524 research outputs found
Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems
Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER
SINR degradation in MIMO-OFDM systems with channel estimation errors and partial phase noise compensation
The phase noise effect in multiple-input-multipleoutput
systems employing orthogonal frequency division multiplexing
is considered in a realistic scenario where the estimated
channel matrix is affected by an error. The analytical SINR
degradation due to phase noise and channel estimation is
obtained for linear receivers (ZF and MMSE)This work is partly funded by the projects "COMONSENS" CSD2008-
00010 and "MULTI-ADAPTIVE" TEC2008-06327-C03-02Publicad
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Laboratory and field trials evaluation of transmit delay Diversity applied to DVB-T/H networks
This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The requirements for future DVB-T/H networks demand that broadcasters design and
deploy networks that provide ubiquitous reception in challenging indoors and other
obstructed situations. It is essential that such networks are designed cost-effectively and with minimized environmental impact. The use of transmit diversity techniques with
multiple antennas have long been proposed to improve the performance and capacity of
wireless systems. Transmit diversity exploits the scattering effect inherent in the channel by means of transmitting multiple signals in a controlled manner from spatially separated antennas, allowing independently faded signals to arrive at the receiver and improves the chances of decoding a signal of acceptable quality. Transmit diversity can complement receive diversity by adding an additional diversity gain and in situations where receiver diversity is not practical, transmit diversity alone delivers a comparable amount of diversity gain. Transmit Delay Diversity (DD) can be applied to systems employing the
DVB standard without receiver equipment modifications. Although transmit DD can
provide a gain in NLOS situations, it can introduce degradation in LOS situation. The aim of this thesis is to investigate the effectiveness in real-word applications of novel diversity techniques for broadcast transmitter networks. Tests involved laboratory experiments using a wireless MIMO channel emulator and the deployment of a field measurement campaign dedicated to driving, indoor and rooftop reception. The relationship between the diversity gain, the propagation environment and several parameters such as the transmit antenna separation, the receiver speed and the Forward Error Correction Codes (FEC) configuration are investigated. Results includes the effect of real-word parameter usually not modeled in the software simulation analysis, such as antenna radiation patterns and mutual coupling, scattering vegetation impact, non-Gaussian noise sources and receiver implementation. Moreover, a practical analysis of the effectiveness of experimental techniques to mitigate the loss due to transmit DD loss in rooftop reception is presented. The results of this thesis confirmed, completed and extended the existing predictions with real word measurement results
Indoor wireless communications and applications
Chapter 3 addresses challenges in radio link and system design in indoor scenarios. Given the fact that most human activities take place in indoor environments, the need for supporting ubiquitous indoor data connectivity and location/tracking service becomes even more important than in the previous decades. Specific technical challenges addressed in this section are(i), modelling complex indoor radio channels for effective antenna deployment, (ii), potential of millimeter-wave (mm-wave) radios for supporting higher data rates, and (iii), feasible indoor localisation and tracking techniques, which are summarised in three dedicated sections of this chapter
Multiuser MIMO-OFDM for Next-Generation Wireless Systems
This overview portrays the 40-year evolution of orthogonal frequency division multiplexing (OFDM) research. The amelioration of powerful multicarrier OFDM arrangements with multiple-input multiple-output (MIMO) systems has numerous benefits, which are detailed in this treatise. We continue by highlighting the limitations of conventional detection and channel estimation techniques designed for multiuser MIMO OFDM systems in the so-called rank-deficient scenarios, where the number of users supported or the number of transmit antennas employed exceeds the number of receiver antennas. This is often encountered in practice, unless we limit the number of users granted access in the base station’s or radio port’s coverage area. Following a historical perspective on the associated design problems and their state-of-the-art solutions, the second half of this treatise details a range of classic multiuser detectors (MUDs) designed for MIMO-OFDM systems and characterizes their achievable performance. A further section aims for identifying novel cutting-edge genetic algorithm (GA)-aided detector solutions, which have found numerous applications in wireless communications in recent years. In an effort to stimulate the cross pollination of ideas across the machine learning, optimization, signal processing, and wireless communications research communities, we will review the broadly applicable principles of various GA-assisted optimization techniques, which were recently proposed also for employment inmultiuser MIMO OFDM. In order to stimulate new research, we demonstrate that the family of GA-aided MUDs is capable of achieving a near-optimum performance at the cost of a significantly lower computational complexity than that imposed by their optimum maximum-likelihood (ML) MUD aided counterparts. The paper is concluded by outlining a range of future research options that may find their way into next-generation wireless systems
MIMO Transmission with Residual Transmit-RF Impairments
Physical transceiver implementations for multiple-input multiple-output
(MIMO) wireless communication systems suffer from transmit-RF (Tx-RF)
impairments. In this paper, we study the effect on channel capacity and
error-rate performance of residual Tx-RF impairments that defy proper
compensation. In particular, we demonstrate that such residual distortions
severely degrade the performance of (near-)optimum MIMO detection algorithms.
