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Communication Theory (EC 2252)

Communication Theory (EC 2252). Prof.J.B.Bhattacharjee K.Senthil Kumar ECE Department Rajalakshmi Engineering College. Review of Spectral characteristics. Periodic and Non-periodic Signals: A signal is said to be periodic, if it exhibits periodicity. i.e.,

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Communication Theory (EC 2252)

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  1. Communication Theory (EC 2252) Prof.J.B.Bhattacharjee K.Senthil Kumar ECE Department Rajalakshmi Engineering College

  2. Review of Spectral characteristics Periodic and Non-periodic Signals: A signal is said to be periodic, if it exhibits periodicity. i.e., x(t +T)=x(t) , for all values of t. Periodic signal has the property that it is unchanged by a time shift of T. A signal that does not satisfy the above periodicity property is called a non-periodic signal. Periodic signals can be represented using the Fourier Series. Non-periodic signals can be represented using the Fourier Transform. Both Fourier series and Fourier Transform deal with the representation of the signals as a combination of sine and cosine waves.

  3. Fourier Series • Fourier series: a complicated waveform analyzed into a number of harmonically related sine and cosine functions • A continuous periodic signal x(t) with a period T may be represented by: • x(t)=Σ∞k=1 (Akcos kω t + Bksin kω t)+ A0 • Dirichlet conditions must be placed on x(t) for the series to be valid: the integral of the magnitude of x(t) over a complete period must be finite, and the signal can only have a finite number of discontinuities in any finite interval

  4. Fourier Series Equations The Fourier series represents a periodic signal Tp in terms of frequency components: We get the Fourier series coefficients as follows: The complex exponential Fourier coefficients are a sequence of complex numbers representing the frequency component ω0k.

  5. Periodic signals represented by Fourier Series have Discrete spectra.

  6. The Fourier Transform Fourier transform is used for the non-periodic signals. A Fourier transform converts the signal from the time domain to the spectral domain. Continuous Fourier Transform:

  7. Non-periodic signals represented by Fourier transform have Continuous spectra.

  8. Fourier Transform PairsNote: Π stands for rectangular function. Λ stands for triangular function.

  9. Introduction to Communication Systems • Communication – Basic process of exchanging information from one location (source) to destination (receiving end). • Refers – process of sending, receiving and processing of information/signal/input from one point to another point. Source Flow of information Destination Figure 1 : A simple communication system

  10. Electronic Communication System – defined as the whole mechanism of sending and receiving as well as processing of information electronically from source to destination. • Example – Radiotelephony, broadcasting, point-to-point, mobile communications, computer communications, radar and satellite systems.

  11. Objectives • Communication System – to produce an accurate replica of the transmitted information that is to transfer information between two or more points (destinations) through a communication channel, with minimum error.

  12. NEED FOR COMMUNICATION • Interaction purposes – enables people to interact in a timely fashion on a global level in social, political, economic and scientific areas, through telephones, electronic-mail and video conference. • Transfer Information – Tx in the form of audio, video, texts, computer data and picture through facsimile, telegraph or telex and internet. • Broadcasting – Broadcast information to masses, through radio, television or teletext.

  13. Terms Related To Communications • Message – physical manifestation produced by the information source and then converted to electrical signal before transmission by the transducer in the transmitter. • Transducer – Device that converts one form of energy into another form. • Input Transducer – placed at the transmitter which convert an input message into an electrical signal. • Example – Microphone which converts sound energy to electrical energy. Input Transducer Electrical Signal Message

  14. Output Transducer – placed at the receiver which converts the electrical signal into the original message. • Example – Loudspeaker which converts electrical energy into sound energy. • Signal – electrical voltage or current which varies with time and is used to carry message or information from one point to another. Electrical Signal Output Transducer Message

  15. Elements of a Communication System • The basic elements are : Source, Transmitter, Channel, Receiver and Destination. Information Source Transmitter Channel Transmission Medium Receiver Destination Noise Figure : Basic Block Diagram of a Communication System

  16. Function of each Element. • Information Source – the communication system exists to send messages. Messages come from voice, data, video and other types of information. • Transmitter – Transmit the input message into electrical signals such as voltage or current into electromagnetic waves such as radio waves, microwaves that is suitable for transmission and compatible with the channel. Besides, the transmitter also do the modulation and encoding (for digital signal).

