to obtain the same error rate with the internal or external

b. Additional transmitter filtering may be required

coder. For example, if a digital user requires an error

for system operation.

rate equal to or less than 1 x 10-5, the minimum (Eb/No),,

c. The signal power levels and variations expected

ratio needed to support this requirement using only

at the 70 MHz input to the receiver must be determined.

differential coding is +9.8 dB.

If the internal

This information is necessary so that the correct internal

coder/decoder is used, the (Eb/No),, ratio required is

range switch setting may be made during installation.

reduced by 2.7 dB to +7.1 dB. If an external KY-

d. If the fault alarm monitor output is to be used,

801/GSC (Viterbi) coder/decoder is used, the (Eb/No),,

the installer must be provided with sufficient information

ratio required is further reduced an additional 2.6 dB to

to connect and test this function.

+4.5 dB.

e. If the modem is not located in or near the earth

c. The requirement for use of error correcting

terminal, insure that the power losses caused by the

coding. equipment is determined by several factors.

interconnecting 70 MHz input and output cables or

These factors are the signal-to-noise density ratio (C/KT)

networks do not significantly degrade equipment

provided by the communications link, the implementation

operation. Also, insure that interconnecting cables or

loss of the PSK modem, the data rate, the bit error rate

networks provide sufficient bandwidth to support the

required by the digital user, and the link margin required.

expected symbol rates of the 70 MHz input and the 70

To aid in understanding how these factors enter into the

MHz output signals.

determination, consider the case in which a digital

communications system must support a 50 kb/s digitized

voice link. The link margin required is 1.0 dB, and the

3

digital user needs a worst-case bit error rate of 1 x 10-

The signal-to-noise density ratio (C/KT) provided by the

system, which is determined by the earth terminal figure

f. Interface cables or networks, grounding devices,

of merit, geographical location at the terminal,

cable routes, and patching devices must be carefully

implementation losses of the terminal equipment, and

chosen to minimize cross talk between the 70 MHz

power capability or allocation of the satellite, must first be

determined. Further assuming that analysis of the

system indicates an available C/KT of 57.1 dB-Hz, the

effective C/KT may then be determined by subtracting

a. The transmission of digital data over a satellite

the link margin and the PSK Modem implementation loss

communications link typically results in random errors in

as follows:

the data sent to the digital user from the PSK modem

receiver. These errors are primarily caused by the noise

Effective C/KT = System C/KT (Link Margin + PSK

inherent in the satellite link. The performance of a digital

modem Implementation Loss)

communication link is generally measured in terms of the

Effective C/KT = 57.1 (1.0 + 1.1) = 55 dB Hz

average bit error rate at the digital output of the link.

Knowing the data rate (50 kb/s), the equivalent signal-to-

Average bit error rate is determined by dividing the

number of bit errors occurring in a large number of bits

equal to the data rate (R1))can be determined from the

by the total number of bits in the sample. The resulting

formula:

number is the probability of error associated with each

bit. For example, if it is determined that 40 errors have

(Eb/No) = C/KT 10 log R1)

occurred in a total of 10,000 bits, the average bit error

rate (or bit error probability) is 40/10,000, or 4 x 10-3.

This function is plotted for convenience in figure 2-5.

b. The bit error rate produced by a satellite

The resulting (Eb/No) is +8 dB. As shown in the

communications link is a function of the data rate and the

performance curves of figure 2-4, the PSK modem

signal-to-noise ratio present at the PSK modem receiver

operating only with differential coding will provide an

4

input. If a noise bandwidth equal to the bit rate is always

error rate of 4 x 10- at Eb/No, = 8 dB. Error-correcting

used as a reference, the bit error rate as a function of

coding would therefore not be required to provide a bit

-3

signal-to-noise ratio (Eb/No) for any data rate can be

error rate of less than 1 X 10 .

shown on one curve (figure 2-4). As shown in figure 2-4,

if only differential coding is used, the E N,, ratio required

to obtain a low bit error rate is higher than that required

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