Free space optics

OPTICAL WIRELESS COMMUNICATION :FREE SPACE OPTICS

Prepared by : Romil Shah (10BEC093) Pritesh Desai (10BEC128)

Guided by:Dr. D K Kothari

PRESENTATION LAYOUT

Optical Wireless Communcation : Types Introduction to the concepts of Free Space

Optics (FSO). Propagation concepts, Link Budget calculations. FSO: Last Mile Bottleneck Solution. Configurations of FSO systems. Chaining in FSO Systems DATA security/ Safety considerations for FSO

systems. Signal Propagation impediments. Advantages of FSO as regards to other widely

used systems. Physical Applications of FSO systems Manufacturers/Players in field of FSO.

2

Optical Communications

Optical FibreCommunications

Photonic Switching

Indoor Free-Space Optics(FSO)

Free-Space Optics(FSO)• Chromatic dispersion

compensation using optical signal processing• Pulse Modulations• Optical buffers• Optical CDMA

• Pulse Modulations• Equalisation• Error control coding• Artificial neural network & Wavelet based receivers

• Fast switches• All optical routers

Subcarrier modulation Spatial diversity Artificial neural network/Wavelet based receivers

3

Wired Wireless

3

Optical Wireless Communication : What does it offer?

4

What

does it

offer?

Abundance of unregulated bandwidth – 200 THz in the 1500-700 nm range.

No multipath fading – Intensity Modulation and Direct Detection.

High data rate – in particular line of sight(in and out doors).

Improved wavelength reuse capability.

Flexibility in installation - Deployment in a wide variety of network architecture and installation on roof to roof, window to window, roof to window, etc.

Secure transmission.

4

Optical Wireless Communication : Drawbacks

5

Drawback s

Multipath induced dispersion (non-line of sight, indoor) - Limiting data

SNR can vary significantly with the distance and the ambient noise

Limited transmitted power - Eye safety (indoor) Receiver sensitivity Large area photo-detectors - Limits the

bandwidth May be high cost - Compared with RF Limited range: Indoor: ambient noise is the

dominant (20-30 dB larger than the signal level . Outdoor: Fog and other factors

High transmitted power - Outdoor

5

Requirements of a good Transmission System:

High Bandwidth Low BER High SNR Power efficient Provide Data Security. Low cost Easy to install and maintain.

6

Introduction to the concepts of Free Space

Optics (FSO)7

FSO is a line-of-sight technology which uses LASERS and Photo detectors to provide optical connections between two points—without the fiber.

FSO can transmit data, voice or video at speeds capable of reaching 2.5 Gbps. Products capable of speeds upto 10 Gbps are expected to hit the markets within one year.

FSO units consist of an optical transceiver with a laser (transmitter) and a Photo detector (receiver) to provide full duplex (bi-directional) capability.

FSO systems use invisible infrared laser light wavelengths in the 750nm to 1550nm range.

FSO - Characteristics8

Narrow low power transmit beam- inherent security Narrow field-of-view receiver Similar bandwidth/data rate as optical fibre No multi-path induced distortion in LOS Efficient optical noise rejection and a high optical signal

gain Suitable to point-to-point communications only (out-

door and in-door) Can support mobile users using steering and tracking

capabilities Used in the following protocols:

- Ethernet, Fast Ethernet, Gigabit Ethernet, FDDI, ATM- Optical Carriers (OC)-3, 12, 24, and 48.

Cheap (cost about $4/Mbps/Month according to fSONA)

8

FSO - Applications9

9

In addition to bringing huge bandwidth to businesses /homes FSO also finds applications in :

Multi-campus universityHospitals

Others: Inter-satellite communication Disaster recovery Fibre communication back-up Video conferencing Links in difficult terrains Temporary links e.g. conferences

Cellular communication back-haul FSO challenges…FSO challenges…

Applications Of FSO Systems

Disaster management as was exhibited during the Sept 11 attacks.

Merill Lynch & Co. has set up FSO system from its Vesey Street office towers across the Hudson River to an alternate site in New Jersey.

TeraBeam, a major producer of FSO equipment, successfully deployed FSO at the Sydney Summer Olympic Games.

A network of FSO devices is fast coming up in Seattle which is touted as the Capital of Fog. Manufacturers believe that if an FSO system can successfully work in Seattle then it can do so in any part of the world.

