摘要
中国数字电视国家标准(Digital Television/Terrestrial Multimedia Broadcasting, DTMB)及其核心技术时域同步的正交频分复用(Time-Domain Synchronous Orthogonal Frequency Division Multiplexing, TDS-OFDM)已经在全国施行,日趋成熟的国标数字电视系统也在向国际市场迈进。而欧美等发达技术国家和地区在不断完善一代数字电视系统并出口其他地区的同时,已经纷纷推出涵盖大量先进技术的第二代数字电视标准。为了进一步改良现有DTMB标准中的关键技术以及研究二代标准中分集技术,本文在TDS-OFDM系统中快速傅立叶变换(Fast Fourier Transform, FFT)算法研究、3780点处理器资源优化和空时编码技术等方面展开了研究。
首先,TDS-OFDM系统是DTMB标准的核心技术,与其他OFDM技术相比具有独到的优势,采用3780点FFT构造的数据帧结构也是契合我国数字电视频带分配的规定。然而3780并不是一个整数次幂,所以无法直接调用常用的基-2或者基-4的FFT处理模块,必须单独设计专用的快速算法。本文提出的一种基于迭代Winograd快速算法(Winograd Fourier Transform Algorithm, WFTA)计算的3780点FFT算法,优化了计算复杂度。与典型的3780点算法比较,本文提出的方案将乘法计算量降低了45%,其代价仅仅是增加了1%的加法计算量。相比同类的其他优化算法,本文的方案也有明显的优势。新的算法更有助于3780点FFT处理器实现的资源优化。
其次,由于3780点FFT计算需要拆分到小点数DFT运算后再进行合并,多级运算不可避免。用于级联运算模块间的整序缓存和旋转因子乘法都需要开销大量的硬件资源。本文设计了匹配迭代WFTA运算模块的整序算法,小点数离散傅立叶变换(Discrete Fourier Transform, DFT)运算模块的输入输出都优化为自然数顺序,同时还采用改进的正交旋转数字计算(Coordinate Rotation Digital Computer, CORDIC)模块来替代旋转因子复数乘法器,进一步优化了处理器的整体资源开销。分析结果表明,本文设计的3780点FFT处理器所开销的逻辑资源为两种同类方案的53%和81%,并且在系统延迟和DSP资源节省方面也更加出色。本文提出的方案同时也满足TDS-OFDM系统的技术指标,是一种有效的国标DTMB核心处理器装置。
再次,分集发射技术是全球二代数字电视标准中先进技术的重要组成部分。应用于复杂信道条件的差分正交空时编码技术也将会在我国数字电视演进标准中有巨大的应用前景。差分算法消除了正交空时分组码(Space-Time Block Code, STBC)编解码对信道状态信息(Channel State Information, CSI)的依赖,但是却引入了星座点扩散的问题。本文在保证码率满速率的前提下,提出了一种基于双映射的差分正交空时分组码(Differential Space-Time Block Codes, DSTBC)编码算法,成功的抑制了星座点扩散。仿真结果显示,本文提出的抑制星座点扩散的DSTBC方案,是同类方案中映射流程最为简洁的,而且是所有满速率算法中性能最好的。相比未抑制星座点扩散的典型算法,仅在低信噪比条件下有1dB的损失,但是在高信噪比条件下性能相当。
Chinese terrestrial digital television (DTV) standard:Digital Television/Terrestrial Multimedia Broadcasting (DTMB), with its key technology Time-Domain Synchronous Orthogonal Frequency Division Multiplexing (TDS-OFDM) have been carried into effect in China. The DTMB system begins to enter the global market with optimized equipments. However, the Europe and American first generation DTV standards has been fully developed and exported to many countries all over the world. What is more, their second generation DTV standards, with advanced communication technology, are carried out. To face this challenge, we prepare to further optimize the DTMB key equipment and research transmitter diversity technology in the second generation standards. This paper launch the key technology research in the Fast Fourier Transform (FFT) of the TDS-OFDM,3780-point FFT processor optimization, space time block codes (STBC).
Firstly, TDS-OFDM is the key technology in DTMB standard. TDS-OFDM has unique advantage comparing with other OFDM system. The3780-point FFT architecture in TDS-OFDM is designed for DTMB standard and Chinese DTV frequency band.3780is not an integer's power, so the common radix-2and radix-4FFT processor module can not be used in the3780-point FFT. A special algorithm should be designed for the3780-point FFT. This paper presents a novel3780-point FFT processor scheme to reduce the computational complexity. Comparing with the typical3780-point FFT scheme, our proposed3780-point FFT processor scheme reduces45%of the multiplications in the price of1%increase of the additions. The analysis results show that our proposed scheme has the best performance among all the existing optimize algorithms.
Secondly, the3780-point FFT should be decomposed into small points DFT modules, so the3780-point FFT structure is always cascade connected. The reordering cache and twiddle factor multipliers consume a great amount of hardware resources. This paper designs a mapping method suitable for the iterative Winograd Fourier Transform Algorithm (WFTA) architecture. The small point FDT module's input and output data are designed to be natural number order. An optimized Coordinate Rotation Digital Computer (CORDIC) module is designed to do the twiddle factor multiplication to further reduce the resource consumption. The simulation results show that, the resource consumption of our proposed3780-point FFT processor is53%and81%of two similar existing designs. The proposed scheme also performs the best in system delay and DSPs consumption. It is an efficient design for the Chinese DTMB standard.
Thirdly, the transmitter diversity technology is essential part of the advanced communication technology in the second generation DVB standard. The differential space-time block codes (DSTBC) is designed for complicated channel situation. DSTBC has great prospect in the next generation Chinese DVB standard. However, the DSTBC dose not rely on CSI but the differential method causes constellation expansion. This paper presents a double mapping method DSTBC, which can avoid constellation expansion and achieve full code rate. The proposed double mapping method is the simplest structure among all similar codes. The simulation results show that the proposed DSTBC performs best among all the existing codes without constellation expansion. Comparing with the typical DSTBC with constellation expansion, our proposed DSTBC has1dB loss in low SNR situation but can achieve the same performance in high SNR situation.
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