University of Twente Student Theses


A time-frequency localized signal basis for multi-carrier communication

Korevaar, Cornelis Willem (2010) A time-frequency localized signal basis for multi-carrier communication.

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Abstract:The radio-spectrum has been untouched for centuries, but in recent years wireless devices have been competing more and more for some scarce bandwidth. As bandwidth auctions are billion-dollar affaires,wireless devices pop-up literally everywhere and forecasts state a 66x increase of data usage in just four years, an efficient use of the radio-spectrum is of ever increasing importance. To arrive at a more efficient usage of the radio-spectrum, the presented work analyzes spectral leakage associated with Orthogonal Frequency Division Multiplexing (OFDM) and discusses solutions. Conventional solutions target the consequences, reducing sidelobes, rather than targeting the problems, the signals themselves. Instead, this thesis aims to arrive at a set of signals localized in time-frequency. The localization in time and frequency is lower-bounded by the uncertainty principle. The Hermite functions form a set of solutions to this lower-bound. Although Hermite functions are optimally localized in time-frequency, that does not necessarily imply that the signals are also suitable for communication. Based on the discussion of ten signal attributes, criteria are formulated for a set of basis signals for communication. The Hermite functions are assessed based on these criteria and subsequently modified in order to meet the criteria. The resulting set of time-frequency localized signals, referred to as STFL, are in discrete-time, orthogonal, zero-mean, of equal energy and are localized in time and frequency. Both OFDM and STFL signals asymptotically approach the optimum of 2 degrees of modulation freedom per time-bandwidth product. However, in case the spectrum becomes more and more utilized, mutual interference caused by conventional OFDM sidelobes severely degrades the effective data-throughput. Unlike OFDM, the signals STFL have a near-optimal localization and allow multiple users to communicate efficiently over time and frequency. The performance of STFL in mobile radio channels, the transceiver power efficiency and hardware complexity are discussed and compared to conventional OFDM, leading to minor differences between the two. After all, given the increasing competition for some scarce bandwidth, there is good evidence to believe that the realization of transceivers employing Hermite functions, or their practical counterparts STFL, could be a major improvement in communication.
Item Type:Essay (Master)
Faculty:EEMCS: Electrical Engineering, Mathematics and Computer Science
Subject:54 computer science
Programme:Computer Science MSc (60300)
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