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  1. 1. GSM FUNCTIONALITY ANDPARAMETER FINE-TUNING:A CASE STUDY Issue Date: September 2004 Abstract—This paper documents the effect on the performance of an Ericsson™ global system for mobile communication (GSM) network realized by evaluating functionality and fine-tuning parameters after completing the pre- and post-launch optimization phases. These actions were carried out during a base station subsystem (BSS) performance optimization project attempting to further improve the quality of service (QoS) because network traffic was increasing. Each feature changed is addressed separately, followed by a short technical description of the philosophy of the changes performed, the exact settings selected, and a statistical evaluation of the results. This paper demonstrates that network performance gains can be achieved from optimum use of the available functionality and parameter tuning. This paper can also serve as a reference for optimizing Ericsson CME201 systems. INTRODUCTION Ongoing changes in functionality and parameter S ince first deployed in 1992, European global settings are necessary to provide optimum and system for mobile communication (GSM) constant quality of service (QoS). All system networks have become a major commercial vendors continuously seek to improve success. Currently, penetration levels approach functionality by adding improved features with 100 percent in some European countries. The every base station subsystem (BSS) software rapid increase in subscriber numbers prompted release. If fully exploited, this continuous network operators to increase investment in evolution of functionality can result in network infrastructure by embarking on significantly improved QoS and more efficient aggressive network rollout projects aiming to use of network infrastructure. expand system coverage and capacity. Increasing This paper describes a review of the functionality demand for mobile services and competition for and parameter values of an Ericsson™ GSM market share led many operators to dedicate network containing approximately 150 base most of their resources to network deployment. transceiver stations (BTSs) with three cells per Under these circumstances, the industry BTS and main sector configuration of 0-120-240 eventually developed the mindset of “roll out degrees [1]. This review began as an optimization now and optimize later.” project 6 months after completion of the post- Until the late 1990s, most network operators launch optimization phase. During this period, competed purely on coverage, considered the traffic increased substantially and the network most important differentiator among the offered was expanded to satisfy capacity demand as well services. Having “signal bars” on phones was all as to extend coverage. that mattered, even though calls were failing in A number of features were evaluated and fine- many cases. Users expected to see signal bars on tuned. These features are listed below, followed by their phones everywhere. Focused mainly on an a short technical description of each and the agressive buildout strategy, several operators philosophy of the performed changes, the exact continued to use default parameter values settings selected, and a statistical evaluation of the without fully exploiting the available function- results. The seven functions discussed below apply ality of the given system. to Ericsson BSS R8 and R9 (Releases 8 and 9). Network pre- and post-launch optimization is a • Frequency hopping useful mechanism to ensure good performance after commercial launch of the service. However, • Mobile station dynamic power control Michael Pipikakis as the network expands and traffic increases, the • Cell load sharing dmpipika@bechtel.com benefits of post-launch optimization may be lost. • Locating penalty timers ________________________________ 1 Ericsson’s GSM application • Flow control timers © 2004 Bechtel Corporation. All rights reserved. 17
