caused by it depends upon several conditions. Weather conditions are the main cause of attenuation. The region in which a link is being established has some specific weather conditions so that the preceding knowledge of attenuation can be gained; for example, fog and heavy snow are the two primary weather conditions in temperate regions. In tropical regions, heavy rain and haze are two main weather conditions and have major effect on the availability of FSO link in that region [13]. Some of the weather conditions are described below.

(a) Fog. Fog substantially attenuates visible radiation. Optical beam of light is absorbed, scattered, and reflected by the hindrance caused by fog. Scattering caused by fog, also known as Mie scattering [1], is largely a matter of boosting the transmitted power.

(b) Rain. Rain attenuation exists due to rain fall and is a nonselective scattering. This type of attenuation is wavelength independent [11]. Rain has the ability to produce the fluctuation effects in laser delivery. The visibility of FSO system depends upon the quantity of the rain. In case of heavy rain, water droplets have solid composed and it can either modify the optical beam characteristics or restrict the passage of beam as optical beam is absorbed, scattered, and reflected [8].

(c) Haze. Haze particles can stay longer time in the air and lead to the atmospheric attenuation. So, attenuation values depend upon the visibility level at that time. There are two ways to gather information about attenuation for checking the performance of FSO system: first, by installing system temporary at the site and check its performance and, second, by using Kim and Kruse model [11].

(d) Smoke. It is generated by the combustion of different substances like carbon, glycerol, and household emission. It affects the visibility of transmission medium [14].

(e) Sandstorms. Sandstorms are the well-known problem in outdoor link communication. These can be characterized by two ways: first, the size of the wind particles which depends on the soil texture and, second, necessary wind speed in order to blow the particles up during a minimum period of time [15].

(f) Clouds. Cloud layers are main part of earth atmosphere. The formation of clouds is done by the condensation or deposition of water above earth’s surface. It can completely block the fractions of optical beam transmitted from earth to the space. The attenuation caused by clouds is difficult to calculate because of the diversity and inhomogeneity of the cloud particles [16].

(g) Snow. Snow has larger particles which causes the geometric scattering. The snow particles have impact similar to Rayleigh scattering [17].

5. Different Studies Based on Attenuation Effect

Different studies are going on different weather condition to design new models based on the effectiveness of the system. The main focuses of these studies are fog, haze, rain, and snow weather conditions. Based on these studies results, measures can be taken in practical system.

In a study authors followed theoretical and experimental research to study the effect of fog and smoke. Experimental results validated the laboratory-based empirical model that 830, 940, and 1550nm are most durable wavelength windows. Empirical model is used to compare the experimental result for the continuous attenuation spectrum of fog and smoke conditions and results show that the disambiguation is decreasing linearly [14]. In another study, author studies whether fog is wavelength dependent or not. A fog-like environment is developed in a chamber for experimenting. It is verified that attenuation caused by fog is wavelength dependent parameter. FSO link employed with 830nm and 1550nm in parallel in the same chamber and power is measured at the receiving end for both the cases: with fog and without fog. Fog particles lead to Mie scattering so Mie theory is applicable to measure the scattering. One model from the three famous models, that is, empirical, Kim, and Ferdinandov, can be used to calculate the attenuation due to fog [18].

In rain based study, a correlation of precipitation rates with rain attenuation is studied on the short wavelength (785nm). The four-existing-model rain attenuation is utilized to find the result and measured data is compared with calculated results to determine the turbulence model [8]. The effects of rain intensity variation on its attenuation prediction are the focus of another study. The analysis of 7 reduction format models is done to study the FSO link with rain intensity variations. Six of the models have a reduction factor value of unity where one model has 0.7. It reduces the effective path length of FSO link. Rainfall distribution for longer path seems to be more widespread in case of low rain rate and more concentrated in case higher rain rate [19]. In a study, single and multiple transceiver concept is used to study the effect of tropical Malaysian weather on FSO link based on the value of link distance and received power. It is concluded that four-beam FSO system can successfully operate under heavy rain for larger distance depending upon the value of signal to noise ratio (SNR), geometrical and atmospheric losses, and bit error rate (BER) [17].

