4-PAM/FM modulator with DSP: a solution for ERMES

A new standard for paging systems is being released in Europe. This new ERMES system uses 4-PAM (pulse amplitude modulation)/FM modulation at 6250 b/s. This high bit rate requires a very careful design of the modulation scheme and the modulator itself to allow the usual methods of quasi-synchronous operation to cover large zones of terrain. For these reasons, the authors have designed a modulator for this system based on DSP (digital signal processing) that guarantees accuracy and stability. It is shown that one of the best choices for implementing the modulator of the ERMES system transmitters will be to use a quadrature modulator, with I-Q signals generated digitally. The structure of the modulator is presented together with simulations of its performance and some experimental results