6 research outputs found
Classical and Bayesian Linear Data Estimators for Unique Word OFDM
Unique word - orthogonal frequency division multiplexing (UW-OFDM) is a novel
OFDM signaling concept, where the guard interval is built of a deterministic
sequence - the so-called unique word - instead of the conventional random
cyclic prefix. In contrast to previous attempts with deterministic sequences in
the guard interval the addressed UW-OFDM signaling approach introduces
correlations between the subcarrier symbols, which can be exploited by the
receiver in order to improve the bit error ratio performance. In this paper we
develop several linear data estimators specifically designed for UW-OFDM, some
based on classical and some based on Bayesian estimation theory. Furthermore,
we derive complexity optimized versions of these estimators, and we study their
individual complex multiplication count in detail. Finally, we evaluate the
estimators' performance for the additive white Gaussian noise channel as well
as for selected indoor multipath channel scenarios.Comment: Preprint, 13 page
Design and analysis of UW-OFDM signals
AbstractUnique word-orthogonal frequency division multiplexing (UW-OFDM) is a novel signaling concept where the guard interval is implemented as a deterministic sequence, the so-called unique word. The UW is generated by introducing a certain level of redundancy in the frequency domain. Different data estimation strategies and the favourable bit error ratio (BER) performance of UW-OFDM, as well as comparisons to competing concepts have already extensively been discussed in previous papers. This work focuses on the different possibilities on how to generate UW-OFDM signals. The optimality of the two-step over the direct approach in systematic UW-OFDM is proved analytically, we present a heuristic algorithm that allows a fast numerical optimization of the redundant subcarrier positions, and we show that our original intuitive approach of spreading the redundant subcarriers in systematically encoded UW-OFDM by minimizing the mean redundant energy is practically also optimum w.r.t. transceiver based cost functions. Finally, we derive closed form approximations of the statistical symbol distributions on individual subcarriers as well as the redundant energy distribution and compare them with numerically found results
Receiver concepts for Unique Word OFDM
Unique Word OFDM (orthogonal frequency division multiplexing) is an attractive alternative to OFDM with cyclic prefix, which is adopted for data transmission in standards like DSL, LTE, DVB and IEEE 802.11 (WLAN). In this signaling concept, a deterministic sequence, a ``unique word'' (UW), is inserted into the transmit stream, instead of a cyclic copy of the data. Furthermore, this UW is part of the IDFT (inverse discrete Fourier transform) interval.
This property distinguishes UW-OFDM from most other OFDM variants, while it offers the same advantages as the conventional OFDM (free of inter-symbol interference, diagonalization of the channel matrix).
By defining of a sequence in time domain, some capacity has to be allocated for redundancy in frequency domain. This redundancy solely depends on the transmit data (and defined system parameters) and can be utilized for a reliable recovery of the data.
In order to exploit this redundancy, sophisticated receiver structures need to be employed, which is topic of this work. The achieved gain can be used for a higher data rate, range, reliability, capacity or battery lifespan.
Methods to generate valid UW-OFDM symbols are introduced in two variants: The systematic generation of UW-OFDM symbols, which can be done directly or in two steps, and the non-systematic generation. An analysis of the mean transmit energy of all generation methods sheds light on their suitability for communication systems and reveals possibilities for optimization.
The main part of this work is about suited receivers for UW-OFDM that are able to reconstruct the data reliably, after transmission over a dispersive channel. Besides the estimated transmit symbols, all these receivers need to provide reliability information, which enables a channel decoder to achieve better decoding results. All receivers are investigated regarding their bit error performance with and without channel coding, in the AWGN (additive white Gaussian noise) channel as well as in a multipath environment.
Besides two rather intuitively derived, two more optimum linear receivers are discussed, which emerge from classical as well as Bayesian estimation theory: The BLUE (best linear unbiased estimator) and the LMMSE (linear minimum mean square error) estimator. The computational complexity of all these receivers is analyzed for both OFDM symbol generation approaches and compared numerically.
When using real transmit symbol constellations, the LMMSE estimator can be outperformed by the WLMMSE (widely LMMSE) estimator. Furthermore, a symbol scaling effect can be identified for these Bayesian receivers.
This turns out to be harmful for the detection quality with higher order constellations, such as 16-QAM or 4-ASK. Symbol scaling compensated versions of the LMMSE and WLMMSE estimators are introduced and their performance documented.
As another main topic of this work, a few nonlinear receivers are discussed, starting with two decision directed concepts. First, a method for noise interpolation is introduced, which exploits the correlation of the data symbols after an LMMSE estimation, in order to obtain improved estimates. It turns out that the selection of the samples which are used for estimation is decisive for the performance of this receiver.
In decision feedback equalization, the influence of detected data symbols on the receive signal is subtracted iteratively, in order to allow for a more reliable decision of the remaining symbols.
Here, the order of detection is crucial for the decision quality.
For the derived linear UW-OFDM system model, a maximum-likelihood sequence estimation (MLSE) yields the best estimates possible. However, due to its computational complexity, it is unsuitable for practical application.
As a practical realization of the MLSE, sphere decoding is presented, which obtains the same results with acceptable effort.
For the determination of reliability information, however, a mathematical approximation and a limitation of parameter dynamics has to be applied, to keep the complexity in adequate limits, which destroys the optimality of the method.
