7225056 multiple reuse patterns for frequency planning in gsm networks

8/14/2019 7225056 Multiple Reuse Patterns for Frequency Planning in GSM Networks

1/5

MULTIPLE USE PATTFOR FREQUENCY PLANNING IN G S

Stefan Engstrom, Thomas Johansson, Fredric Kronestedt,Magnus Larsson, Stefan Lidbrink, H&an Olofsson

Ericsson Radio Systems ABS-164 80StockholmSwedenAbstract - Providing high capacity in GSM networks at lowcosts using existing macrocells is of increasing importance in thenear future due to the competition between operators. Thispaper shows that by applying frequency hopping in combinationwith an advanced frequency planning method, Multiple ReusePatterns (MRP), very high traffic levels in the existing macro-cells can be supported. Field experience from live networksshows that an average frequency reuse factor as lowas7.5 is pos-sible without negatively affecting the network quality. Thus, he

network capacity can be doubled compared to a non hoppingnetwork with reuse 4/12 using 10 MH z frequency spectrum. In amacrocell, it is possible to carry as much as 43 Erlang at 2%blocking.I. INTRODUCTION

The cellular market has experienced an enormous subscribergrowth in the recent years. Today, GSM networks in morethan 100 countries serve approximately 65 millionsubscribers. It is of significant importance for the networkoperators to support high capacity in their networks atminimum costs due to the increasing competition [11.There are several ways to increase capacity from a cellplanning point of view. Methods in use today include cellsplit, overlaidunderlaid cells and hierarchical cell structures.In general, these methods can be divided into two groups,where one requires addition of new cell sites, while the otheronly implies installation of new transceivers in already exist-ing base stations. Deployment of new cell sites is often afairly slow process, due to site acquisition problems. This,together with the cost of new site equipment makes thisoption less efficient from a cost and implementation perspec-tive. The alternative method, to reuse existing cells and onlyadding transceivers, is thus an attractive option.Addition of transceivers to existing cells can be facilitated byapplying radio network features such as e.g. overlaidunderlaid cells, frequency hopping, power control andDiscontinuous Transmission (DTX). These features reduceandor change the characteristics of the network interferenceso that a tighter frequency plan can be applied and hence moretransceivers can be added.In this paper, a solution for providing high capacity in GSMusing existing macrocells is highlighted. The solution, knownas Multiple Reuse Patterns (MRP) [1-31, uses tight frequencyreuse in combination with frequency hopping. The paper alsoprovides results from MRP field trials. Finally, discussions

regarding 1/3 frequency reuse [4-51, a similar solution, areincluded.

11. FREQUENCYHOPPINGIncreasing network capacity by tightening the frequencyreuse results in a heavily interfered radio environment. Thismakes it more difficult to produce a frequency plan of goodquality. In the end, interference managing techniques such as

frequency hopping, power control and DTX are required inorder to secure the quality in the network. This paper considersonly the use of frequency hopping.With frequency hopping, frequency diversity will occur,which balances the quality between slow and fast movingusers. This implies that a cell planning margin for fast fading(Rayleigh fading) is not needed. Thus, an approximatecoverage gain equal to the fast fading margin can be achievedfrom the frequency diversity effect. Today, cell plannerstypically use 3 dB as the fast fading margin.Furthermore, frequency hopping also introduces interference

diversity [4]. Two aspects of interference diversity combine toimprove performance. Strong interferers are shared betweendifferent users, which is often referred to as intetfierenceaveraging. In addition, the time varying interference as suchincreases the interleaving efficiency and thus improvesreceiver performance.Altogether, a frequency plan with tighter reuse can beimplemented in a frequency hopping network, resulting inimproved capacity compared to a non hopping network.

