5. the Multipath Challenge

| University of Dar es Salaam | Department of Telecommunications Engineering |_______________________________________________________________________________

------------------------------------------------------------------------------------------------------------------------| TE 412 Digital Telecommunications II | Christine Mwase | 29/02/2008 | Slide 1

Digital Telecommunications II

Multipath Propagation



Mobile Radio Propagation - Outline

• Introduction

• Radiowave Propagation Challenges– Free Space Attenuation

– Multipath and Shadowing

– Time Delay Spread

• Radiowave Propagation Mitigation Strategies

• Conclusion




• Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal

• Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave

• Scattering – occurs when incoming signal hits an object whose size is in the order of the wavelength of the signal

or less




Reflection (R), Scattering (S), Diffraction (D)



• Received signal at mobile

= many attenuated,

reflected, diffracted versions of original

signal

� multipath fading.

Multipath propagation



Shadowing / Log-normal fading (Large-scale signal variation)

Important when Non-LOS scenario.

• Shadowing = slow variations in received signal power as

user moves through environment.

• Caused by blockages e.g. large buildings, nearby hills.

• Average signal strength often assumed to follow an inverse

fourth law rather than inverse square.

• Shadowing can severely attenuate the received power level, but its rate of change is generally low (<2 times per second).



Fast Fading

• Caused by phasor additions of multipath signals.

• Assuming two independent paths only,

– arrival phase for each of these rays will be different.

– as user moves, d will vary, hence phase will alter.

– If rays in phase = constructive interference � signal peak.

– If antiphase (180 degrees apart) = destructive interference �

signals will cancel (fade).

λπθθ do 2−=



Example of

received

signal level variation



Rayleigh vs. Rician



Rayleigh vs. Rician

• Rayleigh• Rays have similar amplitudes (Non-LOS)

• Rician

• One dominant multipath exists (LOS).

• Vector summation deterministic

( )22 2log10 σAK =

A2 = power in LOS.

2σ2 = power in random components

Rician: K large

Rayleigh: K small



Rayleigh: Rician:

Rayleigh vs. Rician



received power

dBm-30-35-40-45-50-55-60-65

14 15 16 17 18 19 20 21 22 23 24 25 26 d (meters)

Small Scale & Large Scale Fading

fc = 2.4GHz � λ ≈ 0.125m

1 point = average over 5λ = 0.625m



Propagation Characteristics

The three scales of mobile signal variation







• Path Loss (includes average shadowing)

• Shadowing (due to obstructions)

• Multipath Fading

Pr/Pt

d

PrPt

d

v Very slow

Slow

Fast



Mobile Radio Propagation - Outline

• Introduction

• Radiowave Propagation Challenges– Free Space Attenuation

– Multipath and Shadowing

– Time Delay Spread

• Radiowave Propagation Mitigation Strategies

• Conclusion

5. the Multipath Challenge

Documents

Transcript of 5. the Multipath Challenge