Mw

0

Electro Magnetic wave theory (Antenna-Radiation pattern etc.), Brief introduction to UHF, M/W, Satellite systems and V-SAT.

Electro Magnetic wave theory (Antenna-Radiation pattern etc.) 1.0

INTRODUCTION A microwave antenna system consists of the antenna itself, some form of transmission lines connecting the antenna to the transmitter and receiver, plus some sort of coupling device, either a circulator or an isolator. This handout describes different microwave antenna, their characteristics, construction and mounting arrangements. Also different kinds of transmission lines ie. feeders are described.

2.0

Characteristics of Microwave Antennas Highly directional antennas are used with point- to-point microwave systems. By focusing the radio energy into a narrow beam that can be directed towards the receiving antenna, the transmitting antenna can increase the effective radiated power by several orders of magnitude over that of an omni directional antenna. The receiving antenna also, in a manner analogous to that of a telescope, can increase the effective received power by a similar amount. Although gain is a primary characteristic, there are other antenna characteristics which are of importance in communications systems. Antenna beam width, side-lobe magnitudes, off-axis radiation, directivity patterns and polarization discrimination are of a, great significance for frequency coordination purposes. Impedance match [usually expressed' as VSWR although return loss is a much more useful parameter] across the band to' be used is of great importance in situations where echo distortion is significant. Consequently it is no longer sufficient merely to select an antenna for optimum gain efficiency. Lastly, antennas must be moderate in cost, easy to install and strong enough to give service in rugged environments for over twenty years.

2.1

PARABOLIC ANTENNA The parabolic antenna is used almost universally in point-to-point systems. The parabolic antenna utilizes a reflector consisting of a paraboloid of revolution and primary radiator at the focal point [Fig.1]. The reflector converts the spherical wave radiating from the focus to the planar wave across the face of the paraboloid to concentrate the energy in a beam much like a searchlight beam as discussed below.

The parabola is a plane curve, defined as the locus of a point which moves so that its distance from another point (called the focus) plus its distance from a straight line (directrix) is constant. These geometric properties yield an excellent microwave or light reflector, as will be seen, 2.1.1 Geometry of the parabola Figure 1 shows a parabola CAD whose focus is at F and whose axis is AB. It follows from the definition of the parabola that FP+ PP1 = FQ.+ QQ' - FR + RR' = K Where k= a constant, which may be changed if-a different shape of parabola is required AF= focal length of the parabola.

FIGURE: - 1 Geometry of the parabola Consider a source of radiation placed at the focus. All waves coming from the source and reflected by the parabola will have traveled the same distance by the time they reach the directrix, no matter from what point on the parabola they are reflected. All such waves will thus be in phase. As a result, radiation is very strong and concentrated along the AB axis, but cancellation will take place in any other direction, because of path-length differences. Thus the parabola lead to the production of-concentrated beams .

A practical reflector employing the properties of the parabola will be a three dimensional surface, obtained by revolving the parabola about the axis AB. The resulting geometric surface is the paraboloid, often called a parabolic reflector or microwave dish. When it is used for reception, exactly the same behavior is manifested, so that this is also a high- gain receiving directional antenna reflector. Such behavior is, of course, predicted by the principle of reciprocity, which states that the properties of an antenna are independent of wheather it is used for transmission or reception. The reflector is directional for reception i.e., rays normal to the directrix, are brought together at the focus. On the other hand, rays from any other direction are cancelled at that point, again owing to path-length differences. The reflector provides a high gain because, like the mirror of a reflecting telescope, it collects radiation from a large area and concentrates it all at the local point. 2.2.2 Feed Mechanism As already discussed, the primary antenna is placed at the focus of the paraboloid for best results in transmission or reception. However, the direct radiation from the feed, which is not reflected by the paraboloid, tends to spread out in all directions and hence partially spoils the directivity. Several methods are used to prevent this, one of them being the provision of a small spherical reflector to redirect all such radiation back to the paraboloid. Figure 2 shows yet another way of dealing with the problem, a horn antenna pointing to the main reflector. It has a midly directional pattern in the direction in which its mouth points; thus direct radiation from the feed antenna is once again avoided. lt should be mentioned at this point that although the feed antenna and its reflector obstruct a certain amount of reflection from the paraboloid when they are placed at its focus, the obstruction is slight indeed. For example, if a 3Ocm diameter reflector is placed at the centre of a 3-m dish, simple arithmetic shows that the area obstructed is only 1 percent of the total. Similar reasoning is applied to the horn primary, which obstructs an equally small proportion of the total area. 2.2.3 There are different types of feed designs for various frequency bands and different system applications. Feeds for the 890 to 2,300 MHz bands are generally coaxial dipoles, slot excited circular wave guide reverse horns or printed circuit arrays. The Cassegrain feed is used when it is desired to place the primary antenna at a convenient position and to shorten the length of the transmission line or wave guide connecting the receiver (or transmitter) to the primary. This requirement in the line or waveguid may not be tolerated, specially over lengths which may exceed 30 m in large antennas. Another solution to the problem is to place the active part of the transmitter or receiver at the focus.

With transmitters this can almost never be done because of their size, and it may also be difficult to place the RF amplifier of the receiver there. This is either because of its size or because of the need for cooling apparatus for very low-noise applications in which case the RF amplifier may be small enough, but the ancillary equipment is not. In any case, such placement of the RF amplifier causes servicing and replacement difficulties, and the Cassegrain feed is often the best solution. As shown in Fig.3 an obvious difficulty results from the use of a secondary reflector namely, the obstruction of some of the radiations from the main reflectors, because the dimensions of the hyperboloid are determined by its distances from the horn primary feed and the mouth diameter of the horn itself, which in turn is governed by frequency used: One of the ways to overcome this obstruction is by means of a large primary reflector together with feed placed as close to the sub-reflector as possible.

3.0

ELECTRICAL CHARACTERISTICS OF PARABOLIC ANTENNA

3.1

Antenna Gain

Microwave antenna gain is stated in dBi, indicating decibels relative to the gain of an isotropic antenna. This is a hypothetical 'ideal antenna' or 'point source' which radiates equally in all directions. In some literature, 'dBi1 is shortened to just 'dB'. The gain of a parabolic antenna depends upon its size, frequency and illumination. Maximum gain would occur if the .illumination was uniform in phase and equal in amplitude across the aperture of the parabola. In this case, the gain would merely be the ratio of the aperture of the parabola to the area of the hypothetical isotropic antenna. The gain, G, is given by the equation :

G=

=

Where A = Area of parabolic aperture = Area of isotropic antenna λ = Wavelength of operating frequency.

