System Level Modeling and Optimization of the LTE Downlink

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2. 3GPP Long Term EvolutionIn LTE, multi-user diversity is exploited in both the time and frequency domains.UEs are assigned a set of RBs over time, thus exploiting both degrees of freedom(individual subcarrier allocation would require an excessive amount of signaling).While the exact RB allocation mechanism can vary between different modes [53],the procedure comprises the allocation of frequency resources to one or more UEs,as well the number of spatial layers, precoding, and MCS [56]. In the time domain,a scheduling granularity of one millisecond, corresponding to the subframe duration,is applied.According to the feedback received from the UEs, a scheduler must appropriatelyassign transmit mode, MCS, PHY resource allocation, and, if applicable, a suitablenumber of spatial layers and precoding matrix. Exploiting these degrees of freedom,the goal of a scheduler is typically to try to achieve maximum throughput whilemaintaining a certain degree of fairness [56–59].22
3. Physical Layer Modeling and LTE System Level Simulation3. Physical Layer Modeling and LTESystem Level SimulationIn order to evaluate the system level performance of a wireless network, complexsimulations encompassing a high number of network elements and its interconnectinglinks are employed. By upscaling the number of simulated links and networkelements, it is not only possible to assert if link level improvements do also improvenetwork performance, but also to test and evaluate the algorithms controlling thePHY and MAC layers. Most commonly, system level simulations are employed toevaluate the performance of scheduling and PHY resource allocation [59–61], theimpact of traffic models [62], or multi-user gain [63].A straightforward and conceptually simple approach to system level performanceevaluation would be to, for each link, perform all of the PHY and MAC layer procedures.Despite being simple, this approach does not scale well and results in impracticalsimulation times due to the high computational complexity of the channelcoding/decoding procedures and specially the MIMO receiver [64–66].Link level simulations are normally evaluated for a range of Signal to Noise Ratios(SNRs) or similar measures such as E b /N 0 [67], for which link performance isevaluated in terms of throughput. For the smallest defined LTE system bandwidthof 1.4 MHz, which results in smallest possible simulation run time, a typical link levelsimulation lasts in the order of hours, depending on the employed MCSs employed,MIMO configuration and receiver [68].In order to generate an interference-limited scenario analogous to a network deployment,typically a tri-sectorized hexagonal cell layout with no less than two rings ofsites, each with three sectors is employed [69], such as the setup depicted in Figure3.1. However, in order to correctly capture the effects of Multi-User (MU) gain23
Page 1: DissertationSystem Level Modeling a
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Page 13 and 14: ContentsContents1 Motivation and Sc
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Page 23 and 24: BibliographyThe combined throughput
Page 25 and 26: 2. 3GPP Long Term Evolution2. 3GPP
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5. Performance Evaluation of Fracti
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6. Summary and Outlook6. Summary an
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6. Summary and Outlook6.2. OutlookW
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A. SNR-independence of the CLSM Pre
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B. Correlation Matrices for Shadow
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C. Taylor Expansion of the ZF MSEC.
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C. Taylor Expansion of the ZF MSEAp
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D. Evaluation of Multi-User GainD.
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D. Evaluation of Multi-User GainAvg
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Abbreviations and AcronymsAbbreviat
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Abbreviations and AcronymsKPILLRL2S
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Abbreviations and AcronymsU-PlaneUT
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Abbreviations and AcronymsBibliogra
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Abbreviations and Acronymsdescripti
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Abbreviations and Acronyms[69] Tech
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Abbreviations and Acronymsfemto cel
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Abbreviations and Acronyms[134] Z.
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System Level Modeling and Optimization of the LTE Downlink

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