Signaltogaussiannoise Ratio On Pcm Repeatered Lines

As mentioned earlier, noise accumulation on PCM systems is not a crucial issue. However, this does not mean that Gaussian noise (or crosstalk or impulse noise) is unimportant.7 Indeed, it will affect error performance expressed as error rate. Errors are cumulative, and as we go down a PCM-repeatered line, the error performance degrades. A decision in error, whether a 1 or a 0, made anywhere in the digital system, is not recoverable. Thus such an incorrect decision made by one regenerative repeater adds to the existing error rate on the line, and errors taking place in subsequent repeaters further down the line add in a cumulative manner, thus deteriorating the received signal.

In a purely binary transmission system, if a 22-dB signal-to-noise ratio is maintained, the system operates nearly error free.8 In this respect, consider Table 6.1.

Table 6.1 Error Rate of a Binary Transmission System Versus Signal-to-rms-Noise Ratio

Error Rate

S/N (dB)

Error Rate

S/N (dB)

10-2

13.5

10-7

20.3

10-3

16.0

10-8

21.0

10-4

17.5

10-9

21.6

10-5

18.7

10-10

22.0

10-6

19.6

10-11

22.2

As discussed in Section 6.4, PCM, in practice, is transmitted on-line with alternate mark inversion (in the bipolar mode). The marks (1s) have a 50% duty cycle, permitting signal energy concentration at a frequency equivalent to half the transmitted bit rate. Thus it is advisable to add 1 dB or 2 dB to the values shown in Table 6.1 to achieve the desired error performance in a practical system.

7Gaussian noise is the same as thermal noise.

8It is against the laws of physics to have a completely error-free systems.

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