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PN OFFSET PLANNING STRATEGIES FORNON-UNIFORM CDMA NETWORKSChu Rui Chang, Jane Zhen Wan and MengF. YeeNORTEL Wireless Engineering Services

Richardson,TX 75083-3805Abstract - This paper presents a novelmethodology in planning the PN-offsets of a highlynon-uniform CDMA network and discusses themajor related issues. The strategy is applicable toany real life PCS and cellular networks. It isshownthat by optimally choosing/ arranging parameters, itisnot only possible to mitigate theCO- and adjacentPN-offset interferences, sufficient PN-offset valuescan also be reserved for new cells to be added intoexisting clusters for future network development.

I. INTRODUCTIONIn a CDMA system, pilots from all sectors arespreaded by the same PN Short Code, and a mobiledistinguishes the pilots via their distinct time shiftsof the basic sequence (PN-offsets). Therefore, acareful PN-offset planning should be designed toavoid pilot confusion for a CDMA network. In orderto distinguish a pilot from a remote BTS from amultipath component of the home pilot, enoughseparation between the adjacent E"-offsets must beprovided to avoid adjacent PN-offset confusion. Incases where PN-offset values must be re-used, there-used distance must be large enough to avoidPN-offset confusion.For a uniform network where all cells having similarradii, the PN-offset planning is relatively straightforward since the Short Code should providesufficient number of distinct PN-offsets toaccommodate a large PN re-use pattern, whilemaintaining a reasonably large adjacent PNseparations.However, the virtual PCS/Cellular network isseldom uniform. A typical situation isthat the cellsin rural and highway areas are much larger thancells in urban areas, as shown in Fig. 1. Forextremely non-uniform networks, the task of I"-offset planning becomes much more challenging. Inthe real network, the co-offset confusionis the majorconcern for small cell clusters, but the adjacent offset

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confusionismore likely to happen in large cells. Ifthe small cell clusters and large cell clusters areforced to use the same re-use pattern, then it ispossible that a sufficient reuse distance cannot bemaintained, particularly in between of small cellclusters and large cell clusters, as shown in Fig. 2.Therefore, a more sophisticated planning strategy isneeded.In this paper we will derive the criterion foravoiding the CO and adjacent PN confusions, examseveral possible PN-related problems that canhappen in a real system and present a practicalplanning strategy for a non-uniform CDMAnetwork.

11.CRITERION FOR AVOIDINGADJACENTAND CO-PN CONFUSIONSThe major task of PN-offset planning is to avoid thePN confusion problem. There is a significantdifference between a "I"confusion" and anordinary "interference".The interference caused byaPN confusion is 19.3 dB worse than an ordinaryinterference, assuming a 13.3 kbits vocoder isused.A CDMA signal that is not inside the Active SetSearch Window (SRCH-WIN-A) cannot bedespreaded by the fingers of the RAKE receiver, andwill only contribute to the background noise. Ontheother hand, a CDMA signal in the search windowdespreaded by a finger will obtain a 19.3dBprocessing gain. If a remote CDMA signal whichdoes not belong to the home cell but falls into theSRCH-WIN-A and becomes one of the threestrongest components, the mobile will treat thisremote signal as one of the multipath components ofthe home signal, and will perform coherentcombining on the two unrelated signals, resulting instrong interference (Fig. 3). It is the task of the PNoffset planning to avoid theI"confusion. However,the ordinary interference can only be controlled, butnot avoided.

