Locating

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Locating

  1. 1. User Description, Locating USER DESCRIPTION 219/1553-HSC 103 12/21 Uen A
  2. 2. Copyright © Ericsson AB 2009-2012. All rights reserved. No part of this document may be reproduced in any form without the written permission of the copyright owner. Disclaimer The contents of this document are subject to revision without notice due to continued progress in methodology, design and manufacturing. Ericsson shall have no liability for any error or damage of any kind resulting from the use of this document. Trademark List Ericsson is a trademark or registered trademark of Telefonaktiebolaget LM Ericsson. All other trademarks mentioned herein arethe property of their respective companies. 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  3. 3. Contents Contents 1 Introduction 1 1.1 General 1 1.2 Scope 2 1.3 Conventions 2 1.4 Main Changes in Ericsson BSS 08A 3 1.5 Main Changes in Ericsson BSS G13A 3 2 Capabilities 5 3 Technical Description 7 3.1 General 7 3.2 Algorithm 12 3.3 Disconnection Criteria 61 3.4 Related Statistics 62 4 Engineering Guidelines 65 4.1 Locating and Power Balance 65 4.2 Reference Point 66 4.3 Cell Selection Procedure 67 4.4 Optimizing the Handover Performance 69 4.5 High Capacity Networks 75 4.6 Examples 76 5 Parameters 87 5.1 Main Controlling Parameters 87 5.2 Parameters for Special Adjustments 89 5.3 Value Ranges and Default Values 94 6 Appendix A 99 7 Appendix B 105 8 Appendix C 109 9 Concepts 111 Glossary 113 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  4. 4. User Description, Locating Reference List 115 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  5. 5. Introduction 1 Introduction 1.1 General A mobile telephony connection must be handed over between cells as the person using the phone moves around. There are several criteria that can be used for initiating this handover. The criteria serve different purposes, which in turn arise from a range of requirements that must be put on a mobile telephony system. The requirements are, broadly speaking, coverage, speech quality and capacity. Therefore, the purpose of the criteria is to provide a connection with sufficient signal strength (coverage and speech quality), to avoid disturbances (speech quality), to maximize the Carrier-to-Interferer (C/I) ratio (speech quality and capacity) and to even out the traffic load (capacity). The locating algorithm works out the basis for assignment and handover decisions. It is implemented in the Base Station Controller (BSC). It is the algorithm for cell selection for active mobile stations (that is ongoing connections) after immediate assignment. The cell selection in Ericsson GSM Systems has two main objectives • Quality and continuity of calls, • "Cell size" control in order to minimize total interference in the network. The input to the algorithm is signal strength and quality measurements from the MS and from the base station currently serving the connection. The output is a list of cells that the algorithm judges to be possible candidates for handover. The cells in the list are ranked and sorted in descending order of preference for handover. The algorithm works continuously, completing a calculation cycle in general every 480 ms. Most frequently, the algorithm will recommend not to perform a handover. There are several reasons why a handover should be performed. The handover criteria are based on three different types of measurements: • Field strength (signal strength and/or path loss) of the connection and of the Broadcast Control Channel (BCCH) carriers for neighbours, • Signal quality of the connection (bit error rate estimation mapped on a logarithmic scale), • Timing advance used by the mobile station (MS). Another type of measurement used in the handling of active MSs is based on the result of the decoding of Slow Associated Control Channel (SACCH) 1219/1553-HSC 103 12/21 Uen A | 2012-05-29
  6. 6. User Description, Locating messages in the MS and measurement reports in the Base Transceiver Station (BTS). These measurements are the input to a separate algorithm, "leaky bucket", which is used for recommending disconnection at low transmission quality in the base station or the MS. There is a large number of parameters in the locating algorithm. The majority of these relate to quantities connected to individual cells and to cell-to-cell neighbour relationships. The purpose of these parameters is to adapt the locating algorithm to the reality of the cellular network. In the final analysis, it is the locating algorithm with its parameter setting, that shall put into practice the cell structure that has been planned by the operators for their customers. For High Speed Circuit Switched Data (HSCSD) several timeslots can be assigned to one connection. The first of these is called the main channel. Locating is performed for the entire connection using serving cell measurements only for the main channel. Only Circuit Switched (CS) connections in Active Mode are controlled by the locating procedure. Idle Mode Mobility and cell reselection for MS in packet transfer mode, that is when there is an ongoing packet transfer uplink or down link, are handled by the MS autonomous cell reselection, see Reference [11]. 1.2 Scope The locating algorithm covers the Locating feature available for Ericsson GSM Systems. The locating algorithm serves as the basis for a number of other radio network features. These features will be mentioned as auxiliary radio network functions, and parts of their functionality will be explained when relevant to the locating algorithm. Full descriptions of these features are available as separate User Descriptions, and will be referred to as appropriate. 1.3 Conventions The following conventions apply to measurement quantities that are used in the locating algorithm: • Uppercase italics when the quantity is without dimension (as in signals): 0 RXLEV [0 to 63] 0 RXQUAL [0 to 7] 0 MEAN_BEP [0 to 31] • Lowercase italics when the quantity has dimension (as in calculations): 0 rxlev [-110 to -47 dBm] 0 rxqual [0 to 100 deci-transformed quality units (dtqu)] 2 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  7. 7. Introduction 0 ta [0 to 63 bit periods] • Punished quantities have an indication preceding the quantity: 0 p_rxlev 0 p_SS_DOWN 1.4 Main Changes in Ericsson BSS 08A AMR Wideband was introduced, see Section 3.2.5 on page 44, Section 3.3 on page 61 and Section 4.4.8 on page 72. Default algorithm for basic ranking changed from Ericsson1 to Ericsson3, i.e. default value for EVALTYPE changed from 1 to 3. 1.5 Main Changes in Ericsson BSS G13A BCCH Power Savings has been introduced as an optional feature in BSS G13A: • Locating with BCCH Power Savings has been described. 3219/1553-HSC 103 12/21 Uen A | 2012-05-29
  8. 8. User Description, Locating 4 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  9. 9. Capabilities 2 Capabilities Locating is the basic radio feature providing mobility in the radio network. Locating is also the base on which other features such as HCS and Assignment to Another Cell are built upon. The designed cell plan is implemented in locating with parameters for powers settings, handover borders etc. Ericsson's locating algorithm provides the user with a powerful tool to implement a very flexible cell plan. Big differences in behaviour of the radio network and improvements in performance can be achieved with parameter changes and activation of different radio network features. There are two different basic algorithms to choose between, Ericsson 1 and Ericsson 3. Ericsson 1 is based on the GSM specification and it is possible to use either pathloss or signal strength or both for the handover decision. The Ericsson 3 algorithm is based on the experience that by only considering signal strength at the handover decision, a better network performance is achieved. It is possible, by parameter settings, to achieve exactly the same evaluation in Ericsson 1 as in Ericsson 3. The main benefit with Ericsson 3 is less complexity, that is less parameters, and thereby an easier maintained radio network. With locating it is possible to have a flexible cell planning, with handover borders adapted to the radio environment, minimizing the interference in the network and maximizing the capacity. The auxiliary radio network features though, are very important to be able to optimize the functionality and performance of the radio network. For details see Technical description below. 5219/1553-HSC 103 12/21 Uen A | 2012-05-29
