Multipath propagation is a fact of life in any terrestrial radio scenario. While the direct or line of sight path is often the main wanted signal, a radio receiver will receive different versions of the same signal that have travelled from the transmitter via many different paths.
Multipath propagation basics
The vast number of different signal paths arise from the fact that signals are 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 antennas used for transmission and reception have an effect on the number of paths that the signal can take. Non-directive antennas will radiate the signal in all directions, whereas directive ones will focus the power in one direction reducing the strength of reflected signals away from the main beam.
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 or seek to improve the signal to noise ratio.
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.
- Multipath fading: Multipath fading can be detected on many signals across the frequency spectrum from the HF bands right up to microwaves and beyond. It can cause signals to rise and fall in strength. . Read more about multipath fading.
- Rayleigh fading: Rayleigh fading is the name given to the form of fading that is often experienced in an environment where there is a large number of reflections present. Read more about Rayleigh fading.
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.