FMSCPC- VSAT _L4

8/3/2019 FMSCPC- VSAT _L4

1/45

L - 4

FMSCPC SYSTEM


2/45

JTO Phase-II (6 Weeks) Course, FM-SCPC

FMSCPC SYSTEM

1. Introduction

SCPC stands for Single Channel Per Carrier, i.e. the carrier contains only

one channel. In FMSCPC systems, the carrier is frequency modulated by one

VF channel. These systems are suitable for remote area communications. In this

handout, we shall briefly discuss the principles and operational features of the

FMSCPC systems.

2. Basic SCPC Scheme

Fig.1 illustrates the basic building blocks of an SCPC system.

Fig. 1

The IF of each modulator is different and is set to any frequency

from 52 MHz to 88 MHz. Frequency spacing between adjacent

carriers is kept at 45 KHz. Smaller spacing is also possible.

Each carrier is frequency modulated by a single telephony channel

(4WTRANS). Therefore, each station transmits as many carriers

as the number of channels it operates.

BRBRAITT, Jabalpur, issued January 06 2

IFCOMBINER

IFDIVIDER

SCPCMOD

SCPCMOD

SCPCMOD

SCPCMOD

SCPCMOD

SCPCMOD

4W TX

4W TX

4W TX

4W RX

4W RX

IF

4W RX

IF

IF

IF

IF

* : IN 1+1 CONFIGURATION

IF


3/45


Up/down converter and Solid State Power Amplifier (SSPA) are

common for all the carriers.

On the receive side, the down converter translates the RF band of

a transponder to 5288 MHz IF band. The SCPC demodulator

selects a particular carrier and demodulates it to get the receive

channel (4WREC).

Note the basic advantages of SCPC scheme :

Multiplexing equipment is not required.

Channels can be increased in steps of one by adding a

modulator and a demodulator unit.

3. Special Features of SCPC Operation

To implement the basic SCPC scheme within the constraints of

transponder bandwidth and power, requires some special measures which are

described below. These features are exclusive to FMSCPC system.

3.1VOX Operation

On a satellite transponder having 36 MHz bandwidth, there can be

36000/45 = 800 SCPC carriers. So many carriers if present simultaneously

require a large amount of transponder transmit power which exceeds its

capacity. They also result in unacceptable level of intermodulation noise.

Therefore, VOX (voice operated transmission) mode of operation is used. In this

mode, a carrier is transmitted only when the modulating signal is present. In

case of the speech signal, VOX operation reduces the average number of

carriers in a transponder to about 40% at any instant of time and reduces the

transponder transmit power requirements.



4/45


On the receive side, the carriers are received in bursts. When carrier is

not present, the demodulator generates noise (similar to noise heard on AM/FM

radio receivers used in our homes). A squelch gate is provided at output of the

demodulator to cut out this noise. The gate switches on only when the carrier is

present.

Fig. 3

VOX mode of operation has effect on signaling also. For signaling, 3825

Hz tone is used but the tone cannot be sent continuously. Therefore, pulsed

signaling is used in SCPC systems.

3.2Automatic Frequency Control (AFC)

The RF carriers transmitted to the satellite are in 6 GHz frequency band.

The satellite transponder converts the 6 GHz frequency band to 4 GHz

frequency band. During frequency translation, frequency drifts of the order of 40

KHz are expected. Thus, an SCPC carrier may shift its location in the receive

frequency band by about 40 KHz. Since the SCPC carriers are spaced at 45

KHz, the frequency shift may result in receiving a wrong carrier. Automatic

Frequency Control (AFC) is, therefore, built into SCPC systems. It corrects the

frequency drift experienced in the satellite.

For AFC, one of the SCPC station transmits 70 MHz unmodulated carrier

as a pilot along with other SCPC carriers. At each earth station, this carrier is

used for AFC as explained later.



5/45


3.3Compander

With the limited transponder power distributed over 800 SCPC carriers, it

is not possible to meet the S/N requirements unless some special noise

reduction circuit Compander, is used. Compander consists of a compressorcircuit in the transmit path and an expander circuit in the receive path.

Companders used in SCPC systems provide about 17 dB subjective

improvement in S/N.

4. Overall Block Schematic of an SCPC System

Fig.4 illustrates the overall block diagram of an FMSCPC system. This

schematic and the terminology are based on ITI make SCPC system.

Signal Interface, Modulator/Transmitter, TED/Receiver and the channel

synthesizer are built on a single PCB in ITI SCPC system. Operation of each

block of the system is explained in the following sections.



6/45


Fig. 4

5. Signal Interface Transmit Side

Signal Interface block contains the following circuits in the transmit side:



7/45


band limiting the VF signal to 3003400 Hz,

echo suppressor,

carrier ON signal generation for VOX mode of operation,

compressor,

pulsed signaling tone generation.

