DVB-T standard implementation of high bit rate fixed reception and medium bit rate fixed reception

1 Introduction

The DVB-T standard approved in the autumn of 1996 was the first to launch the market in the UK in the autumn of 1998. It has a total of 280 possible technology combinations. But for more than 8 years, there are only 10 types of engineering realization combinations, accounting for 3.6% of the total technology combination. The important difference between DVB-T and ATSC is that it has a variety of possible technology combinations, which can be flexibly adapted to the specific needs of DT-TB fixed reception in various countries and regions in various countries; ATSC is a single technology combination. DVB-T focuses on the SDTV service, and its total number of applied countries and the total number of home users exceed ATSC (North and Central and South America) or ISDB-T (Japan). The latter two focus on HDTV services. Therefore, the implementation of DVB-T projects in various countries and regions will provide a variety of learning and reference examples for the development of DTTB in China.

This article discusses the first 9 of the 10 engineering implementation combinations of DVB-T, that is, two types of fixed reception with 5 high bit rates (20 to 27 Mbit / s) and 4 medium bit rates (14 to 18 Mbit / s) ( Generally refers to the outdoor roof antenna reception) related parameters (effective bit rate, spectrum utilization, carrier-to-noise ratio threshold, peak and average transmit power and the formation of a single frequency network) and its characteristics, and it is the same as the ATSC standard Compare with the prototype test results of China's DTTB transmission national standard (hereinafter referred to as "Ground National Standard"). Preliminary analysis shows that the Chinese standard is better than DVB-T.

2 DVB-T project realizes fixed reception at medium and high bit rates

2.1 Relevant instructions

The channel bandwidth of DVB-T standard has 3 options: 6, 7, 8 MHz. This article only discusses the 8 MHz channel bandwidth, because China also uses this value. Taiwan that uses 6 MHz has China; and 7 MHz has Australia, Germany VHF band and Italy VHF band.

Secondly, the combination of nine engineering implementations of DVB-T fixed reception basically adopts the option of 8K subcarriers, and only the UK adopts the 2K mode. Because when the UK started DVB-T fixed reception at that time, there were only 2K demodulator chips. But since then, all countries and Taiwan have adopted the 8K mode when they started DVB-T fixed reception. The British government agency OFCOM has decided to change the 2K mode to the 8K mode before fully closing the terrestrial analog TV broadcast in 2012. The advantages of the 8K subcarrier option compared with 2K are: it is more conducive to the formation of a single frequency network (discussed in Section 4); and the 2K mode is more conducive to mobile reception. However, due to technological advances, 8K mode can also be used for mobile reception.

In addition, in order to make a rough comparison between the DVB-T standard and the ATSC standard and China's "Ground National Standard", some minor factors are omitted in the following discussion.

2.2 The highest possible effective bit rate is limited by the peak transmit power

The social benefits (public services) and economic benefits (mainly advertising fee income) of DTTB increase with the increase of effective bit rate. Therefore, DTTB pursues the highest possible effective bit rate. However, for each TV transmitter, its original analog TV broadcast coverage has been formed over the years (generally based on administrative regions and geographical factors), and is achieved under the original frequency planning and assignment, that is, its coverage The scope has been determined for many years.

Now to send terrestrial digital TV signals, the first task is to achieve "simultaneous broadcasting" within the established coverage, that is, to broadcast the original analog TV program content, while broadcasting in digital mode, in order to close the terrestrial analog TV broadcast and free up A large number of terrestrial radio spectrums are prepared. Moreover, within the established coverage, the quality of the images and sounds of watching digital standard definition TV can not only be reduced, but also must be improved, and the number of SDTV programs can be increased by 4 to 10 times, if conditions permit, broadcast during prime time Digital high-definition television.

Under the premise of a given coverage, try to use a transmission system with lower peak and average power to reduce costs and reduce electromagnetic pollution to the environment. Because the peak transmission power is too large, it will interfere with existing terrestrial analog TV broadcasting services (or Future terrestrial digital TV broadcasting services), and this is not allowed in frequency planning. This means that the peak transmit power is limited by interference to "adjacent channels" and "co-channels". In addition to the consideration of the working channel, antenna height and directivity, terrain and other factors, the determination of the average transmission power mainly considers the AWGN carrier-to-noise ratio (C / N) threshold (hereinafter referred to as the threshold) ). After the average design is determined, the peak design is determined by the peak-to-average ratio (PAR) (generally obtained from the laboratory test of the prototype). In this way, it can be verified whether it meets the requirements for "adjacent channel" and "co-channel" interference (during and after the transition).

2.3 High bit rate fixed reception characterized by 64-QAM

The first five types of DVB-T engineering implementation combinations are high bit rate fixed reception, as shown in Table 1 # 1 ~ # 5. The effective bit rate is 20 ~ 27 Mbit / s; the computer simulation thresholds are 18.0 dB and 16.5 dB respectively (the corresponding prototype threshold estimates are 20.5 dB and 19.0 dB, respectively, because it is usually considered: on the computer simulation threshold Add 2.5 dB as the prototype test threshold estimate). Among the five project implementation combinations, if the number of TV home users is considered to be large, number # 3 becomes a typical reception among them.

