DVB-S2 is an upgraded version of DVB-S. The international DVB organization started the upgrade of DVB-S in early 2002. In about two years, it conducted 4 rounds of program competition, and finally the Hughes Network System (HNS) company based on the LDPC+BCH program won. Through the establishment of a large number of simulations and demonstration systems, this program was initially approved by the DVB organization in November 2003. The program text was completed in January 2004 and was recommended by ETSI as the EN 302 207 standard in June 2004.
1. Basic features of DVB-S2
Experiments show that under the given transponder bandwidth and transmission power conditions, the system will get a gain of about 30% according to the selected modulation method and coding rate. The application of variable code modulation technology (VCM) can provide different levels of error protection for different services such as standard-definition television, high-definition television, audio broadcasting, and multimedia. Especially in interactive services or point-to-point services, the variable code modulation technology is combined with the return channel to form adaptive code modulation (ACM, Adaptive Code Modulation). Adaptive coding and modulation technology can provide different levels of error protection coding schemes and modulation methods according to the feedback information of the signal propagation environment in which the receiving terminal is located to achieve the best performance matching, so that the system capacity can be increased by 1 to 2 times. The reliability of various services has also been strengthened. With the development of technology, the improvement of DVB-S/DVB-DSGN single input format (MPEG TS stream) to multi-format data stream (multi-TS or elementary data stream) will not significantly increase the implementation complexity.
As an upgraded system of DVB-S channel coding and modulation technology, DVB-S2 makes full use of the above-mentioned new technologies to provide a new technical standard with strong flexibility and wide satellite service coverage. It has the following five basic characteristics:
1) Flexible input interface matching: It can accept single input stream or multiple input stream in various formats, such as MPEG-2 transport multiplex stream and basic data stream. The input signal can be a discrete data packet or a continuous data stream.
2) High-performance forward error correction system: the inner code uses low-density check code (LDPC code), and the outer code uses Boss-Chaudhuri-Hocquenghem code (BCH, Boss-Chaudhuri-Hocquenghem). This coding scheme differs from Shannon limit in performance by only 0.7-1dB, and the packet error rate (PER) is lower than 10-7 at a given signal-to-noise threshold. It is currently the best-performing coding scheme.
3) Multi-coding rate, multi-modulation, efficient and flexible: coding rate supports 1/4～9/10, etc., 2b/s/Hz～5b/s/Hz symbol mapping modes correspond to QPSK, 8PSK, 16APSK, 32APSK modulation modes respectively, The choice is increased, flexible and efficient, and the non-linearity of the transponder is also specially optimized.
4) Adaptive coding and modulation technology: This technology (ACM) can provide frame-level coding and modulation optimization according to different signal transmission environments, and significantly improve the reliability of system signal transmission.
5) Multiple choices of spectrum roll-off coefficient: 0.35, 0.25, 0.20, which can meet different business needs such as audio, video (SD/HD), and data.
2. DVB-S2 signal processing flow
DVB-S2 adopts the latest channel coding scheme (LDPC+BCH) on the basis of DVB-S, expands signal input mode matching (multiple transport streams and elementary streams), and introduces adaptive signals based on QPSK, 8PSK, 16APSK, and 32APSK Modulation mode (ACM), there are also new breakthroughs in receiver carrier recovery and fast frame synchronization technologies.
The signal processing of DVB-S2 can be roughly divided into five parts to form three format frames (baseband frame, error correction frame and physical frame. The first two types of frames belong to logical frames.). The five parts are:
1) Input mode and transport stream matching part: provide signal input interface, complete input stream synchronization, empty packet deletion, and perform cyclic redundancy check (CRC-8) on the input data packet sequence. If it is multi-input stream mode, it will also be performed The merging or splitting of the input stream is to reorganize the data field of the input stream, and finally insert the baseband mark, fill it, and finally output the baseband frame, the format of which is shown in Figure 4. The baseband frame length Kbch is related to the selected coding rate and modulation mode. In addition, DVB-S2 also provides pseudo-random scrambling for this baseband frame.
2) Forward error correction coding part: mainly completes the channel error protection error correction coding function, mainly divided into three stages: outer code protection (BCH code), inner code protection (LPDC code), and other modulation methods except QPSK The bits are interleaved. The bit interleaving here is very simple. Except for 8PSK with a code rate of 3/5, data input is written serially in columns, and data output is read serially in rows. Write and read are all from the most significant bit (MSB). begin. After the baseband frame undergoes these three steps of error correction coding, a so-called error correction frame (FEC Frame) is formed.
