Antennas and Propagation Lecture 2 G Noubir noubirccs
Antennas and Propagation Lecture 2 G. Noubir noubir@ccs. neu. edu Textbook: Wireless Communications and Networks, William Stallings, Prentice Hall
Outline n n Antennas Propagation Modes Line of Sight Transmission Fading in Mobile Environment and Compensation COM 3525, Winter 2002: lecture 2 2
Decibels n The Decibel Unit: n n n Standard unit describing transmission gain (loss) and relative power levels Gain: N(d. B) = 10 log(P 2/P 1) Decibels above or below 1 W: N (d. BW) = 10 log(P 2/1 W) Decibels above or below 1 Milliwatt: N(d. Bm) = 10 log(P 2/1 m. W) Example: n n P = 1 m. W => P(d. Bm) = ? ; P(d. BW) = ? P = 10 m. W => P(d. Bm) = ? ; P(d. BW) = ? COM 3525, Winter 2002: lecture 2 3
Introduction n An antenna is an electrical conductor or system of conductors n n n 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 COM 3525, Winter 2002: lecture 2 4
Basics of Radio-waves Propagation n Radiowave propagation: n n n Radiowaves: electromagnetic waves Signal energy: electrical field (E) and magnetic field (H) E and H are sinusoidal functions of time The signal is attenuated and affected by the medium Antennas n n Form the link between the guided part and the free space: couple energy Purpose: n n Transmission: efficiently transform the electrical signal into radiated electromagnetic wave (radio/microwave) Reception: efficiently accept the received radiated energy and convert it to an electrical signal COM 3525, Winter 2002: lecture 2 5
Radiation Patterns n Radiation pattern n Beam width (or half-power beam width) n n Graphical representation of radiation properties of an antenna Depicted as two-dimensional cross section Measure of directivity of antenna Reception pattern n Receiving antenna’s equivalent to radiation pattern COM 3525, Winter 2002: lecture 2 6
Types of Antennas n Theoretical reference antenna (isotropic radiator): n n n A point in space radiating with equal power in all directions Points with equal power are located on a sphere with the antenna in the center Real antennas exhibit directive effects: n n Types: omnidirectional (dipole) or directional (pencil beam) Simplest antenna: n n Directional antennas may be more useful: n n Dipole (or Hertzian dipole) of length /4 or /2 To cover a highway, valley, satellite beam Parabolic antenna COM 3525, Winter 2002: lecture 2 7
Antenna Gain n Antenna gain n Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) G = 4 p*Pwr. Direction/All. Radiated. Pwr Effective area n Related to physical size and shape of antenna COM 3525, Winter 2002: lecture 2 8
Antenna Gain n Relationship between antenna gain and effective area n n n G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 ´ 108 m/s) = carrier wavelength Affective area: n n n Isotropic: 2/4 p Half-wave dipole: 1. 64 2/4 p (power gain vs. istropic 1. 64) Parabolic: 0. 56 A (power gain vs. istropic 7 A/ 2) COM 3525, Winter 2002: lecture 2 9
Propagation Modes n n n Ground-wave propagation Sky-wave propagation Line-of-sight propagation COM 3525, Winter 2002: lecture 2 10
Ground Wave Propagation COM 3525, Winter 2002: lecture 2 11
Ground Wave Propagation n n Follows contour of the earth Can Propagate considerable distances Frequencies up to 2 MHz Example n AM radio COM 3525, Winter 2002: lecture 2 12
Sky Wave Propagation COM 3525, Winter 2002: lecture 2 13
Sky Wave Propagation n n 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 n n Amateur radio CB radio COM 3525, Winter 2002: lecture 2 14
Line-of-Sight Propagation COM 3525, Winter 2002: lecture 2 15
Line-of-Sight Propagation n Transmitting and receiving antennas must be within line of sight n n n 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 n n n 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 COM 3525, Winter 2002: lecture 2 16
Line-of-Sight Equations n Optical line of sight n Effective, or radio, line of sight n n n d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction, rule of thumb K = 4/3 COM 3525, Winter 2002: lecture 2 17
Line-of-Sight Equations n Maximum distance between two antennas for LOS propagation: n n h 1 = height of antenna one h 2 = height of antenna two COM 3525, Winter 2002: lecture 2 18
