241 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
Design guidelines for spatial modulation
A new class of low-complexity, yet energyefficient Multiple-Input Multiple-Output (MIMO) transmission techniques, namely the family of Spatial Modulation (SM) aided MIMOs (SM-MIMO) has emerged. These systems are capable of exploiting the spatial dimensions (i.e. the antenna indices) as an additional dimension invoked for transmitting information, apart from the traditional Amplitude and Phase Modulation (APM). SM is capable of efficiently operating in diverse MIMO configurations in the context of future communication systems. It constitutes a promising transmission candidate for large-scale MIMO design and for the indoor optical wireless communication whilst relying on a single-Radio Frequency (RF) chain. Moreover, SM may also be viewed as an entirely new hybrid modulation scheme, which is still in its infancy. This paper aims for providing a general survey of the SM design framework as well as of its intrinsic limits. In particular, we focus our attention on the associated transceiver design, on spatial constellation optimization, on link adaptation techniques, on distributed/ cooperative protocol design issues, and on their meritorious variants
Performance analysis of collaborative hybrid-arq protocols over fading channels
Impairments due to multipath signal propagation on the performance of wireless communications systems can be efficiently mitigated with time, frequency or spatial diversity. To exploit spatial diversity, multiple-antenna technology has been thoroughly investigated and emerged as one of the most mature communications areas. However, the need for smaller user terminals, which results in insufficient spacing for antenna collocation, tends to limit the practical implementation of this technology. Without compromising terminal dimensions, future wireless networks will therefore have to exploit their broadcast nature and rely on collaboration between single-antenna terminals for exploiting spatial diversity.
Among cooperative schemes, Collaborative ARQ transmission protocols, prescribing cooperation only when needed, i.e., upon erroneous decoding by the destination, emerge as an interesting solution in terms of achievable spectral efficiency. In this thesis, an information theoretical approach is presented for assessing the performance of Collaborative Hybrid-ARQ protocols based on Space-Time Block Coding. The expected number of retransmissions and the average throughput for Collaborative Hybrid-ARQ Type I and Chase Combining are derived in explicit form, while lower and upper bound are investigated for Collaborative Hybrid-ARQ Incremental Redundancy protocol, for any number of relays. Numerical results are presented to supplement the analysis and give insight into the performance of the considered scheme. Moreover, the issue of practical implementation of Space-Time Block Coding is investigated
Adaptive Communications for Next Generation Broadband Wireless Access Systems
Un dels aspectes claus en el disseny i gestiΓ³ de les xarxes sense fils d'accΓ©s de banda ampla Γ©s l'ΓΊs eficient dels recursos radio. Des del punt de vista de l'operador, l'ample de banda Γ©s un bΓ© escΓ s i preuat que sΒ΄ha d'explotar i gestionar de la forma mΓ©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 aixΓ un baix retard i una baixa pΓ¨rdua de paquets per cadascun dels fluxos de dades entre la xarxa i l'usuari. Durant els darrers anys sΒ΄han desenvolupat nombroses tΓ¨cniques i algoritmes amb l'objectiu d'incrementar l'eficiΓ¨ncia espectral. Entre aquestes tΓ¨cniques destaca l'ΓΊs de mΓΊltiples 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 optimizaciΓ³ conjunta de la capa d'accΓ©s al medi i la capa fΓsica (fent ΓΊs de l'estat del canal per tal de gestionar de manera optima els recursos) tambΓ© permet incrementar sensiblement l'eficiΓ¨ncia espectral del sistema.L'objectiu d'aquesta tesi Γ©s l'estudi i desenvolupament de noves tΓ¨cniques d'adaptaciΓ³ de l'enllaΓ§ i gestiΓ³ dels recursos rΓ dio aplicades sobre sistemes d'accΓ©s rΓ dio de propera generaciΓ³ (Beyond 3G). Els estudis realitzats parteixen de la premissa que el transmisor