32 research outputs found
Busy burst technology applied to OFDMA–TDD systems
The most significant bottleneck in wireless communication systems is an ever-increasing disproportion
between the bandwidth demand and the available spectrum. A major challenge in
the field of wireless communications is to maximise the spatial reuse of resources whilst avoiding
detrimental co-channel interference (CCI). To this end, frequency planning and centralised
coordination approaches are widely used in wireless networks. However, the networks for the
next generation of wireless communications are often envisioned to be decentralised, randomly
distributed in space, hierarchical and support heterogeneous traffic and service types. Fixed
frequency allocation would not cater for the heterogeneous demands and centralised resource
allocation would be cumbersome and require a lot of signalling. Decentralised radio resource
allocation based on locally available information is considered the key.
In this context, the busy burst (BB) signalling concept is identified as a potential mechanism
for decentralised interference management in future generation networks. Interference aware
allocation of time-frequency slots (chunks) is accomplished by letting receivers transmit a BB
in a time-multiplexed mini-slot, upon successful reception of data. Exploiting channel reciprocity
of the time division duplex (TDD) mode, the transmitters avoid reusing the chunks
where the received BB power is above a pre-determined threshold so as to limit the CCI caused
towards the reserved chunks to a threshold value. In this thesis, the performance of BB signalling
mechanism in orthogonal frequency division multiple access - time division duplexing
(OFDMA-TDD) systems is evaluated by means of system level simulations in networks operating
in ad hoc and cellular scenarios. Comparisons are made against the state-of-the-art centralised
CCI avoidance and mitigation methods, viz. frequency planning, fractional frequency
reuse, and antenna array with switched grid of beams, as well as decentralised methods such as
the carrier sense multiple access method that attempt to avoid CCI by avoiding transmission on
chunks deemed busy. The results demonstrate that with an appropriate choice of threshold parameter,
BB-based techniques outperform all of the above state-of-the-art methods. Moreover,
it is demonstrated that by adjusting the BB-specific threshold parameter, the system throughput
can be traded off for improving throughput for links with worse channel condition, both
in the ad hoc and cellular scenario. Moreover, by utilising a variable BB power that allows a
receiver to signal the maximum CCI it can tolerate, it is shown that a more favourable trade-off
between total system throughput and link throughput can be made. Furthermore, by performing
link adaptation, it is demonstrated that the spatial reuse and the energy efficiency can be traded
off by adjusting the threshold parameter. Although the BB signalling mechanism is shown to
be effective in avoiding detrimental CCI, it cannot mitigate CCI by itself. On the other hand,
multiple antenna techniques such as adaptive beamforming or switched beam approaches allow
CCI to be mitigated but suffer from hidden node problems. The final contribution of this thesis
is that by combining the BB signalling mechanism with multiple antenna techniques, it is
demonstrated that the hybrid approach enhances spatial reusability of resources whilst avoiding
detrimental CCI.
In summary, this thesis has demonstrated that BB provides a flexible radio resource mechanism
that is suitable for future generation networks
Contributions to Analysis and Mitigation of Cochannel Interference in Cellular Wireless Networks
Cellular wireless networks have become a commodity. We use our cellular devices every day to connect to others, to conduct business, for entertainment. Strong demand for wireless access has made corresponding parts of radio spectrum very valuable. Consequently, network operators and their suppliers are constantly being pressured for its efficient use. Unlike the first and second generation cellular networks, current generations do not therefore separate geographical sites in frequency. This universal frequency reuse, combined with continuously increasing spatial density of the transmitters, leads to challenging interference levels in the network.
This dissertation collects several contributions to analysis and mitigation of interference in cellular wireless networks. The contributions are categorized and set in the context of prior art based on key characteristics, then they are treated one by one.
The first contribution encompasses dynamic signaling that measures instantaneous interference situations and allows only for such transmissions that do not harm each other excessively. A novel forward signaling approach is introduced as an alternative to traditional reverse signaling. Forward signaling allows the interference management decisions to be done at the receiver, where there is more relevant information available.
The second contribution analyzes cross-link interference in heterogeneous networks. Cross-link interference is interference between downlink and uplink transmissions that can appear in time-division duplex (TDD) networks. It is shown that uplink reception of small cells can be disturbed considerably by macrocell downlink transmissions. We proposes an intuitive solution to the problem based on power control. Users in small cells have generally enough power headroom as the distance to the small base station is often short.