Affordably extend existing fiber network.

Disaster recovery and temporary applications

10

ADVANTAGES OF FSO SYSTEMS

No licensing required. Installation cost is very low as compared

to laying Fiber. No sunk costs. No capital overhangs. Highly secure transmission possible. High data rates, upto 2.5 Gbps at present

and 10 Gbps in the near future.

11

Block Diagram of a FSO system

12

Optical Link Geometry13

How does the system works?

A source producing data input is to be transmitted to a remote destination. This source has its output modulated onto an optical carrier; laser or LED, which is then transmitted as an optic al field through the atmospheric channel.

The important aspects of the optical transmitter system are size, power, and beam quality, which determine laser intensity and minimum divergence obtainable from the system.

At the receiver, the field is optically collected and detected, generally in the presence of noise interference, signal distortion, and background radiation. On the receiver side, important features are the aperture size and the f/-number, which determine the amount of the collected light and the detector field-of-view (FOV).

The transmit optics consists of lens assembly ( Plano convex lenses ) and receiver Optics consist of telescope units to receive the incident light.

14

LED vs Laser Diode as light source The choice of LED vs. Laser Diode as a light source in

a wireless optical transmission product depends on the target application, and the related performance, cost and reliability requirements of the overall solution being designed.

Long range, very high speed (gigabit or more) point-to-point FSO systems require laser diodes. Such products compete with high-speed RF point-to-point solutions often based on milimeter wave transmission in the 60, 70, 80 and 90 GHz bands.

However, shorter range LED based systems can achieve high-speed optical system performance, while dramatically reducing the overall system size and cost.

15

Receivers and Material System

Compared with transmitters, receiver choices are much more limited.

The two most common detector material systems used in the near-IR spectral range are based on Si or indium gallium arsenide (InGaAs) technology.

Germanium is another material system that covers the operating wavelength range of commercially available FSO systems.

However, germanium technology is not used very often because of the high dark current values of this material.

All these materials have a rather broad spectral response in wavelength, and, unlike lasers, they are not tuned toward a specific wavelength.

16

Detectors in Different Systems

Usually a trans-impedance amplifier is used after the detector because in most cases they provide the highest gain at the fastest speed.

If CCD, CMOS, or quad cell detectors are used as tracking detectors, these relatively large area devices are easy to align to the tracking optics. However, care must be taken in manufacture to co-align these optics with the transmit and receive optical axes.

For building-mounted free-space optical systems, the tracking bandwidth can be very low—sub-hertz—because the bulk of building motion is due to the building’s uneven thermal loading and these effects occur in a time scale of hours.

For systems that are to be mounted on towers or tall poles, the tracking bandwidth should be higher—most likely on the order of several hertz at least—to remove wind-induced vibrations.

Acquisition systems can be as crude as aligning a gunsight to very sophisticated GPS based, high accuracy, fully automated systems. The choice of this subsystem really depends on the application and number of devices to be put into a network.

17

Modulation Technique Used

On and Off Keying (OOK) Modulation : On-off keying (OOK) the simplest form of modulation that represents digital data as the presence or absence of a carrier wave. In its simplest form, the presence of a carrier for a specific duration represents a binary one, while its absence for the same duration represents a binary zero.

Pulse Position Modulation : Pulse-position modulation (PPM) is a form of signal modulation in which M message bits are encoded by transmitting a single pulse in one of possible time-shifts. This is repeated every T seconds, such that the transmitted bit rate is M/T bits per second. It is primarily useful for optical communications systems, where there tends to be little or no multipath interference.

18

OOK Modulation19

PPM Modulation Scheme20

Modulation In Detail21

Working -Modulation

Firstly, the incoming data stream is serial to parallel converted into "n" independent streams. These streams are encoded in parallel by an encoder.

In the parallel encoder, a data block is composed by taking one bit out of each data sequence, each time the data blocks are encoded.

The parity check bits are added and transmitted on "k" exclusive channels, which have same rate as the data sequence and are also generated by the encoder.

Hence, this parallel encoder makes an (11 + k, n) code, where n + k is the codeword length. Secondly, these n + k codeword sequences are modulated into 00K or PPM codes on each channel.

At the optical modulator, these code sequences modulate each diode with a different wavelength and are multiplexed. In the multiplexer, each optical signal from channels is focused on an optical fiber.