coneix (parcialment) l'estat del canal i que la transmissiΓ³ es realitza fent servir un esquema multiportadora amb mΓΊltiples antenes al transmisor i al receptor. En aquesta tesi es presenten dues lΓnies d'investigaciΓ³, la primera per casos d'una sola antenna a cada banda de l'enllaΓ§, i la segona en cas de mΓΊltiples antenes. En el cas d'una sola antena al transmissor i al receptor, un nou esquema d'assignaciΓ³ de recursos rΓ dio i prioritzaciΓ³ dels paquets (scheduling) Γ©s proposat i analitzat integrant totes dues funcions sobre una mateixa entitat (cross-layer). L'esquema proposat tΓ© com a principal caracterΓstica la seva baixa complexitat i que permet operar amb transmissions multimedia. Alhora, posteriors millores realitzades per l'autor sobre l'esquema proposat han permΓ¨s tambΓ© reduir els requeriments de senyalitzaciΓ³ i combinar de forma Γ³ptima usuaris d'alta i baixa mobilitat sobre el mateix accΓ©s rΓ dio, millorant encara mΓ©s l'eficiΓ¨ncia espectral del sistema. En cas d'enllaΓ§os amb mΓΊltiples antenes es proposa un nou esquema que combina la selecciΓ³ del conjunt optim d'antenes transmissores amb la selecciΓ³ de la codificaciΓ³ espai- (frequΓ¨ncia-) temps. Finalment es donen una sΓ¨rie de recomanacions per tal de combinar totes dues lΓnies d'investigaciΓ³, aixΓ con un estat de l'art de les tΓ¨cniques proposades per altres autors que combinen en part la gestiΓ³ dels recursos rΓ dio i els esquemes de transmissiΓ³ amb mΓΊltiples antenes.Uno de los aspectos claves en el diseΓ±o y gestiΓ³n de las redes inalΓ‘mbricas 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 mΓ‘s 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 asΓ 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 ΓΊltimos aΓ±os el nΓΊmero de tΓ©cnicas y algoritmos que tratan de incrementar la eficiencia espectral en dichas redes es bastante amplio. Entre estas tΓ©cnicas destaca el uso de mΓΊltiples antenas en el transmisor y en el receptor con el objetivo de poder transmitir simultΓ‘neamente diferentes flujos de datos sin necesidad de incrementar el ancho de banda. Por otro lado, la optimizaciΓ³n conjunta de la capa de acceso al medio y la capa fΓsica (utilizando informaciΓ³n de estado del canal para gestionar de manera Γ³ptima los recursos) tambiΓ©n permite incrementar sensiblemente la eficiencia espectral del sistema.El objetivo de esta tesis es el estudio y desarrollo de nuevas tΓ©cnicas de adaptaciΓ³n del enlace y la gestiΓ³n de los recursos radio, y su posterior aplicaciΓ³n sobre los sistemas de acceso radio de prΓ³xima generaciΓ³n (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 transmisiΓ³n se realiza sobre un sistema de transmisiΓ³n multiportadora con mΓΊltiple antenas en el transmisor y el receptor. La tesis se centra sobre dos lΓneas de investigaciΓ³n, la primera para casos de una ΓΊnica antena en cada lado del enlace, y la segunda en caso de mΓΊltiples antenas en cada lado. Para el caso de una ΓΊnica antena en el transmisor y en el receptor, se ha desarrollado un nuevo esquema de asignaciΓ³n de los recursos radio asΓ como de priorizaciΓ³n de los paquetes de datos (scheduling) integrando ambas funciones sobre una misma entidad (cross-layer). El esquema propuesto tiene como principal caracterΓstica 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 tambiΓ©n reducir los requisitos de seΓ±alizaciΓ³n asΓ como combinar de forma Γ³ptima usuarios de alta y baja movilidad. Por otro lado, en caso de enlaces con mΓΊltiples antenas en transmisiΓ³n y recepciΓ³n, se presenta un nuevo esquema de adaptaciΓ³n en el cual se combina la selecciΓ³n de la(s) antena(s) transmisora(s) con la selecciΓ³n del esquema de codificaciΓ³n espacio-(frecuencia-) tiempo. Para finalizar, se dan una serie de recomendaciones con el objetivo de combinar ambas lΓneas de investigaciΓ³n, asΓ como un estado del arte de las tΓ©cnicas propuestas por otros autores que combinan en parte la gestiΓ³n de los recursos radio y los esquemas de transmisiΓ³n con mΓΊltiples 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
Near-capacity MIMOs using iterative detection