111. MAXIMIZINGNTERFERENCE IVERSITYInterference diversity due to frequency hopping can be seenas a reduced correlation of the interference signalsexperienced for consecutive bursts. Figure 1 illustrates thissignal correlation decrease for three simplified scenarios, inwhich the uplink of a connection in cell A is interfered bymobile stations in co-channel cells. Cell A is assignedfrequencies 1 and 10 in all scenarios.In the leftmost scenario without frequency hopping, theconnection in cell A continuously uses frequency 1, andtherefore the interference I arises from the same user in cell B

all the time. The correlation of the interference signal onconsecutive bursts is thus high. If the connection has badquality, an improvement can only be made if the co-channelcell stops transmitting on this frequency or if the connection in

0-7803-4320-4/98/$5.000 1998 EEE 2004 VTC 98


2/5

No frequency hoppingI , IO

@, IOMS on frequency1 in cell A.Frequency hoppingwith traditional planning

8, IOMS on requency1 and 10in cell A.Frequency bopping withoutfixed frequency groups

I . 6

MS on frequency1 and IO in cell AFigure 1 . Example of the interference diversity eff ects of differen t fre-quency hopping strategies.

cell A is handovered (by an inter-cell or intra-cell handover).The middle scenario shows the traditional frequency hop-ping case, in which regular frequency groups are used. Theconnection in cell A hops on two frequencies (1 and lo),which are both used in cell B as well. Consequently, the inter-ference origin will alter between two users in cell B, causingthe two interference signals lI and Z2. Since the strength of lland Z2 may be rather different, the interference signal correla-tion may be fairly low for consecutive bursts. In other words,the interference diversity has increased compared to the non-hopping case.Finally, in the rightmost scenario, an irregular frequencyplan is applied together with frequency hopping. Typical forthis case is that there are no fixed sets of frequencies used in acell and its co-channel cells. Thus, cell B is only a partial co-channel cell of cell A, since they have only one frequency incommon. On the other hand, this arrangement creates anincreased number of (partial) co-channel cells, in this exam-ple represented by cell C. In this case, different bursts of aconnection in cell A will be interfered by users in differentcells. Thus, consecutive bursts will experience the interfer-ence signals Zl and Z2, which generally are totally uncorre-lated. Hence, this scenario is superior to traditional planningwith regular frequency groups in terms of maximizing inter-ference diversity.The example above indicates that to obtain maximal inter-ference diversity, a frequency plan without frequency groupsis preferable. However, such frequency plan has apparentdrawbacks, including the extensive re-planning necessary in acontinuously evolving network.By applying the MRP technique, it is possible to providemaximal interference diversity and still maintain a structuredfrequency plan, as will be described in the next section.

Iv. MULTIPLE EUSE PATERNSMultiple Reuse Patterns (MRP) is a generic method forachieving high capacity using tight frequency reuse in combi-nation with frequency hopping [l-31. The MRP techniqueexploits the advantages from frequency hopping in order toincrease the capacity. The fundamental idea with MRP is toapply different separate reuse patterns with different degrees

of tightness and use frequency hopping to combine them intoan average reuse. The goal is to deploy as many transceiversas possible in existing cells to minimize the number of costlynew sites. In this paper, only MRP using baseband frequencyhopping is considered.A. Band split

The first step with MRP is to split the available frequencyspectrum into different bands. One band is the BCCH bandwhich means that a frequency used as a BCCH frequency inone cell will not be used as a TCH frequency in another celland vice versa. The reasons for reserving unique BCCH fre-quencies are:Trafic independent BSZC decoding pelformance:When the mobiles are trying to decode the BSIC(Base Station Identity Code) on the SCH (Synchroni-sation Channel), the performance will not be affectedof the traffic load in the network. The reason is thatthe traffic assigned to the TCH frequencies will neverdisturb any BCCH frequency on which the SCH ismapped. BSIC decoding is very important for thehandover performance. Poor handover performancecould lead to increased number of dropped calls.Simplified neighbor cell list planning: The number ofpossible neighbor cell frequencies decreases with aseparate BCCH band. A simple strategy where all fre-quencies except the own BCCH frequency areincluded in the neighbor cell list can be used. Usingall available frequencies as BCCH frequencies mayresult in longer neighbor cell lists, which has negativeimpact on handover performance [6].Full gain from power control and DTX:Only TCHfrequencies can use DT X and power control in thedownlink. With a dedicated BCCH frequency band,full gain from power control and DTX is achieved inthe downlink [7]. This is not the case if the BCCH fre-quencies interfere with the TCH frequencies. Thus, amore aggressive power control approach can beapplied.Replanning of TCH frequencies will not affect theBCCH frequency plan : If additional transceivers areto be added to already existing cells, the BCCH fre-quency plan is not affected (assuming that the com-biner spacing requirements are neglected). The onlyrestriction to consider is the adjacent frequency inter-ference. Thus, it is possible to keep the same BCCHplan even if additional transceivers are added to thenetwork. The network operator then knows that if theBCCH frequency plan is good it will remain good,independent of the TCH frequencies.