The third contribution provides an extensive analysis of a specific interference managment method that the Long-Term Evolution (LTE) applies in cochannel heterogeneous deployments. We analyze this so-called time muting using a modern stochastic geometry approach and show that performance of the method strongly depends on residual interference in the muted sections of time.
The fourth and last contribution analyzes the impact of interference rank, i.e., number of spatial streams at the interferer, on a beamformed or spatially block coded transmission. It is shown that when the interferer chooses to transmit multiple spatial streams, spreading the power in spatial domain has potential to decrease probability of outage at neighbor receiver, especially if the neighbor transmission uses beamforming
Interference mitigation and interference avoidance for cellular OFDMA-TDD networks
In recent years, cellular systems based on orthogonal frequency division multiple access – time
division duplex (OFDMA-TDD) have gained considerable popularity. Two of the major reasons
for this are, on the one hand, that OFDMA enables the receiver to effectively cope with multipath
propagation while keeping the complexity low. On the other hand, TDD offers efficient
support for cell-specific uplink (UL)/downlink (DL) asymmetry demands by allowing each cell
to independently set its UL/DL switching point (SP). However, cell-independent SP gives rise
to crossed slots. In particular, crossed slots arise when neighbouring cells use the same slot in
opposing link directions, resulting in base station (BS)-to-BS interference and mobile station
(MS)-to-MS interference. BS-to-BS interference, in particular, can be quite detrimental due to
the exposed location of BSs, which leads to high probability of line-of-sight (LOS) conditions.
The aim of this thesis is to address the BS-to-BS interference problem in OFDMA-TDDcellular
networks. A simulation-based approach is used to demonstrate the severity of BS-to-BS interference
and a signal-to-interference-plus-noise ratio (SINR) equation for OFDMA is formulated
to aid system performance analysis. The detrimental effects of crossed slot interference in
OFDMA-TDD cellular networks are highlighted by comparing methods specifically targeting
the crossed slots interference problem. In particular, the interference avoidance method fixed
slot allocation (FSA) is compared against state of the art interference mitigation approaches,
viz: random time slot opposing (RTSO) and zone division (ZD). The comparison is done based
on Monte Carlo simulations and the main comparison metric is spectral efficiency calculated
using the SINR equation formulated in this thesis. The simulation results demonstrate that
when LOS conditions among BSs are present, both RTSO and ZD perform worse than FSA for
all considered performance metrics. It is concluded from the results that current interference
mitigation techniques do not offer an effective solution to the BS-to-BS interference problem.
Hence, new interference avoidance methods, which unlike FSA, do not sacrifice the advantages
of TDD are open research issues addressed in this thesis.
The major contribution of this thesis is a novel cooperative resource balancing technique that
offers a solution to the crossed slot problem. The novel concept, termed asymmetry balancing,
is targeted towards next-generation cellular systems, envisaged to have ad hoc and multi-hop
capabilities. Asymmetry balancing completely avoids crossed slots by keeping the TDD SPs
synchronised among BSs. At the same time, the advantages of TDD are retained, which is
enabled by introducing cooperation among the entities in the network. If a cell faces resource
shortage in one link direction, while having free resources in the opposite link direction, the
free resources can be used to support the overloaded link direction. In particular, traffic can
be offloaded to near-by mobile stations at neighbouring cells that have available resources. To
model the gains attained with asymmetry balancing, a mathematical framework is developed
which is verified by Monte Carlo simulations. In addition, asymmetry balancing is compared
against both ZD and FSA based on simulations and the results demonstrate the superior performance
of asymmetry balancing. It can be concluded that the novel interference avoidance
approach is a very promising candidate t
Multilayer optimization in radio resource allocation for the packet transmission in wireless networks
Doutoramento em Engenharia ElectrotécnicaNa última década tem-se assistido a um crescimento exponencial das redes de
comunicações sem fios, nomeadamente no que se refere a taxa de penetração do serviço prestado e na implementação de novas infra-estruturas em todo o globo. É ponto assente neste momento que esta tendência irá não só continuar como se
fortalecer devido à convergência que é esperada entre as redes móveis sem fio e a disponibilização de serviços de banda larga para a rede Internet fixa, numa evolução para um paradigma de uma arquitectura integrada e baseada em serviços e aplicações IP. Por este motivo, as comunicações móveis sem fios irão ter um papel fundamental no desenvolvimento da sociedade de informação a
médio e longo prazos.
