AnalogDigitalCommunicationRecall_notes.pptx

communication notes

1. Communication systems

2. Outline

3. Introduction Communication system is a system which describes the exchange of information or data between two stations , i.e. between transmitter and receiver. Communication: It is the process of conveying or transferring information from one point to another. ( Or) It is the process of establishing connection or link between two points for information exchange . To transmit signals in communication system, it must be first processed by several stages, beginning from signal representation, to signal shaping until encoding and modulation.

4. Elements of Communication System : Information source : The message or information to be communicated originates in information source . Message can be words, group of words, code, data, symbols, signalsetc . Transmitter : The objective of the transmitter block is to collect the incoming message signal and modify it in a suitable fash Channel : Channel is the physical medium which connects the transmitter with that of the receiver. The physical medium includes copper wire, coaxial cable, fibre optic cable, wave guide and free space or atmosphere. ion ( ifneeded ),suchthat,itcanbetransmittedviathechosenchanneltothereceivingpoint.

5. Channel : Channel is the physical medium which connects the transmitter with that of the receiver. The physical medium includes copper wire, coaxial cable, fibre optic cable, wave guide and free space or atmosphere . Receiver : The receiver block receives the incoming modified version of the message signal from the channel and processes it to recreate the original (non-electrical) form of the message signal .

6. Modes of Communication: S implex, Half-Duplex and Full-Duplex) Simplex (SX) – one direction only, e.g. TV Half Duplex (HDX) – both directions but not at the same time, e.g. CB radio Full Duplex (FDX) – transmit and receive simultaneously between two stations, e.g. standard telephone system. Full/Full Duplex (F/FDX) - transmit and receive simultaneously but not necessarily just between two stations, e.g. data communications circuits

7. Medias for Communication •Telephone Channel •Mobile Radio Channel •Optical Fiber Cable •Satellite Channel

8. Modulation In modulation, a message signal, which contains the information is used to control the parameters of a carrier signal, so as to impress the information onto the carrier . It is the process of varying the characteristics of high frequency carrier in accordance with instantaneous values of modulating or message or baseband signal . (Or) It is a frequency translation technique which converts baseband or low frequency signal to bandpass or high frequency signal . The Messages The message or modulating signal may be either: analogue – denoted by m ( t ) digital – denoted by d ( t ) – i.e. sequences of 1's and 0's The message signal could also be a multilevel signal, rather than binary; this is not considered further at this stage . The Carrier The carrier could be a 'sine wave' or a 'pulse train'. Consider a 'sine wave' carrier: vc (t) = Vc cos ( ωct + φc )  •If the message signal m ( t ) controls amplitude – gives AMPLITUDE MODULATION AM •If the message signal m ( t ) controls frequency – gives FREQUENCY MODULATION FM •If the message signal m ( t ) controls phase- gives PHASE MODULATION PM or

9. Benefits or Need of Modulation To reduce the length or height of antenna For multiplexing or narrow banding or to use antenna with single or same length To reduce noise effect To avoid equipment limitation or to reduce the size of the equipment.

10. Demodulation Demodulation is the reverse process (to modulation) to recover the message signal m ( t ) or d ( t ) at the receiver.

11. Frequency spectrum

12. Low frequencies is used in submarines because low frequencies can penetrate in water more effectively.

13. Advantages of a Communication System 1. Speedy transmission: It requires only a few seconds to communicate through electronic media because it supports quick transmission. 2. Wide coverage: World has become a global village and communication around the globe requires a second only. 3 . Low cost: Electronic communication saves time and money. For example, Text SMS is cheaper than the traditional letter. 4. Exchange of feedback: Electronic communication allows the instant exchange of feedback. So communication becomes perfect using electronic media. 5. Managing global operation: Due to the advancement of electronic media, business managers can easily control operation across the globe. Video or teleconferencing e-mail and mobile communication are helping managers in this regard

