389 research outputs found
Initial synchronisation of wideband and UWB direct sequence systems: single- and multiple-antenna aided solutions
This survey guides the reader through the open literature on the principle of initial synchronisation in single-antenna-assisted single- and multi-carrier Code Division Multiple Access (CDMA) as well as Direct Sequence-Ultra WideBand (DS-UWB) systems, with special emphasis on the DownLink (DL). There is a paucity of up-to-date surveys and review articles on initial synchronization solutions for MIMO-aided and cooperative systems - even though there is a plethora of papers on both MIMOs and on cooperative systems, which assume perfect synchronization. Hence this paper aims to ?ll the related gap in the literature
High Capacity CDMA and Collaborative Techniques
The thesis investigates new approaches to increase the user capacity and improve the error
performance of Code Division Multiple Access (CDMA) by employing adaptive interference cancellation
and collaborative spreading and space diversity techniques. Collaborative Coding Multiple
Access (CCMA) is also investigated as a separate technique and combined with CDMA. The
advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the
uplink and downlink are proposed and evaluated.
Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The
practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive
approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM)
property of digital signals to blindly suppress interference during the despreading process and obtain
amplitude estimation with minimum mean squared error for use in cancellation stages. Two
new blind adaptive receiver designs employing successive and parallel interference cancellation
architectures using the CM algorithm (CMA) referred to as âCMA-SICâ and âBA-PICâ, respectively,
are presented. These techniques have shown to offer near single user performance for large
number of users. It is shown to increase the user capacity by approximately two fold compared
with conventional IC receivers. The spectral efficiency analysis of the techniques based on output
signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore,
an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a
simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI
without any knowledge of channel amplitudes and allows large number of users compared with
equal gain and maximum ratio combining techniques normally used in practice.
New user collaborative schemes are proposed and analysed theoretically and by simulations
in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First,
a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA
are designed and analysed. Next, a new user collaborative scheme with successive interference
cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce
MAI and achieve improved diversity. To further improve the performance of C-SIC under high
system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed.
It is shown to minimize the residual MAI, leading to improved user capacity and a more robust
system. It is known that collaborative diversity schemes incur loss in throughput due to the need of
orthogonal time/frequency slots for relaying sourceâs data. To address this problem, finally a novel
near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time
encoding methods. At the receiver collaborative joint detection is performed to separate each
paired usersâ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth
as inter-user channel SNR becomes high.
A novel approach of âUser Collaborationâ is introduced to increase the user capacity of CDMA
for both the downlink and uplink. First, collaborative group spreading technique for the downlink
of overloaded CDMA system is introduced. It allows the sharing of the same single spreading
sequence for more than one user belonging to the same group. This technique is referred to as
Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collaborative
coding is used for each group to form a composite codeword signal of the users and then a
single orthogonal sequence is used for the group. At each userâs receiver, decoding of composite
codeword is carried out to extract the userâs own information while maintaining a high SINR performance.
To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions,
Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collaborative
coding multiple access and space-time coding principles. A new scheme for uplink of CDMA
using the âUser Collaborationâ approach, referred to as CS-CDMA-UL is presented next. When
usersâ channels are independent (uncorrelated), significantly higher user capacity can be achieved
by grouping multiple users to share the same spreading sequence and performing MUD on per
group basis followed by a low complexity ML decoding at the receiver. This approach has shown
to support much higher number of users than the available sequences while also maintaining the
low receiver complexity. For improved performance under highly correlated channel conditions,
T-user collaborative coding is also investigated within the CS-CDMA-UL system
Capacity, coding and interference cancellation in multiuser multicarrier wireless communications systems
Multicarrier modulation and multiuser systems have generated a great deal of research during the last decade. Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation generated with the inverse Discrete Fourier Transform, which has been adopted for standards in wireless and wire-line communications. Multiuser wireless systems using multicarrier modulation suffer from the effects of dispersive fading channels, which create multi-access, inter-symbol, and inter-carrier interference (MAI, ISI, ICI). Nevertheless, channel dispersion also provides diversity, which can be exploited and has the potential to increase robustness against fading. Multiuser multi-carrier systems can be implemented using Orthogonal Frequency Division Multiple Access (OFDMA), a flexible orthogonal multiplexing scheme that can implement time and frequency division multiplexing, and using multicarrier code division multiple access (MC-CDMA). Coding, interference cancellation, and resource sharing schemes to improve the performance of multiuser multicarrier systems on wireless channels were addressed in this dissertation.
