Doppler shifts Effect on Communication systems Kartik Natarajan

  • Slides: 9
Download presentation
Doppler shifts: Effect on Communication systems Kartik Natarajan

Doppler shifts: Effect on Communication systems Kartik Natarajan

Doppler Overview/Review • Apparent shifts in frequency of transmitted signal due to motion of

Doppler Overview/Review • Apparent shifts in frequency of transmitted signal due to motion of transmitter/receiver or both. • Shift depend on the relative velocity of the transmitter and receiver. • Non-relativistic motion Relativistic motion : • Cellular communication hampered by multipath fading effects and receiver movement (nonrelativistic Doppler).

Small Scale Fading • Rapid fluctuations in receiving conditions due to small movement of

Small Scale Fading • Rapid fluctuations in receiving conditions due to small movement of the receiver. • Fading is caused by phase differences between waves reaching the receiver. • Some causes: – Multipath Fading (Rayleigh and Rician) – Frequency shift due to movement – Doppler

Doppler Fading (1/3) • For a vehicle moving in a straight line at constant

Doppler Fading (1/3) • For a vehicle moving in a straight line at constant velocity v , the Doppler frequency shift, fd is given by : • Typical frequency range : –Most Cellular - 800 to 1500 MHz –UHF – 300 to 3000 MHz (used by TV, PCS etc. ) • Typical Doppler shifts : – 5 Hz to 300 Hz • For example, at for a carrier frequency of 2 GHz and a mobile speed of 68 mph, max fd = 200 Hz

Doppler Fading (2/3) • Doppler Spread (BD) – The difference between the maximum and

Doppler Fading (2/3) • Doppler Spread (BD) – The difference between the maximum and minimum values of fd. • Coherence Time (TC) – Statistical measure of the time duration over which the channel is invariant. – Defined as 1/ BD. • Doppler spread and Coherence time characterizes fading speed and its frequency selectiveness.

Doppler Fading (3/3) • Characterization of fading channels: – Fast fading TS > TC,

Doppler Fading (3/3) • Characterization of fading channels: – Fast fading TS > TC, and BS < BD • Higher the fading speed, more the distortion – Slow fading TS << TC, and BS >> BD – Flat fading BS << BD – Non-Flat of Frequency selective fading BS >= BD

Received Power Spectrum with Doppler (1/3) • Assumptions : – Isotropic antenna with unity

Received Power Spectrum with Doppler (1/3) • Assumptions : – Isotropic antenna with unity gain and receiving average power p (without Doppler). – PDF of the direction of waves reaching the receiver is uniformly distributed between 0 and 2. – Waves coming in from different directions add up to give a PSD S(f). • Received signal frequency, f = f 0 + fd • The PSD for signals in the range f to f+df corresponds to the waves coming in the direction given by +/- ( +d ). => S(f)df = 2* d *(p/2 ) = d *p/

Received Power Spectrum with Doppler (2/3) • Also, df = -fm*sin where fd =

Received Power Spectrum with Doppler (2/3) • Also, df = -fm*sin where fd = fm*cos • But sin =sqrt(1 – cos^2( )) =sqrt(fm^2 – (f- f 0)^2)/ fm • So, df = - sqrt(fm^2 – (f- f 0)^2) • Substituting back, we get |S(f)| = p/( *sqrt(fm^2 – (f- f 0)^2))

Received Power Spectrum with Doppler (3/3) Doppler Power Spectrum :

Received Power Spectrum with Doppler (3/3) Doppler Power Spectrum :