# CS 414 Indian Institute of Technology, Bombay CS 414 Wireless Propagation Basics.

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CS 414 Indian Institute of Technology, Bombay CS 414 Wireless Propagation Basics

CS 414 Indian Institute of Technology, Bombay Last Lecture ● Radio waves – 3KHz to 300 Ghz ● Analog and Digita data and signals – Bandwidth and Data Rate ● Multiplexing and Modulation

CS 414 Indian Institute of Technology, Bombay #3#3 Decibels and Signal Strength ● Signal strength: Magnitude of electric field at a location – Changes with distance in wireless – Approximately like a log-function ● Decibel relative measure of signal strength – db = 10 log 10 (P s /P r ) – P s :Sender power – P r :Receiver power

CS 414 Indian Institute of Technology, Bombay #4#4 Decibel Examples ● Let P s = 10mW – P r = 5mW – G = 10 log 10 (0.5) = - 3 db (i.e., 3db loss) – P r = 1mW, G = -10 db – P r = 0.1mW, G = -20 db – P r = 0.01mW, G = -30 db – P r = 0.001mW, G = -40 db ● If P s = P r = 10 mW, G = ? ● If G =1, Pr = ?

CS 414 Indian Institute of Technology, Bombay #5#5 Decibel Math ● Gain is multplicative – Addition in db space – e.g., signal is reduced by half on first hop then amplifed by factor of 2 – P s = 10 mW, P 1 = 5 mW, P 2 = 10 mW – G 1 = -3db – G 2 = +3db – G at P2 = G 1 + G 2 = 0 db =>P s = P 2

CS 414 Indian Institute of Technology, Bombay #6#6 dbW, dbm, dbi, dbv... ● db = relative maginutes ● Fix reference for to initial signal for absolute compariton ● dbm = 10 log (Power mW / 1mW) – 1 mW = 0 dbm – 10 mW = 10 dbm – 30 dbm = ? ● If P1 – P2 = 10 dbm then, absolute difference is ? ● 1 dbW = ? dbm

CS 414 Indian Institute of Technology, Bombay Last Lecture ● db – relative measure – db = 10 log 10 (ratio) ● if Ps = 10 mW and Pr = – 10mW ~ 0 db – 1 mW ~ -10 db – 100 mW ~ 10 db – 1000 mW ~ 20 db ● dbm, dbv, dbi, dbW... – absolute measure

CS 414 Indian Institute of Technology, Bombay Antennas ● Essential wireless propogation component ● Entry and end-points of RF signals ● Two funtions – Transmission ● Converts electric energy to electro-magnetic – Receiver ● Converts received electro-magnetic energy to electric signal ● Passive element

CS 414 Indian Institute of Technology, Bombay Antenna Types ● Omni-directional – radiation in all directions ● Directional – Focuses (more) power in certain direction – Does not amplify

CS 414 Indian Institute of Technology, Bombay Types of Antennas λ/4 Quarter-wave Antenna λ/4 λ/2 Half-wave Dipole www.cellamericas.com Parabolic Antenna

CS 414 Indian Institute of Technology, Bombay Antenna Gain ● Isotropic Antenna – Idealized omni-directional antenna – Tranmits power in all directions uniformly ● Antenna Gain (G) – ratio of transmit/receive power in a particular direction w.r.t to an isotropic antenna – G = P direction /P isotropic – Measure of reception/tranmission strength of antenna – Transmit Gain = Receive Gain – Typical units: dBi (dB gain w.r.t to isotropic)

CS 414 Indian Institute of Technology, Bombay Antenna Gain ● Isotropic – G = 1 in all directions ● Non-uniform/Directional – G a = ? – G b = ? – G c = ? A B C

CS 414 Indian Institute of Technology, Bombay Antenna Parameters ● Radiation Pattern – Spread of transmitter power ● (Half-power) Beam width – Angle between half power point (direction) and the point (direction) of maximum power – Assumption: Reception beyond beam width is poor and unreliable

CS 414 Indian Institute of Technology, Bombay Antenna Parameters ● Antenna Polarization – Orientation/plane of the electric field (E-plane) ● w.r.t earth's surface Horizontally polarized Vertically polarized image source: wikipedia

CS 414 Indian Institute of Technology, Bombay RF Propagation – Questions: Relation between RF propogation & ● distance ● different environments ● quantification metrics – Goals: Provide connectivity, Estimate link quality (Is this link good?) – Solution Design: ● Antenna placement ● Link quality mapping and monitoring ● Tranmist power, antenna height

CS 414 Indian Institute of Technology, Bombay Propagation Phenomena

CS 414 Indian Institute of Technology, Bombay Path Loss Estimation ● Isotropic Antenna – Free space loss Pt d A eff = product of physical area (mW/m 2 ) and conversion efficiency d in Km and f in MHz

CS 414 Indian Institute of Technology, Bombay Free Space Loss ● With antenna gains G t and G r EIRP regulations to control radiated power Effective isotropic radiated power (EIRP) Free Space Loss

CS 414 Indian Institute of Technology, Bombay Path Loss Example ● P t = 50 mW, f = 2.4 Ghz, d = 2Km, P r = ? ● G t = 24 dBi, G r = 24 dBi, Pr = ?