This upper gain limit is often referred to as 100 percent antenna efficiency. In the practical case, the amplitude and phase illumination errors, spillover loss, reflector surface tolerance error and other losses reduce the gain to slightly over one half this value. Taking the efficiency (n) in to accaunt the gain is given by the formula

G=

To express the power gain in decibels,

G[in dBi] = 10 logη

Conventional feeds provide an illumination of approximately -10 dB at the edge of the parabola from that at the centre, which results in an antenna efficiency of 58 to 63 percent for production antennas. Taking other factors into account, most manufacturers guarantee antenna efficiencies of 55 percent. Typical gains of microwave parabolic antennas are given in Table – 1. TYPICAL ANTENNA GAINS AND BEAMWIDTH FOR VARIOUS SIZES AND FREQUENCIES Antenna

2 GHz

Diameter Gain DBi

6 GHz

11 GHz

Beamwidth Gain degrees dBi

Beamwidth Gain degrees dBi

Beamwidth degrees

1.2m

25.4

8.8

35.0

2.8

40.3

1.6

1.8m

29.0

5.7

38.8

1.9

43.8

1.1

2.5

31.5

4.3

41.2

1.4

46.2

0.8

3.0

33.4

3.5

43.0

1.2

48.1

0.6

3.7

35.0

2.9

44.8

1.0

49.6

0.5

4.6

36.9

2.3

46.2

0.8

-----

----

3.2

The half power beamwidth is the beamwidth, in degrees, at the -3 dB power point. The beamwidth, in degrees for a conventional parabolic antenna is given by the equation : = 70 Where θ = beamwidth in degrees D = the parabolic antenna diameter λ = wave lenght in the same units. The value of θ is approximately 1.1 degree at 6 GHz and 3.4 degree at 2 GHz for a 3.0 m diameter antenna. The main lobe drops off to a null at 1.1 degree beamwidth off axis. This may mean that signal could drop as much as 40 dB if a 3.0 m antenna at 6 GHz is moved 1.1 degree off axis. One can appreciate the need for sturdy mounts and careful tower design.

3.3

VOLTAGE STANDING WAVE RATIO (VSWR) The antenna VSWR is the ratio of the amplitude of the voltage standing wave at the maximum to the amplitude at the minimum. VSWR is always equal to or greater than 1.0 . A 1.000 VSWR indicates that an antenna is perfectly matched to a transmission line. Since the feedhorn, located at the focus, has some physical size, it will catch some reflected energy from the parabola causing a mismatch [Fig. 2]. This is termed dish effect, and the VSWR contribution may be 1.02 or more, depending upon size and frequency. Placing a raised circular plate, of the proper thickness, called a vertex plate, at the centre of the reflector can cancel out these dish reflections. Vertex plates are usually installed along with the feed when the antenna is assembled. The VSWR over the operating frequency bands for standard microwave antennas will be approximately 1.10. Through extreme care in manufacture and additional tuning and matching, low VSWR antennas can achieve a VSWR of 1.04 to 1.06. For some system calculations VSWR in terms of the return loss in decibels is useful. In this case, Return Loss [in db] = 20 log

VSWR +1 VSWR –1

VSWR

R. L.

VSWR

R. L.

VSWR

R. L.

1.02

40.1

1.07

29.4

1.15

23.0

1.03

36.6

1.08

28.3

1.20

20.8

1.04

34.1

1.09

27.3

1.25

19.0

1.05

32.2

1.10

26.4

1.30

17.8

1.06

30.7

1.12

24.9

1.40

15.4

Low VSWR antennas are necessary to ensure minimum echo distortion in long haul microwave systems. A high antenna VSWR will cause some of the transmitted signal to be reflected back down the transmission line. This reflected signal can be again reflected at the RF equipment and sent towards the antenna. This delayed signal causes unwanted noise which can be compounded in a long microwave system. A low VSWR antenna minimizes the amount-of reflected signal. Standard Parabolic Antennas- make a reliable economic choice for the majority of thin route systems. These antennas have a VSWR of about 1.10:1 which is satisfactory for low to medium channel densities and moderate length systems. Low VSWR Parabolic Antennas- use the same reflectors but offer feed design with a VSWR of 1.04 to 1.06 essential for medium to high channel densities and long multi-hop systems. 3.4

RADIATION PATTERNS The radiation patterns of antennas have become more important with the increase in microwave congestion and the need for careful coordination to prevent interference between systems. In planning a route, a system engineer will evaluate the potential interference from microwave systems operating on the same frequencies up to 160 to 320 km. away. If the carrier to interference signal ratio, C/l, is 70 dB or more, interfering noise is negligible. However if the C/l ratio is low, then preventive measures must be taken such as using a high performance or ultra high performance antenna.

Interference coordination analysis normally uses envelopes of the antenna radiation called radiation pattern envelopes [RPE’s] Radiation pattern envelopes

are prepared using the 360 degree azimuth radiation patterns of the antenna at representative frequencies in the band, usually low, middle and high [Fig. 4]. Radiation pattern envelopes are smoothed by drawing a line over all the peaks of all the loves to provide a ‘worst case’ envelope. For symmetrical antennas the envelope is folded to 1800over so that the 180 degree to 360 degree half is super – imposed on the 0 degree half [Fig.5.]. Some antennas, like ultra high performance, are designed to have appreciably different patterns left and right of boresight. In these cases a ful 360 degree radiation pattern envelope may be shown [Fig.6]. The feed ir these antennas may by rotated 180 degree to select the preferred side it cases where the asymmetry can be used to advantage by the system engineer. 00

100

ANTENNA DIRECTVITY : dB BELOW MAIN LOBE FIG : 6 – 3600 PLOT OF RPE FOR ASYMMETRICAL ANTENNA

3.5

The front-to-back ratio of an antenna is defined as the ratio of the power received from [or transmitted to] the main beam of the antenna to the power received from [or transmitted to] the back side. Front-to-back ratio for a standard parabolic antenna is defined at 180 +5 degrees. For high performance or ultra high performance antennas, this is defined as 180 + 80 degrees, in order to operate satisfactorily on a two frequency plan, using the same transmitting frequencies in two directions at a repeater, it is necessary to have high front- to back ratios. While the antenna radiation pattern envelopes whole the expected radiation performance of the particular antenna, the actual pattern achieved in the field is dependent upon site conditions and foreground reflections. Careful site planning is therefore essential.

4,0

Horn Reflector Antennas The horn reflector [cornucopia] antenna has a section of a very large parabola mounted at such an angle that the energy from the feed horn is simultaneously focussed and reflected at right angles. A horn antenna having the equivalent gain of a 3 meter parabolic antenna is over 6 meters in height and causes a much greater load from wind on the tower, However, it has a much higher front- to- back ratio than the standard parabolic antenna, but has about the same front -to-back ratio as higher performance antenna of the same gain'(Fig.8). This type of antenna has good VSWR characteristics and with suitable coupling network's [which are quite complex and very expensive], can be used for multi-band operation on both polarizations. However, there are moding problems, particularly at the higher frequencies which, if uncorrected, can cause severe distortions, Correcting of these moding Problems is a very difficult task. Disadvantages are that this antenna is very big, heavy and complex to mount. The cost of one antenna with suitable coupling networks to provide dual polarization at 4 GHz and 6 GHz band far exceeds the cost of two separate parabolic high performance antennas, providing equivalent or better electrical performance.

6.0

WAVEGUIDES AND TRANSMISSION LINES Wave guide and transmission line is important, not only for its loss characteristics, which enter into the path loss calculation, but also for the degree of impedance matching attainable, because of the effect on echo distortion noise. The later becomes important with high-density systems having long waveguide runs.

