System Level Modeling and Optimization of the LTE Downlink

5. Performance Evaluation of Fractional Frequency Reuse in LTEness loss), throughput and fairness performance has been evaluated over a range ofnumber UEs/cell values. Small confidence intervals are ensured by averaging overenough independent different channel realization sets and UE positions such thateach plotted point is obtained from averaging at least 500 UE throughput points.The results, shown in Figure 5.15, depict on addition the MU-gain analysis resultsfrom Appendix D, so as to compare the FFR MU gain results to those of differentreuse-1 scheduling strategies. Results indicate that, in order for a static FFR schemesuch as the one assumed in this chapter to work, at least 5 UEs per cell are necessary.With less, throughput results do not converge to the FFR result. While for the roundrobin case, fairness can be consistently increased at no throughput cost, it is alsoclear that the same behavior does not hold for PFs. At the optimum trade-off point,both cases do offer similar results. While the achievable fairness is slightly lowerfor PF than round robin (the PF scheduling algorithm pushes aways from extremeresults), throughput is slightly better for PF, as demonstrated in Figure 5.13. BestCQI scheduler results are shown as comparison to the maximum achievable multiusergain, which of course comes also at the expense of fairness.Avg. cell throughput [Mbit/s]240220200180160140120100806040Throughput10.9Fairness of UE throughput0.80.70.60.50.40.30.20.105 10 15 20 25 30number of UEs/cellFairness5 10 15 20 25 30number of UEs/cellRound Robin Proportional fair Best CQIRR no cell thr. loss PF no cell thr. lossRR max. fairness PF max. fairnessFigure 5.15: Performance over number of UEs per cell for the following schedulers: roundrobin, proportional fair, best CQI, FFR-enabled round robin, and FFR-enabledproportional fair. The performance of the the FFR-enabled schedulers is evaluatedfor the FFR configurations were (i) the optimum trade-off between fairnessand mean throughput is achieved and (ii) for the case where no mean throughputloss occurs compared to the reuse-1 case. Left: cell throughput. Right: UEthroughput fairness. Vertical lines: 95 % confidence intervals.The conclusion of this chapter is that, taking into account that PF scheduling isanyway used, due to its increased throughput at the same fairness level compared toround robin scheduling, the usefulness of FFR is limited to allowing a flexible fairness79