S Postgraduate Course in Radio Communications Hierarchical Cell Structures in CDMA Systems

Transcription

1 S Postgraduate Course i Radio Commuicatios Hierarchical Cell Structures i CDA Systems Kimmo Hiltue, 39195V Kimmo.Hiltue@ericsso.fi

2 Hierarchical Cell Structures i CDA Systems Cotets 1 INTRODUCTION WCDA SYSTE EVOLUTION SCENARIO SOE NETWORK OPERATION ASPECTS INTERERENCE PILOT POWER ADJUSTENT HCS DEPLOYED ON A SINGLE CARRIER ICROCELL BOUNDARY DESENSITIZATION UPLINK CAPACITY O ACROCELL/ICROCELL HCS DEPLOYED ON ULTIPLE CARRIERS CELL SELECTION Cell Selectio i Idle ode CELL RESELECTION Cell Reselectio i Idle ode easuremet Rules for Cell Reselectio whe HCS is ot Used easuremet Rules for Cell Reselectio whe HCS is Used Cell Reselectio whe Leavig Coected ode INTER-REQUENCY HANDOVER Iter-requecy Hadover Based o Coverage Iter-requecy Hadover Based o obile Speed ad System Load Iter-requecy easuremets with Compressed ode SUARY REERENCES Kimmo Hiltue

3 Hierarchical Cell Structures i CDA Systems 1 INTRODUCTION Durig iitial system deploymet the majority of WCDA system operators will focus upo providig radio bearer coverage ad quality of service. As the etwork matures, the focus will broade to iclude system capacity. I urba areas where the demad for capacity is especially high ad site acquisitio is particularly difficult, multi-layered etwork deploymet becomes a attractive solutio. The term multi-layered etwork represets etwork deploymets that are build o multiple (hierarchical) cell layers. The differet layers are characterized by features like cell carrier, size, output power, atea positio ad so o. Three mai types of layers havig commoly accepted features ca be foud i the literature: macro, micro ad pico layers. The differet cell types eable a efficiet ad flexible hadlig of traffic with differet characteristics i terms of service ad mobility. or istace, macrocells are suitable for esurig cotiuous coverage ad hadlig low capacity termials with high mobility. icrocells, which are ecessary to achieve good spectral efficiecy, ca be desiged to hadle high capacity termials with low mobility i highly populated areas. ially, picocells, deployed i idoor eviromet, ca serve may termials with very low mobility. Oe of the mai challeges for the radio etwork plaig process is to achieve full coectivity betwee cells belogig to differet layers, while maximizig the total system capacity. This paper will maily focus o a WCDA hierarchical cell structure (HCS) cosistig of macro ad microcells. 2 WCDA SYSTE EVOLUTION SCENARIO The umber of DD carriers affects the operators WCDA etwork deploymet scearios, ad the use of HCS scearios. To start operatig the etwork, the operator would typically begi with just oe carrier deployed o a macrocellular layer to provide cotiuous coverage. This applies especially to a greefield operator who caot rely o a existig GS etwork for coverage. Later, a secod carrier (ad possibly more) is deployed to ehace the capacity. The secod carrier ca be added to the macrocellular layer to create high-capacity sites or it ca be used to build a micro layer. I its first phase, the micro layer is typically deployed oly i traffic hot-spots or where high bit rates are eeded. urthermore, micro cells ca be used to fill coverage holes (black spots) withi the macro layer. ially, i later phases of the etwork deploymet, cotiuous microcellular coverage withi a specific area may be required, ad if further capacity is eeded more carriers must be deployed, usig either a ew frequecy if available, or reusig a carrier that has already bee used i aother layer. A example of a possible WCDA etwork evolutio path is show i igure 1. The required capacity ad coverage trade-off eeds to be carefully cosidered. Typically, withi the HCS i a WCDA etwork, the micro layer provides a very high capacity i a limited area, whereas the macro layer ca offer full coverage but with smaller throughput. Kimmo Hiltue

4 Hierarchical Cell Structures i CDA Systems Aother importat issue is whether the etwork should be able to support mobiles movig at high speed. If there is o such eed, the easiest way to cotiue is to sacrifice the macro layer ad put both frequecies to the micro layer. This alterative might, however, result i icreased ivestmet, which has to be carefully evaluated. O the other had, if a pure microcellular etwork has to support high-mobility users, there would be too may hadovers betwee the cells. Therefore, it is always beeficial to have a umbrella macro layer reserved for such users. The the strategy to icrease capacity further is to reuse oe frequecy i the other layer. f1 f1 f1 Cotiuous macro layer with frequecy f1 f1 f1, f2 f1,f2 Cotiuous macro layer with frequecy f1 High capacity macro cells with f1 ad f2 f1 f1, f2 f1, f2 f2 f1 f1 f1, f2 f2 f2 f2 f2 Cotiuous macro layer with frequecy f1 High capacity macro cells with f1 ad f2 Selected areas with micro cells with f2 Cotiuous macro layer with frequecy f1 High capacity macro cells with f1 ad f2 High capacity areas at traffic hot spots replaced by cotiuous micro layer with f2. igure 1. Example of WCDA etwork evololutio. 3 SOE NETWORK OPERATION ASPECTS 3.1 INTERERENCE It is impossible to cosider ay part of a WCDA system i isolatio. Chages to a part of the system may iclude chages over a large area. I WCDA, system capacity ad coverage are typically limited by the uplik ad/or dowlik iterferece. I uplik the iterferece comes from all the other mobile statios, ad i dowlik from the eighborig base statios. Although the umber of dowlik iterferece sources is low, the iterferece power is relatively high. urthermore, the iterferece power level depeds typically o the locatio of the user. ially, dowlik iterferece level is relatively high also i a low loaded system, sice the base statios always have to trasmit the dowlik commo chaels. I dowlik, the total trasmitted power is shared betwee the users. I uplik, there is a maximum iterferece level tolerable at the base statio receiver. Each user cotributes to the total iterferece, which is the shared betwee all users i the cell. If the performace of some liks ca be improved, the required trasmissio power levels i both uplik ad dowlik, ad as a result of that, the total iterferece are Kimmo Hiltue