Fig.5 shows simplified block schematic of the Signal Interface in the

transmit side. The VF signal is band limited to 0.33.4 KHz. It is then delayed by

8.5 milliseconds. to allow transmission of unmodulated carrier for 8.5

milliseconds. after detection of the speech signal. The delay enables the TED on

the receive side to lock to the carrier within this period.



8/45


Fig. 5



9/45


The audio gate stops the echo when it is opened by the echo suppressor

control circuit. 3825 Hz band stop filter prevents any speech signal at this

frequency else it may interfere with the signaling tone which is inserted later. The

compressor is used for noise reduction. 0 dBm is the unaffected level. The

signaling tone received from the RO/AFC unit is passed through a gate which is

controlled by the pulse signaling received from the MDP card. The signaling tone

and the speech signal are sent to modulator/transmitter block.

On the transmit side, when speech or signaling pulse is detected a Carrier

ON command is generated. It is also applied to the moduator / transmitter block

to switch on the carrier.

The signal interface block requires 30 KHz clock for the delay circuit. This

clock is generated from the reference oscillator at 10 MHz in the RO/AFC unit.

6. Modulator/Transmitter

This block accepts the VF signals which modulate a 45 MHz carrier. This

carrier is then translated to the desired intermediate frequency in the range 52

88 MHz using the output of the frequency synthesizer. For the VOX mode of

operation, carrier is activated by the Carrier ON signal from the Signal Interface

TX block.

Fig.6 shows the block schematic of the modulator. The audio signal is

passed through a preemphasis and a level limiting circuit. 50 Hz energy

dispersal waveform and pulsed 3825 signaling tone are added to it. The

modulator is a phase locked loop (PLL). It generates 45 MHz FM carrier using 30

KHz from RO/AFC unit as reference source. The audio signal is added to the

error voltage so that frequency of the VCO is varied as per the amplitude of the

audio signal.



10/45


The 45 MHz carrier is translated to IF carrier in the frequency band 5288

MHz using output of the frequency synthesizer. The IF is amplified using an IF

amplifier which includes a PIN diode switch. This switch is controlled by the

Carrier ON signal generated in the Signal Interface TX block.

Fig. 6

All the IF carriers are combined using multiport IF combiners. The

combined band of IF carriers is sent to upconverter.

7. Up/Down Converter

The Up/Down converter is in 1+1 redundant configuration. Two stage

frequency translation is used in both the directions. In the transmit direction,

combined IF carriers in the frequency band 5288 MHz are transmitted to 1 GHz

band and then to the desired 6 GHz RF band. In the receive side, one of the



11/45


outputs of the RF divider is applied to the SCPC down converter. The down

converter translates the received RF band to 5288 MHz IF band. This is unlike

FDMFMFDMA where each carrier is separately down converted to 70 MHz

using an independent down converter. Each SCPC carrier occupies theassigned frequency slot within the IF band. Fig.7 shows the frequency translation

scheme of the up/down converter. In the transmit direction, the composite IF

signal, 70+18 MHz from the IF combiner is translated to 1070 MHz band using

1000 MHz local oscillator frequency. The output is applied to a BPF which allows

1070+18 MHz components. The second frequency translation is to 6 GHz band

Fig. 7



12/45


of the SCPC transponder, e.g. transponder 9 which is used for SCPC carriers,

has the centre frequency 6285 MHz. Therefore, 1070 MHz frequency band is

translated to 6285 MHz band using 5215 MHz frequency. This frequency is

generated using a transponder selecting crystal having frequency 108.645833MHz and multiplying it by 48.

On the receive side, the RF band corresponding to the SCPC transponder

9 is 4060+18 MHz. This band is translated to 1155 MHz using the second stage

local oscillator (Frequency 5215 MHz) of the up converter. 1155 MHz band is

passed through a BPF and then applied to another mixer stage having local

oscillator frequency of 1225 MHz to translate 1155 MHz band to 70 MHz IF

band. 1225 MHz is generated using a voltage controlled crystal oscillator(VCXO) operating at 1223711 MHz and multiplying its output by 11. The VCXO

frequency is controlled by the AFC correction voltage. Any frequency drift in the

transponder is taken care of and the IF band at the output of the down converter

is tracked within 2 KHz.

The output of the down converter is applied to the IF divider which

distributes the signal to SCPC TED/Receiver

8. SOLID STATE POWER AMPLIFIER (SSPA)

In remote earth stations, 20 Watts solid state power amplifiers

(SSPA) are used in 1+1 configuration for amplification of the RF signals received

from the upconverter. At other stations capacity of the power amplifier depends

on the number of SCPC carriers being transmitted.

TED/RECEIVER

The TED/receiver block accepts the IF band 52-88 MHz from the IF

divider and selects the desired carrier by translating it to 45 MHz. Frequency

synthesizer provides the local frequency required for translation. Frequency

demodulation is carried out using a phase locked loop which also generates a

carrier present signal for squelch gate operation

Fig. 8 shows the block schematic of the receiver. The IF input from

the IF divider contains all the SCPC carriers in the frequency band 52 to 88 MHz.



