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Title
Design and analysis of unique word OFDM / Christian Hofbauer
AuthorHofbauer, Christian
CensorHuemer, Mario ; Huber, Johannes
PublishedKlagenfurt, 06/2016
Descriptionvi, 204 Seiten : Diagramme
Institutional NoteAlpen Adria Universität Klagenfurt, Dissertation, 2016
LanguageEnglish
Bibl. ReferenceOeBB
Document typeDissertation (PhD)
Keywords (DE)UW-OFDM / OFDM / Übertragungsverfahren / Mehrträgerverfahren / Signalverarbeitung / Nachrichtentechnik / systematisch kodiertes UW-OFDM / nicht-systematisch kodiertes UW-OFDM / Pilotträger / Trägerfrequenzoffset
Keywords (EN)Unique Word / UW-OFDM / OFDM / transmission scheme / multicarrier / signal processing / communications / systematically encoded UW-OFDM / non-systematically encoded UW-OFDM / carrier frequency offset
URNurn:nbn:at:at-ubk:1-23116 Persistent Identifier (URN)
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 The work is publicly available
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Abstract (English)

Unique Word orthogonal frequency division multiplexing (OFDM) is a promising alternative to cyclic prefix based OFDM (CP-OFDM), which currently denotes the method of choice for many digital communication standards, with applications ranging from audio and video broadcasting, last mile internet access to modern cellular networks. In this signaling concept, the guard interval (GI) is filled with an arbitrary deterministic sequence - the so-called "unique word" (UW) - instead of the random CP. This sequence provides the same advantages as a CP (no intersymbol interference and diagonalization of the channel matrix), but can additionally be designed to optimally meet synchronization and estimation tasks. Furthermore, most important, and different to almost all signaling schemes of the OFDM family, the UW is already part of the discrete Fourier transform (DFT) interval. Ensuring such time domain properties entails the introduction of a certain redundancy in the frequency domain. This redundancy can be exploited beneficially to enhance range, reliability, capacity or battery lifespan. In this sense, UW-OFDM transforms the usually disregarded guard interval into a multipurpose sequence, thus tackling the well-known inefficiency problem of guard intervals in current communication systems. Moreover, adapting the UW and therefore the GI length to different channel conditions will not impact the DFT length and thus keeps relevant processing chain structures untouched. Hence, UW-OFDM allows supporting a wide range of communication scenarios while still ensuring high efficiency. The implementation of the inherent redundancy - the primary root of the special UW-OFDM properties - is ambiguous, giving rise to a variety of different signal variants. Main topic of this work is the investigation of the signal generation process of UW-OFDM symbols and its impact on the performance. The first principle approach of generating OFDM symbols with unique word is the concept of systematically encoded UW-OFDM. This concept is based on the idea of a systematic block code, leading to dedicated data and redundant subcarriers. This redundancy translates to beneficial properties regarding spectral behavior and bit error ratio (BER) performance. Systematically encoded UW-OFDM shows a superior sidelobe suppression over conventional CP-OFDM and outperforms it in terms of the BER performance in a multipath environment, for coded as well as uncoded transmission. Still, the required energy to load the redundant subcarriers is significantly higher than for the data subcarriers, showing further potential for improvement. Introducing additional redundant subcarriers or allowing systematic noise in the guard interval reduce the OFDM symbol energy further. The resulting BER enhancement comes at the price of either reduced bandwidth efficiency or an inevitable error floor. The second principle approach of generating OFDM symbols with unique word is denoted as non-systematically encoded UW-OFDM, which annihilates the flaws of the systematic approach. Inspired by a non-systematic code, the idea of dedicated data and redundant subcarriers is discarded and all-purpose subcarriers introduced instead. A non-systematic generation translates to an additional gain in terms of spectral properties and BER behavior. BER results are obtained for various setups, channel conditions, data symbol constellations and imperfect channel knowledge, all delivering results significantly in favor of UW-OFDM. An extension of the UW-OFDM framework enables the inclusion of pilot tones into the frequency domain symbol, while still preserving all beneficial properties known from a pilotless UW-OFDM concept. A mean square error (MSE) analysis of an exemplary carrier frequency offset (CFO) estimation task reveals a significant better performance of pilot tone based estimation concepts in UW-OFDM than in CP-OFDM, a result inherited from the introduced redundancy. The effects of a CFO on UW-OFDM are studied in detail and compared to those in single-carrier and OFDM signaling schemes. The CFO induced error after data estimation is on the one hand due to subtracting a disturbed UW and pilot offset, and on the other hand due to insufficient CFO compensation. Both error sources are investigated independently and alternative approaches with different computational complexity are presented. MSE results confirm UW-OFDM to achieve a higher robustness against CFO than conventional CP-OFDM. BER simulations additionally illuminate the effects of CFO impairments, again identifying UW-OFDM as the clearly better performing alternative.

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