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Keywords: Turbo encoder, digital TV, channel coding, T-TCM
1. Foreword
Channel coding is a very important part in digital TV, and it plays a decisive role in the overall performance of digital TV systems. The Turbo code is the best of all current channel coding schemes. Based on the research of digital TV channel coding theory and Turbo codec theory, we design a Turbo code encoder chip that can be used in digital TV transmission system. In the design, we completed the design process of the entire chip with the EDA front-end design tool. The circuit function of the Turbo encoder chip is to implement Turbo channel coding and its transmission process, and the encoded data stream structure is 16 bits/block. In order to meet the real-time processing requirements of actual digital TV signals, the chip must work at a higher frequency, so the pipeline technology is adopted in the design.
2. Channel coding in digital television
2.1 Digital TV transmission standard
(1) The American Advanced Digital Television Systems Committee standard, ATSC, uses lattice-coded eight-level vestigial sideband modulation (8-VSB).
(2) The European digital terrestrial video broadcasting standard DVB-T uses coded Orthogonal Frequency Division Multiplexing (COFDM).
(3) Japan's terrestrial integrated services digital broadcasting ISDB-T, which uses band-segment transmission orthogonal multiplexing modulation (BST-OFDM).
The above are the three major international digital TV standards.
On August 30, 2006, the DMB-T standard and the ADTB-T standard of Shanghai Jiaotong University were proposed by the Digital TV Transmission Technology R&D Center of Tsinghua University, and finally established as the national standard for digital TV terrestrial transmission in China. Terrestrial Broadcast Transmission System Frame, Channel Coding and Modulation (referred to as DMB-TH standard). According to the international announcement issued by the National Standardization Administration, the digital TV standard with the standard number GB20600-2006 has been officially approved as a mandatory national standard and will be officially implemented on August 1 next year. It marks the industrialization of China's digital TV independent intellectual property standards.
2.2 Digital TV system channel coding scheme
The channel coding scheme of the three major standards of digital television international and the channel coding scheme of the new digital television standard in China (using the channel coding scheme of Tsinghua) basically use the RS code as the outer code, plus the interleaver, plus the inner code. The structure of a typical concatenated code (convolutional code or TCM) [2] . The protection capability of the convolutional code/TCM as the inner code largely determines the white noise threshold of the system. Its error correction ability has a certain distance compared with the fragrance limit.
The design of the Turbo code encoder chip in this paper is before the introduction of the new national standard, but before we designed it, we have thoroughly studied the Tsinghua DMB-T standard and the ADTB-T standard of Shanghai Jiaotong University. Based on this, the design is available. The Turbo code encoder chip of the digital television transmission system uses it as an inner code, thereby greatly reducing the noise threshold of the system.
2.3 New ideas for digital TV channel coding
Grid Coded Modulation (TCM) overcomes the shortcomings of traditional channel coding. Combining modulation and coding, it can obtain 3~6dB coding gain without increasing bandwidth and reducing data transmission rate. Efficient modulation scheme. As a channel coding method with excellent performance under low SNR conditions, Turbo is an ideal scheme for implementing trellis code modulation, which provides a new idea for code modulation [1] .
Turbo-TCM technology (also known as T-TCM technology) combined with modulation techniques using Turbo codes and multi-layer coding using Subcodes with Turbo codes has become a new direction for implementing code modulation schemes. Figure 1 is a block diagram of a two-dimensional T-TCM encoder.
Figure 1 Encoder structure of T-TCM
Figure 2 Turbo encoder circuit block diagram
In the past digital TV solutions, the TCM solution (such as the ATSC standard in the US) has been adopted in the internal code. If the T-TCM solution combining the Turbo code and the TCM is applied to the digital television system, better performance will be obtained. which performed.
3. Hardware implementation of Turbo code coding
3.1 Design ideas
Due to the time limit of the film, the design of this paper only designs the Turbo encoder chip.
In the digital television system, the transport code stream (TC) formed by the MPEG-2 coding is interleaved by the inner code and the inner code after being interleaved by the outer code and the outer code to complete the channel coding. In our design, the Turbo code replaces the original inner code (convolution code/TCM) in the digital TV system.
3.2 design process
Our design is based on the ASIC design flow, and the design flow is shown in Figure 3. From the design system behavior level description, through the system behavior level of functional verification, design synthesis, integrated post-simulation, automated place and route, post-layout simulation, and DRC and LVS verification. The simulation tool we used in the design is NC-Verilog of Candence, the synthesis tool is Synopses' Design Compiler, and the layout and wiring is Cadence's Silicon Ensemble, which adopts 0.6μm technology.
3.3 chip main function module
Figure 2 shows the block diagram of the Turbo code encoder chip.
1) Component encoder: is a recursive system convolutional code (RSC) encoder that encodes the recursive system convolutional code of the system input code stream information, and uses a constraint length of K=3, (7, 5) code.
2) Interleaver: Data interleaving is performed on the system input code stream information before the second-dimensional RSC coding. S parity random interleaving is used in this design, S=14.
3) Encoding output module: The encoded data stream is sent out of the encoder in a certain order.
3.4 chip parameters
Chip Name: Turbo Coder
Area: 1.70 × 1.70 = 2.89 mm 2
Logical unit: about 5000 doors
Package: CDIP20
Pins: 19
Technical process: CSMC 0.6μm 2p2m
The maximum operating frequency of the chip is 44.24Mhz, and the actual working frequency can reach 28.6Mhz. Figure 4 shows the Turbo Coder chip layout.
Figure 3 design flow chart
4.Turbo code chip layout
4. Chip simulation results
In order to ensure the correct function of the entire Turbo code encoder chip, we have carried out rigorous simulation and verification at each stage. Here we use Cadence's simulation tool NC_Verilog for pre- and post-synthesis simulation and post-layout simulation.
Figure 5 Simulation waveform of Turbo Coder after synthesis
Fig. 5 is a waveform of the integrated simulation. The simulation results after the layout are consistent with the synthesized simulation results. Indicates that the function of the chip meets the design requirements.
5. Conclusion
We use the above Turbo code chip to replace the original internal code (convolution code / TCM) in the digital TV system, and get the highest working frequency of the system with a relatively low hardware resource of 44.24Mhz, the actual working frequency is 28.6Mhz, which is far Far superior to existing digital TV systems.
However, since only the original code in the digital television system is replaced by the Turbo code, the bandwidth utilization in the practical application is low. To improve bandwidth utilization and achieve better error correction performance, the Turbo-TCM solution can be considered, but its decoding is more complicated, especially when working at high speeds, the hardware requirements are higher. Our next work goal will be based on the hardware implementation of T-TCM for digital TV.
references
[1]. Liu Donghua. Principle and Application Technology of Turbo Code [M] Beijing: Publishing House of Electronics Industry, January 2004.
[2]. Terrestrial digital TV transmission technology (white paper) Beijing: Digital TV Transmission Technology R&D Center of Tsinghua University, August 2004.
[3] . Zheng Zhihang, Digital High Definition Television and DVB [M], Beijing: China Radio and Television Press, 1997.
[4]. C. Berrou, A. Glavieux, and P. Thitimajshima Near Shannon Limit Error-Correcting Coding and Decoding : Turbo-Codes[Z]. Proc ICC'93, Geneva, Switzerland , May 1993, PP. 1064-1070 .
[5]. æ™ Yuchun, JIANG Yuzhong. Simulation of Turbo Code Application in Low SNR Communication System[J]. Microcomputer Information, 2006, 1-1: P238-239, P102
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