6.1

Coaxial Transmission Lines In bands up to 2 GHz, coaxial cable is usually used, and except for very short runs, it is usually of the air dielectric type. Typical sizes are: 2.2 c.m. diameter. Andrew type HJ 5-50, with attenuation of about 6dB per 100 meters at 2 GHz, and 4.1 c.m. diameter, Andrew type HJ 7-50 with an attenuation of about 3 dB per 100 meters. These cables are normally ordered in the exact length required with factory installed and sealed terminal connectors. Both these types of cables are flexible enough to provide direct connection at the rear of the antenna provided that the mount allows direct access in horizontable plane. If the vertical run of the coaxial cable is down the side of the tower away frOm the antenna, this can be easily accomplished. In any coaxial cable system where VSWR is important, the number of connections should be kept to a minimum. However, at the equipment end, it mAy be necessary to reduce the larger size cable if used, to the smaller size cable with a suitable runs, with suitable transitions for flexibility in connection to the radio equipment. In some rare circumtences, where high power are necessary, it may be \nacessary to use elliptical flexible waveguides with vertical runs, with suitable transitions to coaxial cable at the top of the run and at the bottom of the run.

6.2

Waveguides Bands higher than 2 GHz require the use of waveguides almost exclusively and one of three basic types may be used rigid rectangular, rigid circular, and flexible elliptical. The latter is of continuous construction, having the advantages of minimising the number of flange connection usually of two. one at the antenna end, one at the equipment end. If rigid rectangular or circular waveguides are used, it is necessary to use short section" of flexible waveguide for connection to the antennas and to the equipments. In some cases, it may be necessary to use rigid rectangular waveguide inside the equipment building because of restrictions of space. However, in all cases it is desireable to keep the number of flanges and length of flexible sections as small as possible since each flange and each flexible section, besides having higher, losses, have poor VSWR than the main waveguide types.

6 .2.1 Rectangular Guide Rigid rectangular waveguide is the most commonly used, with oxygen-free, high conductivity copper (OFHC), the recommended material. The types and approximate characteristics are as follows: 4 GHz band :

WR 229 is standard for most installations. It has a loss of approximately 2.79 dB per 100 meters.

6 GHz band :

WR 137 is normally used. It has a loss of approximately 6.6 dB per 100 meters. In cases where, due to high towers, a reduced transmission loss is required, transitions can be supplied for use with WR 159, which has a loss of about 4.6 dB per 100 meters.

7-8 GHz

:

WR 112 is normally used. Attenuation is approximately 8.8 dB per 100 meters.

11 GHz

:

WR 90 is normally used. Attenuation is approximately 11.5 dB per 100 meters.

12-13 Ghz

:

WR 75 is normally used. Attenuation is approximately 14.7 dB per 100 meters.

For the most critical applications, where extremely low VSWR is required to meet stringejit noise performance specifications, special precision waveguide, manufactured to very tight tolerance, is recommended. 6.2.2 Circular Guide Circular waveguide has the lowest loss of all, and in addition, it can support two orthogonal polarizations within the single guide. It is also capable of carrying more than one frequency band in the same guide. For example, WC 281 circular, guide is normally used with horn reflector antennas to provide two polarizations at 6 GHz. But circular guide has certain disadvantages. It is practical only for straight runs, requires rather complicated and extremely critical networks to make the transitions from rectangular to circular and can have significant moding problems, when the guide is large enough to support

more than one mode for the frequency range in use. Consequently, though circular waveguide is available in several different sizes, and its low losses to make it attractive, it is recommended that it be used with considerable caution. 6.2.3 Elliptical Guide Semi-flexible elliptical waveguide is available in sizes comparable to most of the standard rectangular guides, with attenuations differing very little from the rectangular equivalents. The distinctive features of elliptical guide is that it can be provided and installed as a single continuous run, with no intermediate flanges. When carefully transported and installed it can provide good VSWR performance but relatively small deformations can introduce enough impedance mismatch to produce severe echo distortion noise. However, usually the effect of small deformations can be 'tuned' out. The most commonly used types and their approximate characteristics are as follows: 4

GHz band EW - 37 :

Approximately

2.8

dB per 100 meters

6

GHz band EW – 56 :

Approximately

5.7

dB per 100 meters

7-8

GHz band EW – 71 :

Approximately

8.2

dB per 100 meters

11

GHz band EW – 107 :

Approximately

12.1

dB per 100 meters

1213

GHz band EW – 122 :

Approximately

14.7

dB per 100 meters

All attenuation figures given at mid band. In all types of waveguide systems it is desirable to keep the number of bends, twists, and flexible sections to a minimum. It is also vitally important to use great care in installation, since even very slight misalignments, dents, or introduction of foreign material into the guides can create severe discontinuities.As manufacturing techniques improve, and installers become more familiar with elliptical guide, the return losses have been significantly reduced to the point that waveguide systems utilizing premium elliptical waveguides provide minimum noise contributions to the overall transmission system.

Brief introduction to UHF, M/W, Satellite systems and V-SAT With the advent of mass scale industrialization in our country, the demand for more communication facilities came up. Several new telephone exchanges have been installed throughout the country for local communication and more and more carrier channels have been provided for carrying the trunk traffic. With the planned introduction of Subscriber Trunk Dialing throughout the country, the number of carrier chls required to interconnect different cities became too high to be accomplished by overhead lines. Thus, U/G Cables Carrier Systems were introduced, the first of them being the symmetrical pair Cable Carrier System between Calcutta and Asansol with an ultimate capacity of 480 channels. Then came the Co–axial Cable Carrier System linking all major cities in the country.With the development of Microwave technique, which can provide large block of circuits at comparative cost, the problem of long distance communication circuits appear virtually solved. A brief description of the Microwave technique is attempted in the following paragraphs. 1. Electromagnetic waves can be broadly classified in terms of frequencies as follows : RANGE 0–30 KHz

NAME WAVELENGTH V.L.F. Upto 10 km.

30–300 KHz L.F. 0.3–3 MHz M.F.

10 km to 1 km 1 km to 100 m

3–30 MHz

H.F.

100 m to 10 m

30–300 MHz 0.3–3 GHz 3–30 GHz

V.H.F.

10 m to 1 m

U.H.F. S.H.F.

1 m to 10 cm. 10 cm to 1 cm.

E.H.F.

1 cm to 1 mm.

30–300 GHz

USES Used for long communication. Has limited information. Bandwidth require very high power. Radio Broadcast, Marine Power in KW, ground wave propagation, i.e. follows the curvature of the Earth. Long haul point to point communication. Propagation is by one or more reflections from inosphere layers and so subject to variations. Line of sight, Troposcatter communication. –––––– do –––––– Line of sight, terrestrial M/W and Satellite communication. Experimental.

The term SHF corresponds to "MICROWAVE" Centrimetric waves. As a convention frequencies, above 1 GHz and upto 40 GHz are termed as Microwave. However, most of the m/w systems available are in the range of 1 to 18 GHz. APPLICATIONS: M/W frequency bands are used for the following services : (i)

Fixed Radio Communication Services.

(ii)

Fixed Satellite Services.

