512 research outputs found

    Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems

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    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

    Multiuser MIMO-OFDM for Next-Generation Wireless Systems

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    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

    A Linear Multi-User Detector for STBC MC-CDMA Systems based on the Adaptive Implementation of the Minimum-Conditional Bit-Error-Rate Criterion and on Genetic Algorithm-assisted MMSE Channel Estimation

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    The implementation of efficient baseband receivers characterized by affordable computational load is a crucial point in the development of transmission systems exploiting diversity in different domains. In this paper, we are proposing a linear multi-user detector for MIMO MC-CDMA systems with Alamouti’s Space-Time Block Coding, inspired by the concept of Minimum Conditional Bit-Error-Rate (MCBER) and relying on Genetic-Algorithm (GA)-assisted MMSE channel estimation. The MCBER combiner has been implemented in adaptive way by using Least-Mean-Square (LMS) optimization. Firstly, we shall analyze the proposed adaptive MCBER MUD receiver with ideal knowledge of Channel Status Information (CSI). Afterwards, we shall consider the complete receiver structure, encompassing also the non-ideal GA-assisted channel estimation. Simulation results evidenced that the proposed MCBER receiver always outperforms state-of-the-art receiver schemes based on EGC and MMSE criterion exploiting the same degree of channel knowledge (i.e. ideal or estimated CSI)

    Blind reception of multicarrier DS-CDMA using antenna arrays

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    MMSE multiuser detection for array multicarrier DS-CDMA in fading channels

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    Initial synchronisation of wideband and UWB direct sequence systems: single- and multiple-antenna aided solutions

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    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

    Esquemas de pré-codificação IA com IB-DFE para sistemas MC-CDMA

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    Mestrado em Engenharia EletrĂłnica e TelecomunicaçÔesTo achieve high bit rates, needed to meet the quality of service requirements of future multimedia applications, multi-carrier code division multiple access (MC-CDMA) has been considered as a candidate air-interface. Interference alignment (IA) is a promising technique that allows high capacity gains in interfering channels. On the other hand, iterative block decision feedback equalization (IB-DFE) based receivers can efficiently exploit the inherent space-frequency diversity of the MIMO MC-CDMA systems. In this thesis we proposed an IA precoding at the transmitter with IB-DFE based processing at the receiver for MC-CDMA systems. The IA precoding is applied at chip level instead of the data symbols level, as in the conventional IA based systems. The receiver is designed in two steps: first the equalizers based on zero forcing (ZF) or minimum mean square error (MMSE) are used to remove the aligned usersÂŽ interference. Then and after a whitening noise process, an IB-DFE based equalizer is designed to remove both the residual inter-user aligned and inter-carrier interferences. The results have shown that the obtained performance is very close to the one obtained by the optimal matched filter, with few iterations at the receiver side.Para atingir maiores ritmos de transmissĂŁo, as futures aplicaçÔes multimĂ©dia necessitam de atingir a qualidade de serviço necessĂĄria. Para isso, o multi-carrier code division multiple access (MC-CDMA) tem sido apontado como um forte candidato para interface ar dos futuros sistemas celulares. O Interference Alignment (IA) ou alinhamento de interferĂȘncia Ă© uma tĂ©cnica promissora que permite ter altos ganhos de capacidade em canais com interferĂȘncia. Por outro lado, temos receptores baseados no conceito iterative block decision feedback equalization(IB-DFE) que conseguem tirar partido, de uma forma eficiente, da inerente diversidade espaço-frequĂȘncia dos sistemas MIMO MC-CDMA. Nesta dissertação Ă© implementada uma prĂ©-codificação baseada no conceito de IA considerando trĂȘs transmissores (ou estaçÔes base) juntamente, com um processamento IB-DFE no receptor para sistemas MC-CDMA.A prĂ©-codificação Ă© aplicada ao nĂ­vel de chip em vez de ser aplicado ao nĂ­vel dos dados. O receptor Ă© projectado em dois passos: em primeiro lugar equalizadores baseados em ZF ou em MMSE sĂŁo utilizados para remover a interferĂȘncia alinhada dos restantes utilizadores. De seguida, e apĂłs aplicar um processo de branqueamento do ruĂ­do ao sinal Ă  saĂ­da do primeiro equalizador, um segundo equalizador baseado em IB-DFE Ă© projectado para remover a interferĂȘncia inter-utilizador residual e tambĂ©m a interferĂȘncia residual entre portadoras. Os resultados obtidos mostraram-se satisfatĂłrios na remoção da interferĂȘncia obtendo-se um desempenho muito prĂłximo do obtido considerando um filtro adaptado