2.4 Comparison of DVB-T and ATSC at high bit rates

In Table 1, number # 3 is compared with number (1): the former's spectrum utilization rate is 7.0% lower, and the prototype threshold is 3.8 dB higher. Therefore, DVB-T number # 3 is not as good as ATSC number (1).

2.5 Comparison of DVB-T and Chinese standards at high bit rates

In Table 1, the numbers (2) to (5) of China's "Ground National Standard" are arranged in order of the effective bit rate, and the prototype threshold test values ​​are also listed at the same time. It can be seen that:

1) Number # 3 is compared with numbers (2) and (3): the effective bit rate of the latter is higher than 7.6% or 1.0%, respectively, while the prototype threshold is reduced by about 3.5 dR or 3.7 dB. It can be seen that China's "Ground National Standard" number (2), (3) is better than DVB-T number # 3.

2) Comparison between No. # 3 and Nos. (4) and (5): Although the effective bit rate and spectrum utilization of the latter are reduced by 10.3% or 13.9%, respectively, the prototype threshold is reduced more (about 7.7 dB or 7.6 dB) (see discussion in Section 3!).

3) In addition, the number (2) is compared with the number (1): when the threshold is close, the spectrum utilization rate of the former increases by 5%. Its performance is the best in Table 1.

However, because the number (2) of 595/3 780 = 1 / 6.353 is greater than the number 1/32 of the guard interval, it is greater than the number (1) of 1/207, that is, the head "overhead" is slightly larger. If you want to increase the effective bit rate, you can add (595/3 3 780) × (1/2, 1/4 or 1/8) 3 options to 595/3 780, and change the effective bit rate from 25.989 Mbit / s, Increased to 27.89 Mbit / s (+ 7.30%), 28.94 Mbit / s (+ 11.36%) or 29.50 Mbit / s (+ 13.51%).

In short, from the overall view of Table 1, China's "Ground National Standard" numbers (2) and (3) are better than DVB-T number # 3, and the performance of number (2) is optimal (and there is room for improvement!) .

3 Mid-bit rate fixed reception realized by DVB-T project

3.1 Medium bit rate fixed reception characterized by 16-QAM

The four combinations of DVB-T engineering implementations are fixed bit-rate reception (see Table 2), whose effective bit rate is 14 to 18 Mbit / s; and the prototype threshold is close to ATSC (# 6, # 7, # 8) or slightly lower (# 9 is about 1.6 dB lower). Among the four combinations, if it is considered that there are a large number of TV home users, the numbers # 6 and # 9 are two typical of the four types. Because its threshold value is 4 dB or more lower than that of No. # 3, it is easier to achieve "simultaneous broadcasting" and improved coverage within a given coverage, but at the cost of reduced spectrum utilization.

3.2 Britain uses 16QAM modulation to improve coverage

When the UK launched terrestrial digital TV broadcasting in the autumn of 1998, it used 64QAM modulation (No. # 3) (currently still used in a few services). However, OnDigital's "Paid Watching" service was announced a year and a half after its failure, and the coverage was insufficient (see Table 3). Therefore, when the BBC and Castle obtained the OFCOM authorization cooperation and restarted the free viewing service Freeview, most regions changed to 16QAM modulation (No. # 6), which can increase the coverage of the established coverage from 66% to 83% (see table 3).

This may be: 1) Remedial measures taken during the transition period to limit the peak value of digital transmission power due to interference to adjacent channels or co-channels of existing terrestrial analogue TV broadcasts; 2) Lower threshold 16.5-12.5 = 4.0 (dB) In order to increase the coverage within a given coverage (see Table 3); 3) The price paid is: the effective bit rate in the original 8 MHz channel is reduced from 24 Mbit / s to 18 Mbit / s (from 5 sets of SDTV to 4 set).

The switch to 16QAM in the UK has taken effect. According to reports, by the third quarter of 2006, there were 18.538 million digital TV households in the UK (accounting for 74.8% of the total number of TV households). Among them: terrestrial 37.8%; cable 15.8%; satellite 46.4% (including both). In this way, the UK has become the country with the highest proportion of DTTB households and digital TV.

3.3 Germany focuses on the development of fixed bit rate reception to take into account indoor reception (including portable reception) and mobile reception

From the spring of 2003, Germany started DTTB with 16QAM modulation (No. # 9). Due to its lower threshold (the difference between numbers # 3 and # 9, there is a gap of 5.4dB), it is more conducive to achieving "simultaneous broadcasting" and improving coverage within a given coverage. The basic national conditions of German TV viewing are (July 2005 data): 33.9 million TV households, of which 51.7% are cable; 43.1% are satellite; and 9.7% are on the ground (including both)

According to the ranking of DVB website information, the positioning of German DTTB business is: indoor reception (including portable reception), mobile reception and fixed reception (generally refers to outdoor roof antenna reception).

