Background on wireless PHY layer

Transcription

1 Facoltà di Sciene Matematiche, Fisiche e Naturali Dipartimento di Sciene dell Informaione Corso di Laurea Specialistica in Sciene di Internet (SdI) e Informatica (Inf) Sistemi e Reti Wireless Luciano Bononi (bononi@cs.unibo.it) Ricevimento: sempre aperto. Si consiglia di concordare via almeno un giorno prima (informaioni in tempo reale sulla home page personale) Background on wireless PHY laer Figure-credits: some figures have been taken from slides published on the Web, b the following authors (in alfabethical order): J.J. Garcia Luna Aceves (ucsc), James F. Kurose & Keith W. Ross, Jochen Schiller (fub), Nitin Vaida (uiuc) RF Properties RF Properties Understanding Radio Frequenc Generation, coverage and propagation issues Higher amplitude RF signals go farther V Fundamental for wireless planning and management Radio Frequenc Signals Electromagnetic energ generated b high frequenc alternate current (AC) in s Antenna: converts the wired current to RF and viceversa RF energ (amplitude) RF Propagation medium? V Transmission Power (Watts) = Energ / Time = Joule / Sec More energ (voltage) moves more electrons (current) Power = Voltage * Current RF energ (amplitude) RF Propagation medium? Voltage (electric energ amplitude) i 3 Voltage (electric energ amplitude) i 4

2 RF Properties RF propagation f Frequenc (and Wavelength) Wireless Spectrum (see net slides) Portion of wireless spectrum regulated b regional authorities and assigned to wireless technologies Higher Frequencies Lower Frequencies 3...Time (sec) Wavelength = c / frequenc E.g..4 GhZ (ISM band) Wave Length = (m/s) / H = 0.5 m =.5 cm In practice: Antennas work better with sie =, ½, ¼ of wavelegth (tr to measure sie of our IEEE 0. device) 5 Radio transmission coverage The range is a function of power transmission (Pt) Signal strength reduces with d^k (K>=..3, no obstacles, isotropic radiator) A d signal detection limit A ~4 Pt Pt d d In 3D, sphere: V=(4 π r 3 /3) S=(4 π r ) 6 Wireless networks technolog RF Properties Radio transmission coverage obstacles can reflect or absorbe waves depending on materials and wave frequencies A??? Rules of thumb: high frequencies are good for short distances and are affected b abstacles low frequencies are good for long distances and are less affected b abstacles 7 Phase: shift of the wave (in degrees or radians) Positive phase (left-shift), earl wavefront Negative phase (right-shift), late wavefront Phase shift different phase...time Eample of signal composition with phase variations Sum Main and +0 degrees) Sum Main and +90 degrees) Signal (phase +0 degrees) Signal (phase +90 degrees) Main Signal (phase 0) = Time (phase=degrees) In practice: RF echoes arriving at receivers with different phase ma have positive or negative effects... Wh?

3 RF Properties Phase: shift of the wave (in degrees or radians) Positive phase (left-shift), earl wavefront Negative phase (right-shift), late wavefront Phase shift different phase...time Eample of signal composition with phase variations Sum Main and +0 degrees) Sum Main and +90 degrees) Signal (phase +0 degrees) Signal (phase +90 degrees) Main Signal (phase 0) = Time (phase=degrees) In practice: RF echoes arriving at receivers with different phase ma have positive or negative effects... Wh? 9 RF Properties Phase: shift of the wave (in degrees or radians) Positive phase (left-shift), earl wavefront Negative phase (right-shift), late wavefront Phase shift different phase...time Eample of signal composition with phase variations Sum Main and +0 degrees) Sum Main and +90 degrees) Signal (phase +0 degrees) Signal (phase +90 degrees) Main Signal (phase 0) = Time (phase=degrees) In practice: RF echoes arriving at receivers with different phase ma have positive or negative effects... Wh? 0 RF Properties RF Properties Polariation: (phsical orientation of ) RF waves are made b two perpendicular fields: Electric field and Magnetic field Magnetic Field (perpendicular to ) Horiontal Polariation (electric field is parallel to ground) Vertical Polariation: tpicall used in WLANs OK Transfered radiation OK 00% radiation captured Intuitivel... Radiating element Electric Field (parallel to ) Vertical Polariation (electric field is perpendicular to ground) Vertical Polariation (electric field is perpendicular to ground) E-plane (parallel to ) OK Transfered radiation NO 0% radiation captured H-plane (perpendicular to ) 3

