Thushara K. Gunaratne

Multidimensional (MD) Circuits and Systems for Emerging Applications Including Cognitive Radio, Radio Astronomy, Robot Vision and Imaging

Advances in the performance of VLsi circuits are leading to a number of emerging applications of multidimensional (md) filters. Early progress was focused on the numerical design of two dimensional (2-d) transfer functions and the challenging stability issues associated with low-complexity infinite impulse response (iir) implementations. However, over the last decade or so, important practical advances have occurred in the design of 3-d and 4-d iir filters, leading to some important emerging applications. in this tutorial article, some of these applications are described, with emphasis on 2-d spatio-temporal beamforming and 4-d light field processing. in particular, advances in spatio-temporal beamforming for cognitive radio systems and for synthetic aperture radio telescopes are considered. in the 4-d case, we describe a class of 4-d light field filters for image processing, 4-d hyper-fan filters for low-light imaging, depth filtering, denoising and the attenuation of distracting objects, with applications in computational photography and habitat monitoring. Both analog and digital systolic VLsi circuit implementations are described with emphasis on recent progress using field programmable gate array (fPgA)-based and digital VLsi circuits that can potentially operate at radio frequencies in the multi-gHz range. these new innovations open up exciting possibilities for real-time md filters having frames rates in the multi-gHz for emerging radio frequency (rf) antenna signal processing and imaging systems.

Beamforming of Broad-Band Bandpass Plane Waves Using Polyphase 2-D FIR Trapezoidal Filters

A new discrete-domain method is proposed for the beamforming of temporally broad-band bandpass plane waves (PWs) using a real-coefficient 2-D spatio-temporal (ST) finite-impulse response (FIR) filter having a novel rectangularly symmetric double-trapezoidal-shaped passband. The arriving temporally broad-band-bandpass ST PWs are received by a 1-D uniformly distributed sensor array. The sensor signals are pre-filtered, down-shifted to the intermediate frequency (IF) band, low-pass filtered and synchronously sampled by the real IF tri-stage temporal sampler array, resulting in a real-valued 2-D sampled sequence. The beamforming operation is then carried out on this 2-D sampled sequence using the real-coefficient 2-D FIR double-trapezoidal filter. Arithmetic complexity in the hardware implementation of the 2-D FIR double-trapezoidal filter is significantly reduced by using an array of real-coefficient polyphase 1-D FIR filters. Experimental results have confirmed that this method is capable of enhancing the desired temporally broad-band-bandpass ST PWs according to their directions of arrival under severe co-channel interference.

Broadband beamforming of dense aperture array (DAA) and focal plane array (FPA) signals using 3D spatio-temporal filters for applications in aperture synthesis radio astronomy

It is shown that 3D spatio-temporal filters have potential applications in aperture synthesis radio astronomy for the broadband-beamforming of the array of signals that is received from dense aperture arrays (DAAs) and also from focal plane arrays (FPAs). In particular, we consider possible applications for the planned Square Kilometer Array (SKA) project where broadband beamforming is required at the front-end of the signal processing system for some experiments such as pulsar timing. In the case of a synthesized aperture that is composed of DAAs, the required 3D magnitude frequency response has a non-separable narrow-cone-shaped (or narrow-frustum-shaped) passband whereas, for FPAs, the required 3D magnitude frequency response has a non-separable wide-cone-shaped (or wide-frustum-shaped) passband. The corresponding 3D passbands are designed to faithfully transmit the celestial signals of interest (SOIs), whereas the 3D stopbands are designed to significantly attenuate such undesired signal components such as natural and artificial sources of radio frequency interference (RFI) and the dominant part of the 3D electronic broadband noise that is contributed by millions of low noise amplifiers (LNAs), each of which amplifies the signal received in each elemental antenna of the DAAs or FPAs. The criteria for designing both narrow- and wide-cone/frustal filters, in order to achieve optimal sensitivity, are presented in terms of the power of the recovered signal and the power of the contaminating noise.

Low-Complexity Maximally-Decimated Multirate 3-D Spatio-Temporal FIR Cone and Frustum Filters

A low-complexity multirate 3-D spatio-temporal FIR cone and frustum filter structure is proposed having potential applications as a spatio-temporal directional filter. The cone filter structure employs a 1-D modified discrete Fourier transform (DFT) filter bank and 2-D spatial filters. The frustum filter having a double-frustum-shaped passband oriented along the temporal frequency axis is approximated by employing an appropriate subset of subbands. Low computational complexity is achieved by maximal decimation in the temporal dimension, and by employing the DFT-polyphase realization to implement the 1-D modified DFT filter bank. The cone and frustum filters are almost alias free, and provide near-perfect reconstruction. The reduction in computational complexity, relative to undecimated and under-decimated realizations, is numerically confirmed by means of a potential application involving the attenuation of strong broadband plane wave interference.

Broadband Beamforming of Focal Plane Array (FPA) Signals Using Real-Time Spatio-Temporal 3D FIR Frustum Digital Filters

A real-time spatio-temporal (ST) 3D FIR Frustum filter is proposed for improving the SNR of broadband focal plane array (FPA) signals. The 3D stop band of the proposed filter attenuates non-reflected off-dish signals, such as radio frequency interference (RFI), and also attenuates noise due to local thermal sources (e.g., ground) as well as receiver noise. The frustum-shaped 3D pass band transmits, with low distortion, desired broadband reflected celestial signals, as received by the focal plane array (FPA) of receiving elements. It is shown that, for pulsar-timing and pulsar-profile studies, the pre-processing of FPA signals using the proposed 3D ST Frustum filter-based beamforming method, prior to coherent de-dispersion, yields lower distortion than the 3D conjugate field matching (CFM) method (when appropriately extended from 2D to 3D) and also lower distortion than the conventional 2D spatial-only beamforming method.

