What is fading, multipath fading and Doppler shift?

When buying wireless transmission equipment, many times, such as: interference, channel, gain, fading, Doppler effect and other terms are heard. The definitions of interference, channel, gain, etc. have been introduced in the previous small series. Today, Xiaobian will introduce what is the fading and Doppler effect.

What is the decline? When we transmit wirelessly, the signal level is weakened by time due to some factors, which is the decline. The decline is divided into two kinds of fading states: fast and slow.

There are many reasons for fading. In wireless transmission, wireless signals can easily receive obstructions (such as buildings, mountains, and trees). In addition, there are various bad weather such as haze. Rain and other weather.

Common fading is multipath signal fading and Doppler shift (also called Doppler effect).

Multipath fading process

As shown in the above figure, a multipath fading process occurs. When a wireless signal encounters an obstruction such as a building, a mountain, a forest, or a running vehicle, the wireless signal is reflected or scattered, thereby forming multiple signals to be transmitted and received. The time and phase of the multipath signal reaching the receiving end are different, and the different signals of the phase are weakened by the superposition, thereby generating signal fading. According to different application scenarios, selecting different wireless microwave channels can effectively reduce fading.

Doppler effect

As shown in the above figure, the Doppler effect is. When the wireless signal is wirelessly propagated in the form of waves, the relative motion of the transmitting end and the receiving end occurs, and the frequency of the signal received by the receiving end changes due to motion, which is called Puller effect. Basically, all fluctuations have this characteristic, as do electromagnetic waves. The COFDM technology can restrain this type of fading very well. Interested friends can learn about the advantages of COFDM equipment.

The small-range fading caused by these two fading modes is extremely destructive to the receiving signal of the mobile receiving device. In order to solve these two types of fading, many times we choose to avoid the occlusion or use the COFDM device.