FDM demultiplexer using oversampled digital filters

Claim 1

An FDM demultiplexer comprising: means for converting an N-channel FDM (frequency division multiplexed) signal into complex signals of baseband frequencies, the channels of said FDM signal being spaced at frequency Δf; analog-to-digital converting means for converting said complex signals into digital samples having a frequency NΔf; serial-to-parallel converter means for converting a series of N digital samples of said complex signals into N parallel digital signals; a plurality of first finite impulse response (FIR) subfilter means for respectively filtering said parallel digital signals at frequency Δf to produce a first series of filtered digital signals from each of said first FIR subfilter means; (m-1) groups of second FIR subfilter means, the second FIR subfilter means of each of said (m-1) groups respectively filtering said parallel digital signals at frequency Δf and delivering a second series of filtered digital samples from each of said second FIR subfilter means at timing displaced with respect to said first series by a/mΔf, where a is an integer ranging from unity to (m-1) and m is an integer equal to or greater than 2; a plurality of adders for respectively summing outputs of said first FIR subfilter means with outputs of said second FIR subfilter means to produce N summation outputs at frequency mΔf; and an N-point Fast Fourier Transform (FFT) processor connected to said adders for performing fast Fourier transform on said N summation outputs at frequency mΔf.