perspectives in communication and fundamental limits

PERSPECTIVES IN COMMUNICATIONS AND FUNDAMENTAL LIMITS

By

Dr. S PalGroup Director

Communication Systems Group

ISRO Satellite Centre

Bangalore, INDIA

10th October 2000

Dr. S. Pal : Communication Systems Group ISAC.

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• For mankind to survive, three things are essential:

»Energy

»Love

»Communications


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• COMMUNICATION– Communication is at the root of the progress of every society.

– Speech is inherent part of human nature & biggest advantage over others

– It is communication, more than anything else, which has been responsible for the shrinking of time and distance and with the development of space technology, time and distance have lost their conventional meaning, permitting men and women all over the world to share their experiences, frustrations & successes. Society is often described as essentially people in communication. Communication word comes from Latin Communico -

meaning share.


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When we study communication, we study people relating to each other and to their groups, organizations and societies, influencing each other and being influenced, informing and being informed. To understand human communication we must understand how people relate to one another.

Communication in simple terms is nothing but discriminating response to a stimulus.

In natural form, the process of communication exists both internal and external to every biological organism and also in matters.

It exists internally within the atomic structure in the form of interaction between electrons, protons and neutrons and externally between matters with their own peculiar absorption and reflection characteristics.


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In plants it exists through a low speed hormonal communication where external communication is carried through transfer of molecules when the fragrance of flower attracts bees and pollen is physically transferred.

In case of human being, the development of communication is undoubtedly of very high degree. Here the human being collects the information, it compares, synthesizes, analyses, stores and generates new action items. it is human being's this tremendous capability / ability to build on previous experience of their ancestors, storage of knowledge through spoken and written words which has resulted in the VECTORIAL development. The large contribution has been made by books and printed materials.


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• HISTORY OF COMMUNICATIONS

– Early historic times, long distance communication linking whole empire was essentially confined to:

Military Diplomatic Governmental agencies but hardly for personal or social

welfare


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Earlier days `Communications’ used to be done using visual or audio signals or sending messengers. With the advent of Telegraph (1837) the communication era really started becoming `Telecommunications’ era. In olden days the long distance communications used to be for the whole empire essentially confined to Military, Diplomatic & Governmental agencies. It was hardly used for personal or Social causes. No evidence is available to show that “Humanitarian Assistance” was any day on the agenda of the olden systems. In the present scenario with socialistic governments, UN agencies & NGOs associated with humanitarian causes use the modern day telecommunication techniques very effectively for humanitarian assistance and disaster management.

Today the communication has broken all the limits and besides the above age old purposes it is equally or more utilized for personal and social welfare causes.


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* LET US HAVE A LOOK INTO THE HISTORY

Printing press was introduced by William Caxton in 1476.

New paper started in 1650 News papers became popular only in 19th century 1st regular mail was established. In early 19th century with the introduction of railway

and telegraphy (Morse 1837) the communications improved.


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With Graham Bell (Telephone 1876), Marconi (First wireless signal across Atlantic 1901) and others the electrical communication became popular. Today it has increased in volume and speed by almost 100 million fold.

Second World War was responsible for the tremendous growth of electrical communication which is being also termed as Electronic Communication.

Arthur Clark gave a concept of Three Space Stations to cover the Globe.


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After 1948 there is a virtual communication explosion. DIGITAL OR Capability of digitization has made almost whole of our past technologically OBSOLETE.

FIRST TRANSATLANTIC TELEPHONE calls of 36 channels cable in 1956.

First artificial satellite was launched in 1957.

Introduction of space technology in 1964-65, 1st active experimental communication satellite relayed 1st LIVE picture between USA & Europe.

Early Bird (1965) - 240 channels commercial TRANSATLANTIC Communication.


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Today almost >50,000 VOICE CHANNELS FOR INTER CONTINENTAL COMMUNICATION and >1000 TV CHANNELS.

20th July 1969 (Moon landing).

Communication satellites & emergence of V-SAT, DATA CONNECTING NETWORKS, Computer networking, satellite based remote sensing & processing, Deep space network & communication etc.

HAND HELD PERSONALIZED TELEPHONE SYSTEM.