13/45


The carriers have 45 KHz frequency spacing. The required IF carrier is selected

by translating it first to 45 MHz and then to 10.71 MHz. The local oscillator

frequency of the first mixer stage is taken from the output of frequncy synthesizer

which is set at 45 MHz difference from the desired carrier frequency. Note that

there is common frequency synthesizer for transmit and receive directions and

the frequency setting is also same. Therefore, the transmit carrier is translated to

45 MHz. The filter which follows the mixer stage is so designed that two more

carriers adjacent to the transmit carrier are passed by the filter and applied to the

second stage of frequency translation.

The second stage selects the appropriate carrier from these three

carriers by proper setting of the local oscillator frequency of the second stage.

The LO frequency can be so set that

transmit frequency is selected, or

the next higher carrier is selected, or

the next lower carrier is selected.



14/45


Fig. 8



15/45




16/45


In the first case, the channel operates in loop back mode so that

channel testing may be carried out. For communication purposes between two

stations, the receive carrier can either be next higher or the lower carrier

adjacent to the transmit carrier.

After carrier selection, the demodulation is carried out using a PLL

threshold extension demodulator operating at 455 KHz. 10.71 MHz to 455 KHz

translation is done using another stage of down conversion with LO frequency at

10.255 MHz. The demodulator contains AGC amplifier and a quadrature detector

which generates the carrier present signal for the squelch control. The

demodulated output is sent to the Signal Interface described below.

SIGNAL INTERFACE - RECEIVE SIDE

On the receive side, the demodulated signals from the TED are

processed to get back the squelch signal and the signalling pulses. The receive

side of the interface contains the following circuits :

squelch gate,

echo suppressor,

expander,

3825 Hz signalling one detection.

Fig. shows the block schematic of the Signal Interface - Receive

Side. Output of the TED is filtered and delayed by 8.5 msec. The delay is

provided to ensure that the squelch gate is closed before arrival of the speech

signal. Squelch gate is controlled by squelch control circuit which receives its

input from the TED/Receiver. 3825 Hz tone is filtered out, detected and sent to

the MDP card for conversion to usual E/M signaling. The VF signal is applied to

expander circuit through a band stop filter to remove 3825 signaling tone.

Expander restores the VF signal which was "compressed" at the time of

transmission. From expander, it goes echo suppressor gate which gives 0 dB or

6 dB loss depending on the input from the E/S control circuit.



17/45


CHANNEL SYNTHESIZER

It generates the local oscillator signal in the frequency range 97-

133 MHz using a Phase Locked Loop (PLL). The frequency can be set in the

steps of 22.5 KHz. There is a common frequency synthesizer for both transmit

and receive side. Fig. 9 shows its block schematic.

Fig. 9

The reference input is 30 KHz. It is divided by 32 to get 0.9375 KHz. The

VCO frequency is divided by 24 and then by a programmable divider to get

0.9375 KHz. When the PLL is locked, the two input frequencies to the phase

detector are equal. Otherwise it generates an output error voltage which is

proportional to the phase difference of the two input signals. The error voltage is

filtered using a low pass filter and applied to the VCO whose frequency is

controlled by the error voltage. The VCO frequency is given by 0.9375 x 24 x N


REFERENCE

INPUT

PHASEDETECTOR

LOOP

FILTER VCO*

* VOLTAGECONTROLLED

OSCILLATOR

LO OUTPUTADJUSTABLE FROM

97 TO 133 MHz

IN STEPS OF 22.5 KHz30

KHz

32

PROGRAMMABLE

DIVIDER

DIVIDER

N

24


18/45


KHz. When N is incremented or decremented by 1, the VCO frequency changes

by 0.9375 x 24 = 22.5 KHz. By changing the divisor N, the frequency of the VCO

can be set to any frequency within the band 97 to 133 MHz in the steps of 22.5

KHz.

RO/AFC UNIT

All the reference frequencies used within the SCPC system, and

the AFC correction voltage applied to the down converter, are provided by the

RO/AFC unit. It incorporates :

10 MHz high stability reference oscillator,

3825 Hz signalling tone generator,

AFC circuit.

The RO/AFC unit is in 1+1 reduntant configuration.

Reference Oscillator

The reference oscillator consists of a 10 MHz frequency standard

with associated circuitry to generate 30 KHz for the AFC card and for the

channel synthesizer (Fig. 10). The 3825 Hz signalling tone is also generated in

this unit and distributed to all the modem cards.

Fig. 10


10 X 3 100

REF OSC

10 MHz

10 MHz REF

30 KHz REF


19/45


9.1 Automatic Frequency Control

For AFC, a pilot modem card is used. It receives its input from the

IF divider. The receiver chain is same as in the other modem cards. The channel

synthesizer is set to receive 70 MHz pilot. The pilot is transmitted to 45 KHz andapplied to the AFC card in the RC, AFC unit (Fig. 11). A drift in the received RF

band is reflected in the pilot signal and, therefore, in the 455 KHz signal. In the

AFC circuit, 455 KHz signal is divided by 455000 to get 1 Hz signal. This signal

is compared with 1 second reference time slot generated using 30 KHz.