6. Various Techniques to Enhance System Performance

Various techniques to enhance the system performance are being introduced. Some of these techniques are discussed below in detail and their comparison is done in the following section.

(a) Performance of SAC OCDMA Based FSO System. Spectral Amplitude Coding Optical Code Division Multiple Access technique is used in FSO system by the researchers. This multiplexing scheme has several advantages like flexibility of channel allocation, asynchronously operative ability, privacy enhancement, and network capacity increment. KS (Khazani-Syed) codes are used with SDD (spectral direct decoding) technique. An optical external modulator (OEM) is used to modulate the code sequence with data. The data is an independent unipolar digital signal. Mach-Zehnder Modulator (MZM) is used and combination of modulated code sequences is transmitted through the FSO link and these sequences are separated by an optical splitter at the receiver end. The overlapping chips are discarded to avoid the interference at receiver end and decoder will only filter the nonoverlapping chips. Optical band pass filters serve the purpose of encoders and decoders. A low pass filter (LPF) is used to recover the original data. The performance of this system with SDD technique is analyzed along with FSO system using intensity modulation with direct detection (IM/DD) technique. SDD technique performs better and the link distance is improved by 22.7% [5].

(b) High Speed, Long Reach OFDM-FSO Transmission Link Incorporating OSSB and OTSB Schemes. By introducing the OFDM scheme, an effort has been made to probe the impact of the environment conditions and to design a high speed and long reach FSO system free from the multipath fading. Different weather conditions like clear, foggy, and hazy channel are used to model different types of condition in system. CW laser diode is used at the line-width of 10MHz and 1550nm wavelength. The power to be used by hybrid system is 0dBm and ideal antenna aperture is 15cm. The data rate is 5Gbps and a 4-QAM sequence generator generates the data and OFDM modulator using 512 subcarriers is used. The data is transmitted over FSO link using OTSB/OSSB schemes instead of ODSB scheme which is prone to fading problem. This modulation is done by Dual Electrode Mach-Zehnder Modulator (DEMZM) and a phase shifter. It is concluded that hybrid OFDM-FSO system performs better in diverse channel conditions and upon comparing both OSSB and OTSB schemes OSSB performs better than OTSB at high data rate as it has more immunity against fading due to weather conditions [4].

(c) Optimization of Free Space Optics Parameters Using WDM System. A unidirectional WDM system is designed by the investigators. Different characteristics like data rate, power, link range, number of users, and channel spacing are needed to be optimized according to the weather conditions. The attenuation for different type of rain is 6.27, 9.64, and 19.28dB/km for light, medium, and heavy rain, respectively. 1550nm wavelength is best for both rain and haze as there is less attenuation than any other wavelength. The priority for optimization of parameters is required to be done for the better performance of system. Geometric losses are not considered during this work. Optical Amplifier Gain is having the highest priority and the rest of priority decrementing series is laser power, data rate, and aperture size and link length is having the lowest priority. A 622Mbps of data rate is maximized for all types of rain as concluded from results. For clear weather condition, data rate could be 2.5Gbps for the distance of 150km. For critical weather conditions, short link distance and lower data rate can be used to optimize the FSO system for successful transmission [11].

Comparison of these studies is done based on the different parameters like wavelength, power level, data rate, and link distance. Summarization of all parameters with different techniques is done in Table 2.

Table 2: Comparison table of various techniques based on system parameters on wavelength of 1550 nm [4, 5, 11, 21].

From Table 2, it can be concluded that the more the attenuation the smaller the link distance. With increase in the data rate, link distance reduces. If power level increases, then link distance improves depending upon the value of power level. Effect of attenuation is lesser if power level is high but power level cannot be increased more than value defined by various organizations that define the principle of laser safety. Such as a human eye can be affected by Laser when eye comes in direct contact with it on a particular wavelength at a particular power like 10 mW power for Class 1 M laser in 1550 nm wavelength permissible by IEC (International Electro technical Commission) standards [20].