In August 2004, the Berlin and Brandenburg regions took nine months to complete the transition from terrestrial analog TV broadcasting to digital TV broadcasting. It is currently being promoted on a large scale in various regions of the country. According to reports, by mid-January 2007, 7 million terrestrial digital TV receivers (including set-top boxes) were in operation in Germany (the direct ratio of this number to the total number of TV households was 20.6%). The statistics of digital TV households in July 2005 are: ground 4.0%; cable 5.0%; satellite 16.7%. It can be seen that the DTTB business in Germany has also been progressing rapidly in the past year and a half.

3.4 Comparison of DVB-T and Chinese standards at medium bit rate

1) Comparison between Table 2 No. # 6 and Table 1 Nos. (4) and (5): the effective bit rate (spectrum utilization) of the latter two is increased by at least 15%, while the threshold is reduced by at least 2.6 dB, which is China The high bit rate mode of "Ground Standard" numbers (4) and (5) is significantly better than DVB-T's # 6 medium bit rate mode.

2) Comparison between number # 7 and number (6): The effective bit rate of the latter is close to the former, but the threshold value is about 5 dB lower. It can be seen that China's "Ground Standard" number (6) is also better than DVB-T's # 7.

3) In addition, the number (7) is more suitable for fixed bit rate reception, taking into account indoor reception (including portable reception) and mobile reception. Because its prototype threshold is lower at 5.8 dB, and it is very close to the estimated value of 5.6 dB for mobile reception in Singapore (2K mode, GI = 1/4, FEC = 1/4, QPSK modulation) (computer simulation value 3.1 dB Plus 2.5 dB); and its effective bit rate is more than doubled (10.396 Mbit / s compared to 4.98 Mbit / s)! It can be seen that China ’s “Ground National Standard” number (7) fixed bit rate fixed reception and mobile reception mode , Significantly better than DVB-T's low bit rate mobile reception mode.

4 Discussion on other parameters in DVB-T engineering implementation

4.1 The average value and peak value of the transmit power

For the determined coverage area, due to the difference between the carrier-to-noise ratio threshold and the transmit power peak-to-average ratio (PAR), when using different engineering implementation combinations for terrestrial digital TV broadcasting, the average value and peak value of the required transmitter are different (See Table 4. For a given coverage area, assume that the average transmission power required for DVB-T number # 3 is 1 kW).

Note: The values ​​in Fangjian are for reference.

It can be seen from Table 4:

1) Comparison between No. # 3 and No. (1): the threshold of the former is about 3.8 dB higher; and the peak-to-average power of transmission is 3.7 dB higher than (PAR). Therefore, for the same coverage, the average and peak transmit power of the former are 3.4 times and 7.9 times of the latter, respectively! It can be seen that ATSC number (1) is better than DVB-T number # 3.

2) Comparison between No. # 3 and Nos. (2) and (3): the threshold of the former should be at least about 3.5 dB higher; the PAR should be 2.76 dB higher or close. Therefore, the average transmit power of the former is more than twice that of the latter, while the peak transmit power of the former is more than 4 times or twice that of the latter. It can be seen that China's "Ground National Standard" number (2), (3) is better than DVB-T number # 3, and the number (2) has a greater advantage.

4.2 Related parameters when setting up a single frequency network (SFN)

It can be seen from Table 5: DVB-T number # 3 (8K mode, excluding the United Kingdom) and number # 9 (8K mode) when forming the SFN, their coverage radius is 8.4 km or 33.6 km respectively; and number # 6 ( 2K mode) only 2.1 km (too small). This result shows that 8K is more conducive to the formation of SFN than 2K mode.

Note: The data of DVB-T comes from its standard document [7]; while the data of China ’s “Ground Standard” comes from literature [5].

The coverage radius of China's "Ground National Standard" numbers (2), (5), (7) and (3), (4) and (6) are 23.6 km and 16.7 km respectively, which are also conducive to the formation of SFN.

In addition, DVB-T is a multi-carrier system, which requires a working clock accuracy of about 10-12; while ATSC or China ’s “Ground National Standard” C = 1 options are single carrier systems, which require a working clock accuracy of about 10-9 . The difference between the two is 3 orders of magnitude. Because there are three orders of magnitude difference in the adverse consequences caused by phase noise: 2K mode is 2 048 times that of single carrier system; 4K mode is 4 096 times; 8K mode is 8 192 times.

5 Summary

1) DVB-T's 64QAM high bit rate fixed reception (No. # 3 is an example) can achieve "simultaneous broadcasting" within a given coverage range and obtain a higher effective bit rate when the peak transmit power is limited.

2) DVB-T's 16QAM fixed bit rate reception is beneficial to achieve "simultaneous broadcasting" (number # 6 as an example) within a given coverage and increase coverage; or to take into account indoor reception, portable reception, mobile reception and formation SFN (No. # 9 is an example).

3) Various examples of the implementation of DTTB's high-bit or medium-bit-rate fixed reception projects using the DVB-T standard "according to local conditions" are available for us to learn from and learn from.

4) China's "Ground National Standard" is superior to the DVB-T standard. This may be due to the improvement obtained by using LDPC encoding.

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