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Unformatted text preview: E4703 Wireless Communications Lecture 3 Outline • Summary of last lecture. • Signal representation: bandpass and lowpass. • Stochastic processes. • Statistical smallscale model: ¾ Multipath statistical model. ¾ Doppler spread and coherence time. ¾ Delay spread. • Narrowband channels. ¾ Mean and autocorrelation. ¾ Power spectral density. ¾ ClarkeJakes uniform scattering environment. ¾ Amplitude and power distributions: Rayleigh, Rician and Nakagami. • Wideband channels. ¾ Power delay profile. ¾ Coherence bandwidth • Depolarization. Summary of Lecture 2 • Radiation pattern and gain characterize ability to concentrate power in certain directions in detriment of others. • Polarization determines orientation of the fields composing the electromagnetic wave. • Exact Maxwell equations unsolvable. Approximate (e.g. ray tracing) methods available for controlled environments (indoor). Accuracy entails complexity. • More generally, resort to statistical modeling. Regard channel as stochastic. Distinguish between large scale and smallscale features. • Largescale features: path loss (deterministic) and shadow fading (stochastic). • Variety of path loss models: LOS, 2ray, empirical, simplified. • Power falloff with d 2 in free space, d 4 in two path model evidencing crucial role of ground. • Appealing simple model P r = P t K [ d / d ] γ • Random attenuation due to shadowing modeled as log normal (empirical std). • Shadow fading decorrelates over decorrelation distance roughly the size of obstacles. • Combined path loss and shadow fading leads to outage probabilities and amoebalike cell shapes. • Cell outage area dictates average % of locations in outage. Cell coverage indicates average % of locations not in outage. Announcements • Location for TA office hours changed to 8 th floor lounge of the Schapiro building. • Need to repeat questions for benefit of CVN students. Please remind. Signal Representation • Communication typically occurs within a bandwidth 2B around a center frequency f c as per spectrum regulations. Passband signal s I ( t )cos( 2 π f c t ). • Most processing takes place at baseband, before signal is upconverted to f c (modulated) and sent to antenna. 1 st receiver stage is to downconvert back to baseband. • Convenient to work with baseband signal s I ( t ) . • Possible to superimpose a second signal without interference. • Inphase and quadrature signals are orthogonal. ) 2 sin( ) ( Q t f t s c π − • The aggregate signal with inphase and quadrature components is where s ( t ) is the passband signal and u ( t ) the lowpass (or baseband) equivalent signal, also termed the complex envelope of s ( t ) ....
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This note was uploaded on 08/05/2008 for the course ELEN E4702 taught by Professor Lazano during the Summer '08 term at Columbia.
 Summer '08
 LAZANO

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