y when the SINR collapses as shown in Figure 13.21. The rapid increase of required preambles also causes preamble collision probability (two UEs randomly select the same preamble) to increase and due to that increased amount of required preambles per RRC attempt and increased interference levels. LTE Small Cell Optimization Avg RSSI for PUSCH Random access setup success rate Avg SINR PUSCH RRC setup success rate Figure 13.20 Average received signal strength indicator (RSSI), signal to interference and noise ratio (SINR), RACH setup success rate and RRC setup success rate of highest loaded cells for aggressive uplink power control; 15-minute data samples are used The preamble collision probability as given in [5] is shown in Figure 13.22 as a function of RAO (random access opportunity, i.e. the amount of random access TTIs per radio frame) and random access intensity per second (sent preambles per second). In order to have very low col- lision probability and retransmissions of preambles due to that, the amount of random access TTIs per radio frame should be increased for mass events. Typically collision probability of below 1-2% is recommended in order to limit the additional interference increase. Number of contention-based preambles per RRC attempt Random access setup success rate RRC setup success rate Figure 13.21 Number of required preambles per RRC setup attempt for highest loaded cells using 15-minute data samples Capacity Optimization 10 RAO per 10 ms 5 RAO per 10 ms 3 RAO per 10 ms -2 RAO per 10 ms 1 RAO per 10 ms Random access intensity per second Figure 13.22 Preamble collision probability versus sent preambles per second In order to calculate the peak random access intensity per second based on the, for example, 15-minute period statistics a peak (highest 1-second value) to average (average per second measured over 15-minute period) ratio should be used. Figure 13.23 shows one example where peak to average ratio is ~5.6 for highest loaded time for highest loaded cell for number of received RRC connect