  2. 2. ABBREVIATIONS, ACRONYMS, AND TERMS BCCH broadcast control channel GSM global system for mobile communication BSC base station controller HSN hopping sequence number BSIC base station identity code MAHO mobile assisted handover BSS base station subsystem MS mobile station BTS base transceiver station QoS quality of service C/I carrier-to-interference (ratio) SACCH slow associated control CLS cell load sharing channel CP central processor SDCCH standalone dedicated control CTR cell traffic recording channel DCR dropped call rate TCH traffic channel DPC dynamic power control UL uplink FH frequency hopping ERICSSON PARAMETERS ACCMIN minimum signal strength to PSSBQ penalty value for bad quality access the cell PSSHF penalty value for failed BSRXSUFF received by the BTS handover sufficient signal strength PTIMBQ penalty timer for bad signal level quality CLSACC CLS traffic accept PTIMHF penalty timer for handover CLSLEVEL CLS level failure EVALTYPE evaluation type QCOMPUL uplink signal quality compensation factor HIHYST high-signal-strength hysteresis QDESUL quality desired for uplink HODWNQA handovers due to downlink signal quality RHYST region hysteresis HOTOKCL handovers to K cells RXLEV measured signal strength level HOTOLCL handovers to L cells RXQUAL measured signal quality HOUPLQA handovers due to uplink signal quality SSDES signal strength desired HYSTSEP signal strength level between TALLOC time between TCH high and low strength cells allocations KHYST K-criterion hysteresis TCALLS counter for TCH allocation attempts LCOMPUL uplink signal strength compensation factor TCONGAS congestion timer for immediate TCH assignments LHYST L-criterion hysteresis TCONGHO congestion timer for handover LOHYST low-signal-strength TCH assignments hysteresis TURGEN time for urgent handover MSRXSUFF received by the mobile sufficient signal strength level • Cell selection and access different frequencies. Each cell uses a predefined • Signal strength measurement criteria in the set of frequencies, among which the connection locating algorithm hops according to a specified pattern (i.e., cyclic or random) 217 times per second. The radio environment between a mobile station (MS) and a FREQUENCY HOPPING BTS is subject to variations due to multipath F requency hopping (FH) means that multiple frequencies are used to transmit speech or data in a single connection. The basic principle fading and cumulative interference. FH can improve the radio environment, providing frequency diversity against the multipath fading involves transmitting consecutive bursts at and averaging the overall interference. See [2].18 Bechtel Telecommunications Technical Journal
  3. 3. Parameter Adjustment and EvaluationCyclically sequenced baseband FH was 2.3introduced at launch in traffic channels (TCHs) 2.0and standalone dedicated control channels(SDCCHs). With this pattern, all available 1.7 DCR (%)frequencies of a cell are used with a consecutiveorder in a call or signaling connection. For 1.4instance, a connection in a three-frequency (f1, f2,f3) cell will show the following burst-to-burst 1.1pattern: 0.8 …f3, f2, f1, f3, f2, f1, f3, f2, f1, f3, f2, f1,… Cyclic FH Random FH 0.5With reuse-pattern frequency planning, cyclic 1/12 1/22 2/1 2/11 2/21 3/3 3/13 3/23 4/2 4/12 Date (MM/DD)hopping may result in connections in cells thatare reusing the same frequencies to get in phase Figure 1. Network DCR After Implementation of Random Hopping Sequencewith one another, hopping “hand in hand” but both TCHs and SDCCHs. The algorithmlosing the benefit of interference averaging. calculates a power order according to BTS received signal strength and BTS measuredRandom FH was proposed, which introduces a quality. The first term introduces MS powerpseudo random hopping sequence, according to reduction based on a desired value—signalparameter hopping sequence number (HSN). Up strength desired (SSDES)2. The second termto 63 different FH patterns not correlated with introduces compensation for bad quality,one another can be defined. The burst-to-burst according to a desired value for signal quality—pattern would look as follows: quality desired for uplink (QDESUL)2. The MS …f3, f1, f2, f2, f1, f3, f3, f2, f1, f1, f2, f1,… power capabilities are a limiting factor. The MS power cannot be reduced beyond the minimumCarefully choosing HSN values for cells using the output power of the MS (for phase 2 MSs, thesame frequency groups was expected to increase dynamic