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The adjacent PN-offset confusion can happen due tolarge differences in the propagation delay.Assuming two pilots with adjacent PN-offsets, whenreaching the mobile, the pilot with an earlier phasepropagates anextra distance that is large enoughsothat its phase shifts behind anditfalls into the searchwindow of the pilot with a later phase, the mobilewill confuse these two pilots as two multipathcomponents of the same pilot. Therefore a necessarycondition for the adjacent PN confusion is asufficient large difference in propagation delaybetween two signals.IS95 specifies that the minimum separation betweentwo adjacent valid I"-offsets is 64 chips, which islarger than most of multipath spread. To furtherreduce the chance of the adjacent PN confusion, theminimum separation can be increased by assigning aglobal parameter PILOT-INC > 1, so that theminimum adjacent PN separation becomes64*PILOT_INC [chips]. Since the RF wavepropagates 244 meters per chip, if two pilots withadjacent PN-offset separation of 64*PILOT_INCchips are to have confusion, the pilot with the earlierphase will have to travel an extra distance ofAD[m]= PILOT I NC X 64-- x 244 (I )[ - " I

where AD isthe differenceinpropagation distancebetween the home pilot and the remote pilot, andWA is SRCH-WIN-A of the mobile. In (l), theeffective PN-offset separationSisexpressed as:S[chip]= PILOT- INC x 64--1 (2)[ 2Note that hal f of the window size, W A /~,s usedsince the mobile always centers its Active Set SearchWindow to the earliest usable multipath componentof the arriving signal.

To avoid the PN confusion, itissufficient to requirethat either the PN separation S is bigger than thedifference in propagation distance; or the remotesignal isat least21dB weaker than the home signal.It can be proven that the minimum adjacent PNseparationmustbe:

S>R x -11 (3)L 2Where, cx is the propagation path loss exponent.From (3) t is clear that the minimum required PNseparation is proportional to the cell radius R. Ifusing the same adjacent PN separation through out

the system, the large cells are more vulnerable toadjacent PN confusion than small cells.Next we derive the criterion to avoid the CO-PNconfusion. In Fig.4, Cell A and Cell B both uses thesame PN-offset (omni-cells are the worst case) andCO-I"onfusion problem will happen if the pilotsfrom two different cells fall into the sameSRCH-WIN-A of the mobile, and both become oneof the three strongest signal components. Note thatifone pilot signal propagates much longer than theother, so that the difference in propagation delaycauses the remote pilot to "fall out" of the mobile'sSRCH-WIN-A, the mobile's search finger will neverfind the remote pilot and theCO-E"onfusion can beavoided. In Fig. 4, assuming the distance betweenCell A and Cell B is D. Further assume the worstcase scenario that a mobileis located between thesetwo cells and its distance to the home Cell A (the cellin which the mobile establishes its time referencefrom) isdb and its distance to the remote Cell B is(D - dH). To guarantee the remote pilot "fall out" ofthe Active Set Search Window, one needs

(D-d,)-d, >-WA2Since d , I R , from the above formula we obtainthe minimum required physical separation betweentwo BTS that reuse the same PN-offsets to be:

rrrD>%+2R2 (4)If Cell A and Cell B have different radii, then Rshould be the maximum cell radius.Assuming the number of cells in a PN reuse clusterisK, then the reuse distanceisapproximatelywhich reduces proportional to the cell radius R.Since the search window WA should be independentof R, the required CO-PNdistance, D=- 2R ,does not decrease proportionally with R. Thereforesmall cells will be much more vulnerable to CO-PNconfusion since they tend to haveavery small reusedistance.

D=R& (5)

WA2

A third type of PN-confusion is the so-calledconfused handoff. Assume two sectors A and Bhaving the same PN-offsets, Sector A isclose to themobile and is in the Neighbor List, and SectorB isnot. However, due to different antenna orientations

