In the newest generation of DSL systems, crosstalk is the dominant source of performance degradation. Many crosstalk cancellation schemes have been proposed. These schemes typically employ some form of co-ordination between modems and lead to large performance gains. The use of crosstalk cancellation means that power allocation should be viewed as a multi-user problem. In this paper we investigate optimal (i.e. capacity maximizing) power allocation in DSL systems which employ co-ordination to facilitate crosstalk cancellation. By exploiting certain properties of the DSL channel, it is shown that power allocation can be simplified considerably. The result has each user waterfilling against the background noise only, explicitly ignoring the interference from other users. We show this to be near-optimal for upstream DSL when Central Office (CO) modems are co-ordinated. Compared with conventional waterfilling which is done against the background noise and interference, the performance gains are significant
