652 research outputs found
Adaptive Modulation Schemes for Underwater Acoustic OFDM Communication
High data rate communication is challenging in underwater acoustic (UA) communication as UA channels vary fast along with the environmental factors. A real-time Orthogonal frequency-division multiplexing (OFDM) based adaptive UA communication system is studied in this research employing the National Instruments (NI) LabVIEW software and NI CompactDAQ device. The developed adaptive modulation schemes enhance the reliability of communication, guarantee continuous connectivity, ensure maximum performance under a fixed BER at all times and boost data rate
Real-Time Adaptive Modulation Schemes for Underwater Acoustic OFDM Communication
Adaptive modulation received significant attention for underwater acoustic (UA) communication systems with the aim of increasing the system efficiency. It is challenging to attain a high data rate in UA communication, as UA channels vary fast, along with the environmental factors. For a time-varying UA channel, a self-adaptive system is an attractive option, which can choose the best method according to the channel condition to guarantee the continuous connectivity and high performance constantly. A real-time orthogonal frequency-division multiplexing (OFDM)-based adaptive UA communication system is presented in this paper, employing the National Instruments (NI) LabVIEW software and NI CompactDAQ device. In this paper, the received SNR is considered as a performance metric to select the transmission parameters, which are sent back to the transmitter for data transmission. In this research, a UA OFDM communication system is developed, employing adaptive modulation schemes for a nonstationary UA environment which allows to select subcarriers, modulation size, and allocate power adaptively to enhance the reliability of communication, guarantee continuous connectivity, and boost data rate. The recent UA communication experiments carried out in the Canning River, Western Australia, verify the performance of the proposed adaptive UA OFDM system, and the experimental results confirm the superiority of the proposed adaptive scheme
Effect of Synchronization error in Multi Carrier systems
Communications as an important aspect life plays a major role in our daily routine. With the progress in age and growth in its demand, there has been rapid development in the field of communications. Analog signals which were used to send information previously are now sent in digital domain in a much wider range. For better performance in terms of transmissions, single carriers are replaced by multi-carriers Some of the methods that use multi-carriers for transmission are Orthogonal Division Frequency Multiplexing (OFDM), Code Division Multiple Access (CDMA). In OFDM system, orthogonally placed subcarriers are used to carry the data from the transmitter to the receiver. The guard band present in this system avoids the system from being effected by Inter Symbol Interference (ISI). However, Doppler Shifts and delays induced in the channel due to various reasons impose frequency offset to the carrier. This results in an error in the Synchronization between the transmitter and receiver. This leads to the loss of orthogonally between the subcarriers and thus degrades the performance of the OFDM system.In this project, the Synchronization error in OFDM is discussed in particular and a technique of estimating the Carrier Frequency Offset (CFO) using Null Subcarriers is studied
Analysis and Performance Comparison of DVB-T and DTMB Systems for Terrestrial Digital TV
Orthogonal frequency-division multiplexing (OFDM) is the most popular
transmission technology in digital terrestrial broadcasting (DTTB), adopted by
many DTTB standards. In this paper, the bit error rate (BER) performance of two
DTTB systems, namely cyclic prefix OFDM (CP-OFDM) based DVB-T and time domain
synchronous OFDM (TDS-OFDM) based DTMB, is evaluated in different channel
conditions. Spectrum utilization and power efficiency are also discussed to
demonstrate the transmission overhead of both systems. Simulation results show
that the performances of the two systems are much close. Given the same ratio
of guard interval (GI), the DVB-T outperforms DTMB in terms of signal to noise
ratio (SNR) in Gaussian and Ricean channels, while DTMB behaves better
performance in Rayleigh channel in higher code rates and higher orders of
constellation thanks to its efficient channel coding and interleaving scheme
Simulation framework for multigigabit applications at 60 GHz
This dissertation describes the implementation of a OFDM-based simulation framework
for multigigabit applications at 60 GHz band over indoor multipath fading channels.
