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
Technical Foundation of WiMAX
Technical Foundation of WiMAX
� Broadband Wireless Channel: Pathloss and
Shadowing
� Model: Simple tap-delay line (TDL)
� Output of channel:
Technical Foundation of WiMAX
� Some questions
Technical Foundation of WiMAX
� Pathloss: free-space pathloss formula (Friis formula)
Technical Foundation of WiMAX
� 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
Technical Foundation of WiMAX
Technical Foundation of WiMAX
� Shadowing
Technical Foundation of WiMAX
� 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
Technical Foundation of WiMAX
Technical Foundation of WiMAX
Technical Foundation of WiMAX
Technical Foundation of WiMAX
Technical Foundation of WiMAX
� 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
Technical Foundation of WiMAX� Sectoring (instead split cell): using directional antenna
Technical Foundation of WiMAX
� The Broadband Wireless Channel: Fading (caused by
multipath and local scattering)
� Reflections may arrive nearly simultaneously
Technical Foundation of WiMAX
Technical Foundation of WiMAX
� 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
Technical Foundation of WiMAX
� Delay Spread and Coherence Bandwidth
Technical Foundation of WiMAX� Doppler Spread and Coherence Time
Technical Foundation of WiMAX
Technical Foundation of WiMAX
� 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]
Technical Foundation of WiMAX
� LOS Channels: Ricean distribution (there is a dominant path LOS)
� A more general model: Nakagami-m Fading
Technical Foundation of WiMAX
Technical Foundation of WiMAX
� Statistical Correlation of the Received Signal
� Time Correlation, Frequency correlation, The selectivity/dispersion
duality
Technical Foundation of WiMAX� Multidimensional Correlation
Technical Foundation of WiMAX
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
Technical Foundation of WiMAX
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
Technical Foundation of WiMAX
� 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