(iii)

Mobile Services.

(iv)

Broadcasting Services.

(v)

Radio Navigation Services.

(vi)

Meteorological Services.

(vii)

Radio Astronomy Services.

To meet the requirements of all above mentioned services, co–ordination among the users of M/W spectrum is necessary. In this regard (in the national context) the wireless planning and co–ordination wing (WPC) of the ministry of communication has allotted m/w frequencies spectrum, on the basis of various wireless users classified as general users and major users. Wireless users who are permitted to plan their services and take action for the development of the required equipments are major users. BSNL has been nominated as a major wireless user by the WPC in 1981 in the following sub baseband of the m/w spectrum for fixed radio communication. Microwave Spectrum Available for BSNL Band 2 GHz 4 GHz 6 GHz 7 GHz 11 GHz 13 GHz

Bandwidth Available 300 MHz 900 MHz 1185 MHz 300 MHz 1000 MHz 500 MHz

Spectrum Space 2000–2300 MHz 3300–4200 MHz 5925–7110 MHz 7425–7725 MHz 10,700–11,700 HHz 12,750–13,250 MHz

In India the first M/w System was completed in December, 1965 between Kolkata and Asansol with a system capacity of 1200 channels. At present many kilometers of M/W systems are scattered throughout the country and further expansion is taking place at a very large rate. Frequency Characteristics Microwaves are very short frequency radio waves that have many of the characteristics of light wave in that they travel in line–of–sight paths and can be reflected, boomed and focussed. By focussing these ultra high radio waves into a narrow beam, their energies are concentrated and relatively low transmitting power is required for reliable transmission over long distance. System Capacity Microwave communication systems are used to carry telephony, television and data signals. Majority of the systems, however, carry multi–channel telephone signals. The spectrum of the multichannel telephone signal is shown in Fig.1. This signal is also called base band (Fig. also shows the TV spectrum). Individual telephone channels, 4 KHz wide (300 to 3400 Hz for speech and the remaining for signalling and guard band) are multiplexed together in a multiplex equipment to get the base band. The base band frequency given in Table below : Channel capacity 60 channels 60 channels 120 channels 300 channels 600 channels 960 channels 1800 channels 2700 channels

Base band frequency in KHz 12–252 60–300 60–555 60–1300 60–2540 60–4028 312–8120/316–8204 312–12336/316–12388

The system capacity of line of sight systems ranges from 60 telephone channels to 2700 channels over a Radio bearer with a few systems of lower capacities varying from 60 to 60 channels. On the same m/w route one can use more than one radio channels, thus getting still larger capacity. As an example one can accommodate 8 go and 8 return RF channels each with a capacity of 1800 telephone channels in a 500 MHz bandwidth. Of course, in such cases usually one or two RF channels are kept as a standby which are switched over automatically on fading or equipment failure. Usually the system with capacities upto 300 channels is called

narrow band system and the systems providing more than 300 channels are called wide band system. M/W systems used to provide communication on major trunk routes with high traffic density and serving long distances are classified as long haul m/w systems. 2, 4, and 6 GHz systems are long haul systems. Systems used to provide communication over short distances for trunk routes with light traffic density are classified as short haul system. 7 and 11 GHz systems are short haul systems.

TELEPHONY SUB BASE BAND 960CHLS

4 20

60

BASE BAND

312

4028

PILOT

8120

9023

1800 TELEPHONE CHANNELS

(TELEVISION)

330 KHz

VIDEO

KHz 5000

7020 7500 8065 8590 9023 (4 SOUND CHANNELS) PILOT SOUND SUBCARRIERS

FIG : 1 BASE BAND SPECTRUM

The salient features of various long distance communication systems are summarised below to make a comparative study.

Fig. 2 A Typical Microwave Radio System

Fig. 3 Schematic of a Microwave Terminal

Digital Radio 1.

Introduction The Department of Telecommunication at the time of formulation of the 7th

Five Year Plan took a decision that the long term perspective for the country would be an integrated services digital network. The approach adopted for achieving this objective is to first proceed towards integrated digital network in which both the switch and the transmission media would be of digital type. Subsequently, through further developments and improvements in technology, it was proposed to bring in the other necessary requirements, viz. capability of the switch to handle data, introduction of No.7 common channel signalling and extension of the digital media up to the subscriber premises for converting the network into ISDN. 1.1

There were several reasons for the decision to go in for ISDN network. Some of these are :

–

The expected growth in data traffic where the source information is in digital form. The main source of this data traffic is from the use of computers. This has been very evident abroad but also been noticed over the last few years in India.

–

Over all economics in the use of fully digital environment as compared to the analogue environment. It is to be noted, however, that economy is not feasible in the mixed environment of analogue and digital.

–

The technical performance expected from the new digital equipment is superior to the analogue equipment because of the rapid technological developments in the micro–electronics area and in so far as microwave systems are concerned the immunity of signal from noise even in faded conditions.

–

Possibility of providing a large variety of new services to the customer.

–

Development of optic fibre technology which for all practical purposes is a digital technology and in this form offers revolutionary advantages in the network.

2.

Available Transmission Media The major reliable terrestrial transmission media which are available today are

: (i)

PCM on copper cable.

(ii)

Fibre optic systems.

(iii)

Digital radio systems.

The choice of the transmission media depends on the capacity required, the cost economics for the required capacity and distances and the requirement of media diversity for reliability purposes. The choice made also varies depending on application area such as inter–city, intra–city requirements. 3.

Transmission Capacities Available on the Radio Systems The transmission capacities available on digital radio systems are, of course,

integral multiples of PCM hierarchical bit rates and are classified into small, medium and large or high capacity systems. Specifically this categorization covers : Low capacity

–

704 kbps, 2 mbps and 8 mbps

Medium capacity

–

34 mbps

High capacity

–

140 mbps

Some manufacturers and some administrations have used some other integral multiples also such as 2 x 8 and 2 x 34 mbps systems but these are not being considered in the Indian network. The 704 kbps system is not other wise a standard system but has been proposed in the Indian network context, because for the rural network it is found that a 2 mbps system corresponding to 30 channels was too large and wasteful of frequency resource. This 704 kbps system corresponds to capacity of 10 channels, which is quite adequate in the rural network of the country..

4.

Frequency Bands The frequency bands and the capacities which are proposed to be used by

digital microwave and UHF systems in the country are given below :

Small capacity Small capacity

Bit rate Mb/s. 0.704 2.048

Small capacity

8.448

120

Small capacity

8.448

120

Medium capacity

34.368

480

Medium capacity

34.368

480

Medium capacity

34.368

480

High capacity

139.264

1920

High capacity

139.264

1920

High capacity

139.264

1920

Nomenclature

No. of chls. 10 30

Frequency band 658–712 MHz (UHF) 400 MHz band (UHF) 520–585 MHz (UHF) 622–712 MHz (UHF) 2 GHz band (M/W) (2.0–2.3 GHz) 7 GHz band (M/W) (7.425–7.725 GHz) 13 GHz band (M/W) (12.75–13.25 GHz) band M/W 15 GHz band (M/W) (14.75–15.75 GHz) 4 GHz band (M/W) (3.3–3.8 and 3.8–4.2 GHz) 6 GHz band (M/W) (5.925–6.425 GHz; Lower) (6.430–7.110 GHz; Upper) 11 GHz band (M/W) (10.7–11.7 GHz)

8.