power range is 8 dBm to 33 dBm).the interference averaging gains of FH. Parameter Adjustment and EvaluationRandom FH was introduced in all cells, with an MS DPC was initially introduced with theHSN per cell based on the base station identity following settings for desired values andcode (BSIC) plan (HSN = 63 – BSIC), which was weighting factors:also planned to differentiate between co-channelcells. • SSDES = –94 dBmOld values: HOP = ON, HSN = 0 => Results in • QDESUL = 10cyclic hopping (HOP is the Ericsson cell level • Uplink signal strength compensation factor Considerableparameter to enable hopping) (LCOMPUL)2 = 50 networkNew values: HOP = ON, HSN = (63 – BSIC) => • Uplink signal quality compensation factor performanceResults in random hopping (QCOMPUL)2 = 30 gains can beAs can be seen from the results in Figure 1, a These initial values correspond to an aggressive made by fullyconsiderable improvement in QoS was achieved. power-down regulation aiming to minimize utilizing theThe dropped call rate (DCR) decreased by uplink interference. However, it was observed availableapproximately 20 percent. from analyzing drive test files and cell traffic recording (CTR) files that the settings could lead functionality to performance deterioration. For instance, a and fine-tuningMOBILE STATION DYNAMIC POWER CONTROL connection with received signal strength the network (RXLEV)2 = –80 dBm and received signal qualityM S dynamic power control (DPC) is a feature parameters (RXQUAL)2 = 5, given the previous settings, that controls the output power of an MS so would be further down-regulated in steps of using statisticsthat the BTS receives a desired uplink signal to evaluate 2 dB, despite the obvious quality problem.strength level. MS DPC helps reduce MS batteryconsumption, protects against possible BTS After studying the case, a more reasonable value the results.receiver saturation, and reduces overall uplink of SSDES = –88 dBm was introduced, whileinterference. QDESUL was set to zero. Also, compensation factor LCOMPUL, which introduces a slope in theThe MS DPC algorithm is implemented on the _________________________________base station controller (BSC) and performed for 2 An Ericsson DPC parameterSeptember 2004 • Volume 2, Number 2 19
  4. 4. power reduction, was set to 100. This setting optimization activities. The minute-Erlang per corresponds to maximum uplink regulation (no drop index is inversely proportional to the slope) because the algorithm was expected to DCR index. work rapidly on “good” signals. Quality Due to the new settings for SSDES, the average compensation parameter QCOMPUL was set to power received on the uplink is greater than 60 to enhance up-regulation in case of inter- before, so the risk of a connection dropping due to ference and to give the connection a chance to weak signal strength on the uplink should overcome the bad quality by increasing the decrease. Since the main reason for uplink quality output power. For a more detailed description of is also believed to be the strength of the MS the algorithm, see [3]. transmitted signal power, bad quality drops on the Figure 2 shows the positive effect of the changes uplink should also decrease with the new settings. on the MS DPC settings in terms of dropped In Figure 3, the improvement trend can be connections due to uplink quality and uplink verified by examining the handover reasons due signal strength. The indices “min-ERLANG/ to uplink (UL) quality. UL_QUAL-DROP” (minutes of traffic carried before a call drop due to uplink signal quality occurs) and “min-ERLANG/UL_SS-DROP” CELL LOAD SHARING (minutes of traffic carried before a call drop due 1,400 to uplink signal strength occurs) were used. 300 C ell load sharing (CLS) is a feature that distributes traffic among neighboring cells at high traffic load to reduce congestion and better minErl/UL_QA_DROP 1,300 280 use the available resources. minErl/UL_SS_DROP 1,200 The CLS algorithm works by monitoring trafficNumber of Minutes minErl/UL_SS_DROP 260 1,100 load for every cell in terms of idle TCHs. When 1,000 240 the number of idle TCHs in a given cell, expressed as a percentage of the total, falls below 900 220 the CLS level (CLSLEVEL)3, traffic is shifted from 800 200 this cell to prevent it from being congested. 