CHAPTER 6 THE WIRELESS CHANNEL

Transcription

1 CHAPTER 6 THE WIRELESS CHANNEL These slides are made available to faculty in PowerPoint form. Slides can be freely added, modified, and deleted to suit student needs. They represent substantial work on the part of the authors; therefore, we request the following. If these slides are used in a class setting or posted on an internal or external www site, please mention the source textbook and note our copyright of this material. All material copyright 2016 Cory Beard and William Stallings, All Rights Reserved Wireless Communication Networks and Systems 1 st edition Cory Beard, William Stallings 2016 Pearson Higher Education, Inc. The Wireless Channel 6-1

2 ANTENNAS An antenna is an electrical conductor or system of conductors Transmission - radiates electromagnetic energy into space Reception - collects electromagnetic energy from space In two-way communication, the same antenna can be used for transmission and reception The Wireless Channel 6-2

3 RADIATION PATTERNS Radiation pattern Graphical representation of radiation properties of an antenna Depicted as two-dimensional cross section Beam width (or half-power beam width) Measure of directivity of antenna Reception pattern Receiving antenna s equivalent to radiation pattern Sidelobes Extra energy in directions outside the mainlobe Nulls Very low energy in between mainlobe and sidelobes The Wireless Channel 6-3

4 A A B z B Antenna location (a) Omnidirectional (b) Directional 6.1 ANTENNA RADIATION PATTERNS The Wireless Channel 6-4

5 TYPES OF ANTENNAS Isotropic antenna (idealized) Radiates power equally in all directions Dipole antennas Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi antenna) Parabolic Reflective Antenna Directional Antennas Arrays of antennas In a linear array or other configuration Signal amplitudes and phases to each antenna are adjusted to create a directional pattern Very useful in modern systems The Wireless Channel 6-5

6 ISOTROPIC RADIATOR Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna Real antennas always have directive effects (vertically and/or horizontally) Radiation pattern: measurement of radiation around an antenna y z x z y x ideal isotropic radiator

7 l/2 l/4 (a) Half-wave dipole (b) Quarter-wave antenna 6.2 SIMPLE ANTENNAS The Wireless Channel 6-7

8 y y z x z x Side view (xy-plane) Side view (zy-plane) (a) Simple dipole Top view (xz-plane) y y z x z x Side view (xy-plane) Side view (zy-plane) (b) Directed antenna Top view (xz-plane) 6.3 RADIATION PATTERN IN THREE DIMENSIONS The Wireless Channel 6-8

9 y a Directrix b c f f c b a Focus x (a) Parabola (b) Cross section of parabolic antenna showing reflective property (c) Cross section of parabolic antenna showing radiation pattern 6.4 PARABOLIC REFLECTIVE ANTENNAS The Wireless Channel 6-9

10 ANTENNA GAIN Antenna gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) Effective area Related to physical size and shape of antenna The Wireless Channel 6-10

11 ANTENNA GAIN Relationship between antenna gain and effective area G = antenna gain A e = effective area G = 4π A e! λ 2 f = carrier frequency = 4π f c = speed of light ( m/s) λ = carrier wavelength c 2 2 A e The Wireless Channel 6-11

12 SPECTRUM CONSIDERATIONS Controlled by regulatory bodies Carrier frequency Signal Power Multiple Access Scheme Divide into time slots Time Division Multiple Access (TDMA) Divide into frequency bands Frequency Division Multiple Access (FDMA) Different signal encodings Code Division Multiple Access (CDMA) The Wireless Channel 6-12

13 SPECTRUM CONSIDERATIONS Industrial, Scientific, and Medical (ISM) bands Can be used without a license As long as power and spread spectrum regulations are followed ISM bands are used for WLANs Wireless Personal Area networks Internet of Things The Wireless Channel 6-13

