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Systems and Techniques for Multicell-MIMO and Cooperative Relaying in Wireless Networks

Abstract : The constantly increasing demand for wireless services, the scarcity of radio spectrum and the characteristics of the global wireless market, necessitate that future wireless systems (Fourth Generation Mobile - 4G) provide higher peak data rates and better QoS, especially for the cell-edge users. Furthermore it is essential that they achieve high spectral efficiencies and they are easily deployed. In order to be able to accomplish these objectives, wireless systems need to incorporate technologies that increase the cell throughput without increasing spectral consumption. A very promising technique that can achieve the aforementioned targets is Multicell Cooperative Processing (MCP) or Multicell-MIMO. MCP has the potential to mitigate Inter-Cell Interference (ICI) and augment data rates without sacrificing additional spectrum but at the cost of some overhead and complexity. According to the concept of clustered MCP proposed in this thesis, Base Stations (BSs) are grouped into cooperation clusters, each of which contains a subset of the network BSs. The BSs of each cluster exchange information and jointly process signals as they form virtual antenna arrays distributed in space. In these systems, each user receives useful signals from several BSs and therefore the notion of a cell transcends the one of the conventional cellular systems. Although Multicell-MIMO is a technique that can help meet a lot of the challenges towards 4G systems, it has some intrinsic drawbacks that need to be addressed in order for it to be brought into practice; this is the main focus of the present thesis. Firstly the problem of how to optimally form BS cooperation clusters of limited size has been investigated. MCP's overheads are proportional to the size of cooperation clusters, therefore this size should be kept limited. The straightforward solution of forcing neighboring BSs to collaborate provides limited gains. In this thesis it is proposed that the BSs which interfere the most with each other should cooperate rather the ones that are in close proximity. This is shown to lead to significant spectral efficiency gains while cluster sizes are kept very small. The typical centralized architectural conception for MCP entails that the BSs of each cooperation cluster should be inter-connected through a control unit and exchange Channel State Information (CSI). This conception impedes the deployment of MCP systems as it implies additional infrastructural costs. In this thesis a new decentralized framework has been proposed that allows the incorporation of MCP by the conventional cellular systems with very few changes upon their architecture. Mobile Stations (MSs) feed back their CSI not only to one BS as in current systems, but they broadcast this information to all collaborating BSs, and the resulting inter-BS CSI information exchange requirement is minimal. In the downlink, a major overhead of MCP that needs to be mitigated is the one of CSI over-the-air feedback (i.e. mobile to base). Furthermore the collaborating BSs need to exchange the user data to be transmitted through the backhaul (backhaul overhead). For downlink communication under Frequency Division Duplexing (FDD), each user needs to estimate and feed back to the system infrastructure (one or more BSs) a number of channel coefficients, equal at least to the number of collaborating antennas at each subcarrier in Orthogonal Frequency Division Multiplexing (OFDM). This feedback load renders the deployment of MCP prohibitive in large scale deployments. In this thesis we suggest the use of a selective feedback approach. In this setup only the significant coefficients are fed back by the users; the ones whose channel gain exceeds a threshold. This approach can be also exploited in reducing backhaul overhead through scheduling or precoding design. It is shown that this is a good tradeoff between performance and overheads that can facilitate the incorporation of MCP by future systems. Another promising technique that can increase spectral efficiency of wireless systems is cooperative relaying. In this thesis the utilization of dynamic relays (user terminals relay signals) in cellular systems is investigated. Dynamic relays are more cost effective than static ones, as they bring the gains of relaying without the need for costly new infrastructure. However their utilization entails very high overheads and complexities (CSI feedback requirements, relay selection process). In the present dissertation the performance of dynamic relays in different cellular environments is assessed from a system level point of view and some novel techniques that exploit dynamic relays while requiring minimal overhead are presented. The overheads of relaying are proportional to the number of considered relay candidates (relay selection process). It is suggested that for a specific transmission only a small but suitable set of relay nodes are considered as relaying candidates. This is an efficient method to benefit from dynamic relays while circumventing their drawbacks.
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Submitted on : Sunday, June 5, 2011 - 4:41:19 PM
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  • HAL Id : pastel-00598244, version 1

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Agisilaos Papadogiannis. Systems and Techniques for Multicell-MIMO and Cooperative Relaying in Wireless Networks. Electronics. Télécom ParisTech, 2009. English. ⟨pastel-00598244⟩

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