UNIT I – Wireless channels

TOPICS Path loss models: Free Space and Two-Ray models Link Budget design Parameters of mobile multipath channels Time dispersion parameters Coherence bandwidth Doppler spread & Coherence time Small Scale fading Fading effects due to multipath time delay spread Fading effects due to doppler spread

Introduction Wireless communication – broadcasting and reception of signals by an appropriate receiver through wireless medium (air)

Characteristics of wireless channels Path loss Fading Interference Doppler shift

Path Loss Ratio of power of the transmitted signal to the power of same signal received by reciever. Dependent on radio frequency used and nature of terrain Path loss models – Free space & Two ray Free space – no attenuation of signals, only direct path Two ray – direct path & Reflected path

Fading Fluctuations in signal strength when received at the receiver. Types – (1) Fast/small scale fading (2) slow/large scale fading. Fast fading – rapid fluctuations in amplitude, phase or multipath delays. Slow fading – objects partially absorb the transmissions Counter measure – Diversity & Adaptive modulation

Interference Types : (1) ACI (2) CCI (3) ISI Interference Counter measure ACI Guard band CCI Usage of directional antennas, dynamic channel allocation ISI Adaptive equalization

Doppler shift Change in frequency when transmitter and receiver are mobile in nature. Higher frequency – when both move towards Lower frequency – when both move away