120 research outputs found
New hybrid automatic repeat request (HARQ) scheme for 4x4 MIMO system, based on the extended alamouti quasi-orthogonal space-time bloc coding (Q-STBC), in invariant and variant fading channel
A new Hybrid Automatic Repeat reQuest (HARQ) combining scheme for a 4x4 Multiple Input Multiple Output (MIMO) system in invariant and variant fading channel conditions is proposed and analized. Based on the Extended Alamouti Quasi-orthogonal Space-Time Block Coding (Q-STBC), the use of the so-called Alternative Matrices for transmission, depending on the Channel State Information (CSI) received as feedback, is compared to other existing solutions.
Sign changes and permutations in the retransmission sequences allow reducing the interference while exploiting the spatial diversity to introduce some gain in the signal power. The best transmission order is selected by the Determinant Criterion, which optimizes the SNR in each receiver antenna to minimize the Bit Error Rate (BER) and maximize the throughput.
Studying the performance of a priori different alternatives, both analytically and empirically, several equivalents are found. Finally, the simulation results show that the proposed scheme achieves an improvement for the case of an invariant channel, but not for the time varying model, where the Auto-Regressive of order 1 (AR-i) is chosen for simplicity
Ülekande süsteemi: Alamouti indeks-kodeerimine
Consider a wireless broadcast channel with a number of receivers, where each receiver possesses
some side information. In an index coding problem, the transmitter aims at delivering
different messages to different receivers. It is desirable to minimize the total number of message
transmissions in order to improve the bandwidth efficiency. It is known that the minimum
number of transmissions can be achieved by solving a minimum rank problem for a given side
information graph, which is a known NP-hard problem.
In this thesis, the index coding problem over additive white Gaussian noise and Rayleigh fading
channels is studied. Modulation techniques, such as phase shift keying and quadrature amplitude
modulation, are tested, and it is shown that a careful choice of modulation can improve the
performance.
It is also shown that a careful choice of a generator matrix can provide for further performance
gains, in particular for ”prioritized” receivers. A probabilistic soft information detection (PSID)
is compared to a simple hard decision scheme, and the PSID performance is shown superior in
achieving a lower error rate. Additional improvement in performance is achieved by using diversity,
when employing two transmit and two receive antenna system in conjunction with the
Alamouti code. All these techniques provide for the lower error rates and higher throughput
when compared to the traditional schemes
Interference Mitigation in Wireless Communications
The primary objective of this thesis is to design advanced interference resilient schemes for asynchronous slow frequency hopping wireless personal area networks (FH-WPAN) and time division multiple access (TDMA) cellular systems in interference dominant environments. We also propose an interference-resilient power allocation method for multiple-input-multiple-output (MIMO) systems.
For asynchronous FH-WPANs in the presence of frequent packet collisions, we propose a single antenna interference canceling dual decision feedback (IC-DDF) receiver based on joint maximum likelihood (ML) detection and recursive least squares (RLS) channel estimation. For the system level performance evaluation, we propose a novel geometric method that combines bit error rate (BER) and the spatial distribution of the traffic load of CCI for the computation of packet error rate (PER). We also derived the probabilities of packet collision in multiple asynchronous FH-WPANs with uniform and nonuniform traffic patterns.
For the design of TDMA receivers resilient to CCI in frequency selective channels, we propose a soft output joint detection interference rejection combining delayed decision feedback sequence estimation (JD IRC-DDFSE) scheme. In the proposed scheme, IRC suppresses the CCI, while DDFSE equalizes ISI with reduced complexity. Also, the soft outputs are generated from IRC-DDFSE decision metric to improve the performance of iterative or non-iterative type soft-input outer code decoders.