Depending on this comparison, a frequency correction voltage is generated and

applied to the VCXO in the down converter.

ADVANTAGES OF FM-SCPC SYSTEM

As mentioned in the beginning, FM-SCPC system provides single

channel add on flexibility. The system is very cost effective for low traffic density

and for remote area communication. Demand Assigned Multiple Access

(DAMA) in which a satellite channel is assigned to a station on call to call basis

is feasible using SCPC system.



20/45


Fig. 11

VSAT NETWORK

What is VSAT ?

It can be defined as class of very small aperture Intelligent Satellite Earth

Station suitable for easy onpremise installation, usually operating in conjunction

with a large size HUB earth station. Capable of supporting a wide range of two

way integrated Telecom Services.


1 SEC TIME

SLOT

AFC CORRECTION

VOLTAGE

TO VCXO

( DOWN

CONVERTER )

AFC

CIRCUIT

1 Hz

FROM

PILOT

MODEM

455

KHz

455000

N

30 KHz

REF


21/45


VSAT is also known as

1. Micro Earth Station

2. Mini Earth Station

3. Personal Earth Station

4. Roof Top Terminal

5. Customer Premise Terminal

Advantages of VSAT

V-SATs provide the following advantages :

1. Can be located in the user premises on roof top or backyard andhence eliminate last mile problem.

2. Superior quality Satellite Based Data Services. BER better than 1 x

107

for 99.5% of time.

3. Quick implementation time.

4. Reliable communication.

5. Broadcast feature of satellite communication.

6. Communication to Remote areas.

7. Flexibility for network growth and changes.

8. Service is distance insensitive.

9. Low cost.

Reasons for VSAT Evolution

The main reasons for VSAT evolution is due to advances in following

areas.

1. Packet transmission and switching.

2. Efficient Multiple Access Protocols.

3. Powerful Microprocessors.

4. KU band RF electronics.

5. Antenna miniaturization.

6. Spread Spectrum Techniques.



22/45


7. Protocol Standardisation and Implementation.

8. LSI Based FEC Codecs and Modems.

9. Higher power satellites.

Classification of V-SATs

V-SATs can be classified based on following :

1. Modulation Type.

2. Application used for.

3. Transmission Rate

4. Cost

Categories of V-SATs

1. Broadcast/Point to Multipoint

Type of services

(A) Broadcast Video.

(B) Program Quality Audio.

(C) Packetised Data.In this mode of operation V-SATs transmit/receive data through a

Centralised Hub. This type of network is called a STAR NETWORK.

2. Pointtopoint

Type of Services

(A) Voice

(B) Data

(C) Image

In this mode of operation V-SATs transmit/receive data without the help of

Hub station. This type of network is called a MESH NETWORK.

3. Twoway Interactive

Type of Services



23/45


(A) Voice

(B) Data

(C) Image

In this configuration, V-SATs can communicate both in STAR as well as

MESH topology.

Salient features of TwoWay Interactive V-SATs

1. Twoway interactive V--SATs use P.S.K. Modulation both in C and

Ku band. They may also employ Spread Spectrum technique.

2. VSAT Antenna Diameter.

1.2 M to 1.8 M in Ku band.

1.8 M to 2.4 M in C band.

3. Solid State Power Amplifier.

Up to 5 watts in C band.

1 to 3 watts in Ku band.

4. Network configuration

STAR or MESH.

5. VSAT to HUB Transmission

In bound carrier On TDMA.

6. HUB to VSAT Transmission.

Out bound carrier On TDMA Carrier.

7. In bound carrier access.

(a) FTDMA

(b) Random Access

(i) Pure Aloha

(ii) Slotted Aloha

(iii) Reservation Aloha



24/45


(iv) Demand Assigned TDMA

(c) CDMA

8. One VSAT several DTEs.

One Inbound carrier Several hundred V-SATs.

One or two outband.

and few inbound Several thousand VSATs.

VSAT Users

(1) Large corporate customers Private Network.

(2) Medium sized users Quasi Private Network.

(3) Small users A Shared Network.

Design Factors for VSAT Systems

The capabilities of a VSAT system depends upon following factors.

1. Satellite characteristics.

2. Geographical and Environmental factors.

3. Transmission link properties.

4. Earth station characteristics.

5. Information encoding and modulation.

6. Total system operation.

Each of these factors is interrelated. As a result, a desire to maximize

one attribute comes often at the expanse of others. The goal should be to

balance them all, in order to obtain a system that is as reliable, resilient and

efficient as possible for a particular application for which the system is designed.

1. Satellite CharacteristicsIt depends upon the following :

Antenna design Operating frequency Receiver sensitivity Transponder power and B.W.



25/45


2. Geographical and Environmental Factors Area of earth covered. Fixed and variable atmospheric weather conditions. The effects of terrestrial microwave links.

