Analysis of frequency domain frame detection and synchronization in OQAM-OFDM systems

For future communication systems, filter bank multicarrier schemes offer the flexibility to increase spectrum utilization in heterogeneous wireless environments by good separation of signals in the frequency domain. To fully exploit this property for frame detection and synchronization, the advantage of the filter bank should be taken at the receiver side. In this work, the concept of frequency domain processing for frame detection and synchronization is analyzed and a suitable preamble design as well as corresponding estimation algorithms is discussed. The theoretical performance of the detection and estimation schemes is derived and compared with simulation-based assessments. The results show that, even though the frequency domain algorithms are sensitive to carrier frequency offsets, satisfactory frame detection and synchronization can be achieved in the frequency domain. In comparison to time domain synchronization methods, the computational complexity increases; however, enhanced robustness in shared spectrum access scenarios is gained in case the described frequency domain approach is utilized

Reliability oriented OTFS-based LEO satellites joint transmission scheme

This paper investigates a dual satellite transmission scheme with coherent reception. The receiver has a single synchronization circuit and is locked to only one of the satellites. Beam-centric pre-compensation techniques are considered in the paper. The cooperation area in which coherent reception is feasible is characterized analytically. The application of precoding to the orthogonal time and frequency space (OTFS) waveform is considered to counteract the residual offsets, which result from the displacement of the receiver from the selected reference point. Numerical evaluations show that the dual satellite scheme improves the system spectral efficiency as well the link reliability in comparison with the single satellite transmission scheme.This paper is part of the R+D+i project (PID2020-115323RB-C31) funded by MCIN/AEI/ 10.13039/501100011033.Peer ReviewedPostprint (author's final draft

Towards Massive Connectivity Support for Scalable mMTC Communications in 5G networks

The fifth generation of cellular communication systems is foreseen to enable a multitude of new applications and use cases with very different requirements. A new 5G multiservice air interface needs to enhance broadband performance as well as provide new levels of reliability, latency and supported number of users. In this paper we focus on the massive Machine Type Communications (mMTC) service within a multi-service air interface. Specifically, we present an overview of different physical and medium access techniques to address the problem of a massive number of access attempts in mMTC and discuss the protocol performance of these solutions in a common evaluation framework

D4.1 Draft air interface harmonization and user plane design

The METIS-II project envisions the design of a new air interface in order to fulfil all the performance requirements of the envisioned 5G use cases including some extreme low latency use cases and ultra-reliable transmission, xMBB requiring additional capacity that is only available in very high frequencies, as well as mMTC with extremely densely distributed sensors and very long battery life requirements. Designing an adaptable and flexible 5G Air Interface (AI), which will tackle these use cases while offering native multi-service support, is one of the key tasks of METIS-II WP4. This deliverable will highlight the challenges of designing an AI required to operate in a wide range of spectrum bands and cell sizes, capable of addressing the diverse services with often diverging requirements, and propose a design and suitability assessment framework for 5G AI candidates.Aydin, O.; Gebert, J.; Belschner, J.; Bazzi, J.; Weitkemper, P.; Kilinc, C.; Leonardo Da Silva, I.... (2016). D4.1 Draft air interface harmonization and user plane design. https://doi.org/10.13140/RG.2.2.24542.0288

Mehrnutzer MIMO-OFDM in der Praxis:Der Weg zu spektral effizienter Übertragung über zeitvariante Kanälen

