Required Transmit Filter Bandwidths in Digital Radio Systems

Widely used digital radio modulations such as quaternary phase shift keying (QPSK), minimum shift keying (MSK), and sinusoidal frequency shift keying (SFSK) usually require some form of transmit filtering to ensure compliance with spectrum emission rules. Typically, radio transmit filters are of a specified type, such as fourthorder Butterworth, where the bandwidth parameter is a design variable. We specify β to be optimal for a particular filter when it has the largest value for which the transmitted signal satisfies the emission rules, and we present numerical solutions for β over a wide range of conditions. These data can be used in design studies that compare contending modulation/transmit filter combinations. The solutions given are for systems subject to the spectral limits of FCC Docket 19311. The types of modulation considered are generalizations of those cited above; the types of transmit filters considered are Butterworth and Chebyshev filters of various orders; and β is tabulated as a function of channel width-to-symbol rate ratio for numerous modulation/filter combinations. While computed specifically, for the 6 GHz common carrier band, the results are also valid, to within a few percent, for the 4 and 11 GHz bands.