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

2. 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

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

4. 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

5. 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

6. 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

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

8. 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...

9. 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

10. 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

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

12. 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

13. 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

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

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

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

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

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

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

20. 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

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

22. 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

23. 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