  10. 10. User Description, Locating 6 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  11. 11. Technical Description 3 Technical Description 3.1 General The handover decisions are based on signal strength and signal quality measurements performed by the mobile station. The MS measures the strength of the radio signal from the base station that serves the connection. In addition, it measures the signal strength of the BCCH carrier frequency from the surrounding cells. These measurements are used in a comparison, in order to find the "best" server. 3.1.1 Precise Handover Borders Since the handover algorithm is based on comparing the available handover candidates, the handover borders are fixed in space, and independent of in which direction the MS is moving. An adjustable safety margin against fluctuating signal strength, a hysteresis, is added. The main reasons for such fluctuations is fading due to movements of the MS or movements of objects in the surroundings. A low hysteresis yields a sharp handover border, but a larger amount of fluctuating handovers. In addition to hysteresis, timers regulate the minimum time allowed between handovers. The benefit of using the MS as the probe for the measurements used in the comparisons, is that the systematic errors in the measurement devices are cancelled. Thus the handover borders will be constant and independent of the MS. 3.1.2 Handover Borders Adapted to the Radio Environment The signal strength measurements provided by a MS to the BSC, allow comparisons between the serving cell and the neighbouring cells to which the MS is listening. In Locating there are two algorithms available for basic ranking of handover candidates. One is based both on signal strength and path loss (the so called K/L-ranking). The second is strictly based on signal strength. When the ranking is based on signal strength, the handover borders are influenced if the Effective Isotropic Radiated Power (EIRP) of one or several base stations is changed. An increase in the output power in one cell thus means an increase in the area of that cell. However, if the output power is changed by the same amount everywhere, the borders will not be affected. The path loss is calculated by subtracting the received signal strength from the assumed EIRP in each base station. When basing the ranking on path loss, the actual output power in the different base stations do not influence the position of 7219/1553-HSC 103 12/21 Uen A | 2012-05-29
  12. 12. User Description, Locating the handover borders. Thus, if the output EIRP of one or several base stations independently of each other is changed, the handover borders are not affected. 3.1.3 Handover Borders that Yield Low Interference The comparisons in the locating algorithm serve to find the cell with the highest signal strength or the lowest path loss (K/L-ranking only). Both strategies aim at achieving a high C/I ratio in the total system. The effect of maximizing signal strength relies on the fact that C can be seen as an approximation to C/I. Thus maximizing C for each connection will approximate maximizing C/I for each connection. This is true at least in a statistical sense. Using the path loss mode of the K/L-ranking favours base stations of low output EIRP. Less energy will be emitted into the air, as compared to strictly maximizing signal strength, thus decreasing the "I" part of C/I. However, minimizing path loss can result in areas where the C/I level is locally lower than it would have been using the highest signal strength strategy. 3.1.4 Flexible Cell Planning The base station output power for the frequency that carries BCCH can be set to a different level than that for all other frequencies. All cell borders can be individually moved by cell-to-cell related handover border offset parameters in order to adjust them to the topography or to the traffic situation. In addition they have an individual hysteresis, also by cell-to-cell definable parameters. Separate hysteresis values can be used at low received signal strength and at high signal strength. How this is achieved depends on the selected basic ranking algorithm. 3.1.5 Urgent Handover in Bad Quality Situations Signal quality measurements are provided by the MS (downlink) as well as by the base station (uplink). These measurements are only provided for the radio path connected to the serving cell. The quality measurements are used to detect occurrences of bad quality. When this happens, the locating algorithm may propose handovers even to worse cells. An urgency handover is only carried out if the MS is within a region close to the border between the serving cell and the candidate cell. One reason for this is to ensure that the connection does not disrupt the normal cell plan and thus cause too much interference. Another reason is to reduce the risk that the quality for the call in the new serving cell is even worse than in the old. 8 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  13. 13. Technical Description 3.1.6 Urgent Handover at Excessive Timing Advance In order for MSs to be able to synchronize the transmission of their bursts with the moment in time when the base station expects to receive them, the time it takes for the radio signals to travel from the MS to the base station has to be taken into account. This is done by the base station. It calculates this time interval, and sends an order to the MS to start transmitting its bursts a little in advance of what it might have done if the speed of the radio signals had been infinite. The time interval is called timing advance. The GSM Time Division Multiple Access (TDMA) protocol allows for a maximum timing advance that corresponds to a base-to-mobile distance of approximately 35 km. Note that in Extended Range cells (see Reference [10]) the maximum timing advance sent to the MS is the same as in normal cells. The increased range is achieved by allowing a connection to occupy two consecutive time slots. The timing advance value calculated by the base station is available in the locating algorithm, and is used as a measure of the base-to-mobile distance. In this way, a maximum geographical cell radius can be specified. If exceeded, the locating algorithm proposes a handover. 3.1.7 Auxiliary Radio Network Functions Locating is the basic algorithm for determining the best cell to serve the connection. However, as implemented in Ericsson GSM Systems, Locating also incorporates all or parts of the logic for a number of other radio network features, namely those radio network features that involve a cell, subcell or channel change: • Assignment to Another Cell, see Reference [2] • Hierarchical Cell Structures, see Reference [13] • Overlaid/Underlaid Subcells, see Reference [16] • Intra-cell Handover, see Reference [14] • Extended Range, see Reference [10] • Cell Load Sharing, see Reference [3] 3.1.8 Handover to UTRAN The support of handover to UTRAN by the feature GSM-UMTS Cell Reselection and Handover is not considered as a part of the locating algorithm but includes some dependencies and restrictions on the neighbouring cell measurements and building of candidate lists. For Multi-RAT MSs (that is GSM and UMTS), the reporting of measurements on the neighbouring UTRAN cells is made reusing the measurement report. When using the message MEASUREMENT REPORT, this means there will 9219/1553-HSC 103 12/21 Uen A | 2012-05-29
