Talking about the technology of mobile TV and CMMB system
1 System characteristics Radio and television system proposed in 2006, China Mobile Multimedia Broadcasting (CMMB) system based on independent intellectual property rights, which adopts the technology system of “integration of heaven and earthâ€, that is, using high-power S-band satellites to pass the broadcast channel. And the distribution channel realizes coverage of mobile multimedia broadcast signals covering 100% of the country's land, adopts s-band ground supplementation network for S-band satellite coverage shadow area signal forwarding coverage, U-band ground coverage network for effective coverage of urban densely populated areas; and uses wireless mobile The communication network constructs a return channel to realize interaction, forming a combination of one-way broadcast and two-way interaction, and a seamless coverage system combining central and local is shown in FIG.
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Its main technical features can be summarized as:
1) Applicable to the frequency range of 30Mg to 3000Hg, supporting 8Hg and 2Mg broadband, flexible application;
2) Highly structured, low-density parity code (Lopc) of advanced channel coding technology with low complexity is implemented to reduce the signal reception threshold.
3) Using slot-based frame structure and orthogonal frequency division multiplexing (OFDM) modulation technology, it provides a good mechanism for terminal power saving.
4) Adopting logical channel technology, it can be flexibly configured, and is suitable for real-time broadcasting, data broadcasting and other services.
5) Using the separate transmission and control parameters of the control parameters and service data, the key service auxiliary information and channel scheduling control information are placed in a dedicated high protection rate channel for transmission, which can well adapt to the harsh environment of wireless transmission.
6) Using beacon technology to achieve fast synchronization of complex and variable wireless channels.
Compared to other existing mobile multimedia broadcast technologies, CMMB's "integrated world" technology has a natural system-level advantage. We know that the current mobile media broadcasting technology is improved on the basis of existing terrestrial digital TV/sound broadcast transmission, such as DVB-H developed on the basis of DVB-T and changed from DVB. T-DMB, TMMB, etc.
In order to maintain consistency or compatibility with the original transmission technology system, they are inevitably restricted in technological advancement, and the other is to adopt a new technical system, such as MediaFLO. However, note that these standards use a single terrestrial transmission system for signal coverage. There are several constraints:
1) Coverage cost: When covering with a single ground launch, a large number of base stations are needed to solve the coverage of the suburbs and the outer suburbs. At the same time, the connection between the base stations needs to use the fiber link as shown in Fig. 2, so the construction and maintenance cost is large.
2) Covering blind spots: Just like the mobile communication of terrestrial television, due to geographical constraints, in areas such as deserts, oceans, inland areas, mountains, and other places where it is difficult to set up base stations, signals cannot be effectively covered, forming a blind spot for business.
3) Roaming and business continuity: using a single terrestrial networking mode, on the one hand, the service is interrupted due to the limitation of coverage, on the other hand, the local access of the roaming users (different transmission frequencies), management, etc. will be a Very complicated work requires a unified connection and management of operating platforms across the country through the network.
4) Resilience to disasters: When different natural disasters occur, ground communication may be damaged or the power system may not work properly, which may cause the business to be interrupted.
In comparison, CMMB adopts the technical system of “integration of heaven and earthâ€, which can basically make up for the above shortcomings.
2 Source compression technology Mobile multimedia broadcasting requires low bit rate, high efficiency video and audio compression coding, China's standard AVS, PRA, international standard H. 264. The AAC standard is a source compression standard suitable for mobile multimedia broadcasting. In actual use, it is necessary to configure appropriate coding parameters to achieve appropriate compression efficiency and algorithm complexity to meet system requirements.
As shown in FIG. 3, the source compression section receives the data of the audio and video source and outputs it to the multiplexing section through compression encoding processing. Mobile multimedia broadcasting (MMB) system audio and video services are divided into TV broadcasting service and sound broadcasting service, mobile multimedia broadcasting system source video, audio compression coding standard is used as:
1) In the TV broadcasting service, video compression coding adopts AVS standard, H. 264 standard.
2) In the sound broadcasting service, the audio compression coding adopts the DRA standard and the AAC standard.
2.1 Video compression coding
   CMMB video compression coding uses AVS, H. The 264 standard uses advanced compression techniques and key algorithms in its transformation, quantization, prediction, entropy coding, etc., so that its video frame rate supports at least 25 frames per second, and the image resolution supports QVGA (320×240) and QCIF (176). ×144), the video compression code rate supports 64 kbps to 768 kbps.