5 Hierarchical Cell Structures i CDA Systems immediately reduced. I the ed, this reduced iterferece level results i improved capacity, coverage or lik quality. 3.2 PILOT POWER ADJUSTENT Power allocatio for the dowlik Commo Pilot Chael (CPICH) is aother very importat task i the WCDA etwork desig. Optimum pilot powers esure coverage with miimum iterferece to the eighborig cells. Excessive pilot powers will easily reserve too large portio of the total available base statio trasmissio power so that ot eough power is left for the traffic chaels. urthermore, the cell ca collect distat users, which do ot ecessarily have eough mobile trasmissio power to coect to the base statio, ad would be more optimally served by some other base statio. O the other had, pilot powers that are too low may ot provide wide eough pilot coverage, ad result i smaller coverage areas tha plaed. ially, if lik-power limits are set with respect to the pilot levels, low pilot powers also restrict lik powers. Typically, approximately 5-1% of the maximum base statio power is allocated to the pilot chael, ad roughly the same amout to other commo chaels. If a mobile is i locatio where several pilots are received with roughly equal sigal stregths, it may happe that oe of the pilot sigals is domiat eough to eable the mobile to start a call. Pilot coverage from eighborig base statios must overlap i cell border areas to accommodate hadovers. However, each cell that has sigificat power withi the soft hadover area will icrease the total iterferece power ad decrease the CPICH E c /N (eergy of the pilot sigal divided by the total chael power) for the domiat CPICH. The total chael power icludes the total received power from all base statios ad the thermal oise. Receivig too may pilot sigals ca degrade both the capacity ad quality, ad ca be preveted by proper etwork plaig. It is essetial to create a etwork pla, where cells have clear domiace areas [1]. 4 HCS DEPLOYED ON A SINGLE CARRIER A solutio where macro ad microcells operate o the same carrier frequecy ca be also called as Embedded icrocells. There, cells belogig to differet layers are separated by spatial isolatio, ad soft hadover is eabled betwee the layers. This solutio aims at maximizig the system capacity i case of o-homogeeous traffic demad. The service area of the micro base statio, situated below the rooftops, is surrouded by the service area of a macro base statio, situated above the rooftops. The service areas, with the exceptio of the soft hadover zoe, are disjoied 1, ad full coectivity betwee the layers is guarateed by the soft hadover itself. System capacity gai depeds to a great extet o the isolatio achieved betwee the differet layers. Isolatio ca be improved by: Placig the microcells so that most of the users i its service area have lie-of-sight propagatio coditios towards the servig base statios. 1 I case of embedded microcells, it is imperative that mobiles active withi the microcell area are ot coected to the macrocell, i order to avoid possible system performace degradatio due to iter-layer iterferece (i.e. ear-far problems). Kimmo Hiltue

6 Hierarchical Cell Structures i CDA Systems Lowerig the microcell atea positio ad makig use of the shadowig provided by the buildigs, i order to limit the iter-layer iterferece. Icreasig the spatial separatio amog base statios belogig to differet layers. Besides the isolatio betwee layers, the overall capacity gai depeds o the ability to pla microcells so that the service area is large eough to cover the hot spot traffic. This is due to the fact that the performace of the embedded microcell solutio is sesitive to chages i the geographical traffic distributio. A traffic hot-spot expadig outside the microcell coverage area will partly be served by the macrocell, hece losig at least part of the spectral efficiecy gai provided by the microcells. 4.1 ICROCELL BOUNDARY I case of the dowlik, the boudary betwee the macrocell ad a embedded microcell is defied by the received CPICH. Sice a ormal soft hadover algorithm is applied, the mobile ca be defied to be at the border betwee two cells, whe the received CPICH E c /N (or CPICH RSCP) is the same from both of them 2. Thus, at the cell border the received CPICH RSCP ca be expressed as P Rx ( A + 1α log( d )) PCPICH, + G + GUE, CPICH (4.1) PCPICH, + G + GUE ( A + 1α log( d ) where P Rx,CPICH P CPICH, P CPICH, G G G UE A A α α d d is the received CPICH power [dbm], is the trasmitted CPICH power from the macro cell [dbm], is the trasmitted CPICH power from the micro cell [dbm], is the macro base statio atea gai [db], is the micro base statio atea gai [db], is the mobile statio atea gai [db], is the atteuatio costat for the lik towards the macro base statio [db], is the atteuatio costat for the lik towards the micro base statio [db], is the atteuatio factor for the lik towards the macro base statio [db], is the atteuatio factor for the lik towards the micro base statio [db], is the distace towards the macro base statio [m] is the distace towards the micro base statio [m]. Equatio (4.1) leads to the followig relatioship ( d ) α α ( d ).1 [ ( PCPICH, PCPICH ) + ( G G ) ( A A )] 1 (4.2), 2 I fact, the specificatios allow the use of cell idividual offsets, which ca be used to adjust the locatio of the cell border. Here, however, all cell idividual offsets are assumed to be set to zero. Kimmo Hiltue

7 Hierarchical Cell Structures i CDA Systems Thus, the key issues that determie the cell shape are the path loss expoets, CPICH trasmissio powers ad the shape of the atea beams. urthermore, the size of the microcell depeds o the distace from the overlayig macrocell. igures 2 to 5 show i a simplified maer how the factors listed above cotribute to the shape of the microcell. Impact of path loss expoet o microcell shape ad size 2 acro: alpha3.5 icro: alpha4 icro: alpha6 CPICH received power [dbm] distace [m] igure 2. Effect of the path loss expoet o the microcell shape (P CPICH, 2 W; P CPICH,.2 W) Impact of microcell positio o cell shape ad size 2 acro icro: dm1 m icro: dm18 CPICH received power [dbm] distace [m] igure 3. Effect of micro base statio positio o the microcell shape (P CPICH, 2 W; P CPICH,.2 W, α 3.5, α 5). Kimmo Hiltue

