Technical Foundation of WiMAX (Chapter 3)

Technical Foundation of WiMAX (Chapter 3)

ENE 490

MON 13:30-16:30

Asst. Prof. Suwat Pattaramalai

Technical Foundation of WiMAX

� The Challenge of Broadband Wireless Channels

� Communication System Building Blocks


� Broadband Wireless Channel: Pathloss and

Shadowing

� Model: Simple tap-delay line (TDL)

� Output of channel:


� Some questions


� Pathloss: free-space pathloss formula (Friis formula)


� Two-ray approximation:

� Empirical path loss formula:

� Range versus Bandwidth: opposite direction

� Large pathloss and Increased Capacity: interfering transmitters are father away than the desired transmitter


� Shadowing


� Empirical pathloss formula:

� where is a sample of the shadowing random process and

is typically modeled as a lognormal random variable.

� Shadowing cause large random fluctuations about the

pathloss model

� Beneficial: if an object is blocking interference


� Cellular Systems

� Cellular concept

Technical Foundation of WiMAX� Analysis of Cellular Systems

� Cochannel-reuse ratio

� SIR can be used instead of SINR as

� Received SIR for the worst case (empirical pathloss formula)

� Outage probability

Technical Foundation of WiMAX� Sectoring (instead split cell): using directional antenna


� The Broadband Wireless Channel: Fading (caused by

multipath and local scattering)

� Reflections may arrive nearly simultaneously


� Channel response have two dimentions: delay (τ) and time (t)

� Wide-sense stationary uncorrelated scattering (WSSUS): model for wideband fading channels because of scale τ usually µsec and t usually msec


� Delay Spread and Coherence Bandwidth

Technical Foundation of WiMAX� Doppler Spread and Coherence Time


� Modeling Broadband Fading Channels

� Statistic channel models

� Rayleigh Fading: Quadrature components are two independent

time-correlated Gaussian random process and PDF of r(t) is

� PDF of r(t) is Rayleigh and PDF of received power |r|2 is

exponential

� Phase of r(t) is uniformly distributed from 0 to 2π or [- π, π]


� LOS Channels: Ricean distribution (there is a dominant path LOS)

� A more general model: Nakagami-m Fading


� Statistical Correlation of the Received Signal

� Time Correlation, Frequency correlation, The selectivity/dispersion

duality

Technical Foundation of WiMAX� Multidimensional Correlation

Technical Foundation of WiMAX� Empirical Channel Models (extensive measurement )

� 3GPP channel model: modeling outdoor macro- and microcell

wireless environment (commonly used in WiMAX modeling)

Technical Foundation of WiMAX� Semiempirical Channel Models: 3GPP2 pedestrian A,B and vehicular A,B

� Mitigation of Fading

� Frequency-selective fading is more prominent in wideband channels


� Broadband fading: time dispersion or frequency selectivity

� Narrowband fading: frequency dispersion or time selectivity

� Diversity is the key to overcoming loss from fading

� Time diversity, Spatial diversity, Frequency diversity, Diversity-Types

Interactions

Technical Foundation of WiMAX� Broadband Fading: frequency-selective fading cause ISI

� ISI: Inter-Symbol Interference can fixed by using OFDM and

� Spread spectrum (CDMA) and Rake receivers

� Equalization: linear/nonlinear equalizer, maximum likelihood

sequence detection (MLSD)

� Multicarrier Concept: time-dispersive ISI channel

� Summary and Conclusions� Summary and Conclusions

Technical Foundation of WiMAX (Chapter 3)

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