3) Coding symbol mapping part: It mainly completes the mapping work of transmission bits per modulation symbol. Each input error correction frame performs serial-to-parallel conversion according to different parallelism (2, 3, 4, 5), and the converted parallel sequence performs constellation mapping according to the selected modulation efficiency to generate (I, Q) sequence. In this way, the input error correction (frame) sequence becomes the corresponding complex sequence, consisting of 64800/16200 modulation symbols. After this part of the processing, the input error correction frame (FEC Frame) becomes a complex sequence (I, Q) output, which is called a complex sequence error correction frame (XFEC Frame).
4) Physical frame coding part: Here, the complex sequence error correction frame is divided into S segments in units of 90 symbols. The value of S is determined by the length of the complex sequence (frame) (64800/16200) and the selected modulation efficiency (2/3) /4/5) Joint decision. In order to facilitate the configuration of the receiver, it is also necessary to add a physical frame header (PLHREADER) at the front end of the complex sequence, and the length of the physical frame header is also 90. In the S segments divided into multiple serialization, a pilot block (Pilot Block) is inserted after every 16 segments to help the receiver to synchronize. This pilot block composed of an unmodulated carrier is 36 in length. In this way, the length of the physical frame becomes:
90×(S+1)+P×ent{S/16}
In the formula, P=36, ent{} is the rounding function.
Obviously, the coding efficiency of the physical frame is: when there is no XFEC frame that can be processed, the system will insert a dummy frame to ensure the continuity of receiver processing and the smoothness of signal transmission. . This dummy frame consists of a physical frame header and a 36×90 long unmodulated carrier (I=1/, Q=1/).
Finally, each physical frame must undergo complex sequence scrambling except for the frame header before being sent to the modulator.
5) Signal modulation part
The modulation part mainly completes the two major functions of baseband shaping and quadrature modulation. For scrambled physical frames, different roll-off coefficients (0.35/0.25/0.20) are selected for square root raised cosine filtering and shaping according to different business requirements.
After shaping, the signal I and Q components need to be multiplied by sin(2πfot) and cos(2πfot) respectively (fo is the carrier frequency), and then sent to the modulator to obtain the required modulated signal.
So far, DVB-S2 has completed signal coding and modulation. The output signal of the modulator can be sent to the satellite radio frequency channel for signal transmission.
3. The core new technology adopted by DVB-S2
1) BCH+LDPC coding: The basic scheme of this technology is: the outer code uses BCH code, and the inner code uses LDPC code. The BCH code is a conventional cyclic code, which has the characteristics of simple code word generation and strong error detection and correction capabilities. LDPC code is a type of linear block error correction code that can be defined by a very sparse check matrix or bipartite graph. It was originally discovered by Gallager, so it is also called Gallager code. It is similar to the well-known Turbo code, has performance close to the Shannon limit, and is applicable to almost all channels, so it has become a hot spot in the coding industry in recent years. In addition to superior performance to Turbo codes, an important reason for the success of LDPC codes is its advantages in decoding algorithms.
The decoding algorithm of channel coding is an important factor that determines coding performance and application prospects. Due to the sparseness of the parity check matrix of LDPC code, it has an efficient decoding algorithm. Its decoding complexity has a linear relationship with the code length, which overcomes the huge decoding calculation complexity faced by the block code when the code length is long. The degree of difficulty makes the application of long coded packets possible. Moreover, due to the sparse nature of the check matrix, when a long code group is used, information bits far apart participate in a unified check, which makes continuous burst errors have little effect on decoding, and the code itself is resistant to burst errors. The characteristics of the interleaver do not need to be introduced, and there is no time delay that may be caused by the existence of the interleaver.
2) Adaptive Coding and Modulation (ACM): After adopting this technology, the transmission efficiency is improved. DVB-S only has one modulation method, that is QPSK. One modulation symbol maps 2 bits. Therefore, the ability of satellites to transmit signals is greatly restricted. DVB-S2 expands it into multiple selectable modes, namely QPSK, 8PSK, 16APSK, 32APSK, and correspondingly, the number of bits mapped for each modulation symbol is 2, 3, 4, and 5 respectively. In this way, the satellite transmission capacity is greatly improved. It is especially meaningful for operators of direct broadcast satellite systems, that is, the same number of satellites and transponders can double the number of transmitted signals or programs.