Performance of multiple access schemes applied to a downlink multiuser wireless system was studied from an information theory perspective and from a more practical perspective. For time, frequency, and code division, implemented using OFDMA and MC-CDMA, the system outage capacity region was calculated for a correlated fading channel. It was found that receiver complexity determines which scheme offers larger capacity regions, and that OFDMA results in a better compromise between complexity and performance than MC-CDMA. From the more practical perspective of bit error rate, the effects of channel coding and interleaving were investigated. Results in terms of coding bounds as well as simulation were obtained, showing that OFDMAbased orthogonal multiple access schemes are more sensitive to the effectiveness of the code to provide diversity than non-orthogonal, MC-CDMA-based schemes.
While cellular multiuser schemes suffer mainly from MAI, OFDM-based broadcasting systems suffer from ICI, in particular when operating as a single frequency network (SFN). It was found that for SFN the performance of a conventional OFDM receiver rapidly degrades when transmitters have frequency synchronization errors. Several methods based on linear and decision-feedback ICI cancellation were proposed and evaluated, showing improved robustness against ICI.
System function characterization of time-variant dispersive channels is important for understanding their effects on single carrier and multicarrier modulation. Using time-frequency duality it was shown that MC-CDMA and DS-CDMA are strictly dual on dispersive channels. This property was used to derive optimal matched filter structures, and to determine a criterion for the selection of spreading sequences for both DS and MC CDMA. The analysis of multiple antenna systems provided a unified framework for the study of DS-CDMA and MC-CDMA on time and frequency dispersive channels, which can also be used to compare their performance
Adaptive Communications for Next Generation Broadband Wireless Access Systems
Un dels aspectes claus en el disseny i gestiĂł de les xarxes sense fils d'accĂ©s de banda ampla Ă©s l'Ășs eficient dels recursos radio. Des del punt de vista de l'operador, l'ample de banda Ă©s un bĂ© escĂ s i preuat que sÂŽha d'explotar i gestionar de la forma mĂ©s eficient possible tot garantint la qualitat del servei que es vol proporcionar. Per altra banda, des del punt de vista del usuari, la qualitat del servei ofert ha de ser comparable al de les xarxes fixes, requerint aixĂ un baix retard i una baixa pĂšrdua de paquets per cadascun dels fluxos de dades entre la xarxa i l'usuari. Durant els darrers anys sÂŽhan desenvolupat nombroses tĂšcniques i algoritmes amb l'objectiu d'incrementar l'eficiĂšncia espectral. Entre aquestes tĂšcniques destaca l'Ășs de mĂșltiples antenes al transmissor i al receptor amb l'objectiu de transmetre diferents fluxos de dades simultaneament sense necessitat d'augmentar l'ample de banda. Per altra banda, la optimizaciĂł conjunta de la capa d'accĂ©s al medi i la capa fĂsica (fent Ășs de l'estat del canal per tal de gestionar de manera optima els recursos) tambĂ© permet incrementar sensiblement l'eficiĂšncia espectral del sistema.L'objectiu d'aquesta tesi Ă©s l'estudi i desenvolupament de noves tĂšcniques d'adaptaciĂł de l'enllaç i gestiĂł dels recursos rĂ dio aplicades sobre sistemes d'accĂ©s rĂ dio de propera generaciĂł (Beyond 3G). Els estudis realitzats parteixen de la premissa que el transmisor coneix (parcialment) l'estat del canal i que la transmissiĂł es realitza fent servir un esquema multiportadora