Constellation time division multiplexing for the 6 Mb/s ADSL

A new multiplexing scheme, based on the quadrature amplitude modulation or carrierless AM/PM line codes is presented for the 6 Mb/s asymmetric digital subscriber line (ADSL). The scheme can provide an acceptable level of error performance for both the video and ISDN channels while satisfying the system delay requirements in ADSL. The proposed multiplexing scheme, which is called constellation time division multiplexing, uses two constellations of different sizes for transmission of two types of data, one-way entertainment video and two-way interactive video signals such as video telephony. A constellation with less number of signal points is allocated for the transmission of the latter which is delay sensitive and is assumed to utilize no channel coding. Another constellation with a larger number of points is allocated for the former which is not delay-sensitive and uses channel coding. The greater distance between signal points in the smaller constellation provides more immunity against crosstalk and impulse noise interference as well as white Gaussian noise as compared to when the larger constellation is used. This helps compensate for the fact that the delay sensitive data do not utilize any channel coding or interleaving which introduces delay in the system. A frame formatting algorithm that can format the two types of data with different data rates is also presented