INTRODUCTION TO COMMUNICATION SYSTEMS AND TRANSMISSION MEDIA

Transcription

1 COMM.ENG INTRODUCTION TO COMMUNICATION SYSTEMS AND TRANSMISSION MEDIA 9/6/2014 LECTURES 1

2 Objectives To give a background on Communication system components and channels (media) A distinction between analogue and digital communication systems Modulation techniques Limitation imposed on communication system performance Multiplexing (MUX) and demultiplexing (DMUX) Guided and unguided media

3 1-Long distance communication. FIRE SIGNALS ARRANGED SIGNS SMOKE SIGNALS OPTICAL TELEGRAPH DRUM TELEGRAPH WIRE COMM SYS WIRELESS COMM SYS SATELLITE COMM SYS MOBILE COMM SYS 9/6/2014 LECTURES 3

4 2-History of communication systems 1837 Telegraph; first electronic comm. system which transfer information in form of dots, dashes and spaces (Samuel Morse) 1876 Telephone; transfer human conversation (Alexander Graham Bell & Thomas A. Watson) 1894 First Wireless Radio Signal (Guglielmo Marconi) 1908 Triode Vacuum Tube Amplifier (Lee Forest) 1920 AM Radio Broadcasting 1933 FM modulation (Edwin Howard Armstrong) 1936 TV Broadcast; FM broadcasting 1947 Transistor Invention 1960 Digital communications 1965 First Commercial Satellite Systems 1970 First Internet Node; Darpa-net, Aloha-net 1980 Development of TCP/IP; Transport Control Protocol, Internet Protocol 1990 First Digital Mobile System 1993 Invention of Web 9/6/2014 LECTURES 4

5 Transmitter baseband Input sensor message Information source 3-Communication system passband Information source: Generates the message to be transmitted This message could be words, codes, symbols, sound signal, music, video i.e., the source produces the required message to be transmitted Input Sensor: Converts message into electric signal (baseband signal ) Transmitter (Tx): Converts (Process) baseband signal into format appropriate for channel transmission (passband signal) and/or to enhance the communication link performance 9/6/2014 LECTURES 5

6 Transmitter baseband Input sensor message Information source passband Channel Transmission media RECEIVER Channel: It is the medium between transmitter and receiver in which the pass band signal is going to be transmitted through it. Most channels introduce distortion, noise, and interference Important characteristic of the channel -Type of channel (linear, nonlinear, ) -Power required to achieve S/N ratio -BW of the channel -Amplitude and phase response of the channel -Effects of the interference on channel performance 9/6/2014 LECTURES 6

7 Amplitude Transmitter baseband Input sensor message Information source passband Channel Transmission media RECEIVER Telephone channel -Provides service for voice signal as telephones -BW: KHz -S/N: about 30 db -Response : approximately linear Frequency, KHz Satellite channel -Provides service different communications, can reach remote places and can communicate with mobile devices -Operates at low GHz bands -Acts as repeaters with Rx antenna, transponder and Tx antenna -Uplink from ground station to satellite and downlink is the opposite way of transmission -Subjected to attenuation depending on power, frequency, antenna size and atmospheric condition i.e., the channel is considered as the center of operation of a communication system that determines the system capacity and quality of services (QoS) 9/6/2014 LECTURES 7

8 Transmitter Channel Receiver Input sensor passband baseband baseband Transmission media Output RECEIVER sensor message message Information source Destination information Receiver (Rx): Process received signal to estimate the baseband signal O/p sensor: Convert baseband signal to a message similar to generated one 9/6/2014 LECTURES 8

9 Amplitude 4- Modulation Baseband signal The signal which is not modulated is the baseband signal All electrical equivalent signal of the voice, data and picture signal are called baseband signal. Its spectrum occupies the frequency spectrum right from zero Hz Frequency, Hz Bandpass signal The modulated signal is the bandpass signal Shifting the baseband signal in frequency domain gives a band pass signal. Its spectrum occupies the frequency spectrum from certain lowest frequency f 1 to the highest frequency f 2 The signal in nature like ultrasound, visible light and radio waves are other form of not modulated bandpass signal Frequency, Hz

10 Baseband transmission The original information (sound) is converted into electrical signal (baseband signal) and placed directly on the telephone lines for transmission Also the computer data (baseband) is transmitted on the coaxial cables in the computer network The point is, the baseband signal of sound ( low frequency) cannot be transmitted for long distances in free space, so modulation should be used Why we do modulation? -Possible radiation low frequency signals from an antenna in a form of electromagnetic energy - The information signal is often occupy the same frequency so, it is subjected to interference - Reduction of the antenna physical dimension (l λ /2) - Possible multiplexing and demultiplexing - Improves the quality of reception

11 EX: Modulation avoids interference

12 Modulation Modulating signal (Low frequency) Modulator Modulated signal Carrier signal (High frequency) In modulation process, some parameters of the carrier ( amplitude, frequency and phase) are varied in accordance with the modulating signal In demodulation process, is the opposite to that in the transmitter

13 AM modulation

14 FM modulation

15 PM modulation

16 ASK,FSK,PSK modulation Information Carrier ASK modulation FSK modulation PSK modulation

17 5-Classifications of communications systems According to channel media: GUIDED CH. UNGUIDED CH. Copper Cables Optical Fiber RF BAND MW BAND Twisted pair Single mode cable Co-axial Multi-mode cable Waveguide Terrestrial Satellite Cellular Mobile LOS Portable NLOS Fixed 9/6/2014 LECTURES 17

