Mobile TV Dual Antenna Diversity Reception Technology
With the development of DVB-T in outdoor broadcast fields such as mobile TV, car TV, building TV, subway TV, etc., within these receiving ranges, small-scale fading such as multipath fading and Doppler frequency shift is inevitable. Addressing these fading and interference becomes a matter of great concern. In order to solve the fading and improve the signal quality of digital TV broadcast mobile reception , various measures are used on the receiving equipment, such as channel decoding error correction technology, anti-fading reception technology, etc., but the dual/multi-antenna diversity reception technology is the most obvious and effective solution.
1. Multipath Signal Fading and Doppler Effect
What is fading? To put it simply, it is called fading when the signal level changes with time due to the influence of various factors. Fading is divided into slow fading and fast fading. There are many reasons for fading. Wireless ground transmission signals are easily affected by obstacles such as high-rise buildings, mountains and hills, clouds and rain, and these effects will cause fading. Two common types of fading in DVB-T mobile reception are multipath signal fading and Doppler frequency shift.
figure 1
The above figure (Figure 1) is a process of multipath fading. When the ground wave signal is blocked by multiple obstacles such as tall buildings, hills, and moving vehicles in the transmission path, reflection or scattering will occur, resulting in multi-path signal arrival. The receiving antenna, due to the different arrival times and different phases of the receiving antenna, different signals with opposite phases weaken each other due to superposition, resulting in signal fading.
figure 2
Another unavoidable problem in mobile reception is the Doppler effect. If the signal propagates in the form of waves, when the receiver and the generator move relative to each other, the frequency of the signal received by the receiver will change due to the movement, which is called the Doppler effect. This is a property of any wave process, and so is electromagnetic waves. If the receiver is relative to the transmitter, the signal received by the receiver will also experience Doppler effect, depending on the direction and speed of the receiver moving relative to the transmitter (as shown in Figure 2 above), when the receiver and transmitter When the machine is moving towards each other, the frequency of the signal it receives is higher than the frequency of transmission. On the contrary, when they move in opposite directions, the frequency of the signal received is lower than the frequency of transmission. This frequency change is also called Doppler frequency shift. The resulting fading can make it difficult for a receiver to accurately decode the signal.
The small-scale fading caused by multipath fading and Doppler frequency shift is extremely destructive to the received signal of mobile receiving equipment, and can cause large intersymbol interference and frequency vector reduction, so the signal power is required to be strong enough when receiving Or the receiver sensitivity is high enough. The fading caused by multipath fading and Doppler frequency shift belongs to fast fading in a small range. Theory and actual measurement show that the amplitude of fast fading obeys Rayleigh distribution, and the phase obeys uniform distribution. The effective way to overcome the influence of fast fading is diversity reception .
Two-Diversity Receiving Technical Thought
The reason for the signal fading is explained above. Since the multipath component signal is generated due to interference such as reflection during the signal transmission process, the receiving end uses multiple antennas to receive signals of different paths at the same time, and then selects and combines these signals into a total signal. To reduce the impact of signal fading, this is called diversity reception. Diversity is to combine the scattered signals together. As long as several signals are independent of each other, the maximum signal gain can be obtained after proper combination.
The methods of diversification are:
(1) Space diversity: the received signals of different antennas are independent of each other
(2) Polarization diversity: the signals of horizontal polarization and vertical polarization are independent of each other
(3) Frequency diversity: the received signals of different frequencies are independent of each other
(4) Time diversity: The received signals at different times are independent of each other.
The merge methods are:
(1) The best choice
(2) Equal gain addition formula
(3) Maximum ratio addition formula
Among the three merging methods, the largest ratio addition merging has the best performance. The specific comparison is as follows (Figure 3)
image 3
Application Theory of Diversity Reception Technology in Three DVB-T
DVB-T (Digital Video Broadcasting Terrestrial ) is a digital TV terrestrial transmission standard in the DVB standard . The DVB-T standard was established in 1997, using COFDM (orthogonal frequency division multiplexing) modulation and transmission, inner coding using convolutional code, outer coding using RS code, and interleaver using convolutional interleaving.
When the coding and interleaving schemes in the DVB-T standard have been determined, to improve its mobile receiving performance, it can only be improved on the receiver. Generally, there are two basic ways to improve:
(1) Under the structure of the existing internal receiver, improve the frequency and timing synchronization and tracking algorithms in the receiver , and improve the efficiency of channel estimation and compensation algorithms for Rayleigh fading channels;
(2) Change the structure of the existing receiver.
Experimental studies have shown that dual/multi-antenna diversity technology can effectively combat signal fading and greatly improve the mobile reception performance of DVB-T. The reason is that the signal strength at a known point is the vector sum (addition or subtraction) of the main signal and the multipath signal, so using moving antennas or using multiple antennas separated by at least one wavelength, at each antenna Different signal levels will be generated. At this time, moving the antenna can cause changes in the receiving level, and a strongest signal can always be found, which can significantly improve the receiving quality compared to using a single antenna. In order to significantly improve the output signal of the combined antenna, the maximum ratio combination technology MRC is used to solve the problem of optimal combination of diversity signals, but only the French company DIBCOM has successfully used this technology.
Figure 4
Fig. 4 is a solution for improving DVB-T performance by antenna diversity technology. The signal is first partially demodulated, and then the maximum ratio synthesis technique MRC is used, thus solving the problem of optimal synthesis of the signal. This technology can directly improve the sensitivity, reduce the C/N threshold, and improve the anti-interference performance of multi-mirror channels.
Application of Antenna Diversity Receiving Technology in Four DIBCOM
At present, in the field of DVB-T, the application of antenna diversity receiving technology is the most advanced and mature company in France, DIBCOM. France DIBCOM is a professional design company of DVB demodulation IC, DIBCOM has a number of patents in this field . Set-top boxes, P VR , subways, buildings and other occasions, and have completed the matching with most mainstream back-end chips in the market , such as the back-end of Freescale, ST, Telechips, Zoran, Fujitsu and other companies.
DIBCOM's dmodulator is fully applicable to DVB-T, DVB-H, DVB-SH, ISDB-T, T-DMB, CMMB, etc. based on COFDM modulation. DIBCOM's antenna diversity reception technology and basic principles of communication have not changed much, but it is far ahead of other schemes in the synthesis algorithm of scattered signals.
1 Diversity reception principle and application of DIBCOM
DIBCOM's diversity reception adopts the maximum ratio combining algorithm, as shown in the figure below (Figure 5). The dual-antenna diversity reception controls the receiving gain of each branch separately to obtain the best signal from different antennas, and at the same time provides the signal-to-noise signal of each branch signal in real time. Ratio, after the signal is phase-adjusted and combined in-phase with an appropriate gain factor, a high-quality signal with the maximum signal-to-noise ratio can be obtained.
Figure 5
For environments with large channel fading or higher mobile speed requirements, DIBCOM can also connect multiple ICs in a daisy chain to achieve diversity reception, that is, each chip can maximize the signal received by itself to the signal received by other chips The combination of ratios is also a patented technology of DIBCOM.
Figure 6
2. DIBCOM diversity receiving antenna processing
Practical research shows that even if the distance between the two antennas is about λ/5, 80% of the efficiency can still be obtained. For example, the distance between the two antennas is 8cm. For UHF, it still has high efficiency, which is impossible for other solutions. (Diversity reception requires that the antenna body be separated by at least 1 wavelength).
If the antenna receives two signals with different polarities, the efficiency will be higher. For example, if the two antennas are placed at 90 degrees, or antennas with different polarization directions, a better diversity reception effect will be obtained.