RachRootSequence Planning Prepared ByRAJESH KUMAR Random access preamble transmission • First step in the random access procedure is the transmission of a random access preamble • Indicates to the eNodeB the presence of a random access attempt and to allows the eNodeB to estimate the delay between the eNodeB and the UE • Random access preamble is transmitted on Physical Random Access Channel (PRACH) • In a first step of the random access procedure, the UE selects one preamble to transmit on the PRACH Preamble sequence generation • The preamble consists of two parts: – Preamble sequence – Cyclic prefix • • In each cell, there are 64 preamble sequences available Preamble sequences are constructed from one or several root Zadoff-Chu (ZC) sequences, by combining different cyclic shifts of each root sequence – Logical root ZC sequence index is broadcasted as part of the System Information – There are 838 different root ZC sequences used for random access and each ZC sequence consists of NZC = 839 samples – A new parameter is introduced in the current release for the Random access process: rachRootSequence Planning It can be beneficial to plan rachRootSequence such that the same root sequences are not used in neighboring cells – If the same root sequences are used in neighboring cells, and the PRACH allocation overlaps, this will lead to an increased false alarm ratio, where a random access preamble transmitted by a UE in the neighboring cell could be detected by the eNodeB – This is mainly a problem when the random access load is high – In cells, where UEs are traversing with high speed, the restricted set of root ZC sequences should be used • It could be applicable for cells that cover railways and highways • It is not applicable in USA because speeds on railways and highways do not exceed 105 mph (limit for 1900 MHz) 500 450 400 350 UE speed [km/h] • 300 250 200 150 100 50 0 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500 2600 Carrier frequency [MHz] rachRootSequence Planning RECOMENDATION › It is recommended to set the parameter rachRootSequence to different values in neighboring cells to reduce the probability for false RACH detections › The values must differ by at least 10 between any two neighbors (and at most 827 since there is a wrap-around between the first and last value in the value range) rachRootSequence GROUPING rachRootSequence Group rachRootSequence Group 1 5 15 25 15 425 435 445 2 35 45 55 16 455 465 475 3 65 75 85 17 485 495 505 4 95 105 115 18 515 525 535 5 125 135 145 19 545 555 565 6 155 165 175 20 575 585 595 7 185 195 205 21 605 615 625 8 215 225 235 22 635 645 655 9 245 255 265 23 665 675 685 10 275 285 295 24 695 705 715 11 305 315 325 25 725 735 745 12 335 345 355 26 755 765 775 13 365 375 385 27 785 795 805 14 395 405 415 28 815 825 835 rachRootSequence PLANNING Process › PN planning tool in Planet will be used to generate a PN plan and then convert the PN plan to the rachRootSequence plan 1. 2. 3. – – 4. – Best server layer will be generated Neighbor list will be created by utilizing best server layer in Planet A PN plan with an increment of 7 will be generated Gives 24 groups, no reserves rachRootSequence grouping includes 28 groups (see slide 6) Take the first 24 groups and replace the PN Plan created in Planet with values listed on slide 6 4 spare groups left THANKS