Digital matched filter

A hierarchical filter comprising:

• a first filter operating to correlate a received input signal with a first pattern to produce a plurality of intermediate values in a natural order of the received input signal;
• a memory for temporarily storing the plurality of intermediate values;
• and
• a second filter operating to retrieve certain ones of the plurality of intermediate values stored in the memory and correlate the retrieved certain ones of the plurality of intermediate values with a second pattern to produce a plurality of output values, those output values produced in a permuted order with respect to the received input signal so as to allow reuse in the second filter of some of the previously retrieved certain ones of the plurality of intermediate values in successive second filter correlations.

A filtering method comprising the steps of:

• receiving an input signal in a natural order;
• correlating the received input signal against a code;
• and
• producing a plurality of output signals as a result of the correlation step, those output signals being produced in a permuted order with respect to the input signal.