摘要
以太数字用户线(Ethernet Digital Subscriber Line,EDSL)结合了两大通信技术(DSL和Ethernet)的精华,可以在双绞线上实现高速的数据传输,有效的解决了通信网络中的“最后一公里”问题。为了实现高质量的数据传输,EDSL使用了自适应调制技术,它通过研究信道的信噪比(SNR)等参数动态地改变调制方式,从而有效地改善误比特率和提高信息传输速率。
本文的主要研究对象是EDSL系统中的自适应多进制正交振幅调制(Multiple Quadrature Amplitude Modulation, MQAM)模块。首先概述了EDSL系统的基本结构及特点,详细说明了EDSL收发器单元(ETU)的工作过程,论述了变速率MQAM在EDSL系统中的具体应用。然后,研究了MQAM的基本原理及系统的组成结构,并重点分析了自适应调制的原理和调制参数的选择。在此基础上,提出了一种适用于EDSL系统的可变符号率、可变星座点数的全数字MQAM调制器的实现方案。
采用美国Altera公司推出的快速FPGA开发环境DSP Builder作为设计工具,介绍了在该环境下进行数字系统设计的一般方法。对MQAM调制器的主要模块(包括扰码、串并转换、差分编码、星座映射、内插器、脉冲成型滤波、载波发生器)进行了深入研究和算法上的改进之后,利用该方法对MQAM的各个功能模块进行了建模设计。仿真分析证明,该方法极大的简化了硬件实现流程,开辟了一条新的DSP处理器及系统的设计方法,为设计具有自主知识产权的DSP处理芯片做了有意义的尝试。
本文设计的MQAM调制系统的通信指标参数如下:系统的调制方式采用QPSK、16QAM、64QAM ;传输比特率不低于125kbps ;中频载波为62.5kHz～1666.67kHz,共12级;基带成形滤波器采用滚降系数为0.3。接口设置为:单路码流串口输入,单路已调模拟信号输出,调制方式和DDS控制字均可设置。本系统不需要改变硬件和软件结构,只需要改变系统时钟,就可以实现调制方式和符号率的改变。另外,本文介绍的MQAM设计方法也适用于其他系统的调制应用。
最后,本文对研究工作的继续深入进行提出了设想。
EDSL combined the elite of two- communications technology—DSL and Ethernet, and it can implement high-speed data transmission on the twisted-pair, so it solved the“last kilometre”problem in the communications network effectively. For the sake of high quality of data transmission, EDSL adopts adaptive modulation technique. Modulation mode can be adaptively adjusted by estimating such parameters as SNR and etc. By this way, performance of bit error rate can be effectively improved and the information transmission bit rate can be increased.
Studying of adaptive MQAM modulation module in EDSL system has been more concerned in this paper. Firstly, the basic struture and characteristics of EDSL is outlined, and variable rate QAM, which can improve the spectral efficiency distinctly, is introduced, its particular application in EDSL system and the work process of ETU is also discussed in detail. Then, the principle and structure of MQAM is studied. After that, the elements of adptive modulation and the seleceting formula to the modulation parameters is emphasized. Based on the above, an all-digitized implementation algorithm of MQAM for EDSL system is presented. This implemented modulator can bear a variable symbol rate and multiple constellation patterns.
A new software, named DSP Builder, which is detruded by Altera, is chosen as the designing tool. The general method of how to use the altera DSP Builder system to design a digital system is introduced. The main modules of the modulator, including scrambler, serial to parallel, differential encoding, signal constellations and mapping , interpolator, impulse shaping filter, carrier wave generator are studied deeply, and the algorithms are ameliorated. Then, model design of each MQAM function module is made by DSP Builder. At last, the simulation and synthesis of MQAM modulator based on FPGA is given. The whole system is simulated and tested, and the result indicates that the flow of the hardware realization is predigested greatly. Moreover, this method inaugurates a new way of designing DSP processor and its system, and it does a meaningful attempt in designing DSP chip with individual property right.
The parameter of the MQAM modulator system designed in this paper is as follows: three modulator modes—QPSK,16QAM,64QAM; the low limit transmission bit rate is 125kbps; intermediate frequency of the carrier wave is 62.5kHz～1666.67kHz, 12 classes; rolls coefficient of the Base Band shaping filter is 0.3. Interface setting is as follows: single road code stream serial input, single road modulated analog signals output, both modulation mode and control words of DDS are programmable. Holding the hardware and software structure, just by changing the system clock, can we change the modulation mode and simble rate. In addition, the MQAM design method introduced by this paper is the same with other system.
At last, we draw out the conclusion of this paper and point out the further research issues.
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