amb mĂșltiples antenes al transmisor i al receptor. En aquesta tesi es presenten dues lĂnies d'investigaciĂł, la primera per casos d'una sola antenna a cada banda de l'enllaç, i la segona en cas de mĂșltiples antenes. En el cas d'una sola antena al transmissor i al receptor, un nou esquema d'assignaciĂł de recursos rĂ dio i prioritzaciĂł dels paquets (scheduling) Ă©s proposat i analitzat integrant totes dues funcions sobre una mateixa entitat (cross-layer). L'esquema proposat tĂ© com a principal caracterĂstica la seva baixa complexitat i que permet operar amb transmissions multimedia. Alhora, posteriors millores realitzades per l'autor sobre l'esquema proposat han permĂšs tambĂ© reduir els requeriments de senyalitzaciĂł i combinar de forma Ăłptima usuaris d'alta i baixa mobilitat sobre el mateix accĂ©s rĂ dio, millorant encara mĂ©s l'eficiĂšncia espectral del sistema. En cas d'enllaços amb mĂșltiples antenes es proposa un nou esquema que combina la selecciĂł del conjunt optim d'antenes transmissores amb la selecciĂł de la codificaciĂł espai- (frequĂšncia-) temps. Finalment es donen una sĂšrie de recomanacions per tal de combinar totes dues lĂnies d'investigaciĂł, aixĂ con un estat de l'art de les tĂšcniques proposades per altres autors que combinen en part la gestiĂł dels recursos rĂ dio i els esquemes de transmissiĂł amb mĂșltiples antenes.Uno de los aspectos claves en el diseño y gestiĂłn de las redes inalĂĄmbricas de banda ancha es el uso eficiente de los recursos radio. Desde el punto de vista del operador, el ancho de banda es un bien escaso y valioso que se debe explotar y gestionar de la forma mĂĄs eficiente posible sin afectar a la calidad del servicio ofrecido. Por otro lado, desde el punto de vista del usuario, la calidad del servicio ha de ser comparable al ofrecido por las redes fijas, requiriendo asĂ un bajo retardo y una baja tasa de perdida de paquetes para cada uno de los flujos de datos entre la red y el usuario. Durante los Ășltimos años el nĂșmero de tĂ©cnicas y algoritmos que tratan de incrementar la eficiencia espectral en dichas redes es bastante amplio. Entre estas tĂ©cnicas destaca el uso de mĂșltiples antenas en el transmisor y en el receptor con el objetivo de poder transmitir simultĂĄneamente diferentes flujos de datos sin necesidad de incrementar el ancho de banda. Por otro lado, la optimizaciĂłn conjunta de la capa de acceso al medio y la capa fĂsica (utilizando informaciĂłn de estado del canal para gestionar de manera Ăłptima los recursos) tambiĂ©n permite incrementar sensiblemente la eficiencia espectral del sistema.El objetivo de esta tesis es el estudio y desarrollo de nuevas tĂ©cnicas de adaptaciĂłn del enlace y la gestiĂłn de los recursos radio, y su posterior aplicaciĂłn sobre los sistemas de acceso radio de prĂłxima generaciĂłn (Beyond 3G). Los estudios realizados parten de la premisa de que el transmisor conoce (parcialmente) el estado del canal a la vez que se considera que la transmisiĂłn se realiza sobre un sistema de transmisiĂłn multiportadora con mĂșltiple antenas en el transmisor y el receptor. La tesis se centra sobre dos lĂneas de investigaciĂłn, la primera para casos de una Ășnica antena en cada lado del enlace, y la segunda en caso de mĂșltiples antenas en cada lado. Para el caso de una Ășnica antena en el transmisor y en el receptor, se ha desarrollado un nuevo esquema de asignaciĂłn de los recursos radio asĂ como de priorizaciĂłn de los paquetes de datos (scheduling) integrando ambas funciones sobre una misma entidad (cross-layer). El esquema propuesto tiene como principal caracterĂstica su bajo coste computacional a