Wireless data networks Physical Layer

Transcription

1 Wireless data networks Physical Layer Martin Heusse X L ATEX E

2 Attenuation / Propagation Ethernet (twisted pair), attenuation < 10dB for 100m Fiber: typically 1dB/km Radio waves in the air: 10 2 db/km But the signal is not guided: the energy is projected on a surface that grows with the distance ( ) λ 2 P r = P t G t G r 4πd (Friis transmission equation) P r,t received / transmitted power; G r,t rec./trans. antenna gain; d: distance, λ: wavelength (G r λ is the effective area of the receiver) PHY. 2

3 Consequences d = 1km 80dB d = 10m 40dB d = 1m 30dB I m deaf when I m talking Relatively high gain input amplification PHY. 3

4 Real world propagation Reflections Diffraction Absorption Loss at 5GHz Wood house siding: 8 db Concrete wall: 22dB Scattering Doppler (People movement) PHY. 4

5 Fresnel zone Elliptically shaped surface of revolution where the reflected/diffracted path length is different by a multiple of 180 from the direct path n = 3 n = 2 n = 1 d 1 d 2 Transmitter a r 1 b Receiver Figure 2.19 Propagation path (can be curved) Fresnel zones around a propagation path shown in 2 dimensions. Surface such that: d 1 + d 2 + nλ/2 = a + b r n nλd 1 d 2 d 1 +d 2 PHY. 5

6 Two ray ground h t d 1 d d 2 h r ( P r = P t G t G λ ) 2 ( ) 1 r 4πd d d 1 +d 2 e jδϕ 2 If d 1 + d 2 d then 1 ( d d 1 +d 2 ) and ( ) δϕ = 2π λ (d 1 + d 2 d) 2π λ Then: (No more proportional to λ ) e jδϕ 2 δϕ 2 2 h th r d P r = P t G t G r (h t h r ) 2 d 4 PHY. 6

7 Antennas Current charge displacement Electrical field Current in a two-wire transmission line _ + Corpuscular version: electrons emit photons when shaken! PHY. 7

8 Antennas (cont.) Eletrical field lines λ/2 Dipole antenna PHY. 8

9 Antennas (cont.) Figure 4.11 Three-dimensional pattern of a λ/2 dipole. (SOURCE: C. A. Balanis, Antenna Theory: A Review Proc. IEEE, Vol. 80, No 1. Jan IEEE). Do not point it toward the intended receiver! Marconi antenna λ/4 Same thing, using the reflection on a ground plane. PHY. 9

10 Antennas (cont.) θ θ Relative power (db down) Two-dimensional amplitude patterns for a thin dipole of l = 1.25λ (Copied from Balanis) PHY. 10

11 Other antennas Horn July 11, 1962, first transmission from the US to france of a TV signal, bounced back by the Telstar satellite. The satellites were low orbiting so that required a tracking antenna. No positioning system on the satellite: they were spheres and no parabolic antenna!! Antenna weight: 280 tons! 6.69 GHz uplink, 4.12 GHz downlink. The radome is inflated. Still the biggest radomes ever built, and still there in Plemeur-Bodou! Reflector Driven element Directors Yagi PHY. 11

12 Other antennas (cont.) + Parabola All points equidistant from a fixed line and a fixed point PHY. 12

13 Modulation techniques ASK FSK PSK (BPSK, QPSK) QAM PHY. 13

14 Signal shaping PSK exemple: abrupt transitions on symbol boundaries f (x) t Infinite spectrum! () Signal shaping Ex.: raised cosine spectrum. The impulse response of a raised cosine is 0 at every nt (no interference at sampling instant) PHY. 14

15 Example: QAM modulation R bps Shaping Shaping sin(2πfct) cos(2πfct) + PHY. 15

16 What is fading? Additive white Gaussian noise (AWGN) Thermal noise, electronics, propagation Rayleigh fading: multiple indirect paths; no dominant (LOS) path The signal enveloppe follows a Rayleigh distribution Rician: there is a dominant path; parameter K = power(lospat) powerotherpaths PHY. 16

17 What is fading? (cont.) PHY. 17

18 Spread spectrum Use as much of the channel as possible Make E b b/n 0 as large as possible Helps in presence of narrow band interferences Spread the transmitted power over a wider freq. range (ISM bands ) PHY. 18

19 FHSS PHY. 19

20 DSSS XOR each bit of data with a predefined sequence (or code) Same operation at the receiver (x c c = x) Used by b Related to CDMA PHY. 20

21 OFDM Orthogonal Frequency Division Multiplexing Many narrow band symbols sent in parallel ( gain and phase is constant in each sub-channel) Good spectrum utilization Limited complexity at both the emitter and receiver Works great with severe channel conditions But: Sensitive to Doppler shift Requires linear amplifiers, high peak to average power ratio (the power peaks when the sine waves add to each other) PHY. 21

22 OFDM Orthogonal Frequency Division Multiplexing (cont.) 1 sin(x+pi)/(x+pi) sin(x)/x sin(x-pi)/(x-pi) PHY. 22

23 OFDM Orthogonal Frequency Division Multiplexing (cont.) R bps Serial to parallel T IFFT T Parallel to serial T DAC fc PHY. 23

24 OFDM Orthogonal Frequency Division Multiplexing (cont.) Applications (A)DSL (250 sub-carriers split between uplink and downlink) Symbol duration: µs sub-carrier spacing: khz a, g (48 sub-carriers) DVB-T (up to 8192 sub-carriers), DAB WiMAX LTE PHY. 24