connection requests 25,000 20,000 15,000 10,000 Day 1 Day 2 Day 3 Figure 12.10 Average random access setup success rate in a small cluster of cells and average number of radio resource connection setup requests received by eNodeB per hour per cell over a period of 3 days LTE Key Performance Indicator Optimization The eNodeB receiver algorithms do not support high UE velocity, that is, large Doppler shift compensation is missing and therefore the PRACH, PUCCH and PUSCH performance is not adequate. The UE cannot receive the random access messages due to either PDCCH or PDSCH message problems caused by coverage or interference conditions and therefore random access procedure needs to be repeated several times. The eNodeB cannot receive the RRC connection request message because of excessive interference (caused by external or intra-LTE interference) or wrong timing detection from received preamble (due to, e.g. too small cyclic shift length for random access preambles) or too low power offset between received preamble and RRC connection request message. Selecting the Preamble The random access procedure is started by the UE selecting one preamble from all available preambles. A fixed amount of 64 preambles is allocated per each cell and depending on the cyclic shift length, Ncs, one or several random access root sequences are needed per cell to generate all 64 preambles. It should be noted that the cyclic shift length defines the cell range for eNodeB to correctly estimate the timing advance. If eNodeB receives preambles beyond the defined cell range the timing advance estimation will be wrong and random access procedure will fail causing additional reattempts from the UE. Preambles per cell are divided into two subsets: preambles for contention-free random access procedure used for handover procedure and for reacquiring the uplink synchronization preambles for contention-based random access procedure used for signalling scenarios where initial synchronization establishment is required as well as for contention-free