Differentially matched filter for a spread spectrum system

Abstract

A differentially matched filter for spread spectrum or code division multiple access communication systems comprises a delay line, a multiplication stage, two adders, and an additional delay element. In one embodiment, the delay line comprises elements of delay T with taps therebetween connected to respective elements of the multiplication stage. In another embodiment, the delay line comprises elements of delay T/M with taps after each Mth element connected to respective elements of the multiplication stage. The multiplication stage is connected to a first adder connected in turn to a first input of a second adder. The output of the second adder is the output of the filter and is also connected through the additional delay element to a second input of the second adder, to produce an output sequence. Also disclosed is a method of filtering spread signals at a receiving side of a transmission system comprising the steps of fixing a received sequence of spread signals in a delay line; multiplying successive or predetermined values of the received sequence by respective weighting coefficients constituting a second code sequence that is a replica to a first code sequence used for coding a base band signal at a transmitting side of the system; adding together products of the multiplication, to produce an output sequence; delaying the output sequence for a time of delay equal to delay of one element of the delay line to produce a delayed output sequence; and adding the output sequence to the delayed output sequence.