A estratégia seguida no projecto e implementação das redes móveis celulares da actual geração (2G e 3G) foi a da estratificação da sua arquitectura protocolar numa estrutura modular em camadas estanques, onde cada camada do modelo é
responsável pela implementação de um conjunto de funcionalidades. Neste modelo a comunicação dá-se apenas entre camadas adjacentes através de primitivas de comunicação pré-estabelecidas. Este modelo de arquitectura resulta numa mais
fácil implementação e introdução de novas funcionalidades na rede. Entretanto, o facto das camadas inferiores do modelo protocolar não utilizarem informação disponibilizada pelas camadas superiores, e vice-versa acarreta uma degradação
no desempenho do sistema. Este paradigma é particularmente importante quando sistemas de antenas múltiplas são implementados (sistemas MIMO). Sistemas de antenas múltiplas introduzem um grau adicional de liberdade no que respeita a
atribuição de recursos rádio: o domínio espacial. Contrariamente a atribuição de recursos no domínio do tempo e da frequência, no domínio espacial os recursos rádio mapeados no domínio espacial não podem ser assumidos como sendo completamente ortogonais, devido a interferência resultante do facto de vários terminais transmitirem no mesmo canal e/ou slots temporais mas em feixes espaciais diferentes. Sendo assim, a disponibilidade de informação relativa ao estado dos recursos rádio às camadas superiores do modelo protocolar é de
fundamental importância na satisfação dos critérios de qualidade de serviço exigidos.
Uma forma eficiente de gestão dos recursos rádio exige a implementação de algoritmos de agendamento de pacotes de baixo grau de complexidade, que definem os níveis de prioridade no acesso a esses recursos por base dos
utilizadores com base na informação disponibilizada quer pelas camadas inferiores quer pelas camadas superiores do modelo. Este novo paradigma de comunicação, designado por cross-layer resulta na maximização da capacidade de transporte de
dados por parte do canal rádio móvel, bem como a satisfação dos requisitos de qualidade de serviço derivados a partir da camada de aplicação do modelo.
Na sua elaboração, procurou-se que o standard IEEE 802.16e, conhecido por Mobile WiMAX respeitasse as especificações associadas aos sistemas móveis celulares de quarta geração. A arquitectura escalonável, o baixo custo de
implementação e as elevadas taxas de transmissão de dados resultam num processo de multiplexagem de dados e valores baixos no atraso decorrente da
transmissão de pacotes, os quais são atributos fundamentais para a disponibilização de serviços de banda larga. Da mesma forma a comunicação
orientada à comutação de pacotes, inenente na camada de acesso ao meio, é totalmente compatível com as exigências em termos da qualidade de serviço dessas aplicações. Sendo assim, o Mobile WiMAX parece satisfazer os requisitos exigentes das redes móveis de quarta geração.
Nesta tese procede-se à investigação, projecto e implementação de algoritmos de encaminhamento de pacotes tendo em vista a eficiente gestão do conjunto de recursos rádio nos domínios do tempo, frequência e espacial das redes móveis celulares, tendo como caso prático as redes móveis celulares suportadas no standard IEEE802.16e. Os algoritmos propostos combinam métricas provenientes da camada física bem como os requisitos de qualidade de serviço das camadas
superiores, de acordo com a arquitectura de redes baseadas no paradigma do cross-layer. O desempenho desses algoritmos é analisado a partir de simulações efectuadas por um simulador de sistema, numa plataforma que implementa as
camadas física e de acesso ao meio do standard IEEE802.16e.In the last decade mobile wireless communications have witnessed an explosive growth in the user’s penetration rate and their widespread deployment around the globe. It is expected that this tendency will continue to increase with the convergence of fixed Internet wired networks with mobile ones and with the evolution to the full IP architecture paradigm. Therefore mobile wireless communications will be of paramount importance on the development of the information society of the near future.