8 Hierarchical Cell Structures i CDA Systems Impact of CPICH power o microcell shape ad size 2 acro: PCPICH2W icro: PCPICH.2W icro: PCPICH.1W CPICH received power [dbm] distace [m] igure 4. Effect of CPICH power o the microcell shape (α 3.5, α 4). Impact of the atea beam o the microcell shape ad size 2 acro icro: o beam gai icro: 1 db atea gai i the iward directio CPICH received power [dbm] distace [m] igure 5. Effect of beam shape o the microcell shape (α 3.5, α 4, 1 db atea gai i the iward directio). By lookig at the figures, the followig observatios ca be made: The outward boudary of the microcell is farther away from the micro site tha the iward boudary, as the differece i the slopes of the path loss curves is smaller. Whe α icreases (e.g. as a result of lower atea heights), the path loss curve becomes steeper, ad the size of the microcell becomes smaller. The larger the distace betwee the macro ad micro base statios, the larger the microcell. The larger the radio P CPICH, /P CPICH,, the smaller the microcell. The shape of the microcell shape depeds also o the directivity of the base statio ateas. The effect of directivity is equivalet to the icrease or decrease i trasmissio power i a certai directio, accordig to the atea gai. Kimmo Hiltue

9 Hierarchical Cell Structures i CDA Systems I igure 6 the impact of the distace from the macro site ad the atea diagram o the shape ad size of the microcell is visualized i a more efficiet way. There, is assumed to be equal to 1 db, ad the followig path loss models are applied: ( d ) 15 L log1 [db] (4.3) ( ) 5 L log1 [db] (4.4) d urthermore, the impact of log-ormal shadow fadig is igored. The dark areas surroudig the micro base statios describe the soft hadover zoes, assumig a soft hadover widow equal to 3 db igure 6. Shape of the microcell as a fuctio of the distace betwee the macro ad the micro base statio. acro base statio is marked with o [m] [m] igure 7. Coverage area of a microcellular system deployed uder a macrocellular system. acro base statios are marked with o. I a urba eviromet, buildigs ad other obstacles ca be used to limit the iter-cell (micro-micro) ad iter-layer (micro-macro) iterferece, still providig coverage where eeded. or example, i case of buildig blocks formig straight street cayos, the sigals ca propagate alog them for several blocks, while the microcell sigal might ot eve be detectable i the street o the other side of a buildig. igure 7 shows Kimmo Hiltue

10 Hierarchical Cell Structures i CDA Systems the total coverage area of a microcellular system cosistig of four base statios located uder a macrocell. rom the uplik poit of view the boudary betwee the cells is defied as the equilibrium poit where the trasmissio power P Tx required from the user by both cells is the same. Thus, P Tx where C C C C ( G + G ) + A + 1α log( d ) UE UE α ( G + G ) + A + 1 log( d ) is the required carrier power at the macro base statio [dbm], is the required carrier power at the micro base statio [dbm]. (4.5) Accordig to [2] balacig betwee uplik ad dowlik is recommeded, i order to esure smooth hadover ad to avoid orpha cases, where the mobile could be better covered by oe cell but better received by aother oe. However, exact lik balacig is ot achievable at all times, for example due to traffic variatios. Still, some rough (average) balacig ca be appropriate. Cosiderig both the uplik ad dowlik, the followig equatios ca be writte: C P G + L C G + L (4.6) CPICH, + G L PCPICH, + G L (4.7) where L ad L are the path losses towards the macro ad micro base statio, respectively. Whe the equatios are combied, the followig lik balacig relatioship is obtaied: P CPICH, PCPICH, C C (4.8) 4.2 DESENSITIZATION As described i [3] the miimum couplig loss (CL) betwee the cell site ad the mobile depeds o the cell-site atea height ad its gai. The CL is roughly equal to 7 db for macrocells, while for micro ad picocells, where the atea heights are much lower, the CL is approximately 53 ad 45 db, respectively. Such a low trasmissio loss makes the cell-site receiver susceptible to iterferece from various sources ad to saturatio by earby uits. It could therefore be ecessary to desesitize the microcell. However, the amout of desesitizatio required is less tha the excess couplig due to the lower atea placemet of the microcell [2]. A cell ca be desesitized by icreasig the oise figure of the receiver, or by addig a atteuator. The dowside of the desesitizatio is that it icreases the average mobile trasmit power withi the microcell. As a result of that, the uplik iterferece from the microcell towards other co- ad adjacet chael cells ad systems icreases, resultig for example i a lower macrocell capacity. Desesitizatio affects also the rage of the Kimmo Hiltue

11 Hierarchical Cell Structures i CDA Systems microcell for a give mobile statio trasmit power, but capacity rather tha rage is the primary issue cosidered i the desig of a microcell. 4.3 UPLINK CAPACITY O ACROCELL/ICROCELL Assume the simplified sceario show i igure 8. There, the macrocell layer cosists of omidirectioal macrocells with radius R, each oe uiformly loaded by the same umber of users,. urthermore, a sigle (circular) microcell with radius r ad the umber of users equal to K is deployed uder oe of the microcells. R D igure 8. Assumed system sceario [4]. Assumig ow a sigle service system with perfect ad ucostraied power cotrol, the received carrier power at the servig (primary) base statio is the same for all users. I this paper, C is the received carrier power at the macro base statio, while C m is the received carrier power at the micro base statio. Now, the total iterferece powers at the macro ad the micro base statio ca be approximated as ( + ) + KC N I 1 + (4.9) I C KC + C + N (4.1) where is the macrocell-to-macrocell iterferece factor, is the microcell-tomacrocell iterferece factor, ad is the macrocell-to-microcell iterferece factor [4]. The iterferece factors will deped e.g. o the microcell shape ad size, the distace D betwee the macro ad microcell ad the propagatio characteristics. or a sigle layer system, the followig uplik pole capacities ca be obtaied: 1 1 (4.11) ρ for the macrocell, ad Kimmo Hiltue

