LVR/P-98:0360 Rev A 1998-06-05 1(14)

DUAL BAND PARAMETER TESTS

INTERNAL WITHINERICSSON

Abstract

This report presents different dual band tests performed in 1998. The aim wasto evaluate different strategies for distributing the traffic between the GSM900 and 1800 bands.

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Table of Contents1 INTRODUCTION ..........................................................................................3

2 CRO TESTS....................................................................................................3

2.1 OBJECTIVE ................................................................................................32.2 TEST CASE.................................................................................................32.3 RESULT .....................................................................................................32.4 CONCLUSION .............................................................................................6

3 LEVTHR TESTS............................................................................................6

3.1 OBJECTIVE ................................................................................................63.2 TEST CASE.................................................................................................63.3 RESULT .....................................................................................................73.4 CONCLUSION .............................................................................................9

4 HCS LEVEL 2 TEST IN THE CITY ...........................................................10

4.1 OBJECTIVE ..............................................................................................104.2 TEST CASE...............................................................................................104.3 RESULT ...................................................................................................114.4 CONCLUSION ...........................................................................................11

5 CLS TESTS IN THE CITY ..........................................................................11

5.1 OBJECTIVE ..............................................................................................115.2 TEST CASE...............................................................................................125.3 RESULT ...................................................................................................13

5.3.1 Handover.....................................................................................13

6 APPENDICES...............................................................................................14

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1 IntroductionDifferent tests have been performed to further evaluate how different dualband strategies can be used.

STS and TEMS measurements have been used for evaluation of every testcase. Data from STS have been collected during the 6 busiest hours, 14.00-20.00, of the day.

The tests were performed in BSC1 and BSC4. BSC1 was used for CRO andLEVTHR tests and the test sites were; 001, 002, 003, 004, 005 and 006. BSC4was used for CLS and HCS same layer tests and the sites were; 010, 011 and012.

Since dual band mobiles only are a smaller part of all mobiles in the networkand the counters count every event from all mobiles, it is sometimes difficultto see larger impacts of the changes.

2 CRO tests

2.1 ObjectiveThe objective with the test was to be able to decide if a low CRO (~8dB) in the1800 band is enough to keep the dual band mobiles in the desired band (1800)for a special traffic distribution decided by LEVTHR.

The objective was also to verify if a lower CRO in the 1800 band wouldimprove the call set-up performance.

2.2 Test caseTwo tests, during 2 separate days, were performed in BSC1.

Day 4 CRO1800=40 (80dB)

Day 5 CRO1800=4 (8dB)

HCS with 2 layers was used and LEVTHR was set to -95dBm for the 1800band. A CRO1800=40 was set in the network. Day 4 was then used as areference day. A change to CRO1800=4 was made during Day 5. A CRO1800=4will make it possible for the dual band mobiles to set-up the call also on the900 band.

2.3 ResultThe traffic on TCH was almost the same for the different bands during the 2test days, see Table 2.1. It is therefore assumed that these two days are verysimilar from a traffic behaviour point of view.

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A CRO=40 in the 1800 band will always force a dual band mobile to set-up acall in the 1800 band even though the signal strength is better in the 900 band.The assumption was that a lower CRO in the 1800 band should improve thecall set-up performance when the dual band mobile have a signal strengthclose to or worse than the sensitivity level of the mobile. Since ACCMIN is setto -110dBm in the network it is possible to set-up a call when the receivedsignal strength is less than –110dBm.

No improvements have been seen for the call set-up performance, due to toolow percentage of dual band mobiles that tries to set-up a call with low signalstrength. Random access performance and successful establishment of SDCCHand TCH are the same for the different test cases. The only change is the droprate on SDCCH, see Table 2.1, but this cannot be explained by the change ofCRO. The SDCCH drop rate on the 900 band is better when CRO=4.

DATE Day 4CRO1800=40

Day 5CRO1800=4

SYSTEM 900 1800 900 1800

TCHTraffic[Erlang]

29,7 46,7 29,4 48,8

Drop onSDCCH

1,8 0,2 1,3 0,3

Table 2.1 Traffic and SDCCH drop rate.

Changes have been found when it comes to find an equal performance of theidle and active mode behaviour.

The number of dual band assignment handovers have increased slightly for the900 to 1800 relations, but the number of assignment handovers have decreaseda lot more on the 1800 to 900 relations, see Figure 2.1. The same behaviourcan be seen when it comes to all dual band handovers. If CRO=4 is used, notthat many calls are set-up in the wrong cell according to locating. When a callis set-up in the wrong cell either an assignment handover to a better cell willbe performed or a normal handover will be performed after assignment (if thecall set-up time was too short for evaluating an assignment handover).