  17. Block Diagram of a Transmitter Transmitting Antenna 5 minutes exercise; Describe the sequence of events that happen at the radio waves station during news broadcast? Audio Amplifier Modulator RF Amplifier Modulating Signal Carrier Signal

  18. Channel/Medium – is the link or path over which information flows from the source to destination. Many links combined will establish a communication networks. • There are 5 criteria of a transmission system; Capacity, Performance, Distance, Security and Cost which includes the installation, operation and maintenance. • 2 main categories of channel that commonly used are; line (guided media) and free space (unguided media)

  19. Receiver – Receives the electrical signals or electromagnetic waves that are sent by the transmitter through the channel. It is also separate the information from the received signal and sent the information to the destination. • Basically, a receiver consists of several stages of amplification, frequency conversion and filtering.

  20. Block Diagram of a Receiver Receiving Antenna • Destination – is where the user receives the information, such as loud speaker, visual display, computer monitor, plotter and printer. RF Amplifier Intermediate Frequency Amplifier Demodulator Destination Audio Amplifier Mixer Local Oscillator

  21. Analog Modulation • Baseband Transmission • Baseband signal is the information either in a digital or analogue form. • Transmission of original information whether analogue or digital, directly into transmission medium is called baseband transmission. • Example: intercom (figure below) Voice Speaker Audio Amplifier Voice Microphone Audio Amplifier Wire

  22. Baseband signal is not suitable for long distance communication…. • Hardware limitations • Requires very long antenna • Baseband signal is an audio signal of low frequency. For example voice, range of frequency is 0.3 kHz to 3.4 kHz. The length of the antenna required to transmit any signal at least 1/10 of its wavelength (λ). Therefore, L = 100km (impossible!) • Interference with other waves • Simultaneous transmission of audio signals will cause interference with each other. This is due to audio signals having the same frequency range and receiver stations cannot distinguish the signals.

  23. Modulation • Modulation – defined as the process of modifying a carrier wave (radio wave) systematically by the modulating signal. • This process makes the signal suitable for transmission and compatible with the channel. • Resultant signal – modulated signal • 2 types of modulation; Analog Modulation and Digital Modulation. • Analogue Modulation – to transfer an analogue low pass signal over an analogue bandpass channel. • Digital Modulation – to transfer a digital bit stream the carrier is a periodic train and one of the pulse parameter (amplitude, width or position) changes according to the audio signal.

  24. Purpose of Modulation Process in Communication Systems • To generate modulated signal that is suitable for transmission and compatible with the channel. • To allow efficient transmission – increase transmission speed and distance, eg; • By using high frequency carrier signal, the information (voice) can travel and propagate through the air at greater distances and shorter transmission time • Also, high frequency signal is less prone to noise and interference. Certain types of modulation have the useful property of suppressing both noise and interference • For example, FM use limiter to reduce noise and keep the signal’s amplitude constant. PCM systems use repeaters to generate the signal along the transmission path.

  25. Amplitude Modulation (AM) • Objectives:- • Recognize AM signal in the time domain, frequency domain and trigonometric equation form • Calculate the percentage of modulation index • Calculate the upper sidebands, lower sidebands and bandwidth of an AM signal by given the carrier and modulating signal frequencies • Calculate the power related in AM signal • Define the terms of DSBSC, SSB and VSB • Understand the modulator and demodulator operations

  26. Introduction • Modulation • The alteration of the amplitude, phase or frequency of an oscillator in accordance with another signal. • Input signal is encoded in a format suitable for transmission • A low frequency information signal is encoded over a higher frequency signal • Carrier Signal • Sinusoidal wave, • Modulating Signal/Base band • Information signal, • Modulated Wave • Higher frequency signal which is being modulated • Modulation Schemes • To counter the effects of multi path fading and time-delay spread