4 RF Properties Vertical Polariation: the PCMCIA device problem OK Transfered radiation OK 00% radiation captured Intuitivel... RF Behaviors Radio transmission interference OK Transfered radiation NO N.B. the polariation problem is ver much important when using distant devices and directional s. With short distances signal reflections help!??% radiation captured 3 4 Wireless transmission: Electromagnetic waves Wireless transmission Different parameters of electromagnetic waves: amplitude M proportional to transmission energ (loudness) frequenc f (tone) measured in Hert (Ccle/sec) phase φ (peak shift with respect to reference signal) (rad) Signal Gain: (measured in Decibels, Db) Increase in amplitude M proportional to transmission energ Active gain (amplifiers) Passive gain (s focusing signal energ, and additive signal effects) same frequenc, different amplitude Frequenc Spectrum f Higher Frequencies source Signal Amplifier same frequenc, different phase Additional energ Lower Frequencies passive gain: a pitfall vs. regulations? φ Phase...Time (sec) 3...Time (sec) 5 6 4

5 Wireless transmission Wireless signal propagation ranges Signal Loss: (Db) source Decrease in amplitude M proportional to energ waste Intentional (resistance, signal attenuation -> heat) Obstacles, e.g. (walls, water for.4 Gh) and distance (wireless) Signal reflected energ (return loss) connector Wireless propagation loss A d High impedance medium A d 7 Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal ma not be detected signal adds to the background noise sender transmission detection interference distance Ranges depend on receiver s sensitivit! Wireless Signal propagation effects Real world eample Propagation in free space alwas like light (straight line) Receiving power proportional to /d² (d = distance between sender and receiver) Receiving power additionall influenced b fading (frequenc dependent) shadowing Ratracing eamples Low signal reflection at large obstacles refraction depending on the densit of a medium scattering at small obstacles diffraction at edges high signal shadowing reflection refraction scattering diffraction 9 0 5

6 Multipath propagation Signal can take man different paths between sender and receiver due to reflection, scattering, diffraction LOS pulses multipath pulses Effects of mobilit Channel characteristics change over time and location signal paths change different dela variations of different signal parts different phases of signal parts signal at sender signal at receiver quick changes in the power received (short term fading) power long term fading Time dispersion: signal is dispersed over time interference with neighbor smbols, Inter Smbol Interference (ISI) The signal reaches a receiver directl and phase shifted distorted signal depending on the phases of the different parts Additional changes in distance to sender obstacles further awa t short term fading slow changes in the average power received (long term fading) Voltage Standing Wave Ratio (VSWR) Intentional radiator and EIRP VSWR occurs with different impedance (Ohm) = resistance to AC current flow between transmitter and VSWR is the cause of return loss energ towards the transmitter Measured as ratio between impedance (before and after) E.g.,5: (impedance ratio before/after is,5 times the ideal value) = normalied ideal impedance (: means perfect VSWR) VSWR Causes burnout of transmitter circuits, and unstable output levels VSWR solution: alwas use same impedance circuits, cables, connectors (tpical 50 Ω in LANs) (Intentional) radiator: (def.) RF device specificall designed to generate and radiate RF signals....includes T RF device, cables and connectors ( ecluded) IR Power output: (subject to regulations) is the power output of last connector just before the Equivalent Isotropicall Radiated Power (EIRP): the power radiated b the (including the passive gain effect of directional s) EIRP ( gain +0dBi): 60 EIRP (isotropic ): 6 IR Power output: 30-4 = 6 Transmitter Impedance before Impedance after Out imp. 50 Ω cable imp. 50 Ω cable imp. 75 Ω VSWR : VSWR,5: 3 Connectors: - Transmitter T Out power 30 Cable: -3mw Cable: -5mw connector Reti Wireless Luciano Bononi 007 Sistemi e 4 6

