System Level Modeling and Optimization of the LTE Downlink

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