3. Transmission link properties Noise sources, magnitude and frequency. Signal strength losses Signal to noise ratio.

4. Earth Station Characteristics Antenna size Receiver sensitivity Satellite elevation relative to earth.

5. Information encoding and modulation Information encoding and compression technique. Modulation method. Transmission technology both analog and digital.

6. Total system operation Efficiency of endtoend information flow. Information protocols and flow control. Multiple user access techniques. System operation under heavy loadings.

When evaluating a VSAT system each of the above attributes should be

given attention. However, few of more important attributes are Operational aspects Frequency bands Satellite Access Methods.

Operational aspects

Considering a VSAT system from its operational aspects, there are five

broad functions that determine how the system works. These are : Bandwidth allocation Multiplexing Network management Protocol handling Transmission



26/45


(a) Bandwidth allocation

Bandwidth allocation refers to the allocating of communication channels

between V-SATs and the satellite so as to maintain network operation at peak

levels with minimum delays.

(b) Multiplexing

Multiplexing is the ability to mix transmissions to and from multiple

sources so as to share satellite channel between several V-SATs. There are

several methods of doing this, the goal being to optimise the utilisation of the

network by reducing wasted satellite time.

(c) Network management

Network management concerns handling the information necessary for

automatic or manual optimisation of the networks performance as conditions

change.

(d) Protocol handling

Protocol handling differs widely from one VSAT system to the next.

Protocol refers to procedures that enable control to the actual transmission such

as error checking, retransmission and identifying parties, etc. Generally, these

are two levels of protocols in use.

(i) The network protocol used by the ground stations and the satellite.

(ii) The user protocol any one of many international standards for

information inter change used to encode and verify transmissions.

Examples are Asynchronous, SNA/SDLE & X.25 etc.

(e) Transmission

Transmission involves establishing communication channels between

VSAT terminal and the satellite. Typically a satellite system uses one wide band

out bound channel and multiple inbound channels.



27/45


Frequency Bands

Several frequency bands are commonly used by VSAT systems. They are

generally referred to by a letter designation rather than by citing their precise

frequencies. For example

L Band is used for mobile satellite communication.

S Band because of its low BW and vulnerability to interference, is not

commonly used.

C Band

The most common frequency bands in use today, especially for receive

only terminals. However, it can be affected by terrestrial microwave, often a

problem in cities and urban areas. C band offers the advantages of world wide

coverage.

KU Band

More recently introduced KU band offers greater flexibility of applications

in that it supports data, video and voice with smaller dishes.

At a higher frequency is X band which is reserved solely for specialised

government use such as deep space mission. Highest of all is Ka band which

remains largely under development and will allow the use of even smaller

terminals known as VVSAT (Very Very) MSAT (micro) and USAT (Ultra Small)

aperture terminals.

Each band can be affected by atmosphere conditions such as rain. This is

not so much a problem for C band, but is especially troublesome for KU Band.

Falling snow does not have the same effect, although a few millimeters of ice or

a few centimeters of snow on the antenna can cause major degradation to signal

strength.

Before information can be sent using one of these bands, it must first be

made suitable for transmission. This process is known as modulation. After



28/45


being modulated, the signal can be packaged in a number of different ways to

transmit to the satellite, these are known as satellite access methods.

Satellite Access Technique

To share the bandwidth between multiple users and to have more efficient

utilisation of the satellite, different access techniques have been devised.



29/45


REMOTE AREA BUSINESS MESSAGE NETWORK

Telecommunication has been recognised as a necessary infrastructural

input for growth. The emphasis is shifting from urban to rural areas for industrial

development to provide jobs to the rural unemployed and stem the migration

from the rural to urban centres. This necessitates the provision of reliable and

cost effective means of communication to and from rural area.

The Department of Telecom has approved a satellite based data network

that will ensure the availability of message communication services to the areas

where they do not exist today. The network has been named REMOTE AREA

BUSINESS MESSAGE NETWORK. It provides interactive data communication

between computers and also FAX between the users as well as access to Public

Telex and Data Network.

Introduction

Government of India has given additional emphasis on industrial

development of remote and backward areas by providing increased incentives tothe entrepreneurs in order to provide employment to the rural poor and prevent

their immigration to already overburdened cities. For the development of

industries communication is a must. At present it is practically nonexistent in

the rural area. To give further impetus to the rural growth by providing reliable

communication, the Department of Telecom has approved a satellite based Data

Network as a first step. It would ensure a very reliable message communication

services to such areas where no other communication facilities are available.

The Remote Area Business Message Network ensures utmost reliability,

it is independent of terrain, it works via satellite media with a small antenna

located in the users premises. The technology used for this network is such that

it is free from interference due to the terrestrial systems. This network optimally

utilises the space segment of which there is a shortage at present.



