System Level Modeling and Optimization of the LTE Downlink

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6. Summary and Outlookmodel. A run-time comparison has been carried out for a single-cell, single-userscenario, comparing link and system level simulator run times. The estimated simulationrun-time speed-ups for each of the implemented mode and antenna configurationare shown in Table 6.1 and range between 140x and 1 150x overall comparedto link level results. For the most complex case of CLSM, simulation run time gainhas been estimated at 359x.Table 6.1.: System level simulator speed-up compared to link level simulation run time,20 MHz bandwidth scenario.Single TX TxD OLSM CLSM1×1 1×2 2×2 2×2 4×2 4×4 2×2 4×2 4×4speed-up 153x 140x 351x 665x 836x 1 150x 200x 259x 359xIn Chapter 4, two enhancements to the L2S model are proposed. In the first part,the link performance model is extended to include HARQ in the BLER calculation.The joint processing of a received HARQ retransmission is modeled as a combinationof an inner channel code and an outer repetition code, the rates of which are derivedfrom link level results. The model is based on the structure of the LTE rate matcher,as well as a mutual information-based combining of the post-equalization SINRs ofeach of the retransmissions to estimate the BLER of the combined data packet.Additionally, the link quality model is extended to take into account imperfect channelinformation into the calculation of the post-equalization SINR of the data symbolpositions. The model is also based on the ZF receiver and characterizes the totalchannel estimation error as a linear function of the sum received interference powerand the noise level.In Chapter 5, the performance of FFR applied to LTE is evaluated. On a networklayout consisting of a hexagonal cell layout, all possible FFR configurations are testedcombined with (i) round robin scheduling and (ii) proportional fair scheduling. Inorder to avoid skewed throughput distributions, a fairness metric is introduced,which adds the constrain of no fairness loss with respect to the reuse-1 case.Simulation results show that, while with the round robin scheduler an overall increasein throughput and fairness is possible, with proportional fair scheduling, the gain islimited to the flexibility of exploiting a fairness-throughput trade-off, without thepossibility of a gain in both.In this static scenario, it is shown that FFR cannot properly function if less than5 UEs/cell are present, thus reducing its practical applicability to highly-loadedscenarios unless combined with dynamic allocation.82
6. Summary and Outlook6.2. OutlookWith LTE, as in the case of UMTS, enhancements to the standard began as soonas the first release (Release 8) was specified. Releases 10 and 11, termed LTE-Advanced, are already following a clear standardization path, with research proposalsfor Beyond-LTE (LTE-B) already trying to address future issues.The currently-implemented model is already adequate for single user MIMO andcorrectly scales for higher number of cells, which is expected as more dense cell deploymentsof small-cell sizes (pico-, femto-) are adopted to increase network throughput[138]. Minor enhancements to the model will allow it to evaluate energy efficiencyand latency issues, already hot topics of discussion for LTE-A/B [139, 140],as well as Carrier Aggregation (CA), which can bring the usable bandwidth to upto 100 MHz [83, 141].However, improvements in the PHY will provide new modeling challenges, as MU-MIMO and interference-coordination schemes such as Interference Alignment (IA)or Cooperative Multi-Point (CoMP) need to be included into the L2S model as well.As the number of small cells becomes higher, cellular networks will, up to a certainpoint, stop being planned and will rely more on self-organizing spectrum allocationto avoid interference, for which an extra modeling layer of inter-cell coordination andcommunication could be designed. Additionally, new models may also be neededto include the effect of Machine-to-Machine (M2M) communications where up to1000+ machines/cell may be sporadically communicating.In prior standards, it remained the duty of the physical layer to provide a link with anever-increasing spectral efficiency. Currently, the importance of MAC and inter-cellcoordination schemes in order to exploit multi-user gain and spatial reuse (smallercells) has steadily increased. With it grows also the importance of accurate andlow-complexity models such those presented in this thesis. Future link abstractionmodels will continue to allow performance evaluation of ever-more-complex networkdeployments with an ever-increasing degree of self-organization, interference coordinationand, as a result, increased performance.83
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DissertationSystem Level Modeling a
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I hereby certify that the work repo
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KurzfassungDie vorliegende Arbeit p
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ContentsContents1 Motivation and Sc
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Contentsix
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1. Motivation and Scope of Work1. M
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1. Motivation and Scope of Worklaye
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1. Motivation and Scope of Workperf
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BibliographyThe combined throughput
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2. 3GPP Long Term Evolution2. 3GPP
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2. 3GPP Long Term Evolutionup to 30
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2. 3GPP Long Term EvolutionNAS proc
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2. 3GPP Long Term Evolutionchannel
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2. 3GPP Long Term EvolutionSince th
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2. 3GPP Long Term EvolutionThe chan
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2. 3GPP Long Term EvolutionTable 2.
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3. Physical Layer Modeling and LTE
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System Level Modeling and Optimization of the LTE Downlink

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