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With the development of DVB-T in outdoor broadcasting fields such as mobile TV, car TV, building TV, subway TV, etc., in these reception ranges, small-scale fading such as multipath fading and Doppler frequency shift is an inevitable problem. Solving these fading and interference has become a matter of great concern. In order to solve the fading and improve the signal quality of digital TV broadcasting mobile reception, a variety of measures are used on the receiving equipment, such as channel decoding error correction technology, anti-fading receiving technology, etc., but the dual / multi-antenna diversity receiving technology is the most obvious and effective solution.
A multipath signal fading and Doppler effect
What is fading? Simply speaking, the signal level changes with time due to various factors. It is called fading. Fading is divided into slow fading and fast fading. There are many reasons for fading, and wireless ground transmission signals are easily affected by weather such as clouds and rain due to obstacles such as high-rise buildings, hilly terrain, etc. These effects will cause fading. Two types of fading common in DVB-T mobile reception are multipath fading and Doppler frequency shift.
figure 1
The above picture (Figure 1) is a multipath fading process. When the surface wave signal is blocked by many obstacles such as tall buildings, hills, moving vehicles, etc. in the transmission path, it will be reflected or scattered, forming a multi-channel signal arrival In the receiving antenna, due to the different time and phase of the arrival at the receiving antenna, different signals of opposite phases weaken each other due to superposition, resulting in signal fading.
figure 2
Another inevitable problem in the mobile reception process is the Doppler effect. If the signal propagates in the form of waves, when the receiver and the generator are in relative motion, the frequency of the signal received by the receiver will change due to the movement, which is called the Doppler effect. This is a characteristic of any wave process, as is electromagnetic waves. If the receiver is relative to the transmitter, the Doppler effect will also occur on the signal received by the receiver, depending on the direction and speed of the receiver's movement relative to the transmitter (as shown in Figure 2 above). When the machine moves towards each other, the frequency of the signal he receives is higher than the transmission frequency. Conversely, when they move oppositely, the frequency of the received signal is lower than the transmission frequency. This frequency change is also called Doppler frequency shift. The resulting fading will make it difficult for the receiver to accurately resolve the signal.
The small-scale fading caused by multi-path fading and Doppler frequency shift is extremely destructive to the received signal of the mobile receiving device, and can cause large intersymbol interference and frequency vector reduction, so the signal power is required to be strong enough during reception Or the receiver sensitivity is high enough. Fading caused by multipath fading and Doppler frequency shift belongs to fast fading in a small range. Theoretical and practical measurements show that the amplitude of fast fading follows Rayleigh distribution, and the phase follows uniform distribution. The effective way to overcome the effect of fast fading is diversity reception .
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