700 Connections close to the cell border, within an 180 area determined by region hysteresis (RHYST)3, 600 Old MS DPC Settings New MS DPC Settings are handed over to any neighboring cell 500 160 5/5 5/7 5/9 5/11 5/13 5/15 5/17 5/19 5/21 5/23 5/25 5/27 considered suitable to accept traffic, i.e., whose Date (MM/DD) percentage of idle TCHs is greater than the value Figure 2. Effect of New MS Power Control Settings CLS traffic accept (CLSACC)3. The indices presented in Figure 2 are TCH drops Drawbacks of the feature are the increased due to bad uplink quality and low uplink signal number of handovers and a considerable increase strength related to the traffic carried by the in BSC central processor (CP) load. For a detailed system. The indices “minErl/UL_QA_DROP” description of the functionality and algorithm, and “minErl/UL_SS_DROP” express the minutes see [4]. of traffic the system carries before a drop occurs due to bad uplink quality or low uplink signal Parameter Adjustment and Evaluation strength. The minute-Erlang method was used The CLS feature was introduced networkwide to because it is more sensitive to changes and thus cope with unevenly distributed traffic among more accurately evaluates the effectiveness of cells, to use the available resources efficiently, 24 and to increase the total capacity. The New MS DPC Settings original parameter set was CLSLEVEL = 23, 23 CLSACC = 55, and RHYST = 75, meaning that 22 CLS evaluations for a cell started when the idle number of TCHs fell below 23 percent, while a (%) 21 cell accepted CLS traffic only if 55 percent or 20 more of its resources were idle. 19 Statistical analysis indicated that with these settings, the success rate of CLS handovers was 18 4/25 5/2 5/9 5/16 5/23 5/30 poor, mainly because of the high values of Date (MM/DD) _________________________________ Figure 3. Effect on Handovers due to UL Quality of new MS Power Control Settings 3 An Ericsson CLS parameter 20 Bechtel Telecommunications Technical Journal
  5. 5. CLSLEVEL and CLSACC. A more reasonablesetting was introduced, where a cell would more 1,000,000 CLS Attemptseasily accept CLS handovers (CLSACC = 25) and CLS Successwould not start CLS evaluations as soon 800,000 Number of Attempts(CLSLEVEL = 15). Also, RHYST was set to 100, New CLS Settingsmaximizing the area around the nominal cell 600,000border where CLS could take place.In Figure 4, the impact of the change can be seen. 400,000Cell load sharing became more effective, sinceCLS calculations were limited, practically 200,000maintaining the same number of successful CLShandovers. This development had a positive 0effect on the BSC CP load. 3/6 3/13 3/20 3/27 4/3 4/10 4/17 4/24 5/1 5/8 5/15 5/22 5/29 Date (MM/DD) Figure 4. Effect of CLS Parameter ChangesLOCATING PENALTY TIMERS originating channel within 10 seconds. Hand-P enalty timers for bad signal quality over success improved as a direct result of (PTIMBQ)4 and for handover failure reducing the possibility of attempted connections(PTIMHF)4 specify the time in seconds for which to a cell suffering from poor quality.the respective penalty values in decibels, namely The reduction of mobile connections lost duringpenalty value for bad quality (PSSBQ)4 and handover can be seen in Figure 6. In addition topenalty value for failed handover (PSSHF)4, are the improvements in ping-pong effect andapplied to a cell’s neighbors. handover success rate, the timer change alsoWhen an urgent handover is successfully had a positive effect on network dropoutperformed that resulted from bad quality due to performance. In a typical GSM network, nearlydownlink, uplink, or both, the originating cell ispenalized with PSSBQ decibels to prevent 96.5 25immediate hand-back to this cell. The original cell Success 96.0is penalized because bad radio conditions might Ping-pong 20 Ping-pong Handovers (%)still be in effect there; also, the original bad 95.5quality cell is most likely the best cell from 15a strictly signal strength point of view. Under a 95.0 (%)similar philosophy, handover to a cell where 94.5 10a handover failure occurred is inhibited for a timedetermined by timer PTIMHF [5]. 