Efficient and flexible oversampled filterbank with near perfect reconstruction constraint

A filterbank system is provided for reordering subbands of a wideband digital signal. The filterbank system extracts the subbands from a wideband digital input signal into an even channel number group of even channel number subbands and an odd channel number group of odd channel number subbands. The separation of even and odd channel number subbands provides an even channel grouping with guard bands between the even channel number subbands and an odd channel number grouping with guard bands between the odd channel number subbands. The filterbank system then reorders the even channel number subbands and the odd channel number subbands. The reordered subbands are then combined by combining the reordered even channel number subbands and odd channel number subbands into a wideband digital output signal.

TECHNICAL FIELD

The present invention relates generally to communication systems, and more particularly to a near perfect reconstruction filterbank architecture for recombining a plurality of subbands into a wideband signal.

BACKGROUND OF THE INVENTION

Filterbanks have been employed in many applications such as transmultiplexers, audio/image compression, and adaptive filtering. A typical perfect reconstruction filterbank is designed to filter a wide band signal comprised of a plurality of subbands or subchannels into the subbands or subchannels, process the subbands or subchannels (e.g., compress/decompress), and then recombine the subbands or subchannels into a wide band signal with an attempt to minimize distortion. Oversampled filterbanks are widely employed to reduce the computational complexity of signal processing algorithms, such as subband adaptive filtering techniques utilized in many audio/image compression techniques. After the signal channeling and signal combining algorithms are performed, the wideband signal is typically transmitted over a radio frequency (RF) wireless link to one or more other radio devices (e.g., user terminals). The transmission frequency of the RF wireless link is at a substantially higher frequency than the processing frequency of the wideband signal.

In certain situations it is desirable to reorder the subchannels or introduce subchannels from one or more independent filterbanks. For example, in satellite transmissions the transmission frequencies (e.g., downlink frequencies) are different than the receiving frequencies (e.g., uplink frequencies). Therefore, signals that are received from ground stations with subchannels over certain frequency bands are retransmitted to user terminals over different frequency bands. Additionally, in certain applications is desirable to reorder the subchannels based on transmission order which requires the channels to be reordered not only into different frequency bands but into different frequency locations. These requirements cannot be met with conventional modulated filterbanks due to aliasing distortion of the signal caused by recombining the signal into a wideband signal.

Conventional oversampled modulated filterbanks employ “aliasing cancellation” methods to remove the aliasing distortion. This works quite well as long as the subchannels are not reordered. When the subchannels are reordered, “aliasing cancellation” cannot be utilized. Furthermore, algorithms that employ alias cancellation, when combined with subchannel switching, exhibit amplitude and group delay distortion. The distortion results because, during signal re-synthesis, the adjacent subchannels can be from different sources and do not necessarily contain the information needed for aliasing cancellation.

SUMMARY OF THE INVENTION

The present invention relates to a filterbank system for reordering subbands of a wideband digital signal. The filterbank system oversamples (e.g., by downsampling by less than the number of channels) and extracts the subbands from a wideband digital input signal into an even channel number group of even channel number subbands and an odd channel number group of odd channel number subbands. The separation of even and odd channel number subbands provides an even channel grouping with guard bands between the even channel number subbands and an odd channel number grouping with guard bands between the odd channel number subbands. The filterbank system then reorders the even channel number subbands and the odd channel number subbands. The reordered subbands are then combined by combining the reordered even channel number subbands and odd channel number subbands into a wideband digital output signal.

In one aspect of the present invention, the filterbank system is a near perfect reconstruction filterbank (NPRFB). The NPRFB includes an analysis filterbank portion with an even channelizer and an odd channelizer. The even channelizer and the odd channelizer downsample the wideband digital input signal to respective channel filter functions associated with respective even and odd channel number subbands. The even channelizer includes an Inverse Discrete Fourier Transform (IDFT) component that cooperates with the even channel filter functions to produce a group of even channel number oversampled subbands separated by guard bands. Alternatively, the IDFT can be replaced with a conjugate, DFT component conjugate combination. The odd channelizer includes an IDFT component that cooperates with the odd channel filter functions to produce a group of even channel number oversample subbands separated by a guard band. The NPRFB includes a switch bank operative to reorder the subbands and synthesis portion operative to recombine the reordered subbands into a wideband digital output signal.