The main goal of the framework is to provide a modular simulation tool designed
for high data rate application in order to be easily adapted to a speci c standard or
technology, such as 5G. The performance of OFDM using mmWave signals is severely
a ected by non-linearities of the RF front-ends. This work analyses the impact of RF
impairments in an OFDM system over multipath fading channels at 60 GHz using the
proposed simulation framework. The impact of those impairments is evaluated through
the metrics of BER, CFR, operation range and PSNR for residential and kiosk scenarios,
suggested by the standard for LOS and NLOS. The presented framework allows
the employment of 16 QAM or 64 QAM modulation scheme, and the length of the
cyclic pre x extension is also con gurable. In order to simulate a realistic multipath
fading channel, the proposed framework allows the insertion of a channel impulse response
de ned by the user. The channel estimation can be performed either using
pilot subcarriers or Golay sequence as channel estimation sequences. Independently of
the channel estimation technique selected, frequency domain equalization is available
through ZF approach or MMSE. The simulation framework also allows channel coding
techniques in order to provide a more robustness transmission and to improve the link
budget
MIMO signal processing in offset-QAM based filter bank multicarrier systems
Next-generation communication systems have to comply with very strict requirements for increased flexibility in heterogeneous environments, high spectral efficiency, and agility of carrier aggregation. This fact motivates research in advanced multicarrier modulation (MCM) schemes, such as filter bank-based multicarrier (FBMC) modulation. This paper focuses on the offset quadrature amplitude modulation (OQAM)-based FBMC variant, known as FBMC/OQAM, which presents outstanding spectral efficiency and confinement in a number of channels and applications. Its special nature, however, generates a number of new signal processing challenges that are not present in other MCM schemes, notably, in orthogonal-frequency-division multiplexing (OFDM). In multiple-input multiple-output (MIMO) architectures, which are expected to play a primary role in future communication systems, these challenges are intensified, creating new interesting research problems and calling for new ideas and methods that are adapted to the particularities of the MIMO-FBMC/OQAM system. The goal of this paper is to focus on these signal processing problems and provide a concise yet comprehensive overview of the recent advances in this area. Open problems and associated directions for future research are also discussed.Peer ReviewedPostprint (author's final draft
Advances in Multi-User Scheduling and Turbo Equalization for Wireless MIMO Systems
Nach einer Einleitung behandelt Teil 2 Mehrbenutzer-Scheduling für die
Abwärtsstrecke von drahtlosen MIMO Systemen mit einer Sendestation und
kanaladaptivem precoding: In jeder Zeit- oder Frequenzressource kann eine
andere Nutzergruppe gleichzeitig bedient werden, räumlich getrennt durch
unterschiedliche Antennengewichte. Nutzer mit korrelierten Kanälen sollten
nicht gleichzeitig bedient werden, da dies die räumliche Trennbarkeit
erschwert. Die Summenrate einer Nutzermenge hängt von den Antennengewichten
ab, die wiederum von der Nutzerauswahl abhängen. Zur Entkopplung des
Problems schlägt diese Arbeit Metriken vor basierend auf einer geschätzten
Rate mit ZF precoding. Diese lässt sich mit Hilfe von wiederholten
orthogonalen Projektionen abschätzen, wodurch die Berechnung von
Antennengewichten beim Scheduling entfällt. Die Ratenschätzung kann
basierend auf momentanen Kanalmessungen oder auf gemittelter Kanalkenntnis
berechnet werden und es können Datenraten- und Fairness-Kriterien
berücksichtig werden. Effiziente Suchalgorithmen werden vorgestellt, die
die gesamte Systembandbreite auf einmal bearbeiten können und zur
Komplexitätsreduktion die Lösung in Zeit- und Frequenz nachführen können.
Teil 3 zeigt wie mehrere Sendestationen koordiniertes Scheduling und
kooperative Signalverarbeitung einsetzen können. Mittels orthogonalen
Projektionen ist es möglich, Inter-Site Interferenz zu schätzen, ohne
Antennengewichte berechnen zu müssen. Durch ein Konzept virtueller Nutzer
kann der obige Scheduling-Ansatz auf mehrere Sendestationen und sogar
Relays mit SDMA erweitert werden. Auf den benötigten Signalisierungsaufwand
wird kurz eingegangen und eine Methode zur Schätzung der Summenrate eines
Systems ohne Koordination besprochen. Teil4 entwickelt Optimierungen für
Turbo Entzerrer. Diese Nutzen Signalkorrelation als Quelle von Redundanz.
Trotzdem kann eine Kombination mit MIMO precoding sinnvoll sein, da bei
Annahme realistischer Fehler in der Kanalkenntnis am Sender keine optimale
Interferenzunterdrückung möglich ist. Mit Hilfe von EXIT Charts wird eine
neuartige Methode zur adaptiven Nutzung von a-priori-Information zwischen
Iterationen entwickelt, die die Konvergenz verbessert. Dabei wird gezeigt,
wie man semi-blinde Kanalschätzung im EXIT chart berücksichtigen kann.
In Computersimulationen werden alle Verfahren basierend auf
4G-Systemparametern überprüft.After an introduction, part 2 of this thesis deals with downlink multi-user
scheduling for wireless MIMO systems with one transmitting station
performing channel adaptive precoding:Different user subsets can be served
in each time or frequency resource by separating them in space with
different antenna weight vectors. Users with correlated channel matrices
should not be served jointly since correlation impairs the spatial
separability.The resulting sum rate for each user subset depends on the
precoding weights, which in turn depend on the user subset. This thesis
manages to decouple this problem by proposing a scheduling metric based on
the rate with ZF precoding such as BD, written with the help of orthogonal
projection matrices. It allows estimating rates without computing any
antenna weights by using a repeated projection approximation.This rate
estimate allows considering user rate requirements and fairness criteria
and can work with either instantaneous or long term averaged channel
knowledge.Search algorithms are presented to efficiently solve user
grouping or selection problems jointly for the entire system bandwidth
while being able to track the solution in time and frequency for complexity
reduction.
Part 3 shows how multiple transmitting stations can benefit from
cooperative scheduling or joint signal processing. An orthogonal projection
based estimate of the inter-site interference power, again without
computing any antenna weights, and a virtual user concept extends the
scheduling approach to cooperative base stations and finally included SDMA
half-duplex relays in the scheduling.Signalling overhead is discussed and a
method to estimate the sum rate without coordination.
Part 4 presents optimizations for Turbo Equalizers. There, correlation
between user signals can be exploited as a source of redundancy.
Nevertheless a combination with transmit precoding which aims at reducing
correlation can be beneficial when the channel knowledge at the transmitter
contains a realistic error, leading to increased correlation. A novel
method for adaptive re-use of a-priori information between is developed to
increase convergence by tracking the iterations online with EXIT charts.A
method is proposed to model semi-blind channel estimation updates in an
EXIT chart.
Computer simulations with 4G system parameters illustrate the methods using realistic channel models.Im Buchhandel erhältlich:
Advances in Multi-User Scheduling and Turbo Equalization for Wireless MIMO Systems / Fuchs-Lautensack,Martin
Ilmenau: ISLE, 2009,116 S.
ISBN 978-3-938843-43-
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