14 PROPAGATION MODES Ground-wave propagation Sky-wave propagation Line-of-sight propagation The Wireless Channel 6-14

15 Signal propagation Transmit antenna Earth Receive antenna (a) Ground wave propagation (below 2 MHz) Ionosphere Signal propagation Transmit antenna Earth Receive antenna (b) Sky wave propagation (2 to 30 MHz) Signal propagation Transmit antenna Earth Receive antenna (c) Line-of-sight (LOS) propagation (above 30 MHz) 6.5 WIRELESS PROPAGATION MODES The Wireless Channel 6-15

16 GROUND WAVE PROPAGATION Follows contour of the earth Can propagate considerable distances Frequencies up to 2 MHz Example AM radio The Wireless Channel 6-16

17 SKY WAVE PROPAGATION Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and forth between ionosphere and earth s surface Reflection effect caused by refraction Examples Amateur radio CB radio The Wireless Channel 6-17

18 LINE-OF-SIGHT PROPAGATION Transmitting and receiving antennas must be within line of sight Satellite communication signal above 30 MHz not reflected by ionosphere Ground communication antennas within effective line of site due to refraction Refraction bending of microwaves by the atmosphere Velocity of electromagnetic wave is a function of the density of the medium When wave changes medium, speed changes Wave bends at the boundary between mediums The Wireless Channel 6-18

19 FIVE BASIC PROPAGATION MECHANISMS 1. Free-space propagation 2. Transmission Through a medium Refraction occurs at boundaries 3. Reflections Waves impinge upon surfaces that are large compared to the signal wavelength 4. Diffraction Secondary waves behind objects with sharp edges 5. Scattering Interactions between small objects or rough surfaces The Wireless Channel 6-19

20 Area of lower refractive index Incident direction i Area of higher refractive index r Refracted direction 6.6 REFRACTION OF AN ELECTROMAGNETIC WAVE The Wireless Channel 6-20

21 LOS WIRELESS TRANSMISSION IMPAIRMENTS Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise The Wireless Channel 6-21

22 ATTENUATION Strength of signal falls off with distance over transmission medium Attenuation factors for unguided media: Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal Signal must maintain a level sufficiently higher than noise to be received without error Attenuation is greater at higher frequencies, causing distortion The Wireless Channel 6-22

23 FREE SPACE LOSS Free space loss (Friis Model): P P t r = (4πd ) G G t r 2 λ 2 = (4πdf G G t r ) c 2 2 P t = signal power at transmitting antenna P r = signal power at receiving antenna λ = carrier wavelength d = propagation distance between antennas c = speed of light ( m/s) G t = transmitter antenna gain (=1 for isotropic antenna) G t = receiver antenna gain (=1 for isotropic antenna) where d and λ are in the same units (e.g., meters) The Wireless Channel 6-23

24 FREE SPACE LOSS Free space loss (isotropic antenna) equation can be recast: L db = 10log P t = 20log 4πd P! r λ ( λ) + 20log( ) 21.98dB = 20 log d + 4πfd = 20 log = 20 log d c ( f ) + 20 log( ) db The Wireless Channel 6-24

25 f = 300 GHz f = 30 GHz Loss (db) f = 3 GHz f = 300 MHz f = 30 MHz Distance (km) FREE SPACE LOSS The Wireless Channel 6-25

26 DERIVATION OF THE FRIIS EQUATION Power Flux Density: power spread over the sphere s surface: p= P t /4π r 2 G t Antenna s Apperture or Effective Area: A eff = λ 2 /4π G r = P o /p Where P o P r is the antenna s output power that feeds the receiver circuit s load. Note: Antenna s apperture efficiency (0 e a 1): e a = A eff / A phys, where A phys is the physical apperture of e.g., parabolic dish or horn Friis Equation: P r =p A eff

27 PATH LOSS EXPONENT IN PRACTICAL SYSTEMS Practical systems reflections, scattering, etc. Beyond a certain distance, received power decreases logarithmically with distance Based on many measurement studies P t P r = 4π λ 2 d n = 4πf c 2 d n L db = 20log( f ) +10nlog( d) db The Wireless Channel 6-27