More importantly, DVB-S2 uses an adaptive coding and modulation (ACM) scheme, which makes signal transmission more flexible and reliable. This coding and modulation method can reach the frame level (Frame-by-Frame). In other words, in its entire transmission sequence, the coding rate and modulation mode of each single frame can be different. The flexibility of this method is manifested in that different receiving environments (sunny, cloudy, and thunderstorm weather) can provide different coding rates and modulation methods, so that the receiving terminal can receive the most ideal and reliable signal in this environment. For those mobile receiving terminals, this modulation method is more effective. When the receiving terminal travels through different weather environments, its receiving effect will not change drastically due to weather. We can also see that the worse the receiving environment is, the greater the redundancy in the received frame in order to improve the anti-interference of the transmission. Of course, the implementation complexity of each terminal has also increased. It must make full use of the possible return channel to feed back the current receiving environment parameters in real time; on the other hand, because the code rate and modulation mode of the frame and the frame may be different, DVB-S2 provides fast frame synchronization and efficient carrier recovery for the receiving terminal Technology to help the realization of smooth reception.
However, the expansion of modulation methods and coding rates has also increased the requirements for receivers and the entire satellite system. In terms of theoretical simulation, the carrier-to-noise ratio C/N of DVB-S (QPSK) generally ranges from 3.5 to 7.5 dB. Due to the use of BCH and LDPC, the carrier-to-noise ratio C/N range of DVB-S2 (QPSK) is correspondingly reduced, about 1 ~ 5dB, or in other words, in the case of QPSK, DVB-S2 is in a worse channel situation Can also work. According to the industry standard GY/T 148-2000 "Technical Requirements for Satellite Digital TV Receivers", the Eb/N0 threshold is required to be less than 5.5dB (when FEC=3/4), and there is a conversion relationship between Eb/N0 and C/N . Therefore, the working threshold of the demodulator of the future DVB-S2 receiver must be lower, that is, the requirements for the receiver are increased. In other words, DVB-S2 receivers must adopt more advanced and effective technologies to deal with frame synchronization and other issues. Of course, for DVB-S2 modulation methods that are more efficient than QPSK, such as 8PSK, 16APSK, etc., the requirements for the direct broadcast satellite system itself are also greatly improved, or the corresponding minimum carrier-to-noise ratio must also be improved. For example, the carrier-to-noise ratio C/N required by 8PSK is about 5.5-10dB; the carrier-to-noise ratio C/N required by 16APSK is about 10-14dB; that is to say, the satellite transponder, coverage beam design, etc. should also be used More advanced technology, with better signal or coverage field strength when receiving, can achieve greater transmission capacity or total transmission.
4. Comparison of DVB-S2 with other systems
1) Comparison of DVB-S2 and DVB-S: Given the transmission bits per symbol (same modulation method), DVB-S2 has a C/N improvement of 3dB compared to DVB-S. Take QPSK as an example, when bits/symbol=1.5, (C/N)DVB-S≈7dB, (C/N)DVB-S2≈4dB. From the perspective of channel capacity, since DVB-S uses a single QPSK modulation method, the channel capacity provided is very small; while DVB-S2 can use QPSK, 8PSK, 16APSK or even 32APSK, it can provide a larger channel capacity, which is in the DVB -S mid-to-high end that is beyond reach. DVB-S2 can not only provide a higher bandwidth output, but also increase the isolation of the output and increase the stability of the system. Hughes Network Systems (HNS) found through a large number of simulation studies that the LDPC+BCH coding scheme adopted by DVB-S2 is only 0.7 to 1 dB away from the ideal Shannon limit. This shows that we no longer need a new satellite broadcasting coding system for a long time in the future.
Under the condition of the same satellite transponder power and bandwidth, DVB-S2 provides more program sets (channels) and higher symbol rate than DVB-S, increasing the gain by 25% to 35%. At the same time, DVB-S2 also extends DVB-S on the input signal interface. In addition to MPEG-2 transport stream (TS), it also supports the input of basic data packets or streaming data, which increases service flexibility and interoperability.
2) Comparison of DVB-S2 on Internet and other strong interactive services: The latest survey shows that from 2005 to 2009, there may be 1.3 billion US dollars invested in DVB-S2 and related equipment. However, 70% of this investment will be used for interactive services (IS). At present, in terms of interactive services (especially broadband high-speed Internet access), ADSL is the biggest competitor. For ADSL, the access method is simple, the equipment is low, and it provides a wide range of access speeds.
Assuming that the annual rental of a Ka-band spot beam satellite transponder with a bandwidth of 72MHz is 2.6 million euros, it can support 8500 users for DVB-S systems and 22,000 users for DVB-S2, then each user per month Satellite channel cost: 25.5 Euros for DVB-S and 9.85 Euros for DVB-S2.
The current European ADSL package costs 444 Euros per year and 37 Euros per month. It should be noted that this number is all costs (channel + information) of ADSL users, and the above two numbers are just channel costs.Even so, we can see that DVB-S2 is still very competitive (especially in the suburbs or remote areas), and the advantages of DVB-S are not obvious.
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