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 networks will be characterized by the support of
multiple radio access technologies in a core network fully compliant with the Internet Protocol (all IP paradigm). Such networks will sustain the stringent quality of service (QoS) requirements and the expected high data rates from the type of multimedia applications to be available in the near future. The approach followed in the design and implementation of the mobile wireless networks of current generation (2G and 3G) has been the stratification of the architecture into a communication protocol model composed by a set of layers, in which each one encompasses some set of functionalities. In such protocol layered model, communications is only allowed between adjacent layers and through specific interface service points. This modular concept eases the
implementation of new functionalities as the behaviour of each layer in the protocol stack is not affected by the others. However, the fact that lower layers in the protocol stack model do not utilize information available from upper layers, and vice versa, downgrades the performance achieved. This is particularly relevant if multiple antenna systems, in a MIMO (Multiple Input Multiple Output) configuration, are implemented. MIMO schemes introduce another degree of freedom for radio resource allocation: the space domain.
Contrary to the time and frequency domains, radio resources mapped into the spatial domain cannot be assumed as completely orthogonal, due to the
amount of interference resulting from users transmitting in the same frequency sub-channel and/or time slots but in different spatial beams. Therefore, the availability of information regarding the state of radio resources, from lower to upper layers, is of fundamental importance in the prosecution of the levels of QoS expected from those multimedia applications.
In order to match applications requirements and the constraints of the mobile radio channel, in the last few years researches have proposed a new paradigm for the layered architecture for communications: the cross-layer design framework. In a general way, the cross-layer design paradigm refers to a protocol design in which the dependence between protocol layers is actively
exploited, by breaking out the stringent rules which restrict the communication only between adjacent layers in the original reference model, and allowing
direct interaction among different layers of the stack.
An efficient management of the set of available radio resources demand for the implementation of efficient and low complexity packet schedulers which prioritize user’s transmissions according to inputs provided from lower as well as upper layers in the protocol stack, fully compliant with the cross-layer design paradigm. Specifically, efficiently designed packet schedulers for 4G networks
should result in the maximization of the capacity available, through the consideration of the limitations imposed by the mobile radio channel and
comply with the set of QoS requirements from the application layer.
IEEE 802.16e standard, also named as Mobile WiMAX, seems to comply with the specifications of 4G mobile networks. The scalable architecture, low cost implementation and high data throughput, enable efficient data multiplexing and low data latency, which are attributes essential to enable broadband data services. Also, the connection oriented approach of Its medium access layer is
fully compliant with the quality of service demands from such applications.
Therefore, Mobile WiMAX seems to be a promising 4G mobile wireless networks candidate.
In this thesis it is proposed the investigation, design and implementation of packet scheduling algorithms for the efficient management of the set of
available radio resources, in time, frequency and spatial domains of the Mobile WiMAX networks. The proposed algorithms combine input metrics from physical layer and QoS requirements from upper layers, according to the crosslayer design paradigm. Proposed schedulers are evaluated by means of system level simulations, conducted in a system level simulation platform implementing the physical and medium access control layers of the
IEEE802.16e standard
TV White Spaces: A Pragmatic Approach
190 pages The editors and publisher have taken due care in preparation of this book, but
make no expressed or implied warranty of any kind and assume no
responsibility for errors or omissions. No liability is assumed for incidental or
consequential damages in connection with or arising out of the use of the
information contained herein. Links to websites imply neither responsibility
for, nor approval of, the information contained in those other web sites on the
part of ICTP. No intellectual property rights are transferred to ICTP via this
book, and the authors/readers will be free to use the given material for
educational purposes. e ICTP will not transfer rights to other organizations,
nor will it be used for any commercial purposes. ICTP is not to endorse or
sponsor any particular commercial product, service or activity mentioned in
this book. This book is released under the Attribution-NonCommercial-NoDerivatives
¦.þ International license. For more details regarding your rights to use and
redistribute this work, see
http://creativecommons.org/licenses/by-nc-nd/4.0/
Improving the Performance of Wireless LANs
This book quantifies the key factors of WLAN performance and describes methods for improvement. It provides theoretical background and empirical results for the optimum planning and deployment of indoor WLAN systems, explaining the fundamentals while supplying guidelines for design, modeling, and performance evaluation. It discusses environmental effects on WLAN systems, protocol redesign for routing and MAC, and traffic distribution; examines emerging and future network technologies; and includes radio propagation and site measurements, simulations for various network design scenarios, numerous illustrations, practical examples, and learning aids
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