    Técnicas de equalização e pré-codificação para sistemas MC-CDMA

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    Mestrado em Engenharia EletrĂłnica e TelecomunicaçÔesO nĂșmero de dispositivos com ligaçÔes e aplicaçÔes sem fios estĂĄ a aumentar exponencialmente, causando problemas de interferĂȘncia e diminuindo a capacidade do sistema. Isto desencadeou uma procura por uma eficiĂȘncia espectral superior e, consequentemente, tornou-se necessĂĄrio desenvolver novas arquitecturas celulares que suportem estas novas exigĂȘncias. Coordenação ou cooperação multicelular Ă© uma arquitectura promissora para sistemas celulares sem fios. Esta ajuda a mitigar a interferĂȘncia entre cĂ©lulas, melhorando a equidade e a capacidade do sistema. É, portanto, uma arquitectura jĂĄ em estudo ao abrigo da tecnologia LTE-Advanced sob o conceito de coordenação multiponto (CoMP). Nesta dissertação, considerĂĄmos um sistema coordenado MC-CDMA com prĂ©-codificação e equalização iterativas. Uma das tĂ©cnicas mais eficientes de prĂ©-codificação Ă© o alinhamento de interferĂȘncias (IA). Este Ă© um conceito relativamente novo que permite aumentar a capacidade do sistema em canais de elevada interferĂȘncia. Sabe-se que, para os sistemas MC-CDMA, os equalizadores lineares convencionais nĂŁo sĂŁo os mais eficientes, devido Ă  interferĂȘncia residual entre portadoras (ICI). No entanto, a equalização iterativa no domĂ­nio da frequĂȘncia (FDE) foi identificada como sendo uma das tĂ©cnicas mais eficientes para lidar com ICI e explorar a diversidade oferecida pelos sistemas MIMO MC-CDMA. Esta tĂ©cnica Ă© baseada no conceito Iterative Block Decision Feedback Equalization (IB-DFE). Nesta dissertação, Ă© proposto um sistema MC-CDMA que une a prĂ©-codificação iterativa do alinhamento de interferĂȘncias no transmissor ao equalizador baseado no IB-DFE, com cancelamento sucessivo de interferĂȘncias (SIC) no receptor. Este Ă© construĂ­do por dois blocos: um filtro linear, que mitiga a interferĂȘncia inter-utilizador, seguido por um bloco iterativo no domĂ­nio da frequĂȘncia, que separa eficientemente os fluxos de dados espaciais na presença de interferĂȘncia residual inter-utilizador alinhada. Este esquema permite atingir o nĂșmero mĂĄximo de graus de liberdade e permite simultaneamente um ganho Ăłptimo de diversidade espacial. O desempenho deste esquema estĂĄ perto do filtro adaptado- Matched Filter Bound (MFB).The number of devices with wireless connections and applications is increasing exponentially, causing interference problems and reducing the system’s capacity gain. This initiated a search for a higher spectral efficiency and therefore it became necessary to develop new cellular architectures that support these new requirements. Multicell cooperation or coordination is a promising architecture for cellular wireless systems to mitigate intercell interference, improving system fairness and increasing capacity, and thus is already under study in LTE-Advanced under the coordinated multipoint (CoMP) concept. In this thesis, efficient iterative precoding and equalization is considered for coordinated MC-CDMA based systems. One of the most efficient precoding techniques is interference alignment (IA), which is a relatively new concept that allows high capacity gains in interfering channels. It is well known that for MC-CDMA systems standard linear equalizers are not the most efficient due to residual inter carrier interference (ICI). However, iterative frequency-domain equalization (FDE) has been identified as one of the most efficient technique to deal with ICI and exploit the inherent space-frequency diversity of the MIMO MC-CDMA systems, namely the one based on Iterative Block Decision Feedback Equalization (IB-DFE) concept. In this thesis, it is proposed a MC-CDMA system that joins iterative IA precoding at the transmitter with IB-DFE successive interference cancellation (SIC) based receiver structure. The receiver is implemented in two steps: a linear filter, which mitigates the inter-user aligned interference, followed by an iterative frequency-domain receiver, which efficiently separates the spatial streams in the presence of residual inter-user aligned interference. This scheme provides the maximum degrees of freedom (DoF) and allows almost the optimum space-diversity gain. The scheme performance is close to the matched filter bound (MFB)
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