"This paper briefly introduces the new generation of portable mobile TV receiving standard DVB-H and the source compression coding standard of mobile TV, focusing on the composition of DVB-H system, time slicing technology, MPE-FEC, 4K transmission mode and several main technical characteristics that can adapt to mobile TV receiving.
DVB-H、
It used to be a dream to use portable communication tools such as mobile phones to watch TV anytime and anywhere. With the establishment of source coding technology, channel transmission and a new generation of basic communication network, the portable mobile receiving subsystem has also changed from a single text and picture form to a more colorful video and audio form. In order to adapt to this trend, the television industry has also carried out standard formulation and technology R & D for related technologies. Now do the following discussion on related technologies.
To watch TV on mobile phones, we need to deal with three links technically: signal source, transmission path and receiving terminal. In the aspect of signal source, a source compression coding standard with high compression ratio is needed; In terms of transmission channels, there are wireless microwave and network transmission. In order to realize mobile reception, digital modulation and channel processing technology with strong anti-interference ability are needed. In terms of receiving terminals, we must develop chips with high integration, small volume, light weight and low power consumption, as well as rechargeable batteries with small volume and high capacity.
At present, there are three main ways to realize the service:
1. Implementation method using mobile network
At present, the mobile TV business launched by mobile operators in the United States and China mainly depends on the existing mobile network. China Mobile's mobile TV service is based on its GPRS network, while China Unicom relies on its CDMA network. Whether GPRS mobile phone or CDMA mobile phone, it is necessary to install corresponding playback software on the mobile phone terminal equipped with operating system (generally PDA mobile phone and other high-end products), and the corresponding TV program source is organized and provided by mobile communication companies or through corresponding service providers.
2. The way of using satellite network
It is a very new idea to use mobile phones to receive TV program signals broadcast by satellites. At present, only South Korea is pushing mobile TV broadcasting (DMB). The DMB receiver can provide high-quality images, and the receiver module can enable users to receive the signals of terrestrial wireless TV broadcasting and satellite TV broadcasting at the same time.
3. Installation of digital TV receiving module in mobile phone
At present, the most promising mobile TV technology is through the integration of digital TV and mobile phone. In this way, the microwave digital TV receiving module needs to be installed on the mobile terminal, and the digital TV signal can be obtained directly without the link of the mobile communication network. At present, the mobile digital TV standard is only DVB-H in Europe and single band broadcasting standard in Japan.
In China, only CCTV and a few mobile companies have launched mobile TV business one after another. Taking CCTV as an example, since there is no DVB-H digital broadcasting network in China, they broadcast "mobile TV" programs through 2.5G or 2.75g network transmission technology, that is, they provide users with online live or on-demand streaming audio and video programs through WAP Portal through China mobile GPRS / edge network or China Unicom CDMA network.
The transmission standards and coding standards of mobile TV are discussed below:
1、 Transmission standard of mobile TV -- DVB-H
The full name of DVB-H (dvb-x in the early days) standard is digital video broadcasting handheld. It is a transmission standard formulated by DVB organization to provide multimedia services to portable / handheld terminals through terrestrial digital broadcasting network. Based on DVB-T, DVB-H is a system that uses IP packets (datagrams) to transmit data (mainly digital multimedia data). This standard is considered as an extended application of DVB-T standard. However, compared with DVB-T, DVB-H terminal has lower power consumption and superior mobile reception and anti-interference performance. Therefore, this standard is suitable for small portable devices such as mobile phones and handheld computers to receive signals through terrestrial digital TV broadcasting network. In fact, DVB-H is a standard to support multimedia services. Besides TV business, DVB-H can also carry out various comprehensive services such as e-newspaper, electronic auction, tourism guide, games, video on demand and interaction. In short, the DVB-H standard is based on the current DVB-T transmission system to enable mobile phones and other portable devices to stably receive radio and television signals by adding certain additional functions and improved technologies.
