Multipath Fading

Multipath fading occurs when signals reach a receiver via many paths & their relative strengths & phases change.

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Multipath fading affects most forms of radio communications links in one form or another.

Multipath fading can affect signals on frequencies from the LF portion of the spectrum and below right up into the microwave portion of the spectrum.

Multipath fading occurs in any environment where there is multipath propagation and the paths change for some reason. This will change not only their relative strengths but also their phases, as the path lengths will change.

Multipath fading may also cause distortion to the radio signal. As the various paths that can be taken by the signals vary in length, the signal transmitted at a particular instance will arrive at the receiver over a spread of times. This can cause problems with phase distortion and inter-symbol interference when data transmissions are made. As a result, it may be necessary to incorporate features within the radio communications system that enables the effects of these problems to be minimised.

Multipath fading basics

Multipath fading is a feature that needs to be taken into account when designing or developing a radio communications system. In any terrestrial radio communications system, the signal will reach the receiver not only via the direct path, but also as a result of reflections from objects such as buildings, hills, ground, water, etc that are adjacent to the main path.

The overall signal at the radio receiver is a summation of the variety of signals being received. As they all have different path lengths, the signals will add and subtract from the total dependent upon their relative phases.

At times there will be changes in the relative path lengths. This could result from either the radio transmitter or receiver moving, or any of the objects that provides a reflective surface moving. This will result in the phases of the signals arriving at the receiver changing, and in turn this will result in the signal strength varying as a result of the different way in which the signals will sum together. It is this that causes the fading that is present on many signals.

Selective and flat fading

Multipath fading can affect radio communications channels in two main ways. This can given the way in which the effects of the multipath fading are mitigated.

Flat fading: This form of multipath fading affects all the frequencies across a given channel either equally or almost equally. When flat multipath fading is experienced, the signal will just change in amplitude, rising and falling over a period of time, or with movement from one position to another.
Selective fading: Selective fading occurs when the multipath fading affects different frequencies across the channel to different degrees. It will mean that the phases and amplitudes of the signal will vary across the channel. Sometimes relatively deep nulls may be experienced, and this can give rise to some reception problems. Simply maintaining the overall amplitude of the received signal will not overcome the effects of selective fading, and some form of equalization may be needed. Some digital signal formats, e.g. OFDM are able to spread the data over a wide channel so that only a portion of the data is lost by any nulls. This can be reconstituted using forward error correction techniques and in this way it can mitigate the effects of selective multipath fading.

Selective multipath fading occurs because even though the path length will be change by the same physical length (e.g. the same number of metres, yards, miles, etc) this represents a different proportion of a wavelength. Accordingly the phase will change across the bandwidth used.

Selective fading can occur over many frequencies. It can often be noticed when medium wave broadcast stations are received in the evening via ground wave and skywave. The phases of the signals received via the two means of propagation change with time and this causes the overall received signal to change. As the multipath fading is very dependent on path length, it is found that it affects the frequencies over even the bandwidth of an AM broadcast signal to be affected differently and distortion results.

Selective multipath fading is also experienced at higher frequencies, and with high data rate signals becoming commonplace wider bandwidths are needed. As a result nulls and peaks may occur across the bandwidth of a single signal.

Cell phone signal fading

Mobile phone communications are subject to multipath fading. There are a variety of reasons for this.

Mobile user is moving:, The first is that the mobile station or user is likely to be moving, and as a result the path lengths of all the signals being received are changing. The second is that many objects around may also be moving. Automobiles and even people will cause reflections that will have a significant effect on the received signal. Accordingly multipath fading has a major bearing on cellular telecommunications.
Other objects moving: Often the multipath fading that affects cellular phones is known as fast fading because it occurs over a relatively short distance. Slow fading occurs as a cell phone moves behind an obstruction and the signal slowly fades out.

The fast signal variations caused by multipath fading can be detected even over a short distance. Assume a frequency of 2 GHz (e.g. a typical approximate frequency value for many phones). The wavelength can be calculated as:

λ = \frac{c}{f}

λ = \frac{3 \cdot 10^{8}}{2 \cdot 10^{9}}

λ = 0.15 metres

Where:
c = speed of light in metres per second
f = frequency in Hertz

To move from a signal being in phase to a signal being out of phase is equivalent to increasing the path length by half a wavelength or 0.075m, or 7.5 cms. This example looks at a very simplified example. In reality the situation is far more complicated with signals being received via many paths. However it does give an indication of the distances involved to change from an in-phase to an out of phase situation.

Ionospheric fading

Short wave radio communications is renowned for its fading. Signals that are reflected via the ionosphere, vary considerably in signal strength. These variations in strength are primarily caused by multipath fading.

When signals are propagated via the ionosphere it is possible for the energy to be propagated from the transmitter to the receiver via very many different paths. Simple diagrams show a single ray or path that the signal takes. In reality the profile of the electron density of the ionosphere (it is the electron density profile that causes the signals to be refracted) is not smooth and as a result any signals entering the ionosphere will be scattered and will take multiple paths to reach a particular receiver. With changes in the ionosphere causing the path lengths to change, this will result in the phases changing and the overall summation at the receiver changing.

The changes in the ionosphere arise from a number of factors. One is that the levels of ionisation vary, although these changes normally occur relatively slowly, but nevertheless have an effect. In addition to this there are winds or air movements in the ionosphere. As the levels of ionisation are not constant, any air movement will cause changes in the profile of the electron density in the ionosphere. In turn this will affect the path lengths.

It is for this reason that signals on the short wave bands are constantly changing in strength.

Tropospheric fading

Many signals using frequencies at VHF and above are affected by the troposphere. The signal is refracted as a result of the changes in refractive index occurring, especially within the first kilometres above the ground. This can cause signals to travel beyond the line of sight. In fact for broadcast applications a figure of 4/3 of the visual line of sight is used for the radio horizon. However under some circumstances relatively abrupt changes in refractive index occurring as a result of weather conditions can cause the distances over which signals travel to be increased. Signals may then be "ducted" by the ionosphere over distances up to a few hundred kilometres.

When signals are ducted in this way, they will be subject to multipath fading. Here, heat rising from the Earth's surface will ensure that the path is always changing and signals will vary in strength. Typically these changes may be relatively slow with signals falling and rising in strength over a period of a number of minutes.

Multipath radio fading is factor that appears on most signals to a greater or lesser degree. As radio signals tend to reach a receiver via multiple paths regardless of how good the path appears to be there are always likely to be reflections from other objects. The only exception is in outer space where there are very few significant objects that are likely to cause major issues.

In view of the fact that signals take multiple paths and there is always likely to be some movement causing path lengths to change and signal strengths to vary, multipath fading will be an issue in many instances.