  14. 14. User Description, Locating be fewer positions in the measurement report for GSM cells, since still only six cells (including UTRAN) can be reported in the measurement report and valid UTRAN cells are reported first by the Multi-RAT MS. When using the message ENHANCED MEASUREMENT REPORT, more than six cells can be reported and valid GSM cells are reported with higher priority. Another restriction is that the maximum number of GSM neighbouring cell frequencies per cell is limited to 31 (instead of 32) when one or more UTRAN cells are also defined as neighbours. The evaluation of UTRAN handover candidates are processed separately. This is done by filtering out the UTRAN cell measurements and applying different conditions such as the load level in the serving GSM cell, checking service indicator, excessive TA urgency (see Section 3.2.5 on page 44), and bad quality urgency (see Section 3.2.5 on page 44). A final candidate list is created by adding UTRAN cells together with the GSM candidate cells from the locating algorithm. For more detailed information about GSM-UMTS handover see Reference [12]. 3.1.9 Locating in a Multi Band Cell The feature Multi Band Cell may have impact on serving cells basic ranking, but this feature is not seen as an auxiliary function to Locating. The Multi Band Cell feature makes it possible to have more than one frequency band in a cell and this makes it possible to have different frequency bands allocated for the BCCH frequency and the TCH frequencies. The pathloss may be different for the different frequency bands and in order to still have the same handover borders for all frequencies in a Multi Band Cell it is desired that all frequencies shall have the same coverage as the BCCH frequency. To achieve this when the active channel resides in the non-BCCH frequency band group, the own BCCH rxlev measurements are used at filtering of serving cell if it is reported and the cell's own BCCH frequency is present in the active mode BA-list. If own rxlev measurements are not included all the time or if the cell's own BCCH frequency is not included in the active mode BA-list, a frequency band offset is used. The signal strength (SS) is then compensated by adding the frequency band group offset value for the active channel at filtering of serving cell. It is possible to have a mix of the 800 and 900 MHz frequency bands within a subcell. For these frequency bands it is assumed that the two pathlosses resemble and it is not necessary to neither compensate nor measure the BCCH frequency. All the subcell parameters are shared between the two frequency bands, that is, the same link budget is assumed. For more detailed information about Multi Band Cell see Reference [15]. 10 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  15. 15. Technical Description 3.1.10 Handover Control in MSC The Mobile services Switching Centre (MSC) has a number of AXE parameters that control some properties in the MSC at intra- and inter-MSC handover. These parameters include time supervision of the switching in the group switch, handover allowed or not allowed in certain situations, etc. For more detailed descriptions, see Section 5.2 on page 89. 3.1.11 Loacting and BCCH Power Savings The feature BCCH Power Savings is not an auxiliary function to Locating. The BCCH Power Savings makes it possible to reduce / control the power of the channels on BCCH Carrying TRX, see Reference [22] for more details. Mobile stations being carried on BCCH TRX, will measure reduced signal strength on the serving as well as the neighbor cells on the channels when BCCH Power Savings is active. The Ranking part of Locating algorithm, due to BCCH Power Savings, may have different signal strength and pathloss measurements resulting in change in the ranking order of the cells. This may result in difference in assignment and handover decisions. 3.1.12 Measurement Procedure The BSC sends a list to the MSs containing the Absolute Radio Frequency Channel Number (ARFCN) for the handover candidates, the so called BCCH Allocation (BA) list (see Reference [7]). The maximum number of frequencies in this list is 32 (or 31 for Multi-RAT MSs see Section 3.1.8 on page 9). Every 120 ms (once in a 26 frames multiframe) there is an idle frame in the TCH that allows the MS to tune to the frequencies specified in the BA list and try to decode the synchronization bursts. This burst contains the Base Station Identity Code (BSIC) that includes the Network Colour Code (NCC). Parameter NCCPERM defines the permitted NCCs. If the MS can detect the synchronization burst and decode it, it checks if the NCC is permitted. The MS reports every SACCH period the six (or less for Multi-RAT MSs see Section 3.1.8 on page 9) strongest candidates for which it during the last 10 seconds has succeeded to decode the BSIC (containing a permitted NCC). For ENHANCED MEASUREMENT REPORT more than six candidates can be reported by the MS but only the six strongest candidates are evaluated by Locating. Fore more information of how the selection of candidates are made, see Reference [12]. 11219/1553-HSC 103 12/21 Uen A | 2012-05-29
  16. 16. User Description, Locating 3.2 Algorithm 3.2.1 Overview The locating algorithm serves the purpose of providing a list of possible cell candidates, in descending ranking order, for handover. The channel allocation and handover signalling is not considered part of the locating algorithm. The algorithm consists of eight stages, corresponding to the eight boxes in the main flow chart, Figure 1. The stages are processed roughly in a chronological manner. 12 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  17. 17. Technical Description Initiations Filtering Basic Ranking Auxiliary radio network functions evaluations Organizing the list Sending the list Allocation reply Penalty list Measurement results Urgency Condition Figure 1 Main Flow of the Locating Algorithm 13219/1553-HSC 103 12/21 Uen A | 2012-05-29
  18. 18. User Description, Locating Initiations A locating individual (see Section 3.2.2 on page 15) is created. If there has been a previous locating individual handling the same connection, a penalty list can be received. Filtering Measured values (for signal strength, quality and timing advance) are filtered by performing an averaging of a number of consecutive measurements. Urgency Conditions Two types of urgency conditions are evaluated: bad signal quality and excessive timing advance. The signal quality is evaluated in the uplink as well as in the downlink. Basic Ranking A basic ranking list of cell candidates is prepared. The Ericsson GSM Systems provides two algorithms for basic ranking: Ericsson1 and Ericsson3. The Ericsson1 ranking (also called K/L-ranking) takes both signal strength and path loss into account. The Ericsson3 ranking only considers signal strength. Auxiliary Radio Network Functions Evaluations The criteria for Overlaid/Underlaid Subcell Change, Hierarchical Cell Structures, Intra-cell Handover, Assignment to Another Cell, Extended Range and Cell Load Sharing are evaluated. Organizing the List All cells are organized into one final candidate list according to rules that are defined by the outcome of the urgency conditions, the overlaid/underlaid evaluations, the hierarchical cell structures evaluations, intra-cell handover evaluations and cell load sharing evaluations. Additional locating criteria are applied in order to remove unsuitable candidates. Sending the List The candidate list is sent for further processing to be used for channel allocation. Allocation Reply The outcome of the channel allocation determines the action. At success, the connection is transferred to another channel and the locating processing is transferred to a new locating individual. At congestion or signalling failure, the connection remains. 14 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  19. 19. Technical Description 3.2.2 Initiations A locating individual is the software process that handles Locating and the auxiliary radio network functions. It is activated as a result of immediate assignment, assignment or handover. This means that all types of circuit switched connections: speech connections, data traffic, SMS messages, location updating, supplementary services, emergency calls etc.