6,611 research outputs found

    Quasi-Dynamic Frame Coordination For Ultra- Reliability and Low-Latency in 5G TDD Systems

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    The fifth generation (5G) mobile technology features the ultra-reliable and low-latency communications (URLLC) as a major service class. URLLC applications demand a tight radio latency with extreme link reliability. In 5G dynamic time division duplexing (TDD) systems, URLLC requirements become further challenging to achieve due to the severe and fast-varying cross link interference (CLI) and the switching time of the radio frame configurations (RFCs). In this work, we propose a quasi-dynamic inter-cell frame coordination algorithm using hybrid frame design and a cyclic-offset-based RFC code-book. The proposed solution adaptively updates the RFCs in time such that both the average CLI and the user-centric radio latency are minimized. Compared to state-of-the-art dynamic TDD studies, the proposed scheme shows a significant improvement in the URLLC outage latency, i.e., 92% reduction gain, while boosting the cell-edge capacity by 189% and with a greatly reduced coordination overhead space, limited to B-bit

    Evaluating the effectiveness of Cooperative/Coordinated Multipoint (CoMP) LTE feature in uplink and downlink transmissions

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    Shannon demonstrated that the channel capacity depends of the ratio of the received signal power to interference plus noise power (SINR). Inter-cell interference caused by neighbouring base stations (BSs) has been identified as one of the most severe problem towards the deployment of LTE technology as it can significantly deteriorate the performance of cellside User Equipment (UE). However, because of regulatory and radiation restrictions as well as operational costs, signal power may only be increased only up to a certain limit to reduce the interference. The other common radio propagation impairment is multipath. Multipath refers to a scenario where multiple copies of a signal propagate to a receiver using different paths. The paths can be created due to signal reflection, scattering and diffraction. As will be discussed later the effects of multipath contribute little to intercell interference because multipath characteristics such as delay spread are compensated for using cyclic prefixes. In this work, we will limit our scope to interference as it has been identified as the main cause of performance degradation for cell edge users due to the full frequency reuse technique used in LTE. To mitigate interference 3GPP devised options of increasing the capacity in LTEAdvanced Release 12 which include the use of spectral aggregation, employing Multiple Input and Multiple Output (MIMO) Antenna techniques, deploying more base stations and micro and femto cells, increasing the degree of sectorisation and Coordinated Multipoint (CoMP). We are primarily interested in evaluating performance improvements introduced when uplink (UL) and downlink (DL) coordinated/cooperative multipoint (CoMP) is enabled in LTE Advanced Release 12 as a way of reducing interference among sites. The CoMP option of reducing interference does not require deployment of new equipment compared to the other options mentioned above hence network deployment costs are minimal. CoMP in theory is known to reduce interference especially for cell edge users and therefore improves network fairness. With CoMP, multiple points coordinate with each other such that transmission of signals to and from other points do not incur serious interference or the interference can even be exploited as a meaningful signal. In September 2011 work on specifications for CoMP support was started in 3GPP LTEAdvanced as one of the core features in LTE-Advanced Release 11 to improve cell edge user throughput as well as the average network throughput. We set to do field measurements in the evaluation of the effectiveness of CoMP in LTE. 3GPP LTE Release 12 was used and cell edge users' performance was the focus. The network operates in 2330 - 2350 MHz band (Channel 40). From the field measurements, it was demonstrated that the CoMP (Scenario 2) feature indeed effective in improving service quality/user experience/fairness for cell edge users. CoMP inherently improves network capacity. A seven (7) percent throughput was noticed

