Allocation of adaptivity of multistage digital filters

Many communications receivers employ a cascade of two or more filters. They are typically designed and implemented separately, each accomplishing its own specific function. Some are fixed in their spectral characteristics, some are selectable, and some, such as data equalizers, are data-adaptive. The incredible growth in the speed and capability of digital signal processing (DSP) devices allows cost-effective digital implementations of many of these filters. This growing digitization would appear to permit the combination of many heretofore separate functions into the same physical filter realization, leading to yet further savings size, weight, power consumption, and cost. The authors examine the case of a digitally implemented tally implemented demodulator which combines the digital filters used for conversion from real sample to complex-valued samples, predecimation filtering, and adaptive equalization. It is shown that it is feasible to combine these functions into a single adaptive filter, but that computational savings are not always attained and that memory requirements almost always grow