Digital Radio Applications

8.1

Small Capacity Digital Radio Systems

–

10 channel systems in the UHF range are being developed indigenously with a view to utilize them in the rural area where channel requirements are very small, for example, linking an RAX to the nearest large exchange. The systems are expected to work in the 1+0 unprotected configuration.

–

2 Mbps system in the UHF band is expected to serve the purpose of linking secondary switching area centre to concentration points in the rural areas of a

secondary switching area. These are expected to be manufactured by ITI, BEL and PCL. . –

2 and 8 Mbps systems are expected to be available in the 18 to 20 GHz range. These are likely to be applied with integral antenna, mounted on a mast and will have point to multipoint application. These will be suitable for business network in large urban centres. The hop lengths are likely to be a few kilometres depending on rainfall statistics in a given area.

–

8 Mbps system in 2 GHz band is suitable for application as a short haul system and will find application in the rural network, for linking either secondary switching areas to their next higher TAXs or linking the secondary switching centre to trunk concentration points in the rural area. The advantage of this system is the possibility of using long hops. The equipment is to be manufactured by ITI and the expected cost per terminal is not yet established. The type of antenna used will be grid paraboloid.

8.2

Medium Capacity Digital Systems The systems being used in the BSNL in the medium capacity range are 2 GHz

and 13 GHz 34 Mbps equipments. Their applications are as follows : –

2 GHz, 34 Mb/s is to be used in the trunk network with longer hops than those feasible in the higher frequency bands.

–

7 GHz, 34 Mbps system is being used in the trunk network to connect primary centres to secondary switching centres. It is possible to use 4 frequency channels with one standby channel but the equipment currently expected to be available in the country is suitable for 1+1 RF bearer. The modulation method used in 4 PSK, Hop lengths which sometimes tend to be as much diversity.

as 40 kms requiring space diversity along with frequency

–

13 GHz 34 Mbps equipment is being used almost exclusively in the junction network in large urban telephone systems. The rain statistics do dictate the hop length but the use being in the urban network does not cause much problem even in cities like Calcutta where the rainfall is heavy. These are operated in N+1 mode with the possibility of N=7. Because of the small hop length no multipath fading problem is observed. The modulation method used is mostly 4 PSK.

8.3

High Capacity Digital Radio Systems The preferred application in the BSNL network is as follows : Presently, 6 GHz band 140 Mbps system is being introduced for long haul

trunk routes between major cities. This equipment because of its large capacity requires several specific features in its design. These include the use of adaptive equalisers including base band transversal equaliser to minimize intersymbol interference and IF band resonance equaliser to equalise notch and slope besides using space diversity. Presently, these are being used in the N+1 mode with N=7. 5.

Comparison Between Digital and Analogue Radio Relay System With reference to radio relay systems in particular, digital systems have certain

advantages and disadvantages. The major advantages include : –

The ability to regenerate at each repeater with the result that circuit performance becomes essentially independent of length.

–

The plentiful capacity for data traffic and the ability to support an IDN and subsequent potential involvement into an Integrated Services Digital Networks (ISDN).

–

A higher immunity to noise and interference which amongst other things allows operation at higher carrier frequencies and in metropolitan areas.

Associated with the use of higher frequencies for digital radio are reductions in spectrum congestion and equipment size making such equipment easy to transport and install. On the other hand, DRRS have certain disadvantages. These include : –

The sensitivity of high capacity systems to frequency selective fading which can result in reduction of the effective fade margin by some 20 dB below the flat fade margin for typical analogue hop lengths. To restore such systems to acceptable performance, it is necessary to add various combinations of combining space diversity, adaptive equalizers at IF and/or transversal equalizers.

–

The absence of sub–baseband which makes it more costly to drop and insert small numbers of circuits typically used for wayside traffic.

–

Higher power requirement when compared to currently available low drain IF or RF repeating analogue radio–relay equipment. Making it uneconomic to power by solar cell arrays.

Most of the disadvantages of high capacity DRRS are being eliminated with the second generation of equipment coming onto the market. Major power drain reduction has occurred, more powerful equalizers have been incorporated as a standard part of the equipment and additional drop and insert capacity is being introduced. The salient characteristics for the analogue bearer are that the basic noise and intermodulation noise from each hop are cumulative, the voice frequency (VF) channel signal–to–noise (S/N) ratio depends on the received input signal level and more particularly, on the carrier–to–noise (C/N) ratio and that co–channel carrier–to– interference ratio of 30 dB makes the circuit quality unacceptable. The salient characteristics of the digital bearer are that the performance is uniform over a wide range of receive input levels and deteriorates rapidly over a small range of C/N ratios near the threshold. In addition, the introduction of even a 30 db C/N ratio has only a marginal effect in worsening performance near the threshold.

6.

Performance Requirements of Digital Microwave System in Comparison to Analogue Microwave Systems The performance requirements for digital and analogue microwave systems

differ because the definitions of quality differ in the two cases. In the case of analog microwave systems, the quality is measured in terms of the signal–to–noise. For digital microwave systems, the quality is measured in terms of bit error rate (BER). For digital microwave systems, the S/N does effect the performance, but the bit by bit faithful reproduction (which is the ultimate objective) is also influenced by other parameters such as coding, modulation scheme, inter symbol interference properties, etc. Thus, a knowledge of S/N alone may not be adequate to determine the actual BER of the system. As already mentioned in Section 5, for an analog microwave system the quality is more or less a direct function of the fade, i.e., as fade increases, the S/N deteriorates. On the other hand, in the case of digital microwave system, as the fade increases the quality of the equipment, i.e. BER remains nearly constant upto a value close to the threshold at which point the BER rises rapidly and the system performance collapses. In view of the above, different definitions of the quality, the CCIR definitions for the performance of hypothetical reference circuit (HRC) for analog system and hypothetical reference digital path (HRDP) for digital system are as follows : Analog System Following noise figures are not to be exceeded for the time percentages indicated : (a)

7,500 pwop for more than 20% of any month.

(b)

47,500 pwop for more than 0.1% of any month.

(c)

10,00,000 pw (unweighted, with an integrating time of 5 ms, for more than 0.01% of any month).

Digital System Following BERs not to be exceeded for the indicated time percentage as given below : (a)

1 x 10–7 BER for more than 1% of any month.

(b)

1 x 10–3 BER for more than 0.5% of any month.

For actual paths which differ from the HRDP in composition or are much smaller in length the performance criterion under consideration by the CCIR is as follows : When a path is established over a link which is less than the HRDP (2500 kms), but greater than 280 kms and which differs in composition from the HRDP, the allowable time percentage should be proportional to the link length L (kms) of the link. (a)

1 x 10–7 BER for more than (L/2500) x 1% of any month.

(b)

1 x 10–3 BER for more than (L/2500) x 0.05% of any month.

