Design and implementation of high performance DTMB receiver chip

Since the release of China's national terrestrial digital TV broadcasting standard (DTMB) with independent intellectual property rights in 2006, it has made great progress. Combined with the gradually mature AVS source coding standard, DTMB has the ability to replace the original analog TV broadcasting, so it has also attracted the attention of many TV manufacturers. 2010 will be the year when the sales of DTMB digital TV sets will begin to explode. In the in-vehicle mobile reception market, DTMB will gradually replace the original DVB-T system in some cities and become the best choice for in-vehicle TV.

As DTMB applications become more and more popular, the complexity of urban ground wireless signal transmission begins to emerge, and higher requirements are placed on receiving chips. As the most critical part of a DTMB receiver, the performance of the receiving chip directly determines the receiving effect of the entire machine and the popularity of terrestrial digital TV. In March 2008, Hangzhou Guoxin took the lead in launching the first domestic three-mode demodulation receiving chip that integrates DTMB single-carrier/multi-carrier and cable digital TV. It has been widely used in markets such as Hong Kong, Hangzhou, Jingmen, and Hunan, and has sufficient experience and full understanding of how to design chips that meet the domestic receiving environment.

Advantages of DTMB Standard

DVB-T is the most widely used terrestrial digital TV broadcasting standard in the world. It has a history of 13 years since its birth. It was also the most widely used terrestrial digital TV standard in my country before the DTMB standard was proposed. Although DVB-T technology is very mature, the requirements for digital TV reception have changed significantly in the past 10 years compared to that time. In addition, the large-scale integrated circuit industry has also experienced rapid development, allowing more advanced technologies to be applied to receiving chips. DTMB is a new generation of digital TV transmission standard, which has the following advantages over DVB-T:

1. Higher spectrum utilization. DTMB uses TDS-OFDM modulation, which does not require pilot signals for channel estimation. Compared with DVB-T, spectrum utilization is increased by about 10%, and more TV programs can be sent under the same transmission bandwidth.

2. Better signal coverage. Terrestrial digital television uses wireless signal transmission, which is easily affected by signal attenuation and other wireless signals. The FEC (forward error correction code) in DTMB is a combination of LDPC (low-density parity check matrix code) and BCH code. Compared with the combination of convolutional code and RS code in DVB-T, it has a lower signal-to-noise ratio threshold, thus greatly improving the signal coverage under the same transmission power.

3. Better mobile reception and anti-interference capabilities. The subcarrier spacing of DTMB's multi-carrier mode is equivalent to the 4K mode of DVB-T (applied in DVB-H). From the perspective of channel estimation, its maximum theoretical anti-Doppler frequency deviation capability that complies with the Nyquist criterion is around 900Hz. Converted to a signal frequency of 704MHz, it can reach a theoretical speed of more than 1,300 kilometers per hour. In addition, because it has a maximum interleaving depth of 250ms, it also greatly improves DTMB's ability to resist sudden interference and shadow fading.

Disadvantages of DTMB Standard

Compared with the DVB-T standard, the disadvantage of the DTMB standard is that the cyclic prefix structure used by DVB-T enables it to resist echo interference of up to 224s, while the multi-carrier mode of DTMB can only reach 125?s (frame mode 3) at most. From the analysis of the PN sequence characteristics of the DTMB frame header, the reception performance can only be guaranteed to be in the best state when the echo length is less than 57.4?s (corresponding to a distance of 17.2Km). If the echo length is greater than 57.4?s, although the channel estimation has a maximum spread spectrum gain of 20dB, once the number of echoes increases (as shown in Figure 1), the interference between echoes increases, and the gain brought by the channel estimation will drop sharply, and even become negative in extreme cases. The single-carrier mode of DTMB uses time domain equalization technology to eliminate the impact of echoes, so it is less sensitive to echo length. In our design, it can resist echo interference of nearly 300?s, which will not be described in detail in this article.

Figure 1. Ground signal echo interference.

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High performance DTMB receiver chip

As for the forward echo and multipath Doppler interference mentioned by most designers, which lead to the decline of DTMB reception capability, our research found that if the delay range of all echoes is less than 57.4?s, using a reasonable frame header area detection algorithm, the gain of channel estimation is very high regardless of the characteristics of the echo distribution. At this time, the signal coverage is mainly determined by the signal field strength. In general, the Doppler interference introduced by vehicle movement does not cause reception failure. After theoretical derivation and field testing, when MFN (multi-frequency network) is used and the echo delay range is within 30?s, the reception quality is only related to the signal field strength.

Long-delay multipath echoes are more likely to appear in SFN (single frequency network). In order to effectively cover a larger area and make reasonable frequency planning, SFN is usually used to lay the network. According to the previous analysis, when the distance between two transmission towers exceeds 17.2 km, it is easy to have strong echoes with a delay of more than 57.4?s. In addition, there are many dense multipath echoes introduced by reflections from other objects (such as high-rise buildings and water surfaces), and the field strength is often sufficient, but it cannot be received normally. This is why some cities still cannot achieve good reception effects after completing signal coverage. In addition, during the process of mobile reception on the vehicle, the number and position of echoes are prone to drastic changes, which can easily cause the DTMB receiver frame header detection module to be unable to accurately locate, thereby affecting the channel estimation and timely tracking of changes in the receiving environment, reducing the mobile reception effect.

From the above analysis, it can be seen that in the mobile reception and multipath echo complex environment, the accuracy of frame header detection is crucial. GX1501B adopts a frame header detection mechanism specially optimized for SFN, which can accurately locate the main path position with the strongest signal and a large number of echo ranges, providing the most effective data for the channel estimation module. The channel estimation module adopts a unique iterative algorithm to make full use of the data of each echo and improve the accuracy of channel estimation; combined with the design of time domain/frequency domain joint equalization, both the single carrier mode of C=1 and the multi-carrier mode of C=3780 can provide outstanding anti-echo performance, which completely compensates for the decline in reception capacity caused by the introduction of high-rise buildings, water surface reflections, and multiple transmission towers and supplementary point repeaters in the urban reception environment. This also makes GX1501B the DTMB receiving chip with the best comprehensive reception performance.