Digital implementation of baseband pulse-shaping filters and modulators for high-capacity QAM digital radio systems

In conventional digital radio modems currently in production, baseband filtering and modulation are implemented in analogue technology. The feasibility of implementing these functions digitally is considered, as this offers a route to VLSI realisation, and the benefits that are known to accrue from such an approach. The example chosen is a typical high-capacity 140 Mbit/s 16QAM configuration. The system under consideration is shown. It differs from the conventional system in that the functions of pulse-shaping filtering, modulation and inphase-quadrature channel addition are all performed in a fully digital fashion. In this example, the implementation of the modulation subsystems has been simplified by choosing the sampling frequency f_s=280 Msamples/s to be four times the intermediate frequency f₀=70 MHz, whereupon the modulation process becomes one of multiplication by ±1 or 0. Because the resulting inphase and quadrature signals have interleaving zeros, their addition can be reduced to simply multiplexing between the two outputs of the modulators. The filtering arrangement is interpolation in the transmitter and decimation in the receiver