David Tipper. Graduate Telecommunications and Networking Program

Transcription

1 Wireless Communication Fundamentals David Tipper Associate Professor Graduate Telecommunications and Networking Program University it of Pittsburgh Telcom 2700 Slides 2 Wireless Networks Wireless Wide Area Networks (WWANs) Cellular Networks : GSM, cdmaone (IS-95), UMTS, cdma2000 EVDO Satellite Networks: Iridium, Globalstar, GPS, etc. Wireless Metro Area Networks (WMANs) IEEE WiMAX Wireless Local Area Networks (WLANs) IEEE , a, b, g, etc. (infrastructure, ad hoc, sensor) Wireless Personal Area Networks (WPANs) IEEE (Bluetooth), IrDa, Zigbee, etc. Telcom

2 Wireless Issues Wireless link implications communications channel is the air poor quality: fading, shadowing, weather, etc. regulated by governments frequency allocated, licensing, etc. limited bandwidth Low bit rate, frequency planning and reuse, interference power limitations Power levels regulated, must conserve mobile terminal battery life security issues wireless channel is a broadcast medium! Wireless link implications for communications How to send a signal? How to clean up the signal in order to have good quality? How to deal with limited data rate and limited bandwidth? Telcom Typical Wireless Communication System Source Source Encoder Channel Encoder Modulator Channel Destination Source Decoder Channel Decoder Demod -ulator Telcom

3 Components of Communication system Source Produces information for transmission (e.g., voice, keypad entry, etc.) Source encoder Removes the redundancies and efficiently encodes the source info Example: In English, you may encode the alphabet e with fewer bits than you would q using a vocoder Channel encoder Adds redundant bits to the source bits to recover from any error that the channel may introduce Modulator Converts the encoded bits into a signal suitable for transmission over the channel Antenna A transducer for converting signals in a transmission line into electromagnetic radiation in an unbounded medium or vice versa Channel Carries the signal, but will usually distort it Receiver reverses the operations Telcom Signals Signal - physical representation of data Mathematically, a signal is represented as a function of time or can be expressed as a function of frequency Any electromagnetic signal can be shown to consist of a collection of sinusoids at different amplitudes, frequencies, and phases (Fourier Series or Transform) Communication systems perform the tasks of Signal generation Signal transmission Signal reception Signal detection Telcom

4 Terminology Consider a periodic signal (e.g., a sine wave) Period (T) - amount of time it takes for one repetition of the signal T = 1/frequency = 1/f Phase ( ) - measure of the relative position in time within a single period of the signal Wavelength ( ) - distance occupied by a single cycle of the signal Or, the distance between two points of corresponding phase of two consecutive cycles For electromagnetic waves in air or free space, = c/f where c is the speed of light = 3 x 10 8 m/sec Telcom General sine wave Consider a Sinusoid s(t) = A cos(2 ft + ) Next slide shows the effect of varying each of the three parameters A = 1, f = 1 Hz, = 0 => T = 1s Increased peak amplitude; A=2 Increased frequency; f = 2 => T = ½ Phase shift; = /4 radians (45 degrees) Note: 2 radians = 360 = 1 period Telcom

5 The sinusoid Acos(2 ft + ) 2 cos(2 t) 2 cos(2 2 t) Amplitude cos(2 t) cos(2 t + /4) time Telcom Frequency-Domain Concepts Frequencies measured by number of cycles per second unit is Hertz 5KH KHz 5000 times per second 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 tmost of the signal s energy is contained in Example: Human Voice absolute bandwidth 0-20 KHz, effective bandwidth Hz. Telcom

6 Frequencies for Communication twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible light UV VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wavelength: = c/f Wavelength, speed of light c 3x10 8 m/s, frequency f in Hz Telcom Radio Frequency Bands Telcom

