Study Of Adaptive MQAM Modulator In The EDSL System And Its Implementation With FPGA

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.