For the design of interference resilient power allocation scheme in MIMO systems, we investigate an adaptive power allocation method using subset antenna transmission (SAT) techniques. Motivated by the observation of capacity imbalance among the multiple parallel sub-channels, the SAT method achieves high spectral efficiency by allocating power on a selected transmit antenna subset. For 4 x 4 V-BLAST MIMO systems, the proposed scheme with SAT showed analogous results. Adaptive modulation schemes combined with the proposed method increase the capacity gains. From a feasibility viewpoint, the proposed method is a practical solution to CCI-limited MIMO systems since it does not require the channel state information (CSI) of CCI.Ph.D.Committee Chair: Professor Gordon L. StBe
Técnicas com múltiplas antenas distribuídas para sistemas sem fios
Mestrado em Engenharia Electrónica e TelecomunicaçõesTransmissão cooperativa, em que uma fonte e um relay cooperam para enviar uma mensagem para o destino, pode proporcionar diversidade espacial contra o desvanecimento nas comunicações sem fios. O objectivo deste projecto é estudar a performance de um sistema de transmissão cooperativo com dois relays equipados com duas antenas, entre o transmissor e o utilizador. Considera-se que a estação base está equipada com duas antenas e o terminal móvel apenas com uma. O sistema cooperativo foi implementado de acordo com as especificações do LTE e avaliado em diversos cenários de propagação, considerando canais com diferentes Relação Sinal Ruído (SNR). Verificou-se que o desempenho do sistema proposto é melhor, quando comparado com o sistema não cooperativo, na maior parte dos cenários estudados.Cooperative transmission, in which a source and relay cooperate to sent a mensage to destination, can provide spatial diversity against fading in wirless telecomunications. The goal of this project is to study the perfomance of a cooperative tranmition systems with two relays equiped with two antennas, between transmitter and user. It is considered that the base station is equipped with two antennas and the mobile terminal with only one. The cooperative system was implemented according to the specifications of the LTE and evaluated at several propagation scenarios, considering channels with diferents Signal to Noise Ratio (SNR). It was found that the perfomance of the proposed system is better when compared with the non-cooperative ones, in most scenarios considered.CODIV/FP7-ICT-200
Distributed space-time coding including the golden code with application in cooperative networks
This thesis presents new methodologies to improve performance of wireless cooperative networks using the Golden Code. As a form of space-time coding, the Golden Code can achieve diversity-multiplexing tradeoff and the data rate can be twice that of the Alamouti code. In practice, however, asynchronism between relay nodes may reduce performance and channel quality can be degraded from certain antennas.
Firstly, a simple offset transmission scheme, which employs full interference cancellation (FIC) and orthogonal frequency division multiplexing (OFDM), is enhanced through the use of four relay nodes and receiver processing to mitigate asynchronism. Then, the potential reduction in diversity gain due to the dependent channel matrix elements in the distributed Golden Code transmission, and the rate penalty of multihop transmission, are mitigated by relay selection based on two-way transmission. The Golden Code is also implemented in an asynchronous one-way relay network over frequency flat and selective channels, and a simple approach to overcome asynchronism is proposed. In one-way communication with computationally efficient sphere decoding, the maximum of the channel parameter means is shown to achieve the best performance for the relay selection through bit error rate simulations.
Secondly, to reduce the cost of hardware when multiple antennas are available in a cooperative network, multi-antenna selection is exploited. In this context, maximum-sum transmit antenna selection is proposed. End-to-end signal-to-noise ratio (SNR) is calculated and outage probability analysis is performed when the links are modelled as Rayleigh fading frequency flat channels. The numerical results support the analysis and for a MIMO system
maximum-sum selection is shown to outperform maximum-minimum selection. Additionally, pairwise error probability (PEP) analysis is performed for maximum-sum transmit antenna selection with the Golden Code and the diversity order is obtained.
Finally, with the assumption of fibre-connected multiple antennas with finite buffers, multiple-antenna selection is implemented on the basis of maximum-sum antenna selection. Frequency flat Rayleigh fading channels are assumed together with a decode and forward transmission scheme. Outage probability analysis is performed by exploiting the steady-state stationarity of a Markov Chain model
Diversity techniques for a free-space optical communication system in correlated log-normal channels
International audiencePerformance analysis of free-space optical (FSO) communication systems in different channel conditions has gained significant attention in literature. Nevertheless, most existing studies consider uncorrelated channel conditions. An uncorrelated channel requires sufficient spacing between transmitters and limits the receiver field of view and link distance. However, this might not be feasible in all applications. Thereby, this paper studies repetition code (RC) and orthogonal space time block code (OSTBC) performance in correlated log-normal FSO channels using intensity modulation and direct detection. An approximate analytical expressions using moment generating function for the average bit error probability are derived. Our simulation results show that RCs are superior to OSTBCs in correlated channel conditions
Implementação de um sistema de comunicações móveis para o Uplink
Mestrado em Engenharia Electrónica e TelecomunicaçõesÉ evidente que actualmente cada vez mais a internet móvel está presente na
vida das sociedades. Hoje em dia é relativamente fácil estar ligado à internet
sempre que se quiser, independentemente do lugar onde se encontra
(conceito: anytime and anywhere). Desta forma existe um número crescente
de utilizadores que acedem a serviços e aplicações interactivas a partir dos
seus terminais móveis. Há, portanto, uma necessidade de adaptar o mundo
das telecomunicações a esta nova realidade, para isso é necessário
implementar novas arquitecturas que sejam capazes de fornecer maior largura
de banda e reduzir os atrasos das comunicações, maximizando a utilização
dos recursos disponíveis do meio/rede e melhorando assim a experiência do
utilizador final.