12 Hierarchical Cell Structures i CDA Systems 1 K (4.12) 1 1+ ρ for the microcell. Keepig i mid that C ( I C ) ρ ad C ( I ) the ρ C capacity of the macrocell as a fuctio of the umber of users coected to the microcell ca be solved as max 1+ K ( K K )( 1+ ) (4.13) ad the capacity of the microcell as a fuctio of the umber of users coected to the macrocell as K max 1+ K ( )( 1+ ) (4.14) Equatios (4.13) ad (4.14) ca be re-writte usig the relative capacities, η ad η K K (see igure 9, where it is assumed that.6,, ,, ,, ad, [4]): K max max ( η 1)( 1+ ) K ( η 1)( 1+ ) (4.15) (4.16) 1.8 Relative macro cell load D.4 D Relative micro cell load igure 9. The relative load i the macrocell versus the relative load i the microcell (r.2r, D (.4, 1)R). Kimmo Hiltue

13 Hierarchical Cell Structures i CDA Systems ially, the equatios estimatig the uplik oise rise ca be writte as I N K N ( K K ) N K ( K K )( 1+ ) 1 η ( η 1) N N 1 ( η 1)( 1+ ) (4.17) for the macrocell, ad I N 1 1+ K K ( )( 1+ ) 1+ N N ( )( 1+ ) 1+ 1 η 1+ 1 ( η 1)( 1+ ) N N 1 ( η )( + ) 1 1 (4.18) Assumig a certai maximum allowed values for the I /N ad I /N, the curves show i igure 1 ca be obtaied. There, it is assumed that N /N 1 (i.e. micro base statio is assumed to desesitized by 1 db) ad D.4R. or example, it ca be oticed that if the maximum allowed oise rise is 6 db for both layers, i balaced sceario the relative loadig level is.51 for the macro layer ad.64 for the micro layer. Thus, the macro layer capacity suffers clearly more from the iter-layer iterferece tha the micro layer capacity db 1 db db.7 6 db.6.6 Relative macro cell load db Relative macro cell load db.2 2 db Relative micro cell load Relative micro cell load igure 1. Relative macrocell capacity as a fuctio of the relative microcell load (left) ad relative microcell capacity as a fuctio of the relative macrocell load (right) assumig certai maximum oise rise levels. I this chapter oly oe microcell has bee assumed. However, the aalysis ca be exteded to the case of multiple microcells. That is, if microcells are sufficietly separated, the microcell-to-microcell iterferece ca be igored. However, if microcells are clustered, the microcell-to-microcell iterferece factor should be added i (4.1) as i (4.9). urthermore, the values for ad have to be adjusted based o the exteded sceario with multiple microcells. Kimmo Hiltue

14 Hierarchical Cell Structures i CDA Systems 5 HCS DEPLOYED ON ULTIPLE CARRIERS I this deploymet the hierarchical cell layers are operatig o differet (i most of the cases adjacet) carrier frequecies, ad the separatio betwee the layers is provided by the receiver ad trasmitter filters. As a result is this, there is o hard requiremet for disjoit service areas for the differet layers. Thus, users located withi the microcellular service area ca still be coected to the macrocell for e.g. mobility or load sharig reasos. However, as poited out i [3], some ear-far problems may still exist due to the implemetatio imperfectios i the trasmitters ad the receivers. or example, the micro base statios ca be surrouded by dowlik dead zoes for the mobiles coected to a adjacet chael macro base statio. Oe of the mai advatages of multi-carrier HCS deploymet is the ability to have a overflow of traffic from oe layer to a alterative oe, based o specific capacity maagemet ad load sharig strategies. The coectivity amog layers is esured by the iter-frequecy hadover fuctioality. I a system with overflow capability, a blocked call ca trigger a redirectio to the other layer if the capacity limit i the origial layer is reached. Besides that, a blocked itra-layer hadover request ca also trigger a overflow to the other layer. ially, a sigificat chage i speed ca lead to a overflow as well. As a cosequece of the better resource sharig betwee macro ad micro layers, the overflow capability ca improve the system Grade of Service (GoS) metrics, i.e. blockig ad droppig probabilities, which i the ed leads to a higher system capacity. urthermore, a careful selectio of the mobiles to be overflowed ca reduce the sigalig load ad/or promote the applicatio of specific service maagemet strategies. Next, some of the cetral Radio Resource aagemet algorithms eeded to fully utilize the HCS are described: Cell selectio, Cell reselectio, Iter-frequecy hadover 5.1 CELL SELECTION Defiitio 1: Cell selectio criterio The cell selectio criterio S is fulfilled whe [5] ad where S Q Q > (5.1) qual qualmeas qualmi S Q Q P > (5.2) rxlev rxlevmeas rxlevmi compesatio S qual S rxlev is the cell selectio quality value (db), is cell selectio Rx level value (db), Kimmo Hiltue

15 Hierarchical Cell Structures i CDA Systems Q qualmeas Q qualmi Q rxlevmeas Q rxlevmi is the measured CPICH E c /N (db), is the miimum required quality level i the cell (db), set i system iformatio, is the received sigal stregth, CPICH RSCP (dbm), is the miimum required Rx level i the cell (dbm), set i system iformatio. Parameter P compesatio is calculated as P compesatio ( UE _ TXPWR _ AX _ RACH P _ AX, ) max (5.3) where P_AX is the maximum R output power of the mobile (dbm) ad UE_TXPWR_AX_RACH is the maximum trasmit power (dbm) the mobile may use whe accessig the cell o RACH. The value for UE_TXPWR_AX_RACH is broadcasted i system iformatio. Hece, P compesatio is a compesatio value for the mobile that caot trasmit at the maximum allowed power o the RACH i the cells. The cell will shrik for those mobiles. Defiitio 2: Suitable cell A suitable cell is a cell which the mobile may camp o to obtai ormal service. Such a cell shall fulfill all the followig requiremets [5]: The cell is a part of the selected PLN. The cell is ot barred. The cell is ot part of a forbidde registratio area. The cell selectio criteria are fulfilled Cell Selectio i Idle ode Whe the mobile has selected the PLN to use, it shall create a cadidate list of possible cells to camp o. The list ca be created with either Iitial Cell Selectio or Stored Iformatio Cell Selectio [5]. Iitial Cell Selectio This procedure requires o prior kowledge of which R chaels are UTRA carriers. The mobile shall sca all R chaels i the UTRA bad to fid a suitable cell. O each carrier, the mobile searches first for the strogest cell ad reads its system iformatio, i order to fid out which PLN the cell belogs to. If the selected PLN is foud, the search of the rest of carriers may be stopped. Oce the mobile has foud a suitable cell (see Defiitio 1) for the selected PLN, the mobile shall select it. Stored Iformatio Cell Selectio This procedure requires stored iformatio of carrier frequecies ad optioally also iformatio o cell parameters, e.g. scramblig codes, from previously received measuremet cotrol iformatio elemets. Oce the mobile has foud a suitable cell Kimmo Hiltue