2.2 Audio compression coding
   CMMB audio compression coding uses the key algorithms of DRA and AAC, so that the number of audio channels supports mono and stereo. The audio compression code rate supports 32 kbps to 128 kbps. The audio sampling rate supports 48khz, 44.1khz, 32khz.
3 Reuse multiplexing is an important part of the CMMB system. It mainly completes the information encapsulation and arrangement of audio, video, data, and electronic service guides, so that it can be transmitted on the mobile multimedia broadcast channel, as shown in Figure 4.
The components of the CMMB multiplexing protocol include a multiplex frame, a multiplex subframe, a video segment, an audio segment, and a data segment. Each of the physically transmitted broadcast channel frames is composed of multiplexed frames, and the plurality of multiplexed subframes or control information tables constitute a multiplexed frame. The multiplex subframe includes audio, video, and data segments. There may be up to 40 multiplexed frames in a CMMB broadcast channel frame. According to different bearer contents, it can be divided into two types of multiplexed frames. The first multiplexed frame (frame identifier = 0) is called control. Frames, other multiplexed frames are called service frames. The payload of the control frame is a type of control information table, including a network information table, a persistent service multiplexing configuration table, a persistent service configuration table, a short-time service multiplexing configuration table, a short-time service configuration table, an ESG basic description table, and an emergency broadcast. Provide various corresponding control information for the terminal. The payload of a service frame is one or more multiplexed subframes (up to 15), and each multiplex subframe carries video and audio or data information, which is called a service, that is, each multiplex subframe is a service application. The audio elementary stream of the same service, the video elementary stream and the data stream are encapsulated in the same multiplex subframe, and the service multiplex frame is a collection of multiple services. In summary, CMMB reuse is the basic key protocol in the system, and the efficient and flexible sealing of multimedia data such as video and audio will lay a solid foundation for the smooth development of China's mobile multimedia broadcasting industry.
4 Electronic Service Guide In CMMB, the Electronic Service Guide (ESG) is a multimedia broadcast information navigation service. Mainly used to provide users with information on all services. It is composed of basic description information, data information and program prompt information as shown in Fig. 5.
The basic description information describes the allocation of the data information in the ESG service, the update status, and the like, and is transmitted in the control logical channel, that is, in the multiplex control frame.
The data information is represented by XML. It mainly describes the business information, business extension information, programming information, content information and business parameter information related to the mobile multimedia broadcasting service. The XML information is used to describe the ESG data information as a special mobile in the CMMB system. Multimedia broadcast service transmission. The service information is composed of several service elements, each of which describes the attributes of the service, such as the service identifier, name, language, etc.; the service extension information is composed of several service extension elements, each element describes the detailed attribute of the service; Arrangement elements, each element describes the start time of the program, the name of the program, etc.; the content information is composed of several content elements, each element describes the attributes of the program, such as duration, program introduction, keywords, etc.; The information consists of several service parameter elements, each of which describes the parameters required to access the service, such as video, audio bit rate, frame rate, and so on.
The program prompt information describes the summary information of the current time period and the next time period of the service, and is transmitted along with the mobile multimedia broadcast video and audio services.
5 Conclusion Different types of standards have mobile TV based on this standard chip. The advantage of the 3G mobile phone that transmits video signals through the telecommunication network is that it can realize two-way transmission, and it can easily provide interactive services such as on-demand broadcasting. The disadvantage is that there is a broadband neck strength, and the cost is closely related to the traffic, and the advantages of the mobile TV that transmits the television signal through the broadcast channel There is no bandwidth limitation, and the cost has nothing to do with the traffic. The disadvantage is that the signal must be transmitted back through the telecommunication network to achieve two-way interaction. Considering the tariff and network environment, broadcast mobile TV is now more popular. It is worth noting that the excellent performance of CMMB during the Olympics made us feel that the prospects of CMMB have become clearer. After the successful trial operation during the Beijing Olympics and Paralympics, the CMMB mobile multimedia service led by the broadcasting and TV system has been in 37 cities across the country. Investment operation, in the near future, a unified standard, unified planning, unified construction, unified operation, the mobile multimedia mode of the national roaming CMMB system will be presented to the world.
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