A novel non-linear adaptive structure for time variant equalisation

A considerable effort has been devoted to research into adaptive equalisers. This research has been dominated by the needs of either cable or fixed radio links. Such communications links have two key characteristics of relevance to the topic of this paper: (i) the exact characteristic (transfer function) of the channel is a-priori unknown. (ii) The channel is essentially time invariant, or at least only slowly time-variant. The first of these characteristics results directly in the need for an adaptive solution to the equalisation problem, However, the second characteristic leads to an easing of the problem difficulty by permitting the use of adaptive solutions which invoke the presumption of wide sense stationarity, In the more challenging environment of mobile communications the previously common assumption of slow time variation is no longer tenable. Equaliser design must be considered within the constraints imposed by a fast fading environment. The authors discuss the design of a new equaliser structure which restricts the possible movement of the individual adapted coefficients in the time variant case. This has to be done in the context of maintaining the full equaliser coefficient range. This can only be achieved by increasing the number of coefficients (but not the number of time samples). In this way each sub-coefficient covers a much smaller range of values but one must then devise a technique for switching between sub-coefficients. It is this technique (Cowan, 1994) involving nonlinear adaptive filters which is the subject of this paper.