# Handover-Driven Interference Management for Co-Channel Deployment of Femto- and Macro-Cells
## 6 Performance Evaluation
### Numerical Results  
Figure 4 shows the sample-time evolutions of the UL interference at the macrocell and three example femtocells: (a) without handover; (b) with per-user handover control (HO-FIM); and (c) with range control. Thirty macrocell users and thirty femtocells are distributed in the macrocell. The target SINR of macrocell users is set to \(-5\) dB while that of femtocell users is 5 dB (it is assumed that indoor femtocell users tend to use data-centric services requiring a higher data-rate). As shown in Figure 4, without handover, the macrocell's interference is \(-83.9\) dBm. When the per-user handover control is applied, however, a significant reduction of the interference is obtained every 100 slots (i.e., when handovers are triggered), and the macrocell's interference converges to \(-114.4\) dBm. The range control also results in a considerably reduced interference (\(-103.2\) dBm), but the achieved gain is smaller than the per-user control due to the reduced optimality, as explained above. Figure 4a reveals that the UL interference at some femtocells (depending on the presence of a nearby macrocell or femtocell user) could be serious (\(-67.1\) dBm at Femtocell 3). With HO-FIM, a similar result is observed for femtocells, i.e., convergence of the interferences at femtocells to significantly reduced levels. The achieved SINRs of different handover mechanisms are compared in Figure 5 for macrocell users (top) and femtocell users (bottom) when the target SINRs of macrocell and femtocell users are set to 0 and 10 dB, respectively. First, it is shown that the conventional handover control based on the strongest received power achieves much better SINRs of macrocell users than the case without using handovers. This result encourages exploitation of handovers to solve the femtocell interference problem, since even a non-optimal control of handovers improves performance. Such an improvement comes from: (1) reduced loss of path to serving cells; and (2) macrocell offloading. Either per-user (HO-FIM) or range control, as expected from the ability to reduce interference, further improves the achieved SINRs of macrocell users by making handover decisions towards the minimum system load. HO-FIM also improves the achieved SINRs of femtocell users. That is, HO-FIM benefits both macrocell and femtocell users, instead of sacrificing femtocell users to protect macrocell users, as many of the existing approaches do. The effects of the number of macrocell users (\(M\)) and femtocells (\(F\)) are explored in Figures 6 (no handover), 7 (per-user control), and 8 (range control). The results show a common trend that the increasing number of macrocell users deteriorates the SINRs of all users due to the increase in the number of interference sources. As the number of femtocells increases, the users' achieved SINRs get deteriorated when handovers are not used, while the SINRs are improved with HO-FIM. Figure 6 clearly shows a degradation of macrocell users' performance when \(F\) is increased from 10 to 50. In contrast, in Figure 7, the increase in \(F\) improves the SINRs of macrocell users with HO-FIM. This results from the increased possibility for macrocell users to be served by local femtocells, thus further offloading the macrocell and enhancing spatial reuse of channel resources. When 30 macrocell users  
Figure 4: Time evolution of macrocell (**left**) and femtocell (**right**) uplink interference for different mechanisms. Figure 5: Probability distribution of achieved SINRs of macrocell users (**left**) and femtocell users (**right**) for different mechanisms (\(M=20\), \(F=30\)). and 50 femtocell users within a macrocell are considered, more than 90% of macrocell users achieve higher SINRs with HO-FIM than the case without using handovers by 23.3 dB on average, which is 3.7 dB higher than the range control. The entire cohort of femtocell users with HO-FIM is shown to achieve 2.9 dB higher SINRs, on average.