7 Sstem design (under power viewpoint) Man factors must be considered in the design of Wireless sstems: Power of transmitting device Loss and gain of connectivit devices (cables, connectors, attenuators, amplifiers, splitters) between transmission device and transmitter s Power of the intentional radiator (last connector just before ) Power radiated b element (EIRP) Propagation properties of the medium (attenuation before signal reception) Loss and gain of connectivit devices (cables, connectors, attenuators, amplifiers, splitters) between receiver s and receiver device. WATT: electric power unit Watt = Ampere * Volt (P=V*I) also P= R*I^ and P = L/t Current (ampere) is the amount of charge (electrons) flowing as current in a wire Voltage (Volt) is the pressure applied to the flow of charge Resistance (impedance) is the obstacle to current flow Power is the energ needed (in a given time unit) to appl a given pressure to a given amount of charge, b resulting in a flow of current. Watt and are units used for absolute power measurement Transmitter 5 Tpical RF power for WLANs: AP: (up to 50 outdoor), PCMCIA: Decibel (db): a power measurement unit designed to epress power loss It is more practical to use given the logarithmic deca of wireless signals It allows to make eas calculations on resulting power Decibel (db) measures the logarithmic relative strength between two signals ( are a linear absolute measure a energ) Log 0 (X) = Y <==> 0 Y = X Eponential growth = 0 0, log 0 () = 0 0 = 0, log 0 (0) = 00 = 0, log 0 (00) = 000 = 0 3, log 0 (000) = 3 How strong is a 0 db signal? (it depends on the reference signal) Linear growth BEL units (B) 7 Decibel (db): /0 of a Bel E.g. 000 is one Bel greater than 00 => 000 is 0 db greater than 00 Linear signal difference (factor) = 0 0, log 0 () = 0 0 = 0, log 0 (0) = 00 = 0, log 0 (00) = 000 = 0 3, log 0 (000) = 3 How strong is a 0 db signal? (it depends on the reference signal) Positive db value is power gain, negative db value is power loss e.g. given 7 power, a +0 db signal gain is 70 e.g. given 7 power, a -0 db signal gain (loss) is 0.7 Power Difference (in db) between T and R signal: Power Difference (db) = 0 * log(power T(Watt) / Power R (Watt)) Gain and Loss are relative power measurements: db is the unit 0 db 0 db 0 db 0 db 0 db 30 db 7