The optical pulses from the fiber are spread on the optical channel and suffer form the effects of ISI.

22

Working - Demodulation

At the receiver, the transmitted pulses are received together with the ambient light noise. These multiplexed signals are separated in accordance with their carrier wavelength.

The optical filter is used as the de-multiplexer. These optical band-pass filters are usually constructed of multiple thin dielectric layers, and can achieve narrow bandwidths.

These separated signals passed to the photo diode array, demodulated by pulse demodulator, and then decoded in parallel by the parallel decoder. Finally, these parallel data blocks are parallel to serial converted to retrieve the original data.

23

DR

IVE

R

CIR

CU

IT

SIG

NA

LPR

OC

ES

SIN

GPH

OTO

DETEC

TO

R

Link Range L

FSO - Basics Cloud Rain Smoke Gases Temperature variations Fog and aerosol

Transmission of optical radiation through the atmosphere obeys the Beer-Lamberts’s law:

α : Attenuation coefficient dB/km – Not controllable and is roughly independent of wavelength in heavy attenuation conditions.d1 and d2: Transmit and receive aperture diameters (m)D: Beam divergence (mrad)(1/e for Gaussian beams; FWHA for flat top beams),

This equation fundamentally ties FSO to the atmospheric weather conditions

10/22

1

22 10)(

Ltr LDd

dPP

Dominant term at 99.9% availability

Dominant term at 99.9% availability

24

Theoretical Maximum Range:25

LAST MILE BOTTLENECKS 26

Less then 5% of all buildings in the US have a direct connection to the very high speed (2.5-10 Gbps) fiber optic backbone, yet more than 75% of businesses are within 1 mile of the fiber backbone.

Most of these businesses are running some high speed data network within their building, such as fast Ethernet (100 Mbps), or Gigabit Ethernet (1.0 Gbps).

Yet, their Internet access is only provided by much lower bandwidth technologies available though the existing copper wire infrastructure (T-1 (1.5 Mbps), cable modem (5 Mbps shared) DSL (6 Mbps one way) ), etc.

The last mile problem is to connect the high bandwidth from the fiber optic backbone to all of the businesses with high bandwidth networks.

27

DSL and cable modems cannot provide true broadband services. Cable modems enjoy higher capacity, yet the channel is shared and the amount of bandwidth at any given time is not guaranteed.

Copper lines provide data rates to a fraction of 1 Mbps.

T1 lines can reach upto a few Mbps but are still far away from the Gbps speed which the fiber backbone can support.

The chart below shows how these technologies address different market segments based on technology, technical capabilities (reach, bandwidth), and economic realities.

28

29

Different Topologies of FSO Networks

Point to Multipoint Topology Point to Point Topology Ring with Spurs Topology Mesh Topology Metro Network

30

Point-to-Multipoint Topology31

Point-to-Point Topology32

Ring with Spurs Topology33

Mesh Topology34

Typical Topology in a Metro35

A high-bandwidth cost-effective solution to the last mile problem is to use free-space laser communication (also known as or optical wireless) in a mesh architecture to get the high bandwidth quickly to the customers.

36

DATA SECURITY

To overcome the security in a network two conditions are necessary:

(1) Intercept enough of the signal to reconstruct data packets and

(2) Be able to decode that information.

37

Directional transmission:

Narrow divergence of the FSO transmit path (shown in red) as compared to a typical Radio Frequency (RF) path (shown in blue). The tightly collimated FSO beam ensures that the signal energy is focused on the receiving unit, making interception of the beam extremely difficult.

Preventing Interception of the Signal38

39

Another view of the narrow beam divergence inherent in FSO transmission. (For clarity only one transit beam is shown.)

