ncy) measurements and UEs can be moved from lower frequency band to higher or vice versa blindly. Figure 13.49 shows how call setup attempts can be balanced between frequency bands using idle mode load balancing. In Figure 13.49 the idle mode balancing is enabled at sample 27 and disabled at sample 40. As can be seen from Figure 13.49 the high band cells collect call setups faster than the low band cells. This is because in idle mode the high band is prioritized SO that all UEs that support high band and low band are camping on the high band cells. y = 7.6265x-0.098 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Estimated average number of subscribers per cell Figure 13.48 RRC setup attempts per subscriber versus estimated number of subscribers per cell per 15 minutes LTE Small Cell Optimization Low band (10 MHz) High band (10 MHz) 200,000 180,000 160,000 140,000 120,000 100,000 80,000 Idle mode load 60,000 balancing activated from 40,000 low band cells to 20,000 high band cells Figure 13.49 Load balancing activated to equalize the number of call attempts for both carrier fre- quencies. Idle mode load balancing activated for a cluster of cells and 15-minute statistics samples 13.7 Capacity Bottleneck Analysis During the mass events the PRB utilization typically reaches 100% (all PRBs from all available, e.g. 50 for 10 MHz, LTE are used) especially for uplink but also possibly for downlink as shown in Figure 13.50. The circled area 1 contains the samples for which the uplink data volume is fast increasing as indicated in Figure 13.2b. The average number of users with data Downlink Uplink 0 10 20 30 40 50 60 70 80 90 1001101201301 140 Average number of users with data in buffer UL/DL per cell Figure 13.50 Uplink and downlink physical resource block utilization versus average number of users with data in buffer per cell and per 15 minutes Capacity Optimization Downlink Uplink UL/DL data volume per cell (MB) Figure 13.51 Uplink and downlink physical resource block utilization versus uplink and downlink data volume 