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Handover

0

500

1000

1500

2000

2500

3000

3500

Day 4900 - 1800

Day 41800 - 900

Day 5900 - 1800

Day 51800 - 900

Nu

mb

er of h

and

overs

0

50

100

150

200

250 Nu

mb

er of ass. h

and

overs to

better cell

HOVERCNTHOASBCL

Figure 2.1 Handover and assignment handover .

The traffic distribution on SDCCH, see Figure 2.2, has changed. There is anincrease of the traffic on the 900 band, when CRO=4 is used, but not higherthan the traffic on the 1800 band with CRO=40. Also the traffic on the 1800band was decreased, when CRO=4 was used. The increase on the 900 band ishowever less than the decrease on the 1800 band. An overall decrease of theSDCCH traffic and an equal distribution of the traffic between the two bandsis accomplished with CRO=4.

Traffic on SDCCH

0

1

2

3

4

5

6

CRO=40Day 4

CRO=4Day 5

Traffic [E

rlang]

GSM900GSM1800

Figure 2.2 Traffic on SDCCH.

The number of establishments on SDCCH have increased slightly on the 900band but decreased more on the 1800 band when CRO=4 was used, see Figure2.3. The SDCCH establishment is more evenly distributed between the bandsand an overall decrease of the number of establishments can be seen. Thedecrease is also reflected in the decrease of traffic on SDCCH, see Figure 2.2.

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SDCCH Establishment

0

5000

10000

15000

20000

25000

30000

35000

40000

CRO=40Day 4

CRO=4Day 5

GSM900GSM1800

Figure 2.3 SDCCH establishment.

2.4 ConclusionA CRO=4 is better when it comes to call set-up distribution. Calls will not beset-up in the wrong band as frequently as it did when a CRO=40 was used. Ahigh decrease in SDCCH traffic, SDCCH establishments, assignmenthandovers and handovers can be seen on the 1800 band. This decrease is in allcases, a lot higher than the increase of traffic, SDCCH establishments,assignment handovers and handovers on the 900 band.

3 LEVTHR tests

3.1 ObjectiveThe objective with the tests with different values of the HCS threshold is tofind a value that distributes the traffic in the best way. The congestion shall beas low as possible in both bands and no quality problems shall be introduced.

3.2 Test caseThree tests, in 4 days, were performed in BSC1.

Day 1 LEVTHR=95 TEMS referenceDay 2 LEVTHR=75Day 3 LEVTHR=85Day 4 LEVTHR=95 STS reference

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HCS with 2 layers was used, LEVEL=3 in the 900 band and LEVEL=2 in the1800 band. LEVTHR was changed for the 1800 band. The first day wasinitially used as a reference but the traffic is higher than the test days and istherefore only used as reference during the TEMS measurements. Anotherday, Day 4, was used as reference for STS measurement. This day had almostthe same amount of traffic as the other test days.

The TEMS log-file from Day 3 (LEVTHR=85) was not complete. Thismeasurement can therefore not be used to make any exact conclusionscompared to the other test cases, but gives a good estimate of the overalldistribution.

3.3 Result

DATE Day 2LEVTHR=75

Day 3LEVTHR=85

Day 4LEVTHR=95

SYSTEM 900 1800 900 1800 900 1800

TCH Traffic[Erlang]

34,5 42,0 31,1 46,3 29,7 46,6

TimeCongestion

1,0% 0% 0,6% 0% 0,6% 0%

Table 3.1 Traffic and congestion for the different test cases.

From STS point of view the traffic distribution is almost the same when usingLEVTHR=95 and LEVTHR=85, see Table 3.1 and Figure 3.1. The changecomes when LEVTHR=75. A larger part of the dual band traffic is then movedto the 900 band. There is no increase in congestion in any band whenLEVTHR is changed from 95 to 85. The congestion in the 900 band isincreased when LEVTHR is changed from 85 to 75, see Table 3.1.

The TEMS measurements show a slightly different result, see Appendix A.1,B.1 and C.1. The distribution between the bands changes the same amountwhen LEVTHR is changed from 95 to 85 as it does when LEVTHR is changedfrom 85 to 75.

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TCH Traffic distribution

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

LEVTHR=75Day 2

LEVTHR=85Day 3

LEVTHR=95Day 4

Percentage/1800Percentage/900

Figure 3.1 Traffic distribution between the bands.

The signal strength from the TEMS test drives, see Appendix A.2, B.2 andC.2, shows almost an equal distribution. But the TEMS dual band mobile findsthe 1800 band almost as strong as the 900 band. The average signal strengthwill therefore almost be the same regardless of the change of LEVTHR.