Die Kombination der MIMO Mehrantennentechnik mit der OFDM Mehrträgerübertragung verspricht das Erzielen hoher spektraler Effizienzen bei moderater Komplexität von Sender- und Empfängerstrukturen. Weiterhin erlaubt das MIMO-OFDM Systemkonzept die flexible Vergabe von Übertragungsressourcen in Zeit, Frequenz und Raum an unterschiedliche Nutzer. Diese Dissertation behandelt Verfahren zur Signalübertragung in Mehrnutzer MIMO-OFDM Systemen, die die Realisierung solcher Systeme in der Praxis ermöglichen sollen. Betrachtet wird hierbei eine isolierte Zelle, bestehend aus einer einzelnen Basisstation, die mehrere in der Zelle verteilte Nutzerterminals versorgt. Im Allgemeinen sind praktische Mobilfunkkanäle geprägt durch zeitvariantes Verhalten, das von der Bewegung der Nutzerterminals oder aber von Unstimmigkeiten innerhalb der Übertragungskette herrührt. Jene Zeitvarianz kann zu massiven Störungen im Übertragungssystem führen, die die praktisch erzielbare spektrale Effizienz beschränken. Um dennoch möglichst nah an die von der theoretischen Analyse versprochenen spektralen Effizienzen heranzukommen, müssen geeignete Verfahren gefunden werden, mit denen die der Zeitvarianz geschuldeten Degradationseffekte wirksam überwunden werden können. Die Dissertation gliedert sich in drei Hauptkapitel, in denen jeweils unterschiedliche Grade der Kanal-Zeitvarianz betrachtet werden. Die vorgestellten Ergebnisse zeigen, dass sich für alle betrachteten Fälle Lösun-gen mit moderaten Komplexitätsanforderungen für die Signalübertragung in MIMO-OFDM Systemen finden lassen, mit denen hohe spektrale Effizienzen auch in der Praxis erzielt werden können. Die spezifischen Inhalte und Ergebnisse der drei Kapitel werden im Folgenden kurz zusammengefasst: 1. Die einfachste Form der Zeitvarianz im Kanal wird verursacht durch einen Frequenzversatz der Signale an Sender und Empfänger. Kompensation der durch diesen Versatz hervorgerufenen Störeffekte ist Aufgabe der Synchronisation. Das erste Kapitel befasst sich daher mit der Entwicklung eines geschlossenen Konzep-tes für die Synchronisation des Mehrnutzer MIMO-OFDM Gesamtsystems bestehend aus Vorwärtsstrecke (Downlink) und Rückwärtsstrecke (Uplink). Für die Präambel-basierte Downlink-Synchronisation wird mit Hilfe analytischer und simulativer Mittel der Einfluss von Frequenzdiversität, räumlicher Diversität sowie der Präambellänge auf die Synchronisationsperformanz untersucht. Verwenden die Terminals in der Uplink-Phase den im Downlink geschätzten Frequenzversatz zur Vorkompensation ihrer Sendesignale, können die durch Schätzfehler verbleibenden Störungen direkt an der Basisstation kompensiert werden. Basierend auf einer Analyse der Signalbedingungen im Uplink wird ein einfaches Verfahren hierfür hergeleitet. 2. Nach erfolgreicher Synchronisation kann im Fall von nahezu statischen Verhältnissen in der Ausbreitungsumgebung ein adaptives Übertragungskonzept mit hoher spektraler Effizienz umgesetzt werden, das entsprechend der aktuellen Kanalbedingungen für jeden Nutzer die geeigneten Ressourcen sowie den bestmöglichen MIMO Übertragungsmodus auswählt. Ein solches Konzept wird für den Downlink eines Frequenz-Duplex Systems entwickelt, in dem der aktuelle Kanalzustand aller beteiligten Nutzer über einen begrenzten Feedback-Kanal bereitgestellt wird. Es wird gezeigt, dass mit diesem praktikablen Konzept unter idealen Bedingungen ein hoher Anteil der theoretisch erzielbaren Kapazität des MIMO Broadcast-Kanals realisiert werden kann. Mit zunehmender Mobilität der Nutzer und der damit einhergehenden schnellen Änderung der Kanalzustände erfährt dieses adaptive System jedoch eine Degradation der Systemleistung. Für moderate Fahrzeuggeschwindigkeiten und eine feste Antennenkonfiguration wird eine mögliche Erweiterung des Konzeptes aufgezeigt, die auf linearer Prädiktion der Nutzerkanäle beruht. Hiermit kann eine zuverlässige Funktion der adaptiven Übertragung auch bei Mobilität aufrechterhalten werden. 