Method of, and transmitter for, transmitting a digital signal

摘要

A digital modulator in which symbols representing bits are encoded and assigned to respective points of constellation of points representing the modulation scheme. The quadrature components of a vector representing each of the constellation points are contained in a truth table (14) in which the columns comprise the respective constellation points and the rows comprise the relative in-phase and quadrature component values. In response to the assignment of a symbol to a constellation point the appropriate column in the truth table is read out in parallel as a binary word and the bits of the word are applied to respective first stages (S10) of a plurality of shift registers (SR1 to SR8) which store the histories of the respective component values for the previous (N-1) symbols. Outputs of each of the stages (S1 to S10) of each shift register are applied as an address to a look-up table (16) storing the filter response sequences to all combinations of N bits to obtain weighted filter response sequences representative of the particular combination of bits currently stored in the stages (S1 to S 10) of each shift register (SR1 to SR8). The in-phase and quadrature phase weighted filter response sequences are combined and converted to analogue signals which are applied to a respective frequency up-conversion stage (18,20).