- are handled by Locating. Handover on a Standalone Dedicated Control Channel (SDCCH) can however be inhibited with parameter SCHO. At a change of channel (assignment, handover, subcell change and intra-cell handover), a new locating individual is created, and takes over the handling of the connection. The old locating individual is terminated. If the new locating individual was activated as a result of handover, a list of penalties is transferred from the old locating individual, see Section 3.2.4 on page 24. Limited penalty information is also transferred at urgency handover to a cell in another BSC, see Section 3.2.9 on page 58. Immediately after an assignment, a handover, a subcell change or a intra-cell handover, it is desirable to remain on the same channel for a while. The reason is that the filtering of measurements needs some time to produce reliable estimates on which to base further action. Therefore, at initiation of a locating individual, a timer TINIT is started. The timer inhibits handovers, until it expires. After immediate assignment handovers are inhibited before TINIT has expired, but assignment to own or other cell are allowed. 3.2.3 Filtering Measurement Preparation Locating is based on a number of quantities that are reported to the BSC, see Table 1. When auxiliary radio network functions are used additional data is necessary, for example, uplink signal strength is also evaluated if Intra-cell Handover is used. Table 1 Data that Is Used for the Locating Evaluations Data description Source signal strength downlink own cell full set MS signal strength downlink own cell subset MS signal strength downlink six strongest neighbours MS quality downlink own cell full set MS quality downlink own cell subset MS quality uplink own cell full set BTS 15219/1553-HSC 103 12/21 Uen A | 2012-05-29
  20. 20. User Description, Locating Data description Source quality uplink own cell subset BTS timing advance BTS MS power capability (according to classmark) BSC (MS) DTX used by base station during the measurement period BTS DTX used by mobile during the measurement period MS The signal strength, signal quality and timing advance measurements are made and reported once for each SACCH period, that is every 0.48 s. The MS is able to measure signal strength from up to 32 neighbouring cells, but can only report the six strongest in each MEASUREMENT REPORT. In case of Multi-RAT MSs these figures can be reduced see Section 3.1.8 on page 9. When using ENHANCED MEASUREMENT REPORT more than 6 neighbouring cells can be reported but Locating only evaluates the six strongest GSM neighbouring cells. Please note that when Enhanced AMR Coverage is activated and the MS is in Repeated SACCH mode the same Measurement Report is sent twice in order to increase the probability of correct decoding. This means that a Measurement Report is still sent every SACCH period, but new Measurement Reports are sent every second SACCH period. The repeated SACCH mode could be seen as having a BLER of 50% on normal SACCH transmissions. In order to use it when it is only neccessary the repeated SACCH mode is switched on adaptively when needed. For more information about Enhanced AMR Coverage, see Reference [20]. The signal strength measurements from serving cell and the quality (RXQUAL) measurements (from the MS as well as from the BTS) are available in two sets, the full set and the subset, if EMR is not supported. The full set is measurements based on all TDMA frames during the SACCH period, and the subset is measurements based on those TDMA frames where transmission is guaranteed even when DTX is active. The locating algorithm selects either the full set or the subset. In general, the full set is used if DTX has not been used during the measurement period (SACCH period), and the subset if DTX has been used any time during the period, see Reference [6]. When EMR is supported the signal strength measurements from serving cell and the quality (MEAN_BEP) measurements are valid throughout the whole measurement periods and thus not divided in full set and subset. All signal strength measurement reports are delivered as integer values from 0 to 63 (so called rxlev). This corresponds to signal strength of -110 dBm to -47 dBm. Measurement values above -47 dBm are set to 63, and below -110 dBm are set to 0. 16 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  21. 21. Technical Description Note: In the ENHANCED MEASUREMENT REPORT (see 3GPP Technical Specification 44.018) the element SCALE is an offset which might add +10 dB to the downlink signal strength value resulting in a value of the signal strength of -100 dBm to -37 dBm. When BTS power control is used in serving cell, the reported downlink rxlev value is before filtering compensated for the down regulation. The result is that the filter input corresponds to what the signal strength would have been if maximum output power was used. For further details, see Appendix C on Section 8 on page 109. The quantity used as the measure of quality is the bit error rate (BER) that is estimated in the signal decoding process mapped on a logarithmic scale. The quality measurements are delivered from the MS and the BTS. When EMR is not supported the quality measurements (RXQUAL) are represented as an integer value between 0 and 7, where 0 corresponds to good quality (low BER) and 7 to bad quality (high BER). The locating algorithm transforms these values linearly to a 0 to 70 scale. Note that the transformed quality values cannot be higher than 70, whereas the threshold parameters used in conjunction with the quality values have a range up to 100. This provides a mechanism to turn off certain functionality, see Section 3.2.5 on page 44. When EMR is supported the quality measurements (MEAN_BEP) have a higher granularity than when EMR is not supported, being represented as an integer value between 0 and 31. The locating algorithm transforms these values to a 0 to 76 scale. In the further locating evaluations the quality measurement, indicated as rxqual, is the transformed RXQUAL value if EMR is not supported, or, the transformed MEAN_BEP value if EMR is supported. The timing advance values are delivered from the BTS as values from 0 to 63 bit periods. The MS transmits the downlink measurement to the BTS. The BTS adds the serving cell uplink measurements and transmits further to the BSC. Parameter MISSNM controls the handling of missing reports, for neighbours as well as for serving cell. A neighbour is not eligible for handover if consecutive reports are missing. If the reporting is resumed again before MISSNM report periods have elapsed, missing measurement reports are replaced by a linear interpolation and the neighbour becomes eligible again. If more consecutive measurements are missing than what is specified by the parameter, the filtering for the neighbouring cell in question is terminated. If measurements from that neighbour start arriving again, the filter is re-initiated. The handling of missing reports for serving cell is the same as for neighbours. Furthermore, if measurements from serving cell are missing, locating is suspended until the measurements start arriving again. If an urgency condition occurs, and if there are no neighbours available in the current measurement report, the last received measurement report containing 17219/1553-HSC 103 12/21 Uen A | 2012-05-29
