Sistemi di Trasmissione Radio. Università di Pavia. Sistemi di Trasmissione Radio

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2 Programma del corso Tecniche di trasmissione Modulazioni numeriche Sistemi ad allargameneto di banda Sistemi multi-tono Codifica di canale Codifica di sorgente (vocoder)

3 Programma del corso Sistemi di trasmissione senza filo Sistemi radiomobili cellulari comunicazioni satellitari Cordless e Wireless Local Loop Mobile IP e WAP WLAN Sensor Networks

4 Testi consigliati O. Bertazioli,, L. Favalli, GSM. Hoepli,, Seconda Edizione, W. Stallings, Wireless Communications and Networks. Pearson-Prentice Hall, Second Editions,, 2005.

5 Storia delle comunicazioni radio Guglielmo Marconi invented the wireless telegraph in 1896 Communication by encoding alphanumeric characters in analog signal Sent telegraphic signals across the Atlantic Ocean Communications satellites launched in 1960s Advances in wireless technology Radio, television, mobile telephone, communication satellites More recently Satellite communications, wireless networking, cellular technology

6 Electromagnetic Signal Function of time Can also be expressed as a function of frequency Signal consists of components of different frequencies

7 Time-Domain Concepts Analog signal - signal intensity varies in a smooth fashion over time No breaks or discontinuities in the signal Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level Periodic signal - analog or digital signal pattern that repeats over time s(t +T ) = s(t ) where T is the period of the signal

8 Frequency-Domain Concepts Spectrum - range of frequencies that a signal contains Absolute bandwidth - width of the spectrum of a signal Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal s s energy is contained in

9 Relationship between Data Rate and Bandwidth The greater the bandwidth, the higher the information-carrying capacity BUT the transmission system will limit the bandwidth that can be transmitted AND, for any given medium, the greater the bandwidth transmitted, the greater the cost HOWEVER, limiting the bandwidth creates distortions

10 Relazione tra data-rate e banda Sistemi limitati in banda Sistemi limitati in potenza Sistemi limitati sia in banda sia in potenza

11 Data Communication Terms Data - entities that convey meaning, or information Signals - electric or electromagnetic representations of data Transmission - communication of data by the propagation and processing of signals

12 Segnali analogici e digitali

13 Segnali digitali

14 Analog Transmission Transmit analog signals without regard to content Attenuation limits length of transmission link Cascaded amplifiers boost signal s s energy for longer distances but cause distortion Analog data can tolerate distortion Introduces errors in digital data

15 Digital Transmission Concerned with the content of the signal Attenuation endangers integrity of data Digital Signal Repeaters achieve greater distance Repeaters recover the signal and retransmit Analog signal carrying digital data Retransmission device recovers the digital data from analog signal Generates new, clean analog signal

16 About Channel Capacity Impairments, such as noise, limit data rate that can be achieved For digital data, to what extent do impairments limit data rate? Channel Capacity the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions

17 Concepts Related to Channel Capacity Data rate - rate at which data can be communicated (bps) Bandwidth - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz) Noise - average level of noise over the communications path Error rate - rate at which errors occur Error = transmit 1 and receive 0; transmit 0 and receive 1

18 Nyquist Bandwidth For binary signals (two voltage levels) C = 2B2 With multilevel signaling C = 2B 2 log 2 M M = number of discrete signal or voltage levels

19 Signal-to-Noise Ratio Ratio of the power in a signal to the power contained in the noise Typically measured at a receiver Signal-to-noise ratio (SNR, or S/N) ( SNR ) = db 10log10 signal power noise power A high SNR means a high-quality signal, low number of required intermediate repeaters SNR sets upper bound on achievable data rate

20 Shannon Capacity Formula Equation: C = B log 2 + ( 1 SNR) Represents theoretical maximum that can be achieved In practice, only much lower rates achieved Formula assumes white noise (thermal noise)

21 Example of Nyquist and Shannon Formulations Spectrum of a channel between 3 MHz and 4 MHz; SNR db = 24 db B = 4 MHz 3 MHz = 1MHz SNR SNR db = = 251 Using Shannon s s formula 24 db = 10log 10 ( SNR) C = 10 6 log 2 ( ) = 8Mbps

22 Example of Nyquist and Shannon Formulations How many signaling levels are required? C = 2B = M 6 log 2 = 16 log = 2 2 M M ( 6 10 ) log 2 M

23 Wireless transmissions Transmission and reception are achieved by means of antennas Configurations for wireless transmission Directional Omnidirectional