28 PATH LOSS EXPONENT IN PRACTICAL SYSTEMS The Wireless Channel 6-28

29 MODELS DERIVED FROM EMPIRICAL MEASUREMENTS Need to design systems based on empirical data applied to a particular environment To determine power levels, tower heights, height of mobile antennas Okumura developed a model, later refined by Hata Detailed measurement and analysis of the Tokyo area Among the best accuracy in a wide variety of situations Predicts path loss for typical environments Urban Small, medium sized city Large city Suburban Rural The Wireless Channel 6-29

30 CATEGORIES OF NOISE Thermal Noise Intermodulation noise Crosstalk Impulse Noise The Wireless Channel 6-30

31 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 The Wireless Channel 6-31

32 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) The Wireless Channel 6-32

33 THERMAL NOISE Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts N = ktb = dbw + 10 log T + 10 log B N = 10 logk + 10 log T + 10 log B The Wireless Channel 6-33

34 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 The Wireless Channel 6-34

35 EXPRESSION E b /N 0 Ratio of signal energy per bit to noise power density per Hertz E b N 0 S / R = N 0 S TR The bit error rate (i.e., bit error probability) 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 = k The Wireless Channel 6-35

36 Worse performance Probability of bit error (BER) Better performance (Eb/N0) (db) 6.9 GENERAL SHAPE OF BER VERSUS E b /N 0 CURVES The Wireless Channel 6-36

37 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 The Wireless Channel 6-37

38 THE EFFECTS OF MULTIPATH PROPAGATION Reflection, diffraction, and scattering 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 Rapid signal fluctuations Over a few centimeters The Wireless Channel 6-38

39 (a) Microwave line of sight (b) Mobile radio 6.10 EXAMPLES OF MULTIPATH INTERFERENCE The Wireless Channel 6-39

40 R Lamp post S D R 6.11 SKETCH OF THREE IMPORTANT PROPAGATION MECHANISMS The Wireless Channel 6-40

41 REAL WORLD EXAMPLES

42 Transmitted pulse Transmitted pulse Time Received LOS pulse Received multipath pulses Received LOS pulse Received multipath pulses Time 6.12 TWO PULSES IN TIME-VARIANT MULTIPATH The Wireless Channel 6-42

43 80 Received power (dbm) Position (m) TYPICAL LARGE-SCALE AND SMALL-SCALE FADING IN AN URBAN MOBILE ENVIRONMENT The Wireless Channel 6-43

44 TYPES OF FADING Large-scale fading Signal variations over large distances Path loss L db as we have seen already Shadowing Statistical variations Rayleigh fading Ricean fading The Wireless Channel 6-44

45 TYPES OF FADING Doppler Spread Frequency fluctuations caused by movement Coherence time T c characterizes Doppler shift How long a channel remains the same Coherence time T c >> T b bit time à slow fading The channel does not change during the bit time Otherwise fast fading Example 6.11: T c = 70 ms, bit rate r b = 100 kbs Bit time T b = 1/ = 10 µs T c >> T b? 70 ms >> 10 µs? True, so slow fading The Wireless Channel 6-45

46 TYPES OF FADING Multipath fading Multiple signals arrive at the receiver Coherence bandwidth B c characterizes multipath Bandwidth over which the channel response remains relatively constant Related to delay spread, the spread in time of the arrivals of multipath signals Signal bandwidth B s is proportional to the bit rate If B c >> B s, then flat fading The signal bandwidth fits well within the channel bandwidth Otherwise, frequency selective fading Example 6.11: B c = 150 khz, bit rate r b = 100 kbs Assume signal bandwidth B s r b, B s = 100 khz B c >> B s? 150 khz >> 100 khz? Using a factor of 10 for >>, 150 khz is not more than khz False, so frequency selective fading The Wireless Channel 6-46

47 Signal Spectrum Signal Spectrum A A -B B -B B Flat Fading Channel Frequency selective channel B B -B B Flat fading output from the channel Frequency selective output from the channel 0.1 A 0.1 A -B B -B B 6.14 FLAT AND FREQUENCY SELECTIVE FADING The Wireless Channel 6-47

