FDMA IN SATELLITE COMMUNICATION SYSTEM

Artificial satellite of the earth is an object, orbiting the earth, containing radio equipment such as receiver, amplifier, signal processor, transmitter etc, and powered by solar photovoltaic module. First commercial satellite was launched in August 1965. Since then numerous satellites have been launched for communication navigation, defense, broadcasting and other purposes.

Satellite systems may be domestic (used by a single country like Canadian Telecast System or Indian Satellite system-INSAT), regional system used by two or more countries (French-German Symphonic system) and global systems of intercontinental character like INTELSAT, INMARSAT, ASIASAT, INTERSPUTNIK etc. The uses of satellite systems space segment allocation and frequency allocation are coordinated by International Telecommunication Union (ITU) based in Geneva. Specific frequency bands are allocated for use in satellite system:

S-band -2GHz
C-band -4GHz
Ku-band -11GHz
Ka- band -18GHz

In general up-link and down-link frequencies are different to avoid interference.
Satellite stays in the orbit in its position because of balance of centripetal forces on the satellite and the gravitational force of the earth. For this happen the height of the satellite has to be greater than 600 KM from the earth’s surface.

Orbit of Satellite

Orbit is an imaginary path in the space along which the satellite rotates the earth. The basic orbits are elliptical inclined, circular polar and circular equatorial (or as a special case of it-geo-stationary orbit). Circular equatorial orbit situated at the height of about 36,000 KM is called geostationary orbit (or synchronous orbit). Any satellite located at this height will rotate around its axis (23 Hrs. 56 Mins.). In this case the satellite is seen stationary with respect to the earth station. Therefore in practice geostationary orbit satellites are extensively used. Elliptically inclined orbits are used for communication with earth stations located near polar region. Weather forecasting satellites are usually located at lower orbits (Low Earth Orbits-LEO).

Satellite Communication Systems

Satellite contains active elements like receiver, amplifier, signal processor, transmitter antenna etc. Each set of these equipments (transmitter and receiver) is called Transponder. The antenna is common device shared by both receiver and transmitter. The area in the earth surface where the level of signal from the satellite is greater or equal to satisfactory level is called “Foot Print”. As the signal level transmitted by the satellite is very low, the level of received signal is also extremely low. Therefore a special front end amplifier, called Low Noise Amplifiers (LNA) is used in the satellite receivers.

The basic types of communication in which satellites are used as active reflectors are:

Direct Satellite Broadcasting Television (DSB-TV)

In DSB-TV system, the TV program generated from the Television studio is transmitted to the satellite. The received signal is then amplified and re-transmitted to the earth by the satellite. The coverage area of the transmitted signal will be wide and individuals can receive this signal and obtain high quality pictures. Examples of DSB-TV are programs of Doordarshan from INSAT and STAR programs from ASIASAT. Terrestrial TV broadcasting stations can receive these signals and retransmit using local transmitters. In this case all the individual users do not need to use expensive satellite receivers.

Fixed Assignment Communication Channel

If heavy communication traffic is known to exist between any two points in the earth, a fixed transponder may be assigned to relay multiplexed signal between these two points. This is called fixed assignment satellite communication. This system does not permit flexibility and has low utilization factor.

Multiple Access System

To increase the satellite utilization factor, most of the satellite systems use Demand Assigned Multiple Access (DAMA) system. In this system the earth station A desirous to establish communication with another earth station B send request signal to satellite. The satellite assigns the station A any of the free channel, presently unoccupied, to establish link between A and B. The assignment could be in frequency slot (FDM) or in time slot (TDM). If the assignment is in frequency slot, the system is called Frequency Division Multiple Access (FDMA). In case of time slot assignment, the system is called Time Division Multiple Access (TDMA).

A good example of FDMA is SPADE System (Single Channel Per Carrier PCM Multiple Access Demand Assignment Equipment). In this system a common pool of about 800 channels (carriers) are available to all ground stations having common signaling channel. If station A wishes to establish link with station B, the former selects a free channel randomly and though the signaling link send the information on the selected channel. When the station B confirms it, the link is established. Because of various constraints in using FDM, present satellite systems use only TDMA.

SATELLITE COMMUNICATIONS SYSTEM

In a geostationary satellite communication system, a message signal is transmitted from an earth station via an uplink to a satellite amplified in a transponder (i.e. electronic circuitry) on board the satellite, and then retransmitted from the satellite via a downlink to another earth station. The most popular frequency band for satellite communication is 6 GHZ (c-band) for the uplink and 4 GHZ for the downlink.

