JPL's Wireless Communication Reference Website

Chapter: Wireless Channels
Section: Propagation Mechanisms

Multipath Reception

Experiments with mobile communication were done at VHF frequencies, near 50 MHz, already in the 1920s. Results of these tests revealed a very hostile propagation environment, particularly in urban centers. The signal quality varied from "excellent" to "no signal". Moving the vehicle over a few meters resulted in dramatic changes of the received field strength.

The mobile or indoor radio channel is characterized by 'multipath reception': The signal offered to the receiver contains not only a direct line-of-sight radio wave, but also a large number of reflected radio waves. Even worse in urban centers, the line-of-sight is often blocked by obstacles, and a collected of differently delayed waves is all what is received by a mobile antenna. These reflected waves interfere with the direct wave, which causes significant degradation of the performance of the link. If the antenna moves the channel varies with location and time, because the relative phases of the reflected waves change. This leads to fading: time variations of the received amplitude and phase.

In a non-fading (thus fixed) radio channel the BER decreases rapidly when the signal-to-noise (or signal-to-interference) ratio is increased. In a fading channel, every now and then the received signal is very weak and many bit errors occur. This phenomenon remains present, even if the (average) signal-to-noise ratio is large. So the BER only improves very slowly, and with a fixed slope, if plotted on a log-log scale. (Diversity or error correction can help to make the slope steeper, hence improve performance.)

A wireless system has to be designed in such way that the adverse effect of multipath fading is minimized. In the past, multipath has notoriously hindered the development of reliable and inexpensive mass-product systems. A better understanding of these phenomena, and the advent of powerful signal processing techniques contributed to the explosion of digital wireless communication since the 1980s.

 
The basic model of Rayleigh fading assumes a received multipath signal to consist of a (theoretically infinitely) large number of reflected waves with independent and identically distributed inphase and quadrature amplitudes. This model has played a major role in our understanding of mobile propagation. The model was first proposed in a comment paper written by Lord Rayleigh in 1889, describing the resulting signal if many violinists in an orchestra play in unison, long before its application to mobile radio reception was recognized. Lord Rayleigh, "On the resultant of a large number of vibrations of the same pitch and of arbitrary phase", Phil. Mag., Vol. 10, August 1880, pp. 73-78 and Vol. 27, June 1889, pp. 460-469.

More recently, this model of many randomly phased sinusoids appeared to appropriately describe the wireless radio channel, and to allow calculation of outage probabilities, fade durations and many other critical parameters of wireless links. It greatly facilitated the development systems that can reliably communicate despite the anomalies and unpredictability of the mobile communication channel.

As the demand for mobile communication increases, systems have to be more efficient and cell sizes are chosen smaller and smaller. To describe microcellular propagation, the Rayleigh model lacked the effect of a dominant line-of-sight component, and Rician model appeared to be more appropriate.


1'17"
Multipath modeling.
Although channel fading is experienced as an unpredictable and stochastic phenomenon to the user or the system planner, powerful models have been developed that can accurately predict average system performance. Countermeasures can be used to avoid system failure, even if the channel exhibits fades at particular frequencies of particular locations.

Quicktime
Multipath reception leads to "fades" which are spots where the signal is weak. Such fades usually are limited in spatial dimension and frequency bandwidth.

Most conventional digital modulation techniques are sensitive to intersymbol interference unless the channel symbol rate is small compared to the delay spread of the channel. On the other hand a narrowband signal with bit durations much longer than the delay spread may vanish completely in fade. A signal received at a frequency and location where reflected waves cancel each other, is heavily attenuated and may thus suffer large bit error rates.

Models for multipath reception

Narrowband Rayleigh, or Rician models mostly address the channel behavior at one frequency only. Time dispersion, or the Doppler spread is the critical phenomenon. Frequency dispersion and intersymbol interference, on the other hand, are modeled by the delay spread. A model that combines these aspects is the scatter diagram.
See also: Table of Contents; chapter on channel modeling

The effect of multipath reception

 for a fast moving user: rapid fluctuations of the signal amplitude and phase  
 for a wideband (digital) signal: dispersion and intersymbol interference
 for an analog television signal: "ghost" images (shifted slightly to the right)
  for a MultiCarrier signal: different attenuation at different (sub-)carriers and at different locations
 for a stationary user of a narrowband system: good reception at some locations and frequencies; poor reception at other locations and frequencies.
 for a satellite positioning system: strong delayed reflections may cause a severe miscalculation of the distance between user and satellite. This can result in a wrong "fix", i.e., a wrong estimate of the position.

Exercise

After you studied the pages of Rayleigh fading, have a look at our design exercise and quizzes.



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