Causes and preventive measures of multipath weakness

In the communication system, due to the wide antenna beam of the communication ground station, affected by many factors such as ground objects, landforms and sea conditions, the receiver receives electromagnetic waves that arrive through several paths such as refraction, reflection and direct reflection. Multipath effect. The electromagnetic wave rays arriving by these different paths are inconsistent and time-varying, resulting in the fading state of the received signals; these electromagnetic wave rays arrive at different delays, which in turn leads to inter-symbol interference. If the multi-ray intensity is large and the delay difference cannot be ignored, a bit error will be generated. This error cannot be eliminated by increasing the transmission power, and the fading caused by the multipath effect is called multipath fading, which is also generated. The source of intersymbol interference. For digital communications, radar best detection, etc. will have a very serious impact.

Causes of multipath fading

Radio wave propagation for mobile communications includes direct waves, diffracted waves, scattered waves, and reflected waves. When there are only direct waves and one reflected wave, if the reflected wave path changes, the path difference changes, and the phase of the two signals at the receiving point changes. In terrestrial mobile communication systems, mobile stations often work in urban buildings and other complex terrain. Due to the low height of the mobile station antenna, most of the time is "submerged" below the height of the urban building, there is no line of sight at all. Therefore, the radio wave propagation between the base station and the mobile station has almost no direct wave form, but a reflected signal of a plurality of paths appears, so that the signal arriving at the receiving antenna is a composite wave of each electric wave from a different propagation path.

Propagation path of short-wave signal reflected from the ionosphere

Due to the different propagation paths, the properties of the reflectors are different, so that the amplitude and phase of each reflected wave arriving at the receiving point are random. There may be a direct wave and a plurality of reflected waves of different paths. The field strengths at different positions in a small range may be added in the same phase and become larger, and sometimes reversed to cancel and become smaller, forming a standing wave distribution. In the mobile communication environment, even if the surrounding environment is unchanged, the rapid movement of the mobile station in the standing wave field causes the amplitude of the composite wave received by the receiving antenna to change rapidly and in a wide range. This forms a multipath fast fading phenomenon of the signal received by the receiver. For different bands and different modes of propagation, the mechanism for forming multipath propagation is not the same. The drawings illustrate the principles of short-wave ionospheric reflection channels and multipath fading in ultrashort wave, microwave troposcatter channels and mobile communications.

Mobile communication path

Multipath fading prevention measures

1, diversity reception

As a kind of multiplicative interference, fading seriously affects the performance of communication systems, so corresponding measures must be taken to overcome them. The more effective anti-fading measures are: diversity reception is to synthesize several composite signals with different fading conditions (relatively statistically independent) at the receiving end, and then combine them in a certain way to make the signal noise of the total received signals. The ratio is reduced and the effect of fading is reduced. This is an effective method for overcoming the effects of fading in a long history and widely used. Available diversity methods are: spatial diversity, frequency diversity, angular diversity, polarization diversity, time diversity, and so on.

2, signal design

The so-called signal design is to design a signal with strong anti-fading capability for the channel, and adopt the corresponding modulation and detection technology at the end of the transmission. Such as multi-ary signal, time-frequency phase modulation technology and time-frequency modulation signal, pseudo-noise coding (pseudo-random coding) and other spread spectrum communication technology.

3. Adaptive communication technology

The main adaptive equalization technique is to adjust the receiver parameters according to the influence of the channel on the signal to offset the above effects. For example, in a medium-capacity system of digital microwave communication, the frequency characteristics of the channel are often compensated by a frequency domain adaptive equalizer. In the large-capacity system, in addition to the frequency domain equalizer, a time-domain adaptive equalizer that compensates the waveform is also used, and the effect is remarkable.

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