As a next step, the MRP method implies that the remaining(TCH) frequencies are divided into different bands. Thus, oneBCCH and several TCH bands will exist. The main idea with

0-78034320-4/98/$5.000 998 IEEE 2005 VTC '98


3/5

several TCH bands is to apply different reuse patterns ondifferent TCH transceivers. The first TCH transceiver in allcells will use frequencies from the first TCH band and so on.The reasons for splitting the TCH frequencies into differentbands are:Dimension the a verage frequency reuse according tothe transceiver distribution of the network: The trans-ceiver distribution determines the average frequencyreuse that can be applied in the network. The averagefrequency reuse is adjusted according to the maxi-mum number of transceivers needed per cell and thenumber of cells requiring this number. In this way, thequality can be controlled in an effective manner in thefrequency planning process.Small impact on existing frequency p lan w hen addingmore transceivers: The TCH band split will limit therequired amount of frequency planning work whenmore transceivers are added. Only the cells with thesame number of transceivers or more will be affectedif more transceivers are added. For example, adding afourth transceiver to a cell with three transceivers willonly have an influence on the cells with four and moretransceivers.A structured way offrequency planning: It is possible,for instance, to make a frequency plan for the firstTCH transceiver without modifying the BCCH planor the plans for the other TCH transceivers. Thisstructure will aid the frequency planner in hisherwork by making it easier to produce a new frequencyplan and to detect a bad frequency plan.

B. Frequency AssignmenlThe M RP frequency assignment can be exemplified by

means of Figure 2, which shows a schematic picture of howthe different frequencies can be allocated to an MFWconfiguration with maximum four transceivers per cell. Theexample is referred to as a 12/10/8/6 plan. This means thatthere are 12 BCCH frequencies (frequencies 1,3, ..,23), 10TCH frequencies in group 1 (frequencies 2,4, ..,20), 8 TCHfrequencies in group 2 (frequencies 22,24, ..,36) and 6 TCHfrequencies in group 3 (frequencies 25,27,...,35). Figure 2shows only the frequency allocations for two cells A and Bwhich have two and four transceivers respectively. Cell A isassigned the BCCH frequency 1 and the TCH frequency 6.Thus, cell A uses baseband frequency hopping over twofrequencies. Further, cell B is allocated the BCCH frequency23 and the TCH frequencies 6, 26 and 35. Consequently, cellB uses baseband frequency hopping over four frequencies.Note that the frequencies defined as BCCH frequencies do nothave to be defined as shown in Figure 2. Hence, any frequencyfrom the available spectrum can selected as a BCCH as longas the BCCHRCH separation is fulfilled.There is no need to strictly adhere to the MRF technique allthe time. If a cell with quality problems exists, it is acceptable Diversity gam

Slow baseband hoppingover 4 frequencies8 e31 e 2 3 , 6 2 6 . 3 5 )Slow baseband hoppingover 2 frequencies( l e L6 )

small larger largest

v vCell A Cell BFigure 2. An exam ple of frequency planning with MRP.to solve this problem by changing a frequency in that cell withan illegal frequency which initially is used in anothertransceiver group. However, it is recommended that the MRPstructure is followed as much as possible.C. Tailoring the frequency plan