16 Hierarchical Cell Structures i CDA Systems (see Defiitio 1) for the selected PLN the mobile shall select it. If o suitable cell of the selected PLN is foud the Iitial Cell Selectio procedure shall be started. Thus, the iitially selected cell is the strogest suitable cell that has bee foud withi a frequecy bad belogig to a allowed PLN. Whe a suitable cell has bee foud, the mobile shall perform ecessary NAS registratio procedures. Whe the mobile has registered successfully, it shall camp o the cell (state Camped Normally). I this state, the mobile shall moitor pagig iformatio, moitor system iformatio ad perform radio measuremets. The measuremets shall be used i evaluatio of the cell reselectio criteria. The etwork cotrols what the mobile shall measure by sedig measuremet cotrol iformatio i the system iformatio. 5.2 CELL RESELECTION Cell Reselectio i Idle ode After the mobile has foud oe suitable cell for the selected PLN, it shall create a cadidate list cosistig of the selected cell ad its eighborig cells, as received i the measuremet cotrol iformatio via the selected cell. Whe the mobile triggers a cell reselectio evaluatio process (at certai time itervals), the mobile shall perform rakig of eighborig cells that fulfill the cell reselectio criteria. Defiitio 3: Cell Reselectio Criteria Whe judgig for the eed of cell reselectio, the followig two reselectio criteria are applied [5]: irstly, the quality level threshold criterio H for hierarchical cell structures is used to determie whether prioritized rakig accordig to hierarchical cell reselectio rules shall apply, ad is defied as H H s Q Q meas, s meas, Q Q hcs, s hcs, TO L (5.4) for the servig ad eighborig cell, respectively. If it is idicated i system iformatio that HCS is ot used, the quality level threshold criterio H is ot applied. Secodly, the cell rakig R is defied as R R s Q Q meas, s meas, Q Q hyst, s offset, s, TO ( 1 L ) (5.5) I the equatios above: Q meas Q hcs Q hyst is the quality of the received sigal, is the quality threshold level for applyig prioritized hierarchical cell reselectio, is the hysteresis value. Ca be used to expad the cell borders Kimmo Hiltue

17 Hierarchical Cell Structures i CDA Systems Q offset urthermore, i the equatios above TO of the servig cell to achieve a hysteresis effect ad to avoid pig-pog effects is the offset betwee two cells. Ca be used to move the cell border betwee two cells. ( PENALTY TIE T ) TEPORARY _ OSET W _ L L W W 1 if HCS _ PRIO if HCS _ PRIO ( x) for x < ( x) 1 for x HCS _ PRIO HCS _ PRIO s s Parameter TEPORARY_OSET applies a offset to the H ad R criteria for the duratio of PENALTY_TIE after a timer T has started for that eighborig cell. Oe should ote that TEPORARY_OSET ad PENALTY_TIE are oly applicable if the usage of HCS is idicated i system iformatio. The quality value of the received sigal, Q meas, is derived either from the averaged CPICH E c /N or the CPICH RSCP. The measuremet that is used to derive the quality value is set by the Cell_selectio_ad_reselectio_quality_measure i system iformatio. Depedig o the applied quality measuremet, differet parameter values are sigaled i the system iformatio broadcasts, see Table 1. Table 1. Cell reselectio parameters i system iformatio broadcasts depedig o the measure for Q meas. Parameter CPICH RSCP CPICH E c /N Q offset,s, Q offset1,s, Q offset2,s, Q hyst,s Q hyst1,s Q hyst2,s TEPORARY_OSET TEPORARY_OSET1 TEPORARY_OSET2 The timer T is implemeted for each eighborig cell. The timer T shall be started from zero whe oe of the followig coditios becomes true: if HCS_PRIO HCS_PRIO s ad Q Q meas, > hcs, Or if HCS_PRIO HCS_PRIO s ad Q meas, > Qmeas, s + Qoffset, s, Timer T for the associated eighbor cell shall be stopped as soo as ay of the above coditios are o loger fulfilled. Ay value calculated for TO is valid oly if the associated timer T is still ruig. Otherwise, TO shall be set to zero. Kimmo Hiltue

18 Hierarchical Cell Structures i CDA Systems At cell reselectio, a timer T is stopped oly if the correspodig cell is ot a eighbor cell of the ew servig cell, or if the criteria give above for startig timer T for the correspodig cell is o loger fulfilled with the parameters of the ew servig cell. O cell reselectio, timer T shall be cotiued to be ru for the correspodig cells but the criteria give above shall be evaluated with parameters broadcast i the ew servig cell if the correspodig cells are eighbors of the ew servig cell. The mobile shall perform rakig of all cells that fulfill the cell selectio criterio S amog: All cells that have the highest HCS_PRIO amog those cells that fulfill the criterio H. Note that this rule is ot valid whe UE high-mobility is detected (see Chapter 5.2.3). All cells, ot cosiderig HCS priority levels, if o cell fulfill the criterio H. This case is also valid whe it is idicated i system iformatio that HCS is ot used, that is whe servig cell does ot belog to a hierarchical cell structure. The cells shall be raked accordig to the R criteria specified above, derivig Q meas, ad Q meas,s ad calculatig the R values usig either CPICH RSCP or CPICH E c /N measuremets. ially, the best raked cell is the oe with the highest R value, ad the mobile shall perform cell reselectio to that cell. I all cases, the mobile shall reselect the ew cell, oly if the followig coditios are met: The ew cell is better raked tha the servig cell durig a time iterval T reselectio. ore tha 1 secod has elapsed sice the mobile camped o the curret servig cell easuremet Rules for Cell Reselectio whe HCS is ot Used If the system iformatio broadcast i the servig cell idicates that HCS is ot used, the for itra-frequecy ad iter-frequecy measuremets ad iter-rat measuremets, the mobiles shall use S qual for DD cells ad S rxlev for TDD for S x ad apply the followig rules [5]: 1. If S x > S itrasearch, the mobiles eed ot perform itra-frequecy measuremets. If S x S itrasearch, perform itra-frequecy measuremets. If S itrasearch is ot set for servig cell, perform itra-frequecy measuremets. 2. If S x > S itersearch, the mobiles eed ot perform iter-frequecy measuremets If S x S itersearch, perform iter-frequecy measuremets. If S itersearch is ot set for servig cell, perform iter-frequecy measuremets. 3. If S x > S searchrat,m the mobiles eed ot perform measuremets o cells of RAT m. If S x S searchrat,m, perform measuremets o cells of RAT m. If S searchrat,m is ot set for servig cell, perform measuremets o cells of RAT m. If HCS is ot used ad if S limit,searchrat,m is set for servig cell, UE shall igore it easuremet Rules for Cell Reselectio whe HCS is Used Assumig that system iformatio broadcast i the servig cell idicates that HCS is used, iter-frequecy cell reselectio measuremets are triggered whe the S rxlev of the Kimmo Hiltue

