System Level Modeling and Optimization of the LTE Downlink

4. Extensions to the L2S Model1140.94-QAM16-QAM64-QAM124-QAM16-QAM64-QAM0.80.7108m=0m=1m=2m=30.660.540.420.31 2 3 4 5 6 7 8 9 10 11 12 13 14 15MCS01 2 3 4 5 6 7 8 9 10 11 12 13 14 15MCSFigure 4.5: Inner (r m ) and outer (N m rep) code rates for HARQ modeling.Table 4.1.: Turbo code code rates (r m ) for HARQ modeling.Modulation Total r m4-QAM 4 1/1.70, 1/2.29, 1/2.44, 1/3.0016-QAM 6 1/1.67, 1/2.09, 1/2.42, 1/2.72, 1/2.83, 1/3.0064-QAM 161/1.08, 1/1.18, 1/1.34, 1/1.53, 1/1.81, 1/1.83,1/1.93, 1/2.08, 1/2.21, 1/2.28, 1/2.56, 1/2.58,1/2.68, 1/2.83, 1/2.96, 1/3.0026Figure 4.6 shows, for MCS 6, a BLER comparison after each retransmission and thecoding/repetition gain (10% BLER points marked). It is seen that most of the codinggain is always concentrated on the first retransmissions, while for retransmissionindexes higher than rv 1 , almost no r c -encoded bits remain to be transmitted. whilewith a repetition code, a gain of 3 dB would be obtained, due to the coding gain ofthe IR bits, a higher 5.7 dB gain is obtained.As new r c -encoded bits become depleted, r m converges to r c = 1/3, as observed onthe second and third retransmissions.The accuracy of the model is evaluated, for each MCS, in terms of the deviationof the simulated and modeled 10 % BLER points. It is evaluated for AWGN andITU Pedestrian-B channels and is depicted in Figure 4.7. Alternatively, Table 4.2lists the average deviation in dB obtained for each retransmission index and MCS,grouped by modulation alphabet.Focusing on the more realistic frequency selective case, it is observed that exceptfor the case of 64-QAM and rv idx > 2, the predicted BLER values show an averagedeviation lower than 0.4 dB. However, as in the case of the precoder precalculationshown in Appendix A, what is important is an analysis of the relevance of the56