7 Licensed Vs. Unlicensed Licensed Spectrum need to buy right to use spectrum allocation in a specific geographic location from the government (e.g., AM/FM radio) Prevents interference licensee can control signal quality Unlicensed spectrum Anyone can operate in the spectrum (e.g. ISM band for WLANs) but must maintain proper behavior in spectrum (max power level and frequency leakage, etc.) Can have interference problems Licensed Unlicensed Guaranteed access Better coverage and quality Higher barriers for entrance Fast Rollout Coverage and quality inconsistent More worldwide options Telcom Frequency Allocations Europe USA Japan WWANs Licensed WMANs Licensed Unlicensed Cellular: MHz, MHz; PCS: MHz, MHz; MHz, MHz 3G: MHz MHz IEEE GHz SAME as WLANs Cellular MHz, MHz; PCS MHz, MHz; IEEE GHz, GHz Same as WLANs Cellular MHz, MHz; MHz, MHz 3G MHz MHz IEEE GHz Same as WLANS WLANs IEEE IEEE IEEE Unlicensed MHz MHz (b, g) MHz (b, g) GHz GHz (a) GHz (a) HIPERLAN MHz WPANs Unlicensed IEEE MHz IEEE MHz IEEE MHz Telcom

8 What is Signal Propagation? Signal Propagation describes how a radio signal is transformed from the time it leaves a transmitter to the time it reaches the receiver Important for the design, operation and analysis of wireless networks Where should transmitters (i.e., base stations/access points) be placed What transmit powers should be used What frequency channels need be assigned to a transmitter How are handoff decision algorithms affected Propagation in free open space like light rays In general make analogy to light and sound waves Telcom 2700 Signal propagation Received signal strength (RSS) influenced by Fading signal weakens with distance received power proportional to 1/d² (d = distance between sender and receiver) Frequency dependent fading signal weakens with increase in f Shadowing (no line of sight path) Reflection off of large obstacles Scattering at small obstacles Diffraction at edges shadowing reflection scattering diffraction Telcom

9 Signal Propagation Scattering Tx Transmission Diffraction Reflection Rx Effects are similar indoors and out Several paths from Tx to Rx Different delays, phases and amplitudes Add motion makes it very complicated Termed a multi-path propagation environment Difficult to look at all of the effects in a composite way In practice Ray Tracing Approach: Breakdown phenomena into different categories use physics model for each path Use empirical based models Telcom Multipath Propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction signal at sender signal at receiver Time dispersion: signal is dispersed over time interference with neighbor symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts Limits the data rate on the channel Telcom

10 Effects of mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts Results in quick changes in the power received Called short term or fast fading Results in sudden burst of errors on the channel limits the goodput of the channel. power short term fading long term fading t Telcom The Radio Channel Three main issues in radio channel Achievable signal coverage What is geographic area covered by the signal Governed by path loss Achievable channel rates (bps) Governed by multipath delay spread Channel fluctuations effect data rate Governed by Doppler spread and multipath Consider the first one only two and three impact physical and link layer and will be studied later. Telcom

11 Coverage Determines Transmit power required to provide service in a given area (link budget) Interference from other transmitters Number of base stations or access points that are required Parameters of importance (Large Scale/Term Fading effects) Path loss (long term fading) Shadow fading Telcom Signal Propagation Ranges Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise sender transmission detection interference distance Telcom

12 Decibels Power (signal strength) is expressed in decibels (db) for ease of calculation Values relative to 1 mw are expressed in dbm Power in dbm = log 10 (Power in W / 1 mw) Values relative to 1 W are expressed in dbw Power in dbw = log 10 (Power in W / 1 W) Other values are simply expressed in db (i.e., Gains of Antennas, loss due to obstacles, etc.) Example 1: Express 2 W in dbm and dbw dbm: 10 log 10 (2 W / 1 mw) = 10 log 10 (2000) = 33 dbm dbw: 10 log 10 (2 W / 1 W) = 10 log 10 (2) = 3 dbw In general dbm value = 30 + dbw value Note 3 db implies doubling/halving power Telcom Free Space Loss Model Assumptions Transmitter and receiver are in free open space No obstructing objects in between The earth is at an infinite distance! The transmitted power is P t The received power is P r Isotropic antennas Antennas radiate and receive equally in all directions with unit gain The path loss is the difference between the received signal strength and the transmitted signal strength PL = P t (db) P r (db) d Telcom