19 Hierarchical Cell Structures i CDA Systems servig cell drops below the threshold S searchhcs or whe the S qual of the servig cell drops below the threshold S itersearch. urthermore, if these parameters are ot set i the system iformatio the mobile has to measure all iter-frequecy cells all the time [5]. A special HCS priority (HCS_PRIO) ca be defied for the servig ad the eighborig cells. If there are cells i the eighbor list with higher HCS_PRIO tha the servig cell, these cells are measured all the time. With these priorities it is possible to force the mobiles to camp to micro layer wheever it is available. This approach makes sure, together with iter-frequecy hadovers, that the micro layer ca be fully utilized. Users with high mobile speeds ca be directed from micro to macro layer already i idle mode: If the umber of cell reselectios durig time period T CRmax exceeds N CR, high mobility has bee detected. Durig this high mobility state, the mobile will perform cell reselectio measuremets o all itra- ad iter-frequecy cells, which have a lower HCS priority level tha the servig cell. urthermore, it will prioritize the reselectio of itra- ad iter-frequecy eighborig cells o a lower HCS priority level before the eighborig cells o the same HCS priority level [5] Cell Reselectio whe Leavig Coected ode Whe returig to idle mode from coected mode, the mobile shall select a suitable cell to camp o. Cadidate cells for this selectio are the cell(s) used immediately before leavig coected mode. If o suitable cell is foud, the mobile shall use the Stored Iformatio Cell Selectio procedure i order to fid a suitable cell to camp o. Whe returig to idle mode after a emergecy call o ay PLN, the mobile shall select a acceptable cell to camp o. Cadidate cells for this selectio are the cell(s) used immediately before leavig coected mode. If o acceptable cell is foud, the mobile shall cotiue to search for a acceptable cell of ay PLN i state Ay Cell Selectio. 5.3 INTER-REQUENCY HANDOVER Cell Selectio process aims at fidig a cell for the mobile to camp o, whe it is returig from out of coverage or whe it is switched o. urthermore, the purpose of Cell Reselectio procedure is to make the mobile to camp o a cell, which provides sufficiet quality i terms of CPICH E c /N ad/or CPICH RSCP, eve if this is ot the optimal cell all the time. Hece, by applyig the idle mode cotrol with the usage of HCS parameters the mobile ca be made to camp to micro cell wheever it is available. While i Cell_DCH state itra-frequecy ad iter-frequecy hadover algorithms are required to support user mobility. A umber of iter-frequecy hadover scearios ca be highlighted, see igure 11. These differet scearios are discussed i the ext chapters. The dowside of the iter-frequecy hadover is that it is a hard hadover, i.e. ot seamless. urthermore, i order to be able to perform iter-frequecy measuremets, the mobile must eter ito a compressed mode trasmissio, which results i reduced quality. The compressed mode is discussed further i Chapter Kimmo Hiltue

20 Hierarchical Cell Structures i CDA Systems Load- ad mobility-based hadover Urba Rural f1/gs f1/gs f1/gs f1/gs f2 f2 f3 f3 f3 f3 Coverage-based hadover Coverage-based hadover igure 11. Iter-frequecy hadovers withi WCDA Iter-requecy Hadover Based o Coverage Two types of coverage-based iter-frequecy hadover scearios ca be highlighted: 1. obile coected to a micro cell moves out of micro layer coverage area. A coverage-based iter-frequecy hadover from micro to macro layer is required, see igure obile coected to a macro cell moves ito a micro layer coverage area. Due to the adjacet chael iterferece the macro cell dowlik coverage area might be shruk (see [3]), ad evetually, a coverage-based iterfrequecy hadover from macro to micro (or secod macro) layer is required, see igure 13. Urba Rural f1/gs f1/gs f1/gs f1/gs f2 f2 f3 f3 f3 f3 f3 f2 f1/gs Iitiate call i micro layer Coverage-based hadover Coverage-based due to HCS priorities from micro to macro hadover igure 12. icro-to-macro iter-frequecy hadover based o coverage. Kimmo Hiltue

21 Hierarchical Cell Structures i CDA Systems acro icro Poor DCH or CPICH coverage from macrocell due to adjacet chael iterferece igure 13. acro-to-micro iter-frequecy hadover based o coverage. The coverage-based iter-frequecy measuremets ad hadovers are typically triggered by the trasmitted power levels ad the lik quality measuremets i the followig way: Uplik: obile trasmissio power reported to RNC (triggered e.g. by the UE iteral measuremet evet 6a, The UE Tx power becomes larger tha a absolute threshold, or evet 6d, The UE Tx power reaches maximum value [6]) or uplik quality obtaied from the outer loop power cotrol. Dowlik: Trasmitted code power reported from the base statio to RNC or the dowlik quality (e.g. CPICH E c /N ) reported by the mobile As a example, assume that the macro layer cosists of B base statios ad the micro layer cosists of B u base statios, the CPICH E c /N for base statio b measured by mobile m ca be expressed as E N c m, b N m + CPICH, b m, b B Ptot, kgm, k B u P ACIR G + P tot, G ACIR m, k 1 m, k 1 m, (5.6) where P CPICH,b is the CPICH trasmit power, G m,b is the path gai betwee mobile m ad base statio b, N m is the oise power of mobile m, P tot,k is the total output power of base statio k, ACIR m,k is the Adjacet Chael Iterferece power Ratio betwee mobile m ad base statio k. Alteratively, the triggerig ca be based o trasmitted dowlik code power P m,b, which ca be modeled as Kimmo Hiltue

