Design and Implementation of a TV Diversity Antenna System

With the advancement and popularity of cable TV technology, the inherent shortcomings of wireless TV transmission have caused the technology research to be at a standstill. Due to the relatively fixed characteristics of wireless TV receiving terminals, the research is much more difficult than mobile communication. At present, the research on wireless TV can be divided into three kinds of schemes: one is to abandon the original simulation system, adopting a new digital system, namely digital compression and digital modulation, and new technologies such as channel and source coding. However, this needs to be introduced with the new generation of digital TV receivers, and it is not compatible with the current analog TV sets. Second, it is to improve on the original analog system, and fully adopt the techniques of diversity and spectrum compression in the fourth generation mobile communication. . This also requires major technological transformations of TV transmitters and televisions, which are not easy to popularize. Third, the latest research results using contemporary communication technologies based on existing simulation systems, such as multi-antenna antenna technology, low-noise amplification technology, Spectrum processing technology, etc., has increased the viewing performance of a large number of analog TV sets by an order of magnitude.

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1 TV propagation channel analysis

At present, the development and popularization speed of digital TV technology is progressing slowly. The main reason is that its technology is too complicated and cannot be compatible with the simulation system. The cost of network transformation and use is too high. Due to the deterioration of the electromagnetic environment and the outdated transmission equipment, the quality of wireless TV broadcast transmission has dropped drastically, and it has become impossible to watch the TV festival destination without cable TV. This research program can make full use of the existing analog TV network resources, optimize the existing wireless network through technological transformation, and play a good bridge role in the simulation and digital transition period. Especially in the vast rural areas of China, the use of diversity technology as a simple and efficient way of viewing will have a large market space.

The radio wave propagation of the radio video segment is carried out by various modes of propagation such as direct reflection, reflection, and scattering. The amplitude, delay and phase of the receiving terminal signal change anytime and anywhere, so that the level fluctuation of the signal is unstable, and the multipath signals superimpose each other to form a fading. That is, the phenomenon of flickering, ghosting, etc. of the TV screen seen. Therefore, it is generally defined that a wireless television propagation channel is a multipath fading channel.

Studies have shown that wireless TV propagation channels are "slow fading" and this fading obeys a lognormal distribution. Diversity technology is an effective way to overcome multipath fading. In this way, the television receiver can decide on a plurality of received signals carrying the same information and having fading characteristics independent of each other after the merging process. Since fading has frequency, time, and spatial selectivity, diversity techniques include frequency diversity, time diversity, and spatial diversity.

To reduce slow fading, spatial diversity should be used, that is, using several independent antennas or transmitting and receiving signals separately at different sites to ensure the fading independence between signals. Due to the different geographical environments during the transmission of these signals, the fading of each signal is different. The selective synthesis technique is used to select an output with a strong signal, thereby greatly reducing the influence of terrain and other factors on the signal.

2 Space diversity technology

Spatial diversity: Also known as antenna diversity, it is a form of diversity that is used in communication. To put it simply, it is to use multiple receiving antennas to receive signals and then combine them. In order to ensure the uncorrelation of the received signals, the distance between the antennas is required to be sufficiently large, so as to ensure that the fading characteristics of the received multipath signals are different. Ideally, the distance between the receiving antennas is only half of the wavelength λ.

According to the principle of signal theory, if a copy of the original transmitted signal with other degrees of attenuation is provided to the receiver, it is helpful to correctly determine the received signal. This method of increasing the correct decision rate of a received signal by providing multiple copies of the transmitted signal is referred to as diversity. Diversity technology is used to compensate for fading channel loss. It usually uses the uncorrelated characteristics of independent samples of the same signal in a wireless propagation environment, and uses certain signal combining techniques to improve the received signal to resist the adverse effects caused by fading. Spatial diversity means can overcome spatially selective fading, but the distance between diversity receivers must satisfy the basic conditions of more than 3 times the wavelength.

