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Doppler shifts: Effect on Communication systems

Doppler shifts: Effect on Communication systems. Kartik Natarajan. 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.

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Doppler shifts: Effect on Communication systems

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  1. Doppler shifts: Effect on Communication systems Kartik Natarajan

  2. 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 (non-relativistic Doppler).

  3. 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

  4. 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 1500MHz • UHF – 300 to 3000MHz (used by TV, PCS etc.) • Typical Doppler shifts : • 5Hz to 300 Hz • For example, at for a carrier frequency of 2GHz and a mobile speed of 68 mph, max fd = 200Hz

  5. 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.

  6. 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

  7. 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 = f0 + 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/

  8. 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- f0)^2)/ fm • So, df = - sqrt(fm^2 – (f- f0)^2) • Substituting back, we get |S(f)| = p/(*sqrt(fm^2 – (f- f0)^2))

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

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