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or terrain conditions, the arriving signal from SectorB is strong (>T-add), but the arriving signal fromSector A isweak. If the PilotB falls into the mobile'sNeighbor Set Search Window W,, the mobile willconfuse PilotB as Pilot A and will handoff to SectorA, while despreading signal from Sector B. Thisresults in strong forward link interference (Fig. 5).Proper antenna down tilting which reduces thespilled RF energy to remote cells, isan effective wayto reduce the likelihood of confused handoff.Increasing the reuse distance, and proper allocationof the PN-offsets also helps to mitigate the problem.111I"-OFFSET ALLOCATION SCHEMESInl"planning, the first parameter to be determinedis PILOT-INC. The setting tradeoff is that a largePILOT-INC will increase the adjacent PN-offsetseparation, but will reduce the number of valid PN-offsets, which in turn will reduce the reuse distance.A low setting will do just the opposite. Also there isa lower bound on PILOT-INC, that isPILOT- INC x 64[chips]>max(W,, W,>Where W,,W, are the search window sizes for theRemaining and Neighbor Set. If this condition isviolated, the two adjacent search windows willoverlap and the measured value of the PN-offset of apilot found in the overlapping region will have anambiguity, as it belongs to both search windows.In the following, we present the I"-planningstrategy via an example. Assume the maximum cellradius 5 15 km (=61 chips), and further assumethat for the worst case the path lossexponenta =3.2,and (3)givesS=216 chips. If the search window WA= 28 chips, from (2) the PILOT-INC = 4, since(4x 64-14)=242>216. The total number ofvalid PN-offsets is then 512/PILOT_INC = 128.Assuming 3-sectors/cell, 128 valid PN-offsets willyield a total of 42 cells with distinct PN-offsets.The maximum search window size that can beaccommodated for PILOT-INC = 4 is 226 chips,whichismore than sufficient for most cases.For uniform networks (ideal case), we may deploy42 cells/cluster, or 37 cells/cluster plus somereserved PN-offsets for future use. It is desirable toreservecertain number of PN-offsets so that a new

cell can be inserted into theexisting cluster withoutdisturbingtheothers.A s mentioned previously (Fig. 1, 2), for a highlynon-uniform network, it is often not desirable toforce large cell clusters and small cell clusters intothe same reuse pattern. Typically the shape of thesmall cell clusters and large cell clusters aredifferent: small cells clusters usually cover an area(2-D), but large cells often cover the highways (1-D),as showninFig.1.We propose to divide the total available PN-offsetsinto two disjointsets,one for small cells, and one forlarge cells. In this way, thesmall cells and large cellsno longer share the PNs from the same set, the reusedistance from large cells to small cellsisno longer aconcern. Also the small cell clusters can have adifferent reuse pattem from the large cell clusters.The number of cells per reuse cluster for small cellsmust be much higher than that for large cells, sincesmall cells are more vulnerable to CO-PN onfusion.Depending on the actual cell configuration, werecommend K , =27-32 cells per reuse clusterfor small cells. For large cells, the number of cells perclusterKL can be much smaller, specially if thelarge cells are used only to cover the highways.Usually K , =7-12 issufficient. The resulting re-used distance must satisfy (4), also K , +K L 42.In frequency planning, the adjacent channels shouldnot be allocated totheadjacent sectors. The situationis just the opposite for I" planning: the adjacentsectors are _least vulnerable to adjacent PNconfusion. A large path difference is a necessarycondition for adjacent pilot confusion and it is leastlikely for two pilots from the adjacent sectors toproduce a large path difference. Since the twosectors are facing different directions and in orderfor two adjacent pilots starting from the same point(BTS tower), and ending at the same point (mobile),one pilot mustgothrough at least one reflection andbecome much weaker, as shown in Fig. 6. Also thepilot with earlier phase has to travel an extradistance of about 59 km [(l) gives U =(4x 64-14)x244=59km1, which is highlyunlikely. Therefore, one should allocate adjacent PN-offsets to adjacent sectors, and allocate non-adjacentPNstoremotesectors.

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Fig.2shows that it ismore likely for large cells withhigh antennas to cause interference to small cells,since signals from high antenna can propagate muchfarther. Therefore it is best to allocate theE" withlater phases to large cells and those with earlierphases to small cells. This further reduces thelikelihood of confusion because the propagationdelay will tend to shift the phase behind, soa pilotfrom a large cell with later phase, after propagationdelay, will appear with an even later phase and willbe even less likely get confused with the pilot fromsmall cells with early phases. The small cell pilots,with lower antennas, cannot propagate very far. Theonly exception is that because the Short Code isperiodic with the period =512*64 [chips], so a PN-offset of 512*64isthe same as I"-offset of 0. Thus itisnot desirable to use PN offset of 0.We recommendthe first I"-offset value starts at 4 [x 64 chips],and the last one be 508[x 64 chips1.The general expression for allocating PILOT-PN toeach sector is:{a,p, y } -Sector'sOffset