22 Hierarchical Cell Structures i CDA Systems B B Ptot kg u, m, k Ptot, Gm, N + + m k 1 ACIRm, k 1 ACIRm, Pm, b ρ m, b ( 1 α m, b ) Ptot, b G (5.7) m, b where ρ m,b is the required CIR for mobile m coected to base statio b, α m,b is the dowlik orthogoality factor for mobile m coected to base statio b (α meas perfect orthogoality) Iter-requecy Hadover Based o obile Speed ad System Load If fast movig mobiles are coected to small micro cells, the amout of sigalig related to e.g. hadovers ca be cosiderable. urthermore, due to the sigalig ad processig delays, resultig i hadover delay, the uplik iterferece ca icrease, or the dowlik ca be lost. Therefore, fast movig mobiles located withi the micro layer coverage area should be haded over from micro to macro layer, see igure 14. The high user mobility ca be detected e.g. by lookig at the frequecy of the active set updates. acro f1 icro f2 icro f2 icro f2 icro f2 X Too frequet active set updates withi micro layer. IHO to macro layer iitiated. igure 14. obility-based hadover I case of HCS solutio, the cells are orgaized i layers with differet priority levels accordig to the spectral efficiecy each layer ca provide; the highest priority level beig allocated to the layer that esures the best spectral efficiecy (e.g. micro layer). Normally, traffic is iitially directed to the highest priority layer available so that the total capacity per uit area is maximized. However, due to e.g. user mobility, load sharig betwee the layers may be eeded. Sice the micro layer ca provide a higher spectrum efficiecy tha the macro layer, a proper goal would be to aim at full utilizatio of the micro layer, where applicable. urthermore, the loadig betwee the layers should be balaced before iter-system hadovers are iitiated. The traffic steerig fuctio takig care of the overflow of traffic betwee the layers may be ivoked e.g. at every call set-up, or oly whe the target cell is lackig resources for a ew call or soft hadover leg. I the latter case, the mobile ca be Kimmo Hiltue

23 Hierarchical Cell Structures i CDA Systems overflowed to the alterative layer, if applicable. Otherwise, the call/leg ca be admitted i the target cell, oly if the required amout of radio resources ca be released first. Typically, the resources ca be released by triggerig overflow (i.e. iter-frequecy hadover) of mobiles havig a service or mobility profile suitable for the other layer. I certai cases, droppig may be eeded to eforce a specific service maagemet strategy, e.g. to give priority to a high demadig service over a low priority oe. I [7] a policy has bee proposed, which aims at esurig fair access amog differet service classes ad protectig itra-layer hadover traffic i the presece of complete resource sharig amog various services. Overload i the micro layer is maaged by forcig arrow badwidth calls to be haded over to the macro layer or dropped, i order to serve hadover ad/or ew call requests of a wider badwidth class. The aalysis poits out that ufairess amog services ca be reduced without resortig to resource partitio, thus avoidig trukig iefficiecy. As for the trade-off betwee capacity ad sigalig load, the use of a threshold velocity at call set-up determies the portios of the total traffic, which will be offered to differet hierarchical layers. The assessmet of the threshold velocity is a optimizatio issue, where the goal is to miimize the hadover rate while keepig the GoS above a acceptable level. If this optimizatio problem is solved durig the etwork plaig process, the optimal threshold will be obtaied assumig a certai traffic ad mobility parameters. Ufortuately, the characteristics of a system i operatio are chagig dyamically. Therefore, [8] proposes a method for a dyamical adaptatio of the threshold velocity, based o the above-metioed optimizatio goal. As a result, the threshold is modified accordig to the chages i the traffic load ad the mobility properties of the mobiles Iter-requecy easuremets with Compressed ode WCDA uses cotiuous trasmissio ad receptio ad ca ot make iter-frequecy measuremets with sigle receiver if there are o gaps geerated to the WCDA sigals. Therefore, a method called compressed mode is eeded for both iter-frequecy ad iter-system measuremets. The compressed mode meas that trasmissio ad receptio are halted for a short time i order to perform measuremets o the other frequecies, see igure 15. The itetio is ot to lose ay data but to compress the data trasmissio i the time domai. The stadard proposes three possibilities for the trasmissio time reductio [9]: Pucturig. A umber of bits of the coded data are simply discarded, resultig i a lower performace of the codig. I practice, this method is limited to rather short Trasmissio Gap Legths, sice the pucturig has some practical limits. The beefit is that the existig spreadig factor is kept ad therefore o ew requiremets are caused for the chaelisatio code usage. Icreasig the data rate by reducig the spreadig factor by 2. This method is suitable also for loger Trasmissio Gap Legths. Higher layer schedulig. Higher layers set restrictios so that oly a subset of the allowed Trasport ormat Combiatios are used i a compressed frame. The maximum umber of bits that will be delivered to the physical layer durig the compressed radio frame is the kow ad a trasmissio gap ca be geerated. Kimmo Hiltue