24 General Frequency Ranges Microwave frequency range 1 GHz to 60 GHz Directional beams possible Suitable for point-to-point transmission Used for satellite communications Radio frequency range 30 MHz to 1 GHz Suitable for omnidirectional applications Infrared frequency range Roughly, 3x10 11 to 2x10 14 Hz Useful in local point-to-point multipoint applications within confined areas

25 Terrestrial Microwave Description of common microwave antenna Parabolic "dish", 3 m in diameter Fixed rigidly and focuses a narrow beam Achieves line-of-sight transmission to receiving antenna Located at substantial heights above ground level Applications Long haul telecommunications service Short point-to-point links between buildings

26 Satellite Microwave Description of communication satellite Microwave relay station Used to link two or more ground-based microwave transmitter/receivers Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink) Applications Television distribution Long-distance telephone transmission Private business networks

27 Multiplexing Capacity of transmission medium usually exceeds capacity required for transmission of a single signal Multiplexing - carrying multiple signals on a single medium More efficient use of transmission medium

28 Multiplexing

29 Multiplexing Techniques Frequency-division multiplexing (FDM) Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal Time-division multiplexing (TDM) Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal

30 Frequency-division Multiplexing

31 Time-division Multiplexing

32 Antenna Gain Relationship between antenna gain and effective area G = 4 A 2 G = antenna gain A e = effective area f = carrier frequency c = speed of light (» m/s) = carrier wavelength e = 4 f c 2 2 A e

33 Free Space Loss Free space loss, ideal isotropic antenna P t P r = ( ) 2 4 d ( 4 fd) P t = signal power at transmitting antenna P r = signal power at receiving antenna = carrier wavelength d = propagation distance between antennas c = speed of light (» m/s) where d and are in the same units (e.g., meters) 2 = c 2 2

34 Free Space Loss Free space loss accounting for gain of other antennas P t P r = ( )( 2 ) 2 ( ) 2 4 d d ( cd) G G r t 2 = A G t = gain of transmitting antenna G r = gain of receiving antenna A t = effective area of transmitting antenna A r = effective area of receiving antenna r A t = f 2 A r 2 A t

35 Categories of Noise Thermal Noise Intermodulation noise Crosstalk Impulse Noise

36 Noise Terminology Intermodulation noise occurs if signals with different frequencies share the same medium Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies Crosstalk unwanted coupling between signal paths Impulse noise irregular pulses or noise spikes Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and flaws in the communications system

37 Thermal Noise Thermal noise due to agitation of electrons Present in all electronic devices and transmission media Cannot be eliminated Function of temperature Particularly significant for satellite communication

38 Thermal Noise Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: N = kt 0 ( W/Hz) N 0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = J/K T = temperature, in kelvins (absolute temperature)

39 Thermal Noise Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): N = ktb or, in decibel-watts N = 10 log k + 10 log T + 10log B = dbw + 10 log T + 10log B

40 Expression E b /N 0 Ratio of signal energy per bit to noise power density per Hertz E b S / R S = = N0 N0 ktr The bit error rate for digital data is a function of E b /N 0 Given a value for E b /N 0 to achieve a desired error rate, parameters of this formula can be selected As bit rate R increases, transmitted signal power must increase to maintain required E b /N 0

41 Other Impairments Atmospheric absorption water vapor and oxygen contribute to attenuation Multipath obstacles reflect signals so that multiple copies with varying delays are received Refraction bending of radio waves as they propagate through the atmosphere

42 Multipath Propagation

43 Multipath Propagation Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave Scattering occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

44 The Effects of Multipath Propagation Multiple copies of a signal may arrive at different phases If phases add destructively, the signal level relative to noise declines, making detection more difficult Intersymbol interference (ISI) One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

45 Types of Fading Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading

46 Error Compensation Mechanisms Forward error correction Adaptive equalization Diversity techniques

47 Forward Error Correction Transmitter adds error-correcting code to data block Code is a function of the data bits Receiver calculates error-correcting code from incoming data bits If calculated code matches incoming code, no error occurred If error-correcting codes don t t match, receiver attempts to determine bits in error and correct

48 Adaptive Equalization Can be applied to transmissions that carry analog or digital information Analog voice or video Digital data, digitized voice or video Used to combat intersymbol interference Involves gathering dispersed symbol energy back into its original time interval Techniques Lumped analog circuits Sophisticated digital signal processing algorithms

49 Diversity Techniques Diversity is based on the fact that individual channels experience independent fading events Space diversity techniques involving physical transmission path Frequency diversity techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers Time diversity techniques aimed at spreading the data out over time