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• BASIC CONCEPTS OF ELECTRICAL COMMUNICATIONS

TWO FUNDAMENTAL CONCEPTS OF ELECTRICAL COMMUNICATION ARE:

FREQUENCY & BANDWIDTH Human voice : 4 KHz, Music :15 KHz, TV : 6 MHz B.W etc.


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• TOTAL PROCESS OF ELECTRICAL COMMUNICATION REQUIRES:

* 1. Choice of adequate bandwidth* 2. Appropriate method multiplexing signals* 3. Modulating carrier waves* 4. The path loss needs to be compensated using

suitable repeaters


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To include versatility for any communication channel to deal with a complex and wide variety of information such as data, TV, messages & voice communication, techniques such as FDMA, TDMA etc., are employed.

A variety of modulation schemes such as AM, FM, PM, PCM, MPSK, FSK, MGSK etc., have been evolved to modulate the carrier.

Availability and overcrowding of EM spectra, use of higher frequencies and efficient antennas, LNA'S and power amplification devices, use of Spread Spectrum techniques to reduce the difficulties associated with overcrowding of spectrum.

When we talk about communications, we must also talk about the limits of the modern communications.


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• FUNDAMENTAL LIMITS OF COMMUNICATION• FUNDAMENTAL LIMITATIONS IMPOSED ARE:

– Thermal noise of the receiver (e2n = 4KTRB)and also the background noise

– Limitation due to quantum uncertainty principle.– Limitations in Space Communications– For deep space missions speed of electrical signal imposes

another limitation.– Limitations on antennas– Limitations due to atmosphere / propagation / rains etc.


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• LIMITATIONS IN SATELLITE COMMUNICATIONS

• OTHER THAN SPACECRAFT CONSTRAINS IMPOSED:

– Signal Attenuation due to rain– Peak power limited nature of satellite amplifiers– Co-channel interference among neighboring beams– Constrains on ground terminal size and regulatory

constraints– Nonuniformly distributed traffic – Orbit overcrowding & co-location


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• S/C IMPOSED CONSTRAINTS– Available RF power and antenna size

– Capacity reduction caused by nonlinearity of S/C - power amplifiers.

– Frequency Reuse and interference.

• The limitation due to the quantum uncertainty principle becomes important only when the frequency `’ is high, as to make `H KT’ where `h' is plank's constant (6.626 x 10-34 JS), `K' is Boltzman's constant (1.381 x 10 23 J/Ko i.e., the quantum energy of the radiation exceeds the Boltzman energy of an harmonic oscillator at a noise temperature T. For T = 300 deg.K, the corresponding frequency beyond which quantum effects completely swamp the thermal noise is equal to 6 THz.


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• The limitation on the signal design for communication imposed by the extension of quantum uncertainty principle for a wave train, states that : the product of the time duration of the wave train (a bit) and the energy spectral spread is constant ( E.t = h ).

• As the spectrum is finite resource for electromagnetic wave propagation, one cannot arbitrarily keep on increasing the information rate for a fixed spectral bandwidth. This fact coupled with the famous Shannon's Channel capacity limit theorem which states that information cannot be transmitted above a rate `C' determined by the bandwidth `B' & S/N and given by

C = B log2 (1+S/N) bits/sec ......(1)


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and this becomes the fundamental limit in the design of communication system.

(The Shannon's limit is still quite far away)

Here N is the white Gaussian noise which can originate from thermal, atmospheric and galactic sources.

The lower limit on thermal noise is the well known Nyquist formula `KTB'. It can be reduced by decreasing `B' which Unfortunately reduces the potential communication rate.

From (1) we see that `C' increases much faster with `B' than `S' or power and reduction in N.


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For `SPACE COMMUNICATION’ that is the reason satellite capacity is increased by using multiple beams. The capacity in bits/seconds of a single power limited communication channel computed for Guassian noise

• C1 = B log2 {1+P/No B} where `P' is the received power.

• If S/C power is divided among `N' independent beams then

• `C' = NB log 2{ 1 + P/NNoB}.


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• In other words reusing frequency band N-times has the same effect on theoretical capacity as increasing the bandwidth of a single channel by a factor of `N’

Formula for back ground Noise for kT >>h

(At microwave frequencies thermal and background noise are important)

• At optical frequencies it is the quantum effect that sets

the upper limit

• NB=KT watts at 1 micrometer wavelengths, h is 50 times as great as KT at 300oK h is important.