The uses of this frequency band others the following advantages.
Relatively in expensive microwave equipments.
Low attenuation due to rainfall, rainfall is the primary cause of signal degradation.
Insignificant sky background noise; the sky background noise due the random noise emissions from galactic, solar, and terrestrial sources) reaches at its lowest level between 1 to 10 GHZ.

However, radio interference limits the applications of communication satellite operating in the 6/4 GHZ band coincide with those used for terrestrial microwave systems. This problem is eliminated in the more powerful “second generation” communication satellites that operate in the 14/12 GHZ band (i.e. ku-band); moreover, the use of these higher frequencies makes it possible to build smaller and therefore less expensive antennas.

The basic components of a single transponder channel of a typical communication satellite. Specifically, the receiving antenna output of the uplink is applied to the cascade connection of the following components.

· Band pass filter, designed to separate the received signal from among the different radio channels.
· Low-noise amplifier.
· Frequency down converter, the purpose of which is to convert the received radio frequency (RF) signal to the desired downlink frequency.
· Traveling- wave Tube amplifier, which provides high gain over a wide band of frequencies.
· In traveling –wave Tube (TWT) an electromagnetic signal travels along a helix (i.e. a spring shaped coil of wire), while electrons in a high voltage beam travel through the helix at a speed close to that of the signal wave; the net result is the transfer power from the electrons to the wave, which grows rapidly as the signal wave travels down the helix.
· In a satellite, non linearity of the transponder is the primary cause of interference between users. To contain this serious problem, the traveling-wave tube amplifier in the transponder is purposely operated below capacity. Consequently we find that an in FDMA system the power efficiency of the system is reduced because of the necessary power back off of the traveling wave tube amplifier.

MULTIPLE ACCESS TECHNIQUES

Multiple access is a techniques where by many subscribers or local stations can share the use of a communication channel at the same time of a communication channel at the same time or nearly so, despite the fact that their individual transmissions may originate from widely different locations. Stated in another way a multiple-access technique permits the communication resources of the channel to be shared by a large number of users seeking to communicate with each other.

Multiple Access refers to the remote sharing of a communication channel such as dispersed locations. On the other hand, multiplexing refers to the sharing of a channel such as telephone channel by uses confined to a local site.
In a multiplexed system, user requirements are ordinary fixed. In contrast, in a multiple-access system user requirements can change dynamically with time, in which can provisions are necessary for dynamic channel allocation.

1. FREQUENCY-DIVISION MULTIPLE ACCESS (FDMA)
In this technique, disjoint sub bands of frequency are allocated to the different users on a continuous-time basis. In order to reduce interference between users allocated adjacent channel bands, guard bands are used to act as buffer zones. These guard bands are necessary because of impossibility to achieving ideal filtering for separating the different users.

2. TIME DIVISION MULTIPLE ACCESS (TDMA)
In this technique, each user is allocated the full spectral occupancy of the channel, but only for a short duration of the called a time slot, buffer zones in the form of guard times are inserted between the assigned time slots. This is done to reduce interference between users by allowing for time uncertainty that arise due to system imperfections especially in synchronization schemes.

Frequency division Multiplexing

Multiplexing is a technique whereby a number of independent signals can be combined into a composite signal suitable for transmission over a common channel voice frequencies transmitted over telephone systems, for example, range from 300 to 3100 HZ. To transmit a number of these signals over the same channel, the signals must be kept apart so that they do not interfere with eachother, and thus they can be serperated at the receiving end. Thies is accomplished by separating these signals either in frequency or in time. The technique fo separating the signals in frequency is referred to as frequency-division multiplexing (FDM), whereas, the technique of separating the signals in time is referred as time-division multiplexing (TDM).

The Components of a FDM System are
Message signal
Transmitter consists of product modulators and oscillators
Side band Filters
Band pass Filters
Receiver or detector
Low pass filter

The incoming message signals are assumed to be of the low pass type, but their spectra donot necessarily non zero values all the way down to zero frequency. A low pass filter, when is designed to remove high-frequency component. These low pass filters may be omitted only if the input signals are sufficiency band limited initially. The filtered signals are applied to modulators that shift the frequency ranges of the signals so as to occupy mutually exclusively frequency intervals. The necessary carrier frequencies needed to perform these frequency translation are obtained from carrier supply. The most widely used method of modulation in frequency-division multiplexing is single side band modulation, which in the case of voice signals, requires a bandwidth that is approximately equal to that of the original voice signal. In practice, each voice input is usually assigned a bandwidth of 4KHZ. The band to restrict the band of each modulated wave to its prescribed range. The resulting band pass filters outputs are next combined in parallel to form the input to the common channel. At the receiving terminal, a bank of band-pass filters with their inputs connected in parallel is used to separate the message signals on a frequency occupancy basis. Finally the original message signals are recovered by individual demodulators.