In order to reduce the power consumption of small handheld devices, DVB-H adopts a technology called "time slicing", which transmits IP packets in the form of data burst data burst in a very short period of time (time slots). The front end circuit of the receiver will be turned on only during the selected service data burst period. During this very short period, the data is received at high speed and can be stored in the buffer provided by the device. This buffer can store the downloaded content or directly play the live broadcast data file.
1. DVB-H system structure
DVB-H supports small mobile terminal devices such as mobile phones and is the standard for mobile digital TV transmission. DVB-H is a standard based on DVB data broadcasting and DVB-T transmission, and pays more attention to the implementation of the protocol. The front end of the system is composed of DVB-H encapsulation machine and DVB-H modulator. DVB-H encapsulation machine is responsible for encapsulating IP data into MPEG-2 system transmission stream, and DVB-H modulator is responsible for channel coding and modulation; The system terminal is composed of DVB-H demodulator and DVB-H terminal. DVB-H demodulator is responsible for channel demodulation and decoding, and DVB-H terminal is responsible for relevant business display and processing.
DVB-H transmission system also has the following special requirements:
(1) In order to prolong the service time of the battery, the terminal periodically turns off part of the receiving circuit to save power consumption;
(2) Can roam, and can still receive DVB-H service very smoothly during roaming;
(3) The transmission system can ensure the smooth reception of DVB-H services at various mobile rates;
(4) The system has strong anti-interference ability;
(5) the system has considerable flexibility to accommodate simultaneous interpreting of different transmission bandwidth and channel bandwidth.
2. Protocol hierarchy
DVB-H standard will implement data link layer and physical layer.
(1) Data link layer - time slicing technology is adopted to reduce average power consumption and facilitate smooth and seamless service exchange; MPE (multi protocol encapsulation) forward error correction technology is adopted to improve the C / N threshold and Doppler performance in mobile use and enhance the ability of anti pulse interference.
(2) Physical layer - compared with DVB-T, 4K transmission mode and deep symbol interleaving are added.
Other technical features include: adding DVB-H signaling to transmission parameter signaling (TPS) bits to improve service development speed; Cellular identification (in TPS) is used to support fast signal scanning and frequency exchange during mobile reception; Add 4K mode to adapt to mobile reception and single frequency cellular network, and improve the flexibility of network design and planning; 2k and 4K modes carry out deep symbol interleaving to further improve the robustness in mobile environment and impact noise environment.
3. Key new technologies
(1) Power consumption: DVB-H requires the power consumption of RF receiving, channel demodulation and decoding to be less than 100MW.
(2) Network design
Because the receiving antenna of DVB-H terminal is small and single when moving in the network, the single frequency network must be optimized. To this end, new technical modules are added to DVB-H, mainly including:
① Time slicing - based on time division multiplexing technology, which saves power consumption of receiving terminal and facilitates network switching;
② MPE-FEC - based on RS error correction coding technology, additional forward error correction coding is added to improve the mobile and anti pulse interference ability of the system;
③ 4K mode - used to improve the flexibility of network design;
④ DVB-H TPS - dedicated transmission parameter signaling for DVB-H to improve system synchronization and service access speed.
Time slicing, MPE-FEC, 4K mode and DVB-H TPS are described in detail below:
① Time slicing
Time slicing technology is the most important new technology module in dvb-h. it uses burst mode to transmit data. Each burst time slice transmits a service. In the service transmission time slice, the service will occupy all data bandwidth separately, and point out the time when the next same service time slice is generated, so that the handheld terminal can receive the selected service at the specified time, Do energy-saving processing in the idle time of the service, so as to reduce the total average power consumption. During this period, the front-end radiology machine has been working. Other service data will be transmitted between two time slices of the same service. DVB-H signal is composed of many such time slices. From the perspective of the receiver, the received service data is not the traditional continuous input mode with constant rate. The data arrive at intervals in a discrete manner, which is called burst transmission. If the decoding terminal requires a low data rate but must be a constant code rate, the receiver can buffer the received burst data first and then generate a data stream with constant rate. It can not only effectively reduce the average power consumption of handheld terminals, but also realize the smooth and seamless service exchange between different networks.