When it comes to quality it is very difficult to see any changes from the STSmeasurement for the dual band mobiles since the STS measurements are donefor all mobiles. When the dual band mobiles are moved to a worse or betterband the quality in that band will not be significantly affected. However whendual band mobiles are moved to a worse or better band, the performance ofthose dual band mobiles will change. The difference for dropped calls and theamount of dropped calls due to bad quality is shown in Figure 3.2 and Figure3.3. The changes of the drop call rate might depend on other effects than thechange of LEVTHR since the changes of the drop call rate are the same for the900 and 1800 band.

Drop rate 900 & quality drop

0,00

1,00

2,00

3,00

4,00

5,00

6,00

7,00

8,00

LEVTHR=75 LEVTHR=85 LEVTHR=95

[%] Drop 900

Drop quality 900

Figure 3.2 Drop rates 900 band.

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Drop rate 1800 & quality drops

0,00

0,20

0,40

0,60

0,80

1,00

1,20

1,40

1,60

1,80

2,00

LEVTHR=75 LEVTHR=85 LEVTHR=95

[%] Drop 1800

Drop quality 1800

Figure 3.3 Drop rate 1800 band.

Another way too see the quality is to look at all normal terminations and seehow many of them that are done at bad quality, see Figure 3.4. There is a smallimprovement when LEVTHR is set to a better signal strength.

Normal termination with bad quality

0,00

1,00

2,00

3,00

4,00

5,00

6,00

LEVTHR=75 LEVTHR=85 LEVTHR=95

Per

cent

whe

n ba

d qu

ality

Term quality 900Term quality 1800

Figure 3.4 Normal termination with bad quality present.

The speech quality from the TEMS test drives , see Appendix A.3, B.3 and C.3shows almost equal distribution for the different tests. The speech quality isonly measured on the downlink and the quality problems are on the uplink forthe 900 band.

3.4 ConclusionThe traffic distribution is changed in favour of the 900 band first when theLEVTHR is changed to 75. However the congestion is also then increased for

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the 900 band. No changes can be seen when it comes to dropped calls andquality for the different bands. A LEVTHR=85 compared to a LEVTHR=95seems to be better since indoor mobiles have a higher probability to stay on the900 band, with higher signal strength. But the quality on the 900 band isworse than that on the 1800 band and the LEVTHR should be kept at 95 untilthe 900 band is improved.

4 HCS level 2 test in a city test area

4.1 ObjectiveTo see whether traffic control with HCS can be achieved within the same layerwith the same effect as with 1800 in level 2 and 900 in level 3.

4.2 Test caseThe test was performed in a city on Day 3, while the Statistics from the daybefore, Day 4,was used as reference. The 3 sites in the circle in Figure 4.1were used for the test. The setting of HCS is listed in Table 5.1.

Reference day: Day 3 Test day: Day 4Band 900 1800 900 1800

LEVEL 3 2 2 2LEVTHR - 95 0 85

Table 4.1. HCS setting during the trial and the reference day.

The three test sites and the surrounding four sites, 900 cells and co-sited 1800cells, were set to HCS LEVEL=2. The 1800 cells had LEVTHR=85 while the900 cells had LEVTHR=0. Therefore the 900 cell were always below the HCSthreshold and worse than 1800 (above –85 dBm). Signal strength ranking wasused when 1800 was below -85 dBm. The reference day had a slightlydifferent setting than desired. For 1800 LEVTHR was 95 instead. The changeof LEVTHR from 95 to 85 could have caused problems to the resultinterpretation.

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010

012

011

Test area ThecirclelLEVEL 1800/900 = 2

LEVTHR 1800 = -85 dBmLEVTHR 900 = 0 dBm

013014

016

015

LEVEL1800/900 = 2gradual decrease ofLEVTHR

Highway

Figure 4.1. The three sites in the city test area.

4.3 ResultThere was no change in statistics between the test and the reference day exceptnormal fluctuations. There was little traffic in total: around 1 Erlang/cell inaverage over the 6 busiest hours (14.00-20.00). The drive test with TEMSshowed expected behaviour regarding band selection. The traffic distributionwas also the same as on the reference day. Hence, the change of threshold in1800 from -95 to -85 dBm had no influence on the traffic levels. That was alsothe experience in the LEVTHR test.

4.4 ConclusionThe solution with both 900 and 1800 in layer 2 showed the same performanceas the reference with 900 cells in layer 3. There was no indication ofdifferences in quality or traffic distribution. The method with both bands inlayer 2 but 900 always below the threshold can in principal be appliedsystematically but that banns layer 3 to be used in the surrounding area whenHCS is used.

5 CLS tests in the same city as the level 2tests

5.1 ObjectiveThe objective was to move traffic from the 900 band to the 1800 band by usingCLS and to verify that no Ping-Pong handovers or instabilities occurred. Thehope was also to see how well CLS could be combined with HCS. That wouldhowever require more testing than only one day.