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• Following are the three important fundamental limits for antennas:• (1) ELECTRICALLY SMALL ANTENNAS• With the miniaturization of components endemic in almost all parts of

electronics today, it is important to recognize the limits upon size reduction of antenna elements. These are related to the basic fact that the element's purpose is to couple to a free space wave and the space wavelength has not yet been miniaturized. Chu & Harington have shown that any radiating field can be written as a sum of spherical modes, the antenna of any type is to be enclosed in a sphere. The radiated power can be calculated from the propagating modes in the sphere. When the sphere is too small to allow propagating modes; all modes are then evanescent & the Q becomes large, as the evanescent modes contribute little real power:


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331

1)221(33

2231

rKQ

or

KrforrKrK

rKQ

This indeed represent a fundamental limit which hasonly been approached but never even equaled.


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BW 1/QFor an octave bandwidth Q = 2 & with no losses, this requires a minimum antenna length of 0.365 . Since most small antennas are loops or dipoles, which do not use the spherical volume efficiently, an actual octave antenna is significantly larger and often larger than /2. Actually improving bandwidth for an electrically small antenna is only possible by totally utilizing the volume in establishing a TM & TE mode, or by reducing efficiency. The latter is typified by the ELF SQUID which can be extremely small in wavelengths yet possess a sizable bandwidth with very low efficiency.


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• (2) SUPER DIRECTIVITY

A useful operational definition of antenna superdirectivity is directivity higher than that obtained with the same antenna configuration uniformly excited (constant amplitude and linear phase). Excessive array superdirectivity inflicts major problems in low radiation resistance (hence low efficiency), sensitive to excitation & position tolerances and narrow bandwidth. Superdirectivity applies in principle to arrays of isotropic elements although, of course actual antenna arrays are composed of noniotropic elements. The directivity of arrays is increased by exciting elements using various phase and amplitude distributions. For a 10dB SLR, 2 wavelength array and 20dB SLR, 2 wavelength long array with decreasing, number of elements increasing the Q increases. The approximate relationship between Q and G is log Q = a (G-2)

log Q = a ( G-2 )


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TABLE - 1

2 - Array with SLR = 20 dB

N d/ G(dB) Q

5 0.5 4.69 1.7

7 0.333 5.18 7.0

9 0.250 6.21 1.2 x 104

13 0.167 8.54 5.5 x 1011


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TABLE -2

2 - Array with SLR = 10 dB

N d/ G(dB) Q

5 0.5 4.89 0

7 0.333 5.40 37

9 0.250 6.19 7.6 x 104

11 0.200 6.99 3.5 x 108

13 0.167 7.74 3.3 x 1012

The directivity can be maximized to certain extent bymanipulating d/ , Q and the side lobe levels.


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• (3) HIGH GAIN ANTENNAS:

• Here the fundamental limitations concerns large antennas which exhibit high gain not due to super directivity but due to large areas in square wavelengths. Almost all high gain antennas are reflectors since large arrays are more expensive.

The cost of reflectors usually varies with

D 2.8

The cost is also a function of the manufacturing tolerance ratio `R', which is the ratio of largest dimension to the one sigma error `’ .

R = D/

• Usually for low side lobes (/40)


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2)/4/(11(ln2/4

1

2)/4exp(

12

2

1

1/3200

22

DfDfAWhere

A

ADG

Df

RG

Typical reflector efficiency varies from 0.4 to 0.8 (0.8 for a shaped high efficiency reflectors).

For a broad band reflector the typical value is 0.5.

= (/40)

The typical high gains range in between 90-100 dB


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• Technology in current use includes homologous design in which a large reflector deforms in to a new paraboloid with elevation angle change so that a feed refocus restores performance and computer design of light weight support structure.


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EFFECTS BY RAIN ATTENUATION

Attenuation of signal

Reduction in CNR (clear sky)

Depolarization due to rainsThe CNR - clear sky can get degraded by 10 to 15 dB


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• having talked about electrical communication and its fundamental limits, one can classify the present day communications under the following category.

The age old copper line communicationSurfaceOcean

The Radio Wave communicationsHF, VHF CommunicationLine of Sight - Microwave CommunicationSatellite and Space CommunicationOptical Fibre Communication


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• THE NEWER VERSIONS OF COMMUNICATIONS WHICH OF COURSE DEPEND ON ALL THE ABOVE ARE:

• Computer Communications

Information highways and superhighways Paperless offices, virtual offices, manless factories, etc.


