  The paper is devoted to the analysis of problems appearing in optimisation
and improvement of the power-bandwidth efficiency of digital communication
feedback systems (FCS). There is shown that unlike digital systems, adaptive
FCS with the analogue forward transmission allow full optimisation and
derivation of optimal transmission-reception algorithm approaching their
efficiency to the Shannon boundary. Differences between the forward channel
capacity and capacity of adaptive FCS as communication unit, as well as their
influence of the power-bandwidth efficiency of transmission are considered.
