US 4,785,447 AGranted

FDM demultiplexer using oversampled digital filters

AssigneeNEC Corporation

InventorOsamu Ichiyoshi

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Abstract

An N-channel FDM signal is converted into complex signals of baseband frequencies spaced at intervals equal to frequency .DELTA.f. The complex baseband signals are converted first into digital samples having a frequency N.DELTA.f and then into N parallel digital signals. A plurality of first FIR subfilters respectively perform filtering on each of the parallel digital signals at frequency .DELTA.f to produce a first series of filtered digital signals from each of the first FIR subfilters, and (m-1) groups of second FIR subfilters respectively perform filtering on each of the parallel digital signals at frequency .DELTA.f to produce a second series of filtered digital samples from each of the second FIR subfilters at timing displaced with respect to the first series by a/m.DELTA.f, where is an integer ranging from unity to (m-1) and m is an integer equal to or greater than 2. Outputs of the first FIR subfilters are combined with outputs of the second FIR subfilters to produce N summation outputs at frequency m.DELTA.f. An N-point Fast Fourier Transform processor performs fast Fourier transform on the N summation outputs at frequency m.DELTA.f to derive digital channels. Because of the oversampling at frequency m.DELTA.f, each of the digital channels has a frequency response which can be made flat over the bandwidth .DELTA.f.

Claims6

Dependent claims

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 .DELTA.f;
analog-to-digital converting means for converting said complex signals into digital samples having a frequency N.DELTA.f;
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 .DELTA.f 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 .DELTA.f 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/m.DELTA.f, 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 m.DELTA.f;
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 m.DELTA.f.

5 more elements

Dependencies are inferred from the claim text.