191 research outputs found
Adaptive relay techniques for OFDM-based cooperative communication systems
Cooperative communication has been considered as a cost-effective manner to exploit the spatial diversity, improve the quality-of-service and extend transmission coverage. However, there are many challenges faced by cooperative systems which use relays to forward signals to the destination, such as the accumulation of multipath channels, complex resource allocation with the bidirectional asymmetric traffic and reduction of transmission efficiency caused by additional relay overhead. In this thesis, we aim to address the above challenges of cooperative communications, and design the efficient relay systems.
Starting with the channel accumulation problem in the amplify-and-forward relay system, we proposed two adaptive schemes for single/multiple-relay networks respectively. These schemes exploit an adaptive guard interval (GI) technique to cover the accumulated delay spread and enhance the transmission efficiency by limiting the overhead. The proposed GI scheme can be implemented without any extra control signal. Extending the adaptive GI scheme to multiple-relay systems, we propose a relay selection strategy which achieves the trade-off between the transmission reliability and overhead by considering both the channel gain and the accumulated delay spread. We then consider resource allocation problem in the two-way decode-and-forward relay system with asymmetric traffic loads. Two allocation algorithms are respectively investigated for time-division and frequency-division relay systems to maximize the end-to-end capacity of the two-way system under a capacity ratio constraint. For the frequency-division systems, a balanced end-to-end capacity is defined as the objective function which combines the requirements of maximizing the end-to-end capacity and achieving the capacity ratio. A suboptimal algorithm is proposed for the frequency-division systems which separates subcarrier allocation and time/power allocation. It can achieve the similar performance with the optimal one with reduced complexity. In order to further enhance the transmission reliability and maintaining low processing delay, we propose an equalize-and-forward (EF) relay scheme. The EF relay equalizes the channel between source and relay to eliminate the channel accumulation without signal regeneration. To reduce the processing time, an efficient parallel structure is applied in the EF relay. Numerical results show that the EF relay exhibits low outage probability at the same data rate as compared to AF and DF schemes
Optimization and Applications of Modern Wireless Networks and Symmetry
Due to the future demands of wireless communications, this book focuses on channel coding, multi-access, network protocol, and the related techniques for IoT/5G. Channel coding is widely used to enhance reliability and spectral efficiency. In particular, low-density parity check (LDPC) codes and polar codes are optimized for next wireless standard. Moreover, advanced network protocol is developed to improve wireless throughput. This invokes a great deal of attention on modern communications
Time diversity solutions to cope with lost packets
A dissertation submitted to Departamento de Engenharia Electrotécnica of Faculdade de Ciências e Tecnologia of Universidade Nova de Lisboa in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Engenharia Electrotécnica e de ComputadoresModern broadband wireless systems require high throughputs and can also have very high
Quality-of-Service (QoS) requirements, namely small error rates and short delays. A high spectral efficiency is needed to meet these requirements. Lost packets, either due to errors or collisions, are usually discarded and need to be retransmitted, leading to performance degradation.
An alternative to simple retransmission that can improve both power and spectral
efficiency is to combine the signals associated to different transmission attempts.
This thesis analyses two time diversity approaches to cope with lost packets that are
relatively similar at physical layer but handle different packet loss causes. The first is a lowcomplexity Diversity-Combining (DC) Automatic Repeat reQuest (ARQ) scheme employed in a Time Division Multiple Access (TDMA) architecture, adapted for channels dedicated to a single user. The second is a Network-assisted Diversity Multiple Access (NDMA) scheme, which is a multi-packet detection approach able to separate multiple mobile terminals transmitting simultaneously in one slot using temporal diversity. This thesis combines these techniques with Single Carrier with Frequency Division Equalizer (SC-FDE) systems, which are widely recognized as the best candidates for the uplink of future broadband wireless systems.
It proposes a new NDMA scheme capable of handling more Mobile Terminals (MTs)
than the user separation capacity of the receiver. This thesis also proposes a set of analytical tools that can be used to analyse and optimize the use of these two systems. These tools are then employed to compare both approaches in terms of error rate, throughput and delay performances, and taking the implementation complexity into consideration.