30/45


This is a packet switched data network in star configuration which uses

STDM/CDMA (Statistical Time Division Multiplexing/Code Division Multiple

Access) transmission with spread spectrum technique. The network consists of

very small aperture terminals called micro earth stations which are connected to

a master earth station called HUB. The master earth station is collocated with

BSNL Earth Station complex at Sikandrabad in Bulandshahar district. This

network is suitable for thin route applications.

System Concept

The Fig. depicts the system concept of RABMN. Basically, it provides

connectivity between a remotely located Data Terminal Equipment (DTE) to a

host computer. The DTE may be a PC or a mini computer.

Computer to computer traffic generally needs intermittent use of high

band width channel. Also, the interactive computer may have different data

rates. Generally, the remote station originates a short query which can be

transmitted to the host computer at low bit rate whereas the response from the

host may generally be large which may require a higher bit rate for transmission.

The host computer may be located in Delhi or to any other places in the

country.

If the host computer is located in Delhi, it can be connected to the master

earth station through the rearward/backhaul link. In this case, the connection

between micro and host terminal uses one satellite hop.

If the host computer is located out of Delhi, it can be connected to the

network through a VSAT and the call is established from VSAT to VSAT through

the master earth station. Technically, it is possible to connect host located

anywhere in India to the hub through leased lines but this will not be cost

effective and reduce the reliability.



31/45


It is also possible to communicate between two PCs connected to two

different VSATs. The call between two VSATs is established through packet

switch which is also a part of master earth station electronics.

Services Offered

Following services are possible on this network.

1. Interactive Data Communication between a host and remote orbetween remote terminals.

2. Access to Public Telex Network.

3. Access to Public Data Network.

4. FAX services between the users.

5. Messaging service.

It is possible at the micro terminal end to have all this services available

from a single Multi Function Workstation having suitable software.

Details of Micro Earth Station

As the name suggests micro earth station is a very small earth station.

The parabolic reflector of the antenna is of the size 1.2 by 1.8 mtrs. It is made of

a strong plastic material coated with reflecting material. It is offset fed. VSATs

electronics consists of the power amplifier for transmit, low noise amplifier for

receive (120 K) up/down converters and the controller. Except the controller

every other unit is a part of the antenna complex. The controller which is the

heart of the micro terminal is normally kept in a controlled environment like that

of a PC. The antenna may be placed on the roof top or any flat surface from

where it can look directly into the satellite without any obstruction in between.

The controller and the antenna module are connected by means of a pair of

coaxial cables. The size of the cables should not exceed 100 mtrs.

The controller comprises of Mod/Demod, and three microcomputer based

processors called (i) Space processors, (ii) Network processor, and (iii) I/O

Processor.



32/45


The space processor accomplishes control and timing functions. Network

processor establishes the X.25 network protocol whereas the I/O processor can

be made to accommodate any of the large variety of customer protocols.

The control module has two user ports which offer connection to the users

data terminal equipment. The micro earth station type C201 permits a

maximum data transmission rate of 1200 bps. The data which is in BPSK is

chipped at a rate of 1:2048 so that the spectrum occupies a band width of 5

MHz. This signal which is at IF goes to the up converter where it is converted to

transmission frequency. The amplifier amplifies it to the required power level

before it is transmitted to satellite. The rated power output of the amplifier in case

of C201 is 1 Watt.

This network can support a very small number of C251 type of VSAT

which can work up to 9600 bps. The power amplifier rating in this case is 5

watts.

Both the type of VSATs can receive data up to 19.2 kbps. The VSATs

transmit BPSK/CDMA carrier in bursts in absolutely random access mode and at

the same frequency.

Master Earth Station

The master earth station also known as hub is located in the DOTs earth

station complex at Sikandrabad.

The earth station antenna is of the size of 11 mtr. and Low Noise Amplifier

of 40 K. The antenna and the LNAs are shared by the other services at the

master earth station. The high power amplifier is of the rated capacity of 400

watts. It is TWT type.

The carrier from the MES is called outbound and the carrier to the MES is

called inbound. On the outbound side double conversion technique is used. The

first IF is 150 MHz and the second IF is 1100 MHz. On the inbound side the twin

IFs are 1100 MHz and 70 MHz. On the receive side AFC controller is used to



33/45


correct the offset frequency which is caused by the doppler shift due to satellite

movement, space craft oscillator tolerances and path length variations.

The Mod/Demod are BPSK/SSMA units. The outbound and inbound MUX

transmit and receive raw data at 153.6 kbps. All the processing is done in the

digital section.

The OB MUX includes a chipper, with a chipping ratio of 1:16 which

spreads the data spectrum with a PN sequence and packetises the chipped data

stream to be transmitted to the modulator. On the receive side the demodulator

card or the ear card identifies the signal from the associated VSAT based on

identical PN sequence which is unique for the VSAT and the associated ear

card. The inbound MUX provides function of calibrating, monitoring and reporting

the operational status of the demod cards.

The digital section consists of NPG (Network Processing Group), HCI

(Host Computer Interface), PSX (Packet Switch) and Admin MUX. The NPG

provides the FEC for the outbound data. One NPG can handle 220 VSATs.