7. Conclusion

FSO offers many advantages over existing techniques which can be either optical or radio or microwave. Less cost and time to setup are the main attraction of FSO system. Optical equipment can be used in FSO system with some modification. Merits of FSO communication system and its application area make it a hot technology but there are some problems arising due to the attenuation caused by medium. FSO system poses some problem like attenuation in medium that can affect the performance of transmission as power loss would be there. But extra care and prestudy of the medium can guide what type of parameters to be considered before setting up the system. Many studies are going in this perspective to minimize the effect of attenuation by introducing new system design like WDM based FSO system.

Different models based on these studies are used to study the system performance before installing it at the location. This can lead to the improvement of the system. Different techniques like OFDM-FSO, WDM-FSO based system are new approach to improve the system performance with high speed and longer distance. So new techniques can be designed by combination of these and, by enhancing these techniques, system designing can be improved and the demerits of FSO system can be reduced to a minimum level.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Free-space optical communication (FSO) is an optical communication technology that uses light propagating in free space to wirelessly transmit data for telecommunications or computer networking. "Free space" means air, outer space, vacuum, or something similar. This contrasts with using solids such as optical fiber cable.

The technology is useful where the physical connections are impractical due to high costs or other considerations.

History[edit]

Optical communications, in various forms, have been used for thousands of years. The Ancient Greeks used a coded alphabetic system of signalling with torches developed by Cleoxenus, Democleitus and Polybius.[1] In the modern era, semaphores and wireless solar telegraphs called heliographs were developed, using coded signals to communicate with their recipients.

In 1880, Alexander Graham Bell and his assistant Charles Sumner Tainter created the photophone, at Bell's newly established Volta Laboratory in Washington, DC. Bell considered it his most important invention. The device allowed for the transmission of sound on a beam of light. On June 3, 1880, Bell conducted the world's first wireless telephone transmission between two buildings, some 213 meters (700 feet) apart.[2][3]

Its first practical use came in military communication systems many decades later, first for optical telegraphy. German colonial troops used heliograph telegraphy transmitters during the Herero and Namaqua genocide starting in 1904, in German South-West Africa (today's Namibia) as did British, French, US or Ottoman signals.

During the trench warfare of World War I when wire communications were often cut, German signals used three types of optical Morse transmitters called Blinkgerät, the intermediate type for distances of up to 4 km (2.5 miles) at daylight and of up to 8 km (5 miles) at night, using red filters for undetected communications. Optical telephone communications were tested at the end of the war, but not introduced at troop level. In addition, special blinkgeräts were used for communication with airplanes, balloons, and tanks, with varying success.[citation needed]

A major technological step was to replace the Morse code by modulating optical waves in speech transmission. Carl Zeiss, Jena developed the Lichtsprechgerät 80/80 (literal translation: optical speaking device) that the German army used in their World War II anti-aircraft defense units, or in bunkers at the Atlantic Wall.[4]

The invention of lasers in the 1960s, revolutionized free space optics. Military organizations were particularly interested and boosted their development. However the technology lost market momentum when the installation of optical fiber networks for civilian uses was at its peak.

Many simple and inexpensive consumer remote controls use low-speed communication using infrared (IR) light. This is known as consumer IR technologies.

Usage and technologies[edit]

Free-space point-to-point optical links can be implemented using infrared laser light, although low-data-rate communication over short distances is possible using LEDs. Infrared Data Association (IrDA) technology is a very simple form of free-space optical communications. On the communications side the FSO technology is considered as a part of the optical wireless communications applications. Free-space optics can be used for communications between spacecraft.[5]

Commercial products[edit]