  22. 22. User Description, Locating neighbour measurements is used. This is done only if the old measurements report is not older than MISSNM. Uplink signal strength measurements are available for serving cell. They are used for Intra-Cell Handover and MS Power Control. Signal Strength and Quality Filtering The signal strength and quality measurement values are filtered in order to smooth out measurement noise. In addition, some fading components of a duration of about the same as the filter response time, are filtered out. Five types of filter are available: • General FIR filters • Recursive straight average • Recursive exponential • Recursive 1st. order Butterworth • Median General FIR Filters The general FIR filters are of the following type (shown for signal strength measurements): Srxlev = c w ×(signal strength) (1)n i i = 1 n i where n is the filter length in SACCH periods, w i are weight coefficients and c n are normalization coefficients. These filters are implemented as straight average filters, that is the weight coefficients w i are equal. Recursive Straight Average Filter The recursive straight average filter is of the following type: rxlev t = rxlev t - 1 + [(signal strength )t - (signal strength )t-n ] / n (2) where n is the filter length and t the time of arrival of the latest measurement report (in SACCH periods). The straight average filter has a rather slow step and impulse response, but steep flanks. The general FIR filters and the recursive straight average filter have the same properties. The reason for including the recursive filter is to provide for a 18 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  23. 23. Technical Description straight average filter with maximum computational efficiency, that is minimal load on the BSC hardware. Recursive Exponential Filter The exponential filter is implemented as a recursive filter as well: rxlev t = α · rxlev t -1 + β · (signal strength ) t (3) where α is the filter coefficient and β = 1 0 α. The filter length n is given in SACCH periods (parameter SSLENSD etc., see Table 2). This value is then mapped internally to the filter coefficient α. In this way, the exponential filter will be comparable to the other filter types regarding the response time. The exponential filter has a quick step and impulse response, but is less sharp than the straight average filters. Recursive 1st. Order Butterworth Filter The 1st. order Butterworth filter is given by the following recursion formula: rxlev t = α · rxlev t01 + β · [(signal strength) t + (signal strength) t-1 ] (4) where α is the filter coefficient, and β = (1 0 α)/2. α is mapped from the filter length n in the same manner as for the exponential filter (but with a different mapping). The 1st. order Butterworth filter has an intermediate step and impulse response, and intermediate steepness. Median Filter The median filter selects the median value from the set last n measurements. The median filter is less sensitive to freak outliers and to measurement errors. Initiations Before a filter is filled, that is when less than n measurement reports have arrived, the filter is modified slightly. The general FIR filters and the straight average filter are used with the number of measurement reports available. In equations 1 and 2, the filter length n is thus modified. The exponential and Butterworth filters are initiated in the same manner, that is a straight averaging procedure with the number of measurements available. The initiation of the median filter is also done by using it with the number of measurements available. This is the initiation scheme for filtering of the serving cell measurements. For the neighbouring cells the filtering is modified with additional initiation schemes. When only one measurement report from a specific neighbour has 19219/1553-HSC 103 12/21 Uen A | 2012-05-29
  24. 24. User Description, Locating arrived, the neighbour is retained as "not valid". When two measurement reports have arrived in sequence, a linear ramping up procedure is started. The output from the filter is ramped up from half of the signal strength (RXLEV = 0, corresponding to rxlev = 0110 dBm) to the correctly normalized value when the filter is filled. As a result, the signal strength from the neighbouring cells will be underestimated during the first few reported measurements. This is a safety measure, so as not to perform a handover based on an unreliable estimate. The duration of the ramping up can be controlled by adjusting the number of SACCH periods during which the ramping up shall be active. In this way, one can obtain a quick step response at initiation even with a slow filter. These initiation schemes result in responses to a step function input that are shown schematically in Figure 2. The first measurement report arrives at t 1 in the figure, and the ramping up is active during K SACCH periods. If K is equal to 1 no ramping is applied. The figure shows the behaviour of neighbour cell filter for different values of K. 20 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  25. 25. Technical Description Output from filter 1.0 0.5 0 to t + 0.48*K sec.o time (SACCH frames) Output from filter 1.0 0.5 0 t t time (SACCH frames) Serving cell filter Neighbour cell filter to 1 1t Output from filter 1.0 0.5 0 t t + 0.48*K sec.o time (SACCH frames) Neighbour cell filter to 1 Output from filter 1.0 0.5 0 t t + 0.48*K sec.o time (SACCH frames) Neighbour cell filter to 1 Output from filter 1.0 0.5 0 t time (SACCH frames) Neighbour cell filter to 1 K=1 K=2 K=3 K>3 (here with 2 Figure 2 Step Response of the Serving and Neighbouring Cell Filters at Initiation 21219/1553-HSC 103 12/21 Uen A | 2012-05-29
  26. 26. User Description, Locating Quality Filtering Quality measurements are available for uplink as well as downlink, but only for serving cell. The quality filtering yields the filtered quantities rxqual(uplink) and rxqual(downlink). The quality filtering employs the same method as the signal strength filtering, equations 1 to 4, including the initiation scheme for serving cell. Filter Type Selection and Filter Length Selection Table 2 is a summary of the filters and the parameters used for selection of signal strength filter type and filter length: Table 2 Signal Strength Filter Selection and Filter Length Selection. Filter selection parameter value SSEVALSI, SSEVALSD Filter type Filter length, SACCH periods 1 general FIR 2 2 general FIR 6 3 general FIR 10 4 general FIR 14 5 general FIR 18 6 recursive straight average SSLENSI, SSLENSD 7 recursive exponential SSLENSI, SSLENSD 8 recursive 1st. order Butterworth SSLENSI, SSLENSD 9 median SSLENSI, SSLENSD The signal strength filter types are selected by parameters SSEVALSI and SSEVALSD. SSEVALSI selects a filter that is used during the signalling phase of a connection. In general, this phase occurs when the connection runs on an SDCCH channel. SSEVALSD selects the filter during the speech/data phase of a connection, that is when the connection runs on a TCH channel. SSEVALSI and SSEVALSD have a range from 1 to 9. Values 1 through 5 correspond to the general FIR filters. These filters are straight average filters with filter lengths 2, 6, 10, 14 and 18 SACCH periods (1, 3, 5, 7 and 9 sec). The filter length for the general FIR filters is selected by the filter selection parameter. Values 6 through 9 correspond to the recursive straight average, recursive exponential, 1st. order Butterworth and median filter respectively. The filter length for these filters (n in equations 1 to 4) is selected by parameters 22 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  27. 27. Technical Description SSLENSI and SSLENSD. SSLENSI is used to specify the length of the SSEVALSI filter, and SSLENSD is used for the SSEVALSD filter. The ramping for the general FIR filters (filter selection parameter values 1 to 5) is fixed to the entire filter length, that is K in Figure 2 is equal to n. For the remaining filters (filter selection parameter values 6 to 9), parameters SSRAMPSI or SSRAMPSD determine the number of SACCH periods during which the ramping shall be active, that is K in Figure 2. Table 3 is a summary of the filters and the parameters used for selection of quality filter type and filter length: Table 3 Quality Filter Selection and Filter Length Selection. Filter selection parameter value QEVALSI, QEVALSD Filter type Filter length, SACCH periods 1 general FIR 4 2 general FIR 8 3 general FIR 12 4 general FIR 16 5 general FIR 20 6 recursive straight average QLENSD, QLENSI 7 recursive exponential QLENSD, QLENSI 8 recursive 1st. order Butterworth QLENSD, QLENSI 9 median QLENSD, QLENSI The quality filter types are selected with parameters QEVALSI and QEVALSD in the same manner as for the signal strength filters. Filters 1 through 5, the general FIR quality filters, have filter lengths 4, 8, 12, 16 and 20 SACCH periods (2, 4, 6, 8 and 10 sec.). The filter length of filters 6 through 9 is selected with parameters QLENSI and QLENSD. When filtering signal strength for a neighbouring cell, the filter parameter setting specified for that neighbour is used. In this way serving cell and neighbouring cells can be filtered differently. This could be valuable when for example serving cell is a large macro cell and a neighbour is a small micro cell. However, for external neighbours it is not possible to use their own filter parameter setting instead the serving cell filter parameter setting is used. 23219/1553-HSC 103 12/21 Uen A | 2012-05-29
