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Table 4.4
Typical Forms of Encoding Applied to Microwave Links over Communications
Satellites
Impact on Data
Encoding Technique
Application
Benefits
Throughput
Forward error
Error rate reduction
Lower error rate, or
Increases the output
correction
reduced power
bit rate to
requirement (
E
b
/
N
o
)
accommodate
redundant bits
Compression
Reduces the total bit
Less data to send or
Increases throughput;
count, either lossless
store—better
may reduce quality
or lossy
utilization of the link
and introduce delay
and system
Encryption
Information security
Makes data private
Generally, no increase
and difficult to
in data rate; involves
corrupt
complex management
and could introduce
delay
Protocol adaptation
Data communications
Improves the user
May increase actual
networks, typically
experience by
throughput;
using the Internet
countering satellite
introduces
Protocol suite
link impairments,
complexity; tied to the
(TCP/IP)
such as bit errors and
type of information
delay
transfer or application
136
Microwave Link Engineering
of encoding along with their benefits and characteristics. Forward error correction
(FEC) is one type of encoding that is always applied as it can only improve the
quality of transmission in terms of the bit error rate (BER). A reduction in BER
can be taken either as a quality enhancement or to reduce the amount of transmitted
power (alternatively, the antenna size). Modern FEC techniques use combinations
of individual error correcting codes through the process of concatenation.
In many cases, FEC is incorporated into the modulation and demodulation
function, which are performed by the modem. Compression techniques fall into
two categories: lossless compression, which only removes useless bits that convey
no information, and lossy compression, where information is removed but is
intended to provide sufficient quality to satisfy the user and application. Information
that starts out in analog form, such as voice, image, and video, can employ lossy
compression as users are accustomed to some amount of impairment. However,
the amount of compression deserves careful review by the intended audience to
assure that the service is not compromised as to acceptability. Encryption is highly
desirable on satellite links as it is a relatively simple matter to intercept satellite
transmissions and potentially to introduce either bogus data or to otherwise disrupt
information transfer. Protocol adaptation was discussed in Chapter 3 and is very
effective for reducing the perceived delay of Internet services over GEO satellites.
The output of the encoding stage is referred to as the baseband. It is the purpose
of the modulator to take the baseband and apply it to an RF carrier. The type of
modulator used in a microwave station often handles a wideband baseband input
such as that obtained from a high-capacity data stream measured in megabits per
second. In SCPC service, the output of the modem is kept on to allow a continuous
stream of data to be uplinked to the satellite. TDMA operation, on the other hand,
requires that the modem transmit in noncontinuous bursts. That is because the RF
channel is being shared by multiple Earth stations that transmit in different time
slots.
The opposite of modulation is simply demodulation, which is the process
whereby the baseband is removed from the carrier. The demodulator intially must
acquire the incoming carrier, demodulate the bit stream, and then produce the
baseband in a form that can be used by the decoder. The process is repeated
for each received transmission, particularly for TDMA operation. The decoding
circuitry corrects a majority of the errors produced by noise and interference on
the uplink and downlink.
4.3.1.2
Frequency Conversion and RF Amplication
The RF carrier coming from the modulator typically is not at microwave frequencies
but rather is centered within a standard frequency channel, the intermediate fre-
quency (IF). Most transmitting and receiving stations use 70 MHz as the IF, allowing
modulators and demodulators to be conveniently interchanged and interconnected
by patch cords and coaxial switches. In low-cost consumer equipment, the IF is
internal to the unit and hence need not follow any particular standard. Another
point is that there are cases where the RF bandwidth is larger than 140 MHz,
making it unfeasible to use 70 MHz as the IF (since the bandwidth would extend
below zero frequency). This can be overcome by directly modulating a carrier at
4.3
Microwave Transmitters and Receivers
137
the microwave frequency of transmission or, more likely, by using an adequately
high IF, such as 140 MHz.
For digital modulation, the bandwidth is directly proportional to the input
data rate:
B
=
a
m
⭈
R
where
R
is the data rate in bits per second and
a
m
is a constant determined by the
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