8 Advantage of db: what is better? E.g.: A signal transmitted at [TX] 00 is received at [RX] Power Difference (db) = 0 * log([rx] /[TX]) = 0 * log( /00) = -73 A signal transmitted at 00 is received with gain (loss) 73 db Advantage of db: what is better? E.g.: A signal transmitted at 00 is received at , then it is amplified (*00) to ??? A signal transmitted at 00 is received with gain (loss) -73+0= -53 db -3 db ½ power in (/ ) +3 db power in (* ) -0 db /0 power in (/ 0) +0 db 0 power in (* 0) Approimated table (values defined for ease of calculations) 9 Practical eample: Signal T at 00, cable 3dB loss, amplifier +0 db gain 00 / (-3dB) = 50 * 0 (+0 db) = 500 IR power output Signal TX at 30 is received at the as 6 (/0 of TX power) Intentional Radiator Gain (loss) = 30 / 0 = 3 * = 6 Intentional Radiator Gain (loss) = -0 db + 3 db = -7 db ( /5, 7dB 5) N.B. dbs are additive measures of gain (loss): e.g. 6dB = +3+3 db, 7dB = 0-3 db E.g db = db = 00 / / = 5 E.g db = db = 00 *0 / = 500 E.g db = 0 ( )dB = 000/3 = 3.5 E.g. 0 + db =? 0 ( )dB = 000/ = 5 E.g. 50 db =? 50 ( )dB = 00/00 = N.B. Approimated values (values defined for ease of calculations) 30 : db-milliwatt, the absolute measure of signal power - : conversion table Assumption: reference signal is = 0 (normaliation factor) Useful for gain/loss calculation without passing through -4O -3O -O -O O +O +O +3O +4O E.g. access point transmits 00 = (*0*0) =+0 PCMCIA card transmits at 30 = (*0*3) = +4.7 E.g. T= 30, cable db, amplifier +9 db: 00 nw µw 0 µw 00 µw O 00 W 0 W 30 = *0 *3 = 4.7 IR power : 4.7 db +9dB =.7 (47.9 ) In general, for converting to and viceversa: O P = 0 log(p ) and P = 0 (P /0) 6,5 µw 5 µw µw 500 µw

9 dbi: db-isotropic, the normalied measure of passive gain Assumption: an isotropic radiator has 00% efficienc in radiating energ in uniform wa in ever direction (e.g. the Sun) Antennas concentrate energ in non-isotropic wa, resulting in passive gain (space dependent). Ideal : ero length dipole dbi: db-isotropic, the normalied measure of passive gain If an located in the origin (0,0,0) has twice the radiated energ of an isotropic radiator in a given point (,,), then the gain in (,,) can be defined as +3 dbi. If the energ is 0 the isotropic radiator, the gain is +0 dbi, etc.etc. Q: If the gain is 7 dbi in (,,)? (A: 5 isotropic energ) Preferred direction dbi: db-isotropic, the normalied measure of passive gain dbd: db-dipole, the normalied passive gain vs.,4 dbi half-wave dipole Real s alwas have a preferred direction where te power is greater than isotropic radiator: gain is alwas positive in the preferred direction! Eample: IR power applied to directional with +0 dbi gain in the preferred direction, would translate in EIRP? Reference is a half wave dipole with.4 dbi gain in preferred direction! Conversion rule: 0 dbd =.4 dbi, dbd = (dbi -.4), dbi = (dbd +.4) EIRP = + 0 dbi = (0) = 0 EIRP N.B. this does not mean that generates more power! Antenna concentrates power in preferred direction. Reference dipole Preferred direction Preferred direction

10 Power monitoring (e.g. IEEE 0. devices) Antennas (received) Power monitoring in IEEE 0. devices is needed for making device driver to work properl (tpical sensitivit range is [ ] ): Detect signal (below or above the sensitivit threshold?) Detect signal power (selection of coding technique... That is bitrate!) Detect channel status: idle? Ok, transmit! Bus? Ok, wait. Received Signal Strength Indicator (RSSI) Inde defined for IEEE 0. devices (check device analer, if an) RSSI = function ( or received) = pure number reported to device driver! Unfortunatel the RSSI scale is not standard, that is, device dependent! This fact does not allow to compare if device A receives better than device B (assuming different manufacturer) based on RSSI mesurement Problem: device A indicates maimum RSSI=55 ( bits) with 0 signal (0. ), and device B indicates maimum RSSI=3 (5 bits) with 5 (0.03 ). Q: when both A and B in (,,) receive 5, which one is better device? That is, which one would ou bu if ou are a sstem admin? A: the one with the minimum cost! Wh? 37 Illustration of general issues Convert electrical energ in RF waves (transmission), and RF waves in eletrical energ (reception) Sie of is related to RF frequenc of transmission and reception Shape (structure) of the is related to RF radiation pattern Radiation patterns of different tpes Positioning s Maimum coverage of workspace Securit issues Real tpes: omni-directional, semi-directional, highl-directional 3 Omnidirectional Omni-directional : radiates RF power equall in all directions around the vertical ais. Most common eample: dipole (see Access Points) See how to make it (disclaimer: do not tr this at home): Info & fun: Omnidirectional s: simple dipoles Real s are not isotropic radiators but, e.g., dipoles shape of proportional to wavelength λ/4 λ/ Eample: Radiation pattern of a simple Hertian dipole More info: TV dipole Q: Wh TV dipole is bigger? A: 00 Mh vs..4 Gh AP dipole 39 side view (-plane) side view (-plane) top view (-plane) Gain: maimum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) simple dipole 40 0

