Multipath Propagation Tutorial
- 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.
At other times, the variety of signal paths arising from the multipath propagation can be used to advantage. Schemes such as MIMO use multipath propagation to increase the capacity of the channels they use. With increasing requirements for spectrum efficiency, the use of multipath propagation for technologies such as MIMO are able to provide significant improvements in channel capacity that are much needed.
Multipath propagation basics
Multipath radio signal propagation occurs on all terrestrial radio links. The radio signals not only travel by the direct line of sight path, but as the transmitted signal does not leave the transmitting antenna in only the direction of the receiver, but over a range of angles even when a directive antenna is used. As a result, the transmitted signals spread out from the transmitter and they will reach other objects: hills, buildings reflective surfaces such as the ground, water, etc. The signals may reflect of a variety of surfaces and reach the receiving antenna via paths other than the direct line of sight path.
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.
It can also be found that the interference may be flat, i.e. applied to all frequencies equally across a given channel, or it may be selective, i.e. applying to more to some frequencies across a channel than others.A more in depth description of multipath fading is given in a page referenced in the "Related Articles" section on the left hand side of this page below the main menu.
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.
Another form of multipath propagation interference that arises when digital transmissions are used is known as Inter Symbol Interference, ISI. This arises when the delay caused by the extended path length of the reflected signal. If the delay is significant proportion of a symbol, then the receiver may receive the direct signal which indicates one part of the symbol or one state, and another signal which is indicating another logical state. If this occurs, then the data can be corrupted.
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.
Using the latest signal processing techniques, a variety of methods can be used to overcome the problems with multipath propagation and the possibilities of interference.
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.
Note on OFDM:
Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low rate data. Normally these signals would be expected to interfere with each other, but by making the signals orthogonal to each other there is no mutual interference. The data to be transmitted is split across all the carriers to give resilience against selective fading from multi-path effects..
Click on the link for an OFDM tutorial
OFDM is the modulation format that is used for many of today's data transmission formats. The applications include 802.11n Wi-Fi, LTE (Long Term Evolution for 3G cellular telecommunications), LTE Advanced (4G), WiMAX and many more. The fact that OFDM is being widely used demonstrates that it is an ideal format to overcome multipath propagation problems.
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.
Note on MIMO:
Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon's Law. MIMO provides a way of utilising the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.
Click on the link for a MIMO tutorial
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.
Multipath propagation is an issue for any radio communications system. Ranging from the short range wireless communications such as Wi-Fi though the cellular and longer range data schemes such as WiMAX though to VHF links where troposheric propagation may affect the signal path, through to HF systems using the ionosphere for reflections. In all of these systems, the effects of multipath propagation can be seen and experienced. Any form of communications, therefore has to be able to accommodate the effects of the multipath propagation in one way or another.
By Ian Poole
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