Optimization of Quadrature-Carrier Modulation for Low Crosstalk and Close Packing of Users

For FDM users with arbitrary quadrature-carrier modulation, signal smoothness at bit transitions is shown to completely determine mean-square crosstalk between users at large frequency spacings. To find modulation forms which reduce interference between users at closer spacings, where crosstalk is not dependent solely on bit transition smoothness, optimum windows are generated by numerical minimization of mean-square crosstalk. First a one-parameter class of modulation which includes MSK and SFSK is optimized. Then optimization is extended to the general class of constant-envelope quadrature modulation. The resulting modulation forms experience lower crosstalk than MSK or SFSK and permit closer spacing of users.