Noncircularity exploitation in signal processing overview and application to radar

International audienceWith new generation of Active Digital Radar Antenna, there is a renewal of waveform generation and processing approaches, and new strategies can be explored to optimize waveform design and waveform analysis and to benefit of all potential waveform diversity. Among these strategies, building and exploitation of the Noncircularity of waveforms is a promising issue. Up to the middle of the nineties, most of the signals encountered in practice are assumed to be second order (SO) circular (or proper), with a zero second correlation function. However, in numerous operational contexts such as in radio communications, the observed signals are either SO noncircular (or improper) or jointly SO noncircular with a particular signal to estimate, to detect or to demodulate, with some information contained in the second correlation function of the signals. Exploitation of this information in the processing of SO noncircular signals may generate dramatic gain in performance with respect to conventional processing and opens new perspective in signal processing. The purpose of this paper is to present a short overview of the interest of taking into account the potential SO noncircularity of the signals in signal processing and to describe the potential interest of SO noncircular waveforms for radar applications

Noncircular Waveforms Exploitation for Radar Signal Processing : Survey and Study for Agile Radar Waveform

International audienceWith new generation of Active Digital Radar Antenna, there is a renewal of waveform generation and processing approaches, and new strategies can be explored to optimize waveform design and waveform analysis and to benefit of all potential waveform diversity. Among these strategies, building and exploitation of the Noncircularity of waveforms is a promising issue. Up to the middle of the nineties, most of the signals encountered in practice are assumed to be second order (SO) circular (or proper), with a zero second correlation function. However, in numerous operational contexts such as in radio communications, the observed signals are either SO noncircular (or improper) or jointly SO noncircular with a particular signal to estimate, to detect or to demodulate, with some information contained in the second correlation function of the signals. Exploitation of this information in the processing of SO noncircular signals may generate dramatic gain in performance with respect to conventional processing and opens new perspective in signal processing. The purpose of this paper is to present a short overview of the interest of taking into account the potential SO noncircularity of the signals in signal processing and to describe the potential interest of SO noncircular waveforms for radar applications

Constellation Design for Widely Linear Transceivers

Constellation design has been previously addressed by assuming that there is a linear equalizer at the receiver side. However, the widely linear equalizer is well known to outperform the linear one with no significant complexity increase; we derive optimum and suboptimum techniques for constellation design in presence of such an equalizer. The proposed techniques adapt the circularity properties of the transmitted signals to the specific channel to be equalized; their performance analysis shows that also the simplest suboptimum procedure provides significant improvements over a fixed-constellation scheme

Equalization for DS-UWB systems

Ultra-wideband wireless transmission has attracted considerable attention both in academia and industry. For high-rate transmission, direct sequence based ultra-wideband (DS-UWB) systems are a strong contender for standardization by the IEEE 802.15.3a Wireless Personal Area Networks (WPAN) committee. The DS-UWB proposal envisages two modulation formats: binary phase shift keying (BPSK) and 4-ary bi-orthogonal shift keying (4BOK). Due to the large transmission bandwidth, the UWB channel is characterized by a long root-mean- square delay spread and the RAKE receiver cannot always overcome the resulting intersymbol interference. We therefore study equalization for DS-UWB systems employing BPSK and 4BOK modulation. In the first part of this work, we consider equalization for DS-UWB with BPSK modulation, which is mandatory for standard-proposal compliant DS-UWB devices. Assuming RAKE preprocessing at the receiver, we analyze the performance limits applicable to any equalizer, taking into account practical constraints such as receiver filtering, sampling, and the number of RAKE fingers. Our results show that chip-rate sampling is sufficient for close-to-optimum performance. For analysis of suboptimum equalization strategies, we further study the distribution of the zeros of the channel transfer function including RAKE combining. Our findings suggest that linear equalization is well suited for the lower data rate modes of DS-UWB systems, whereas nonlinear equalization is required for high-data rate modes. Moreover, we devise equalization schemes with widely linear processing, which improves performance without increasing equalizer complexity. Simulation and numerical results confirm the significance of our analysis and equalizer designs and show that low-complexity (widely) linear and nonlinear equalizers perform close to the pertinent theoretical limit. In the second part, we investigate equalization for DS-UWB with 4BOK. To this end, we first derive expressions for the bit-error rate according to the matched-filter bound for 4BOK DS-UWB, which serve as theoretical performance limits for equalization. We then devise structures and methods for filter optimization for low-complexity linear and nonlinear equalization schemes. In this context, we develop a new equivalent multiple-input multiple-output (MIMO) description of 4BOK DS-UWB, which facilitates the design of efficient equalizers using MIMO filter optimization techniques. Furthermore, we propose the application of widely linear processing to these equalizers. Simulation and semi-analytical results show that (a) MIMO equalization is greatly advantageous over more obvious non- MIMO schemes, and (b) the proposed MIMO equalizers allow for power-efficient 4BOK DSUWB transmission close to the theoretical limits with moderate computational complexity.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat

Frequency domain equalization and multiuser detection techniques for DS-UWB systems

Ultra wideband (UWB) is an emerging technique for high data rate transmissions over short distances. Direct sequence (DS)-UWB approach is one of the two competing high data rate UWB standards, along with multiband orthogonal frequency division multiplexing (OFDM). One of the major challenges in a DS-UWB receiver design is the intersymbol interference (ISI). Several time domain equalization schemes to eliminate IS1 have been proposed in the literature for DS-UWB systems. However, for long dispersive channels, these time domain equalization schemes require very high computational complexity in order to achieve desired bit error performance. Frequency domain equalization schemes which give better performance than time domain equalization schemes for single carrier systems, over highly dispersive channels, are well known in the literature. In this thesis, performances of frequency domain minimum mean square error (MMSE) linear, decision feedback and iterative decision feedback equalizers are studied for uncoded single user BPSK and 4BOK DS-UWB systems. We compare bit error rate (BER) performance of various time domain and frequency domain equalization techniques and evaluate their computational complexity. We show that the frequency domain equalization techniques can offer better trade off between complexity and performance compared to the time domain equalization techniques for DS-UWB systems. We then consider frequency domain multiuser detection techniques for DS-UWB systems. We employ frequency domain successive interference cancellation and parallel interference cancellation schemes combined with frequency domain equalization schemes and study their average BER performance. We derive low complexity frequency domain MMSE turbo equalization schemes for coded BPSK and 4BOK DS-UWB systems. Soft interference cancellation is used in the multiuser systems to remove multiple access interference (MAI). The average BER performance is obtained using simulations. The performance gain due to turbo equalization is shown to be significant, particularly, for DS-UWB systems with lower spreading gain. The improvement in the performance due to turbo detection is found to be very high for multiuser systems.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat