System Level Modeling and Optimization of the LTE Downlink

3. Physical Layer Modeling and LTE System Level Simulationmodel combines the output of the link quality model with that of the applied modulationorder and code rate and predicts the BLER of the received TB. Ultimately,given this frame error probability, the successful or erroneous receiving of the TB israndomly decided via a coin toss corresponding to the BLER probability. Combinedwith the TB size throughput is then determined, as depicted in Figure 3.11.throughputlink quality model......link performance modelBLERsubcarrier SINR vectormodulation & code rateallocated RBsTB sizeFigure 3.11: Link performance model. The output of the link quality model is combined withinformation regarding the allocated RBs, and the employed modulation/coderate.The SINR-to-BLER mapping comprises an (n + 1)-dimensional mapping of n postequalizationsubcarrier SINRs values (γ 1 , . . . , γ n ) and the modulation and codingemployed to a single BLER value.While theoretically possible, it is in practice unfeasibly complex to obtain a mappingtable of the possible combinations of the n SINR values to a BLER value for eachMCS. Additionally, the length of γ varies depending on the number of RBs scheduledto the UE, with a maximum value restricted by the LTE channel bandwidth (seeTable 3.1).Over time, several methods to first map the sub-carrier post-equalization SINRvector, denoted as γ to an effective SINR value (γ eff ) have been proposed [88–91].While different names, such as Actual Value Interface (AVI) and Effective SINRMapping (ESM) exist, both relate to the same concept of mapping γ to an effectiveSINR value γ eff .In order to compress the SINR vector γ into a single value γ eff , Mutual InformationEffective SINR Mapping (MIESM) [90, 91] is employed, as it does not require an empiricalcalibration step such as previous methods as long as codes that perform closeto capacity are employed. The non-linear ESM averaging of MIESM is expressed asγ eff = I −1k(1N)N∑I k (γ n ) , (3.21)where N is the length of the SINR vector and I k the BICM capacity for the chosenmodulation at the given value γ n . The BICM capacity (I k ) for a modulationn=139