Frequency in Telephony
FDM is extensively used in telephone to transmit number of telephone channels simultaneously over a channel of cable or microwave link. Basic telephone channel is bandlimited to 300-3400HZ (bandwidth 3100HZ). A frequency slot of 4 KHZ is assigned to each telephone channel so that there is a guard band of 900HZ for each channel. The first three channels are multiplexed at 12,16 and 20 KHZ to form a pre-group of 3 telephone channels. The multiplexing (frequency shifting) is SSB-USB. The total bandwidth of the pre-group consisting of three telephone channels is 12KHZ.

Four set of pre-group produce 12 channel group carrier frequencies 60, 72, 84 and 96KHZ is assigned to each pre-group. The bandwidth of the 12 group channel is 48KHZ.
Five sets of 12 channel group produce 60 channel super group. The frequencies assigned are 312, 360, 408, 456, 504 and 552 KHZ. The bandwidth of super group is 240KHZ.
Ten sets of super group produces Master group of 600 voice channels with the bandwidth of 2500KHZ. Finally, six sets of master group produce super master group with a bandwidth of 17MHZ.
Television signal can also transmitted over the voice channel. As the composite video signal (video plus Fm audio) bandwidth is about 6MHZ a super master group can be used for simultaneous transmission of two television channels and about 1200 voice channels.

MODULATION

Modulation in general sense is the mixing of two signals. In the communication system modulation is the process of varying some characteristics of carrier wave (Amplitude, phase or frequency) with respect to the modulating wave (message signal). In other words modulation process is viewed as the frequency transformation of message signal from low frequency to high frequency band. The impression can be performed by varying the amplitude, frequency or phase of the carrier signal in accordance with the rate of change of the low frequency signal. The low frequency signal, usually the information bearing signal is called modulating signal.
Need of Modulation or Benefit of Modulation :
Modulation is needed due to following reasons
Practical Antenna length
Mixing up of different voice signals
Unpredictable variation in volume and pitch of voice signal

Practical Antenna Length : The voice frequencies in the band of 20HZ to 20KHZ, For the efficient transmission and reception of radio frequency signals. The antenna length ‘l’ required in terms of wavelength ‘^’ used in transmission is expressed as ‘^’ = c/f.
Where c = velocity of light in free space
F = frequency of wave used in transmission
L = length

L = ^/2,which is very high for the frequency range of 20 HZ to 20KHZ and length of antenna is impractical to construct.
We know that in case of free space is used as a transmitting medium (i.e. channel), messages are transmitted and received with the help of antennas. For efficient radiation and reception the transmitting and receiving antennas must have lengths comparable to a quarter-wavelength of the frequency used. Obviously, it will be totally impracticable to construct and install an antenna of such a height. However, this height of the antenna may be reduced by modulation technique and yet effective radiation and reception is achieved. In modulation process, low frequency of audio signal at radio stations are translated to higher frequency spectrum i.e., radio frequency range. These higher radio frequencies with the small wavelength act as carrier for the audio frequencies (i.e. modulation signal).Thus, the height of the antenna required is much reduced and becomes practical.

Mixing up of different voice signals : All the voice signals (Audible sound wave) is concentrated with in range of 20 Hz to 20KHZ, so that all signals from the different sources would be hopelessly and inseparateably mixed up if they are transmitted without modulation. The modulation helps to separate then in radio frequency spectrum.
Unpredictable variation in volume and pitch of voice signal : The unmodulated carrier wave doesnot transmit any message or intelligence because they have constant amplitude frequency and phase. The voice signal has unpredictable variation in volume and pitch (amplitude and phase) which is impossible to represent these two variables by a set of three constant parameters (amplitude frequency and phase). Thus the message contains in voice is transmitted by varying any one parameter (amplitude frequency and phase) of carrier. According to the variation in the message signal.

Types of Modulation

Modulation is basically of two types:

i. Continuous Wave Modulation : When the carrier wave is continuous in nature, the modulation process is known as continuous wave (CW) modulation or analog modulation. Examples of continuous wave modulation are Amplitude Modulation and Angle Modulation. When the amplitude of the carrier is varied in accordance with the message signal, it is known as amplitude modulation (AM). Also, when the carrier is varied according to the instantaneous value of the modulating signal, it is called angle modulation. Angle modulation may be further subdivided into Frequency Modulation (FM) and Phase Modulation (PM), in which the instantaneous frequency and phase of the carrier, respectively, are varied in accordance with the message signal.

ii. Pulse Modulation : When the carrier wave is a pulse-type waveform, the modulation process is known as pulse modulation. In pulse modulation, the carrier consists of a periodic sequence of rectangular pulses. Pulse modulation can be of an analog or digital type.
In analog pulse modulation, the amplitude, duration or position of a pulse is varied in accordance with sample values of the message signal. The analog pulse modulation may be of following three types: a. Pulse-amplitude modulation (PAM). b. Pulse-duration modulation (PDM).
c. Pulse-position modulation (PPM).

