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Physical layer

Physical layer. Taekyoung Kwon. signal. physical representation of data function of time and location signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift  E.g., sinewave is expressed as s(t) = A t sin(2  f t t +  t ).

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Physical layer

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  1. Physical layer Taekyoung Kwon

  2. signal • physical representation of data • function of time and location • signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift  • E.g., sinewave is expressed as s(t) = At sin(2  ft t + t)

  3. Signal (Fourier representation) 1 1 0 0 t t ideal periodic signal real composition • Digital signals need • infinite frequencies for perfect transmission (UWB?) • modulation with a carrier frequency for transmission (analog signal!)

  4. signal • Different representations of signals • amplitude (amplitude domain) • frequency spectrum (frequency domain) • phase state diagram (amplitude M and phase  in polar coordinates) Q = M sin  A [V] A [V] t[s]  I= M cos   f [Hz]

  5. Radio frequency 직진성

  6. Radio channel type * Ground wave = surface wave + space wave

  7. Radio channel type -> Really? 802.16

  8. Radio channel type

  9. Why 60GHz?

  10. Why 60GHz? Frequency reuse

  11. 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 Xmission distance detection interference

  12. Radio propagation

  13. Attenuation in real world • Exponent “a” can be up to 6, 7

  14. propagation reflection scattering diffraction

  15. Signal propagation models • Slow fading (shadowing) • Distance between Tx-Rx • Signal strength over distance • fast fading • Fluctuations of the signal strength • Short distance • Short time duration • LOS vs. NLOS

  16. Slow fading vs. fast fading • Slow fading = long-term fading • Fast fading = short-term fading long term fading power t short term fading

  17. shadowing • Real world • Main propagation mechanism: reflections • Attenuation of signal strength due to power loss along distance traveled: shadowing • Distribution of power loss in dBs: Log-Normal • Log-Normal shadowing model • Fluctuations around a slowly varying mean

  18. shadowing

  19. Fast fading • T-R separation distances are small • Heavily populated, urban areas • Main propagation mechanism: scattering • Multiple copies of transmitted signal arriving at the transmitted via different paths and at different time-delays, add vector-like at the receiver: fading • Distribution of signal attenuation coefficient: Rayleigh, Ricean. • Short-term fading model • Rapid and severe signal fluctuations around a slowly varying mean

  20. Fast fading

  21. Fast fading

  22. Fast fading

  23. The final propagation model

  24. Real world example

  25. Modulation and demodulation analog baseband signal digital data digital modulation analog modulation radio transmitter 101101001 radio carrier analog baseband signal digital data analog demodulation synchronization decision radio receiver 101101001 radio carrier UWB: no carrier -> low cost, low power

  26. modulation • Digital modulation • digital data is translated into an analog signal (baseband) • ASK, FSK, PSK • differences in spectral efficiency, power efficiency, robustness • Analog modulation • shifts center frequency of baseband signal up to the radio carrier • Motivation • smaller antennas (e.g., /4) • Frequency Division Multiplexing • medium characteristics • Basic schemes • Amplitude Modulation (AM) • Frequency Modulation (FM) • Phase Modulation (PM)

  27. Digital modulation 1 0 1 • Modulation of digital signals known as Shift Keying • Amplitude Shift Keying (ASK): • very simple • low bandwidth requirements • very susceptible to interference • Frequency Shift Keying (FSK): • needs larger bandwidth • Phase Shift Keying (PSK): • more complex • robust against interference t 1 0 1 t 1 0 1 t

  28. antenna • Radiation and reception of electromagnetic waves • Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna • Real antennas always have directive effects (vertically and/or horizontally) z y z ideal isotropic radiator y x x

  29. antenna • Isotropic • Omni-directional • Radiation in every direction on azimuth/horizontal plane • Directional • Narrower beamwidth, higher gain

  30. Omni vs directional

  31. Antenna (directed or sectorized) • E.g. 3 sectors per BS in cellular networks y y z directed antenna x z x side view (xy-plane) side view (yz-plane) top view (xz-plane) z z sectorized antenna x x top view, 3 sector top view, 6 sector

  32. Switched vs. adaptive

  33. Switched vs. adaptive

  34. MIMO?

  35. Why directional antenna? • Wireless channel is a shared one • Transmission along a single multi-hop path inhibits a lot of nodes • Shorter hops help, but to a certain degree • Gupta-Kumar capacity result: • T = O( W / sqrt(nlogn) ) • Major culprit is “omnidirectionality”

  36. Why directional antenna? • Less energy in wrong directions • Higher spatial reuse • Higher throughput • Longer ranges • Less e2e delay • Better immunity to other transmission • Due to “nulling” capability

  37. Directional vs. networks • One-hop wireless environments • Cellular, WLAN infrastructure mode • BS, AP: directional antenna • Mobile: omni-directional • Ad hoc, sensor networking • Every node is directional

  38. Directional antenna types • Switched: can select one from a set of predefined beams/antennas • Adaptive (steerable): • can point in almost any direction • can combine signals received at different antennas • requires more signal processing

  39. Antenna model 2 Operation Modes: OmniandDirectional A node may operate in any one mode at any given time

  40. Antenna model In Omni Mode: • Nodes receive signals with gain Go • While idle a node stays in omni mode In Directional Mode: • Capable of beamforming in specified direction • Directional Gain Gd(Gd > Go) Symmetry: Transmit gain = Receive gain

  41. Potential benefits • Increase “range”, keeping transmit power constant • Reduce transmit power, keeping range comparable with omni mode • Reduces interference, potentially increasing spatial reuse

  42. neighbor • Notion of a “neighbor” needs to be reconsidered • Similarly, the notion of a “broadcast” must also be reconsidered

  43. Directional neighbor Receive Beam Transmit Beam B A C • When C transmits directionally • Node A sufficiently close to receive in omni mode • Node C and A are Directional-Omni (DO) neighbors • Nodes C and B are not DO neighbors

  44. Directional neighbor Transmit Beam Receive Beam A C B • When C transmits directionally • Node B receives packets from C only in directional mode • C and B are Directional-Directional (DD) neighbors

  45. Directional antenna for MAC • Less energy consumption • Within the boundary of omni-directional Xmission range • Same energy consumption • DD neighbor is possible

  46. Directional antenna for routing • same energy consumption • One hop directional transmission across multi-hop omnidirectional transmission • DO neighbor will be the norm

  47. D-MAC Protocol[Ko2000Infocom]

  48. IEEE 802.11 F A B C D E RTS RTS CTS CTS DATA DATA ACK ACK Reserved area

  49. Directional MAC (D-MAC) • Directional antenna can limit transmission to a smaller region (e.g., 90 degrees). • Basic philosophy: MAC protocol similar to IEEE 802.11, but on a per-antenna basis

  50. D-MAC • IEEE802.11: Node X is blocked if node X has received an RTS or CTS for on-going transfer between two other nodes • D-MAC: Antenna T at node X is blocked if antenna T received an RTS or CTS for an on-going transmission • Transfer allowed using unblocked antennas • If multiple transmissions are received on different antennas, they are assumed to interfere

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