The synthesis filterbank portion includes an even combiner and an odd combiner. The even combiner recombines the new or reordered even channel number subbands by moving the even channel number subbands about their respective frequencies. The odd combiner recombines the new or reordered odd channel number subbands by moving the odd channel number subbands about their respective frequencies. The even combiner includes a Discrete Fourier Transform (DFT) component that cooperates with even channel filter functions to produce a group of reordered even channel number subbands, and the odd combiner includes a DFT component that cooperates with odd channel filter function to produce a group of reordered odd channel number subbands. The even channel number subbands and the odd channel number subbands are combined to provide a wideband digital output signal.

DETAILED DESCRIPTION OF INVENTION

The present invention relates to a filterbank architecture that minimizes distortion yet allows subchannels to be switched efficiently. The filterbank architecture employs a near perfect reconstruction filterbank (NPRFB) where the subchannels can be reordered or combined with subchannels from other independent sources without alias distortion. The present invention does not suffer from the amplitude and group delay distortion of conventional information switching or routing devices.

FIG. 1illustrates a block diagram of a filterbank system10in accordance with an aspect of the present invention. The filterbank system10includes an analysis filterbank portion12, a switch bank18and a synthesis filterbank portion20. The analysis filterbank portion12receives a wideband digital input signal (e.g., complex signal) comprised of a plurality of subbands or subchannels over a particular frequency band having a particular channel order. The analysis filterbank portion12includes an even channelizer14and an odd channelizer16.

The even channelizer14extracts the even channel number subbands and moves each even channel number subband to be centered about complex baseband (e.g., zero frequency). The even channelizer14provides an output of the even channel number subbands that are oversampled and separated by a guard band. Additionally, the even channel channelizer14downsamples the wideband digital signal by a downsampling factor of N/2, where N is the number of subbands in the wideband digital signal. The downsampling of the wideband digital signal by less than N effectively provides an oversampled subband signal. The downsampling factor optimizes the oversampling of the subband signals.

The odd channelizer16extracts the odd channel number subbands from the wideband digital input signal and moves each odd channel number subband to be centered about complex baseband. The odd channelizer16provides an output of the odd channel number subbands that are oversampled and separated by a guard band. The odd channel number subbands are also frequency shifted, so that the odd channel number subbands align with the even channel number subbands. Additionally, the odd channel channelizer16can downsample the wideband digital signal by a downsampling factor of N/2, where N is the number of subbands in the wideband digital signal. Again, downsampling by a factor less than N effectively provides oversampled subband signals. The downsampling factor optimizes the oversampling of the subband signals. The centering of the subbands about complex baseband allows the switching and passing through of subbands to the synthesis filterbank portion20for reconstruction without alias distortion. Furthermore, the separating of the odd and even channels, and oversampling provides guard bands between the subbands, so that the odd and even channels can be readily switched and recombined without alias distortion.

The even channel subbands from the even channelizer14and the odd channel subbands from the even channelizer16are provided to the switch bank18. Optionally, the switch bank18can be coupled to a controller28for performing one or more algorithms that can reorder the odd and even subbands, provide additional subbands and/or replace certain subbands. It is to be appreciated that the controller28can be a digital signal processor (DSP) or a plurality of DSPs. Alternatively, the switch bank18can include the necessary functionality for storing, reordering and switching of subbands for reconstruction into a wideband digital output signal (e.g., complex signal).

The new or reordered even channel number subbands are then provided to an even combiner22of the synthesis filterbank20, while the new or reordered odd channel number subbands are provided to an odd combiner24. The even combiner22and the odd combiner24recombine the odd and even subbands into a wideband digital signal via a summer26. It is to be appreciated that the even channelizer14, the odd channelizer16, the even combiner22, and the odd combiner24can employ hardware and/or software to separate and combine the plurality of subbands. Alternatively, the even channelizer14, the odd channelizer16, the even combiner22, and the odd combiner24can employ one or more DSPs executing one or more algorithms to separate and combine the plurality of subbands or subchannels.