Maximum 16 NPGs can be installed in one master earth station. NPG is capable

of switching the VSATs connected to the same NPG. VSATs connected to

different NPGs are switched by means of an external packet switch.

Host Computer Interface serves as a buffer between the master earth

station and the user host computer providing protocol interface, polls,

acknowledgements, retransmission and data buffering on the transmit side as

well as accepting polls from the host computer and forwarding data receivedfrom the VSATs after the previous poll. HCI performs the routing of the call

through sockets which can be a VSAT or any other HCI port.

Admn. MUX

Admn MUX interfaces with the network control centre for adding,

removing and modifying network links and various other parameters to be

received from the demod section and identification of virtual network. Admn



34/45


receives a report on all the alarm conditions from NPG. It provides host of other

control functions such as off line testing of subsystems.

Network Control Centre

As the name suggests the network control centre keeps the control of

various network functions right from the definition, establishment, maintenance

and the status display of various links and various subsystems of the master

earth station are connected by means of a high speed local area network (LAN)

operating at 575 Kbps which provide inter-unit communication as well as

switchover arrangement to switch over to stand by module in case of failure of a

regular module.

The NPG transmits at 153.6 kbps demand based STDM (Statistical Time

Division Multiplexed) data stream generated by polling the HCI ports which

assigned to VSATs or host computer. The data contains various length of

packets addressed to different VSATs. Outbound MUX chips this stream at the

rate of 16 chips per bit resulting a 2.4576 Mbps bit stream. The BPSK modulator

produces an LF whose spectrum is spread to 5 Mhz. This is upconverted to

transmit frequency and transmitted to satellite after suitable amplification by the

High Power Amplifier. The master earth station continuously transmits 153.6

Kbps STDM/BPSK SSMA carrier.

Network Details

The RABMN Network will consist of large number of micro earth stations

linked to the Sikandrabad Master Earth Station via the designated satellite

transponder. At present part of ARABSAT transponder is being used for this

purpose. All the micro earth station transmit to the satellite on a random access

basis in burst mode at the same carrier frequency. Each micro earth station is

assigned an individual code and the transmission packets from a micro earth

station carry both destination as well the origination code. The simultaneous, co

frequency carrier transmissions from a number of micro earth stations, would



35/45


appear as a background self interference to any one desired micro earth stations

carrier. Each spread spectrum receiver or the demodulator card does receive, at

the input, all the carriers but all others except the one whose destination address

matches with that of the demod channel, get rejected. All these carriers amount

to Self Interference. Depending on the satellite link design, the number of such

tolerable simultaneous transmission from micros to avoid network congestion,

could be of the order of 100.

The outbound carrier transmission with a bit rate of 153.6 kbps from the

master earth station carries time division multiplexed data for the active micro

earth stations. Each micro earth receives the data pertaining to it at a maximum

permissible data rate of 19.2 kbps. The bit error rate of the order of 107

with

FEC and 109

with ARQ is achieved.

The link from a micro earth station to another micro earth station or the

main earth station could be established over a Semi Virtual Circuit (SVC) or a

Permanent Virtual Circuit (PVC). At present, this network is configured to support

SVCs only. For establishing a SVC the originating micro earth station first

transmits call request packets. If the destination micro earth station is available,

a SVC is established and the data packets are exchanged. At the end of the

transaction SVC is released.

The protocol employed for the network is X.25 and with appropriate

interfaces the network would provide private data communications in an

interactive X.25 packet switched environment. Host computers operating with the

network must be equipped with serial RS232C interfaces operating with an

X.25 LAPB protocol. The same applies for connecting DTEs to micro terminal

ports.



36/45


Fig. 1VSAT Network



37/45


Fig. 2Block Schematic of Micro Earth Station



38/45


Fig. 3

Block Schematic of Hub Station Eqpts.



39/45


GLOSSARY OF TERMS

Satellite Communications :

An orbiting system in space that receives radio signals from ground

stations and then retransmits them to ground stations at distant locations.

Earth Station :

Equipment on the ground that sends/receives signals to/from Satellites.

Uplink :

Transmission going from an earth station to an orbiting satellite.

Downlink :

Transmission going from a satellite to an earth station.

Multiple Access :

A number of earth stations in different geographical locations sharing a

common resource, the Satellite, simultaneously in a network.

Frequency Division Multiple Access (FDMA) :

Bandwidth is split into narrow frequency bands with multiple users each

allocated frequency range within the larger bandwidth.

Fig.



40/45


Time Division Multiple Access (TDMA) :

Each terminal is allocated a short time slot in which to transmit on a

sequential basis. The time slot is usually a fraction of a second.

Fig.

Code Division Multiple Access (CDMA) :

Each terminal is allocated a unique encryption code. The transmitted

signals are encoded and only the terminal intended to receive it can decode it.

This is also known as Spread Spectrum Multiple Access (SSMA).

