Wireless data networks Physical Layer Martin Heusse X L A TEX E - - PowerPoint PPT Presentation

X E L

ATEX

Wireless data networks Physical Layer

Martin Heusse

• PHY. 2

Attenuation / Propagation

• Ethernet (twisted pair), attenuation < 10dB for 100m
• Fiber: typically 1dB/km
• Radio waves in the air: 10−2dB/km

But—the signal is not guided: the energy is projected on a surface that grows with the distance Pr = PtGtGr ( λ 4πd )2 (Friis transmission equation) Pr,t received / transmitted power; Gr,t rec./trans. antenna gain; d: distance, λ: wavelength (Grλ is the effective area of the receiver)

• PHY. 3

Consequences

• ✓ d = 1km → −80dB

✓ d = 10m → −40dB ✓ d = 1m → −30dB

• I’m deaf when I’m talking
• Relatively high gain input amplification

• PHY. 4

Real world propagation

• Reflections
• Diffraction
• Absorption

Loss at 5GHz

✓ Wood house siding: 8̃dB ✓ Concrete wall: 22dB

• Scattering
• Doppler (People movement)

• PHY. 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 d1 d2 r1 a b n = 1 n = 2 Transmitter Receiver Propagation path (can be curved)

Figure 2.19 Fresnel zones around a propagation path shown in 2 dimensions.

• Surface such that: d1 + d2 + nλ/2 = a + b
• rn ≈

√

nλd1d2 d1+d2

• PHY. 6

Two ray ground

ht hr d d1 d2

• Pr = PtGtGr

( λ

4πd

)2

• 1 −

(

d d1+d2

) e−jδφ

• 2
• If d1 + d2 ≈ d then |1 −

(

d d1+d2

) e−jδφ|2 ≈ δφ2 and (…) δφ = 2π

λ (d1 + d2 − d) ≈ 2π λ 2 hthr d

• Then:

Pr = PtGtGr (hthr)2 d4

(No more proportional to λ…)

• PHY. 7

Antennas

• Current → charge displacement → Electrical field…

Current in a two-wire transmission line

_ +

• Corpuscular version: electrons emit photons when shaken!

• PHY. 8

Antennas (cont.)

• Eletrical field lines
• Dipole antenna

λ/2

• PHY. 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. 1992.  1992 IEEE).

Do not point it toward the intended receiver!

• Marconi antenna

Same thing, using the reflection on a ground plane.

λ/4

• PHY. 10

Antennas (cont.)

θ θ 30° 0° 60° 90° 90° 60° 30° 120° 150° 180° 150° 120° Relative power (dB down) 10 20 30 20 10 30 − − − + + −

Two-dimensional amplitude patterns for a thin dipole of l = 1.25λ (Copied from Balanis)

• PHY. 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!
• Yagi

Directors Driven element Reflector

• PHY. 12

Other antennas (cont.)

• Parabola

+

All points equidistant from a fixed line and a fixed point

• PHY. 13

Modulation techniques

• ASK
• FSK
• PSK (BPSK, QPSK)
• QAM

• PHY. 14

Signal shaping

• PSK exemple: abrupt transitions on symbol boundaries

1 2 3 4 5 −1.0 −0.5 0.0 0.5 1.0 t f (x)

→ 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. 15

Example: QAM modulation

R bps

sin(2πfct)

Shaping Shaping

cos(2πfct)

+

• PHY. 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. 17

What is fading? (cont.)

• PHY. 18

Spread spectrum

• Use as much of the channel as possible

Make Ebb/N0 as large as possible

• Helps in presence of narrow band interferences
• Spread the transmitted power over a wider freq. range (ISM

bands…)

• PHY. 19

FHSS

• PHY. 20

DSSS

• XOR each bit of data with a predefined sequence (or code)
• Same operation at the receiver (x ⊗ c ⊗ c = x)
• Used by 802.11b
• Related to CDMA

• PHY. 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. 22

OFDM Orthogonal Frequency Division Multiplexing (cont.)

• 0.4
• 0.2

0.2 0.4 0.6 0.8 1

• 10
• 5

5 10 sin(x+pi)/(x+pi) sin(x)/x sin(x-pi)/(x-pi)

• PHY. 23

OFDM Orthogonal Frequency Division Multiplexing (cont.)

Serial to parallel R bps IFFT

fc

Parallel to serial

T T T

DAC

• PHY. 24

OFDM Orthogonal Frequency Division Multiplexing (cont.)

• Applications

✓ (A)DSL (250 sub-carriers split between uplink and downlink)

◮ Symbol duration: 231.88µs ↔ sub-carrier spacing: 4.3125

kHz ✓ 802.11a, g (48 sub-carriers) ✓ DVB-T (up to 8192 sub-carriers), DAB ✓ WiMAX ✓ LTE