The functions of the terrestrial broadcasting system: 1. Improve the transmission quality of audio signals; due to the high reliability of digital signals themselves, both the processing and transmission of audio signals are improved. 2. Due to the high reliability and high anti-interference ability of digital signal transmission, the receiver sensitivity can be made higher. Under the same broadcast coverage requirements, its requirements for transmit power are much lower. 3. It can greatly save frequency resources and improve frequency reuse rate.
The operating environment of this tutorial: Windows 7 system, Dell G3 computer.
DTMB (GB 20600-2006, full name Digital Terrestrial Multimedia Broadcast, terrestrial digital multimedia broadcasting), formerly known as DMB-T/H (Digital Multimedia Broadcast-Terrestrial/Handheld, digital multimedia broadcasting-terrestrial/handheld) , is China's digital video broadcasting standard, formulated by the People's Republic of China for digital television and mobile digital broadcasting. The format will serve half of China's TV viewers, especially in suburban and rural areas. DTMB is currently used in mainland China, Hong Kong, Macau and Cuba.
The role of terrestrial broadcasting system
1. Improve the transmission quality of audio signals
and Compared with the existing analog system broadcasting, due to the high reliability of the digital signal itself, the quality of the sound remains unchanged regardless of the processing or transmission of the audio signal, and the quality of the studio can be maintained. Therefore, compared with FM has hundreds or even hundreds of times improvement in several major audio indicators, that is, it can achieve what people often call "CD" quality.
2. Improve power efficiency
Also due to the high reliability and high anti-interference ability of digital signal transmission, the receiver sensitivity can be made higher, thus making the To achieve the same broadcast coverage requirements as existing FM, its transmission power requirements are much lower. Judging from our current broadcasting technology, six sets of high-quality audio programs sharing a 1KW power transmission are close to the coverage of one FM radio using a single 10KW transmission. It can be seen that the power efficiency of DAB is hundreds of times improved compared to FM.
The substantial improvement in power efficiency not only means energy saving, but also a very important point is that it greatly reduces the cost of building a DMB station. The reason is that a single DMB channel can transmit up to six sets of high-power DMB programs and the power level requirements are very low. Therefore, compared with transmitting six sets of high-power FM programs respectively, on the one hand, the occupied area of the computer room is several times smaller; on the other hand, The capacity of its auxiliary power systems such as power supply and distribution, voltage stabilization, and air conditioning is also a hundred times smaller. At the same time, the antenna system that requires a larger investment is also much simpler. In particular, the cost of new stations is reduced and the construction period is also shortened. It is extremely beneficial to promote the use of DMB on a large scale.
3. Improve spectral efficiency
As mentioned before, digital modulation technology has high spectral efficiency and is suitable for DMB, although it is used to solve the problem of mobile reception. QPSK modulation, the spectral efficiency is 2bit/Hz.S, which transmits 6 sets of high-quality audio programs at the same time in the 1.5MHz radio frequency channel. In the 8MHz bandwidth of a TV channel, taking into account the guard interval of each channel, Four DMB channels can be easily arranged to arrange 24 sets of programs, and the number of programs broadcast simultaneously will be much more. This is of course unimaginable under the current simulation system.
In addition, there is an important feature, that is, it is easy to form a single frequency network and use this technology to achieve large-area coverage of the same content, or even province-wide or nationwide coverage. This is actually also It greatly saves frequency resources and improves frequency reuse rate. Taken together, the spectrum efficiency will be more prominent.
Main technical features
1 High transmission efficiency or spectrum efficiency
In Europe DVB- In T, the number of pilot carriers used for synchronization and channel estimation accounts for 10% of the total carriers. The PN sequence of DTMB is placed in the OFDM guard interval, which serves as both frame synchronization and OFDM guard interval. European DVB-T C-OFDM uses 10% of the subcarriers to transmit pilot signals for synchronization and channel estimation, etc., and there is a guard interval for the cyclic prefix, while TDS-OFDM uses the time guard interval for transmitting channel estimation signals at the same time. Therefore, the transmission efficiency of the DVB-T system can only reach 90% of the national standard DTMB system. When comparing multi-carrier technology with single-carrier technology, transmission efficiency is considered a weakness of multi-carrier technology. The core technology of DTMB was developed to solve this problem.
2 Strong anti-multipath interference capability
Compared with single-carrier systems, multi-carrier systems have the ability to resist multi-path interference. The resistance to multi-path interference corresponds to the length of the guard interval. Since the known (after system synchronization) PN sequence is inserted into the national standard time guard interval, given the channel characteristics, the signal of the PN sequence at the receiving end can be directly calculated and removed. The OFDM signal after removing the PN sequence is equivalent to the OFDM signal with the time guard interval filled with zero values. The performance of the OFDM signal with the time guard interval filled with zero values and the OFDM with the time guard interval periodic extension are equivalent under the same channel. of. Moreover, DTMB can still work when the multipath delay exceeds the time guard interval. TDS-OFDM can jointly process the PN sequences of several OFDM frames, so that the delay length against multipath interference is not limited by the length of the guard interval. However, the traditional OFDM guard interval length design requirement must be greater than the delay length of multipath interference. .
3 Good channel estimation performance
Under the AWGN channel, the channel estimation performance of TDS-OFDM is better than C-OFDM. This is because the PN sequence used for channel estimation by TDS-OFDM has a spreading gain of about 20dB, and at the same time, there is no interpolation error unique to C-OFDM for channel estimation. Although the prototype function of DTMB still needs to be improved, the test results of its AWGN channel are still better than those of domestic and foreign systems based on C-OFDM. For multipath channels, the PN sequence of TDS-OFDM and the interference signal caused by the multipath channel are statistically orthogonal. Although the performance of TDS-OFDM channel estimation cannot be directly compared with C-OFDM in principle, it is comparable to the performance of channel estimation using PN sequences in other transmission systems.
4 Suitable for mobile reception
Mobile reception produces Doppler effect and occlusion interference, which makes the transmission channel have characteristics that change with time (time-varying characteristics). What needs to be emphasized is that the signal processing of any OFDM system is based on the assumption that the channel transmission characteristics are time-invariant (the basic condition for applying FFT), that is, within the time of one OFDM symbol, it is assumed that the channel is unchanged, and the changes in the channel are It is considered to occur between OFDM symbols. The channel estimation of TDS-OFDM only depends on the current symbol of OFDM, while the channel estimation of C-OFDM requires 4 consecutive OFDM symbols. Therefore, C-OFDM needs to consider the influence of channel changes of 4 OFDM symbols in mobile situations, while TDS-OFDM only needs to consider the influence of channel changes of 1 OFDM symbol. It can be seen that the DTMB system is more suitable for mobile reception than the European DVB-T.
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