The Study Of Influencing Factor Of ADSL's Transmission Quality

Asymmetric Digital Subscriber Line is an asymmetric broadband access technology. It has very much advantage. ADSL is needless to change existing twisted-pairs, but it can reach high communication rate. Therefore ,not only can it protect the investment of the telecom company and the subscribers, but also can it solve the problem of the "the last one mile".ADSL reaches high communication rate because the advanced modulation technology is adopted. This article introduces the three kinds of modulation technology in ADSL:QAM,CAP and DMT. The DMT has the biggest advantage Comparing with the other two kinds of modulation technology. Therefore, it can become the standard modulation technology, and it is the modulation technology that has the brightest foreground.In order to get good transmission performance in DMT modulation technology, we need to adjust quantity of bits in each sub-channel according to the different SNR .This paper discusses the algorithm that is used to allocate bit in DMT. "The water pouring theorem" is the classical method to solve the problem. But it can't be used in practical system. On the basis of "the water pouring theorem", this paper puts an improved practical bit allocation on the DMT channel. Its computation and running time reduce greatly while maintaining nearly the same comparing with other algorithms.When DMT modulation is used in ADSL system, the distortion will appear at the channel output in the form of interference between the carriers if impulse response of the channel is longer than designed cyclic prefix length. In order to solve this problem, a method to remove this distortion by introducing a precoder at transmitter is presented. Although there will be a power increase after the use of the precoder, the distortion is significantly reduced and an improvement of performance is achieved.