Some applications that are enabled by the present invention include switching subchannels between filterbanks with different sources, frequency translation of subchannels or groups of subchannels within a single filterbank, and efficient fractional oversampled filterbanks (e.g., oversampled by 3/2). Another application enabled by the present invention is to provide efficient Non-uniform filterbanks (i.e., subchannels are oversampled at the rate required by the largest filter bandwidth, and smaller sub-channels are combined to achieve the various bandwidths).

FIG. 2illustrates a block diagram of an eight subchannel implementation of a near perfect reconstruction filterbank (NPRFB)40in accordance of the present invention. It is to be appreciated that the present invention can be extended to any number of subchannels. The NPRFB40is comprised of an analysis filterbank42and a synthesis filterbank54. The analysis filterbank42subdivides the incoming digital input signal or spectrum into subchannels (i.e., frequency bands) and the synthesis filterbank54combines the subchannels into a single wideband digital output.

In the analysis filterbank42ofFIG. 2, the digital input signal is split into odd and even subchannel groupings employing an even channelizer filterbank44and an odd channelizer filterbank48. The digital input signal is filtered into eight subchannels, four from the even channelizer filterbank42and four from the odd channelizer filterbank48. The odd/even processing and the oversampling of the output of each filterbank produces a guard band between the adjacent subchannels, which is not present in conventional reconstruction filterbanks. The guard band mitigates the alias distortion during recombination. A switchbank52reorders the subchannels for frequency translation or to switch in or out and from or to other physically independent filterbanks. The synthesis filterbank54is the mirror image of the analysis filterbank42and performs an inverse function. For example, the analysis filterbank42performs a filter function G(Z) and an IFFT on the subbands, while the synthesis filterbank54performs a FFT and a filter function E(Z) on the reordered subbands.

Each channel of the even channelizer filterbank44and the odd channelizer filterbank48downsamples the incoming signal by N/2, where N is equal to the number of subbands of the wideband input signal. In the present example, the signal is downsampled by four which provides a data rate that is ¼ the data rate and ⅛ of the frequency content of the wideband input signal at each filter function. A conventional device would decimate the input signal by ⅛ the data rate and ⅛ the frequency content for efficient processing. However, the problem with the signal being at ⅛ data rate is that the subband input signals would have transitions bands that would alias back into the wideband output signal. Additionally, the downsampling optimizes the oversampling of the subbands at the output of the even channelizer44.

FIG. 3illustrates a graph70of output amplitude versus frequency of eight subchannels of a wideband digital signal with overlapping transitions bands. The output subbands have a frequency content range from −pi to pi, where pi is the sampled output rate at the Nyquist frequency. The overlapping transition bands72cause alias distortion of the signal during synthesis of the subbands into a single wideband digital output signal. The present invention reduces the downsampling of the signal to mitigate alias distortion of the subband signals upon recombining or synthesis of the subbands into a single wideband digital signal and to optimize the oversampling of the subbands at the output of the analysis filterbank42. Although this is a tradeoff in processing efficiency, the data rate reduction in combination with separate odd/even channel processing assures sufficient guard band to eliminate alias distortion during channel recombination without substantial processing efficiency loss.

Referring again toFIG. 2, the downsampled signals of the even channelizer42are then provided to a corresponding channel polyphase filter unit G0(Z), G1(Z), G2(Z), G3(Z) for polyphase decomposition of the time domain response to that filter of the downsampled wideband digital signal. The decimated downsampled subband signals are then provided to a four point IDFT component46. Alternatively, the IDFT component46can be replaced with a conjugate, DFT component conjugate combination. The four point IDFT component46cooperates with the channel polyphase filter units G0(Z), G1(Z), G2(Z), G3(Z) to center the subbands of the even channels at complex baseband (e.g., zero frequency). The centering of the subbands at complex baseband allows switching and pass through of the switched subbands to the synthesis filterbank54. The outputs of the IDFT component46are then provided to the switch bank52.

FIG. 4illustrates a graph80of output amplitude versus frequency of the even numbered channels centered about complex baseband. The even channel number subbands have a frequency content range from −pi to pi, where pi is the sampled output rate at the Nyquist frequency. As illustrated in the graph80, the zero channel is centered about complex baseband, the second channel and the sixth channel are centered an equidistant from complex baseband with the second channel in a positive frequency region and the sixth channel in a negative frequency region. The fourth channel is split between opposing ends of the baseband in both the positive and negative frequency region. Each of the even channels are separated by a guard band82, which is due to the removal of the odd channels by the even channelizer44during processing in addition to the downsampling of the wideband digital input signal. The guard bands82facilitate reordering since the overlapping transition bands72have been removed.

