Chapter 1: Introduction

1. Chapter 1: Introduction By: Dr. Mouaaz Nahas Introduction to Communication 8022305-3 Umm Al-Qura University Electrical Engineering Department

2. Communication System Model Input Message Input Signal Transmitter Input Transducer Transmitted Signal Distortion & Noise Channel Received Signal Receiver Output Transducer OutputSignal OutputMessage

3. Communication System Components • Source: originates a message, such as a human voice, a television picture, a teletype message (used for telegraph) or data. • Input transducer: converts nonelectrical messages (e.g. human voice, etc.) into electrical waveforms (signals) called baseband or message signal. • Transmitter: modifies (adjusts) the message signal to make it possible (efficient) for transmission. • Channel: is the medium on which the modified (ready) signal is transmitted. E.g. wire, a coaxial cable, an optical fiber or a radio link (i.e. wireless). • Receiver: re-processes the received signal by undoing the modification made at the transmitter and channel. • Output transducer: converts electrical signal into its original nonelectrical baseband form (i.e. message). • Destination: is the unit to which the message is communicated (transmitted).

4. Distortion • Distortion: is any deformation (alteration) of the signal due to traveling in the channel. • Examples: attenuation or expansion in the signal width. Attenuation More Attenuation

5. Attenuation (Example of Distortion) • Attenuation: is a reduction in the strength of a signal due to loss of energy while passing through different materials. • Cables usually have attenuation factor measured per unit distance. The less the attenuation per unit distance, the more efficient the cable. • Repeaters – with amplifiers – can be used to compensate for loss of signal strength (attenuation). However, this is not helpful with the presence of noise (as will be seen later).

6. Noise • Noise: is undesirable (unwanted) signal. • Noise signal is random (probabilistic) and unpredictable.

7. Types of Noise • External noise: • Interference from other signals transmitted on nearby channels. • Automobile ignition radiation. • Other sources. • Internal noise: • Thermal motion of electrons in conductors. • Random emission. • Other sources inside the electronic devices. • With proper care, this type of noise can be minimized or eliminated. • The effect of internal noise can be reduced but can never be eliminated.

8. Signal-to-Noise Ratio (SNR)

9. Analog and Digital Messages • Analog Message: • Definition: it has data which varies (changes) over a continuous range (it can have unlimited number of values). • Examples: temperature, car speed and speech waveform. • Digital Message: • Definition: it has a finite (limited) number of values. • Examples: • Printed English language (50 symbols  M-ary message). • Morse-coded telegraph message (2 symbols  binary message).

10. Example: Morse Code Alphabet Also, punctuation marks have Morse Codes • The space between letters is three units. • The space between words is seven units. • The length of a dot is one unit. • A dash is three units. • The space between parts of the same letter is one unit.

11. Morse Code Alphabet (Cont.) Famous Example

12. Types of Information Bits (Binary Digits)

13. Analog / Digital vs. Continuous / Discrete Signals Digital & Continuous Analog & Continuous Analog & Discrete Digital & Discrete

14. Noise Immunity of Digital Signals In Morse code: Generally, the task of the receiver in any communication system is to extract a message from a distorted and noisy signal at the channel output.

15. Noise Immunity of Digital Signals (Cont.) Distorted signal Due to channel Due to channel Distorted noisy signal Ripples from the addition of noise + + + + – – – – – The data can be recovered correctly as long as the distortion and the noise are within limits.

16. Noise Immunity of Digital Signals (Cont.) Can easily be recovered from the previous signal + + + + – – – – – In contrast, the waveform (signal shape) in analog messages is so important, as even a small distortion or interference can cause error in the received signal Information Clearly, a digital communication system can better withstand (resist) noise and distortion, thus it is more immune to noise.

17. Viability of Regenerative Repeaters in Digital Communication • In digital systems: • Repeater stations are placed along the communication path at distances short enough to ensure that noise and distortion remains within a limit. • At each repeater station: • The incoming pulses are detected. • New clean pulses are generated. • The new pulses are transmitted to the next repeater station. • This process prevents the accumulation of noise and distortion along the path by cleaning the pulses periodically at the repeater station.

18. Viability of Regenerative Repeaters in Digital Communication (Cont.) Tx Rx Tx Rx Tx Rx Tx Rx Ceiling Function

19. Viability of Regenerative Repeaters in Digital Communication (Cont.) • In analog systems: • There is no way to avoid accumulation of noise and distortion along the path. • Amplification is of little help, because it enhances the signal and noise in the same proportion (as discussed earlier). • Signal quality can be improved by filtering the signal then amplifying it. • Repeaters used in analog systems basically consist of filters and then amplifiers (they are not “regenerative” repeaters).

20. Viability of Regenerative Repeaters in Digital Communication (Cont.) For random signals instead of Fourier Transform BPF

21. Viability of Regenerative Repeaters in Digital Communication (Cont.) • Example 1: • At each regenerative repeater station in digital communication system, the received signal is: • a) Filtered and then amplified. • b) Amplified and then filtered. • c) Detected, corrected and retransmitted. • d) Detected and a clean signal is transmitted. Analog repeaters • Example 2: • A digital signal with 1 mV amplitude is to travel along 3000 km. If the amplitude is attenuated by 0.2 mV every 10 km, find the minimum number of regenerative repeaters required to withstand (resist) this distortion (given that the minimum amplitude which can be interpreted by the receiver as high voltage is 0.2 mV). Answer is 74

22. Why Digital over Analog Systems? • Due to the following advantages of digital systems over analog ones: • Noise immunity. • Viability of regenerative repeaters. Almost all communication systems being installed today are digital (very few are excepted)

23. So why do we study analog communication? Old analog communication facilities such as AM and FM radio broadcasting are still in use (quality of the received signal is still acceptable).

