23-02-2013, 09:21 AM
Mobile Radio Propagation: Small-Scale Fading and Multipath
1Mobile Radio Propagation.ppt (Size: 1.14 MB / Downloads: 214)
Small-Scale Multipath Propagation
The three most important effects
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 caused by multipath propagation delays
Factors influencing small-scale fading
Multipath propagation: reflection objects and scatters
Speed of the mobile: Doppler shifts
Speed of surrounding objects
Transmission bandwidth of the signal
The received signal will be distorted if the transmission bandwidth is greater than the bandwidth of the multipath channel.
Coherent bandwidth: bandwidth of the multipath channel.
Impulse Response Model of a Multipath Channel
A mobile radio channel may be modeled as a linear filter with a time varying impulse response
time variation is due to receiver motion in space
filtering is due to multipath
The channel impulse response can be expressed as h(d,t). Let x(t) represent the transmitted signal, then the received signal y(d,t) at position d can be expressed as
For a causal system
Direct RF Pulse System
Direct RF pulse system
This system transmits a repetitive pulse of width , and uses a receiver with a wideband filter with bandwidth
Envelope detector to detect the amplitude response.
Minimum resolvable delay
No phase information can be measured.
Spread Spectrum Sliding Correlator Channel Sounding
System description
A carrier is spread over a large bandwidth by using a pseudo-noise sequence having chip duration and a chip rate .
Despread using a PN sequence identical to that used at the transmitter.
The probing signal is wideband.
Use a narrowband receiver preceded by a wideband mixer.
The transmitter chip clock is run at a slightly faster rate than the receiver chip clock – sliding correlator.
Frequency Domain Channel Sounding
Dual relationship between time domain and frequency domain.
It is possible to measure the channel impulse response in the frequency domain.
Measure the frequency domain response and then converted to the time domain using inverse discrete Fourier transform (IDFT).
Coherent Bandwidth
Coherent bandwidth, is a statistic measure of the range of frequencies over which the channel can be considered to be “flat”.
Two sinusoids with frequency separation greater than are affected quite differently by the channel.
If the coherent bandwidth is defined as the bandwidth over which the frequency correlation function is above 0.9, then the coherent bandwidth is approximately
If the frequency correlation function is above 0.5
Doppler Spread and Coherent Time
Doppler spread and coherent time are parameters which describe the time varying nature of the channel in a small-scale region.
When a pure sinusoidal tone of is transmitted, the received signal spectrum, called the Doppler spectrum, will have components in the range and where is the Doppler shift.
is a function of the relative velocity of the mobile, and the angle between the direction of motion of the mobile and direction of arrival of the scattered waves
Flat Fading
If the channel has a constant gain and linear phase response over a bandwidth which is greater than the bandwidth of the transmitted signal, the received signal will undergo flat fading.
The received signal strength changes with time due to fluctuations in the gain of the channel caused by multipath.
The received signal varies in gain but the spectrum of the transmission is preserved.
Clarke’s Models for Flat Fading
Clark developed a model where the statistical characteristics of the electromagnetic fields of the received signal are deduced from scattering.
The model assumes a fixed transmitter with a vertically polarized antenna.
The received antenna is assumed to comprise of N azimuthal plane waves with arbitrary carrier phase, arbitrary angle of arrival, and each wave having equal average amplitude.
Equal amplitude assumption is based on the fact that in the absence of a direct line-of-sight path, the scattered components arriving at a receiver will experience similar attenuation over small-scale distance.
Simulation of Clarke Fading Model
Produce a simulated signal with spectral and temporal characteristics very close to measured data.
Two independent Gaussian low pass noise are used to produce the in-phase and quadrature fading branches.
Use a spectral filter to sharp the random signal in the frequency domain by using fast Fourier transform (FFT).
Time domain waveforms of Doppler fading can be obtained by using an inverse fast Fourier transform (IFFT).