The odd channelizer48also downsamples the wideband digital signal and provides the downsampled wideband digital signal to a corresponding channel polyphase filter unit G0(Z), G1(Z), G2(Z), G3(Z) for polyphase decomposition of the time domain response to that filter of the downsampled wideband digital signal. The oversampled subband signals are mixed with (−1)Nmultipliers where N is equal to the channel order number and an associated weight factor multiplier W80, W81, W82and W83to shift the odd channels to be centered about baseband.

The decimated downsampled and shifted subband signals are then provided to an IDFT component50. Alternatively, the IDFT component50can be replaced with a conjugate, DFT component conjugate combination. The IDFT component50cooperates with the associated channel filter unit G0(Z), G1(Z), G2(Z), G3(Z) and multipliers to center the subbands of the odd channels at complex baseband (e.g., zero frequency).FIG. 5illustrates a graph90of output amplitude versus frequency of the odd channels centered about complex baseband with and without shifting the frequency. The odd channel number subbands have a frequency content range from −pi to pi, where pi is the sampled output range at the Nyquist frequency. The odd channels without frequency shifting are illustrated with dashed lines, while the frequency shifting causes the odd channels to move along arrows94to be aligned with the even channels. As illustrated in the graph90, the odd channels after shifting include the first channel centered about complex baseband, the third channel and seventh channel centered an equidistant from complex baseband with the third channel in a positive frequency region and the seventh channel in a negative frequency region. The fifth channel is split between opposing ends of the baseband in both the positive and negative frequency region.

Each of the odd channels are separated by a guard band92, which is due to the removal of the odd channels by the odd channelizer48during processing, in addition to the downsampling of the wideband digital signal. The guard band92facilitates reordering since the overlapping transition bands have been removed. The shifting of the frequency allows the odd channels to be centered about baseband, and aligned with the even channels, so that the channels can be readily reordered without alias distortion. The outputs of the IDFT component50are then provided to the switch bank52.

The switch bank52can include capabilities for processing, channel reordering for frequency translation, or for switching in or out from or to other physically independent filterbanks. The switch bank52can be preprogrammed or programmable. Alternatively, the switch bank52can be controlled by one or more other devices. Once the subbands are processed and/or reordered, the switch bank52provides the subband signals to the new channel locations. The subband signals allocated for even channel locations are provided to an even channel combiner56of the synthesis filterbank54and the subband signals allocated for odd channel locations are provided to the odd channel combiner60of the synthesis filterbank54. The even channel combiner56of the synthesis filterbank54includes a four point Discrete Fourier Transform (DFT) component58and associated filter functions E0(Z), E1(Z), E2(Z), E3(Z) for polyphase composition of the frequency domain response to that filter function. The odd channel combiner60of the synthesis filterbank54includes a four point Discrete Fourier Transform (DFT) component62and associated filter functions E0(Z), E1(Z), E2(Z), E3(Z) for polyphase composition of the frequency domain response to that filter function.

The odd channel combiner60also includes multipliers (−1)Nand associated weight factor multipliers W80, W81, W82and W83to provide the desired frequency of the odd channels centered about their associated bands. The odd and even subband signals are then upsampled by four and combined to produce a wideband output signal at the original data rate and frequency content of the wideband input signal.FIG. 6illustrates a graph100of output amplitude versus frequency of a wideband output signal102without alias distortion as illustrated by the flat top region104of the output signal. The wide output signal102has a frequency content that is less than the frequency content range from −pi to pi, where pi is the sampled output range at the Nyquist frequency. The flat top region104extend from −pi/2 to pi/2 and has transition bands106and108that move to zero before reaching the Nyquist frequency (−pi, pi). Therefore, there is no alias distortion of the wideband output signal102.