To mitigate this performance loss, we propose an efficient algorithm, which is
based on an i.i.d. Gaussian model for the distortion caused by these
impairments. In order to validate this model, we provide measurement results
based on a 4-stream Tx-RF chain implementation for MIMO orthogonal
frequency-division multiplexing (OFDM).Comment: to be presented at the International ITG Workshop on Smart Antennas -
WSA 201
Design and evaluation of OFDM radio interfaces for high mobility communications
[Resumo]
Nas dúas últimas décadas, as modulacións multiportadora emerxeron como una solución de
baixa complexidade para combatir os efectos do multitraxecto en comuniacións sen fíos. Entre
elas, Orthogonal Frequency Division Multiplexing (OFOM) é posiblemente o esquema de
modulación máis estudado, e tamén amplamente adoptado como alicerce de estándares da
industria como WiMAX ou LTE. Sen embargo, OFDM é sensible a canles que varian ca tempo,
unha característica dos escenarios con mobilidade, debido á aparición da interferencia entre
portadoras (ICI).
A implementación de equipamento hardware para o usuario final faise normalmente en
chips dedicados, afnda que entornos de investigación, prefírense solucións máis flexibles. Unha
aproximación popular é a coñecida como Software Defined Radio (SOR), onde os algoritmos de
procesado de sinal se implementan en hardware reconfigurable como Digital Signal Processors
(OSPs) e Field Programmable Gate Arrays (FPGAs).
O obxectivo deste traballo é dobre. Por un lado, definir unha arquitectura para
implementacións de tempo real de capas físicas basadas en OFDM usando como referencia
O estándar WiMAX, probada Dunha plataforma composta por OSPs e FPGAs. Por outra banda,
estudar os efectos da selectividade en tempo no sinal OFDM, definindo métodos de estimación
de canle que teñen en conta a ICI, e evaluándoos tanto en simulación como con medidas
experimentais. Seguíronse dúas aproximacións para caracterizar o comportamento de formas de
onda OFDM baixo condicións de mobilidade, unha basada nun emulador de canle que traballa
en tempo real, e outra en inducir grandes ensanchamentos Doppler no sinal mediante a extensión
da duración do símbolo OFOM.[Resumen]
En las dos últimas décadas, las modulaciones multiportadora han emergido como una
solución de baja complejidad para combatir los efectos del multitrayecto en comunicaciones
iDalámbricas. Entre ellas, Orthogonal Frequency Division Mulriplexing (OFDM) es
posiblemente el esquema de modulación más estudiado, y también ampliamente adoptado
como fundamento de estándares de la industria como WiMAX o LTE. Sin embargo, OFDM es
sensible a canales que varían con el tiempo, una característica de los escenarios coo movilidad,
debido a la aparicióo de la interferencia entre portadoras (ICI).
La implementación de equipamiento hardware para el usuario final se hace normalmente en
chips dedicados, aunque eo entornos de investigación, son preferibles soluciones más Hexibles.