Finally, it is shown that both approaches represent viable solutions for future broadband wireless communications complementing each other.Fundação para a Ciência e Tecnologia - PhD grant(SFRH/BD/41515/2007); CTS multi-annual funding project PEst-OE/EEI/UI0066/2011, IT
pluri-annual funding project PEst-OE/EEI/LA0008/2011, U-BOAT project PTDC/EEATEL/
67066/2006, MPSat project PTDC/EEA-TEL/099074/2008 and OPPORTUNISTICCR
project PTDC/EEA-TEL/115981/200
Control Channel Interference Measurement in LTE-TDD Heterogeneous Network
Deploying low power eNodeBs inside macro-cells is an effective way to enhance indoor coverage. By reusing frequency between macro-cells and indoor femto-cells, the efficiency of expensive licensed spectrum can be further increased. This thesis measured Physical Downlink Control Channel (PDCCH) performance in such a heterogeneous LTE-TDD network. Four USRP software radio terminals and connected Linux workstations were deployed to build a test environment. They acted as eNodeB and UE respectively. During the test, the femto-cell was configured to coordinate its radio frame with the macro-cell. Several criteria including received block error rate, payload bit error rate and symbols signal to interference and noise ratio were used to evaluate the PDCCH performance in macro-cell under heterogeneous environment
Técnicas de processamento com múltiplas antenas para o sistema LTE
Mestrado em Engenharia Electrónica e TelecomunicaçõesPerformance, mobilidade e partilha podem ser consideras como as três
palavras-chave nas comunicações móveis de hoje em dia. Uma das necessidades
fundamentais do ser humano é a partilha de experiencias e informação.
Com a evolução ao nível do hardware móvel, a crescente popularidade de
smartphones, tablets e outros dispositivos moveis, fez com que a exigência
em termos de capacidade e taxa de transferência por parte das redes móveis
não parasse de crescer.
As limitações das redes 3G fizeram com que não conseguissem corresponder
a tais exigências e como tal, a transição para uma tecnologia mais robusta e
eficiente passou a ser inevitável. A resposta escolhida como solução a longo
prazo é a rede designada por LTE, desenvolvida pela organização 3GPP é
assumido que será a rede de telecomunicações predominante no futuro. As
vantagens mais sonantes são, naturalmente, elevadas taxas de transmissão,
maior eficiência espectral, redução da latência e de custos de operação. As
principais tecnologias em que o LTE se baseia, são o OFDM e sua variante
para múltiplo acesso, OFDMA, usado para o downlink e o SC-FDMA para
o uplink. Além disso, usa sistemas com múltiplas antenas para impulsionar
a eficiência espectral. Apesar de já implementado em alguns países por
diversas operadoras, constantes pesquisas continuam a ser realizadas com
o intuito de melhorar a sua performance.
Nesta dissertação é proposto um esquema duplo de codificação na frequência
e no espaço (D-SFBC) para um cenário baseado em OFDM com 4
antenas de transmissão e duas antenas de recepção (4 × 2 D-SFBC) para
o downlink. No cenário considerado, 4 símbolos de dados são transmitidos
utilizando unicamente 2 sub-portadoras, fazendo com que, este sistema seja
limitado pela interferência. Para de forma eficiente descodificar os símbolos
de dados transmitidos, foi desenvolvido um equalizador iterativo no domínio
da frequência. Duas abordagens são consideradas: cancelamento da interferência
em paralelo (PIC) e sucessivo cancelamento de interferência (SIC).
Uma vez que apenas 2 sub-portadoras são usadas para transmitir quatro
símbolos de dados em paralelo, o esquema desenvolvido duplica a taxa de
dados quando comparado com o esquema 2 × 2 SFBC, especificado no
standard do LTE.
Os esquemas desenvolvidos foram avaliados sob as especificações para LTE
e usando codificação de canal. Os resultados mostram que os esquemas
implementados neste trabalho utilizando um equalizador iterativo supera os
convencionais equalizadores lineares na eliminação da interferência adicional
introduzida, em apenas 2 ou 3 iterações.Performance, mobility and sharing can be assumed as the three keywords
in the mobile communications nowadays. One of the fundamental needs of
human beings is to share experiences and information. With the evolution of
mobile hardware level, the growing popularity of smartphones, tablets and
other mobile devices, has made that the demand in terms of capacity and
throughput by mobile networks did not stop growing.