CS 414 Indian Institute of Technology, Bombay Next Class ● Loss proportional to d 2... in reality ? ● Reading “The Mistaken axioms of wireless-network research”, D. Kotz, C. Newport, C. Elliott http://pdos.csail.mit.edu/decouto/papers/kotz03.pdf “ Most research on ad-hoc wireless networks makes simplifyingassumptions about radio propagation. The “Flat Earth” model of the world is surprisingly popular: all radioshave circular range, have perfect coverage in thatrange, and travel on a two-dimensional plane.... We then present a set of 802.11 measurements that clearly demonstrate that these “axioms” are contrary to fact.” http://pdos.csail.mit.edu/decouto/papers/kotz03.pdf

CS 414 Indian Institute of Technology, Bombay Last Lecture ● Antenna Gain – G dB = 10 log (P direction /P isotropic ) – Transmit/Receive gain ● Beamwidth – Angle between half-power direction and direction of maximum power

CS 414 Indian Institute of Technology, Bombay Last Lecture ● Path Loss Estimation – Isotropic Antenna d in Km and f in MHz

CS 414 Indian Institute of Technology, Bombay Path Loss Example ● P t = 50 mW, f = 2.4 Ghz, d = 2Km, P r = ? – Path loss = 32.5 + 20 log (2400) + 20 log (2) = 106 dB – P t = 17 dBm – P r = 17dBm – 106 dB = -89 dBm ● G t = 24 dBi, G r = 24 dBi, Pr = ? – P r = 17 dBm + 24 dBi + 24 dBi – 106 dB P r = -41 dBm

CS 414 Indian Institute of Technology, Bombay Near Field & Far Field ● Angular effects at short distances ● At large distances, angular/spherical power density appears flat ● D=1m, f=2.4 Ghz, d f =? Near field Far Field d Valid only when d is in far field D: largest dimension of antenna

CS 414 Indian Institute of Technology, Bombay Path Loss Remars ● Free space path loss is idealistic ● Other losses – Cable (dB/ft) – Environment – Simultaneous tranmissions ● More realistically loss is proportional to d 3 or higher

CS 414 Indian Institute of Technology, Bombay RF Propagaton Phenomena ● Reflection λ < D ● Defraction λ ~ D ● Scattering λ > D ● Refraction

CS 414 Indian Institute of Technology, Bombay Noise ● Change in signal due to random/unwanted distrortions – Medium variations – Frequency interactions – Interference ● Simultaneous transmissions – Impulse noise ● Lightning...

CS 414 Indian Institute of Technology, Bombay Fading ● Variation in received signal strength over time – Short-term fading ● Rapidly changing ● Due to multipath – Long-term fading ● Due to mobility ● Slowly changing (degrading) due to distance

CS 414 Indian Institute of Technology, Bombay Multipath ● Effect of reflection ● Same signal reaches receiver by more than one path – Constructive or destrictive side-effects – Inter-symbol interference T R

CS 414 Indian Institute of Technology, Bombay Received Signal Strength ● Function of --- – Transmit power – Path loss – Short-term fading – Long-term fading

CS 414 Indian Institute of Technology, Bombay RF Propogation in reality ● “The Mistaken axioms of wireless-network research”, D. Kotz, C. Newport, C. Elliott http://pdos.csail.mit.edu/decouto/papers/kotz03.pdf “ Most research on ad-hoc wireless networks makes simplifying assumptions about radio propagation. The “Flat Earth” model of the world is surprisingly popular: all radios have circular range, have perfect coverage in that range, and travel on a two-dimensional plane.... We then present a set of 802.11 measurements that clearly demonstrate that these “axioms” are contrary to fact.” http://pdos.csail.mit.edu/decouto/papers/kotz03.pdf

CS 414 Indian Institute of Technology, Bombay Assumptions of RF Propogation ● Isotropic – radiation pattern is defined (circular for omni) – if I can hear you, you can hear me – if I can hear you, I can hear you perfectly ● Signal strength is a function of distance ● All similar radios have similar range ● World is flat

CS 414 Indian Institute of Technology, Bombay Are omni-antennas isotropic? Isotropic Antenna Real radio radiation patterns

CS 414 Indian Institute of Technology, Bombay Are omni-antennas isotropic?

CS 414 Indian Institute of Technology, Bombay Link quality is binary! ● ??

CS 414 Indian Institute of Technology, Bombay Are Radio ranges same? ● With non-isotropic radiation patterns difficult to estimate range

CS 414 Indian Institute of Technology, Bombay Are links symmteric? ● No

CS 414 Indian Institute of Technology, Bombay Signal Strength ● Simple function of distance ? – NO

CS 414 Indian Institute of Technology, Bombay To Summarize... ● Radiation pattern not circular (spherical) ● Links not symmetric ● Link quality and Signal strength not a simple function of distance ● Radio range cannot be estimated

CS 414 Indian Institute of Technology, Bombay Implications of non-isotropic?

CS 414 Indian Institute of Technology, Bombay Implications of non-isotropic? ● Cannot estimate signal strength based on distance – does link exist? is it probabilistic? – Probability of data/packet reception uncertain ● Simplistic assumption of d^2 used in simulations will not mimic reality ● Routing decisions

CS 414 Indian Institute of Technology, Bombay Next Topics ● 802.11 ● 802.15.4 ● Bluetooth

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