LOS Wireless Transmission Impairments n n n Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction COM 3525, Winter 2002: lecture 2 19
Attenuation (Pathloss) n n Strength of signal falls off with distance over transmission medium Attenuation factors for unguided media: n n n 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 COM 3525, Winter 2002: lecture 2 20
Free Space Loss n Free space loss, ideal isotropic antenna Pt = signal power at transmitting antenna n Pr = signal power at receiving antenna n = carrier wavelength n d = propagation distance between antennas n c = speed of light (» 3 ´ 10 8 m/s) where d and are in the same units (e. g. , meters) n COM 3525, Winter 2002: lecture 2 21
Free Space Loss n Free space loss equation can be recast: COM 3525, Winter 2002: lecture 2 22
Free Space Loss n Free space loss accounting for gain of other antennas n n Gt = gain of transmitting antenna Gr = gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of receiving antenna COM 3525, Winter 2002: lecture 2 23
Free Space Loss n Free space loss accounting for gain of other antennas can be recast as COM 3525, Winter 2002: lecture 2 24
Categories of Noise n n Thermal Noise Intermodulation noise Crosstalk Impulse Noise COM 3525, Winter 2002: lecture 2 25
Thermal Noise n n n 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 COM 3525, Winter 2002: lecture 2 26
Thermal Noise n Amount of thermal noise to be found in a bandwidth of 1 Hz in any device or conductor is: n n n N 0 = noise power density in watts per 1 Hz of bandwidth k = Boltzmann's constant = 1. 3803 ´ 10 -23 J/K T = temperature, in kelvins (absolute temperature) COM 3525, Winter 2002: lecture 2 27
Thermal Noise n n Noise is assumed to be independent of frequency Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts COM 3525, Winter 2002: lecture 2 28
Noise Terminology n Intermodulation noise – occurs if signals with different frequencies share the same medium n n n 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 n n Short duration and of relatively high amplitude Caused by external electromagnetic disturbances, or faults and flaws in the communications system COM 3525, Winter 2002: lecture 2 29
Expression Eb/N 0 n n Ratio of signal energy per bit to noise power density per Hertz The bit error rate for digital data is a function of Eb/N 0 n n Given a value for Eb/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 Eb/N 0 COM 3525, Winter 2002: lecture 2 30
Other Impairments n n n 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 COM 3525, Winter 2002: lecture 2 31
Multipath Propagation
Multipath Propagation n 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 COM 3525, Winter 2002: lecture 2 33
The Effects of Multipath Propagation n Multiple copies of a signal may arrive at different phases n n If phases add destructively, the signal level relative to noise declines, making detection more difficult Intersymbol interference (ISI) n One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit COM 3525, Winter 2002: lecture 2 34
Types of Fading n n n Fast fading Slow fading Flat fading Selective fading Rayleigh fading Rician fading COM 3525, Winter 2002: lecture 2 35
Radio Propagation n n Large scale path loss + small-scale fading Large scale path loss n Outdoor propagation n Indoor propagation (inside buildings) n n Long distance Takes into account terrain profile (e. g. , mountains, hills, large buildings, etc. ) Distances covered are much smaller and the environment is more variable Increasing interest due to PCS and WLAN Classified as line-of-sight (LOS) or obstructed (OBS) Small-scale fading: multi-path COM 3525, Winter 2002: lecture 2 36
Path Loss Models n Log-distance path loss model: average path loss Environment Free space Urban area PCS Shadowed urban PCS In building LOS n Path Loss Exponent n 2 2. 7 to 3. 5 3 to 5 1. 6 to 1. 8 Obstructed in building 4 to 6 Obstructed in factories 2 to 3 Log-normal shadowing (signal level at a specific distance have Gaussian distribution) COM 3525, Winter 2002: lecture 2 37
