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Equalization for DS-UWB systems Parihar, Ambuj


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.

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