Communication Theory course Introduction

Communication theory

1. Dr. Ahmed M. Abdelhaleem @ ECC Dep. Helwan University Communicatio Theory Fall 2022/23 Communications Theory Introduction Faculty of Engineering

2. Block Diagram of A Communication System All communication systems contain three main sub systems: Transmitter Channel Receiver Transmitter Receiver 2

3. Elements of a Communication System Information sources: Voice, music, images, video, and data (baseband signals) Signal: Information converted in electrical form and suitable for transmission is called a signal. Signals can be either analog or digital. Analog signals are continuous variations of voltage or current. Digital signals are those which can take only discrete stepwise values. Binary system that is extensively used in digital electronics employs just two levels of a signal. 3

4. Digital and Analog Sources Basic Definitions: Analog Information Source: An analog information source produces messages which are defined on a continuum. (E.g. :Microphone) Digital Information Source: A digital information source produces a finite set of possible messages. (E.g. :Typewriter) t x ( t ) t x ( t ) Analog Digital 4

5. Transmitter : The signal-processing block is used for more efficient transmission. Examples: In an analog system, the signal processor may be an analog low-pass filter to restrict the bandwidth of m ( t ). In a hybrid system, the signal processor may be an analog-to-digital converter (ADC) to produce digital signals that represent samples of the analog input signal. The transmitter carrier circuit converts the processed base band signal into a frequency band that is appropriate for the transmission medium of the channel( Band Pass). 5 Elements of a Communication System

6. Transmitter : Signal processing for transmission almost always involves modulation and may also include coding . In addition to modulation, other functions performed by the transmitter are amplification , filtering and coupling the modulated signal to the channel. Channel: Channels represents the path in which signals travel from transmitter to receiver. Very general classification of channels are: Wire : Twisted-pair telephone line, coaxial cable, waveguide, and fiber-optic cables. Wireless : Air vacuum, and seawater (Ultra sonic waves) . In general, the channel medium attenuates the signal so that the delivered information deteriorated from that of the source. Elements of a Communication System 6

7. Channel: The signal undergoes some amount of degradation from noise, interference and distortion Propagating signals degrade over distance Repeaters can strengthen signal and reduce noise 7 Elements of a Communication System

8. Noise: Noise refers to the unwanted signals that tend to disturb the transmission and processing of message signals in a communication system. The source generating the noise may be located inside or outside the system. Attenuation: The loss of strength of a signal while propagating through a medium is known as attenuation Range: It is the largest distance between a source and a destination up to which the signal is received with sufficient strength. Elements of a Communication System 8

9. Repeater: A repeater is a combination of a receiver and a transmitter. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Repeaters are used to extend the range of a communication system. Elements of a Communication System 9

10. Communication System Receiver Receiver 10 Receiver: The receiver takes the corrupted signal at the channel output and converts it to be a base band signal that can be handled by the receiver’s base band processor. The base band processor cleans up this signal and delivers an estimate of the source information to the communication system output.  

11. Receiver: Other functions performed by the receiver: amplification (the received signal may be extremely weak), demodulation and filtering. In digital systems, the measure of signal deterioration is usually taken to be the probability of bit error P ( e ) – also called Bit Error Rate (BER) of the delivered data m ( t ). In analog systems, the performance measure is usually taken to be the Signal-to-noise Ratio (SNR) at the receiver output. 11 Elements of a Communication System

12. Digital vs. Analog signals Digital communications: Transmitted signals belong to a finite set of waveforms → The distorted signal can be recovered to its ideal shape, hence removing all the noise. Analog communications: Transmitted signals are analog waveforms, which can take infinite variety of shapes → Once the analog signal is distorted, the distortion cannot be removed. 12

13. Digital Communication Advantages Relatively inexpensive digital circuits may be used; Privacy is preserved by using data encryption; Data from voice, video, and data sources may be merged and transmitted over a common digital transmission system; In long-distance systems, noise dose not accumulate from repeater to repeater. Data regeneration is possible Errors in detected data may be small, even when there is a large amount of noise on the received signal ( Noise immunity ); Errors may often be corrected by the use of coding. Easier to be expanded over larger service areas because of standard interfaces (connectivity) Disadvantages Generally, more bandwidth is required than that for analog systems; Synchronization is required. 13

14. What makes a Communication System GOOD We can measure the “GOODNESS” of a communication system in many ways: How close is the estimate to the original signal m ( t ) Better estimate = higher quality transmission Signal to Noise Ratio (SNR) for analog m(t) Bit Energy to Noise spectral density ( Eb /No) for digital signals, consequently, the Bit Error Rate (BER) in digital signals . How much power is required to transmit s(t)? Lower power = longer battery life, less interference How much bandwidth B is required to transmit s(t)? Less B means more users can share the channel Exception: Spread Spectrum -- users use same B. How much information is transmitted? In analog systems information is related to B of m(t). In digital systems information is expressed in bits/sec. 14

15. Analog Signals 15 Frequency Domains and Spectrum A frequency-domain plot is concerned with only the peak value and the frequency The spectrum of a signal is the range of frequencies that it contains For periodic signal it is obtained using Fourier Series f o 2f o -2f o nf o [nf o ] -f o -nf o

16. Analog Signals 16 Frequency Domains and Spectrum For nonperiodic signals it is obtained using Fourier Transform -40 -30 -20 -10 10 20 30 kHz

17. Bandwidth 17 The spectrum of a signal is the range of frequencies it contains. The absolute bandwidth of a signal is the width of the spectrum. The time domain and frequency domain of a signal are inversely related. In particular, if a signal is strictly limited in time domain, then its spectrum is infinite and vise versa. Note : a signal can not be strictly limited in both domains. BW of a signal provides a measure of the extent of significant spectral content of the signal. When the signal is band limited, the BW is well defined as follows Bandwidth = B.W = f H – f L Hz f H f L f

18. Bandwidth 18 But when the signal is not strictly band limited, we will find difficulties in defining the BW. There is no universally accepted definition of the BW, But there are commonly used definitions, named: Null-to-null (zero crossing) BW 3-db bandwidth Effective (essential) bandwidth. All of these definitions try to find the band in which most of the energy of a signal is contained.

19. Signal to Noise Ratio (SNR or S/N) Signal to noise ratio shows the ratio of signal power to noise power at some point in a Telecom system expressed in decibels (dB) It is typically measured at the receiving end of the communications system BEFORE the detection of signal. Power often expressed in watts S/N = signal power/noise power Two cases of SNR: high SNR and low SNR

20. Signal to Noise Ratio (SNR) Ratio of the signal power level to the noise power level. Express in logarithmic function:  

21. Channel capacity Shannon showed that a channel capacity C (bit/s) could be calculated such that the rate of information R (bits/s) is less than C , the probability of error would approach zero. The equation for C is given by: Where B is the channel bandwidth in hertz (Hz) and S/N (or SNR) is the signal to noise power ratio ( watts/watts, not dB ) at the input to the digital receiver. C is the maximum reliable transmission rate over an ideal channel and reliable means error rate can be made arbitrarily small by proper coding . 21

22. Course Assessment Plan 22 References: Simon Haykin “Communication Systems”,4 th Edition Simon Haykin “Digital Communication systems”, 2014 Leon. W. Couch “ Digital and Analog Communication Systems”, eighth Edition, Pearson Education, 2013. Lab. 20 Midterm 20 Assignments Final exam 100

23. Items/Topics Intro to Digital Comm. Systems Pulse Modulation: Sampling theory. PAM, PWM, and PPM Generation and demodulation. Pulse modulated system & SNR. Analog-to-Digital conversion: Pulse Code Modulation (PCM), and ADPCM. Delta Modulation - & ADM. Linear Predictive Coding. Voice Synthesis and VOCODERS. Syllabus

24. Items/Topics Digital Multiplexing: E- carrier systems T – carrier systems Piesochronous Digital Hierarchy (PDH). Synchronous Digital Hierarchy (SDH).