Bit- and power-allocation algorithm for symmetric operation of DMT-based DSL modems

Discrete multi-tone (DMT) based digital subscriber loop (DSL) modems must allocate power and information bits to the discrete tones for quadrature amplitude modulation. The bitallocation (also referred to as bit-loading) algorithms in the literature are designed for modems that operate in the simplex mode in which the upstream and downstream signals use nonoverlapping frequency bands. However, greater data transmission capacity can be achieved by using overlapping frequency bands and echo cancelled duplex operation. The conventional bit-loading algorithms are not well suited for the allocation of data bits for transmission in the duplex mode, and the problem of bit loading for duplex operation has not yet been considered in the literature. This letter introduces an algorithm suitable for echo-canceled duplex DMT modems that use overlapping frequency bands for upstream and downstream transmissions for the purpose of transmitting equal data rates in both directions. Moreover, the algorithm performs the bit loading while jointly minimizing a weighted sum of the power used by the central office and the customer premise transmitters. Simulation results are included to show the higher data rates that are possible with the duplex operation as a function of varying levels of echo return loss enhancement.