ROM BASED FINITE IMPULSE RESPONSE FILTER FOR USE IN MOBILE TELEPHONE

摘要

PURPOSE: ROM based lookup table is provided to more efficiently implement a finite impulse response filter in a cellular phone using CDMA. CONSTITUTION: The finite impulse response(FIR) filter implemented as a table(104) using read only memory(ROM) is provided to be filtering input signal streams consisting of sequences of antipodal values. The FIR filter table stores a precalculated output filter value for each permissible combination of input values (102) to be filtered. Streams of input values are successively shifted into the table using a shift register (106) and corresponding output values are successively output. The telephone employs a data burst randomize (18) 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.