diversity systems also offer a certain amount of redundancy as the frequency diversity systems.

9.4.3 Tropospheric Scatter Communication

Tropospheric scatter or simply troposcatter systems are single hop systems that operate over
lengths that are greater than the line-of-sight distances. Atmospheric air turbulence and local
variations or irregularities in the refractive index in the tropospheric region of the atmosphere
cause scattering of electromagnetic waves. When a transmitter launches a narrow beam of
signals to its horizon, the beam ‘illuminates’ a region in the troposphere, which is well beyond
the horizon of the transmitter. If a receiving station looks at this scattering region via a high gain
antenna, it is able to receive the signals. This is illustrated in Figure 9.22.

Figure 9.22 Troposcatter phenomenon.

The level of the received signal in troposcatter communication is well below the level of
signal obtained in free space communication for equivalent distance between the stations. Losses
up to 260 dB are not uncommon. To achieve acceptable performance, very high gain antennas,
high power transmitters, and receivers with low noise factors are required. Transmitters with 1
kW to 10 kW power output and parabolic antennas with 5, 10 or 20 m diameter dishes are
common. Further, the received signal level varies considerably with time as the scattering is
random. There are both long term and short term variations in the signals. In other words,
troposcatter communication experiences both slow and fast fading. Slow fading can be corrected
by the automatic gain control mechanism of the receiver. A sophisticated diversity system is
required to reduce fast fading. Most present day operational systems employ quadruple diversity
using both frequency and space diversity. Space diversity is almost universally used. The
physical separation of antennas is normally in the horizontal plane with a separation distance
greater than 100 wavelengths. Frequency diversity may not always be possible due to restrictions
in the availability of multiple frequencies in the crowded RF band. As an alternative, polarisation
diversity is used. The two polarisations together with the space diversity provided by the two
antennas make up for four diversity paths.
A number of methods are available for estimating both long term and short term path losses in
tropospheric communication. These include CCIR (International Consultative Committee for
Radio) Report 238.3 and the US National Bureau of Standards (NBS) Technical Note 101. These
methods are elaborate and complicated and a discussion on these is beyond the scope of this text.
Tropospheric scatter communication generally operates in the 400-MHz and 900-MHz, or 2-
GHz and 4-GHz bands handling 12−240 FDM telephone channels or up to 120 digital channel.
This form of communication is normally used only in thin-traffic routes over difficult terrain or
water. Ministry of Telecommunications in India operates a troposcatter communication link
between New Delhi and Moscow.

9.4.4 Coaxial Cable Transmission

Coaxial cable is a 2-wire transmission line with a unique arrangement of the conductors which
are held in a concentric configuration by a dielectric, with the inner conductor being surrounded
by the outer conductor as shown in Figure 9.23. The outer conductor which is grounded acts as
an electromagnetic shield, almost eliminating the pick up of unwanted interference and reducing
the signal loss due to electromagnetic radiation. Since one of the conductors is grounded, coaxial
cable is basically an unbalanced transmission line. The ungrounded inner conductor carries the
signal voltage with ground as the reference.

Figure 9.23 Coaxial cable structure.

For full duplex transmission, generally a pair of coaxial lines is used, although in some special
cases a single line has been used for 2-way transmission. For long-haul systems, 4, 6, 8 or more
lines (or tubes as the single coaxial lines are called) are placed in a sheath. The characteristic
impedance Z0 of coaxial lines lies between 50 and 100 Ω. It is possible to estimate the line
parameters of a coaxial line from its physical dimensions and the dielectric constant of the
insulating material between the conductors. The line parameters of a transmission line have been
discussed in Section 5.7, and we only present the values for a coaxial line here. Referring to
Figure 9.25, we have the following values for capacitance and inductance:
C = 24ε / log (d1 / d2) pF / m
L = 0.46 log (d1 / d2) μH / m
where ε is the dielectric constant of the insulating material. Therefore, the characteristic
impedance Z0 is given by

(9.21)

The attenuation constant Ac for a coaxial cable at an operating bandwidth of F MHz is given
by
(9.22)
where a, b and c are constants dependent upon the physical parameters of the cable. For long
haul transmission, two standard sizes are used. The dimensions of the standard cables and the
associated values of a, b and c are presented in Table 9.4. The values of phase velocities Vp are
also indicated for the two cables in Table 9.4 for high frequency (see Section 5.7) assuming PVC
as the insulating material.
Table 9.4 Standard Coaxial Cables

d2 (mm) d1 (mm) a b c Vp (m/s)

