Modeling and Simulation of an ADSL transmitter

This paper gives a short introduction of Asymmetric Digital Subscriber Line (ADSL) technology and briefly discusses its importance. ADSL uses Discrete Multitone Modulation technique and enables very high rate data throughput over normal telephone lines. This paper discusses at length the modulation scheme and also describes in detail the functioning of an ADSL transmitter Asymmetric Digital Subscribe Line (ADSL) is used to deliver high-rate digital data over existing ordinary telephone lines. ADSL facilitates use of normal telephone services, ISDN and high-speed data transmission simultaneously. ETSI and ANSI have defined the current ADSL standard. OFDM or Discrete Multi-tone technology is the modulation technology that allows the transmission of high-speed data. The high bandwidth provided by ADSL makes technologies like Video-on-demand and video-conferencing possible. ADSL is seen as a smooth transition between present day telephone lines to the fiber optic links of the future. It is assuming lot of importance because of its economic viability. DSL Architecture ADSL allows data rates of 6Mbps downstream and about 640Kbps upstream. The transmission bandwidth is 1.1MHz downstream and 256KHz upstream. The limit of 1.1MHz is due to power constraints imposed by FCC. The frequency domain is divided into 256 sub-channels of 4kHz. The 0-4kHz channel is dedicated for voice, 2-6 channels are not used. There are two versions of ADSL: • Frequency Division Multiplexed: Upstream and downstream channels use different frequency range. Upstream uses 7-32 and downstream uses 32-256 • Echo cancelled: Upstream and downstream channels overlap. Upstream uses channels 7-32 and downstream uses channels 7-256. So echo-cancellation techniques are used to make the transmitted and received signals independent of