    Técnicas de pré-codificação para sistemas multicelulares coordenados

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    Doutoramento em TelecomunicaçõesCoordenação Multicélula é um tópico de investigação em rápido crescimento e uma solução promissora para controlar a interferência entre células em sistemas celulares, melhorando a equidade do sistema e aumentando a sua capacidade. Esta tecnologia já está em estudo no LTEAdvanced sob o conceito de coordenação multiponto (COMP). Existem várias abordagens sobre coordenação multicélula, dependendo da quantidade e do tipo de informação partilhada pelas estações base, através da rede de suporte (backhaul network), e do local onde essa informação é processada, i.e., numa unidade de processamento central ou de uma forma distribuída em cada estação base. Nesta tese, são propostas técnicas de pré-codificação e alocação de potência considerando várias estratégias: centralizada, todo o processamento é feito na unidade de processamento central; semidistribuída, neste caso apenas parte do processamento é executado na unidade de processamento central, nomeadamente a potência alocada a cada utilizador servido por cada estação base; e distribuída em que o processamento é feito localmente em cada estação base. Os esquemas propostos são projectados em duas fases: primeiro são propostas soluções de pré-codificação para mitigar ou eliminar a interferência entre células, de seguida o sistema é melhorado através do desenvolvimento de vários esquemas de alocação de potência. São propostas três esquemas de alocação de potência centralizada condicionada a cada estação base e com diferentes relações entre desempenho e complexidade. São também derivados esquemas de alocação distribuídos, assumindo que um sistema multicelular pode ser visto como a sobreposição de vários sistemas com uma única célula. Com base neste conceito foi definido uma taxa de erro média virtual para cada um desses sistemas de célula única que compõem o sistema multicelular, permitindo assim projectar esquemas de alocação de potência completamente distribuídos. Todos os esquemas propostos foram avaliados em cenários realistas, bastante próximos dos considerados no LTE. Os resultados mostram que os esquemas propostos são eficientes a remover a interferência entre células e que o desempenho das técnicas de alocação de potência propostas é claramente superior ao caso de não alocação de potência. O desempenho dos sistemas completamente distribuídos é inferior aos baseados num processamento centralizado, mas em contrapartida podem ser usados em sistemas em que a rede de suporte não permita a troca de grandes quantidades de informação.Multicell coordination is a promising solution for cellular wireless systems to mitigate inter-cell interference, improving system fairness and increasing capacity and thus is already under study in LTE-A under the coordinated multipoint (CoMP) concept. There are several coordinated transmission approaches depending on the amount of information shared by the transmitters through the backhaul network and where the processing takes place i.e. in a central processing unit or in a distributed way on each base station. In this thesis, we propose joint precoding and power allocation techniques considering different strategies: Full-centralized, where all the processing takes place at the central unit; Semi-distributed, in this case only some process related with power allocation is done at the central unit; and Fulldistributed, where all the processing is done locally at each base station. The methods are designed in two phases: first the inter-cell interference is removed by applying a set of centralized or distributed precoding vectors; then the system is further optimized by centralized or distributed power allocation schemes. Three centralized power allocation algorithms with per-BS power constraint and different complexity tradeoffs are proposed. Also distributed power allocation schemes are proposed by considering the multicell system as superposition of single cell systems, where we define the average virtual bit error rate (BER) of interference-free single cell system, allowing us to compute the power allocation coefficients in a distributed manner at each BS. All proposed schemes are evaluated in realistic scenarios considering LTE specifications. The numerical evaluations show that the proposed schemes are efficient in removing inter-cell interference and improve system performance comparing to equal power allocation. Furthermore, fulldistributed schemes can be used when the amounts of information to be exchanged over the backhaul is restricted, although system performance is slightly degraded from semi-distributed and full-centralized schemes, but the complexity is considerably lower. Besides that for high degrees of freedom distributed schemes show similar behaviour to centralized ones

    Timing and Carrier Synchronization in Wireless Communication Systems: A Survey and Classification of Research in the Last 5 Years

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    Timing and carrier synchronization is a fundamental requirement for any wireless communication system to work properly. Timing synchronization is the process by which a receiver node determines the correct instants of time at which to sample the incoming signal. Carrier synchronization is the process by which a receiver adapts the frequency and phase of its local carrier oscillator with those of the received signal. In this paper, we survey the literature over the last 5 years (2010–2014) and present a comprehensive literature review and classification of the recent research progress in achieving timing and carrier synchronization in single-input single-output (SISO), multiple-input multiple-output (MIMO), cooperative relaying, and multiuser/multicell interference networks. Considering both single-carrier and multi-carrier communication systems, we survey and categorize the timing and carrier synchronization techniques proposed for the different communication systems focusing on the system model assumptions for synchronization, the synchronization challenges, and the state-of-the-art synchronization solutions and their limitations. Finally, we envision some future research directions