In this thesis, Multiple-Input Multiple-Output (MIMO) techniques designed for transmission over narrowband Rayleigh fading channels are investigated. Specifically, in order to providea diversity gain while eliminating the complexity of MIMO channel estimation, a Differential Space-Time Spreading (DSTS) scheme is designed that employs non-coherent detection. Additionally, in order to maximise the coding advantage of DSTS, it is combined with Sphere Packing (SP) modulation. The related capacity analysis shows that the DSTS-SP scheme exhibits a higher capacity than its counterpart dispensing with SP. Furthermore, in order to attain additional performance gains, the DSTS system invokes iterative detection, where the outer code is constituted by a Recursive Systematic Convolutional (RSC) code, while the inner code is a SP demapper in one of the prototype systems investigated, while the other scheme employs a Unity Rate Code (URC) as its inner code in order to eliminate the error floor exhibited by the system dispensing with URC. EXIT charts are used to analyse the convergence behaviour of the iteratively detected schemes and a novel technique is proposed for computing the maximum achievable rate of the system based on EXIT charts. Explicitly, the four-antenna-aided DSTSSP system employing no URC precoding attains a coding gain of 12 dB at a BER of 10-5 and performs within 1.82 dB from the maximum achievable rate limit. By contrast, the URC aidedprecoded system operates within 0.92 dB from the same limit.On the other hand, in order to maximise the DSTS systemβs throughput, an adaptive DSTSSP scheme is proposed that exploits the advantages of differential encoding, iterative decoding as well as SP modulation. The achievable integrity and bit rate enhancements of the system are determined by the following factors: the specific MIMO configuration used for transmitting data from the four antennas, the spreading factor used and the RSC encoderβs code rate.Additionally, multi-functional MIMO techniques are designed to provide diversity gains, multiplexing gains and beamforming gains by combining the benefits of space-time codes, VBLASTand beamforming. First, a system employing Nt=4 transmit Antenna Arrays (AA) with LAA number of elements per AA and Nr=4 receive antennas is proposed, which is referred to as a Layered Steered Space-Time Code (LSSTC). Three iteratively detected near-capacity LSSTC-SP receiver structures are proposed, which differ in the number of inner iterations employed between the inner decoder and the SP demapper as well as in the choice of the outer code, which is either an RSC code or an Irregular Convolutional Code (IrCC). The three systems are capable of operating within 0.9, 0.4 and 0.6 dB from the maximum achievable rate limit of the system. A comparison between the three iteratively-detected schemes reveals that a carefully designed two-stage iterative detection scheme is capable of operating sufficiently close to capacity at a lower complexity, when compared to a three-stage system employing a RSC or a two-stage system using an IrCC as an outer code. On the other hand, in order to allow the LSSTC scheme to employ less receive antennas than transmit antennas, while still accommodating multiple users, a Layered Steered Space-Time Spreading (LSSTS) scheme is proposed that combines the benefits of space-time spreading, V-BLAST, beamforming and generalised MC DS-CDMA. Furthermore, iteratively detected LSSTS schemes are presented and an LLR post-processing technique is proposed in order to improve the attainable performance of the iteratively detected LSSTS system.Finally, a distributed turbo coding scheme is proposed that combines the benefits of turbo coding and cooperative communication, where iterative detection is employed by exchanging extrinsic information between the decoders of different single-antenna-aided users. Specifically, the effect of the errors induced in the first phase of cooperation, where the two users exchange their data, on the performance of the uplink in studied, while considering different fading channel characteristics
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 stationβs or radio portβs 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
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