Method of, and transmitter for, transmitting a digital signal

A method of transmitting a digital signal, comprising:

• encoding successive bits as symbols,
• modulating said symbols,
• digitally filtering the modulated symbols according to a characteristic which produces a filter response sequence suitable for transmission,
• converting the filter response sequence to analog signals,
• frequency up-converting the analog signals, and
• transmitting the frequency up-converted analog signals,
• wherein the modulating step comprises:
• assigning the symbols to respective points of a modulation constellation using a truth table, each column of the truth table relating to a respective point of the constellation and each row of the truth table representing one of a plurality of possible in-phase and quadrature component values, the component values of each constellation point being indicated by a binary value at the respective intersections of a particular column with the rows, and
• storing the points of the modulation constellation as the in-phase and quadrature components of a vector defining each point, and
• wherein the digitally filtering step comprises:
• inputting the binary value of each row of the truth table into a respective shift register which has a predetermined plurality of stages corresponding to the number N of symbols,
• storing the filter response sequences for all combinations of N bits in a look-up table,
• using the outputs of the stages of the registers to form the input address to the look-up table,
• weighting the filter response sequences for all the in-phase components and for all the quadrature phase components, and obtaining the respective sums of the weighted filter response sequences, and
• applying analog signals corresponding to the respective sums of the weighted filter response sequences to a quadrature related frequency up-converter.

A transmitter for data signals, comprising:

• means for differentially encoding the data signals to produce symbols corresponding to respective points of a differential encoding constellation,
• a modulator comprising:
• a filter,
• means for mapping said symbols as successive constellation points on the filter according to a truth table having columns representing each point on the differential encoding constellation, and rows representing respective component values of constellation vectors, each constellation point being represented by a binary value at the respective intersection of a column and row,
• at least one shift register for storing the binary values corresponding to successive constellation points, and
• weighting means for determining quadrature related weighted filter response sequences from the binary values corresponding to the successive constellation points,
• means for producing analog signals corresponding to the succession of weighted filter response sequences, and
• a frequency up-converter receiving the analog signals.

A transmitter for a digital signal, comprising:

• an encoder for encoding successive bits of the digital signal as symbols,
• a modulator for modulating said symbols, comprising means for forming a truth table representing the components of vectors of a modulation constellation, the truth table having a respective column for each point of the constellation, and having a plurality of rows, each row being assigned a respective one of a plurality of possible in-phase and quadrature component values, the component values of each constellation point being indicated by a binary value at the respective intersections of a particular column with the rows,
• a filter comprising:
• a plurality of shift registers, equal in number to the number of rows in the truth table, for storing the points of the modulation constellation as the in-phase and quadrature components of a vector defining each point, each shift register having a predetermined plurality N of stages,
• means, responsive to receipt of a symbol, for storing an output from a corresponding row of the truth table into a respective shift register,
• at least one look-up table storing the filter response sequences for all combinations of N bits,
• means for using the outputs of each of the stages of the registers to form a respective input address to the look-up table,
• means for weighting the filter response sequences for all the in-phase components and for all the quadrature phase components, to form weighted filter response sequences, and
• means for forming the sum of the weighted filter response sequences for all the in-phase components, and forming the sum for all the quadrature components,
• a digital to analog converter for converting the respective sums of the weighted filter response sequences to respective analog signals, and
• a quadrature related frequency up-converter receiving the respective analog signals.