Digital channelizer with arbitrary output sampling frequency

Details Digital channelizer with arbitrary output sampling frequency Digital channelizer with arbitrary output sampling frequency

1997-12-31

2002-03-12

Kizou, Hassan (Department: 2662)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S210000, C370S343000, C708S313000, C708S400000, C708S405000

Reexamination Certificate

active

06356569

ABSTRACT:

TECHNICAL FIELD
The invention relates to channelizing a digital spectrum into frequency bands when the input Analog to Digital Converter (ADC) sampling rate is constrained to a given value and, a specific but arbitrary, output sampling rate is desired.
BACKGROUND OF THE INVENTION
Frequency Division Multiple Access (FDMA) provides access for multiple users to a single channel by subdividing the channel into multiple sub-channels. Each user is allocated one or a subset of all the channels and the user's signal is used to modulate a carrier of the allocated sub-channel. A single-channel receiver may be used to demodulate a single assigned channel, or a multichannel receiver may demodulate a selected range of channels.
In an analog system, each receiver has a local oscillator (LO) tuned to the frequency of the channel to be demodulated. The LO signal is multiplied by the received signal to generate an intermediate frequency (IF) signal that is applied to a band-pass filter (BPF). The BPF has a passband selected to filter out all signals except the channel to be received. The output signal from the BPF can be digitized and processed subsequently, if desired.
Referring to
FIG. 1
, a typical receiver
100
is shown with a local oscillator (LO)
105
of frequency &ohgr;. A signal mixer
110
mixes the amplified incoming FDMA signal
106
with LO
105
output to produce a signal with a fixed intermediate frequency (IF)
107
. An analog BPF
111
selects the desired channel and an analog-to-digital converter (ADC)
115
converts the resulting output to digital form by sampling the analog signal at an appropriate frequency. Generally, the sampling frequency is at least twice the channel bandwidth to satisfy the Nyquist requirement.
The sampled digital data, x(n), is bandshifted digitally by digital detector
116
by multiplying with a phasor e
−j2&pgr;(kn/N)
denoted by W
N
kn
, where k denotes the channel selected by the receiver. When there are multiple channels contained in the received signal then one such receiver is needed for each of the channels. The resulting signal is low-pass filtered using a digital low pass filter
117
. All of the processes following digitization of the band-passed IF signal can be performed with a digital computer or specialized circuits well known in the art.
Referring to
FIG. 2
, in a conventional polyphase approach to digital channelization, M channels of bandwidth B are received simultaneously in a FDMA signal
106
. (Background on polyphase DSP can be found in R. E. Crochiere and L. E. Rabiner,
Multirate Digital Signal Processing
, Prentice Hall, Englewood Cliffs, N.J., 1983; and N. J. Fiege,
Multirate Digital Signal Processing
, John Wiley & Sons, 1994; the entire contents of each of which is incorporated herein by reference.) After M channels, each of bandwidth B, are passed through an analog BPF having a passband of at least MB (not shown) they are output to an A/D converter
140
which samples at some rate that is at least equal to the Nyquist rate (2MB) for a signal of bandwidth MB. In
FIG. 2
, the data is sampled at 2NB where N is greater than or equal to M. The digital output x(n) is applied to a 2N pole commutator
141
which distributes the input data x(n) to 2N filters
150
. Each filter
150
is updated once every 2N points. Filters
150
perform the channel extraction function. The time series output of filters
150
is applied to respective inputs of an FFT processor
160
which processes the data once every 2N points to produce 2N complex outputs of which M outputs are chosen, each representing the bandshifted subchannel signal at B Hz, the update rate of FFT processor
160
. Only M outputs of FFT processor
160
are required since the sample rate 2NB, as mentioned, can be higher than the minimum required sample rate 2MB.
The prior art approach cannot effectively accommodate signal processing subsequent to FFT processing that requires a sample rate that is independent of the sample rate of the A/D converter.


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