Research On DSL-based Adaptive Echo Cancellation

With the rapid development of the telecommunications industry and the Internetindustry, the demand for Internet bandwidth is growing fast. In order to take fulladvantage of the telephone network which has been used by millions of households inChina, and save the substantial costs caused by the demand re-routed network, thedigital subscriber loop (DSL) broadband access has been updated from asymmetricdigital subscriber line (ADSL) access to very-high-speed digital subscriber line (VDSL)access. The theoretical rate of ADSL which is1.5Mbps in downstream and512Kbps inupstream has been accelerated to55Mbps in downstream and19.2Mbps in upstream byupgraded into VDSL. Then VDSL2technology appears and is quickly maturing, whichgreatly enhances the capacity of the copper transmission. The line rate of VDSL2canreach up to100Mbps in both upstream and downstream. With the rate rapidly rising, thenumber of sub-carriers of the DSL system based on OFDM is increasing too, and theband for transmission has expanded from1.104MHz in ADSL to30MHz in VDSL2. Sothe complexity of the algorithms used in VDSL2is increasing too.The Existing digital subscriber loop system is a “two-line-based full-duplex”transmission system, which means the received signal and the transmitted signalthrough the hybrid circuit are both transmitted over a pair of copper twisted pair. In fact,the impedance of the hybrid circuit is usually mismatch, so some transmitted signalwhich is called echo may be reflected to the received end. In order to cancel the echo,the time domain LMS-based adaptive echo cancellation technology is usually used.However, as the rate of transmission and the number of sub-carriers increase, itscomplexity and the calculation amount are significantly increased, so the LMS-basedmethod will consume a large amount of system resources, and cannot meet the hardwareand software design requirements.In order to reduce the complexity of the echo cancellation, this paper presents a multi-stage wiener filter (MSWF) echo cancellation technology for DSL systems. Comparedto LMS-based echo cancellation technology, the MSWF technology is more convenientwith smaller amount of computation cost, lower complexity and faster convergence. Bycomparing with the existing echo cancellation algorithm in DSL system, like theinterpolated-FIR-based algorithm, and combining the characteristics of the transmittedsignal and the desired signal, a reduced-dimensional multi-stage wiener filter isdesigned for high speed DSL system. In the proposed method, the complexity of the echo cancellation algorithm is reduced, and the simulation results also show that itsBER performance is superior to the existing echo cancellation technology.