la vez que se puede aplicar en caso de transmisiones multimedia. Posteriores mejoras realizadas por el autor sobre el esquema propuesto han permitido tambiĂ©n reducir los requisitos de señalizaciĂłn asĂ como combinar de forma Ăłptima usuarios de alta y baja movilidad. Por otro lado, en caso de enlaces con mĂșltiples antenas en transmisiĂłn y recepciĂłn, se presenta un nuevo esquema de adaptaciĂłn en el cual se combina la selecciĂłn de la(s) antena(s) transmisora(s) con la selecciĂłn del esquema de codificaciĂłn espacio-(frecuencia-) tiempo. Para finalizar, se dan una serie de recomendaciones con el objetivo de combinar ambas lĂneas de investigaciĂłn, asĂ como un estado del arte de las tĂ©cnicas propuestas por otros autores que combinan en parte la gestiĂłn de los recursos radio y los esquemas de transmisiĂłn con mĂșltiples antenas.In Broadband Wireless Access systems the efficient use of the resources is crucial from many points of views. From the operator point of view, the bandwidth is a scarce, valuable, and expensive resource which must be exploited in an efficient manner while the Quality of Service (QoS) provided to the users is guaranteed. On the other hand, a tight delay and link quality constraints are imposed on each data flow hence the user experiences the same quality as in fixed networks. During the last few years many techniques have been developed in order to increase the spectral efficiency and the throughput. Among them, the use of multiple antennas at the transmitter and the receiver (exploiting spatial multiplexing) with the joint optimization of the medium access control layer and the physical layer parameters.In this Ph.D. thesis, different adaptive techniques for B3G multicarrier wireless systems are developed and proposed focusing on the SS-MC-MA and the OFDM(A) (IEEE 802.16a/e/m standards) communication schemes. The research lines emphasize into the adaptation of the transmission having (Partial) knowledge of the Channel State Information for both; single antenna and multiple antenna links. For single antenna links, the implementation of a joint resource allocation and scheduling strategy by including adaptive modulation and coding is investigated. A low complexity resource allocation and scheduling algorithm is proposed with the objective to cope with real- and/or non-real- time requirements and constraints. A special attention is also devoted in reducing the required signalling. However, for multiple antenna links, the performance of a proposed adaptive transmit antenna selection scheme jointly with space-time block coding selection is investigated and compared with conventional structures. In this research line, mainly two optimizations criteria are proposed for spatial link adaptation, one based on the minimum error rate for fixed throughput, and the second focused on the maximisation of the rate for fixed error rate. Finally, some indications are given on how to include the spatial adaptation into the investigated and proposed resource allocation and scheduling process developed for single antenna transmission
Vectorial channel estimation for uplink MC-CDMA in beyond 3G wireless systems
In beyond 3G wireless systems the bandwidth
efficiency can be increased with the use of adaptive antenna
arrays. This paper focus on a key issue for adaptive antenna
arrays, that is, channel parameters estimation including
Direction-Of-Arrival (DOA). In order to estimate DOA. the
channel frequency responses for the links between the mobile
users and each of base station array elements are estimated by
pilot-aided minimum mean square error (MMSE) algorithm.