40

Challenges to FSO Communication

Physical Obstruction Atmospheric Losses

Free space loss Clear air absorption Weather conditions (Fog, rain, snow, etc.) Scattering Scintillation

Building Sway and Seismic activity

41

Physical Obstruction

Construction crane or flying bird comes in path of light beam temporarily

Solution: Receiver can recognize temporary loss of

connection In packet-switched networks such short-

duration interruptions can be handled by higher layers using packet retransmission

42

Free space loss43

Proportion of transmitted power arriving at the receiver

Occurs due to slightly diverging beam

Solution: High receiver gain and large receiver aperture Accurate pointing

Clear Air Absorption

Equivalent to absorption loss in optical fibers

Wavelength dependent Low-loss at wavelengths ~850nm,

~1300nm and ~1550nm Hence these wavelengths are used for

transmission

44

Weather Conditions

Adverse atmospheric conditions increase Bit Error Rate (BER) of an FSO system

Fog causes maximum attenuation Water droplets in fog modify light characteristics

or completely hinder the passage of light Attenuation due to fog is known as Mie scattering

Solution: Increasing transmitter power to maximum

allowable Shorten link length to be between 200-500m

45

Scattering

Caused by collision of wavelength with particles in atmosphere

Causes deviation of light beam

Less power at receiverSignificant for long range communication

46

Scintillation

Heated air rising from the earth or man-made devices such as heating ducts creates temperature variations among different air pockets. This can cause fluctuations in signal amplitude which leads to image fluctuations at the FSO receiver end.

Caused due to different refractive indices of small air pockets at different temperatures along beam path

Air pockets act as prisms and lenses causing refraction of beam

Optical signal scatters preferentially by small angles in the direction of propagation

Distorts the wavefront of received optical signal causing ‘image dancing’

Best observed by the simmering of horizon on a hot day

47

Scintillation (cont…)

Solution: Large receiver diameter to cope with

image dancing Spatial diversity: Sending same

information from several laser transmitters mounted in same housing

Not significant for links < 200m apart, so shorten link length

48

Building Sway and Seismic activity

Movements of buildings upsets transmitter-receiver alignment

Solution: Use slightly divergent beam

Divergence of 3-6 milliradians will have diameter of 3-6 m after traveling 1km

Low cost Active tracking

Feedback mechanism to continuously align transmitter- receiver lenses

Facilitates accelerated installation, but expensive

49

Empirical Design Principles

Use lasers ~850 nm for short distances and ~1550 nm for long distance communication with maximum allowable power

Slightly divergent beam Large receiver aperture Link length between 200-1000m in case of

adverse weather conditions Use multi-beam system

50

Rough Estimate of Power losses in the system Infrared light (765 nm)

: Clear, still air -1 dB/km -5 dB/km Scintillation 0 to -3 dB/km 0 Birds or foliage Impenetrable 0 to -20 dB Window (double-glazed) -3 dB -1 dB Light mist (visibility 400m) -25 dB/km -1 dB/km Medium fog (visibility 100m) -120 dB/km -1

dB/km Thick fog (visibility 40m) -300 dB/km -1

dB/km Light rain (25mm/hour) -10 dB/km -10

dB/km Heavy rain (150mm/hour) -25 dB/km -40

dB/km

51

Limitations of FSO Technology

Requires line-of-sight Limited range (max ~8km) Unreliable bandwidth availability

BER depends on weather conditions Accurate alignment of transmitter-

receiver necessary

52

Manufacturers/ Players in the Field of FSO:

LightPointe: A San Diego based company which received contributions from Cisco Systems and Corning to the tune of $33 million. It has raised a total of $51.5 million.

AirFiber: Another San Diego based company which has received contributions from Nortel Networks to the tune of $50 million. It has raised a total of $92.5 million.

Terabeam: A Kirkland, WA based company has received funding from Luscent technologies to the tune of $450 million and has raised $585 million to date.

53

References

http://en.wikipedia.org/ Lighpointe’s “The phyiscs of free space

optics” white paper Lightpointe Communications Corp.,

"Free Space Optics: A Viable Last-Mile Alternative,"white paper.

"Optical Wireless: Low-Cost, Broadband, Optical Access," white paper, fSONA Communications Corp.

54

References

http://www.lightpointe.com/ http://www.freespaceoptic.com/ http://www.fsonews.com/ http://www.cablefreesolutions.com/ http://www.thefoa.org/ http://www.free-space-optics.org/ http://www.freespaceoptics.com/ http://www.opticsreport.com/

55

Acknowledgment

We thank our Seminar Guide Dr D K Kothari for his valuable guidance and directions in making the seminar resourceful. We would like to express our gratitude to Prof Dhaval Shah who has provided a helping hand in understanding of the topic. We are thankful to our seniors for helping and guiding us and understanding the practical applications of the topic.

56