Spatial diversity is achieved by using the random variation of field strength with space. The larger the spatial distance, the greater the difference in multipath propagation, and the smaller the correlation of received field strength. The correlation mentioned here is a statistical term indicating the degree of similarity between signals, so the necessary spatial distance must be determined. After testing and statistics, CCIR suggested that in order to obtain a satisfactory diversity effect, the distance between the two antennas of the mobile unit is greater than 0.6 wavelengths, that is, d>0.61, and preferably selected near the odd multiple of l/4. If the antenna spacing is reduced, even if it is as small as 1/4, it can achieve a fairly good diversity effect.

The diversity antenna turns the harmful multipath effect in the original wireless communication into a useful factor, separates the multipath signals and makes them irrelevant, and then combines the separated signals by combining techniques to obtain the maximum SNR gain. . Commonly used merge methods include selective merge, switch merge, maximum ratio combining, equal gain combining, and so on.

3 merger criteria

If there are multiple receive branches in the receiver, the signals received by these branches need to be combined to obtain the maximum signal power. In fact, the essence of diversity reception is how to combine these unrelated signal copies at the receiving end.

The combination of signals is shown in Figure 1.

The combined signal r(t) can be expressed by the following formula

Where rk(t) is recorded as a copy of the signal received on the kth (k=1, 2, . . . , M) path, ak represents its weighting coefficient, and r(t) represents the final combined signal. In order to facilitate the analysis and comparison of the impact of diversity techniques on the coefficient transmission performance, it is necessary to normalize the coefficient ak, that is, to satisfy

If only one of the weighting coefficients is not zero, the others are all zero, and the port ak1=1, ak2=0 (k1≠k2; k1, k2=1, 2, ..., M). This combination method is Optimal SelectiON Combining (OSC); if all the received signals are superimposed regardless of the signal quality, ie a1=a2...=ak, the equal gain combining method is obtained ( EGC). If the weighting coefficient ak is automatically adjusted in the shortest time so that the signal to noise ratio of the combined signal r(t) is the highest, the maximum signal to noise ratio combining mode (MRC) is obtained.

It is assumed that the weakening of the envelope ri of each branch obeys the Rayleigh distribution, and its average power is σ2. Define parameters: ωi is the instantaneous signal power/average noise power of each branch; T is the average signal power/average noise power of each branch. Obviously, according to the definition of the parameters ωi and T, the effect of each diversity mode can be quantitatively analyzed.

4 Basic factors affecting diversity gain

(1) Relevance requirements.

Whether or not diversity works depends primarily on the degree of correlation between individual signal copies. The smaller the correlation between the individual copies, the more obvious the diversity effect. In fact, it is unlikely that the copies of the signals in the diversity branch will appear to be completely uncorrelated. As long as the correlation coefficient of two signal envelopes is ≤0.7 for the project, for space diversity, the separation distance of each antenna is generally required to be greater than the coherence distance;

(2) Signal strength requirements.

Similar to the impact of correlation on system performance, the two-branch diversity system is still taken as an example to illustrate, and it is assumed that the two branches are not related to each other. Since the two branches are completely uncorrelated, their average amplitudes must not always be equal. Based on this, a parameter characterizing the relationship between the average amplitudes of the two branches is introduced.

Among them, △=1 means that the average amplitude of the two branch signals is equal, and △=0 means that one of the branches completely disappears.

When △ is small, the weaker branches are almost all noise interference, but EGC still superimposes them with the strong signal in the same ratio in the combined signal, while the OSC mode only selects the stronger branch as the output, so EGC Performance is worse. Therefore, in practical applications, the average amplitude of the branch signal should also be estimated when selecting the diversity mode.

5 antenna amplifier

UHF, wideband, low noise amplifier for use between antennas and feeders. It is used to enhance the weak signal caused by the receiving distance too far, compensate the loss of the transmission distance between the antenna and the TV, and ensure the necessary input power of the distributor in the shared antenna system to improve the receiving quality.

In order to optimize the gain of each antenna, VFH and UFH are usually received separately. The antenna amplifier processes the received signal by: amplifying, mixing, and then amplifying the two circuits. Literally it seems that there is no difference, and only the position of these words is exchanged. In fact, the antenna amplifiers of these two circuits use different effects. In places where the TV signal is strong, the antenna amplifier that is amplified and then mixed is significantly better than the antenna amplifier that is amplified and then amplified. The reason is that the antenna amplifier is amplified after being mixed, and the amplification integrated circuit is too large in the working frequency band and the signal difference between the strong and the weak is too large, so that it is easy to enter the nonlinear state, and the strong signal interferes with the weak signal. The circuit diagram of the former amplifier is given in the paper, and the working principle is shown in Figure 2.

