Combatting multi-user interference in ADSL systems using time-spreading

Asymmetric digital subscriber line (ADSL) is a high-speed wireline access technology capable of supporting up to 8 Mbps downstream and 1 Mbps upstream in a bandwidth of 1.1 MHz. ADSL uses discrete multitone (DMT) modulation, allowing it to optimally adapt to changing line conditions, including line length, line topology and interference. Various techniques, like Reed-Solomon coding, time equalizers, frequency equalizers and cyclic prefixing are used to combat the effect of interference in the system. These methods are limited in their ability to completely remove interference. In this paper we propose a time-spreading (TS) technique, using complex spreading codes. The technique is capable of rebuilding the signal, under severe multi-user interference and fading channel conditions. The technique only requires the addition of time-spreading/despreading modules between the transmitter and receiver in order to function. It can also be used on other xDSL related systems, such as very-high-bit-rate DSL (VDSL).