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5.2 Test caseThe test was performed in a city Day 5. The two days before (Day 3 and 4),described in chapter 4, were used as reference days. The test area and HCSsettings were the same as for the HCS test, see Figure 4.1, except that thethreshold for 900 was -70 dBm instead of 0 dBm. The CRO setting was alsochanged from prioritising 1800 to make dual band MSs to prioritize 900instead.

The reason for the lower 900 threshold was to create two signal strength areas,above -70 dBm and below -85 dBm, where both 900 and 1800 is above orbelow their LEVTHR. K ranking is used in those areas and therefore CLS ispossible. See the dark grey areas in Figure 5.1. The signal strength areabetween –85 dBm (1800 threshold) and -70 dBm (900 threshold) was servedby the 1800 band due to HCS.

LEVTHR900

SS

Distance from site

SS900 – Hyst900-1800 ä

å SS900 + Hyst900-1800

SS1800

å SS900

LEVTHR1800

Negative part of the K hysteresis zone where CLS is possible

Figure 5.1. The expected hysteresis area above and below both HCSthresholds in case of co-siting. The 1800 signal strength (white) is belowthe 900 signal strength (black) but within the RHYST ramping area.

The measurements with TEMS before had showed that the signal strength of1800 was close enough to the 900 signal strength to be within a negativehysteresis of a few dB. KHYST was set to 6 dB and the allowed rampingdown of hysteresis was 80%. That setting gives a negative hysteresis of 3.6 dBthat the co-sited 1800 cell had to be within in order to be a target for a CLShandover.

The parameters regarding CLS are listed in Table 5.1. The idea was to allowhandover from the 900 band when 2 or more channels were occupied (80% ofa 1 TRU cell). The 900 cells were not allowed to accept incoming handoversdue to CLS at all. The 1800 cells were allowed to accept incoming CLShandovers until only 3 TCH channels remained and CLS 1800 to 1800 wasactivated at 1 free channel.

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Band CLSLEVEL CLSACC HOCLSACC RHYST

900 80% 85% OFF 80%1800 20% 50% ON 80%

Table 5.1. The settings of CLS parameters. KHYST was 6 dB.

5.3 Result

5.3.1 Handover resultsCell Load Sharing seems to move traffic from 900 to 1800 as desired. Thenumber of 900 to 1800 handovers increased five times, mostly due to CLS,which contributed to 30% of the outgoing handovers from 900 cells, seeFigure 5.2.

The moved traffic remained in the 1800 layer. No increase in quick handoverback was observed. For the normal 1800 to 900 handover there was a tendencyto quick CLS handover back to 1800 (40% within 10 seconds). This may bedue to the rather low KHYST (6 dB) and high RHYST (80%) that only left asmall hysteresis margin of 2.4 dB between 1800 to 900.

0

500

1000

1500

2000

2500

900-1800900 out total 900-1800900 out total900 CLS HO 1800-9001800-900and back

Reference dayDay 4

CLS Trial dayDay 4

Number of handovers (HOVERCNT)

Figure 5.2. Handovers due to CLS and total number of handovers. Alsothe fraction of quick return back to 1800 after a normal 1800 to 900handover is shown.

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5.3.2 Traffic during CLS testIt is not clear from the one day trial how large fraction that was directed to1800 due to CLS and how much that was due to HCS with decreased impactcompared to the reference days. The 900 to 1800 ratio was completely shiftedtowards 900 during the CLS trial, see Table 5.2. One reason was the lowerHCS threshold in 900 that allowed the unknown fraction of dual band MSsabove -70 dBm to be in the 900 band. The other reason could be the change inidle mode cell selection due to the CRO change. When almost all dual bandMSs do call set-up in 900 they may remain there a short time although 1800 ispreferred by locating. Note the large shift in call set-up band from 40% theearlier two days to 70% when CRO was changed. This change may be anindication of high penetration of dual band MSs, i.e. the fraction able tochange band. The earlier HCS tests described in Chapter 3 could only affecttraffic in the signal strength range –75 down to –95 dBm. The mobiles aboveor below those thresholds could not be seen in the re-distribution of trafficbetween bands. However, the traffic in the test area of the city was low andany figure is associated high a high statistical inaccuracy.

Reference daysDay 3- Day 4

CLS test dayDay 5

900 1800 900 1800TCH traffic (TF_TRAF) 45 % 55 % 55 % 45 %SDCCH traffic (S_TRAF) 45 % 55 % 70 % 30 %SDCCH establishments (CMSESTAB) 40 % 60 % 70 % 30 %

Table 5.2. Traffic on TCH and SDCCH during the CLS test.

6 Appendices

A. LEVTHR tests –95dBm

B. LEVTHR tests –85dBm

C. LEVTHR tests –75dBm

D. Same HCS layer tests

E. CLS tests