System Level Modeling and Optimization of the LTE Downlink

5. Performance Evaluation of Fractional Frequency Reuse in LTEThis work assumes a hexagonal grid of sites with three cells per site, with a constantnumber of UEs per cell and a full-buffer traffic model assumption. This results ina cell layout such as that in the example in Figure 5.1, where each cell contains acenter area employing reuse-1 and an outer area employing reuse-3.5.2. Previous WorkWhile capacity is commonly employed to determine the switching point between theFR and PR zones (simplified into a distance metric for circularly symmetric casesand a SINR threshold for sectorized cases), such a capacity-based approach does nottake into account the presence of multiple UEs and is thus incomplete [133].With the premise that a UE should be assigned to the FR or PR zone so as tomaximize its throughput, and that the PHY resources are shared among all UEsin a cell, capacity density (analogous to UE throughput through the assumptionof a uniform UE density over the cell area) was initially employed to evaluate thepotential benefits of FFR.pos y [m]35030025020015010050FR zone PR zone Combined FR&PR (FFR)pos y [m]pos y [m]350300250200150100503503002502001501005050 100 150 200 250pos x [m]50 100 150 200 250pos x [m]50 100 150 200 250pos x [m]2520151050−5−109008007006005004003002001000Figure 5.2: Capacity density-based FFR results for β FR = 0.75 and Γ thr = 3.20 dB. The cellarea combines a reuse-1 zone (marked green) with a reuse-3 area (marked red).The FR and PR areas of the cell are depicted in terms of SINR (center row) andcapacity density (bottom row). Column-wise, the spatial distribution is shownfor the FR zone (left), the PR zone (middle), and combined (right). FR and PRzone boundaries are marked with dashed lines.67