An investigation of QR decomposition, as it is directly used for sphere decoding and in a version of decision feedback equalization, shows that the way, how the QR decomposition is computed, has significant impact on runtime or detection performance, respectively.
An overall performance investigation reveals that the nonlinear receivers clearly outperform the LMMSE estimator in uncoded transmission. If channel coding is used, they are still able to achieve a small gain over the best performing linear estimator.
However, the LMMSE estimator constitutes a highly reasonable compromise when performance and complexity are taken into account.Unique Word OFDM (orthogonal frequency division multiplexing) ist eine attraktive Alternative zu OFDM mit zyklischem PrĂhrend es die selben Vorteile wie das herkĂmmliche OFDM (Freiheit von Intersymbol-Interferenzen, Diagonalisierung der Kanalmatrix) bietet.
Durch das Definieren einer Sequenz im Zeitbereich, muss im Frequenzbereich etwas KapazitĂngt einzig von den zu sendenden Daten (und festgelegten Systemparametern) ab, die am EmpfĂssliche RĂckgewinnung der Sendedaten genutzt werden kann. Um diese Redundanz zu nutzen sind ausgeklĂgelte EmpfĂsslichkeit, KapazitĂnger fĂr UW-OFDM, die nach der ĂÂbertragung Ăber stĂrende KanĂssig wieder rekonstruieren kĂnnen. All di Sendesymbolen auch VerlĂnger werden hinsichtlich ihrer LeistungsfĂnger vorgestellt, die sich durch eine Herangehensweise Ăber die klassische sowie die Bayes'sche SchĂtzer (linear minimum mean square error). Die rechnerische KomplexitĂnger wird fĂr beide SymbolgenerierungsansĂtzer zeigt sich, dass dieser bei Verwendung von rein reellen Symbolkonstellationen durch den WLMMSE-SchĂngern ein Symbolskalierungseffekt identifiziert werden.
Dieser erweist sich als extrem schĂt bei hĂherwertigen Konstellationen wie 16-QAM oder 4-ASK.
Symbolskalierungskompensierte Versionen des LMMSE und WLMMSE-SchĂhigkeit belegt.
Als weiteres Hauptthema dieser Arbeit werden einige nichtlinearer EmpfĂchst wird ein Verfahren zur Rauschinterpolation vorgestellt, das die Korrelation der Datensymbole nach einer LMMSE-SchĂtzwerte zu erhalten. Dabei stellt sich heraus, dass die Auswahl der Werte, die zur SchĂngers maĂgeblich bestimmt.
Bei der entscheidungsrĂckgekoppelten Entzerrung wird iterativ der Einfluss detektierter Datensymbole auf das Empfangssignal abgezogen, um ein Symbole zu ermĂglichen.
Hier ist die Reihenfolge der Detektion ausschlaggebend fĂr die Entscheidungssicherheit.
FĂr das hergeleitete lineare UW-OFDM-Systemmodell liefert eine Maximum-Likelihood-SequenzschĂtzwerte. Diese ist aber wegen ihrer KomplexitĂsslichkeitsinformationen muss jedoch auf eine mathematische NĂnkung der Parameterdynamik zurĂck gegriffen werden, um die KomplexitĂt der Methode zerstĂrt. OperationszĂt.
Eine Abhandlung Ăber QR-Zerlegung, die direkt in Sphere Decoding wie auch in einer Version der entscheidungsrĂckgekoppelten Entzerrung Anwendung findet, zeigt, dass die Art und Weise der Berechnung der QR-Zerlegung erhebliche Auswirkung auf Laufzeit oder LeistungsfĂnger den LMMSE-SchĂtzer erzielen. Der LMMSE-SchĂuĂerst vernĂnftigen Kompromiss, wenn man Leistung und KomplexitĂ$t in die Betrachtung mit einbezieht.Alexander OnicKlagenfurt, Alpen-Adria-Univ., Diss., 2013OeBB(VLID)241148
FRAMEx. V. Radio Spectral Shape at Central Subparsec Region of Active Galactic Nuclei
We present results from the Very Long Baseline Array multifrequency (1.6, 4.4, 8.6, and 22 GHz), high-sensitivity (âŒ25 ÎŒ Jy beam ^â1 ), subparsec-scale (<1 pc) observations and spectral energy distributions for a sample of 12 local active galactic nuclei (AGNs), a subset from our previous volume-complete sample with hard-X-ray (14â195 keV) luminosities above 10 ^42 erg s ^â1 , out to a distance of 40 Mpc. All 12 of the sources presented here were detected in the C (4.4 GHz) and X (8.6 GHz) bands, 75% in the L band (1.6 GHz), and 50% in the K band (22 GHz). Most sources showed compact, resolved/slightly resolved, central subparsec-scale radio morphology, except for a few with extended outflow-like features. A couple of sources have an additional component that may indicate the presence of a dual-core, single or double-sided jet or a more intricate feature, such as radio emission resulting from interaction with the nearby interstellar medium. The spectral slopes are mostly gigahertz-peaked or curved, with a few showing steep, flat, or inverted spectra. We found that at the subparsec scale, the gigahertz-peaked spectra belong to the low-accreting, radio-loud AGNs, with a tendency to produce strong outflows, possibly small-scale jets, and/or have a coronal origin. In contrast, flat/inverted spectra suggest compact radio emission from the central regions of highly accreting AGNs, possibly associated with radio-quiet AGNs producing winds/shocks or nuclear star formation in the vicinity of black holes