On the other hand, the digital form of pulse modulation is known as Pulse-code modulation (PCM).

Communication Channels

As we have discussed several sources of information which are responsible for the generation of message signals. These message signals need to be transmitted to the destination point. Now, let us discuss various communication channel that carryout the transmission.
Basically, we shall consider following channels:
(i) telephone channels
(ii) optical fibers
(iii) mobile radio channels, and
(iv) satellite channels.

These four different channels have been chosen because they play important roles in modern telecommunications environment.

(i) Telephone Channels : A telephone network makes use of a switching mechanism. This switching mechanism is known as circuit switching and it is used to establish and end –to –end communication link on a temporary basis. Infact, the primary purpose of the network is to ensure that the telephone transmission between a speaker at one end of the link and a listener at the other end, is an acceptable replacement for face- to –face conversion.
In this form if communication, the message source is the sound produced by the speaker’s voice and the destination is the listener’s ear. However, it may be noted that the telephone channel supports only the transmission of electrical signals.
Because of this reason, appropriate transducers are used at the transmitting and receiving ends of the system.
Further, the telephone channel is essentially a linear, bandwidth limited channel. A speech signal is essentially limited to a band from 300 to 3100 Hz in the sense that frequencies in lying outside this band do not contribute much to articulation efficiency. Hence, the particular frequency band may be viewed as a rough guideline for the passband of a telephone channel that provides satisfactory services.

(ii) Optical Fibers : The second communication channel, an optical fiber, is a dielectric waveguide which transports light signal from one place to another just as a metallic wire pair or a co-axial cable, transports electrical signals.
An optical fiber consists of a central core within which the propagating electromagnetic field is confined and which is surrounded by a cladding layer, which is itself surrounding by a thin protective jacket. Basically, the core and cladding are both made of pure silica glass, whereas, the jacket is made of plastic. Optical fibers have unique characteristics that make then highly attractive as a transmission medium. Optical fibers offer the following unique advantages:

(iii) Mobile Radio Channels : The third communincation channel, mobile radio channel, extends the capability of the public telecommunications network by introducing mobility into virtue of its ability to broadcast.

The term ‘mobile radio’ is usually meant to encompass terrestrial situations where a radio transmitter or receiver is capable of being moved, regardless of whether it actually moves or not. Basically, there is no ‘line of sight’ path for communication, rather, radio propagation takes place mainly by way of scattering from the surfaces of the surrounding buildings and by diffraction over and / or around them.

Therefore, the energy reaches the receiving antenna via more than one path. Therefore, in a mobile radio environment, we face a problem of multipath phenomenon in the sense that the various incoming radio waves reach their destination from different directions and with different time delays. Because of this, the received signal strength varies with location in a vary complicated manner and therefore, a mobile radio channel may be viewed as a linear time varying channel that is statistical in nature.

(iv) Satellite Channel

A satellite channel provides broad-area coverage in a continental as well as intercontinental
sense. Moreover, access to remove areas not covered by conventional cable or fiber communications is also a distinct feature of satellites.

In almost all satellite communication systems, the satellites are placed in geostationary orbit. For the orbit to be geostationary, it has to satisfy two requirements. First, the orbit is geosynchronous, which requires the satellite to be at an altitude of 22,300 miles, a geosynchronous satellite orbits the earth in exactly 24 hours. Second, the satellite is placed in orbit directly above the equator on an eastward heading.

Viewed from the earth, a satellite in geostationary orbit appears to be stationary in the sky.
Communications satellites in geostationary orbit offer the following unique system capabilities:
a. broad-area coverage
b. reliable transmission links
c. wide transmission bandwidth

In terms of services, satellites can provide fixed point to point links extending over long distances and into remote areas, communication to mobile platforms (e.g., aircraft, ships), or broadcast capabilities.
In a typical satellite communication system, a message signal is transmitted from an earth station via an uplink to a satellite, amplified in a transponder on board the satellite. It is then retransmitted from the satellite via a downlink to another earth station. In effect the satellite acts as a powerful repeater in the sky. The most popular frequency band for satellite communications is 6 GHz for the uplink and 4 GHz for the downlink.
In fact, the use of this frequency band offers the following advantages:
a. relatively inexpensive microwave equipment
b. low attenuation due to rainfall
c. insignificant sky background noise.

