Finite-length impulse response filter based on ROM used for mobile phone

摘要

The finite impulse response (FIR) filter is implemented as a table using read-only memory (ROM). The FIR filter table stores a pre-calculated output filter value for each permissible combination of input values to be filtered. Streams of input values are successively shifted into the table and corresponding output values are successively output. In an exemplary implementation, the FIR filter is employed within a digital cellular telephone for use in smoothing a digital signal to be transmitted. The size of the required FIR filter table is minimized by exploiting constraints imposed upon the signal to be filtered and by exploiting linearity and symmetry within the filter itself. More specifically, the telephone employs a data burst randomizer to provide a data signal composed of sequences of null or 0 values and sequences of antipodal values (+1's and -1's). Hence, the permissible input combinations for the FIR filter include only patterns containing either all antipodal signals, all null signals, leading antipodal signals followed by trailing null signals, or leading null signals followed by trailing antipodal signals. The FIR filter look up is configured to exploit these limitations on the input streams yielding a look up table having relatively few entries. Method and apparatus implementations are disclosed.