Path Loss Models of Outdoor n Okumura-Hata empirical model: n Valid for f from 150 MHz to 1500 MHz. For urban area: n f: freq, hb: BS height, hm: mobile height, d: BS-MS distance n For a small to medium sized city: n For a large city: n For suburban area: n For rural area: COM 3525, Winter 2002: lecture 2 38
Other Path Loss Models n Euro-COST Extension of Okumura-Hata to PCS (f>1500 MHz): n CM = 0 d. B (medium and suburban), 3 d. B (metropolitan) n Walfish and Bertoni Model n Integrates effect of diffraction on rooftops COM 3525, Winter 2002: lecture 2 39
Path Loss Models for Indoor n Partition losses (same floor with soft or hard partitions) n n Partition losses between floors n n n Uses estimates of path loss Ericsson Multiple breakpoint model: n n Uses estimation of path loss for each material at working frequency 4 breakpoints, range of path-loss: [PLmin, PLmax] Log-distance path loss: n PL(d. B) = PL(d 0) + 10 n log(d/d 0) + Xs n n depend on surrounding environment, Xs is a normal random variable with standard deviation s Attenuation factor model (average path loss) n PL(d. B) = PL(d 0) + 10 n. SF log(d/d 0) + FAF (d. B) = PL(d 0) + 10 n. MF log(d/d 0) Buildings penetration: depends on building height, number of windows, etc. Ray tracing using Geographical Information System (GIS) databases COM 3525, Winter 2002: lecture 2 40
Small Scale Fading Models n Multi-path fading: n n n Multiple reflections from various objects: multiple path => multiple phase shifts Signal strength may vary by as much as 30 -40 d. B in hostile environments when the receiver moves by only a fraction of Main effects: n n n Rapid change in signal strength over a small travel distance Random frequency modulation due to varying Doppler shifts on different multi-path signals Time dispersion (echoes) caused by multipath propagation COM 3525, Winter 2002: lecture 2 41
Small Scale Fading (Cont’d) n Factors: n n n Multipath propagation: multiple version of the signal with different shifts that may add or subtract Speed of the mobile: the relative speed between the mobile and BS results in random frequency modulation due to different incidence angles of paths Speed of surrounding objects Transmission bandwidth of the signal versus channel bandwidth (coherence bandwidth) Characteristics: n Long-term fading x short-term fading: s(t) = m(t)r(t) COM 3525, Winter 2002: lecture 2 42
Fading Statistics n Doppler shift: fd = (v/l) cos q n n mobile trajectory Level crossing rate (LCR): n n q is the angle between the radio wave propagation axis and the Average number of times per second that the signal envelope crosses the level in a positive direction LCR and level crossing duration are important for estimating fading rate and duration => designing error control codes: r = R(Specified level)/Rrms ; fm: max Doppler shift => n n Average fade duration: COM 3525, Winter 2002: lecture 2 43
Bit Error Rate n n n Bit Error Rate (BER): rate of bit errors Estimating the BER is very important: it determines the packet loss (Frame Error Rate: FER) Bit Error Rate is a function of the received energy per bit Estimating path loss statistics allows to estimate the BER Shannon’s Theorem (AWGN): n n A channel with a given SNR has maximum capacity: C = W log (1 + SNR) There exist a coding scheme that allows to achieve the channel capacity COM 3525, Winter 2002: lecture 2 44
Error Compensation Mechanisms n n n Forward error correction Adaptive equalization Diversity techniques COM 3525, Winter 2002: lecture 2 45
Forward Error Correction n Transmitter adds error-correcting code to data block n n Code is a function of the data bits Receiver calculates error-correcting code from incoming data bits n n 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 COM 3525, Winter 2002: lecture 2 46
Adaptive Equalization n Can be applied to transmissions that carry analog or digital information n n 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 n n Lumped analog circuits Sophisticated digital signal processing algorithms COM 3525, Winter 2002: lecture 2 47
Diversity Techniques n n 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` COM 3525, Winter 2002: lecture 2 48
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