BACKGROUND

A full duplex network allows downstream (DS) and upstream (US) signals to be sent within a same frequency spectrum on a network. For example, full duplex Data Over Cable Service Interface Specification (DOCSIS) allows a multiple service operator (MSO) to offer symmetrical services within a frequency spectrum on the network, such as a hybrid fiber coaxial network, to modems. One characteristic of the network is that it is a single-point to multi-point network where multiple modems are connected to a single node through the same network portion, also referred to as the plant. The node transmits the downstream signals to the multiple modems and also receives the upstream signals from the modems connected to the plant.

One of the requirements of a full duplex DOCSIS network was that the network should be an N+0 architecture, which means the fiber node that contains the FDX-capable remote physical device (RPD) is followed only by passive equipment. That is, no amplifiers are placed between the node and the modems. However, when an N+0 architecture is used, it is very costly because the number of subscribers that are serviced by a node in the N+0 architecture is small. In contrast, using an amplifier in between the node and the modems in an N+1/N+2/N+x network (where x is the maximum number of amplifiers in series subtending from the node) allows a node to service more subscribers and becomes more cost efficient. When inserting an amplifier in between the node and the subscribers, echo may result in both directions of the upstream direction and the downstream direction at the amplifier. That is, downstream signals may be coupled or reflected into the upstream path and upstream signals may be coupled or reflected into the downstream path. It may be hard to train the amplifier to cancel the upstream and downstream interference simultaneously while simultaneously amplifying the downstream and the upstream signals.