50 Signal Encoding Techniques What determines how successful a receiver will be in interpreting an incoming signal? Signal-to-noise ratio Data rate Bandwidth An increase in data rate increases bit error rate An increase in SNR decreases bit error rate An increase in bandwidth allows an increase in data rate

51 Factors Used to Compare Encoding Schemes Signal spectrum With lack of high-frequency components, less bandwidth required With no dc component, ac coupling via transformer possible Transfer function of a channel is worse near band edges Clocking Ease of determining beginning and end of each bit position

52 Factors Used to Compare Encoding Schemes Signal interference and noise immunity Performance in the presence of noise Cost and complexity The higher the signal rate to achieve a given data rate, the greater the cost

53 Basic Encoding Techniques Digital data to analog signal Amplitude-shift keying (ASK) Amplitude difference of carrier frequency Frequency-shift keying (FSK) Frequency difference near carrier frequency Phase-shift keying (PSK) Phase of carrier signal shifted

54 Basic Encoding Techniques

55 Amplitude-Shift Keying One binary digit represented by presence of carrier, at constant amplitude Other binary digit represented by absence of carrier Acos () ( 2 f ) ct binary 1 s t = 0 binary 0 where the carrier signal is Acos(2 f c t)

56 Amplitude-Shift Keying Susceptible to sudden gain changes Inefficient modulation technique On voice-grade lines, used up to 1200 bps Used to transmit digital data over optical fiber

57 Binary Frequency-Shift Keying (BFSK) Two binary digits represented by two different frequencies near the carrier frequency s() t = ( 2 f t) ( 2 f t) Acos 1 Acos 2 1 binary binary 0 where f 1 and f 2 are offset from carrier frequency f c by equal but opposite amounts

58 Binary Frequency-Shift Keying (BFSK) Less susceptible to error than ASK On voice-grade lines, used up to 1200bps Used for high-frequency (3 to 30 MHz) radio transmission Can be used at higher frequencies on LANs that use coaxial cable

59 Multiple Frequency-Shift Keying (MFSK)

60 Phase-Shift Keying (PSK) Two-level PSK (BPSK) Uses two phases to represent binary digits s() t = A A cos ( 2 f t) ( c cos 2 f t + ) c binary 1 binary 0 = Acos Acos ( 2 f t) c ( 2 f t) c 1 binary binary 0

61 Phase-Shift Keying (PSK) Differential PSK (DPSK) Phase shift with reference to previous bit Binary 0 signal burst of same phase as previous signal burst Binary 1 signal burst of opposite phase to previous signal burst

62 Phase-Shift Keying (PSK) Four-level PSK (QPSK) s Each element represents more than one bit Acos 2 f ct Acos 2 f ct + 01 t = 4 3 Acos 2 f t 00 c 4 Acos 2 f t 10 c 4 ()

63 Phase-Shift Keying (PSK) Multilevel PSK Using multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved D = R L R log 2 M D = modulation rate, baud R = data rate, bps M = number of different signal elements = 2 L L = number of bits per signal element =

64 Performance Bandwidth of modulated signal (B( T ) ASK, PSK B T =(1+r)R FSK B T =2DF+ F+(1+ (1+r)R R = bit rate 0 < r < 1; related to how signal is filtered DF F = f 2 -f c =f c -f 1

65 Performance Bandwidth of modulated signal (B( T ) MPSK MFSK B T B T 1+ r = L ( 1 r) + = log 2 R M M 1+ = log 2 R r M R L = number of bits encoded per signal element M = number of different signal elements

66 Quadrature Amplitude Modulation QAM is a combination of ASK and PSK Two different signals sent simultaneously on the same carrier frequency s () t = d () t 2 f t d () t sin 2 f t 1 cos c + 2 c

67 Quadrature Amplitude Modulation

68 Reasons for Analog Modulation Modulation of digital signals When only analog transmission facilities are available, digital to analog conversion required Modulation of analog signals A higher frequency may be needed for effective transmission Modulation permits frequency division multiplexing

69 Pulse Code Modulation Based on the sampling theorem Each analog sample is assigned a binary code Analog samples are referred to as pulse amplitude modulation (PAM) samples The digital signal consists of block of n bits, where each n-bit number is the amplitude of a PCM pulse

70

71 Pulse Code Modulation By quantizing the PAM pulse, original signal is only approximated Leads to quantizing noise Signal-to-noise ratio for quantizing noise n SNR db = 20log db = 6.02n db Thus, each additional bit increases SNR by 6 db, or a factor of 4

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