a. Time slicing and power consumption
Time slicing technology uses burst data transmission, which has higher instantaneous rate than traditional data flow services. In order to save power consumption, the burst bandwidth is generally about 10 times of the fixed bandwidth. For example, a service flow with a constant rate of 350kbit / s means that a burst bandwidth of about 4Mbit / S is required. When the burst bandwidth is twice the fixed bandwidth, the power consumption can be saved by 50%. Therefore, if the bandwidth is 10 times, 90% can be saved.
b. Time slicing and PSI / Si
DVB-H standard stipulates that PSI / Si (program specific information, PSI / service information, SI) information will not be processed by time slicing, and they will be allocated a fixed bandwidth for transmission. This is mainly because the currently used psi / Si information does not support time slicing transmission. If it is changed, it will be difficult to be compatible with the current data table. PSI information uses four tables to define the structure of code stream: program association table (PAT), program map table (PMT), network information table (NIT) and conditional access table (CAT).
The handheld terminal needs to access the nit (network information table) and the intermediate code int table in the Si in the DVB-H system. The purpose of nit table is to provide information about the physical network. Its content is fixed. When the handheld terminal joins a new network, it is first necessary to receive the table and determine the network parameters. When switching between different transmission streams, the handheld terminal needs to read the int table. Unless the int table is changed later, the terminal will no longer receive the int table. The int table change information is identified in the PMT (program map table) table of PSI. The PMT table indicates the PID number of each code stream constituting the program service and describes each code stream
Since DVB standard stipulates that PSI information must be retransmitted every 100ms, if the service transmission time of burst pulse is longer than 100ms, the handheld terminal can access all psi information while receiving the service; If the service transmission time is less than 100ms, the handheld terminal shall continue to work for a period of time after the service is received to ensure the receipt of the requested psi table.
c. Time slicing and service exchange
The time slicing technology is adopted to enable the handheld terminal to monitor the adjacent cells in the idle cycle of service transmission, scan other frequency signals and test the strength of signals, but do not interrupt the reception of this service. When users enter new networks, they can switch to simultaneous interpreting of different traffic streams with the same traffic in the idle period according to the monitoring results. If the service is synchronized and accurately arranged at the front end, the same service can appear in different time slots of adjacent peaks and sockets in time, and the user will not notice this change.
d. Time slice and conditional acceptance
DVB-H can realize conditional reception in two ways. One is IP based conditional reception system (ip-cas, IP data broadcast encryption). All CAS (conditional access system) related information is in IP data, and can support time slicing technology to ensure power saving. However, DVB-H standard does not need to support two-way transmission between CAS and receiver, and ip-cas only needs to support broadcast environment.
Another way is the conditional access system (dvb-cas, TV encryption system) using DVB general scrambling algorithm. At this time, transmitting CAS Information in DVB-H system will face some problems. Dvb-cas uses electronic counter measure (ECM) to transmit descrambling key, so ECM cannot perform time slicing. In addition, dvb-cas also uses EMM (EMM entry management message) to transmit authorization management information. Since the time interval of EMM is random, the terminal must work all the time to ensure that EMM will not be lost, And the direct use of dvb-cas will affect the network roaming service.
In order to ensure descrambling, the receiver must complete ECM reception, and the system identifies the minimum repetition period of ECM through ECM repetition rate descriptor. If the handheld terminal has completed at least one ECM minimum cycle reception before starting to receive service data, it can receive at least one ECM to obtain the descrambling key. Generally, the effective time of descrambling key is 10s. Therefore, the receiver must ensure that the descrambling key is received 10s before the service data arrives.
EMM will be transmitted in time slice mode. Firstly, the EMM is encapsulated in the form of IP datagram. After encapsulation, the time slicing method of emm-ip data is the same as that of other IP data,
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