What is Multipath Propagation

Summary: tutorial or overview about the essentials of multipath radio propagation and the ways it affects radio signal transmission.

Multipath propagation is a fact of life in any terrestrial radio scenario. While the direct or line of sight path is normally the main wanted signal, a radio receiver will receive many signals resulting from the signal taking a large number of different paths. These paths may be the result of reflections from buildings, mountains or other reflective surfaces including water, etc. that may be adjacent to the main path. Additionally other effects such as ionospheric reflections give rise to multipath propagation as does tropospheric ducting.

The multipath propagation resulting from the variety of signal paths that may exist between the transmitter and receiver can give rise to interference in a variety of ways including distortion of the signal, loss of data and multipath fading.

Multipath propagation basics

Multipath fading

Signals are received in a terrestrial environment, i.e. where reflections are present and signals arrive at the receiver from the transmitter via a variety of paths. The overall signal received is the sum of all the signals appearing at the antenna. Sometimes these will be in phase with the main signal and will add to it, increasing its strength. At other times they will interfere with each other. This will result in the overall signal strength being reduced.

At times there will be changes in the relative path lengths. This could result from either the 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. It is this that causes the fading that is present on many signals.

Interference caused by multipath propagation

Multipath propagation can give rise to interference that can reduce the signal to noise ratio and reduce bit error rates for digital signals. One cause of a degradation of the signal quality is the multipath fading already described. However there are other ways in which multipath propagation can degrade the signal and affect its integrity.

One of the ways which is particularly obvious when driving in a car and listening to an FM radio. At certain points the signal will become distorted and appear to break up. This arises from the fact that the signal is frequency modulated and at any given time, the frequency of the received signal provides the instantaneous voltage for the audio output. If multipath propagation occurs, then two or more signals will appear at the receiver. One is the direct or line of sight signal, and another is a reflected signal. As these will arrive at different times because of the different path lengths, they will have different frequencies, caused by the fact that the two signals have been transmitted by the transmitter at slightly different times. Accordingly when the two signals are received together, distortion can arise if they have similar signal strength levels.

 

How intersymbol interference can be avoided

One way of overcoming this is to transmit the data at a rate the signal is sampled, only when all the reflections have arrived and the data is stable. This naturally limits the rate at which data can be transmitted, but ensures that data is not corrupted and the bit error rate is minimised. To calculate this the delay time needs to be calculated using estimates of the maximum delays that are likely to be encountered from reflections.

OFDM and multipath propagation

In order to meet the requirements to transmit large amounts of data over a radio channel, it is necessary to choose the most appropriate form of signal bearer format. One form of signal lends itself to radio data transmissions in an environment where reflections may be present is Orthogonal Frequency Division Multiplex, OFDM. An OFDM signal comprises a large number of carriers, each of which are modulated with a low bit rate data stream. In this way the two contracting requirements for high data rate transmission, to meet the capacity requirements, and low bit rate to meet the intersymbol interference requirements can be met.

While multipath propagation creates interference for many radio communications systems, it can also be used to advantage to provide additional capacity on a given channel. Using a scheme known as MIMO, multiple input multiple output, it is possible to multiple the data capacity of a given channel several times by using the multipath propagation that exists.

In view of the advantages that MIMO offers, many current wireless and radio communications schemes are using it to make far more efficient use of the available spectrum. The disadvantage to MIMO is that it requires the use of multiple antennas, and with modern portable equipment such as cell phones being increasingly small, it can be difficult to place tow sufficiently spaced antennas onto them.

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