3. Bei sehr hohen Geschwindigkeiten der Nutzer verletzt die durch Doppler-Effekte hervorgerufene hohe Zeitvarianz des Kanals schließlich die Orthogonalität der OFDM Unterträgersignale; es kommt zur sogenannten Inter-Carrier Interferenz (ICI). Für allgemeine Doppler Kanäle wird für den Downlink ein einfaches Verfahren entwickelt, das die ICI mit Hilfe der am Empfänger verfügbaren Mehrantennen unterdrückt. Dieses Verfahren erhält die unterträgerweise Entzerrung im OFDM System aufrecht, wodurch es einfach zu realisieren ist. Die Schätzung der ICI kann anhand von gewöhnlichen Pilotsignalen durchgeführt werden und zeichnet sich durch einen geringen Realisierungsaufwand aus.The combination of MIMO multiple antenna technique together with OFDM multi-carrier transmission promises to achieve high spectral efficiencies while setting moderate complexity demands on transmitter and receiver structures. Furthermore, the MIMO-OFDM system concept allows to flexibly allocate the resources in time, frequency and space to different users. This dissertation addresses methods for signal transmission in multi-user MIMO-OFDM systems, which are meant to be seen as enablers for the realization of such systems in practice. The focus thereby is on an isolated cell, where a single base station communicates with multiple user terminals distributed over the cell area. In general, practical mobile radio channels are of time-variant nature, which is attributed to movement of the user terminals or to impairments in the transmission chain. The time variance may cause severe distortions in the transmission system, limiting the spectral efficiencies achievable in practice. To get close to spectral efficiencies promised from theoretical analysis though, suitable methods to effectively overcome the degradation induced by the channel’s time variance need to be found. The dissertation is structured into three main chapters that deal with different degrees of the channel’s time variance. The presented results indicate that for all cases considered, solutions with moderate complexity demands can be found which allow to achieve high spectral efficiencies in MIMO-OFDM systems also in practice. The specific content and results of the three chapters are briefly summarized in the following: 1. The simplest form of time variance in the channel is caused by a frequency offset of the signals at transmitter and receiver. Compensation of the offset-induced distortions is the task of the synchronization process. Therefore, the first chapter addresses the development of a self-contained synchronization concept for the multi-user MIMO-OFDM system, consisting of downlink and uplink. For the preamblebased downlink synchronization, the impact of frequency diversity, spatial diversity as well as of the preamble length on the synchronization performance is investigated by analytical and simulative means. If the terminals use the offset estimated in the downlink for a pre-compensation of their transmit signals during the uplink phase, the residual distortions attributed to the estimation error can be compensated directly at the base station. Based on an analysis of the signal conditions in the uplink, a simple method for this purpose is derived. 2. In the case of quasi-static conditions in the propagation environment, the channel can be considered static itself after successful synchronization. Then an adaptive transmission concept with high spectral efficiency can be realized, which selects for each user the most suitable resources as well as the best-suited MIMO transmission mode according to its current channel conditions. Such a concept is developed for the downlink of a frequency division duplex system, where the current channel state information of each user is provided via a control channel with limited feedback. It is shown that under ideal conditions, this practical approach is capable of realizing a large proportion of the theoretically achievable capacity of the MIMO broadcast channel. However, with increasing mobility of the users, causing the user channels to vary rapidly over time, the performance of the adaptive system degrades. For moderate vehicular speeds and for a fixed antenna configuration, a possible extension of the concept is presented, which relies on linear prediction of the user channels. Herewith, a reliable operation of the adaptive transmission concept can be maintained even under user mobility. 3. At very high user speeds, the high time variance of the channel induced by Doppler effects violates the orthogonality of the OFDM subcarrier signals, giving rise to the so-called inter-carrier interference (ICI). For general Doppler channels, we develop a simple method for the downlink, which suppresses the ICI with the aid of the multiple antennas available at the receiver. This approach maintains the subcarrierwise equalization in OFDM, facilitating its implementation. Estimation of the ICI can be carried out based on common pilot signals by a simple method with low demands on computational complexity