Statistical Time Division Multiplexing (STDM) :

In TDM, time slots are alloted to the multiplexed lines even if they have no

data to transmit.

In STDM, a time slot is allotted to the multiplexed line if only there is data

to be sent. The multiplexed circuit is used more efficiently.

VSAT :

Very Small Aperture Terminal, also called Micro Earth Station or Personal

Earth Station.

Master Earth Station :

The earth station at the centre of a VSAT network which monitors and

manages the entire network.

HUB :

The centre of a communication network.



41/45


Inbound Channel :For the master earth station, it is the receive channel.

Outbound Channel : For the master earth station, it is the transmit channel.

Network 1000 VSATs

VSATs transmit on a random access basis in burst mode at the

same frequency. Each VSAT has a unique code, its transmission

packets carry destination and origination addresses.

Simultaneous cofrequency transmissions from a number of

VSATs would appear as background interference to anyone

desired VSATs carrier. Only the one where origination code

matches with that of demod channel is accepted.

Number of simultaneous accesses limited to 100.

Bit error rate objectives :

Raw BER. 103

With FEC 10 7

With ARQ 109

Network Protocol is EQUATOR.


GT 31.7 dB/K

Antenna 11.0 M

LNA 40 K

HPA 400 W

Data Transmission 153.6 Kbit/s

Chipping Rate 1:16

Noise Bandwidth 5.0 MHz

MOD/MULT. Acc. STDM/SSBPSK/FDMA



42/45



Outbound

Each VSAT and Host Computer is allocated an HCI

port.

NPG polls HCI ports and generates a 153.6 Kbps

demand based STDM data stream.

Data Stream contains variable length packets of data

addressed to various VSATs.

OUTMUX chips the data @ 1:16 resulting in 2.4576

Mbps stream.

BPSK modulator produces 5 MHz IF spectrum.

Upconverter converts to 6 GHz RF.

MES transmits 153.6 Kbps STDM/BPSK/SS carriers.

NPG provides FEC encoding for outbound data.

NPG can handle 240 VSATs/16 NPG & MES.

NCC consists the operation of the entire network,

starting from definition, establishment, maintenance and status

display of various links.


Inbound

Inbound signals originate from VSATs.

VSAT has unique PN code.

Data spread by its PN code and transmitted to

satellite in 6 GHz band.



43/45


MES receives the signal in 4 GHz band, converts to

70 MHz IF and sends to demod section.

Demod cards compare the pattern of the signal to the

PN code for its associated VSAT. When the correct pattern is

detected, the original signal is extracted from the IF signal and

transferred to INMUX.

NPG polls INMUX for I/C data packets and transmits

the packets to HCI.

Micro Earth Station

G/T 12.0 dB/K

Antenna 1.2 x 1.8 M

LNA 120 K

TPA 1 W (C

201)

5 W (C250)

Data Trans 1.2 KBps (C

201)

9.6 KBps (C250)

Chipping Rate 1:2048 (C

201)

1:256 (C250)

Noise B.W. 5.0 MHz MOI/MULT Acc. SSBPSK/CDMA

Micro Earth Station

Controller Module

Input/Output Processor

Converts raw data to packets.

Customer protocol to network protocol.



44/45


Network Processor

Trans Side

Generates BFEC and FCS

Retains packets in buffers until ACKs

received and retransmits if required.

Receive Side

Checks BFEC and FCS.

ACKs the packets and requests

retransmission of missing/incorrect packets.

Space processor

Trans Side

Encodes the packets with PN sequence

to 2.45 MBps stream.

BPSK modulation of IF.

Receive Side

Demodulates IF signal to 2.45 MBps.

Despreads to 153.6 KBps stream. Checks destination link address in the

header.

Discards packets addressed to other

links.

Micro earth stations transmit, 1.2 KBps/9.6 KBps

BPSK/CDMA carriers in bursts in absolutely random access mode

in the same frequency.



45/45


Annexure 1

LINK ENGINEERING

(A) Inbound Carrier

Uplink

PA Power 0 dBwMicro E.S. ant. gain 36.5 dbUp path loss 200 dBSatellite G/T 4 dBK(C/No) up 61.1 dBHz

DownlinkSatellite e.i.r.p. 7.0 dBwDown path loss 196 dBMaster Earth Stn. G/T 31.7 dBK

(C/No)dn 57.3 dBHz.

IntermodIM noise density 644 dBW/Hz

(C/No)dn 57.0 dBHz

Self interferenceSelf interference noise

density (Io/I) 65.2 dB/Hz

from 100 simultaneouscarriers (C/I) 20 dB

so (C/Io) 45.2 dBHz

(C/No)T 44.6 dBHz

For a bit rate of 1200 bps

Eb/No = 13.8 db

which is sufficiently more than needed for raw BER of 103

.

Date post:	06-Apr-2018
Category:	Documents
Upload:	motsssss
View:	223 times
Download:	0 times

FMSCPC- VSAT _L4

Documents