1. Chapter 4 Transmission Impairments and Multiplexing

2. The most significant impairments include
With any communications system, the signal that is received may differ from the signal that is transmitted, due to various transmission impairments.
Consequences:
For analog signals: degradation of signal quality
For digital signals: bit errors
The most significant impairments include
Attenuation and attenuation distortion
Delay distortion
Noise

3. Attenuation Attenuation: signal strength falls off with distance.
Depends on medium
For guided media, the attenuation is generally exponential and thus is typically expressed as a constant number of decibels per unit distance.
For unguided media, attenuation is a more complex function of distance and the makeup of the atmosphere.

4. Three considerations for the transmission engineer:
A received signal must have sufficient strength so that the electronic circuitry in the receiver can detect the signal.
The signal must maintain a level sufficiently higher than noise to be received without error.
These two problems are dealt with by the use of amplifiers or repeaters.

5. Attenuation Distortion
Attenuation is often an increasing function of frequency. This leads to attenuation distortion:
some frequency components are attenuated more than other frequency components.
Attenuation distortion is particularly noticeable for analog signals: the attenuation varies as a function of frequency, therefore the received signal is distorted, reducing intelligibility.

6. PHASE DISTORTION
Delay distortion occurs because the velocity of propagation of a signal through a guided medium varies with frequency.
Various frequency components of a signal will arrive at the receiver at different times, resulting in phase shifts between the different frequencies.

7. Delay distortion is particularly critical for digital data
Some of the signal components of one bit position will spill over into other bit positions, causing intersymbol interference, which is a major limitation to maximum bit rate over a transmission channel.

8. Noise
For any data transmission event, the received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere between transmission and reception.
The undesired signals are referred to as noise, which is the major limiting factor in communications system performance.

9. Transmission Impairments

10. Four categories of noise:
Thermal noise
Intermodulation noise
Crosstalk
Impulse noise

11. Noise Thermal noise (or white noise)
Due to thermal agitation of electrons
It is present in all electronic devices and transmission media, and is a function of temperature.
Cannot be eliminated, and therefore places an upper bound on communications system performance.

12. Intermodulation noise
When signals at different frequencies share the same transmission medium, the result may be intermodulation noise.
Signals at a frequency that is the sum or difference of original frequencies or multiples of those frequencies will be produced.
E.g., the mixing of signals at f1 and f2 might produce energy at frequency f1 + f2. This derived signal could interfere with an intended signal at the frequency f1 + f2.

13. Crosstalk
It is an unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pairs.
Typically, crosstalk is of the same order of magnitude as, or less than, thermal noise.

14. Impulse noise
Impulse noise is non-continuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude.
It is generated from a variety of cause, e.g., external electromagnetic disturbances such as lightning.
It is generally only a minor annoyance for analog data.
But it is the primary source of error in digital data communication.

15. LEVEL
Level mean Signal Magnitude, level could be comparative. The output of an amplifier is 20 dB higher than the input.
In telephony it measured in dBm (decible referenced to 1 mW), in wireless system used dBW (decible referenced to 12 watts), in video systems the unit measure is voltage

16. In telecommunication network, if level are too high, amplifiers become overloaded and other types distortion can occur. If the level are too low, customer satisfaction may suffer with a degrades loudness rating.
System level is important parameter for engineering telecommunication system.
dBm = dBm0 + dBr
Where,
dBr – decibels “reference”

17. Signal to Noise Ratio (SNR)
SNR ratio expresses in decibel the amount by which a signal level exceeds the noise within a specified bandwidth.
The types of material to be transmitted, minimum SNR to satisfy the customer and make the receiving instrument function within certain specified criteria.

18. The following SNR with the corresponding and instrument
voice : 40 dB based on customer satisfaction
voice : 45 dB
data : ~ 15 dB, based on a specified error rate and modulation type.
(S/N)dB = level (signal in dBm) – level (noise in dBm)

19. KEY POINTS
A major problem in designing a communications facility is transmission impairment, including attenuation, distortion, and various types of noise. For analog signals, transmission impairments introduce random effects that degrade the quality of the received information and may affect intelligibility. For digital signals, transmission impairments may cause bit errors at the receiver.

20. MULTIPLEXING
Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared.
Multiplexing is the set of techniques that allows the (simultaneous) transmission of multiple signals across a single data link. As data and telecommunications use increases, so does traffic.

21. Dividing a link into channels

22. Categories of multiplexing

23. Frequency-division multiplexing (FDM)

24. FDM process

25. FDM demultiplexing example

26. Example 4.1
Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands.
Solution
We shift (modulate) each of the three voice channels to a different bandwidth, as shown in Figure 6.6. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them as shown in Figure below

27. Example 4.1

28. Example 4.2
Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference?
Solution
For five channels, we need at least four guard bands. This means that the required bandwidth is at least
5 × × 10 = 540 kHz,
as shown in Figure below.

29. Example 4.2

30. Example 4.3
Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz. Design an appropriate configuration, using FDM.
Solution
The satellite channel is analog. We divide it into four channels, each channel having 1M/4=250-kHz bandwidth.
Each digital channel of 1 Mbps must be transmitted over a 250KHz channel. Assuming no noise we can use Nyquist to get:
C = 1Mbps = 2x250K x log2 L -> L = 4 or n = 2 bits/signal element.
One solution is 4-QAM modulation. In Figure 6.8 we show a possible configuration with L = 16.

31. Example 4.3

32. Analog hierarchy

33. FDM Impairment
Noise was the principle impairment of the analog network and a greater portion of this noise derived from the FDM equipment involved.
In digital network noise is secondary issues, degradation of bit error rate become the primary issuse.
Insufficient of SNR is just one of several possible causes of this degradation.