Una aproximación popular es la conocida como Software Defined Radio (SDR), donde los
algOritmos de procesado de señal se implementan en hardware reconfigurable como Digital
Signa! Processors (DSPs) y Field Programmable Gate AIrays (FPGAs).
El objetivo de este trabajo es doble. Por un lado. definir una arquitectura para
implementaciones de tiempo real de capas ¡lSicas basadas en OFDM usando como referencia
el estándar WiMAX, probada en una plataforma compuesta por DSPs y FPGAs. Por otro
lado, estudiar los efectos de la selectividad en tiempo en la señal OFDM, definiendo métodos
de estimacióo de canal que tengan eo cueota la ICI, y evaluándolos tanto en simulación
como con medidas experimenta1es. Se han seguido dos aproximaciones para caracterizar el
comportamiento de formas de onda OFDM bajo condiciones de mobilidad, una basada en
un emulador de canal que trabaja en tiempo real. y otra en inducir grandes ensanchamientos
Doppler en la señal mediante la extensión de la duración del símbolo OFDM.[Abstract]
In Ihe last two decades, multicarrier modulations have emerged as a low complexity solulion
to combal the effects of Ihe multipalh in wireless communicalions. Among Ihem, Orthogonal
Frequency Division Mulliplexing (OFOM) is possibly Ihe mosl sludied modulation scheme,
and has a1so been widely adopted as Ihe foundation of induslry standards such as WiMAX or
LTE. However, OFOM is sensitive lo time selective channels, which are featured in mobility
scenarlos, due lO Ihe appearance of Inler-Carrier Interference (ICI).
Implemenlation of hardware equipmenl for Ihe end user is usually implemenled in dedicaled
chips, bul in researeh environments, more flexible solutions are preferred. One popular
approach is the so ealled Software Defined Radio (SOR), where the signal processing
a1gorithms are implemented in reconfigurable hardware sueh as Digital Signal Processors
(DSPs) and Field Prograrnmable Gate Arrays (FPGAs).
The aim of Ibis work is two-fold. On the one hand, to define an architeclure for Ihe
implementation of real-time OFOM-based physical layers, using as a reference Ihe WiMAX
standard, and it is tested on a platform composed by DSPs and FPGAs. On the olher hand,
to study Ihe effeets of !he time seleetivity on !he OFOM signal, defining channel estimation
me!hods aware of !he ICI, and ils evaluation bo!h in simulation as well as experimental
measuremenls. Two approaches have been followed to assess the behavior of OFOM waveforms
under mobility conditions, one based on a real-time channel emulator, and the other on inducing
large Doppler spreads in !he signal by extending the duration of Ihe OFDM symbols
Adaptive Bit Allocation With Reduced Feedback for Wireless Multicarrier Transceivers
With the increasing demand in the wireless mobile applications came a growing need to transmit information quickly and accurately, while consuming more and more bandwidth. To address this need, communication engineers started employing multicarrier modulation in their designs, which is suitable for high data rate transmission. Multicarrier modulation reduces the system's susceptibility to the frequency-selective fading channel, by transforming it into a collection of approximately flat subchannels. As a result, this makes it easier to compensate for the distortion introduced by the channel. This thesis concentrates on techniques for saving bandwidth usage when employing adaptive multicarrier modulation, where subcarrier parameters (bit and energy allocations) are modulated based on the channel state information feedback obtained from previous burst. Although bit and energy allocations can substantially increase error robustness and throughput of the system, the feedback information required at both ends of the transceiver can be large. The objective of this work is to compare different feedback compression techniques that could reduce the amount of feedback information required to perform adaptive bit and energy allocation in multicarrier transceivers. This thesis employs an approach for reducing the number of feedback transmissions by exploiting the time-correlation properties of a wireless channel and placing a threshold check on bit error rate (BER) values. Using quantization and source coding techniques, such as Huffman coding, Run length encoding and LZWalgorithms, the amount of feedback information has been compressed. These calculations have been done for different quantization levels to understand the relationship between quantization levels and system performance. These techniques have been applied to both OFDM and MIMO-OFDM systems
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