The M RP scheme has been developed to handle the typicalcase when networks have uneven transceiver distributions.This is important since every cellular network differs in char-acteristics regarding e.g. cell sizes, number of available fre-quencies and topography. This means that some cells havemany transceivers while other cells have only a few.To understand how the different cells with different numberof transceivers experience different frequency reuse situa-tions, an example is shown in Table I. A 12/8/6/4 M RP con-figuration which totally requires 30 frequency carriers isselected. There are 12 BCCH frequencies and 3 TCH groupseach containing 8, 6 and 4 requencies. In the example, it isfurther assumed that different fractions of the cells (20%,30% and 50%)have 2, 3 and 4 transceivers respectively.The average frequency reuse factor experienced by a cell isdefined as the total number of frequencies in the groupsassigned to the cell divided by the number of transceivers inthe cell. The different cells will thus encounter differentaverage frequency reuse factors: 10 for the two transceivercells, 8.7 for the three transceiver cells and 7.5 for the four

TABLE I An M RP example with 6 MH z of spectrum,where cells have an uneven distribution of transceivers.

2 1 3 0umberof I(transceivers/cell I 1 I IFraction of cells 20 % I 30% I 50%

MRPgroups ( 1 1 2 / 8 I 1 2 / 8 / 6 I 1 2 / 8 / 6 / 4 IAverage 11 (12+8)/2 (12+8+6)/3 (12+8+6+4)/4frequency reuse =10 I =87 =7.5

, I I I 4)I 10 I 9 2 I 8.9 Ictual reuse(upper bound)0-7803-4320-4/98/$5.000 1998 IEEE 2006 VTC 98


4/5

transceiver cells.The actual average frequency reuse experienced by a cellmay however be sparser, since not all cells are fully equipped.For example, since the third transceiver is only used in 80% ofthe cells, the actual reuse on that transceiver may be as sparseas 6/0.8=7.5,depending of the geographical distribution of thecells with the third transceiver. The upper bound for the actualaverage frequency reuse for cells with three transceivers istherefore (12+8+7.5)/3=9.2 (Table I).The gain from frequency hopping increases with the numberof frequencies included in the hopping sequence [4]. Cellswith a lot of transceivers may experience a tight reuse (whichlead to an increased interference level) but this will bebalanced with a larger interference diversity gain.The above example illustrates how MRP can be adjusted tothe transceiver distribution in the network. That is, the fre-quency plan is adjusted to the network. It should be furthernoticed that MRP do not need to be implemented over theentire network. MR P can just be applied in the area wherehigh capacity is needed. It is also possible to use differentMRP configurations in different locations of the network.

v. RESULTS FROM FIELD MEASUREMENTSThe MRP method is currently being used in more than adozen networks. This section presents MRP experience fromtwo typical live networks.Figure 3 shows results from a network where MRP weretested and later implemented. The traffic in the area causedhigh blocking, around 10-15%, before the trial started. Thetest area was a dense urban area and included 40 cells, allequipped with four transceiverskell. The site-to-site distancewas around 500 meters.The major result is that the dropped call performance (TCHdrop) was unchanged when the average frequency reuse was

decreased from 15 to 8.2. A low TCH drop value reflects agood network performance and a rate of 2% is normallyacceptable. The handover performance was the same for allthe different MRP plans. In addition, call setup, locationupdating and paging performance were not changed eitherover the course of the test. An average reuse of 8.2 was thuspossible.The capacity increase that the combination of frequencyhopping and MRP provide is different from network to net-work. The operator in this case had access to 60 frequencies.With a average reuse of 15, four transceivers per cell was pos-sible. This means that around 21 Erlang per cell was feasibleaccording to the Erlang B table with 2% blocking. With an8.2 reuse, it was possible to have 7 transceivers per cell. Thisfigure results in 43 Erlang per cell, i.e. the capacity was dou-bled compared to the 15 reuse configuration.Figure 4 includes result from MRP tests in another denseurban area. In this case, the trial area consisted of 36 cells, allwith four transceivers. This area had a site-to-site distance of400-500 meters. Again, the results showed that the call drop