When a path is established over a link which is less than 250 kms, it is proposed that BER not to be exceeded for the indicated time percentage as given below : (a)

1 x 10–7 BER for more than (280/2500) x 1% of any month.

(b)

1 x 10–3 BER for more than (280/2500) x 0.05% of any month.

Note : This takes into account fading, interference and all other sources of performance degradation. It does not include BER greater than 1 x 10–3 for periods exceeding 10 consecutive seconds. This condition is included in the availability criterion. The high BERs caused by switching operations are included in the above criterion, but not the ones caused by scheduled switching for maintenance). Availability criterion is 1 x 10–3 BER (measured for 10s time interval) not exceeding 0.3% of a year. It is important to note that during the conditions of fades well above the threshold margin, the system is almost perfect. In interpreting this statement, it should be kept in mind that threshold margin does not necessarily imply flat fade margin. 1.1

SACFA Clearance As stated in the previous paragraph, all the user Departments, like Railways,

Civil Aviation, Defence, Telecommunications Department, etc. are members of the

SACFA Board. There is a Central Board at Delhi and Regional Boards at Madras, Bombay and Hyderabad, etc. The main objective of the function of the SACFA Board is to investigate the interference possibilities, etc. and allot the frequency and spectrum for new routes. All types of Microwave routes should be cleared by this body as far as the frequency to be used, the location, the height of tower are concerned. This body takes the safety aspect from Aviation point of view (of civil as well as Defence flights) also. Hence, while clearing the licence for a new route, this Body specifically mentions whether night warning or both Day and Night warning are to be provided for the Microwave towers. Night warning is by means of aircraft warning lamps and day warning is by means of painting the tower with alternate bonds of international orange and white. The SACFA Board also considers the distance of tower location from the nearby Airports and ensures that the specified minimum distance is maintained from the airport. The SACFA Board takes the individual clearance from the member Departments, before clearing a particular Microwave route. In Project Organization takes up the responsibility of obtaining SACFA clearance for MW routes for BSNL.

Overview of SATELLITE System & V-SAT Introduction Long distance communication using conventional techniques like coaxial cable or microwave radio relay links involves a large number of repeaters. For radio relay links of repeater spacing is limited by line of sight and is of the order of tens of kms. As the number of repeaters increase system performance and reliability are degraded. Tropo scatter propagation can cover several hundred kms. but the channel capacity is limited and costs are high due to necessity of large antennas and high transmit power. HF communication is subject to fading due to ionospheric disturbances and channel capacity is severely restricted due to limited bandwidth available. Large areas could be covered if the height of microwave repeater could be increased by putting it on board an artificial earth satellite (Fig.1). Science Fiction writer Arthur C. Clarke in an article in Wireless World in 1945 proposed that worldwide coverage could be obtained by using three microwave repeaters placed in a geostationary orbit at the height of about 36000 kms. with a period of 24 hours (Fig.2). Satellite Repeater

Inospheric reflection ( HF Radio )

LOS R/R

Tropospheric Scatter

Maximum Coverage Fig. 1

Modes of Communication Geostationary Orbit

Fig. 2 Global Coverage with Geostationary Satellite Satellite communication provide a practical and economical means of long haul communication traffic in a country with a large geographical area. It also enables communication service to those areas which are virtually INACCESSIBLE by other conventional forms of communication system due to natural physical barriers. Principles and Features of Satellite Communications Principles Figure 2 shows the principles of satellite communications. Here, a geostationary satellite with microwave radio repeater equipment receives and amplifies radio waves sent from earth stations and returns them to the earth. A geostationary satellite is launched above the equator 36,000 km high above the earth. Its period round the earth coincides with that of the earth rotation. Therefore, the satellite looks as if it is stationary from the earth. If three (3) communication satellites are launched equidistantly above the equator (See Fig.2), it can serve almost all communication network round the world. Therefore, to facilitate public international telecommunications, INTELSATS IV and V have been launched

above the Atlantic, Pacific, and Indian Oceans. These networks cover almost all countries around the world. Features For international communication, a submarine cable along the Atlantic Ocean was installed in 1857. Also, short–wave radio communication (invented by Marconi in 1886) has been in use. However, short wave radio communication has disadvantages of : (1)

Small transmission capacity; only small telephone channels can be used to transmit.

(2)

Fading in wave propagation; interferes with stability of transmission. Although over–the–horizon propagation is used for short distance international communications, it is impossible to apply it to transoceanic long distance communications.

Unlike

other

system,

geostationary

satellite

communication

systems

summarize as follows : (1)

Stable and large capacity communication.

(2)

Costs of establishment and maintenance do not depend on communication distance. The costs of submarine and over–the–horizon systems are proportional to the length, but those of the satellite system do not affect the communication distance. Therefore, the satellite system is ideal for long distance communications.

(3)

Multiple access is possible. Signals sent from an earth station can be received at several earth stations simultaneously. Therefore, it can transmit signals to many stations simultaneously, such as TV. Actually, increasing of submarine cable's capacity and distance between repeaters, can make submarine cables competitive to satellite communication specially when very large capacity is required but for

small traffic size countries, satellite communication is unavailable for the independent communication services. Advantages of Satellite Communications (i)

Large coverage : Almost one–third of the earth with exception of polar regions is visible from geostationary orbit. It is, thus, possible to cover about 10,000 kms. distance irrespective of intervening terrain with a single satellite.

(ii)

High quality : Satellite links can be designed for high quality performance. The link performance is highly stable since it is free from ionospheric disturbances, multipath effects or fading.

(iii)

High reliability : Reliability is high since there is only one repeater in the link.

(iv)

High capacity : With microwave frequencies, wide bandwidths are available and large communication capacity can be obtained.

(v)

Flexibility : In a terrestrial system, communication is tied down to the links installed. On the other hand, satellite communication is well suited for changing traffic requirements, locations and channel capacities.

(vi)

Speed of installation : Installation of earth terminals can be achieved in a short time as compared to laying of cables or radio relay links.

(vii)

Mobile, short–term or emergency communications : With ariliftable or

road

transportable

terminals,

short–term

or

emergency

communications can be quickly provided. Reliable long distance land mobile, maritime mobile and aeronautical mobile services are feasible only by means of satellite. (viii) Satellite communication is ideally suited for point to multipoint transmission on broadcasting over large areas. Application of satellites for TV broadcasting, audio and video distribution and teleconferencing, facsimile, data and news dissemination is, therefore, increasing rapidly.

(ix)

All types of common services are possible.

Satellite Communication Network Satellite Communication Network could be defined as an ensemble of earth stations of pre–determined size spread over a pre–defined coverage area, interconnected through a suitably designed satellite, placed at a pre–determined location in properly chosen orbit around the earth. Thus, two important elements of a satellite communication network are : (i)

Space Segment

(ii)

Ground Segment

Uplinks and Down Link Uplink is the radio path from Ground segment, i.e. earth station to the Space segment, i.e. satellite, whereas Downlink is the radio path from space segment, i.e. satellite to the ground segment, i.e. earth station. Frequency Bands Choice of Frequency band for space communication depends upon –

Band–width required.

–

Noise consideration

–

Propagation factors

–

Technological developments with regard to component and device.