13 A simple explanation of free space loss Isotropic transmit antenna Radiates signal equally in all directions P t 2 /(4 d) 2 Assume a point source At a distance d from the transmitter, the area of the sphere enclosing the Tx is A = 4 d 2 The power density on this sphere is P t / 4 d 2 Isotropic receive antenna d Captures power equal to the density times the area of the antenna Ideal area of antenna is A ant = 2 P /4 r = P t / L p The received power is: P r = P t / 4 d 2 2 /4 = P t 2 /(4 d) 2 Telcom Free space loss Transmit power P t Received power P r Wavelength of the RF carrier = c/f Over a distance d the relationship between P t and P r is given by: 2 Pt Pr 2 2 ( 4 ) d Where d is in meters In db, we have: P r (dbm)= P t (dbm) log 10 ( ) 20 log 10 (d) Path Loss = PL = P t P r = log 10 ( ) + 20log 10 (d) Telcom

14 Free Space Propagation Notice that factor of 10 increase in distance => 20 db increase in path loss (20 db/decade) Distance 1km 10Km Path Loss at 880 MHz db db Note that higher the frequency the greater the path loss for a fixed distance Distance 880 MHz 1960MHz 1km db db thus 7 db greater path loss for PCS band compared to cellular band in the US Telcom Consider Design of a Point-to- Point link connecting LANs in separate buildings across a freeway Distance.25 mile Line of Sight (LOS) communication Spectrum Unlicensed using b at 2.4GHz Maximum transmit power of AP is P t = 24dBm The minimum received signal strength (RSS) for 11 Mbps operation is -80 dbm Will the signal strength be adequate for communication? Given LOS is available can approximate propagation with Free Space Model as follows Example Telcom

15 Example Example Distance.25 mile ~ 400m Receiver Sensitivity Threshold h = - 80dBm The Received Power P r is given by P r = P t - Path Loss P r = P t log 10 ( ) 20 log 10 (d) = log 10 (3x10 8 /2.4x10 9 ) 20 log 10 (400) = = = P r is well above the required -80 dbm for communication at the maximum data rate so link should work fine Telcom Cell/Radio Footprint The Cell is the area covered by a single transmitter Path loss model roughly determines the size of cell RSS distance Telcom

16 10 Example Can use model to predict coverage area of a base station P t = 5 W f = 900 MHz = m P r in dbm distance from Tx in m If we require -60dbm RSS Telcom Path Loss Models Path Loss Models are commonly used to estimate link budgets, cell sizes and shapes, capacity, handoff criteria etc. Macroscopic or large scale variation of RSS Path loss = loss in signal strength as a function of distance Terrain dependent (urban, rural, mountainous), ground reflection, diffraction, etc. Site dependent (antenna heights for example) Frequency dependent Line of site or not Simple characterization: PL = L log 10 (d) L 0 is termed the frequency dependent component The parameter is called the path loss gradient or exponent The value of determines how quickly the RSS falls with distance Telcom

17 Path Loss Model cont. Can be written in terms of received power: P r = K P t d - is called the path-loss coefficient K depends on the frequency used depends on several factors and is often obtained empirically - Dense shadowed urban = 4 to 5.5 Shadowed Urban area = 3 to 4.5 Suburban area = 2.7 to 3.5 Free Space = 2 More complicated models based on curve fitting to measurements. These models allow for some site dependent parameters (e.g., antenna heights, indoor vs. outdoor, etc.). Consider examples Okumura Hata (outdoor cellular band model) JTC (indoor WLAN band model) Telcom Okumura-Hata Model Okumura collected measurement data ( in Tokyo) and plotted a set of curves for path loss in urban areas Hata came up with an empirical model for Okumura s curves L p = log f c log h te a(h re ) + ( log h te )log d Where f c is in MHz, d is distance in km, and h te is the base station transmitter antenna height in meters and h re is the mobile receiver antenna height in meters a(h re ) is a correction factor for different environments for fc > 400 MHz and large city a(h re ) = 3.2 (log [11.75 h re ]) db Can approximate a(h re ) with a constant C where C = -2 dense urban, -5 urban, -10 suburban, -26 rural Telcom