Multiple CFOs in OFDM-SDMA Uplink: Interference Analysis and Compensation

<p/> <p>In OFDM-based space division multiple access (SDMA) systems, multiple users are served by a multiantenna base station simultaneously on the same frequency resources. In the uplink, each user's signal may be distorted by an independent carrier frequency offset (CFO), which impairs the orthogonality of the subcarrier signals and, if not properly compensated, results in performance degradations. We analyze the influence of multiusers' CFOs on the signal transmission in the OFDM-SDMA uplink and derive suitable bounds for the achievable signal-to-interference conditions. By modifying the signal model suitably, we develop a simple scheme for partial compensation of the CFO distortions. It allows to maintain the subcarrier-wise channel equalization and thus is well suited to be applied for a real-time system implementation. However, as CFOs impair the cyclic structure of the OFDM symbols, our scheme is not able to compensate for the entire distortion. The remaining interference is treated as additional noise, which limits the supported size of the CFOs.</p

Flipped-Underlay in Power-Efficient Industrial IoT Sidelink-Assisted Communication

Future factories will rely on highly reliable wireless communication among the plenty of devices and the network. The 5G networks enable ultra-reliable low-latency communication (URLLC) service to address the reliability and latency requirements in factories, where device-to-device (D2D) connections can provide additional means for improving communication reliability. To this aim, we have proposed a sidelink (SL)-assisted cooperative retransmissions (CoRe) scheme in our previous work, where retransmissions via SL for unsuccessful downlink (DL) transmissions are used to improve communication reliability under strict latency constraints. In this paper, we evaluate the CoRe scheme for a realistic factory scenario using system-level simulations, where we consider interference coordination and an optimal power control (PC) scheme. Inspired by the outcome of small transmit powers needed for the SL-assisted retransmissions, we propose a novel resource management scheme named &#x201C;flipped-underlay&#x201D;, which is realized by underlay communication. While in conventional underlay, a D2D communication with small transmit power is assigned to resources already allocated for uplink (UL) communication and thus underlaid, the D2D resources are allocated firstly in our scheme, and hence called flipped-underlay. Results demonstrate that the gains from CoRe scheme, are three-fold: showing a significant reduction in the total number of SL-assisted retransmissions compared to conventional retransmissions via DL while maintaining the desired reliability and latency performance, reduced transmit power consumption by virtue of the optimal power allocation for retransmissions, and finally yet importantly, reuse of resources in our new flipped-underlay resource allocation (FURA) algorithm substantially reduces the total amount of resources occupied by the system

Pulse shaping design for OFDM systems

Abstract Spectrally contained OFDM-based waveforms are considered key enablers for a flexible air interface design to support a broad range of services and frequencies as envisaged for 5G mobile systems. By allowing for the flexible configuration of physical layer parameters in response to diverse requirements, these waveforms enable the in-band coexistence of different services. One candidate from this category of waveforms is pulse-shaped OFDM, which follows the idea of subcarrier filtering while fully maintaining the compatibility with CP-OFDM. In this paper, we provide an overview of pulse shaping methods in OFDM systems and propose a new pulse-shaped design method with arbitrary length constraint and good time-frequency localization property. Based on the pulse design, we discuss different receiver realizations and present a criterion for pulse shape evaluation. In addition, the parameterizations of OFDM system to address diverse requirements of the services envisaged for the 5G systems are described. Link and system performance results for selected scenarios show that a proper design of the OFDM numerologies and pulse shapes could substantially improve the performance under time and frequency distortions. Furthermore, pulse-shaped OFDM is able to support asynchronous transmissions and reduce the signal sensitivity to Doppler distortions, rendering it beneficial for various applications from the context of vehicular communications and the Internet-of-things