16 J. Salo et alFigure 12. Example of u-hopping. Cluster #1 has hopping-pattern σ = 0 and cluster #2 has hoppingpatternσ = 1. Sequence offset ∆ ss = 0 for all cells. Systematic base sequence collisions within a groupare avoided within a group. Random base sequence collisions are possible in the cluster border.Figure 13. Example of cyclic shift planning. Cells of the same site are allocated the same base sequenceu using the offset parameter ∆ ss . Inter-cell interference between cells of the same site is mitigated bysetting different cell-specific cyclic shift (cs) for cells of a site.
<strong>Practical</strong> <strong>Introduction</strong> <strong>to</strong> <strong>LTE</strong> <strong>Radio</strong> <strong>Planning</strong> 17• Bypass the simple PCI-based base allocationscheme by explicitly defining the basesequence used in the cell. This brings additionalflexibility <strong>to</strong> base sequence allocation,and effectively decouples the PCIplanning from uplink DM RS base sequenceplanning.• The base sequence u can change pseudorandomlyfor every 0.5ms time slot.This planning option randomizes base sequencecollisions and averages inter-cellinterference.• Different cyclic shifts of a ZC sequence areorthogonal. This can be utilized by assigninga different cyclic shift on two cells thatuse the same base sequence u. The cellspecificstatic cyclic shift is broadcasted onBCCH.Defining u independently from PCI: The simplestscheme assigns the base sequence index u <strong>to</strong> acell as modulo-30 of PCI. Optionally, the basesequence can be assigned <strong>to</strong> a cell asu = (PCI+ ∆ ss ) mod 30 ,where ∆ ss = 0 . . . 29 is an offset parametersignalled on BCCH. In the simple PCI-basedscheme ∆ ss = 0. With ∆ ss it is possible <strong>to</strong> avoidcollisions in cells that would otherwise use thesame u due <strong>to</strong> PCI allocation. An example of ∆ ssplanning is shown in Fig. 11.Pseudo-random u-hopping: If u-hopping is activated,the base sequence used in the cell changesat every time slot in a pseudo-random fashion.The index of base sequence in time slot n 8u n = (ν n + PCI+ ∆ ss ) mod 30 ,where ν n = 0 . . . 29 is pseudo-random integerdefined by the hopping-pattern. The hoppingpattern defined used in a cell is defined by theindex σ = ⌊ PCI30⌋. Thus, there are 17 u-hopping8 There are 20 time slots in a radio frame. The hopping patternis re-initialized at the beginning of every radio frame.patterns defined since there are 504 PCIs defined,i.e., σ = 0 . . . 16. A foreseen methodof PCI planning, where near-by cells are assignednear-by PCI values, results in groupingof cells in<strong>to</strong> "clusters-of-30", where within eachcell cluster the same hopping-pattern is used. Toprevent systematic collisions, static part of thesequence group assignment, (PCI + ∆ ss ) mod30, should be different, especially for framesynchronizedcells. At the border of two cellclusters having different σ, random sequencegroup collisions are possible since two differenthopping patterns are utilized. An example ofthis planning scheme is shown in Fig. 12.Cyclic shift planning: So far we have consideredonly how <strong>to</strong> reduce inter-cell interferenceby assigning different base sequences <strong>to</strong> everyneighbouring cell. Another interference reductionmethod follows from the fact that ZC sequenceshave the useful property that two differentcyclic shifts of the same base sequence areorthogonal 9 . Therefore, if a pair of cells havebeen allocated the same base sequence u, intercellinterference can be reduced by assigning adifferent cyclic shift <strong>to</strong> the cells. This scheme canbe applied <strong>to</strong> cells of one site as shown in Fig.13. Other applications, tangential <strong>to</strong> our presentdiscussion, are uplink multi-user MIMO (whereeach UE uses a different cyclic shift) and, in <strong>LTE</strong>-A, uplink single-user MIMO (where each UEtransmit antenna uses a different cyclic shift), ora combination of the two. It should be notedthat cyclic shift planning can also be combinedwith the other two schemes discussed in thissection.6.3. PRACH parameter planningThe random access procedure in <strong>LTE</strong> uplinkbegins when UE transmits a preamble <strong>to</strong> theeNB. The specific preamble is selected randomlyby UE from a pre-defined set of 64 Zadoff-Chusequences. To avoid call setup anomalies, eachcell (within a reuse distance) has its own uniqueset of 64 preambles, and the information of the9 Cyclic shifts of extended ZC sequences used in uplink DMRS are not orthogonal, however.
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