  27. Modulation Schemes Carrier Signal, Vc Modulating Signal, Vm Modulated Signal VAM VPM VFM

  28. Amplitude Modulation • Time Domain • Frequency Domain

  29. AM Modulator Modulator Information Signal Output Carrier Signal

  30. Amplitude Modulation Vc - Vc Vm - Vm Vam - Vam

  31. Modulation Index • Modulation Index, m • Indicates the amount that the carrier signal is modulated. • It is an expression of the amount of power in the sidebands. • Modulation level ranges = 0-1 where • 0 = no modulation • 1 = full modulation • >1 = distortion

  32. Modulation Index

  33. Modulation Index Vmax Vmin Vmax (p-p) Vmin (p-p)

  34. Modulation Index m = 0 m = 0.5 m = 1

  35. Bandwidth VC • Bandwidth for AM signal, fc-fm fc fc+fm

  36. Power Distributions fc-fm fc fc+fm • Total transmitted power, PT • If R= 1,

  37. Double Side Band Suppressed Carrier (DSBSC) • It is a technique where it is transmitting both the sidebands without the carrier (carrier is being suppressed/cut) • Characteristics: • Power content less • Same bandwidth • Disadvantages - receiver is complex and expensive.

  38. Single Side Band (SSB) • Improved DSBSC and standard AM, which waste power and occupy large bandwidth • SSB is a process of transmitting one of the sidebands of the standard AM by suppressing the carrier and one of the sidebands • Advantages: • Saving power • Reduce BW by 50% • Increase efficiency, increase SNR • Disadvantages • Complex circuits for frequency stability

  39. Vestigial Side Band (VSB) • VSB is mainly used in TV broadcasting for their video transmissions. • TV signal consists of • Audio signal – transmitted by FM • Video signal – transmitted by VSB • A video signal consists a range of frequency and fmax = 4.5 MHz. • If it transmitted using conventional AM, the required BW is 9 MHz (BW=2fm). But according to the standard, TV signal is limited to 7 MHz only • So, to reduce the BW, a part of the LSB of picture signal is not fully transmitted.

  40. Vestigial Side Band (VSB) • The frequency spectrum for the TV signal / VSB: Video Carrier Audio Carrier Total TV signal bandwidth = 7 MHz 4.5 MHz Upper Video Bands Lower Video Bands Upper Audio Bands Lower Audio Bands f (MHz) 1.25 0 5.75 6.75 7.0 6.25

  41. B Carrier D A C Modulating Signal Output E Modulator Circuits

  42. Modulator Circuits A. Modulating Signal B. Carrier C. Sum of carrier and modulating signal D. Diode current E. AM output across tuned circuit

  43. A B C AM Signal Demodulator

  44. Demodulator A. AM signal B. Current pulses through diode C. Demodulating signal D. Modulating signal

  45. Frequency Modulation (FM) • Objectives:- • Recognize FM signal in the time domain, frequency domain and trigonometric equation form • Calculate the percentage of modulation index • Calculate the upper sidebands, lower sidebands and bandwidth of an FM signal by Carsons’s Rule and Bessel Function Table • Calculate the power related in FM signal • Understand the modulator and demodulator of FM

  46. Introduction • FM is the process of varying the frequency of a carrier wave in proportion to a modulating signal. • The amplitude of the carrier is kept constant while its frequency is varied by the amplitude of the modulating signal. • In all types of modulation, the carrier wave is varied by the AMPLITUDE of the modulating signal. • FM signal does not have an envelope, therefore the FM receiver does not have to respond to amplitude variations  it can ignore noise to some extent.

  47. Frequency Modulation

  48. Frequency Modulation • The importance features about FM waveforms are: • The frequency varies • The rate of change of carrier frequency changes is the same as the frequency of the information signal • The amount of carrier frequency changes is proportional to the amplitude of the information signal • The amplitude is constant

  49. Frequency Modulation • Carrier Signal • Sinusoidal wave • Modulating Signal/Base band • Information signal • Modulated Wave • Higher frequency signal which is being modulated • Where

  50. Frequency Modulation • Time Domain • Frequency Domain

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