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• Space Communications

Concept of three space stations covering the whole earth was given by Arthur `C' Clark in 1945

The 1st satellite was launched in 1957

The 1st commercial satellite was early bird (1967)

Now there are thousands of satellites orbiting around the earth.


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• in the foreseeable future new equipments and techniques will be used in satellite technology which will extend and improve the possibilities of satellite communications beyond our present imaginations.

Greater Transmit power and more complex antennas on the satellites will make operations possible with a large number of very small earth stations.

Efficient signal processors and switching equipment will enable signal processing to be performed onboard right down to operations similar to switching in exchanges.

Direct connections between satellites will shorten the transmission routes.


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The use of higher frequency bands will enlarge the available bandwidths and thus the transmission capacity.

Miniaturization of electronics.

• With the emergence of optical communications the majority of the existing satellite communication services burden will be taken over by the optical links and the tasks to be performed by future satellites will be:

• Tasks for which it will only be essential to use the satellites to limited basis (Transmission of Telephones and TV to inaccessible places or thin traffic routes etc).


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Tasks for which it will remain advantageous to use satellites (New services with different requirements with respect to BW, partner stations, transmission times. This could include Data Collection and Distribution over a large area, mobile radio services to inaccessible areas, ship, aircrafts, Mobile communication / PCS).


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• FUTURE SATELLITES

• The future Telecommunication spacecrafts will be developed from transmission in to Information Satellite (INFOSAT). They will be given many of the properties of terrestrial telephone exchanges and signal processing equipments and it will be possible to integrate them directly in to future global networks. They will thus permit immediate applications of many existing and future services. Because of their inherent built up flexibility, these satellites will be able to support and speed up the initial construction phase of many new services before their trial on terrestrial networks. These type of satellites will enable new services to be tried out over a large area before being launched on to market and optimally adapted to suit the most appropriate transmission medium.


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• Such a system will obviously have Geosynchronous spacecrafts with some orbiting satellite to take care of North/South pole regions.

The INFOSAT Network is to be an integral component of the planned world-wide broadband telecommunication network. Therefore it will be necessary for planning of the satellite network and the terrestrial network to be closely coordinated. Until now satellites have connected the terrestrial networks of various countries and organizations.

In the new setup more emphasis will be given on standardization of some signal parameters and ground stations to some limited extent, since signal processing will take place in the satellite itself.


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• The future INFOSATS will be of three types:

National / RegionalInternationalRelay

• Future technologies will enable the construction of an INFOSAT network in which the above three will be connected to each other. The onboard processors will ensure that the signals to be exchanged between the satellites and the terrestrial subscribers are combined using TD and SD multiplexing techniques and distributed in accordance with the user requirements. Special coding techniques will ensure the security of transmissions.


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In the area of business communications demand is growing for broad band communication facilities which can be applied flexibly using satellites.

Worldwide Radio paging

Video conferencing and high resolution TV broadcast trials.

Mobile radio services can be combined with location finding services and be used for automatically locating subscribers.

– Earth observation with special warning mechanism and environmental protection services in close coordination with terrestrial sensors.


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• By integrating INFOSAT into terrestrial network, it will be possible to supply all the subscribers of a future global network with all essential information. This will make the location of the user quite irrelevant.

• SERVICES TO BE OFFERED BY INFOSAT Radio and data distribution services to many users

which are spread over a large area. Data collection services for large areas with many data

transmitter stations (multi point-to-point operation : weather, oil, electricity & water meter reading etc.) Telecommunication services for thin routes• Telecommunication services with ships, airplanes,

space vehicles, etc.


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• OPTICAL FIBRE VS SPACE COMMUNICATIONS

• Much is talked about the competition between satellites and optical fibre technology. A closer examination of the specific properties of each transmission medium shows that they could complement each other very well and future world wide terrestrial networks will need to be complemented by a satellite network.

The TV and heavy traffic trunk routes will be taken over by optical fibre.

• The satellites will serve : thin traffic routes extremely large distances / inaccessible areas/weather monitoring / disaster warning / Data collection and distribution etc.


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