TCHDrop [%I 40 ells, 4 transceiverslcellReference= I5 15 I 15 I 15M R P l = 1 3 / 1 2 / 1 0 / 1 0MRP 2 = 13 I 8 / 8 / 8MRP 3 = 13 I 8 I 8 I 6MRP 4 = 13 I 8 I 6 I 6I I

3 .0I

I I I I

I Reuse 15 lReuse -11.21 Reuse-9.2 I Reuse -8.7 I Reuse-8.21 1 I I I IN o M R e f e r e n c e M R P l M R P 2 MRP3 MRP 4

Figure 3. Results from a MRP trial where the average reuse was decreasedfrom 15 to 8.2. The test area dense urban environment with40 cells, allequipped with four transceivers.performance was more or less the same when going from afrequency reuse of 12 down to 7.5. A different call dropmeasure is used in this plot, Erlang*Min/Drop, which reflectshow many accumulated minutes a call can be maintainedbefore it is dropped. Furthermore, all other performanceindicators, such as e.g. handover, paging and call setupperformance, showed no degradation when the frequency planwas tightened.In this case, the potential capacity gain was 100% ifadditional transceivers would be installed as the frequencyreuse was tightened. In the implementation phase, instead ofusing 7 transceivedcell which is possible with a 7.5frequency reuse, 6 transceiverdcell were used. The savedfrequencies were used for implementing microcells in thearea. Thus, unique BCCH frequencies could be used in themicrocells, as recommended for MRP networks includingmicrocells [11 .7.5 reuse is not a lower limit for the reuse. Additional trialshave shown that by applying power control and DTX in thedownlink, the average reuse can be decreased below 7. Notethat the two previously shown examples were not usingdownlink power control or DTX.Additional capacity canmost likely be extracted from the macrocells in these trialareas by tightening the reuse even further.

Erlang*Min/Drop

601 36 cells, 4 transceiverslcellReference= 12 I 12 I 12 I 12MRP 1 = 1 2 1 9 1 9 1 9MRP 2 = 12 18 18 I 6M R P3 = 1 2 / 8 / 6 / 6M R P 4 = 1 2 1 8 1 6 1 4I I I40I I2o tR eu se 12 Reuse-9.7 Reuse8.5 Reuse8 Reuse7.5

Reference MRP 1 MRPZ MRP 3 MRP 4

Figure 4.changed from 15 to 7.5. n this ca se, test area located in dense urban environ-ment consisted of 36 cells, each with four transceivers.Results from a sec ond MRP trial where the average reuse was

0-7803-4320-4/98/$5.00 998IEEE 2007 VTC 98


5/5

The experience from live networks has revealed that thefrequency reuse limit mainly depends on three factors:the number of available frequencies (with a large amountof frequency spectrum, a tighter reuse can be appliedsince more frequencies can be included in the hoppingsequence which increases the diversity gain);the coverage performance (it is easier to perform the fre-quency planning when the coverage is good); andthe site-to-site distance (it is more difficult to create agood frequency plan when the cells are located close toeach other since each cell has many potential neigh-bours).

In the example in Figure 3, MRP was used for boosting themacrocell capacity only. Instead of using all frequencies in themacrocells, some frequencies could be reserved for other cells,e.g. microcells or indoor cells, as was the case in the secondexample (Figure 4). The traffic capacity of the existingmacrocells is in this scenario not doubled, but the new cells(microcellshndoor cells) improve the capacity even more.Using reserved frequencies will also result in easierimplementation of the new cells, since no co-channelinterference will exist between the macrocells and the newmicrocellshndoor cells.