14. Disadvantages of a Communication System 1.The volume of data: The volume of telecommunication information is increasing at such a fast rate that business people are unable to absorb it within the relevant time limit. 2 . The cost of development: Electronic communication requires huge investment for infrastructural development. Frequent change in technology also demands further investment. 3. Legal status: Data or information, if faxed, may be distorted and will cause zero value in the eye of law. 4 . Undelivered data: Data may not be retrieved due to system error or fault with the technology. Hence required service will be delayed

15. Types of AM

16. WHY DIGITAL? Digital techniques need to distinguish between discrete symbols allowing regeneration versus amplification Good processing techniques are available for digital signals, such as medium . Data compression (or source coding ) Error Correction(or channel coding )( A/D conversion ) Equalization Security Easy to mix signals and data using digital techniques

17. Information Representation Communication system converts information into electrical electromagnetic/optical signals appropriate for the transmission medium .  Analog systems convert analog message into signals that can propagate through the channel .  Digital systems convert bits(digits, symbols) into signals

18. Types of information: Voice, data, video, music, email etc. Types of communication systems: Public Switched Telephone Network ( voice,fax,modem ) Satellite systems Radio,TV broadcasting Cellular phones Computer networks (LANs, WANs, WLANs)

19. NOISE IN COMMUNICATION SYSTEMS Noise :It is anun wanted signal which tends to interfere with themodulating signal . Types of noise : Noise is basically divided into , 1.ExternalNoise 2.InternalNoise

20. Classification of Noise 1.External Noise : Atmospheric Noise : Radio noise caused by natural atmospheric processes, primarily lightening discharges in thunderstorms . Extraterrestrial Noise : Radio disturbances from sources other than those related to the Earth. Cosmic Noise : Random noise that originates outside the Earth’s atmosphere . Solar Noise : Noise that originates from the Sun is called Solar noise Industrial Noise : Noise generated by auto mobile ignition, aircrafts, electric motors, Switchgears, welding etc. 2. Internal Noise : Shot Noise : Random motion of electrons in the semiconductor devices generates shot noise. Thermal or Johnson’s Noise : Random motion of electrons in the resistor is called Thermal noise . Vn =KT0BR Where, K=Boltzmann constant , R=Resistance T0=Absolute temperature, B=Bandwidth

21. Noise Temperature and Noise Figure

22. Noise equivalent bandwidth

23. Figure of Merit

24. Receiver Model for Noise calculation •The receiver is combination of Band Pass Filter (BPF) and Demodulator . • The BPF is combination of RF Tuned Amplifier, Mixer and Local Oscillator whose bandwidth is equal to bandwidth of modulated signal at transmitter . • Channel Interconnects transmitter & receiver. Channel adds noise to the modulated signal while transmitting and it is assumed to be white noise whose Power Spectral Density is uniform . • BPF converts white noise into color or Bandpass noise or narrow bandpass noise .

25. Receiver model for noise calculation

26. Communication system model for noise calculation • The communication system model for noise calculation contains transmitter, channel and receiver . • Transmitter is replaced by modulator which converts low frequency modulating signal x(t) into high frequency bandpass signal with the help of carrier signal . • Channel is replaced or modelled as additive noise which adds white noise with PSD η/2 and it contains all frequencies .

27. Radio receiver measurements The important characteristics of super heterodyne radio receiver are , •Sensitivity •Selectivity •Fidelity Sensitivity : •It is defined as the ability of receiver to amplify weak signals •It is defined in terms of voltage which must be applied at the receiver input terminals to provide a standard output power at the receiver output. Sensitivity is expressed in milli volts •For practical receivers sensitivity is expressed in terms of signal power required to produce minimum acceptable output with minimum acceptable noise.

28. Selectivity : It is defined as the ability of receiver to reject unwanted signals. Selectivity depends on • Receiving frequency • Response of IF section

29. Fidelity : It is the ability of a receiver to reproduce all the modulating frequenciese qually .