24. So why do we study analog communication? (Cont.) The only way to understand digital communication is to study the modulation and demodulation techniques used in analog communication systems.

25. Frequency Spectrum • Frequency spectrum allocation is the division of the electromagnetic spectrum into radio frequency bands. • This spectrum management is regulated by governments in most countries. • In Saudi Arabia, the Communications and Information Technology Commission (CITC) is responsible of all communication and information technology issues including the allocation of radio frequencies. • Radio propagation does not stop at national boundaries. Due to technical and economic reasons, governments have sought to harmonize the allocation of RF bands and their standardization.

26. Frequency Spectrum (Cont.) • International Telecommunication Union (ITU) is a specialized agency of the United Nations (UN) that is responsible for issues that concern information and communication technologies, for example: • Coordinates the shared global use of the radio spectrum. • Promotes international cooperation in assigning satellite orbits. • Works to improve telecommunication infrastructure. • assists in the development and coordination of worldwide technical standards.

27. Frequency Spectrum (Cont.) RF MW

28. Frequency Spectrum (Cont.) Microwave (MW) Range Radio Frequency (RF) Range RF Spectrum with Communication Services

29. Channel and Signal Bandwidth • Bandwidth: is a measure of the width of a range of frequencies from the minimum to the maximum frequency values, measured in hertz (Hz ≡ 1/s). • Channel bandwidth: is the range of signal bandwidths allowed by a communication channel without significant loss of energy (attenuation).

30. Channel and Signal Bandwidth (Cont.)

31. Channel and Signal Bandwidth (Cont.) • Likewise, each signal also has a bandwidth B that measures the maximum range of its frequency components (which allows the signal to transmit with reasonable fidelity). This is constrained (limited) by the transmitter. • In continuous-time signals, Hz means number of cycles (oscillations) per second. • In discrete-time signals, Hz means number of samples per second.

32. Channel and Signal Bandwidth (Cont.) • To understand the definition of signal bandwidth, consider the speech (audio) signal which has a maximum frequency of 20 KHz. • However, the bandwidth of audio signal varies depending on application, for example:

33. Channel and Signal Bandwidth (Cont.) Positive Frequency Spectrum The Complete Spectrum Speech Spectrum (for Intelligibility Applications)

34. Channel and Signal Bandwidth (Cont.) The Complete Speech Spectrum (Positive Side Only) Intelligibility System Bandwidth (e.g. Audio Telephone) High-Fidelity System Bandwidth (e.g. Audio CD)

35. Channel and Signal Bandwidth (Cont.) • The faster a signal changes, the higher its maximum frequency (hence bandwidth). • A signal rich in content (that changes quickly) have larger bandwidth than a signal that is dull or varies vary slowly. For example: • High bandwidth: a battle scene in a movie. • Low bandwidth: a video of sleeping animals. • A signal can be successfully sent over a channel if the channel bandwidth exceeds the signal bandwidth.

36. Signal Power

37. Signal Power (Cont.) • In a given communication channel, one resource may be more available than the other. For example: • A typical telephone channel has a limited bandwidth (~3 KHz), but the power is less restrictive (power is taken from the main power supply). • On the other hand, in space vehicles, huge bandwidth is available but the power is severely limited (no power source is available on space, except the batteries installed in the vehicle).

38. Sinusoidal Waveform The sinusoidal signal (waveform) is represented mathematically as:

39. Sinusoidal Waveform (Cont.)

40. Modulation • Analog signal generated by the message source is called “baseband signal” (such a signal has low frequency components, recall the speech spectrum shown above). • Baseband signals may be directly transmitted without modulation (e.g. audio telephone signal with approx. 3 KHz bandwidth is transmitted on twisted pair wire with almost the same bandwidth). • However, baseband signals are not always suitable for direct transmission. • When signal and channel frequencies do not match, channel cannot be moved.

41. Modulation (Cont.) • In this case, the signal must be moved to the right channel bandwidth. • Thus, the message signal must be modified for possible (efficient) transmission. This modification is called “modulation”. The need for modulation

42. Modulation (Cont.)

43. Modulation (Cont.) Deterministic Signal Probabilistic (Random) Signal Probabilistic (Random) Signal

44. Modulation (Cont.)

45. Important Reasons for Modulation Why do we need modulation? Simultaneous transmission of multiple signals (multiplexing). Ease of radiation / transmission on radio waves.

46. Important Reasons for Modulation (Cont.) Ease of radiation / transmission on radio waves: Electromagnetic spectrum in terms of wavelength.

47. Important Reasons for Modulation (Cont.) Speed of Light = 3×108 m/s Frequency in Hz

48. Important Reasons for Modulation (Cont.)

49. Important Reasons for Modulation (Cont.) Simultaneous transmission of multiple signals (multiplexing): • Modulation allows multiple signals to be transmitted at the same time in the same geographical area. • Consider TV signals, without modulation, multiple video signals will be interfering with each other (because all video signals inherently have the same bandwidth of approx. 4.5 MHz). • If carriers are chosen sufficiently far apart in frequency, the TV channels (signals) will not overlap (no interference will occur). • This process is called “frequency division multiplexing” (FDM).

50. Important Reasons for Modulation (Cont.) • FDM is the method of transmitting several signals simultaneously over nonoverlapping frequency bands (similar approach is used in AM and FM radio broadcasting). Example of FDM