94.0 5Parameter Adjustment and Evaluation 93.5 Penalty Timer ChangesPenalty values PSSBQ and PSSHF were both set 93.0 0to 50 dB to remove the penalized cells from the 4/17 4/20 4/23 4/26 4/29 5/2 5/5 5/8 5/11 5/14 5/17 5/20 5/23 5/26 5/29 6/1 Date (MM/DD)locating algorithm evaluations. However, the Figure 5. Effect of Penalty Timer Changes on Network Handover Performancelengths of the timers, PTIMBQ = 10 sec andPTIMHF = 5 sec (original settings), were thoughtto be insufficient to give radio conditions in the 0.95penalized cell a chance to improve. The lengths of 0.90the two timers must be carefully chosen, on the 0.85other hand, to predict handover performance offast-moving subscribers. A very high value may 0.80lead to call drops due to handover being inhibited 0.75 (%)for a time not matching the user’s mobility. The 0.70new time settings selected were PTIMBQ = 15 sec 0.65and PTIMHF = 12 sec. 0.60Figure 5 shows the effect of this change in 0.55handover performance. The term “ping-pong” Penalty Timer Changes 0.50indicates the percentage of handovers back to the 4/15 4/25 5/5 5/15 5/25 6/4_________________________________ Date (MM/DD) Figure 6. Effect of Penalty Timer Changes on Percent of Mobiles Lost During Handover4 An Ericsson locating algorithm parameterSeptember 2004 • Volume 2, Number 2 21
  6. 6. 30 percent of the total dropped calls occur during congestion because congestion timers TCONGAS handover, which is considered a sensitive task in and TCONGHO count every allocation attempt. the radio environment. By increasing TALLOC, the measured figures for congestion during handover and assignment A more reliable way to assess the overall dropout will be closer to the true, customer-perceived performance is to determine the MSs lost during congestion. handover in relation to the total traffic. This data is shown in Figure 7, where a clear and steadily Figure 8 shows the measured congestion trend increasing trend is apparent for the index after the change was performed, indicating that “minErl/MSLOST” (minute-Erlangs per MS lost the overall measured congestion rate during the during handover). busy hour is reduced. Reducing the number of 400 channel allocation attempts can also have a minErl/MSLOST positive effect on the BSC CP load. 350 300 CELL SELECTION AND ACCESS S minErl/MSLOST ome of the parameters controlling MS idle mode 250 behavior during cell selection and system access are critical for the system’s performance. Minimum 200 signal strength to access the cell (ACCMIN)6 is a 150 cell-level parameter that determines the minimum Penalty Timer Changes received signal strength at the MS required to access 100 the system. When an MS first tries to camp to a cell, 4/17 4/24 5/1 5/8 5/15 5/22 5/29 the MS decodes ACCMIN, which is transmitted on Date (MM/DD) the system information messages of the broadcast Figure 7. Effect of Penalty Timer Changes on Mobiles Lost During Handover in Relation to Traffic control channel (BCCH), and compares it to the actual signal strength the MS measures. If ACCMIN is higher, the MS is not allowed to camp to the cell FLOW CONTROL TIMERS because the MS is considered to be at poor radio F low control timer time between TCH allocations (TALLOC)5 gives the time in slow associated control channel (SACCH) periods conditions. Parameter Adjustment and Evaluation (480 msec) between consecutive TCH allocation Depending on the setting of ACCMIN, the cell attempts, from the channel allocation algorithm, radius (in idle mode) can be modified. ACCMIN if the first TCH allocation attempt fails. The timer was originally set to –107 dBm to improve the is used during assignment when the BSC customer perception of the available coverage. attempts to find an idle TCH for data or speech However, such perceived improvement was and also during handover. No candidate list is achieved at the risk of an increased number of call prepared from the locating algorithm before the set-up failures, since MSs at poor radio conditions timer expires unless an urgency is detected, in were allowed to access the system. Additionally, which case the new list for handover is sent the mobile equipment static sensitivity is limited within the time specified by the timer for urgent to approximately –104 dBm for most of