## 5 Performance Evaluation
### Single-Spot Deployment Scenario
#### 5.1.2 Coexistence with Legacy Devices  
Next, we investigate the coexistence performance of MLO. The throughput results for the coexistence evaluation scenario in Figure 17 with and without the STR capability are shown in Figure 19. We consider a network in which four devices coexist. The throughput for the legacy-only case, in which only legacy devices exist, is shown as a baseline. In the other cases, two legacy devices, one on each channel, are replaced with two MLDs, and the per-device throughput performance is shown for both the MLDs (blue bars) and the remaining legacy devices (orange bars). If the orange bar for a particular scheme is shorter than that for the legacy-only case, this means that under this scheme, the MLDs aggressively occupy the channel resources and fail to coexist with the legacy devices in a harmonious manner.  
In the case with the STR capability, as shown in Figure 19a, we can see that most schemes except Sync, Sync-PL, and Sync-FT-Repick + Comp show significantly lower throughput of the legacy devices, implying that their throughput gains result from the decreased channel occupation time of the legacy devices. Sync shows significant performance degradation of the MLDs, while Sync-PL achieves a throughput similar to that with legacy devices only, as also shown in Figure 18. It is seen that Sync-FT performs aggressive operations, and its variants somewhat mitigate such aggressiveness. Among the variants, Sync-FT-Repick + Comp shows the throughput of the legacy devices that is closest to that in the legacy-only case, but its throughput gain is simultaneously reduced. Figure 19b shows the performance results without the STR capability. Most schemes result in a lower throughput of the MLDs than in the case with the STR capability, and thus, the legacy devices achieve more throughput, leading to smaller throughput gaps between the MLDs and legacy devices. The corresponding latency results are presented in Figure 20, showing that MLO is also beneficial in terms of latency. The observed coexistence performance trends in terms of latency are similar to the throughput trends described above.  
Figure 21 shows the per-device throughput performance for an increasing number of total devices in the network. The performance trends are similar between the cases with and without the STR capability. Sync-PL achieves almost the same performance as the legacy-only case. Sync shows poorer performance than the legacy-only case, and its performance degradation increases as the number of devices increases. The other MLO schemes achieve better performance than the legacy-only case. The throughput performance of the legacy devices is exactly the same as for the legacy-only case when the legacy devices are coexisting with MLDs under Sync-PL but worsens with the other schemes. Moreover, the degradation for the legacy devices increases as the number of devices increases. Among the various schemes, Sync-FT-Repick + Comp is shown to  
achieve the best coexistence performance, although it offers poorer coexistence for a higher number of devices since the contention between MLDs is already high and the legacy devices receive fewer channel access opportunities.  
Figure 18: Throughput performance of an MLD network with and without the STR capability when a specified number of legacy devices coexist in the single-spot deployment scenario. (**a**) With STR capability, (**b**) Without STR capability.  
Figure 19: Throughput in the coexistence scenario (Phase 1 vs. 2) with single-spot deployment. (**a**) With STR capability, (**b**) Without STR capability.  
Figure 20: _Cont._  
Figure 21: Throughput with an increasing number of devices in the single-spot deployment scenario (left: MLDs, right: legacy devices). (**a**) With STR capability, (**b**) Without STR capability.  
Figure 20: Latency in the coexistence scenario (Phase 1 vs. 2) with single-spot deployment. (**a**) With STR capability, (**b**) Without STR capability.