Multipath propagation

Multipath or multipath reception (English Multipath ) occurs at a receiver when electromagnetic waves from a transmitter of reflectors (eg buildings, wet leaves ) are deflected in different ways and arrive at the receiver. This results in a mixing of the direct signal to the various staggered reflected echo signals. The level of this mixed signal is weaker than that of the direct path usually.

Special attention has to find with GPS, the accuracy ( mm. .. cm) detour reflected signals may affect significantly the effect with precise satellite positioning.

Causes

A signal is transmitted by radio can take different paths from the transmitter to the receiver. The reasons for this are

  • Reflection ( reflection ) signals, such as on the walls of buildings,
  • Refraction ( refraction ) of signals, such as the ionosphere,
  • Scattering ( scattering) of signals, such as on small objects such as signs,
  • Diffraction ( diffraction ) of signals, such as edges of buildings or mountains.

A so-called co-channel transmitter can act as a further signal source.

Through these different length transmission paths a radio signal may arrive staggered several times and time at a receiver. The individual echo signals that occur at the receiver, depending on the attenuation and path have different amplitudes and durations. A characteristic value for the description of such a ( frequency selective ) fading channel is given by the ratio of the directly received signal power to the sum of the echo signal powers ( Rice factor ).

Measurement

The dispersion time of a radio channel measures the divergence of a rectangular pulse. In the case of digital transmission symbols transmitted at a time which is substantially greater than the dispersion time, almost no fading occurs. One speaks of flat fading and a Rice channel.

Effects

If the dispersion time is very much larger than the symbol duration, the symbol crosstalk plays a negligible role, and the echo signals can even be used productively, by dissolving the individual signals in time and re- composed in the correct phase, and thus the reception power is increased ( multipath diversity). The rake receiver used in spread spectrum, but also for land-based and satellite-based mobile communication systems with CDMA access methods work with this method. Much more common but the Selection combining is in practice used process: if it is possible to resolve the individual signals in time, the other the best signal is evaluated and discarded.

Digital broadcasting uses modern digital modulation methods that are to some degree immune to multipath and use it even for a better quality of the received signal ( eg COFDM, which is used in DVB -T, DAB).

In the analog transmission of radio and television images multipath effects are seen directly eg as ghosting. The superposition of the multi-path signals with the direct signal can cause interference, which affects the signal quality. For shortwave, the selective carrier loss is particularly feared.

If in the position determination (GPS (?) ) Used the method of correlation, so short detours to the indirect paths are critical because the separation of direct and indirect signal is difficult (especially if the direct signal can not be measured ). Long detours due to the high difference in time easier to identify and eliminate.

In general it can be said that the multipath negative impact on the transmission of signals. This is due to the different lengths traveled distance, which causes a delay of the signal in each case.

Where t is the transit time, c is the speed of light in vacuum, and s is the signal path. This formula provides only an approximate value for the normal atmosphere, since the propagation speed in the medium is not air exactly c.

Mathematical Description

For the multipath reception the following formula also can be applied in the time domain ( impulse response ):

Depending on the situation, a multi-path reception of much more indirect paths is possible by the following formula is valid:

In the most general case, the path delays are also time-dependent:

562451
de