18 According to modulation technique( Nature of information signal): ANALOG TR DIGITAL TR AM FM PM BAND PASS BASE BAND ASK FSK PSK SS TDM DS FH AM: Amplitude Modulation FM: Frequency Modulation PM: Phase Modulation ASK: Amplitude Shift Keying FSK: Frequency Shift Keying PSK: Phase Shift Keying SS: Spread Spectrum DS: Direct Sequence FH: Frequency Hoping TDM: Time Division Multiplexing WDM: Wavelength Division Multiplexing 9/6/2014 LECTURES 18

19 Analog devices: maintain exact physical analog of information e.g., microphone at which the voltage v(t) at the output of the microphone is proportional to the sound pressure Sound Sound Microphone (Sensor) Analog Communication System Speaker (Sensor) Digital devices: It uses numbers to process information we should use: Analog-to-digital conversion; ADC in transmission Digital-to-analog conversion; DAC in reception Sound Sound ADC Digital Communication System (Analogue Carrier) DAC Speaker

20 Comparison between analogue and digital transmission The transmitted modulated signal is analogue in nature AM, FM, PM, modulation methods are used Amplitude, frequency and phase variation represents the information or message Poor noise immunity in AM, and improved in FM and PM Difficult to use repeaters because it is difficult to separate out noise and signal Difficult to use coding techniques for error detection and correction Lower BW is required than that for the digital modulation techniques FDM is used for multiplexing Not suitable for military applications The transmitted modulated signal is train of digital pulses ASK, FSK, PSK, modulation methods are used The message is transmitted in the form of code words (bits) noise immunity is better for all types of modulation Repeaters could be used because it is possible to separate out noise and signal Coding techniques could for error detection and correction Higher BW channel due to the required higher bit rates TDM is used for multiplexing Suitable because of coding techniques

21 6-Multiplexing (MUX) and demultiplexing (DMUX) MUX: Combining several signals into a single composite signal and transmits it over a communication medium (single channel) (Sometimes the composite signal is used to modulate a carrier before transmission) Multiple input signals MUX Single communication channel (wire or radio) DMUX Original input signals DMUX: At the receiver end of a communication link the DMUX is used to sort out the signals into their original form MUX Analogue FDM WDM Digital TDM

22 FDMA A unique frequency slot is assigned to each user for the duration of their call TDMA At the same frequency a user can access the channel at any specific time SLOT1 SLOT1 SLOT1 SLOT1 Frequency The number of users within a cell is determined by the number of distinct frequency slots available Applications: High capacity phone lines, AM radio (0.530 MHz MHz, 10 KHz BW per station, FM radio (88 MHz MHz, 200 KHz BW per station, TV broadcasts 6 MHz BW per TV channel, First generation cell phones each user gets two 30 KHz channels sending, receiving Time Each user is assigned a distinct time slot to access the channel Applications: Implemented of telephone data transmission link

23 Examples on FDM: It is required to combine the 3 signals of BW KHz + guard band 0.5 KHz each onto one line of higher BW? Solution: -Allocate 4 KHz for each signal to prevent overlapping -Transform each signal to a different frequency range: Signal 1: KHz Signal 2: KHz Signal 3: KHz -At receiver, filters are used to isolate each channel 9/6/2014 LECTURES 23

24 Examples on TDM: It is required to combine the 3 signals with time unit 1ms for each signal? Solution: - Each signal is split into fixed-size units of time - Send a unit from each signal alternately Send signal 1 for 1ms Send signal 2 for 1ms Send signal 3 for 1ms Each signal has its own time slot; if it has nothing to send, the slot is left empty to preserve synchronization Tx ed Signals Rx ed Signals

25 LECTURES 25

26 7-Transmission modes Simplex (SX): Transmissions only in one direction Transmit-only or receive-only systems TX RX RX RX RX TX RX RX RX Example: -Radio station, TV broadcasting -Telemetry from satellite to earth -Telemetry from satellite to earth 9/6/2014 LECTURES 26

27 Half duplex [HDX]: Transmissions in both direction, not simultaneously Two way alternate systems i.e., at a time the system can either transmit or receive Example: -Security -Walky talky set TX/RX Full duplex [FDX]: Transmissions in both direction, simultaneously -Point-to-point communications -Two way simultaneous systems Example: telephone system TX/RX TX/RX TX/RX Full /Full duplex[f/fdx]: Transmissions in both direction, simultaneously... Multi-point communications TX/RX TX/RX TX/R Two-way simultaneous systems Example: data communications systems

28 8-Limitation on comm. sys. performance Bandwidth: The difference between the highest and lowest frequencies contained in the information Bandwidth of the channel: The bandwidth of the channel must be equal or greater than that of the information Noise: Any undesirable electrical energy that falls within the pass band of the signal For digital signal communication system the correspondence limitations could be the bit rate and the bit error rate Equipment limitation: The cost may exceed what the user of the communication system wants to pay Example: If the voice signal frequencies between 0.3 to 3.5 KHz, then the channel needs band width at least 3.2 KHz

29 9-Electromagnetic spectrum and band designation Spectrum f is increasing λ in m λ is increasing f in Hz hf 9/6/2014 LECTURES 29

30 Band designation 9/6/2014 LECTURES 30

31 9/6/2014 LECTURES 31