the handover (TURGEN)5. handsets available, so lower signals are not practically measurable. Parameter Adjustment and Evaluation Parameter TALLOC specifies the pace at which A lower ACCMIN value also meant that fewer allocation attempts counted by the Ericsson BSC subscribers were able to respond to paging counter for TCH allocation attempts (TCALLS)5 messages and that poor paging performance are repeated when congestion is counted by the could result [6]. Ericsson congestion timer for immediate TCH To improve call set-up performance and assignments (TCONGAS)5 or by the Ericsson minimize the risk of SDCCH dropped congestion timer for handover TCH assignments connections, ACCMIN was set to the quoted (TCONGHO)5. A decision was made to change mobile static sensitivity of –104 dBm and the the original setting from two SAACH periods to SDCCH drop rate was monitored. The expected four to limit the number of allocation attempts improvements were verified by a 22 percent per event (assignment or handover). Multiple reduction in SDCCH drops, as shown in Figure 9. allocation attempts increase the overall measured _________________________________ _________________________________ 5 An Ericsson flow control parameter 6 An Ericsson access parameter22 Bechtel Telecommunications Technical Journal
  7. 7. 50SIGNAL STRENGTH MEASUREMENT CRITERIA IN 45THE LOCATING ALGORITHM Bid Congestion Handover 40T he locating algorithm implemented in the BSC controls cell selection in dedicated (i.e.,call) mode and determines handover decisions. 35 30 Bid Congestion AssignmentThe main objectives of handover are to maintain 25 (%)call continuity and quality and to control cell size 20and handover borders to minimize total network 15interference. 10The inputs to the locating algorithm are signal 5 TALLOC = 2 TALLOC = 4strength and quality measurements from the MS 0(the so-called mobile assisted handover [MAHO]) 5/11 5/13 5/15 5/17 5/19 5/21 5/23 5/25 5/27 5/29 5/31 6/2 6/4 6/6 6/8 Date (MM/DD)and from the BTS. The output is a list of candidate Figure 8. Difference of Measured Congestion After Flow Control Timer Changecells for handover, ranked in descending orderaccording to preferences and constraints intro- strength level (MSRXSUFF)7 and received byduced by other features and by the settings of the the BTS sufficient signal strength levelalgorithm itself. The locating algorithm works (BSRXSUFF)7. High-signal-strength cells arecontinuously for all active MSs and completes a ranked according to the L criterion and thecycle every SAACH period (480 msec). rest according to the K criterion.The signal strength measurements reported by • Ericsson-3, where ranking is performed onlythe MS and the BTS are evaluated according to according to the K criterion, but two separatecomparison criteria that can be selected with hysteresis values are used.different settings in the locating algorithm. Thefirst is the signal strength or K criterion and the Parameter Adjustment and Evaluationsecond is the path loss or L criterion. They are used Before this exercise, only the K criterion was usedto compare reported values for serving and for handover calculations. The hysteresis was setneighboring cells to determine the optimum cell to K-criterion hysteresis (KHYST)7 = 4 dB.ranking and the handover borders. Hysteresis is a signal strength offset that is addedIn the K-criterion mode, the comparisons are to the actual reported value for the serving cell toperformed purely according to the received prevent unnecessary ping-pong handovers at thesignal strength (i.e., cells measured with higher border between two cells. The L criterion wassignal strength are ranked higher). Hence, an introduced in an attempt to further improveincrease in the output power of a cell signifies network handover performance. Sufficientexpansion of its service area. This criterion seeks condition parameters MSRXSUFF = –86 dBm andto maximize the carrier-to-interference (C/I) ratio BSRXSUFF = –92 dBm determine the breakingby maximizing “C.” point between L and K ranking. Cells reporting with signal strength values greater than bothIn the L-criterion mode, path loss is taken into levels are considered suitable for L ranking,account. Cells with lower path