11 Omnidirectional s: simple dipoles Omnidirectional s: simple dipoles Dipole: passive gain is due to concentration (shape) of radiation Short range, verticallow Gain Dipole area coverage High Gain Dipole Long range, horiontal area coverage Dipole: active gain is obtained with power amplifiers (needs eternal source of energ) N.B. near (below) the dipole the signal is weak! And better radiation is obtained in sub-areas around the dipole! 4 Problem: how and when to mount omidirectional s? And which gain is ok? High Gain Dipole (e.g...0 dbi), ver flat coverage Low Gain Dipole (e.g. dbi) low signal in the proimit of the How: Ceiling? Wall? Client positions? Area? Man factors influence the planning... When:???????? Room A need for uniform radio coverage around a central point Outdoor: point-to-multipoint connection (star topolog) Room B 4 Omnidirectional s: simple dipoles Semi-directional s Antenna Tilt: degree of inclination of with respect to perpendicular ais High Gain Dipole (e.g...0 dbi), ver flat coverage mounted on the ceiling High Gain Dipole (e.g...0 dbi), ver flat coverage mounted on the ceiling with downtilt Patch (flat s mounted on walls) Panel (flat s mounted on walls) Yagi (rod with tines sticking out)????? Room A Some s allow a variable degrees downtilt. Half signal disperded in the sk, nd half better eploited. Room B 43 side view (-plane) Patch side view (-plane) Credits: top view (-plane) Yagi Horiontal beamwidth Semi-directional Beamwidth cone: -3dB signal boundar off-ais Vertical beamwidth 44

12 Eample of signal composition with phase variations Main Signal (phase 0) = Time (phase=degrees) Eample of signal composition with phase variations Main Signal (phase 0) = Time (phase=degrees) highl-directional s highl-directional s Parabolic Dish grid Common use: Point-to-point link Out of beam alignment? top view (-plane) dish beamwidth Antenna tpe H beamwidth V beamwidth Semi-directional beamwidth beamwidth Omni-dir. 360 o 7 o.. 0 o Up to 60 km (LOS) Beamwidth Patch/panel 30 o.. 0 o 6 o.. 90 o cone: Yagi 30 o.. 7 o 4 o.. 64 o -3dB signal beamwidth Parabolic dish 4 o.. 5 o 4 o.. o boundar Wind effect: better to have lower gain and wider beam off-ais highl-directional s Line of sight (LOS): Straigth line between transmitter and receiver No obstructions (outdoor long range reduces reflections) Polariation is more important than in indoor scenarios Fresnel Zone: RF is not laser light, RF signals diffuse energ in space Ellipse shaped area centered on the LOS ais Most additive RF signal is concentrated in the Fresnel Zone It is important that Fresnel Zone is free from obstacles Red one: additive phase signal Sum Main and +0 degrees) Sum Main and +90 degrees) Signal (phase +0 degrees) Signal (phase +90 degrees) Yellow one: inverse phase signal Sum Main and +0 degrees) Sum Main and +90 degrees) Signal (phase +0 degrees) Signal (phase +90 degrees) 47 highl-directional s Fresnel Zone (FZ) Blockage of Fresnel Zone causes link disruption Caused b buildings, (growing) trees, foliage, etc. Rule of thumb: < 0% obstruction of Fresnel Zone st FZ Practical rule: calculate the radius of FZ leaving 60% unobstructed radius nd FZ R=radius of 60% central FZ (feet) R60% = 43.3 (d/4f) d=distance(miles), f=freq (GhZ) Point source R00% = 7. (d/4f) R=radius of 00% FZ (feet) d=distance(miles), f=freq (GhZ) 4