Adaptive Complex-Coefficient 2D FIR Trapezoidal Filters for Broadband Beamforming in Cognitive Radio Systems

A novel design method is proposed for an adaptive discrete-domain beamformer for the beamforming of temporally broadband-bandpass signals in cognitive radio (CR) systems. The method is based on a complex-coefficient 2D finite impulse response (FIR) filter having a trapezoidal-shaped passband. The temporally broadband-bandpass signals are received by a 1D uniformly distributed antenna array (1D UDAA), where the outputs of the antennas are complex-quadrature sampled by the front end of the CR system. This CR system is based on a software defined radio (SDR) architecture and can be instantly reconfigured by the control system to select the appropriate frequency band and the required sampling rate. The subsequent beamforming enhances the spectral components of the desired temporally broadband-bandpass signals by arranging for the asymmetric trapezoidal-shaped passband of the 2D filter transfer function to closely enclose the region of support (ROS) of the spectrum of the desired signal, whereas the ROSs of the spectral components of the interfering signals are enclosed by the stopband. The proposed novel closed-form design method facilitates instant adaptation of the shape and orientation of the passband of the beamforming 2D FIR trapezoidal filter in order to match the time-varying frequency band and the time-varying bandwidth of the signal, as well as to track and enhance received signals with time-varying directions of arrival (DOAs). Simulated results confirm that, compared with previously reported methods, the proposed method achieves the best overall tradeoff with respect to the instantaneous adaptations of the operating frequency band, the bandwidth, and the time-varying DOAs, the distortion of the desired passband signal, and the stopband attenuation of interfering signals.

Massively parallel systolic-array architectures for 2d IIR polyphase space–time plane-wave beam digital filters

A systolic architecture has recently been proposed for implementing two-dimensional infinite impulse response (IIR) space–time beam plane-wave filters at a throughput of one-frame-per-clock–cycle for such applications as real-time broadband smart antennas. A novel polyphase systolic architecture is proposed here that further increases the throughput of these IIR beam filters, by a factor of M, to M-frames-per-clock-cycle, where M is the number of polyphases. The proposed method combines the polyphase approach, along with pipelining and look-ahead optimization methods, to achieve frame sample frequencies that are several times higher than the clock-cycle limit of the very large-scale integration (VLSI) technology, thereby potentially allowing multi-GHz frame sample frequencies using current custom VLSI circuits. The implementation of a field programmable gate array-based real-time prototype is described, tested and verified for the two-phase case (M = 2) at a technology-limited clock frequency of 50 MHz which corresponds to a throughput of 100 million-frames-per-clock–cycle. Copyright

Broadband Beamforming of Bandpass Plane Waves using 2D FIR Trapezoidal Filters at Baseband

A new method is proposed for broadband beamforming of two-dimensional (2D) spatio-temporal (ST) bandpass plane waves (PWs) using a complex 2D FIR filter structure having a trapezoidal-shaped passband at baseband and achieves the minimum possible temporal sampling rate for a given bandwidth. Experimental results show that this method is capable of enhancing 2D broadband bandpass ST PWs according to their directions of arrival (DOAs) in the presence of severe co-channel interference

Reducing the Multiplier-Complexity of Massively Parallel Polyphase 2D IIR Broadband Beam Filters

The superior broadband performance of 2D IIR frequency-planar beam filters, relative to conventional 2D FIR true-time-delay beamforming, has recently been reported using computational electromagnetics and real-time emulations on an antenna test range, resulting in significant improvements of bit-error-rates (BERs) in the presence of broadband interference. Further, massively parallel systolic VLSI circuit polyphase architectures have also been reported (Madanayake et al. in Int. J. Circuit Theory Appl. 2010) for the case of the direct-form signal flow graph (SFG) architecture, operating at a maximum throughput of M-(antenna)-frames-per-clock-cycle (MFPCC). The superior broadband performance of 2D IIR frequency-planar beam filters is extended here from the direct-form signal flow graph (SFG) architecture (Madanayake et al. in Int. J. Circuit Theory Appl. 2010) to the novel differential-form SFG architecture in order to reduce overall complexity. The proposed method employs a differential-form polyphase 2D IIR frequency-planar beam SFG, and a corresponding circuit architecture, to implement the required input-output 2D space-time difference equation. The resultant digital hardware has the significant advantage of much-reduced multiplier complexity, relative to the direct-form structure. For example, when look-ahead pipelining is not employed and for polyphase architectures having two, three, and four phases, the corresponding reductions in multiplier complexity are 20%, 28.6% and 33.3%, respectively. A proof-of-concept prototype circuit is designed and implemented on a Xilinx Sx35 FPGA device for the two-phase case, operating at a frame-rate of 132 million linear frames per second on the uniform linear array (ULA), corresponding to 2-frames-per-clock-cycle at a circuit clock frequency of 66 MHz. The circuit is optimized for low critical path delays (CPDs) using look-ahead pipelining of order three. For ultra-wideband (UWB) radio-frequency (RF) implementations, in such fields as radio astronomy, radar and wireless communications, custom VLSI versions of the proposed circuits are required.

Tracking broadband plane waves using 2D adaptive FIR fan filters

A novel composite method is proposed to track a broadband plane wave (PW) based on its direction of arrival (DOA) using 2D adaptive FIR fan filters. This method exploits a closed form expression derived for the unit impulse response of a 2D fan filter so that only one variable of optimization is required. Further reductions in algorithmic complexity are achieved using a unit impulse thresholding technique