48 1 Frequency selective fading or fast fading distortion 10 1 Probability of bit error (BER) 10 2 Flat fading and slow fading Rayleigh limit 10 3 Rician fading K = 4 Additive white Gaussian noise Rician fading K = (E b /N 0 ) (db) 6.15 THEORETICAL BIT ERROR RATE FOR VARIOUS FADING CONDITIONS The Wireless Channel 6-48

49 FRESNEL ZONES 1 st Fresnel Zone Obstruction must be <20% in order to result in propagation loss equivalent to free space. Radius of the n th Fresnel Zone at point P (d1, d2, lambda in meters):

50 TWO-RAY MODEL d< d c : Friis model d> d c : P r = P t G t G r ( h t ) 2 ( h r ) 2 / d 4 L Crossover distance: d c =(4π h t h r )/λ

51 CHANNEL CORRECTION MECHANISMS Forward error correction Adaptive equalization Adaptive modulation and coding Diversity techniques and MIMO OFDM Spread sprectrum Bandwidth expansion The Wireless Channel 6-51

52 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 match, receiver attempts to determine bits in error and correct Subject of Chapter 10 The Wireless Channel 6-52

53 k bits Data Codeword FEC encoder Codeword No error or correctable error FEC decoder Detectable but not correctable error n bits Data Error indication Transmitter Receiver 10.5 FORWARD ERROR CORRECTION PROCESS The Wireless Channel 6-53

54 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 The Wireless Channel 6-54

55 Unequalized input v v v v C 2 C 1 C 0 C 1 C 2 Equalized output Algorithm for tap gain adjustment 6.16 LINEAR EQUALIZER CIRCUIT The Wireless Channel 6-55

56 ADAPTIVE MODULATION AND CODING (AMC) The modulation process formats the signal to best transmit bits To overcome noise To transmit as many bits as possible Coding detects and corrects errors AMC adapts to channel conditions 100 s of times per second Measures channel conditions Sends messages between transmitter and receiver to coordinate changes The Wireless Channel 6-56

57 DIVERSITY TECHNIQUES Diversity is based on the fact that individual channels experience independent fading events Space diversity techniques involving physical transmission path, spacing antennas 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 Use of diversity Selection diversity select the best signal Combining diversity combine the signals The Wireless Channel 6-57

58 MULTIPLE INPUT MULTIPLE OUTPUT (MIMO) ANTENNAS Use antenna arrays for Diversity different signals from different antennas Multiple streams parallel data streams Beamforming directional antennas Multi-user MIMO directional beams to multiple simultaneous users Modern systems 4 4 (4 transmitter and 4 reciever antennas) 8 8 Two dimensional arrays of 64 antennas Future: Massive MIMO with many more antennas The Wireless Channel 6-58

59 Diversity for improved system performance Beam-forming for improved coverage (less cells to cover a given area) Spatial division multiple access ( MU-MIMO ) for improved capacity (more user per cell) 6.18 FOUR USES OF MIMO Multi layer transmission ( SU-MIMO ) for higher data rates in a given bandwidth The Wireless Channel 6-59

60 Object Antenna Transmitter Receiver MIMO signal processing MIMO signal processing 6.19 MIMO SCHEME The Wireless Channel 6-60

61 CHANNEL CORRECTION MECHANISMS Orthogonal Frequency Division Multiplexing (OFDM) Chapter 8 Splits signal into many lower bit rate streams called subcarriers Overcomes frequency selectivity from multipath Spaces subcarriers apart in overlapping yet orthogonal carrier frequencies Spread spectrum (Chapter 9) Expand a signal to 100 times its bandwidth An alternative method to overcome frequency selectivity Users can share the channel by using different spreading codes Code Division Multiple Access (CDMA) The Wireless Channel 6-61

62 SIGNAL PROPAGATION RANGES Transmission range communication possible low error rate Detection range detection of the signal sender possible no communication transmission possible Interference range detection signal may not be detected interference signal adds to the background noise Warning: figure misleading bizarre shaped, time-varying ranges in reality! distance