System Level Modeling and Optimization of the LTE Downlink

3. Physical Layer Modeling and LTE System Level SimulationOf special interest is the feedback-wise more complex case of CLSM, where theoptimum precoder is precalculated at trace generation and the optimum rank chosenat run time, for which the L2S model closely approximates link level results.3.2.1.1. Complexity EvaluationIn this section, the run-time complexity of system level simulations is compared tothat of link level simulations, showing a significant reduction in simulation run timewhen employing the L2S model. Hence, it validates the statement, much-emphasizedin this thesis, that a link abstraction model allows for significantly faster simulationtimes compared to detailed link level simulations.The same single-user, single-cell described above is simulated for the LTE channelbandwidths of 1.4, 3, 5, and 10 MHz (link and system level), and additionally forthe 20 MHz bandwidth case for system level. As from the values, listed in Table 3.4,link level simulation run times scale linearly with the number of RBs, the link levelsimulation time for the 20 MHz case has been extrapolated from the existing values.Table 3.4.: Simulation run time comparison in seconds. Marked in bold face are the systemlevel simulation times, followed by the link level simulation times.Single TX1.4 MHz 3 MHz 5 MHz 10 MHz 20 MHz6 RBs 15 RBs 25 RBs 50 RBs 100 RBs1×1 28/282 28/703 28/1 115 28/2 300 30/-1×2 30/278 28/666 28/1 089 27/2 248 32/-TxD 2×2 27/614 28/1 542 29/2 552 28/5 086 29/-OLSMCLSM2×2 32/1 308 33/3 287 34/5 446 39/10 978 33/-4×2 32/1 426 32/3 842 33/6 598 35/15 040 36/-4×4 33/2 203 37/6 029 36/10 428 40/24 531 44/-2×2 33/358 32/901 33/1 509 32/3 399 34/-4×2 31/493 35/1 257 34/2 020 33/4 394 34/-4×4 37/874 34/2 310 36/3 563 38/7 535 42/-The almost constant simulation run time of the system level simulator can be explainedby the offloading of the most computationally-intensive task, which is thechannel trace generation. Although a detailed complexity analysis of the system levelsimulator has not been performed, the shown simulation results indicate that, whensimulating larger bandwidths, the complexity increase of the bandwidth-dependentpart of the L2S model (in this scenario the link quality model and the CQI and,when applicable, RI feedback 7 ) are almost negligible in comparison to the overallrun time.7 In a single-user scenario, no actual scheduling is performed. However, scheduling algorithms do46