O LTE representa uma das tecnologias mais avançadas e de maior relevância
para o acesso sem fios em banda larga de redes celulares. OFDM é a
tecnologia base que está por traz da técnica de modulação, bem como as
tecnologias adjacentes, OFDMA e SC-FDMA, usadas especificamente no LTE
para a comunicação de dados descendente (downlink) ou ascendente (uplink),
respectivamente. A implementação de múltiplas antenas em ambos os
terminais, potenciam ainda mais o aumento da eficiência espectral do meio
rádio permitindo atingir grandes taxas de transmissão de dados.
Nesta dissertação é feito o estudo, implementação e avaliação do
desempenho da camada física (camada 1 do modelo OSI) do LTE, no entanto
o foco será a comunicação de dados ascendente e a respectiva técnica de
modelação, SC-FDMA. Foi implementada uma plataforma de simulação
baseada nas especificações do LTE UL onde foram considerandos diferentes
esquemas de antenas. Particularmente para o esquema MIMO, usou-se a
técnica de codificação no espaço-frequência proposta por Alamouti. Foram
também implementados vários equalizadores.
Os resultados provenientes da simulação demonstram tanto a eficiência dos
diversos modos de operação em termos da taxa de erro, como o excelente
funcionamento de processos de mapeamento e equalização, que visam
melhorar a taxa de recepção de dados.It is clear that mobile Internet is present in the life of societies. Nowadays it is
relatively easy to be connected to the internet whenever you want, no matter
where you are (concept: anytime and anywhere). Thus, there are a growing
number of users accessing interactive services and applications from their
handsets. Therefore, there is a need to adapt the world of telecommunications
to this new reality, for that it is necessary to implement new architectures that
are able to provide higher bandwidth and reduce communication delays,
maximizing use of available resources in the medium/network and thereby
improving end-user experience.
LTE represents one of the most advanced architectures and most relevant to
wireless broadband cellular networks. OFDM is the technology that is behind
the modulation technique and the underlying technologies, OFDMA and SCFDMA,
used specifically in LTE for data communication downward (downlink)
or upward (uplink), respectively. The implementation of multiple antennas at
both ends further potentiate the increase of spectral efficiency allowing to
achieve high rates of data transmission.
In this dissertation is done the study, implementation and performance
evaluation of the physical layer (OSI Layer 1) of the LTE, but the focus will be
communication and its upstream data modeling technique, SC-FDMA. We
implemented a simulation platform based on LTE UL specifications where were
considered different antenna schemes. Particularly for the MIMO scheme, we
used the technique of space-frequency coding proposed by Alamouti. We also
implemented several equalizers.
The results from the simulation demonstrate both the efficiency of different
modes of operation in terms of error rate, as the excellent operation of mapping
processes and equalization, designed to improve the rate of receiving data
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
Space-time coded cooperation in Wireless Networks
Nowadays, the concept of spatial diversity and cooperative networks attract a lot
of interest because they improve the reliability of transmission in wireless networks.
Spatial diversity is achieved when multiple antennas are at the transmitter. With great
growth and demand for high speed high data rate wireless communication, more and
more antennas are required. In order to achieve maximum diversity, these antennas
should be well separated so that the fading on each link is uncorrelated. This condition
makes it difficult to have more than two antennas on a mobile terminal. The relay's
cooperation helps increase the diversity order without extra hardware cost. However, its
main inconvenience is the use of multiple time slots compared to the direct link
transmission.
In this thesis, we develop a cooperation model which is composed of three
terminals: source, relay and destination. The transmitters (source and relay) are composed
of 2 antennas at the transmitter and the receivers (relay and destination) have 4 antennas.
In the first proposed model, transmitters and decoders are composed of an Alamouti
encoder and decoder respectively. In the second model, we also add a turbo encoder at
transmitters and iterative decoding takes place at receivers. In both cases, the
transmission cycle is composed of two time slots and the decode and forward (DF)
protocol is applied. Multiple scenarios are considered by changing the environment of the
transmission, such as line of sight (LOS) or non line of sight (NLOS) or by modifying the
location of the relay between the source and destination. We also simulate an uplink and
a downlink communication. All the scenarios show a coding gain with the turbo coded
space-time cooperation
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