24 Hierarchical Cell Structures i CDA Systems Normal frame Compressed mode easuremet gap Normal frame igure 15. Compressed mode. Sice more power is eeded durig the compressed mode, the use of compressed mode will affect the WCDA coverage. urthermore, sice the fast power cotrol loop is ot active durig the silet period, ad the effect of iterleavig is decreased, a higher E b /N target is required, which affects the WCDA capacity. Due to the impact o WCDA capacity ad coverage, the compressed mode should be activated by the RNC oly whe there is a real eed to execute a iter-system or iterfrequecy hadover. This ca be doe for example by moitorig the dowlik trasmissio powers for each user, or with the help of mobile measuremets. As a example, the triggerig of compressed mode ca be based o the iter-frequecy reportig evets 2d ad 2f [6]: Evet 2d. The estimated quality of the curretly used frequecy is below a certai threshold. As a result of this evet, iter-frequecy measuremets are iitiated. Evet 2f. The estimated quality of the curretly used frequecy is above a certai threshold. As a result of this evet, iter-frequecy measuremets are termiated if they were iitiated for mobility reasos Parameterisatio of the Compressed ode A trasmissio gap patter sequece cosists of alteratig trasmissio gap patters 1 ad 2. urthermore, each of these patters i tur cosists of oe or two trasmissio gaps, see igure 16. The followig parameters are used to characterize a trasmissio gap patter [1]: TGSN (Trasmissio Gap Startig Slot Number): A trasmissio gap patter begis i a radio frame, heceforward called first radio frame of the trasmissio gap patter, cotaiig at least oe trasmissio gap slot. TGSN is the slot umber of the first trasmissio gap slot withi the first radio frame of the trasmissio gap patter. TGL1 (Trasmissio Gap Legth 1): This is the duratio of the first trasmissio gap withi the trasmissio gap patter, expressed i umber of slots. TGL2 (Trasmissio Gap Legth 2): This is the duratio of the secod trasmissio gap withi the trasmissio gap patter, expressed i umber of slots. If this parameter is ot explicitly set by higher layers, the TGL2 TGL1. TGD (Trasmissio Gap start Distace): This is the duratio betwee the startig slots of two cosecutive trasmissio gaps withi a trasmissio gap patter, expressed i umber of slots. The resultig positio of the secod trasmissio gap withi its radio frame(s) shall comply with the limitatios of [9]. If this parameter is Kimmo Hiltue

25 Hierarchical Cell Structures i CDA Systems ot set by higher layers, the there is oly oe trasmissio gap i the trasmissio gap patter. TGPL1 (Trasmissio Gap Patter Legth): This is the duratio of trasmissio gap patter 1, expressed i umber of frames. TGPL2 (Trasmissio Gap Patter Legth): This is the duratio of trasmissio gap patter 2, expressed i umber of frames. If this parameter is ot explicitly set by higher layers, the TGPL2 TGPL1. The followig parameters cotrol the trasmissio gap patter sequece start ad repetitio: TGPRC (Trasmissio Gap Patter Repetitio Cout): This is the umber of trasmissio gap patters withi the trasmissio gap patter sequece. TGCN (Trasmissio Gap Coectio rame Number): This is the CN of the first radio frame of the first patter 1 withi the trasmissio gap patter sequece. #1 #2 #3 #4 #5 TG patter 1 TG patter 2 TG patter 1 TG patter 2 TG patter 1 #TGPRC TG patter 2 TG patter 1 TG patter 2 Trasmissio gap 1 Trasmissio Trasmissio Trasmissio gap 2 gap 2 gap 1 TGSN TGSN TGL1 TGL2 TGL1 TGL2 TGD TGD TGPL1 TGPL2 igure 16. Illustratio of compressed mode patter parameters. 6 SUARY I this paper a brief overview of the hierarchical cell structures has bee give. Typically, i limited urba areas, where the capacity eed is especially high, ad site acquisitio is particularly difficult, etwork deploymet based o hierarchical cello structures (macro, micro, pico) becomes a attractive solutio. The differet cell layers are characterized by features like carrier frequecy, cell size, output power ad atea locatio. The hierarchical cell structure eables a efficiet ad flexible hadlig of traffic with differet characteristics i terms of service ad mobility. However, i order to achieve Kimmo Hiltue

26 Hierarchical Cell Structures i CDA Systems this, the radio etwork plaig process has to be efficiet eough to obtai full coectivity betwee cells belogig to differet layers, while maximizig the total system capacity. A task, which is ofte easier said tha doe. 7 REERENCES [1] J. Laiho et al., Radio etwork plaig ad optimisatio for UTS, Joh Wiley & Sos, Ltd, 22. [2] J. Shapira, icrocell Egieerig i CDA Cellular Networks, IEEE Tras. Veh. Techol., vol. 43, pp , Nov [3] K. Hiltue, Iterferece i WCDA ulti-operator Eviromets, S Postgraduate Course i Radio Commuicatios [4] D.H. Kim et al., Capacity Aalysis of acro/icrocellular CDA with Power Ratio Cotrol ad Tilted Atea, IEEE Tras. Veh. Techol., vol. 49, pp.34-42, Ja. 2. [5] 3GPP TSG RAN V3.12., UE procedures i Idle ode ad Procedures for Cell Reselectio i Coected ode (Release 1999), December 22. [6] 3GPP TSG RAN V3.13., RRC protocol specificatio (Release 1999), December 22. [7]. Satucci et al., Admissio Cotrol i Wireless Systems with Heterogeous Traffic ad Overlaid Cell Structure, Proceedigs of VTC 2 fall. [8] C. Hartma, O. Schlegelmilch, Hierarchical Cell Structures with Adaptive Radio Resource aagemet, Proceedigs of VTC2 fall. [9] 3GPP TSG RAN V3.11., ultiplexig ad chael codig (DD) (Release 1999), September 22. [1] 3GPP TSG RAN V3.1., Physical layer easuremets (DD) (Release 1999), arch 22. Kimmo Hiltue

27 Hierarchical Cell Structures i CDA Systems HOE EXERCISE Assumig a simple oe-directioal approach, calculate the diameter of the microcell dowlik service area, i.e. the value for R, see the igure below. Thus, here the service area icludes also the soft hadover zoes. Cosider two scearios: Sceario 1: Distace betwee macro ad micro base statio D 5 m Sceario 2: D 15 m Assume the followig parameter values: P CPICH, P CPICH, G G G UE A 32 dbm 22 dbm 15 dbi 5 dbi dbi 2. db (assumig that the uit for d is [m]) α 4. A 28. db (assumig that the uit for d is [m]) α 4. urthermore, the size of the soft hadover widow is 3 db. ially, igore the impact of log-ormal fadig CPICH RSCP [dbm] SoHO widow R SoHO Distace [m] D Kimmo Hiltue