Mobile Radio Environment – Propagation Phenomena M. Schwartz Mobile Wireless Communications Chapter 2 2007 WNRL/KWU

Propagation Behaviors of Radio Waves Ground wave (< 2MHz) Sky wave (2-30MHz) : reflected at the ionosphere LOS (> 30MHz) : most communication systems considered in this course → Why do mobile phones work even without an LOS? 2007 WNRL/KWU

Additional Signal Propagation Effects In real life, we rarely have a LOS between sender and receiver. Receiving power additionally influenced by obstacles and objects. Due to large obstacles (compared to the wavelength) ‘Particle’ behavior of radio signals Blocking (or shadowing) Reflection at big obstacles Refraction depending on the density of a medium: LOS bent to the earth Due to small obstacles ‘Wave’ character of radio signals Scattering at small obstacles Diffraction at edges shadowing reflection refraction scattering diffraction 2007 WNRL/KWU

Signal Fading Time variation of received signal power caused by changes in the transmission medium or path Large-scale fading Long-term fading Average power: Attenuation due to the distance Shadow fading: Due to hills, buildings Small-scale fading Short-term or multi-path fading Due to receiver mobility 2007 WNRL/KWU

2007 WNRL/KWU

Fading Example 2007 WNRL/KWU

Received Power area-mean power local-mean power instantaneous power 2007 WNRL/KWU

2007 WNRL/KWU

2007 WNRL/KWU

2007 WNRL/KWU

2007 WNRL/KWU

2007 WNRL/KWU

2007 WNRL/KWU

2007 WNRL/KWU

Multipath Fading for Large Cell 2007 WNRL/KWU

Multipath Fading for Small Cell 2007 WNRL/KWU

Instantaneous Received Power 2007 WNRL/KWU

2007 WNRL/KWU

Doppler Shift for a Star If objects are moving away from observer, light will be redshifted 2007 WNRL/KWU

2007 WNRL/KWU

Fading on Modulated (Information-bearing) Signal 2007 WNRL/KWU

Information-Bearing Signal: Received Signal Two received scattered signals s1(t) and s2(t) x1, y1 – Gaussian RV a1 – signal envelop; Rayleigh or Rician RV; functions of tk and fm Delay of kth ray = fixed large-scale delay (t1) + additional (incremental) random delay (tk) 2007 WNRL/KWU

2007 WNRL/KWU

Normalized Correlation Function Measure of the connectedness between two RP of s1(t) and s2(t) Assuming Rayleigh fading and exponential incremental delay When r → 1, Two RV spaced t apart are completely correlated The fading channel is non-distorting 2007 WNRL/KWU

2007 WNRL/KWU

Frequency-selective Fading 2007 WNRL/KWU

Flat Fading 2007 WNRL/KWU

Frequency-Selective Fading 2007 WNRL/KWU

Time-selective Fading 2007 WNRL/KWU

Small-Scale Fading Fading The distortion of signal that channel produces Depends on both signal and channel Signal parameters symbol interval (symbol rate) bandwidth Channel parameters delay spread coherence bandwidth Doppler spread coherence time 2007 WNRL/KWU

2 Independent Fading Issues: 4 Basic Types 2007 WNRL/KWU

Four Composite Fadings: Symbol Rate (note) σ – delay spread, Tc – coherence time 2007 WNRL/KWU

Four Composite Fadings: Bandwidth (note) Bd – Doppler spread, Bc – coherence bandwidth 2007 WNRL/KWU