A single transponder can carry at least one color television signal, 1200 voice circuits, or digital data at a rate of 50 Mb/s.

DIGITAL COMMUNICATION SYSTEM

In digital communication system, the message signal to be transmitted is digital in nature. This means that digital communication involves the transmission of information in digital form.

Elements of digital communications system:
Discrete information source
Source Encoder
Channel Encoder
Modulator
Electrical Communication Channel
Noise
Demodulator
Channel Decoder
Source Decoder
Destination

The overall purpose of the system is to transmit the message or sequence of symbols coming out of source to a destination point as a high rate and accuracy as possible. The source and destination point are physically separated in space and a communication channel connects the source to the destination point. The communication channel accepts electrical (i.e., electromagnetic) signals and the output of the channel is usually a smeared of destroyed version of the input due to the non-ideal nature of communication channel. In addition to this, the information bearing signal is also corrupted by unpredictable electrical signals (i.e. noise) from both man-made and natural causes. Thus, the smearing and the noise introduces errors in the information being transmitted and limits the rate at which information can be communicated from the source to the destination.

Discrete Information Source: Information source may be classified into two categories based upon the nature of their output i.e. analog Information sources and discrete information sources. In case of analog communication, the information source is analog. Analog information sources, such as microphone actuated by speech emit one or more continuous amplitude signals.
In case of digital communication system, the information source produces a message signal which is not continuously varying with time. Rather the message signal is intermittent with respect to time. The output of discrete information source such as teletype or the numerical output of the computer consists of a sequence of discrete symbols of letters. An analog information source may be transformed into a discrete information sources through the process of sampling and quantizing. Discrete information sources are characterized by the following parameters:
i. Source Alphabet : These are the letters, digits or special characters available from the information source.
ii. Symbol Rate : It is the rate at which the information source generates source alphabets. It is generally represented in symbols/sec unit.
iii. Source Alphabet Probabilities : Each source alphabet from the source has independent occurrence rate in the sequence. As an example, letters A, E, I etc. occur frequently in the sequence. Hence, probability of the occurrence of each source alphabet can become one of the important property which is useful in digital communication.
iv. Probabilistic Dependence of Symbols in a Sequence: The information carrying capacity of each source alphabet is different in a particular sequence. This parameter defined average information content of the symbols. The entropy of a source describes the average information content per symbol in long message. Entropy may be defined in terms of bits per symbol. Bit is the abbreviation for a binary digit.
This means that the source information rate is the product of symbol rate and source entropy, i.e.,
Information rate = Symbol rate * Source entropy
(Bits/sec) (symbols/sec) (Bits/Symbol)

Thus, the information rate represents minimum average data rate required to transmit information from source to the destination.

Source Encoder and Decoder:
In source coding, the encoder maps the digital signal generated at the source output into another signal in digital form. The mapping is one to one and the objective is to eliminate or reduce the redundancy so as to provide an efficient representation of the source output. The source decoder simply performs the inverse mapping and thereby delivers to the user destination, a reproduction of the digital source output. The advantage of source coding is to reduce the bandwidth of transmission.

Channel Encoder and Decoder:
The purpose of channel encoder is to map the incoming digital signal into a channel input and for the decoder to map the channel output into an output digital signal in such a way that the effect of channel noise is minimized. That is the combined roll of channel encoder and decoder is to provide reliable communication. This provision is satisfied by introducing redundancy in a prescribed fashion. In the channel encoder and exploiting it in the decoder, to reconstruct the original encoder input as accurately as possible.

Other element has described above in element of communication system.


Classification of communication
Regarding the mode of propagation, communication may be divided in the following two forms:
i. Line Communication
ii. Wireless or Radio Communication

Line Communication :
In line communication, the medium of transmission is a pair of conductors called transmission line. This is also called as line channel. This means that in line communication, the transmitter and the receiver are connected through a wire or line. However, the installation and maintenance of a transmission line is not only costly and complex but also overcrowds the open space. Apart from this, its message transmission capability is also limited.

Wireless or Radio Communication :
In wireless or radio communication, a message is transmitted through open space by electromagnetic waves called as radio waves. Radio waves are radiated from the transmitter in open space through a device called antenna. A receiving antenna intercepts the radio waves at the receiver. All the radio, TV and satellite broadcasting are wireless or radio communication. The advantages of wireless communication are cost effectiveness, possible long distance communication and simplicity.