  28. 28. User Description, Locating Timing Advance One single timing advance filter is used for all cells in the BSC. The filter is a straight averaging filter. The timing advance filtering yields the filtered quantity ta. The length of the filter is specified by parameter TAAVELEN. 3.2.4 Basic Ranking 3.2.4.1 Overview There are two algorithms available for basic ranking: Ericsson1 and Ericsson3. They are selected by setting parameter EVALTYPE to 1 and 3 respectively. They differ mainly in that Ericsson1 ranks considering both signal strength and pathloss while Ericsson3 ranks only according to signal strength. They also have certain features in common. For instance, it is allowed to have different maximum output power on a pure TCH carrier than on the BCCH carrier. However, the ranking aims at letting the pure traffic channels (non BCCH) control the cell borders. Furthermore, a neighbour must pass a minimum signal strength check in order to be ranked. The following three first stages are common to both ranking algorithms: • Correction of base station output power, • Evaluation of the minimum signal strength condition, • Subtraction of signal strength penalties. 3.2.4.2 Correction of Base Station Output Power Neighbouring Cells When the MS measures the signal level from a neighbouring cell, that cell transmits a Broadcast Control Channel (BCCH). This channel may have a different output power than the traffic channel that will be seized in case of handover. As already mentioned, the locating algorithms aims at letting the pure traffic frequencies control the cell borders. Therefore the signal strength measurements rxlev for all neighbouring cells are corrected for the difference in output power on the transmitter for the BCCH frequency (BSPWR) and the output power for other frequencies (BSTXPWR). SS_DOWN n = rxlev n + BSTXPWR n 0 BSPWR n (5) where n refers to neighbouring cells and SS_DOWN is the corrected signal strength. If neighbour cell is an OL/UL cell it is BSTXPWR for the underlaid subcell that is used in equation 5. 24 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  29. 29. Technical Description Serving Cell For serving cell there are several situations when correction of the measured signal strength is needed: • If the connection is transmitted on the BCCH frequency (non hopping). Then the same correction is performed as described above for neighbouring cells. • If frequency hopping including the BCCH frequency is used. The result of this compensation is the signal strength that would have been measured if the hopping did not include the BCCH frequency. The reason for this compensation is again the possibility to have different BSTXPWR and BSPWR and the ambition to let BSTXPWR set the cell border. • If serving cell is an OL/UL cell and the MS is connected to the overlaid subcell. The reason is that the two subcells might have different output power. The effect of the compensation is that the cell border is given by BSTXPWR for the underlaid subcell. • If power control is used. The result of the compensation is the signal strength that would have been measured if the BTS was using maximum output power. This compensation is already mentioned, see Section 3.2.3 on page 15. In the rest of the document SS_DOWN will refer to the signal strength corrected according to above. For a detailed description of the formulas used for the different types of signal strength compensation, see Appendix C on Section 8 on page 109. 3.2.4.3 Minimum Signal Strength Condition The output from the neighbouring cell signal strength filtering, rxlev, is tested against two minimum level thresholds, one for the downlink signal strength, MSRXMIN, and one for an estimated uplink signal strength, BSRXMIN. These levels are defined for each cell individually. The weakest of up- and downlink signal levels is used for the minimum level criterion selection. This is to see if the signals can be considered high enough above the sensitivity level for the corresponding cells to be eligible for handover. The minimum level condition is: SS_DOWN n >= MSRXMIN n and SS_UP n >= BSRXMIN n where n refers to a neighbouring cell and SS_DOWN is the downlink signal strength after correction for base station output power. SS_UP is the estimated uplink signal strength. It is estimated by calculating the downlink path loss and subtracting that from the MS output power: 25219/1553-HSC 103 12/21 Uen A | 2012-05-29
  30. 30. User Description, Locating SS_UP n = MS_PWR n 0 L n (6) where MS_PWR is the “nominal” MS output power. It is the lowest of the power capability according to the MS classmark (P) and the highest allowed MS output power in that cell as given by parameter MSTXPWR: MS_PWR n = min (P ,MSTXPWR n ) (7) The path loss L n is L n = BSTXPWR n 0 SS_DOWN n (8) where BSTXPWR indicates base station output power for the transmitter operating on a pure TCH frequency, at a common reference point. Henceforth, we will assume a reference point immediately outside of the antenna. This enables us to use EIRP as the measure of output power. In this modelling of the uplink signal strength, the wave propagation is assumed reciprocal, that is the path loss is the same in the two directions. Note that BSRXMIN must be adjusted to the choice of reference point. 3.2.4.4 Subtraction of Signal Strength Penalties Penalties, or punishments, aggravate handover to cells that for some reason are temporarily undesirable. The aggravation consists of subtracting a signal strength value, a penalty, from the signal strength estimate rxlev for the undesirable cell, so that it appears "worse" than it really is: p_SS_DOWN p = SS_DOWN p 0 LOC_PENALTY p 0 HCS_PENALTY p (9) where p refers to the punished cell. HCS_PENALTY is a punishment which is associated with the feature Hierarchical Cell Structures, see Reference [13]. LOC_PENALTY refers to locating penalties. A locating penalty is given by one of the following three reasons: • Handover failure: If there is a signalling failure at handover, the failure might occur again if a handover to the same cell is attempted too soon. Therefore the cell to which the handover failed, is punished. Section 3.2.9 on page 58, describes handover failure penalty and its controlling parameters. • Bad quality urgency handover: 26 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  31. 31. Technical Description The cell that is abandoned at bad quality urgency is normally the best cell from a signal strength point of view. This means that without precautions, it would be first in the candidate list at the next locating evaluation, and thus cause a handover back to it again. Section 3.2.5 on page 44 describes the penalties associated with bad quality urgency, and the controlling parameters. • Excessive timing advance urgency handover: Excessive timing advance urgency handover is similar to bad quality urgency handover. It is handled in the same manner, with its own set of parameters, see Section 3.2.5 on page 44. However, when a cell is punished due to excessive timing advance, all other co-sited neighbouring cells are checked for excessive timing advance, and punished if necessary. Each penalty is applied to the punished cell for a certain time and only for the connection that experienced the handover failure, the urgency condition or the condition related to Hierarchical Cell Structures. Before the cell ranking, a check is performed to see whether there are any penalties for the neighbouring cells in question. The penalties are subtracted from the estimates for the corresponding cell. The penalty size and duration are set by parameters. For details about penalties and penalty lists, see Section 3.2.9 on page 58. 3.2.4.5 Ericsson1 After the basic stages described above, Ericsson1 first separates “low signal strength” cells from “high signal strength” cells. Cells that do not fulfil a sufficient level condition (low signal strength cells) are called K-cells and are ranked according to their signal strength. Cells that do fulfil the sufficient level condition (high signal strength cells) are called L-cells and ranked according to their path loss. A small pathloss gives a high ranking value. By applying pathloss ranking for cells with sufficient signal strength, the call is connected to the base station closest to the MS, thus enabling optimum power regulation. After the evaluation of the sufficient signal strength condition, the following is performed: • Signal strength evaluation (K-criterion), • Path loss evaluation (L-criterion), • Combination into a basic ranking list. 27219/1553-HSC 103 12/21 Uen A | 2012-05-29