18 Example of Hata s Model Consider the case where h re = 2 m receiver antenna s height h te = 100 m transmitter antenna s height f c = 900 MHz carrier frequency L p = log d The path loss exponent for this particular case is = 3.18 What is the path loss at d = 5 km? d = 5 km L p = log 5 = db p If the maximum allowed path loss is 120 db, what distance can the signal travel? L p = 120 = log d => d = 10 (1.86/31.8) = 1.14 km Telcom Shadow Fading Shadowing occurs when line of site is blocked Modeled by a random signal component X P r = P t L p +X Measurement studies show that X can be modeled with a lognormal distribution normal in db with mean = zero and standard deviation db Thus at the designed cell edge only 50% of the locations have adequate RSS Since X can be modeled in db as normally distributed with mean = zero and standard deviation db determines the behavior d Telcom

19 How shadow fading affects system design Typical values for are rural 3 db, Suburban 6 db, urban 8 db, dense urban 10 db. Since X is normal in db P r is normal P r = P t L p +X Prob {P r (d) > T } can be found from a normal distribution table with mean P r and In order to make at least Y% of the locations have adequate RSS Reduce cell size Increase transmit power Make the receiver more sensitive d Telcom Example of Shadow Calculations The path loss of a system is given by L p = log 10 d 20 log 10 h b h b = 10m, P t = 0.5 W, receiver sensitivity = -100 dbm What is the cell radius? P t = 10 log = 27 dbm The permissible path loss is 27-(-100) = 127 dbm 20 log 10 h b = 20 log = 20 db 127 = log 10 d 20 => d = 316m But the real path loss at any location is X where X is a random variable representing shadowing Negative X = better RSS; Positive X = worse RSS If the shadow fading component is normally distributed with mean zero and standard deviation of 6 db. What should be the shadow margin to have acceptable RSS in 90% of the locations at the cell edge? Telcom

20 Example again Let X be the shadow fading component X = N(0,6) We need to find F such that P{X > F } = 0.1 We need to solve Q(F/ ) = 0.1 Use tables or software In this example F = 7.69 db Increase transmit power to = dbm = 3 W Make the receiver sensitivity dbm Reduce the cell size to m In practice use.9 or.95 quantile vales to determine the Shadow Margin SM SM is the amount of extra path loss added to the path loss budget to account for shadowing.9 SM = SM = Fading Margin F 10% Telcom The JTC Indoor Path Loss Model L Total A B log 10 ( d ) L f ( n ) X Similar to Okumura Hata model in cellular (curve fitting to measure values used to set up model A is an environment dependent fixed loss factor (db) B is the distance dependent loss coefficient, d is separation distance between the base station and portable, in meters L f is a floor/wall penetration loss factor (db) n is the number of floors/walls between the access point and mobile terminal X is a shadowing term due to non-line of sight Telcom

21 JTC Model (Continued) Environment Residential Office Commercial A(dB) B L f (n) (db) 4n (n-1) 6 + 3(n-1) Log Normal Shadowing Std. Dev. (db) Telcom JTC Model (Continued) Example Consider an AP on the first floor of a three story house. The distance to a third floor home office is approximately 8 meters If the AP operates at a power level of.05 W using the JTC model determine the path loss and received signal strength in the office area Using the JTC model with residential parameter set L total = A + B log 10 (d) + L f (n) + 8 = log 10 (8) + 4x2 +8 = db Power received = P r = P t -L total = dbm db = dbm P r is more than adequate. Telcom

22 Cell Coverage modeling Simple path loss model based on environment used as first cut for planning cell locations Refine with measurements to parameterize model Alternately use ray tracing: approximate the radio propagation by means of geometrical opticsconsider line of sight path, reflection effects, diffraction etc. CAD deployment tools widely used to provide prediction of coverage and plan/tune the network Telcom Cellular CAD Tools Use GIS terrain data base, along with vehicle traffic/population density overlays and propagation models Output t map with cell coverage at various signal levels l and interference values To plan out cell coverage area, cell placement, handoff areas, interference level frequency assignment Telcom