loss are ranked where an increased hysteresis value, L-criterionhigher, and the output power of each cell does hysteresis (LHYST)7 = 7 dB, is used. Thenot affect the calculations. The criterion actually remaining cells are K ranked with a hysteresisfavors cells with low output power; thus, KHYST = 4 dB.improvement in C/I ratio is attempted by 3.0decreasing the total interference. However,L ranking can sometimes lead to a locally lower 2.5C/I ratio than K ranking. Two cell ranking SDCCH Drop Rate (%)algorithms are available, set by BSC parameterevaluation type (EVALTYPE)7 [5]: 2.0 • Ericsson-1-2, which uses both L and 1.5 K ranking. The candidate cells are separated into high- and low-signal cells by comparing 1.0 received signals to the following parameters for downlink and uplink, respectively: 0.5 received by the mobile sufficient signal 4/30 5/5 5/10 5/15 5/20 5/25 5/30 6/4 6/9_________________________________ Date (MM/DD) Figure 9. SDCCH Drop Rate Before and After ACCMIN Change7 An Ericsson locating parameterSeptember 2004 • Volume 2, Number 2 23
  8. 8. 3.0 The Ericsson-3 algorithm was also tested. The main difference from the previous K-ranked-only 2.5 algorithm is that, depending on the received downlink signal strength, one of two hysteresis 2.0 values is used. The signal strength level (%) between high and low strength cells 1.5 (HYSTSEP)7 = –86 dBm parameter specifies whether the serving cell is a high or low strength 1.0 K Only K+L Ericsson-3 cell, allowing a larger high-signal-strength Criterion hysteresis (HIHYST)7 = 7 dB or a smaller low- 0.5 signal-strength hysteresis (LOHYST)7 = 4 dB to 5/14 5/16 5/18 5/20 5/22 5/24 5/26 5/28 5/30 6/1 6/3 6/5 6/7 6/9 Date (MM/DD) be applied. The purpose of the high hysteresis Figure 10. Handovers per Call per Evaluation Criterion values for both tested algorithms is to prevent unnecessary handovers in the cell borders when 3.0 x 106 radio conditions permit. 2.5 x 106 In Figure 10 the reduction in the total number of handovers in the system due to the increasedNumber of Handover Causes 2.0 x 106 hysteresis in both testing cases can be verified. It is noteworthy that the L-criterion algorithm 1.5 x 106 seems to introduce the highest (25 percent) reduction in the handovers, as expressed by the 1.0 x 106 handovers per call index. 0.5 x 106 Figure 11 shows the following handover areas: HOTOKCL LOWHYST HIHYST handovers to K cells (HOTOKCL)7, handovers to HOTOLCL HOUPLQA HODWNQA 0 K Only K+L Eric-3 L cells (HOTOLCL)7, low hysteresis (LOWHYST), 5/1 5/5 5/9 5/13 5/17 5/21 5/25 5/29 6/2 6/6 (HIHYST), handovers due to uplink signal quality Date (MM/DD) (HOUPLQA)7, and handovers due to downlink Figure 11. Handover Causes per Evaluation Criterion signal quality (HODWNQA)7. 0.90 280 The portion of handovers performed with the 0.85 L criterion in the first case and with the HIHYST 0.80 250 value in the second can well justify the previous deviation. Up to 30 percent of total handovers in minErl/MSLOST 0.75 220 both cases take place with the use of the increased (%) 0.70 hysteresis values, which means that the 190 0.65 handovers are actually delayed. The result is a 160 total handover reduction, if averaged over the 0.60 Over Total Handovers whole network. 0.55 minErl/MSLOST 130 K Only K+L Eric-3 As already mentioned, handover is considered a 0.50 100 task with a high risk of call drop. Figure 12 shows 5/1 5/4 5/7 5/11 5/14 5/17 5/20 5/23 5/26 5/29 6/1 6/4 6/7 Date (MM/DD) the effect of the tested settings in call drop Figure 12. Mobiles Lost During Handover per Evaluation Criterion performance of the handover algorithm. 96.5 25 Handover dropouts, expressed as a percentage of HO Success 96.0 total handovers, may initially convey that the Ping-pong 20 situation worsened with the new settings. Ping-pong Handovers (%) 95.5 Nevertheless, what matters is the absolute 15 number of failures actually experienced by the 95.0 subscriber; since the total number of handovers 94.5 10 decreased, this difference is not substantial. (%) To emphasize this point, the index 94.0 “minErl/MSLOST,” giving Erlang minutes of 5 93.5 K Only K+L Eric-3 traffic carried out per handover dropout, is also depicted. Inspecting this index, it is clear that the 93.0 0 5/1 5/4 5/7 5/11 5/14 5/17 5/20 5/23 5/26 5/29 6/1 6/4 6/7 L-criterion algorithm appears much improved, Date (MM/DD) while the performance of the Ericsson-3 Figure 13. Handover Success Rate and Ping-pong Rate per Evaluation Criterion algorithm is rather ambiguous. 24 Bechtel Telecommunications Technical Journal