  • In 2008, MRV Communications introduced a free-space optics (FSO)-based system with a data rate of 10 Gbit/s initially claiming a distance of 2 km at high availability.[6] This equipment is no longer available; before end-of-life, the product's useful distance was changed down to 350 m.[7]
  • In 2013, the company MOSTCOM started to serially produce a new wireless communication system[8] that also had a data rate of 10 Gbit/s as well as an improved range of up to 2.5 km, but to get to 99.99% uptime the designers used an RF hybrid solution, meaning the data rate drops to extremely low levels during atmospheric disturbances (typically down to 10 Mbit/s). In April 2014, the company with Scientific and Technological Centre "Fiord" demonstrated the transmission speed 30 Gbit/s under "laboratory conditions".
  • LightPointe offers many similar hybrid solutions to MOSTCOM's offering.[9]

Useful distances[edit]

The reliability of FSO units has always been a problem for commercial telecommunications. Consistently, studies find too many dropped packets and signal errors over small ranges (400 to 500 meters). This is from both independent studies, such as in the Czech republic,[10] as well as formal internal nationwide studies, such as one conducted by MRV FSO staff.[11] Military based studies consistently produce longer estimates for reliability, projecting the maximum range for terrestrial links is of the order of 2 to 3 km (1.2 to 1.9 mi).[12] All studies agree the stability and quality of the link is highly dependent on atmospheric factors such as rain, fog, dust and heat.

Extending the useful distance[edit]

The main reason terrestrial communications have been limited to non-commercial telecommunications functions is fog. Fog consistently keeps FSO laser links over 500 meters from achieving a year-round bit error rate of 1 per 100,000. Several entities are continually attempting to overcome these key disadvantages to FSO communications and field a system with a better quality of service. DARPA has sponsored over US$130 million in research towards this effort, with the ORCA and ORCLE programs.[13][14][15]

Other non-government groups are fielding tests to evaluate different technologies that some claim have the ability to address key FSO adoption challenges. As of October 2014[update], none have fielded a working system that addresses the most common atmospheric events.

FSO research from 1998–2006 in the private sector totaled $407.1 million, divided primarily among four start-up companies. All four failed to deliver products that would meet telecommunications quality and distance standards:[16]

  • Terabeam received approximately $226 million in funding. AT&T and Lucent backed this attempt.[17][18] The work ultimately failed, and the company reorganized in 2004.[19]
  • AirFiber received $96.1 million in funding, and never solved the weather issue. They sold out to MRV communications in 2003, and MRV sold their FSO units until 2012 when the end-of-life was abruptly announced for the Terescope series.[7]
  • LightPointe Communications received $76 million in start-up funds, and eventually reorganized to sell hybrid FSO-RF units to overcome the weather-based challenges.[20]
  • The Maxima Corporation published its operating theory in Science (magazine),[21] and received $9 million in funding before permanently shutting down. No known spin-off or purchase followed this effort.
  • Wireless Excellence developed and launched CableFree UNITY solutions that combine FSO with millimeter wave and radio technologies to extend distance, capacity and availability, with a goal of making FSO a more useful and practical technology.[22]

One private company published a paper on November 20, 2014, claiming they had achieved commercial reliability (99.999% availability) in extreme fog. There is no indication this product is currently commercially available.[23]

[edit]

See also: laser communication in space

The massive advantages of laser communication in space have multiple space agencies racing to develop a stable space communication platform, with many significant demonstrations and achievements.

Operational systems in space:

The first gigabit laser-based communication was achieved by the European Space Agency and called the European Data Relay System (EDRS) on November 28, 2014. The system is operational and is being used on a daily basis.

Demonstrations in space:

NASA's OPALS announced a breakthrough in space-to-ground communication December 9, 2014, uploading 175 megabytes in 3.5 seconds. Their system is also able to re-acquire tracking after the signal was lost due to cloud cover.

In January 2013, NASA used lasers to beam an image of the Mona Lisa to the Lunar Reconnaissance Orbiter roughly 390,000 km (240,000 mi) away. To compensate for atmospheric interference, an error correction code algorithm similar to that used in CDs was implemented.