(6)=(PN, ) j +12k - 8, 4, O}Where (PN,) isthe first I"-offset value from thesets, and k= l , 2, ..., K j , K j is the number ofcells in a reuse cluster. Equation(6) isapplicable forboth the small cell and large cell clusters. For smallcell clusters, the subscript j =S in (6),and for largecell clusters, j =L .

Larae Cells in RurallSub A nSmall CellsNear theCenter

Figure1A typical situation where the urban areas arecovered by large number of small cells and rural/Hwyare covered by much larger cells. Also large cell clustershave different shapesfromsmall cell clusters.

The assignment of PILOTJ'Ns to each cell within asmall cell reuse cluster or within a large cell cluster,should follow a consistent fashion. For example, ifvalues are assigned to each cell within a cluster in aspiral fashion (Fig.2), then the assignment for otherclusters should follow the same fashion as well. Thisproduces an approximately equal reuse distance forevery cell in the cluster. However, the assignmentfashion for large cell clusters and for small cellclusters do not need to be identical, since they usePNs from disjoint sets.

IV . SUMMARY OF PN-OFFSETPLANNING STRATEGIES

There are three types of I"-offset confusionproblem: CO-PN, adjacent I" and confusedhandoff. I" confusion is more harmful thanconventional nterference.Propagation delay, search window sizes andpath loss exponent are the three key factors inPN-offset planning.Sufficient reuse distance and I" separation,proper antenna down tilt, together with properallocation of I"-offset values, reduces thelikelihood of PN confusion.For non-uniform networks, reuse clusters forlarge cells and small cells may have differentshapes and may use I"-offsets from twodisjoint sets; small cell clusters using I"-offsetswith smaller values and large cell clusters usinglarger offset values.

Fime2An exampleof forcing cells with differentsizesintot hesame reuse pattern. In this exampleK=7and R=2r.The reuse distance within small cluster is4.6r, andwithin large cluster is4.6R.But from largecell cluster to small cell cluster isonly2.3R.Thesituation smuch worse if R >>r.

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ShortCodeLength=2 =32768 Chips. . . , . . . .Remot e Pilot (phwe advanced)

'. . I... II '. ._I .._. .I I Ii HomePilot (phase hehind)I I 1~- *I I T h ehlp]4-ActiveSetScar ch Window

HomePilotDelayedRemote'lot ective S e t k i n d o w1

Figure3 Illustration of adjacent PN confusion, wherea pilot with an earlier phase propagated an extra distanceand its phase falls into the home pilot's SRCH-WIN-A.

CellA CellBMobile2! 5473

Figure5 Illustration of confused handoff. Assume thea-sectorof both Cell A and Cell B use the samePN-offset.A mobile located inCell A'sy-sector will haveCell A's a-sector as its neighbor. However, if the signalfrom Cell A's a-sector is weak but the signal from Cell B sa-sector is strong, and if the a-PilotB falls into mobile'sSRCH-WIN-N, the mobile will confuse a-PilotB asa-PilotA and will handoff to a-sectorof Cell A.

Pi l o t A Pilo t B

F i m e4 Illustration of CO-PN onfusion: both cellsuse the same PN-offset and their pilots fall into thesame SRCH-WIN-A. Note that the CO-PN onfusionwill not happen if the remote pilot travels an extradistance and falls out of SRCH-WIN-A.

00r*.III

Figure6 Illustration of allocating adjacent PNs toadjacent sectors. If amobile receivest wopilotsfromtwoadjacent sectors viaa direct path, then there willbe no path difference between them. For there to be anadjacentPN confusion, the signal with earlier phasehas togothrough at least one reflection and becomesmuch weaker; it also has to travel an extra59km.

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