Amplification of the circuit diagram of a hybrid antenna amplifier, as shown in Figure 2. The 1~12 channel TV signal is input at the VHF input terminal, and a simple low-pass filter is composed of L1, C1, L2. The TV signal after 12 channels is filtered out and sent to the IC1 signal input terminal for amplification of about 20 dB, which is amplified. The TV signal is output through the capacitor C3, and is mixed with the UHF signal through the L3, C4, L4 low-pass filter and then sent to the TV signal output terminal. The other 13-57 channel TV signal input from the UHF input terminal is composed of C5, L5, and C6. It is only a high-pass filter composed of 13 channels and later channels in the UHF band, and then applied to the IC2 signal input terminal for about 20 The amplification of dB is output and then mixed with the VHF television signal through a high-pass filter (C7, L6, C8) and sent to the TV OUT terminal. The power supply of this circuit is stepped down by transformer T, U1 rectified, C14 filtered, IC3 regulated, and supplied to amplifiers LED1 and R1 as power indication circuits.

6 Nonlinear adaptive blind algorithm and principle of merging

The adaptive process is a process of constantly approaching the goal. The approach it follows is represented by a mathematical model called an adaptive algorithm. Gradient-based algorithms are commonly used, with the least mean square error algorithm (ie, the LMS algorithm) being especially common. The adaptive algorithm can be implemented in either hardware (processing circuit) or software (program control). The former designs the circuit according to the mathematical model of the algorithm, while the latter formulates the mathematical model of the algorithm into a program and implements it with a computer. There are many algorithms, and its choice is important. It determines the performance quality and feasibility of the processing system.

For wireless communication, in the field of mobile communication, the base station smart antenna has done a lot of research, and many results have been applied. However, research on smart antennas for wireless mobile terminals has been slow. This is mainly because the mobile terminal has a limited volume, so that the number of antennas of the mobile terminal cannot be too much, and the antenna scale cannot be too large. Also consider the mobility of the terminal, so the antenna pattern is as omnidirectional as possible. The cost and complexity of operating at the mobile terminal is one of the primary considerations. In addition, in mobile terminals, the algorithm of adaptive signal processing requires fast. This requires simple calculations and simple hardware processing, which limits the performance of the terminal. On the TV receiver, adaptive antennas are used to suppress interference and improve the quality of the communication link. As shown in Fig. 3, the adaptive algorithm is a nonlinear adaptive blind algorithm, which does not require a priori knowledge of spatial signals or pilot signals as a training sequence, and ensures that the algorithm is simple.

Let the two antenna elements have a space interval of d, and the first unit is used as a reference. Assuming that a signal arrives at the reference antenna from the side at an angle θ, the signal received by the reference antenna is

Where A is the amplitude, ω0 is the central angular frequency, and ρ is a random variable of 0 to 2π. The signal received by antenna unit 2 is

Where ψ=2πdsin θ/λ represents the phase difference between the two elements due to the geometry. The value of θ is -π/2 ≤ θ ≤ π/2. The second antenna unit has a full feedback loop, and the phase shift is adjusted in the output result.

Where α = ψ - φ represents the deviation of ψ from its optimum value. In addition, the output power is normalized to

Obviously, when α = 0, P(α) reaches the maximum. This fully demonstrates that the output power can be maximized by adjusting the value of α. An iterative algorithm is introduced to achieve maximum output power. Discretize the time, then the phase shift iteration is

7 Conclusion

Space diversity has been successfully applied in next-generation mobile communications, and some of these technologies can be ported to the field of wireless TV transmission. At present, under the analog system, how to use diversity antennas and effective combination, how to maximize the receiving signal-to-noise ratio, especially the use of spatial diversity and frequency diversity at the transmitting end to effectively improve the quality of television transmission, is worthy of study. It is believed that in the fixed state, wireless TV transmission will also gain more progress and development.