This estimator is not sensitive to the channel statistics. Based on
frequency response estimatives DOA's of impinging multipath
components to the base station are estimated by a low complex
Wlauimuni Likelihood (ML) approach. Furthermore an uplink
burst structure with specifically designed midamble field for multiuser
channel estimation in MC-CDMA is proposed. The performance is
assessed in terms of channel estimation errors for a MC-CDMA
TDD system over fast and slow fading mobile channels
Multicarrier communication systems with low sensibility to nonlinear amplification
Actualment estem entrant a una nova era de la informaciĂł amb gran demanda de sistemes de comunicaciĂł sense fils. Nous serveis com dades i video requereixen transmissions fiables d'alta velocitat, fins i tot en escenaris d'alta mobilitat. A mĂ©s a mĂ©s, la dificultat d'assignar el limitat espectre radioelĂšctric juntament amb la necessitat d'incrementar el temps de vida de les bateries dels terminals mĂČbils, requereix el diseny de transceptors que usin la potĂšncia i l'ampla de banda disponibles de manera eficient. Les comunicacions multiportadora basades en OFDM sĂłn capaces de satisfer la majoria d'aquests requeriments. PerĂČ, entre altres reptes, reduir la sensibilitat a la amplificaciĂł no-lineal Ă©s un factor clau durant el diseny. En aquesta tesi doctoral s'analitza la sensibilitat dels sistemes multiportadora basats en OFDM a l'amplificaciĂł no-lineal i es consideren formes eficients per superar aquest problema. La tesi s'enfoca principalment al problema de reduir les fluctuacions de l'envolupant del senyal transmĂšs. En aquest sentit es presenta tambĂ© un estudi de les mĂštriques de l'envolupant del senyal, PAPR i CM. A mĂ©s a mĂ©s, basant-nos en l'anĂ lisis presentat es proposen noves tĂšcniques per sistemes OFDM i MC-SS. Per MC-SS, tambĂ© es tracta el diseny d'una tĂšcnica de postprocessament en forma de detector multiusuari per canals no-lineals.Actualmente estamos entrando en una nueva era de la informaciĂłn donde se da una gran demanda de sistemas de comunicaciĂłn inalĂĄmbricos. Nuevos servicios como datos y vĂdeo requieren transmisiones fiables de alta velocidad, incluso en escenarios de alta movilidad. AdemĂĄs, la dificultad de asignar el limitado espectro radioelĂ©ctrico junto con la necesidad de incrementar el tiempo de vida de las baterĂas de los terminales mĂłviles, requiere el diseño de transceptores que usen eficientemente la potencia y el ancho de banda disponibles. Las comunicaciones multiportadora basadas en OFDM son capaces de satisfacer la mayorĂa de dichos requerimientos. Sin embargo, entre otros retos, reducir su sensibilidad a la amplificaciĂłn no-lineal es un factor clave durante el diseño. En esta tesis se analiza la sensibilidad de los sistemas multiportadora basados en OFDM a la amplificaciĂłn no-lineal y se consideran formas eficientes para superar dicho problema. La tesis se enfoca principalmente al problema de reducir las fluctuaciones de la envolvente. En este sentido tambiĂ©n se presenta un estudio de las mĂ©tricas de la señal, PAPR y CM. AdemĂĄs, basĂĄndonos en el anĂĄlisis presentado se proponen nuevas tĂ©cnicas para OFDM y MC-SS. Para MC-SS, tambiĂ©n se trata el diseño de un detector multiusuario para canales no-lineales.We are now facing a new information age with high demand of wireless communication systems. New services such as data and video require achieving reliable high-speed transmissions even in high mobility scenarios. Moreover, the difficulty to allocate so many wireless communication systems in the limited frequency band in addition to the demand for long battery life requires designing spectrum and power efficient transceivers. Multicarrier communications based on OFDM are known to fulfill most of the requirements of such systems. However, among other challenges, reducing the sensitivity to nonlinear amplification has become a design key. In this thesis the sensitivity of OFDM-based multicarrier systems to nonlinear amplification is analyzed and efficient ways to overcome this problem are considered. The focus is mainly on the problem of reducing the envelope fluctuations. Therefore, a study of the signal metrics, namely PAPR and CM, is also presented. From the presented analysis, several new techniques for OFDM and MC-SS are proposed. For MC-SS, the design of a post-processing technique in the form of a multiuser detector for nonlinearly distorted MC-SS symbols is also addressed
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