A two-way distance record for communication was set by the Mercury laser altimeter instrument aboard the MESSENGER spacecraft, and was able to communicate across a distance of 24 million km (15 million miles), as the craft neared Earth on a fly-by in May, 2005. The previous record had been set with a one-way detection of laser light from Earth, by the Galileo probe, of 6 million km in 1992. Quote from Laser Communication in Space Demonstrations (EDRS)

LEDs[edit]

In 2001, Twibright Labs released Ronja Metropolis, an open source DIY 10 Mbit/s full duplex LED FSO over 1.4 km[24][25] In 2004, a Visible Light Communication Consortium was formed in Japan.[26] This was based on work from researchers that used a white LED-based space lighting system for indoor local area network (LAN) communications. These systems present advantages over traditional UHF RF-based systems from improved isolation between systems, the size and cost of receivers/transmitters, RF licensing laws and by combining space lighting and communication into the same system.[27] In January 2009, a task force for visible light communication was formed by the Institute of Electrical and Electronics Engineers working group for wireless personal area network standards known as IEEE 802.15.7.[28] A trial was announced in 2010, in St. Cloud, Minnesota.[29]

Amateur radio operators have achieved significantly farther distances using incoherent sources of light from high-intensity LEDs. One reported 173 miles (278 km) in 2007.[30] However, physical limitations of the equipment used limited bandwidths to about 4 kHz. The high sensitivities required of the detector to cover such distances made the internal capacitance of the photodiode used a dominant factor in the high-impedance amplifier which followed it, thus naturally forming a low-pass filter with a cut-off frequency in the 4 kHz range. Use of lasers can reach very high data rates which are comparable to fiber communications.

Projected data rates and future data rate claims vary. A low-cost white LED (GaN-phosphor) which could be used for space lighting can typically be modulated up to 20 MHz.[31] Data rates of over 100 Mbit/s can be easily achieved using efficient modulation schemes and Siemens claimed to have achieved over 500 Mbit/s in 2010.[32] Research published in 2009, used a similar system for traffic control of automated vehicles with LED traffic lights.[33]

In September 2013, pureLiFi, the Edinburgh start-up working on Li-Fi, also demonstrated high speed point-to-point connectivity using any off-the-shelf LED light bulb. In previous work, high bandwidth specialist LEDs have been used to achieve the high data rates. The new system, the Li-1st, maximizes the available optical bandwidth for any LED device, thereby reducing the cost and improving the performance of deploying indoor FSO systems.[34]

Engineering details[edit]

Typically, best use scenarios for this technology are:

  • LAN-to-LAN connections on campuses at Fast Ethernet or Gigabit Ethernet speeds
  • LAN-to-LAN connections in a city, a metropolitan area network
  • To cross a public road or other barriers which the sender and receiver do not own
  • Speedy service delivery of high-bandwidth access to optical fiber networks
  • Converged Voice-Data-Connection
  • Temporary network installation (for events or other purposes)
  • Reestablish high-speed connection quickly (disaster recovery)
  • As an alternative or upgrade add-on to existing wireless technologies
    • Especially powerful in combination with auto aiming systems, this way you could power moving cars or you can power your laptop while you move or use auto-aiming nodes to create a network with other nodes.
  • As a safety add-on for important fiber connections (redundancy)
  • For communications between spacecraft, including elements of a satellite constellation
  • For inter- and intra-chip communication[35]

The light beam can be very narrow, which makes FSO hard to intercept, improving security. In any case, it is comparatively easy to encrypt any data traveling across the FSO connection for additional security. FSO provides vastly improved electromagnetic interference (EMI) behavior compared to using microwaves.

