PCI & PRACH Root Sequence Planner

A PCI collision is two neighbouring cells with the same PCI: a terminal cannot tell them apart, so handover and measurement reports go to the wrong cell. A PCI confusion is two neighbours of the same cell sharing a PCI, even when they are not neighbours of each other: the serving cell cannot tell which one a measurement report refers to. Different PCIs can still clash on the modulo rules: 151 and 154 are both 1 mod 3, so as neighbours they share a primary synchronisation sequence.

They follow from TS 36.211. PCI mod 3 selects the primary synchronisation sequence and, with two or more antenna ports, which subcarriers carry the reference signals, PCI mod 6 sets the frequency shift of the cell specific reference signals, and PCI mod 30 selects the uplink reference signal sequence group. Neighbours sharing these values can interfere with each other. Because two PCIs equal mod 6 are also equal mod 3, avoiding mod 3 clashes already avoids mod 6 clashes.

The planner gives rules up one at a time, never collision, and reports which ones it gave up and between which cells. For example, a cluster of more than three cells that all neighbour each other cannot avoid sharing a PCI mod 3, because there are only three values. The plan is marked as not meeting the rules you asked for.

Paste them with their PCI and root sequence index filled in. Those cells keep their values and the planner allocates the free cells around them. Conflicts that were already there between existing cells are reported separately as pre-existing, because the new plan did not create them.

It is the smallest cyclic shift whose zero correlation zone covers the round trip delay to the largest cell edge, after the delay spread allowance. A larger shift than needed uses up root sequences, because each root yields fewer preambles. The full table is shown so the trade is visible.

Each cell needs 64 preambles, and one root sequence gives ⌊839 / N_CS⌋ of them in the unrestricted set, so the cell takes ⌈64 / that⌉ consecutive logical roots, starting at its rootSequenceIndex. A 10 km cell on format 0, with a 4.69 µs allowance, needs N_CS 76: 11 preambles per root, 6 roots per cell, and 139 cells before a root is reused. For the high speed restricted sets the preambles a root yields depend on the root, so the planner gives the cyclic shift and cell radius but leaves the roots to the vendor.