  9. 9. The superiority of the L-criterion algorithm over TRADEMARKthe Ericsson-3 algorithm is also apparent in Ericsson is a trademark or registered trademarkFigure 13; the ping-pong handovers (i.e., of Telefonaktiebolaget LM Ericsson.handovers back to the originating cell within10 seconds) are reduced in both cases. Thisreduction is a direct consequence of the hysteresis REFERENCESvalues of 7 dB introduced in both algorithms. [1] “Radio Network Parameters and Cell DesignHowever, the L-criterion algorithm shows the Data” – Ericsson CME20 Documentation.best performance in this field, meaning that more [2] “User description, Frequency Hopping” –accurate and reliable handover decisions Ericsson CME20 Documentation.accompany this algorithm, exactly as predicted [3] “User description, MS Dynamic Power Control” –by theory. Ericsson CME20 Documentation. [4] “User description, Cell Load Sharing” –The only disadvantage of the L-criterion algorithm Ericsson CME20 Documentation.appears to be the handover success percentage, [5] “User description, Locating” – Ericsson CME20half a decimal unit below the previous figures. The Documentation.same applies for the Ericsson-3 algorithm, which [6] “User description, Idle Mode Behaviour” –can be attributed to the lower number of handover Ericsson CME20 Documentation.commands. It can be assumed that, due to variousradio problems, a significant number of handoverfailures always exist in the network. This assumed BIOGRAPHYvalue can be highlighted or hidden, in a statistical Michael Pipikakis is a networksense, depending on the volume of the total planning and wireless tech- nology manager for Bechtelssample. It is believed that careful optimization and Europe, Africa, Middle East,individual neighbor cell inspection of the and Southwest Asia Region. Henetwork’s handover performance can further supports ongoing and newimprove this figure. projects and new business development; writes guidelinesAs a result, the K-L combination algorithm was and procedures for mobileeventually introduced. Further improvement can network design, planning, and optimization; and participates in technology forums.be achieved by fine-tuning sufficient levelparameters BSRXSUFF and MSRXSUFF to Michael is a mobile networks specialist with 17 yearsidentify a balanced breakpoint for cell ranking. of experience in the telecommunications industry, including more than 11 years in RF planning, design,Also, different LHYST values can be tried. optimization, and management of the end-to-end performance of cellular networks. Before joining Bechtel, Michael held variousCONCLUSIONS management positions in the Vodafone Groups radioA ll of the optimization-related changes were system design and optimization department and development department over a 10-year period; made in a controlled manner so that their worked for Cellnet UK and GEC Marconi UK; and waseffectiveness could be measured and evaluated. a telecommunications operator in the Greek Navy.At the end of the project, the average daily DCR From 1999 to 2003, he was a member of the Vodafonewas reduced by 30 percent, and the average Global Forum for UMTS design harmonization.minute-Erlang per drop was increased by almost Michael has a BEng Honors in Electronics Engineering45 percent. At that point, a foundation was with Computing and Business from Kingstoncreated for further fine-tuning as the network University in Surrey, England, and an HND in Radio Communications Systems Design from the Polytechnicexpands in response to increases in traffic and School of Athens, Greece. He is a member of thesubscriber base. Institution of Electrical Engineers.As has been shown, considerable networkperformance gains can be made by fully utilizingthe available functionality and fine-tuning thenetwork parameters using statistics to evaluatethe results.September 2004 • Volume 2, Number 2 25

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