    Esquemas de cooperação entre estações base para o LTE no sentido descendente

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    The explosive growth in wireless traffic and in the number of connected devices as smart phones or computers, are causing a dramatic increase in the levels of interference, which significantly degrades the capacity gains promised by the point-to-point multi input, multi output (MIMO) based techniques. Therefore, it is becoming increasingly clear that major new improvements in spectral efficiency of wireless networks will have to entail addressing intercell interference. So, there is a need for a new cellular architecture that can take these factors under consideration. It is in this context that LTE-Advanced arises. One of the most promising LTE-Advanced technology is Coordinated Multipoint (CoMP), which allows base stations to cooperate among them, in order to mitigate or eliminate the intercell interference and, by doing so, increase the system’s capacity. This thesis intends to study this concept, implementing some schemes that fall under the CoMP concept. In this thesis we consider a distributed precoded multicell approach, where the precoders are computed locally at each BS to mitigate the intercell interference. Two precoder are considered: distributed zero forcing (DZF) and distributed virtual signal-to-interference noise ratio (DVSINR) recently proposed. Then the system is further optimized by computing a power allocation algorithm over the subcarriers that minimizes the average bit error rate (BER). The considered algorithms are also evaluated under imperfect channel state information. A quantized version of the CSI associated to the different links between the BS and the UT is feedback from the UT to the BS. This information is then employed by the different BSs to perform the precoding design. A new DVSINR precoder explicitly designed under imperfect CSI is proposed. The proposed schemes were implemented considering the LTE specifications, and the results show that the considered precoders are efficiently to remove the interference even under imperfect CSI.O crescimento exponencial no tráfego de comunicações sem-fios e no número de dispositivos utilizados (smart phones, computadores portáteis, etc.) está a causar um aumento significativo nos níveis de interferência, que prejudicam significativamente os ganhos de capacidade assegurados pelas tecnologias baseadas em ligações ponto-a-ponto MIMO. Deste modo, torna-se cada vez mais necessário que os grandes aperfeiçoamentos na eficiência espectral de sistemas de comunicações sem-fios tenham em consideração a interferência entre células. De forma a tomar em consideração estes aspectos, uma nova arquitectura celular terá de ser desenvolvida. É assim, neste contexto, que surge o LTE-Advanced. Uma das tecnologias mais promissoras do LTE-Advanced é a Coordenação Multi-Ponto (CoMP), que permite que as estações base cooperem de modo a mitigar a interferência entre células e, deste modo, aumentar a capacidade do sistema. Esta dissertação pretende estudar este conceito, implementando para isso algumas técnicas que se enquadram no conceito do CoMP. Nesta dissertação iremos considerar a implementação de um sistema de pré-codificação em múltiplas células, em que os pré-codificadores são calculados em cada BS, de modo a mitigar a interferência entre células. São considerados dois pré-codificadores: Distributed Zero Forcing (DZF) e Distributed Virtual Signal-to-Interferance Noise Ratio (DVSINR), recentemente proposto. De seguida o sistema é optimizado com a introdução de algoritmos de alocação de potência entre as sub-portadoras com o objectivo de minimizar a taxa média de erros (BER). Os algoritmos considerados são também avaliados em situações em que a informação do estado do canal é imperfeita. Uma versão quantizada da CSI associada a cada uma das diferentes ligações entre as BS e os UT é assim enviada do UT para a BS. Esta informação é então utilizada para calcular os diferentes pré-codificadores em cada BS. Uma nova versão do pré-codificador DVSINR é proposta de modo a lidar com CSI imperfeito. Os esquemas propostos foram implementados considerandos especificações do LTE, e os resultados obtidos demonstram que os pré-codificadores removem de uma forma eficiente a interferência, mesmo em situações em que a CSI é imperfeita

    SCMA for Open-Loop Joint Transmission CoMP

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    Sparse Code Multiple Access (SCMA), a non-orthogonal multiple access scheme, has been introduced as a key 5G technology to improve spectral efficiency. In this work, we propose SCMA to enable open-loop coordinated multipoint (CoMP) joint transmission (JT). The scheme combines CoMP techniques with multi-user SCMA (MU-SCMA) in downlink. This scheme provides open-loop user multiplexing and JT in power and code domains, with robustness to mobility and low overhead of channel state information (CSI) acquisition. The combined scheme is called MU-SCMA-CoMP, in which SCMA layers and transmit power of multiple transmit points (TPs) are shared among multiple users while a user may receive multiple SCMA layers from multiple TPs within a CoMP cluster. The benefits of the proposed scheme includes: i) drastic overhead reduction of CSI acquisition, ii) significant increase in throughput and coverage, and iii) robustness to channel aging. Various algorithms of MU-SCMA-CoMP are presented, including the detection strategy, power sharing optimization, and scheduling. System level evaluation shows that the proposed schemes provide significant throughput and coverage gains over OFDMA for both pedestrian and vehicular users

    Resource allocation and feedback in wireless multiuser networks

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    This thesis focuses on the design of algorithms for resource allocation and feedback in wireless multiuser and heterogeneous networks. In particular, three key design challenges expected to have a major impact on future wireless networks are considered: cross-layer scheduling; structured quantization codebook design for MU-MIMO networks with limited feedback; and resource allocation to provide physical layer security. The first design challenge is cross-layer scheduling, where policies are proposed for two network architectures: user scheduling in single-cell multiuser networks aided by a relay; and base station (BS) scheduling in CoMP. These scheduling policies are then analyzed to guarantee satisfaction of three performance metrics: SEP; packet delay; and packet loss probability (PLP) due to buffer overflow. The concept of the τ-achievable PLP region is also introduced to explicitly describe the tradeoff in PLP between different users. The second design challenge is structured quantization codebook design in wireless networks with limited feedback, for both MU-MIMO and CoMP. In the MU-MIMO network, two codebook constructions are proposed, which are based on structured transformations of a base codebook. In the CoMP network, a low-complexity construction is proposed to solve the problem of variable codebook dimensions due to changes in the number of coordinated BSs. The proposed construction is shown to have comparable performance with the standard approach based on a random search, while only requiring linear instead of exponential complexity. The final design challenge is resource allocation for physical layer security in MU-MIMO. To guarantee physical layer security, the achievable secrecy sum-rate is explicitly derived for the regularized channel inversion (RCI) precoder. To improve performance, power allocation and precoder design are jointly optimized using a new algorithm based on convex optimization techniques
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