As the signal levels from the satellite are expected to be very low, any natural phenomenon to aid the reception of the incoming signals must be exploited. Note in Figure 3 that between the frequencies of 2 GHz to 10 GHz, the level of the sky–noise reduces and this band of frequencies is known as the 'microwave window'. The most of the communication satellites as on today are using a frequency of 6 GHz for "Up link" and 4 GHz for "Down link" transmission. These frequencies are preferred because of –

Less atmospheric absorption than higher frequency.

–

Less noise both galactic and manmade.

–

Less space loss compared to higher frequency.

–

A well developed technology available at these frequencies.

–

6 GHz/4 GHz bands are shared with terrestrial services, creating interference problem.

–

As equatorial orbit is filling with geostationary satellites, RF interference is increasing from one satellite system to another is increasing.

–

14/11 and 30/20 GHz systems for telecommunication and broadcasting satellite services are slowly coming being.

Frequency bands in use for satellite communication are : "L" BAND

1830–2700 MHz

"S" BAND

2500–2700 MHz 5925–6425 MHz UP 3700–4200 MHz DOWN 7900–8400 UP 7250–7750 DOWN 14.000–14.500 Hz. UP 10950–11200 GHz/DN. 11450–11700 GHz/DN. 27.5–30 GHz UP 17.7–21.2 GHz DOWN 6725–7025 UP 4500–4800 DOWN 40–51 GHz UP 40–41 GHz DOWN 59–64 GHz 54–58 GHz

"C" BAND "X" BAND "KU" BAND "K" BAND EXTENDED C BAND V BAND V Band Inter-satellite

INSAT IS USING INSAT IS USING

INSAT IS USING

Time Delay The total earth–satellite–earth path length may be as much as 74,000 km thus giving a one–way propagation delay of 250 ms. The effect of this delay on telephone conversations, where a 500 ms gap can arise between one person asking a question and hearing the other person reply, has been widely investigated, and was found to

be less of a problem than had been anticipated. With geostationary satellites, two– hop operation sometimes unavoidable and gives rise to a delay of over one second.

Fig. 3 Geographical Advantage

Fig. 4.

A station which is located closer to the sub–satellite point,

as demonstrated in Fig.4 will have an advantage in received signal level with respect to one at the edge of the service area of the satellite. For a global coverage satellite, this can be as much as 4.3 dB.

Communication Systems Satellite communication systems classify that : (a)

Communication system (1)

(b)

(c)

Multiplexed telephone channels with one (i)

carrier frequency system, and

(iii)

Single channel per carrier system.

Modulation system (1)

Analog modulation (Frequency Modulation system), and

(2)

Digital modulation system

System configuration (1)

Pre–assignment system

(2)

Demand assignment system, and

(3)

Various other systems combined with those above.

Kinds and Systems of Communication Satellite (1)

Kinds of Communication Satellites – depends on type of orbit and freq. band used. During the early experimental stage of communication satellites, a passive

satellite was used without any amplifiers and it only reflected radio waves sent from the earth station. But, later on active satellite with amplifiers was developed and put into practical use. Communication Satellite can be classified by the orbit used and also by frequency band used. Before discussing satellite orbits in a more generalized manner, however, it is necessary to be aware of the natural laws that control the movement of satellites. These are based on Kepler's laws and basically stated are : (i)

The orbit plane of any earth satellite must bisect the Earth centrally.

(ii)

The Earth must be at the centre of any orbit.

The choice of orbit is restricted to three basic types, namely : polar, equatorial and inclined as illustrated in Fig.5. The actual shape of the orbit is limited to circular and elliptical. Any combination of type and shape is possible but observations are made only of the circular polar, elliptically inclined and the circular equatorial.

Fig. 5 Three Basic Orbits Circular polar orbit This is the only orbit that can provide full global coverage by one satellite, but requires a number of orbits to do so. In a communications sense where instantaneous transfer of information is required, full global coverage could be achieved with a series of satellites, where each satellite is separated in time and angle of its orbit. However, this produces economic, technical and operational disadvantages and is thus not used for telecommunications though it is favoured for some navigation, meteorological and land resource satellite system. Elliptically inclined orbit An orbit of this type has unique properties that have been successfully used for some communications satellite system, notably the Russian domestic system. For this system, the elliptical orbit has an angle of inclination of 63 degrees and a 12– hour orbit period. By design, the satellite is made to be visible for eight of its 12–hour orbit period to minimize the handover problem while providing substantial coverage of the temperate

and polar regions. By using three satellites, suitably phased,

continuous coverage of particular temperate region can be provided that would not be covered by other orbits. The elliptically inclined orbit is used exclusively by the Russians for their Orbital and Molniya systems, but since coverage is limited to particular areas (higher latitudes), it is, therefore, not suitable for a global network. Circular Equatorial Orbit Circular orbits in the equatorial plane permit fewer satellites and ground stations to be used, and satellites with long orbital periods (at high altitudes) have greater mutual visibility. A satellite in a circular orbit at 35,800 km has a period of 24 hours and consequently appears stationary over a fixed point on the earth's surface. The satellite is visible from one third of the earth's surface, up to the Arctic circle, and this orbit is almost universally preferred for satellite communications system. Stabilization of the satellite is necessary since the earth is not truly spherical, and the moon, sun and the earth's tidal motion have gravitational effects on the satellite, tending to make it drift from its correct position. Inclination to the equatorial plane produces a sinusoidal variation in longitude, seen from earth as motion around an ellipse once every 24 hours, with peak deviation equal to the inclination angle. Incorrect velocity results incorrect altitude, and a drift to the east or to the west. When a non re–usable launcher is utilized, injection of the satellite into geostationary orbit requires two rocket burns : the first to get the vehicle into a parking orbit, and the second via an elliptical transfer orbit to geostationary altitude. The spacecraft's own apogee motor then increases its velocity to about 10,000 fps to maintain the geostationary orbit. When launched from the Space Transportation System (Shuttle), a booster rocket is attached to the satellite to boost it to the geostationary orbit. The satellite must then be correctly positioned, and held in position for its required lifetime (typically 7 to 10 years). This is done by using hydrazine (liquid nitrogen plus ammonia) and cold gas jets. About 40 lbs. of hydrazine are required for corrections to maintain geostationary position within q 0.1x for five years, but since hydrazine is also used for initial positioning, the quantity available depends on the

accuracy of the launch. To extend the life of the satellites, less frequent corrections may be made allowing the satellite to drift. F = Noise Figure of Receiver. The antenna noise is expressed in degrees Kelvin and is called noise temperature of antenna. It can be converted to familiar units of power, watts by multiplying it with Boltzmann's constant K = 1.38 x 10–29 joule/kelvin and the bandwidth. Noise temperature of an antenna is of the order of 20–50oK. Geostationary Satellite This satellite revolves above the equator round the earth at a height of 35,790 km. Its period of revolving round the earth is same as that of the earth rotation on its own axis. Therefore, it looks as if it is stationary. This system was contributed to the "WIRELESS WORLD" by Mr. A.C.Clark, Dr. Rosen (an American) and others. It launched a Syncom communication satellite in 1963. Syncom No. 1 failed to launch in February, 1963. But, Syncon No. 2 finally succeeded in July 1963. This satellite centered the equator and moved like a figure eight (8). This was not a complete geostationary satellite, but it came into practical use (24 hours) as synchronous satellite. This satellite is advantageous because : (1)

Its large antenna at an earth station is easy to track.