13 highl-directional s Fresnel Zone (FZ) N.B. the FZ radius depends onl on the distance d between s, and frequenc f of RF signal! Tpe of, beam width (degree), and gain (dbi) have no effects! E.g. +3 dbi Yagi (30 degree beam) vs. +4 dbi Dish (5 degrees) have the same FZ!!!! In practice: if FZ is partiall obstructed, it is not useful to use higher gain s (with small degree beam)!!! highl-directional s Fresnel Zone (FZ) Is not relevant in indoor scenarios (due to reflections...) Consider the Earth bulge!!! Ver long point-to-point connections ma have more than 40% FZ obstucted b Earth surface! Earth Bulge heigth = h (feet) = D / Minimum heigth (link > 7 miles) H = (43.3 D/4F ) + D / D H Sectoried-directional s Aimuth and Elevation charts front Arras of sectoried directional s top view, 3 sector top view, 6 sector Space multipleing (channel reuse) sectoried Charts for understanding coverage pattern Aimuth chart (pattern seen from front/right/behind/left)) Obtained with spectrum analer with central frequenc Signal measured in db around the E.g. Dipole pattern: almost circular E.g. agi pattern: high in front, low beside N.B. distance and T power is not relevant (signal strength in a location is relative to ever other location in the chart, like with db) left left db behind above right right Elevation chart (pattern seen front/below/behind/above) below 5 5 3

14 Antennas: diversit Grouping of or more s multi-element arras Antenna diversit switched diversit, selection diversit receiver chooses with largest output diversit combining combine output power to produce gain cophasing needed to avoid cancellation (phased arra... Requires processor) Path Loss Path Loss: RF signal dispersion (attenuation) as a function of distance E.g. Possible formulas (36.6 or 3.4) Free space: Loss (in db) = 36.6+(0*log0(F))+(0*log0(D)) F (Mh), D (miles) Link budget issue: 6 db rule Each 6 db increase in EIRP (signal 4) implies double T range (e.g. see table below:.4gh Path Loss vs distance) λ/4 λ/ λ/4 λ/ λ/ λ/ 00 meters db 00 meters db -6 db + ground plane meters db 000 meters db 000 meters db 5000 meters - 4. db 0000 meters db -6 db -6 db -6 db 54 Link Budget Calculation Link Budget Calculation: eample Link Budget or Sstem Operating Margin Ecess of signal between transmitter and receiver Calculated for outdoor point-to-point connections Measured in db (relative) or or (absolute) Calculation: Receiver sensitivit RS (weakest detectable signal) The lower the better: e.g. IEEE 0. card (see device manual), -95 (Mbps), -93 ( Mbps), -90 (5.5 Mbps), -7 ( Mbps) Link Budget: received power (in ) - RS (in ) E.g. RS = -, received power = -50 Link budget = -50 (-) = +3 This means the signal has margin of +3 db before it becomes unviable Fade margin: etra margin for link budget (to cope with multipath variation in indoor/outdoor scenarios): tpical [+0..+0] db Eample: design of transmission sstem, needs amplifier? 00% Fresnel one free +4 dbi (free space) 0 Km = -0 db +4 dbi Total Loss= = -30. db -.4 db -0.7 db -.3 db -.0 db Transmitter Total Gain= dbi + 4 dbi = +63 db Condition: (gain + loss) > (RS + fade margin)??? ( ) > - +??? Received Power = -67. Link Budget = -67. (-) = +4. OK! Fade margin = +4. No need for amplifiers or high gain s Transmission power = db -0.7 db -.3 db -.0 db Receiver Receiver sensitivit RS =