VI. 1/3 REUSE WITH FRACTIONALOADINGAnother way of increasing the network capacity by applyingtight frequency reuse and frequency hopping is to use 113reuse on the TCH frequencies 14-51 (the BCCH frequenciesmust still use at least a 12 reuse). However, 1/3 reuse is just aspecial case of MRP where TC H frequencies all have equalreuse; an equivalent MRP configuration is e.g. 12/3/3/3/3.There are some advantages and drawbacks with 1/3 reuse

compared to the MRP configurations described earlier in thispaper. One advantage with 1/3 s an extensive diversity due tothe large number of frequencies in the hopping sequence.Furthermore, reduced frequency planning work may berequired for 1/3 reuse compared to MRP since only one TCHfrequency plan is required, assuming that no new cells areadded. More transceivers can also be added in existing cellswithout additional frequency planning.However, it might also be very difficult to produce a 1/3frequency plan in an irregular network with off-grid cells. Thequality for a problem cell can often not be improved bychanging the frequency plan. In most cases, the problem is dueto that the cell experiences interference from a large numberof other cells.Hence, finding a dominant carrier signal in thisarea may be a problem. Instead, a new site may have to beadded, since it is impossible to solve the problem by changingone or several frequencies. In the MRP case, it is possible tochange the frequency plan in a problem cell, since a sparserfrequency reuse is applied.A further disadvantage with 1/3 reuse is that the cells cannot

be loaded with traffic up to 100%. Full traffic load wouldresult in insufficient user quality. Hence, fractional loading isrequired, i.e. the traffic load must be limited in order tomaintain the quality. The number of frequencies that are usedin a cell is larger than the number of simultaneously availabletraffic resources. Fractional loading can be achieved either byusing hardware or software control [8-91.

MRP with slightly sparser reuse than 1/3 allow the cells to befully loaded, and therefore sparser reuse factors are commonin current MRP networks.VII. CONCLUSIONS

Implementing tight frequency reuse by using Multiple ReusePatterns (MRP) with frequency hopping in GSM has beenproven to be an efficient way to increase the radio networkcapacity with minimal costs for a network operator. Field testsfrom live networks show that it is possible to implement anaverage frequency reuse of 7.5 without jeopardizing thenetwork quality. Features like power control and DTX werenot used in the trials. For comparison reasons, a non-hoppingGSM network can at its best cope with approximately a 12reuse in average.With MRP, it is possible to adjust the tightness of thefrequency plan according to the transceiver distribution. At thesame time, MRP provides a robust frequency plan which isvery insensitive to changes, e.g. addition of transceivers.

REFERENCESM. Madfors et al., High Capacity with Limited Spectrum in CellularSystems , in IEEE Comm unications Magazine, Aug., 1997.J. Dahlin, Ericssons Multiple Reuse Pattem For DCS 1800, inMobile Communications International, Nov., 1996.A. Kolonits, Evaluating the Potential of Multiple Re-Use Patterns forOptimizing Existing Network Capacity, IIR Maximizing CapacityWorkshop,London, June, 1997.H. Olofsson et al., Interference Diversity as Means for IncreasedCapacity in G SM , in Proceedings o 1st EPMCC, Italy, 1995, pp. 97-102.J. Wigard et al., Capacity of a GSM Network with Fractional Loadingand Random Frequency Hopping, in Proceedings of the 7th IEEEPIMRC, 1996, pp. 723-727.H. Olofsson, S. Magnusson, M. Almgren, A Concept for DynamicNeighbor Cell List Planning in a Cellular System, in Proceedings ofthe 7th IEEE PIMRC, 1996, pp. 138-142.F. Kronestedt and M. Frodigh, Frequency Planning Strategies forFrequency Hopping GSM, in Proceedings of the 47th IEEE VTC,1997, pp. 1862-1866.M. Naghshineh and M Schwartz, Distributed Call Admission ControlIn Mo bilelWireless Networks, in Proceedings o the 6th IEEE PIMRC,1995, pp 289-293P. Beming and M. Frodigh, Admission Control in Frequency HoppingGSM Systems, in Proceedings o 47th IEEE VTC, 1997, pp. 1282-1286.

0-7803-4320-4/98/%5.000 998 IEEE 2008 VTC 98

7225056 multiple reuse patterns for frequency planning in gsm networks

Documents