摘要 |
TITLE: CONVERSION FILTERING FOR DIGITAL SIGNALING SYSTEMS Inventor: Michel T. Fattouche One way to increase the bandwidth efficiency of a linear modulation scheme is to increase the number M of possible signaling levels. Such an increase is achieved however at the expense of a smaller minimum distance dmin between the multilevels for a fixed average signal power. The present invention involves the use of a conversion filter as part of a digital signaling system used over a communications channel of limited bandwidth and power. A conventional digital signaling system consists of an encoder, followed by a modulator and a bandpass filter. In the present invention, the digital signaling system consists of an encoder, followed by a modulator, a conversion filter and a bandpass filter. The conversion filter increases the bandwidth efficiency offered by the encoder-modulator combination when the length L of its impulse response extends over several signaling intervals. Such an increase is achieved, however, at the expense of a smaller dmin. By increasing the modulation index h of the conversion filter, it is possible to increase dmin without significantly affecting the bandwidth efficiency. The invention also provides steps for choosing the modulation index h and the cutoff frequency fco of the bandpass filter for a fixed length L of the conversion filing such that dmin is at least locally maximized without significantly affecting the bandwidth efficiency. |