(2)

Twenty–four (24) hours communication can be made with even only one satellite.

(3)

The satellite looks at the earth as if it were stationary, and it radiates highly effective wave power.

(4)

Visibility from one (1) satellite is very wide, and global communication can be made using only three (3) satellites.

Its drawback, however, is its delay caused in long distance transmission. But, the system is economical and accordingly, it is widely used for both international and regional domestic communications.

Figure of Merit (G/T) The earth station are classified on the basis of figure of merit of earth station which is defined by the parameter 'G' by 'T' (G/T). G is the receive gain of the earth station antenna and T is the equivalent noise at LNA input. This noise includes : (i)

Antenna noise

(ii)

Equivalent noise of receive chain (LNA, down converter) referred to LNA input.

The total noise is expressed in terms of noise temperature (Kelvin). Thus, G/T of an earth station in dB/K is given by. G/T = GR – 10 log T

in dB

where G is the receive gain of earth station antenna and T is noise temperature of the receive chain. A high G/T implies that an earth station can receive very weak signal because antenna gain is high and noise is low. Note that an LNA is specified by its noise temperature, i.e. by noise its generates. Noise Temperature The amount of receiver noise present is defined as receiver noise temperature To eq. The parameter To eq is an effective equivalent temperature that an external noise source would have to produce the same amount of receiver noise. The equivalent temperature is written as To eq = Tb0 + (F–1) 290o where, Tb = back ground noise temperature accounting for contribution collected by antenna.

HVNET – BSNL V-SAT NETWORK This is the first High Speed Satellite based VSAT network of Department of Telecom., Govt. of India. It provides for high speed data transfers and voice communication covering the entire country. The BSNL VSAT network consists of a HUB Station located at Yeur Earth Station of BSNL near Thane (about 40 kms from Mumbai) and number of VSATs/Personal Earth Stations (PES) located throughout the country.The VSAT communicate to the Hub through the INSAT Satellite. All VSATs are connected in STAR topology and VSAT to VSAT communication is through HUB at Mumbai. The VSAT which is required to be installed at subscribers premises consists of three units, namely an Outdoor Unit, an Indoor Unit and Inter Facility Link (IFL). Cable interconnecting the two Units along with a 2.4 meter diameter Antenna assembly and can be installed easily in any open space and requires a floor area of about 4 mt x 4 mt. The IFL cable, which carries the telecom signals and power supply, the IFL cable can be up to 100 meters long. HUB OVERVIEW HVNET Data Network Hub station is located at Yeur Earth Station, Yeur in Thane District. HVNET caters to the 200 remote V–SAT Terminals. This Hub consists of the following equipment : 1.

RF Equipment Rack : consisting of UP/DOWN Converters.

2.

Base Band Subsystem Rack.

3.

IF Subsystem Rack.

4.

IPN Switch : (Integrated Packet N/W Switch) which does the switching of V–SAT data calls to different N/W.

5.

EPABX : (Small Telephone Exchange) which switches the V–SAT to V– SAT voice calls and switches to PSTN N/W as per the type of call.

6.

PCM RACK : It is for HVNET Hub connecting to terrestrial network for DATA/VOICE/TLX and GPSS.

7.

System Control Centre : (a)

MicroVAX : It is a central Computer which records the call records and provides these call record details to the Billing System.

(b)

Illuminate Operator Console : This is a workstation used for configuration and monitoring the system parameters in real time basis.

We

can

control

Powers/Frequencies/Identification

numbers, etc. (c)

VT 520 event terminal : It is used to display events in the network.

(d)

GNOC PC : This is connected to packet switch exchange for monitoring and configuring the IPN Ports.

(e)

NARS PC : This is connected to NCP MicroVAX. This gives the network analysis and reports.

(f)

Line printers : These are used for fault event printing and summary printing of the network.

8.

Biling PC : The billing software is installed in this PC. The required call record files are transferred to this PC from MicroVAX Computer and data billing is done on the basis of volume of traffic. The call records consists of calling address, called address, start time and end time with corresponding data and data volume transmitted and received. The back up of call records file are taken on magnetic media regularly.

Services Available on HVNET •

Data communication at speed up to 64 Kbps (Synchronous X.25) and speed up to 19.2 Kbps (Asynchronous X.28).

•

Access to PSTN (National and International Calls) and vice versa in addition to VSAT to VSAT voice calls.

•

Access to RABMN customers.

•

Access to INET – I and II.

•

Access to International Data Networks through GPSS of VSNL.

•

Access to Telex customers of National and International Network.

•

Access to Internet Shell Account.

Indoor Section HVNET VSAT Indoor Section It has indoor unit which is also called PERSONAL EARTH STATION (PES). The PES comprises the following cards. (1)

MPC (Multiport Card)

(2)

VDPC (Voice Data Port Card)

(3)

IFM (Intermediate Frequency Module).

(4)

IOD (Inroute/Outroute Controller CHIP) mounted on backplane.

Function of MPC (a)

Multiport Card provides up to 8 users interface ports.

(b)

Each port can be configured to process a different protocol, e.g. x–28, x–25.

(c)

Each port level converter (PLC) provides the electrical signal conversion for two ports.

(d)

Each junction box attached to the back of card provides 4 nos. of DB– 25 connections.

Function of VDPC (a)

VDPC is used by itself to provide a two wire (RJ–11) or four wire (RJ– 45) telephone interface.

(b)

FIM (Fax Interface Module) card is used to provide fax facility.

(c)

COLC (Central Office Line Card) 600 ohms are used to provide 2–wire (RJ–11) telephone facility.

Function of IFM (Intermediate Frequency Modules) (a)

It is connected to RF head by IFL cable.

(b)

It receive outroute down link signal from RF head, downconverts, demodulates, decodes and sends the signal to the IOC chip on the backplane.

(c)

It receives inroute signal from IOC (Inroute/Outroute controller) chip, modulates to 170–190 MHz and sends signal to rf head.

(d)

It performs overall control and monitoring of the PES.

(e)

It contains EEPROM (Electrically Erasable Programmable Read Only Memory) which stores configuration information.

(f)

It has a Config port (RJ–11) which will be connected to Remote Site Installation Computer and to read and write to the EEPROM.

Function of IOC Chip –

Outroute signal processing •

receives outroute data stream from IFM.

•

descrambles the Outroute bitstream.

•

detects CRC (Cyclic Redundancy Check) errors in the outroute.

•

acquires and maintains received superframe header synchronization.

•

generates system clocks and data packet timing.

•

address filters serial; outroute data packets and converts to parallel for port cards (De–mux).

–

In-route signal processing``````````` •

accepts parallel in-route data packets from port cards (Mux) and convert to serial.

•

scrambles in-route data and provides Forward Error Correction coding (FEC).

•

generates and inserts a preamble sequence for the in-route packet burst.

•

sends data stream to IFM.