Technical advantages[edit]

Range limiting factors[edit]

For terrestrial applications, the principal limiting factors are:

These factors cause an attenuated receiver signal and lead to higher bit error ratio (BER). To overcome these issues, vendors found some solutions, like multi-beam or multi-path architectures, which use more than one sender and more than one receiver. Some state-of-the-art devices also have larger fade margin (extra power, reserved for rain, smog, fog). To keep an eye-safe environment, good FSO systems have a limited laser power density and support laser classes 1 or 1M. Atmospheric and fog attenuation, which are exponential in nature, limit practical range of FSO devices to several kilometres. However the free space optics, based on 1550 nm wavelength, have considerably lower optical loss than free space optics, using 830 nm wavelength, in dense fog conditions. FSO using wavelength 1550 nm system are capable of transmitting several times higher power than systems with 850 nm and are at the same time safe to the human eye (1M class). Additionally, some free space optics, such as EC SYSTEM[36], ensure higher connection reliability in bad weather conditions by constantly monitoring link quality to regulate laser diode transmission power with built-in automatic gain control.[37]

See also[edit]

References[edit]

  1. ^"Book X". The Histories of Polybius. 1889. pp. 43–46. Retrieved 17 November 2014. 
  2. ^Mary Kay Carson (2007). Alexander Graham Bell: Giving Voice To The World. Sterling Biographies. New York: Sterling Publishing. pp. 76–78. ISBN 978-1-4027-3230-0. 
  3. ^Alexander Graham Bell (October 1880). "On the Production and Reproduction of Sound by Light". American Journal of Science, Third Series. XX (118): 305–324.  also published as "Selenium and the Photophone" in Nature, September 1880.
  4. ^"German, WWII, WW2, Lichtsprechgerät 80/80". LAUD Electronic Design AS. Archived from the original on July 24, 2011. Retrieved June 28, 2011. 
  5. ^TerraSAR-X NFIRE test
  6. ^"TereScope 10GE". MRV Terescope. Archived from the original on 2014-08-18. Retrieved October 27, 2014. 
  7. ^ abAn end-of-life notice was posted suddenly and briefly on the MRV Terescope product page in 2011. All references to the Terescope have been completely removed from MRV's official page as of October 27, 2014.
  8. ^"10 Gbps Through The Air". Arto Link. Retrieved October 27, 2014.  
  9. ^"LightPointe main page". Retrieved October 27, 2014. 
  10. ^Miloš Wimmer (13 August 2007). "MRV TereScope 700/G Laser Link". CESNET. Retrieved October 27, 2014. 
  11. ^Eric Korevaar, Isaac I. Kim and Bruce McArthur (2001). "Atmospheric Propagation Characteristics of Highest Importance to Commercial Free Space Optics"(PDF). Optical Wireless Communications IV, SPIE Vol. 4530 p. 84. Retrieved October 27, 2014. 
  12. ^Tom Garlington, Joel Babbitt and George Long (March 2005). "Analysis of Free Space Optics as a Transmission Technology"(PDF). WP No. AMSEL-IE-TS-05001. US Army Information Systems Engineering Command. p. 3. Archived from the original(PDF) on June 13, 2007. Retrieved June 28, 2011. 
  13. ^US Federal Employees. "$86.5M in FY2008 & 2009, Page 350 Department of Defense Fiscal Year (FY) 2010 Budget Estimates, May 2009, Defense Advanced Research Projects Agency, Justification Book Volume 1, Research, Development, Test & Evaluation, Defense-Wide, Fiscal Year (FY) 2010"(PDF). Retrieved October 4, 2014. 
  14. ^US Federal Employees. "US$40.5M in 2010 & 2011, page 273, Department of Defense, Fiscal Year (FY) 2012 Budget Estimates, February 2011, Defense Advanced Research Projects Agency, Justification Book Volume 1, Research, Development, Test & Evaluation, Defense-Wide, Fiscal Year (FY) 2012 Budget Estimates". Retrieved October 4, 2014. 