Thus, the limitations of 3G stops it of being the answer of such demand, and
a transition to a powerful technology has become unavoidable. The answer
chosen is LTE, developed by the 3GPP organization is assumed to be the
predominant telecommunications network in the future. The most relevant
advantages are high transmission rates, higher spectral efficiency, reducing
latency and operating costs. The key technologies in which LTE is based,
are OFDM and its variant schemes for multiple access, OFDMA, used for
downlink, and SC-FDMA for the uplink. It also uses multiple antennas systems
in order to improve spectral efficiency. Although already implemented
in some countries by several operators, continuous research is conducted in
order to improve their performance.
In this dissertation it is proposed a double space-frequency block coding
(D-SFBC) scheme for an OFDM based scenario with 4 transmit antennas
and 2 receive antennas (4×2 D-SFBC) for the downlink. In the considered
scenario, 4 data symbols are transmitted by using only 2 subcarriers and thus
the system is interference limited. To efficiently decode the transmitted data
symbols an iterative equalizer designed in frequency domain is developed.
Two approaches are considered: parallel interference cancellation (PIC) and
successive interference cancellation (SIC). Since only 2 subcarriers are used
to transmit 4 data symbols in parallel the developed scheme achieve the
double data rate when compared with the 2×2 SFBC, specified in the LTE
standard.
The developed schemes were evaluated under the main LTE specifications
and using channel coding. The results have show that the schemes implemented
in this work using an interactive equalizer outperforms the conventional
linear equalizers in the interference removal, just by using 2 or 3
iterations
Radio Communications
In the last decades the restless evolution of information and communication technologies (ICT) brought to a deep transformation of our habits. The growth of the Internet and the advances in hardware and software implementations modified our way to communicate and to share information. In this book, an overview of the major issues faced today by researchers in the field of radio communications is given through 35 high quality chapters written by specialists working in universities and research centers all over the world. Various aspects will be deeply discussed: channel modeling, beamforming, multiple antennas, cooperative networks, opportunistic scheduling, advanced admission control, handover management, systems performance assessment, routing issues in mobility conditions, localization, web security. Advanced techniques for the radio resource management will be discussed both in single and multiple radio technologies; either in infrastructure, mesh or ad hoc networks
Performance analysis of 4G wireless networks using system level simulator
Doutoramento em Engenharia ElectrotécnicaIn the last decade, mobile wireless communications have witnessed an explosive
growth in the user’s penetration rate and their widespread deployment around the
globe. In particular, a research topic of particular relevance in telecommunications
nowadays is related to the design and implementation of mobile communication
systems of 4th generation (4G). 4G networks will be characterized by the support
of multiple radio access technologies in a core network fully compliant with the
Internet Protocol (all IP paradigms). Such networks will sustain the stringent
quality of service (QoS) requirements and the expected high data rates from the
type of multimedia applications (i.e. YouTube and Skype) to be available in the
near future. Therefore, 4G wireless communications system will be of paramount
importance on the development of the information society in the near future.
As 4G wireless services will continue to increase, this will put more and more
pressure on the spectrum availability. There is a worldwide recognition that
methods of spectrum managements have reached their limit and are no longer
optimal, therefore new paradigms must be sought. Studies show that most of the
assigned spectrum is under-utilized, thus the problem in most cases is inefficient
spectrum management rather spectrum shortage. There are currently trends
towards a more liberalized approach of spectrum management, which are tightly
linked to what is commonly termed as Cognitive Radio (CR).
Furthermore, conventional deployment of 4G wireless systems (one BS in cell and
mobile deploy around it) are known to have problems in providing fairness (users
closer to the BS are more benefited relatively to the cell edge users) and in
covering some zones affected by shadowing, therefore the use of relays has been
proposed as a solution.
To evaluate and analyse the performances of 4G wireless systems software tools
are normally used. Software tools have become more and more mature in recent
years and their need to provide a high level evaluation of proposed algorithms and
protocols is now more important. The system level simulation (SLS) tools provide
a fundamental and flexible way to test all the envisioned algorithms and protocols
under realistic conditions, without the need to deal with the problems of live
networks or reduced scope prototypes. Furthermore, the tools allow network
designers a rapid collection of a wide range of performance metrics that are useful
for the analysis and optimization of different algorithms.
This dissertation proposes the design and implementation of conventional system
level simulator (SLS), which afterwards enhances for the 4G wireless technologies
namely cognitive Radios (IEEE802.22) and Relays (IEEE802.16j). SLS is then
used for the analysis of proposed algorithms and protocols.FC
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