Size 1 1.2 4.4 0.066 5.15 0.0047 1.8 × 108

Size 2 2.6 9.5 0.013 2.305 0.003 1.8 × 108

Coaxial cables are usually buried at a depth of 90 −120 cm, depending on frost penetration in
a given locality. Repeaters for coaxial cables are placed at uniform intervals along the route. To
facilitate this, cable lengths are factory cut so that the splice occurs right at repeater locations.
The repeaters are of two types: secondary or dependent repeaters and primary or main repeaters.
Main repeaters have independent power sources and are installed in surface housing. Secondary
repeaters derive their power from the cable itself and are usually buried. Power to the dependent
repeaters is supplied by the main repeaters. A main repeater supplies power to about 15
secondary repeaters in each direction. The repeater spacing is dependent upon the size of the
cable and the frequency of operation. For 2.6/9.5 mm cable at 12 GHz, the dependent repeater
spacing is 4.5 km.
The design of a coaxial system involves the following:

1. Repeater design
2. Repeater spacing
3. Equalisation
4. Temperature considerations
5. Supervision and fault location
6. Power feed
7. Right of way

Consider a coaxial cable system 100 km long using 9.5 mm cable and capable of transmitting up
to 2700 voice frequency (VF) channels totalling to a bandwidth of about 12 MHz. At this
frequency, the cable attenuation is about 8.0 dB/km as calculated from Eq. (9.22) using the data
given in Table 9.4. For the 100-km length, the total attenuation works out to 800 dB. The first
choice is to design a single amplifier of 800 dB gain and plant the same at the front end of the
100-km section. However, it is not practicable to design an amplifier with a gain of 1080.
Obviously, one or two repeaters would not do the job. A practical approach is to use 25−30
repeaters of 26 −32 dB gain, each placed at an interval of 3− 4 km. Each repeater contributes to
thermal noise in the system and hence the number of repeaters is restricted by the upper limit for
noise. The system can be designed with one main repeater which is capable of powering about
15 dependent repeaters on either side. For this purpose, a power feed unit at the repeater usually
uses a supply voltage of 650 V d.c. between the inner and outer conductors and delivers a direct
current of 100 mA. The crosstalk performance of the coaxial cables is very good. Both far end
crosstalk and near end crosstalk attenuation is greater that 140 dB. Crosstalk poses no problem in
coaxial systems unlike shielded wire pairs.
Equation (9.22) indicates that the cable attenuation is proportional to the square root of the
frequency predominantly. As a result, voice channel signals, assigned to the high frequency
segments of the line frequency, are heavily attenuated compared to the ones assigned to the low
frequency carrier segments. Consequently, the voice channels at the high carrier frequency end
are considerably affected by noise. Equalisation ensures that the signal-to-noise ratio in each
voice channel is the same no matter where it is assigned in the line frequency spectrum. In other
words, equalisers attempt to make the amplitude response of the cable uniform throughout the
frequency range and reduce amplitude distortion. Amplitude equalisers may be broadly classified
as fixed and variable equalisers. Three types of coaxial cable amplitude equalisers are
commonly used:

1. Basic equalisers
2. Line build-out (LBO) networks
3. Design deviation equalisations.

Base equalisers are used when cable sections are of uniform length. The equalisation function
is built into the repeaters. The gain of the repeater amplifier is made proportional to the square
root of the frequency, thus compensating for the attenuation which is also proportional to the
square root of the frequency. The LBO networks are used when the cable sections are not
uniform. They are designed to cater to small variations in the nominal length of the cable. The
third type of equalisers compensate for design deviations of nominal design parameters of
standard dependent repeaters. A design deviation equaliser is installed, one for each 10, 15 or 20
repeaters to compensate for gross design deviations over that group of repeaters.
Variable or adjustable equalisers are designed to deal with gain frequency variations with time
caused by temperature variations and component aging. Gain variations are around 0.2% per °C.
Automatic gain regulations are achieved by sending a pilot tone at a fixed frequency periodically.
Any variation in gain noticed for the pilot tone is exactly compensated. This results in automatic
regulation for other frequencies as well although there may be small errors in compensation for
these frequencies. Another method of regulating the gain for temperature variations is to use a
thermistor buried in the ground near the repeater to monitor the temperature variations and adjust
the gain accordingly. Phase vs. frequency characteristics of the coaxial cables are well defined
and the amplitude equaliser may well be designed to perform delay equalisation as well. Usually,
in practical systems, a common equaliser is used. Sometimes, delay equalisers are designed
separately using tapped delay lines to ensure minimum intersymbol interference.
Supervision in coaxial cable system is concerned with monitoring the conditions of repeaters
remotely at a manned location. Different pilot tones are transmitted to different repeaters which
respond thereby giving the status.
Coaxial cable systems are manufactured for transmission rates ranging 34 −565 Mbps
presently. A practical design includes multiplexing/demultiplexing, line coding/decoding,
scrambling/descrambling and pulse shaping units. A typical 140-Mbps systems multiplexes from
34-Mbps streams and uses 7/9 stage scrambler unit and 4B3T line code. With this line code, the
symbol rate on the line works out to be 104 M bauds.