FIG. 7illustrates a digital transponder140that employs a near perfect reconstruction filterbank (NPRFB)144in accordance with an aspect of the present invention. The digital transponder140can be, for example, part of a satellite or terrestrial base station device. The digital transponder140includes a transmitter/receiver component150having an antenna152for receiving and transmitting RF transmission signals. The transmitter/receiver component150receives wideband RF transmission signals and provides the wideband RF transmission signals to a downmixer148. The downmixer148provides the wideband downmixed signal to an analog-to-digital (ADC) converter146. The ADC146converts the wideband downmixed signal to a wideband digital signal for processing by the NPRFB144.

The NPRFB144subdivides the wideband signal into a plurality of odd and even subbands with associated guard bands, as illustrated inFIGS. 1-2. The odd and even subbands can be resorted or reordered and recombined for transmission by the digital transponder140. Alternatively, the odd and even subbands can be stored for reordering and combining with subbands from other wideband digital signals. The recombined signal is then provided to a digital-to-analog converter (DAC)154, which converts the recombined digital signal to a wideband analog signal. The wideband analog signal is then upmixed with an upmixer156to provide an analog signal at a RF transmission frequency. The transmitter/receiver150then transmits the wideband analog signal through the antenna152.

The NPRFB144includes an analysis portion that filters the digital wideband signal into a number of subbands. The subbands are then provided to a central processor unit142. The central processor unit142can process the subbands, resort the subbands based on a transmission order or protocol and provide the resorted or reordered subbands back to the NPRFB144. The NPRFB144includes a synthesis portion that recombines the subbands to provide a wideband digital signal centered at baseband. Alternatively, the intelligence for controlling the resorting order can be provided as part of the NPRFB144and the central processor unit142can be operative to program the NPRFB144and/or perform other functions associated with the digital transponder140.

In one aspect of the present invention, the digital transponder140is operative to receive wideband signals from a plurality of different locations, break the wideband signals into subbands, resort or reorder the subbands with subbands from the wideband signal and/or other wideband signals, combine the reordered subbands into wideband signals, and transmit the wideband signals to one or more locations. For example, a first wideband signal can be received from a first location and a second wideband signal received from a second location. A first portion of the first wideband signal and the second wideband signal are destined for a third location, while a second portion of the first wideband signal and the second wideband signal are destined for a fourth location. The NPRFB144recombines the first portions of the first wideband signal and the second wideband signal, which is then transmitted to the third location. The NPRFB144also recombines the second portions of the first wideband signal and the second wideband signal, which is then transmitted to the fourth location. The NPRFB144can be preprogrammed, for example, via a preprogrammed read only memory (ROM) device, or programmed via the central processing unit142.

FIG. 8illustrates a methodology for reordering subbands in a wideband digital signal in accordance with an aspect of the present invention. The methodology begins at200where the wideband digital input signal is downsampled and decomposed into a plurality of even and odd channels. The wideband digital input signal can be downsampled by N/2, where N is the number of subbands of the complex wideband signal. At210, the even subbands are centered about a complex baseband frequency (e.g., zero frequency). The even subbands can be centered about complex baseband by separating the subbands utilizing polyphase filter function in cooperation with an IDFT component. The combination of the downsampling by N/2, the polyphase filter function and the IDFT component on the even subbands provides a set of subband signals that are oversampled, and separated by a guard band. At220, the odd subbands are frequency shifted and centered about complex baseband frequency. The odd subbands can be centered about complex baseband by separating the subbands utilizing polyphase filter functions in cooperation with an IDFT component. The combination of the downsampling by N/2, the polyphase filter function and the IDFT component on the odd subbands provides a set of subband signals that are oversampled, centered about baseband and separated by a guard band. The odd subbands are frequency shifted to align the odd subbands with the even subbands. The methodology then proceeds to230.

At230, the subband frequency or subband channel order is switched or modified, for example, based on a predetermined protocol. The subbands switched to the even channels are then moved to their respective center frequency at240. The even channel subbands can be moved to their respective center frequency utilizing a DFT in cooperation with a polyphase filter function to position the even channels, as illustrated inFIG. 4. At250, the subbands switched to the odd channels are frequency shifted and moved to their respective center frequencies. The odd channel subbands can be moved to their respective frequency utilizing a DFT in cooperation with a polyphase filter function. The odd channel subbands can be frequency shifted employing multipliers to shift the odd channel frequency to their respective center frequency, as illustrated inFIG. 5. At260, the even and odd channel subbands are upsampled and recombined to provide a wideband output signal (e.g., complex signal).