  32. 32. User Description, Locating Sufficient Level Condition For all neighbouring cells selected according to the minimum level condition, plus serving cell, a sufficient level condition is applied, separating low signal strength cells from high signal strength cells. The sufficient condition is analogous to the minimum level condition, but with a few important differences: • Serving cell is evaluated as well as neighbouring cells. • "Sufficient" signal levels, defined by parameters MSRXSUFF and BSRXSUFF are used as thresholds. Both are set on a per cell basis. • The sufficient thresholds are modified with a transition hysteresis TRHYST and a transition offset TROFFSET, both of which are cell-to-cell relation parameters. The effect of TRHYST is to reduce the risk of ping-pong handovers caused by for example fading. TROFFSET is used to adjust the sufficient level (and thus the corresponding cell border) for a given cell-to-cell relation. Neighbouring Cells The sufficient level condition for a neighbouring cell is as follows: p_SS_DOWN n >= MSRXSUFF n - TROFFSET n,s + TRHYST n,s p_SS_UP n >= BSRXSUFF n - TROFFSET n,s + TRHYST n,s where n refers to a neighbouring cell and s refers to serving cell. p_SS_UP n is defined using the punished signal strength according to p_SS_UP n = MS_PWR n 0 p_L n (10) where p_L n = BSTXPWR n 0 p_SS_DOWN n (11) The expressions on the right hand side in the sufficient level condition can be seen as effective sufficient levels. In this condition, the effective sufficient levels include cell-to-cell relation parameters. This means that one cell might be classified as a K-cell and another as an L-cell, although their reported signal strength is the same. The minimum signal strength level and the sufficient signal strength level can be seen as delineating areas around the base station. Figure 3 shows an example of how the minimum and sufficient levels can appear in an idealized geographical plane, that is in a flat geography without shadow fading. 28 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  33. 33. Technical Description A "high signal strength" B "low signal strength" uplink minimum level downlink minimum level downlink effective sufficient level uplink effective sufficient level Figure 3 Minimum and Sufficient Levels Around a Base Station The solid lines represent the final conditions, that is the combined uplink and downlink conditions. The sufficient level shown is valid only for the neighbouring cell B. Serving Cell The evaluation of the sufficient level condition for serving cell is related to the best neighbouring cell (the neighbour with the highest ranking in the basic ranking list). Therefore the neighbouring cells are ranked before this evaluation takes place. For a description of the serving cell sufficient level evaluation, see Page 33. Ranking According to the Signal Strength Criterion (K-criterion) The K-criterion is based on the same expressions as the sufficient level condition. For neighbouring cells a K-value is calculated for each link by the following expressions: 29219/1553-HSC 103 12/21 Uen A | 2012-05-29
  34. 34. User Description, Locating K_DOWN n = p_SS_DOWN n 0 MSRXSUFF n (12) K_UP n = p_SS_UP n 0 BSRXSUFF n (13) For serving cell K values are calculated according to: K_DOWN s = SS_DOWN s 0 MSRXSUFF s (14) K_UP s = SS_UP s 0 BSRXSUFF s (15) The K-criterion is a relative signal strength criterion, since the K values are signal strength relative to the sufficient level. The K ranking value for serving cell is calculated as the lowest of the uplink and downlink K values: K_RANK s = min (K_DOWN s , K_UP s ) (16) For neighbouring cells, the K ranking value is also the lowest of the uplink and downlink K values, but modified with an offset KOFFSET and a hysteresis KHYST: K_RANK n = min (K_DOWN n , K_UP n ) 0 KOFFSET s,n 0 KHYST s,n (17) K-cells are ranked according to their K_RANK value. The highest K_RANK ranks first. KHYST is used to decrease the ranking values for neighbouring cells, which thus are a little underrated in comparison to the serving cell. This is to prevent ping-pong handovers. KHYST is defined as a cell-to-cell relation, and is always symmetrical, that is the same value for two neighbouring cells: KHYST A,B = KHYST B,A (18) where A and B represent two neighbouring cells. KOFFSET is used to decrease the ranking value for neighbouring cells (or, if KOFFSET is negative, to increase it). This has the effect that the cell border is displaced away from the cell for which the parameter has a positive value. A neighbouring cell n will be underrated if KOFFSET is greater than zero, moving the cell border closer to that cell. It is defined as a cell-to-cell relation, and is always anti-symmetrical, that is the same value but different sign in two neighbouring cells: KOFFSET A,B = 0 KOFFSET B,A (19) KHYST and KOFFSET are used to control cell borders that appear when serving cell and the neighbouring cell are both K-cells. The function of KHYST 30 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  35. 35. Technical Description and KOFFSET is illustrated in Figure 4 in a theoretical signal strength diagram, where it is assumed that the sufficient values are equal in cell A and cell B: signal strength from cell B signal strength from cell A hysteresis corridor handover border, A to B nominal cell border handover border, B to A hysteresis offset hysteresis original cell border without offset Figure 4 Hysteresis and Offset The effect of the offset is to create a nominal cell border displaced away from the one given strictly by the K-value. The effect of the hysteresis is to create an area around the nominal cell border (the shaded area), the so called hysteresis corridor. In this area, the connection belongs to the cell from which the MS entered the area. See also Figure 5 below, Figure 10 and Figure 11. Figure 5 shows an example of how a cell border, together with its hysteresis corridor, can be moved with the offset parameter. The figure shows handover borders in a geographical plane in a more realistic manner. 31219/1553-HSC 103 12/21 Uen A | 2012-05-29
