MULTIPATH ADING AND DISPERSION

MULTIPATH ADING AND DISPERSION

1. Multipath Propagation (NLOS)

   • Multipath propagation
     • Signal arrives at Rx through different paths
     • Paths could arrive with different gains, phase, & delays
     • Small distance variation can have large amplitude variation
   • Physical Phenomena behind Multipath Propagation
     • Reflection (R)
     • Diffraction (D)
     • Scattering (S)

2. Multipath fading

   • Rapid changes in signal strength over a small travel distance or time interval
   • Random frequency modulation due to varying Doppler shifts on different multipath signals
   • Time dispersion (echoes) caused by multipath propagation delays

3. Factors influence fading

   • Multipath propagation
     • result in multiple version of transmitted signal
   • Speed of surrounding
     • – if the surrounding objects move at a greater rate than the mobile
   • Speed of mobile
     • result in random frequency modulation due to different Doppler shifts

4. Doppler effect

   • occurs when transmitter and receiver have relative velocity
   • When a wave source and a receiver are moving towards each other, the frequency of the received signal will not be the same as the source.
     • When they are moving toward each other, the frequency of the received signal is higher than the source.
     • When they are opposing each other, the frequency decreases.

5. Delay Spread

   • When a signal propagates from a transmitter to a receiver, signal suffers one or more reflections.
   • This forces signal to follow different paths.
   • Each path has different path length, so the time of arrival for each path is different.
   • Each path has different path length, so the time of arrival for each path is different.

6. Multipath fading

   • Multiple reflected waves arrive at the receiver
   • Narrowband model
     • Different waves have different phases.
     • These waves may cancel or amplify each other.
     • This results in a fluctuating (“fading”) amplitude of the total received signal.

7. Outdoor Propagation

   • the propagation from a transmitter in an outdoor environment
     • The coverage area around a tranmitter is called a cell.
     • The shape of the cell is modeled as hexagon, but in real life it has much more irregular shapes.
     • By playing with the antenna (tilting and changing the height), the size of the cell can be controlled.
   • propagation characteristics
     • Macrocells
       • Base stations at high-points
       • Coverage of several kilometers
       • The average path loss in dB has normal distribution
         • Avg path loss is result of many forward scattering over a great many of obstacles
         • Sum is normally distributed because of central limit theorem
     • Microcells
       • Okumura
         • a good model for path loss was a simple power law
           • exponent n is a function of the frequency, antenna heights
         • frequencies
           • 100MHz – 1920 MHz
           • distances 1km – 100km
       • Small multipath delay spread
       • feasibility of higher data-rate transmission
       • Mostly used in crowded urban areas
       • If transmitter antenna is lower than the surrounding building than the signals propagate along the streets