  15. ^US Federal Employees. "US$5.9M in 2012, page 250, Department of Defense, Fiscal Year (FY) 2014 President's Budget Submission, April 2013, Defense Advanced Research Projects Agency, Justification Book Volume 1, Research, Development, Test & Evaluation, Defense-Wide". Archived from the original on October 25, 2016. Retrieved October 4, 2014. 
  16. ^Bruce V. Bigelow (June 16, 2006). "Zapped of its potential, Rooftop laser startups falter, but debate on high-speed data technology remains". Retrieved October 26, 2014. 
  17. ^Nancy Gohring (March 27, 2000). "TeraBeam's Light Speed; Telephony, Vol. 238 Issue 13, p16". Retrieved October 27, 2014. 
  18. ^Fred Dawson (May 1, 2000). "TeraBeam, Lucent Extend Bandwidth Limits, Multichannel News, Vol 21 Issue 18 Pg 160". Retrieved October 27, 2014. 
  19. ^Terabeam
  20. ^"LightPointe Website". Retrieved October 27, 2014. 
  21. ^Robert F. Service (21 December 2001). "Hot New Beam May Zap Bandwidth Bottleneck". Retrieved 27 October 2014. 
  22. ^"CableFree UNITY Website". Retrieved September 28, 2016. 
  23. ^Fog Optics staff (20 November 2014). "Fog Laser Field Test"(PDF). Retrieved 21 December 2014. 
  24. ^http://ronja.twibright.com/changelog.php
  25. ^http://www.bizjournals.com/prnewswire/press_releases/2013/01/17/BR44159
  26. ^"Visible Light Communication Consortium". web site. Archived from the original on April 6, 2004.  (Japanese)
  27. ^Tanaka, Y.; Haruyama, S.; Nakagawa, M.; , "Wireless optical transmissions with white colored LED for wireless home links," Personal, Indoor and Mobile Radio Communications, 2000. PIMRC 2000. The 11th IEEE International Symposium on, vol. 2, no., pp. 1325–1329 vol.2, 2000.
  28. ^"IEEE 802.15 WPAN Task Group 7 (TG7) Visible Light Communication". IEEE 802 local and metro area network standards committee. 2009. Retrieved June 28, 2011. 
  29. ^Kari Petrie (November 19, 2010). "City first to sign on to new technology". St. Cloud Times. p. 1. 
  30. ^Clint Turner (October 3, 2007). "A 173-mile 2-way all-electronic optical contact". Modulated light web site. Retrieved June 28, 2011. 
  31. ^J. Grubor; S. Randel; K.-D. Langer; J. W. Walewski (December 15, 2008). "Broadband Information Broadcasting Using LED-Based Interior Lighting". Journal of Lightwave Technology. 26 (24): 3883–3892. Bibcode:2008JLwT...26.3883G. doi:10.1109/JLT.2008.928525. 
  32. ^"500 Megabits/Second with White LED Light". news release. Siemens. January 18, 2010. Archived from the original on March 11, 2013. Retrieved February 2, 2013. 
  33. ^Lee, I.E.; Sim, M.L.; Kung, F.W.L.; , "Performance enhancement of outdoor visible-light communication system using selective combining receiver," Optoelectronics, IET , vol. 3, no. 1, pp. 30–39, February 2009.
  34. ^"Pure LiFi transmits data using light". web site.  (English)
  35. ^Jing Xue, Alok Garg, Berkehan Ciftcioglu, Jianyun Hu, Shang Wang, Ioannis Savidis, Manish Jain, Rebecca Berman, Peng Liu, Michael Huang, Hui Wu, Eby G. Friedman, Gary W. Wicks, Duncan Moore (June 2010). "An Intra-Chip Free-Space Optical Interconnect"(PDF). the 37th International Symposium on Computer Architecture. Retrieved June 30, 2011. 
  36. ^http://www.ecsystem.cz/en/products/free-space-optic-equipment
  37. ^http://www.ecsystem.cz/en/products/free-space-optic-equipment

Further reading[edit]

External links[edit]

An 8-beam free space optics laser link, rated for 1 Gbit/s. The receptor is the large disc in the middle, the transmitters the smaller ones. At the top right corner is a monocular for assisting the alignment of the two heads.
A photophone receiver and headset, one half of Bell and Tainter's optical telecommunication system of 1880
DARPA ORCA official concept art created c. 2008

Leave a Comment

(0 Comments)

Your email address will not be published. Required fields are marked *