ved as the overlap between the cells that allocate the preambles from the same random access preamble root sequence index is reduced and hence the amount of preambles detected by neighbouring cells with wrong timing advance (TA) estimation is reduced. Wrongly estimated TA causes message 3 reception failures and therefore reduced random access setup success rate and increased Random access setup success rate (%) Figure 12.12 Random access setup success rate improvement in a cluster LTE Small Cell Optimization Number of received preambles Random access setup success rate 35,000 30,000 25,000 20,000 15,000 10,000 Figure 12.13 Physical layer optimization impact on random access setup success rate in a cell inter-cell interference. An example of physical layer optimization impact on random access setup success rate and number of received preambles is given in Figure 12.13. The example in Figure 12.13 shows that after some antenna redirection and antenna downtilt tuning done for several cells, one of the cells inside the cluster received three times less preambles and also random access setup success rate improved during the peak times from ~60% to ~80%. The random access procedure signalling, that is, UE sending the preamble and eNodeB sending the random access response, does not contain any retransmissions of those messages but rather always a new preamble transmission is initiated. Therefore, the random access setup success rate (measured from network statistics) typically does not exceed 90%, and due to the resource usage and interference increase optimization the random access setup success rate performance target should be set to 80-90%. Once the RRC connection request has been transmitted, the UE starts the contention reso- lution timer (delivered to the UE in system information block type 2) to supervise message 4 reception as a response to message 3 transmission. The contention resolution timer is restarted at each message 3 HARQ retransmission. The UE uses temporary C-RNTI to monitor the PDCCH for message 4 c