23 Use GIS maps This shows possible location of cell site and possible location of users where signal strength prediction is desired Telcom Outdoor Model CAD Tools provide a variety of propagation models: free space, Okumura- Hata, etc. Telcom

24 Typical City pattern Microcell diamond Radiation pattern Telcom Ray Tracing Mode Telcom

25 Indoor Models Telcom Cellular CAD Tools CAD tool first cut cell site placement, augmented by extensive measurements to refine model and tune location and antenna placement/type Temporary cell Telcom

26 Signal strength prediction for Indoor WLANS Motorola LAN Planner Lucent: WiSE tool Given building/space to be covered and parameters of building and AP predicts signal coverage Telcom Site Survey Tools Software to measure signal strength and recording in order to construct a coverage map of structure must drive/walk around structure to gather data NOKIA site survey tool, Ekahau Site Survey, Motorola LAN survey, etc. Telcom

27 Typical Wireless Communication System Source Source Encoder Channel Encoder Modulator Channel Destination Source Decoder Channel Decoder Demod -ulator Telcom Antennas Antenna Converts analog signals into electromagnetic radiation as efficiently as possible in the direction required Radiation pattern Way in which energy propagates in as a function of direction Any conductor or can serve as an antenna Use materials that result in efficient radiation Thin Dipole Biconical Dipole Loop Parabolic Reflector Microstrip Horn Antenna Telcom

28 Radiation lobes 3 db Main lobe Side lobe 3 db Beamwidth Back lobe Ideal Antenna Ideal antenna Gain = 1 over a certain angle Gain = 0 over the rest of the directions Real antenna Radiates power in unwanted directions Has one or more main lobes and many sidelobes Antenna Beamwidth The beamwidth is the angle of coverage where the radiated energy is 3 db down from the peak of the beam (half-power) Front-to-Back Ratio The ratio of the power in the main lobe to the power in the lobe created at the back of the antenna Telcom Antenna Gain The gain of an antenna in a given direction is the ratio of the power density produced by it in that direction divided by the power density that would be produced by a reference antenna in the same direction Two types of reference antennas are generally used Isotropic antenna: gain is given in dbi Half-wave dipole antenna: gain is given in dbd Manufacturers often use dbi in their marketing To show a slightly higher gain dbi = dbd db 0 dbi 0 dbd Dipole Isotropic Other 5 dbd = 7.15 dbi Telcom

29 Antenna Gains Directional antenna Focused beam high gain Omni-directional signal radiates in all directions equally low gain Telcom Antennas Two factors influence the size and shape of an antenna The frequency of the RF signal A low frequency signal needs a larger antenna The gain desired A high-gain antenna needs a larger antenna and more focused beam than a low-gain antenna Antenna gain adds into path loss calculations Directional antennas can be created using antenna arrays or horn/dish elements 45 0 Beamwidth 19 dbd Gain Panel Antenna Telcom

30 Cellular Antennas Cells are typically sectored into 3 parts each having sector of the cell to cover 1 transmit antenna in middle of each sector face 2 receive antenna at edge of sector face on the tower. This is done to provide antenna diversity it combats fast fading as only 1 antenna will likely be in fade at any point in time. Can get 3-5 db gain in the system Telcom Antenna Examples Monopole Omnidirectional Panel Array of dipoles for sectored cell Grid Reflector Antenna Telcom

31 Link Budget Used to plan useful radio coverage of link/cells Relates transmit power, path losses, margins, interference, etc. Used to find max allowable path loss on each link Typical Factors in Link Budget Transmit Power, Antenna Gain, Diversity Gain, Receiver Sensitivity Shadow Margin, Interference Margin, Vehicle Penetration Loss, Body Loss, Building Penetration, etc.. (Typical values from measurements used) Gains are added, Losses are subtracted must balance Telcom Link Budget Link Up Down TX Power 30dbm 30dbm Antenna Gain 3 5 Antenna Diversity Gain 5 X Shadow Margin Body Attenuation 2 2 Vehicle Penetration 5 5 Receiver Sensitivity Path Loss Budget 126 db 108 db This System Downlink Limited Other maybe Uplink Limited Telcom