  36. 36. User Description, Locating offset original cell border hysteresis corridor nominal cell border B A Figure 5 Handover Borders, Hysteresis and Offset Ranking According to the Path Loss Criterion (L-criterion) L-cells are ranked according to their path loss, calculated as the difference between base station EIRP and the received signal strength in the MS. The path loss criterion is independent of mobile station as well as base station power ratings. At a certain location, every MS will then evaluate the path loss equally, irrespective of power class or base station power changes. This is not the case with the relative signal strength criterion. If two cells have equal EIRP, the K- and L-criterion give the same ranking result with respect to each other. For serving L-cell, the L ranking value is equal to the path loss: 32 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  37. 37. Technical Description L_RANK s = BSTXPWR s 0 SS_DOWN s (20) For neighbouring cells the L ranking value is modified with an offset LOFFSET and a hysteresis LHYST in the same manner as for the K ranking value: L_RANK n = p_L n + LOFFSET s,n + LHYST s,n (21) where p — L is given in equation 11. L-cells are ranked according to their L_RANK value. The lowest L_RANK value ranks first. LOFFSET and LHYST have the same symmetry properties as their K counterparts, that is anti-symmetry and symmetry, respectively and are used in the same manner, see Figure 4. They affect cell borders that appear when serving cell and the neighbouring cell are both L-cells. Ranking of Serving Cell At this point, the ranking of neighbouring cells is known. In order to rank serving cell, it has to be determined whether it is a K- or L-cell. As with neighbouring cells, this is done by evaluating the sufficient level condition. The sufficient level condition for serving cell is as follows: SS_DOWN ³ MSRXSUFF - TROFFSET - TRHYST and SS_UP ³ BSRXSUFF - TROFFSET - TRHYST S S S S s,n1 s,n1 s,n1 s,n1 where n1 refers to the best neighbouring cell and s refers to serving cell. Note that TRHYST is added separately to the sufficient level for the neighbouring cells and the sufficient level for the serving cell. The result is that the hysteresis area that arises as a result of the transition hysteresis will have the width TRHYST on each side of a nominal transition border segment, see Figure 8. If serving cell fulfils the sufficient level condition, it is an L-cell, otherwise it is a K-cell. If serving cell is a K-cell, it is ranked among the other K-cells. If serving cell is an L-cell, it is ranked among the other L-cells. 33219/1553-HSC 103 12/21 Uen A | 2012-05-29
  38. 38. User Description, Locating Basic Ranking List Finally, the basic ranking list is compiled by joining together L-cells and K-cells. L-cells are put on top, with the cell having the ranking value corresponding to the lowest path loss first. K-cells are put at the bottom, with the cell having the ranking value corresponding to the lowest relative signal strength last. This means that an L-cell is always ranked better than a K-cell, no matter the original signal strength values used in the calculations. Figure 6 summarizes the basic ranking procedure in a flow diagram: 34 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  39. 39. Technical Description For serving cell s Best neighbour used for serving cell sufficient condition Wait until ranking of neighbour cell is done L-cells K-cells Serving cell is K-cell Serving cell is L-cell Yes Signal strength above SUFFICIENT level? L-cells K-cells For all reported neighbour cells n Discard n Signal strength above SUFFICIENT level including TROFFSET and TRHYST ? Penalty evaluation Minimum level criterion fulfilled? More neighbours? No Rank n according to K-criterion including KOFFSET and KHYST No No Rank n according to L-criterion including LOFFSET and LHYST Yes Yes No Yes Basic handover candidate list Figure 6 Overview of the Basic Ranking Procedure 35219/1553-HSC 103 12/21 Uen A | 2012-05-29
  40. 40. User Description, Locating Handover Borders This section shows figures representing examples of handover borders and how they appear due to the Ericsson1 locating algorithm. The main use of the sufficient threshold parameters MSRXSUFF and BSRXSUFF is to separate the two regions of signal strength where a neighbouring cell is K-cell or L-cell respectively, that is the K-L-transition. The effect of the different criteria in the K and L regions is that the cell borders that appear in the two regions will not coincide. The two segments, the K-K border segment and the L-L border segment, are joined by a border segment along (one of) the effective sufficient levels, the K-L transition border segment. The reason that the border runs here is that an L-cell is always ranked better than a K-cell. Figure 7 shows the three types of border segment in an idealized geographical plane (cf. Figure 3). K-K border L-L border effective sufficient level from A to B effective sufficient level from B to A transition border (K-L border) transition border (K-L border) equal K B A equal L Figure 7 K-K, K-L Transition and L-L Border Segments 36 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  41. 41. Technical Description For reasons of clarity, the handover borders in Figure 7 are shown without the hysteresis corridors around the nominal cell border. In this example, the base station A has higher EIRP than base station B. Consequently the K-K border (where cell A rank equal to cell B according to the signal strength criterion) will run closer to base B than to base A. However, the L-L border segment (equal ranking according to the path loss criterion) appears halfway between the two base stations. The sufficient thresholds can give rise to K-L transition border segments (see Figure 7), especially when the EIRP varies between base stations. As with all cell borders, it has to be made sure that this type of border has an adequate hysteresis to protect from ping-pong handovers. The transition hysteresis TRHYST is used to achieve this by reducing the effective sufficient level for serving cell and increasing the effective sufficient level for the neighbouring cells. In this way, it will become slightly easier for serving cell to be identified as an L-cell, and slightly more difficult for a neighbouring cell to be identified as an L-cell. The transition offset TROFFSET is used to displace the entire cell border, by analogy with the other types of offset. A positive transition offset from a cell A towards a cell B will decrease the effective sufficient level for cell A as calculated against cell B as neighbouring cell, that is this will make cell A larger. The effective sufficient level from A to B (see Figure 7), will thereby be displaced towards cell B. At the same time, the effective sufficient level for neighbouring cell B as calculated against cell A as neighbouring cell, will increase by the same amount, and thus be displaced towards cell B as well. The transition hysteresis corridors will follow the displaced sufficient values, see Figure 8. 37219/1553-HSC 103 12/21 Uen A | 2012-05-29
  42. 42. User Description, Locating signal strength, cell B signal strength, cell A effective sufficient level from A to B effective sufficient level from B to A TRHYST TRHYST TROFFSET (=-TROFFSET )B,A A,B TROFFSETA,B Figure 8 Displacement of the Sufficient Levels with a Transition Offset TRHYST and TROFFSET have the same symmetry properties as the other types of hysteresis and offset. Figure 9 shows handover borders between two cells A and B in a signal strength plane, as they appear due to the K and L ranking criteria. The case shown is a similar one to that shown in Figure 7, that is cell A has a higher EIRP than cell B. In addition, the three hysteresis parameters have the default values for the A to B cell relation. 38 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  43. 43. Technical Description signal strength, cell B signal strength, cell A effective sufficient level in B effective sufficient level in A handover border from A to B handover border from B to A nominal cell border KHYST TRHYST LHYST Figure 9 Handover Borders in a Signal Strength Plane Figure 10 shows a similar situation, but where KOFFSET B,A has been given a positive value. In addition, the three different hysteresis are different. 39219/1553-HSC 103 12/21 Uen A | 2012-05-29
  44. 44. User Description, Locating signal strength, cell B signal strength, cell A effective sufficient level in B handover border from A to B nominal cell border effective sufficient level in A handover border from B to A TRHYST LHYST KHYST KOFFSETA,B Figure 10 Handover Borders with Offset and Different Hysteresis Figure 11 shows the same as Figure 10, but schematically in an idealized geographical plane. 40 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  45. 45. Technical Description L-L border transition borders (K-L border) AB K-K border sufficient levels KOFFSET B,A Figure 11 Handover Borders in a Geographical Plane Any application of an offset that is larger than the corresponding hysteresis, may lead to oscillating handovers in small areas, the "merry-go-round effect", see Figure 12. 41219/1553-HSC 103 12/21 Uen A | 2012-05-29
  46. 46. User Description, Locating A B C A - B cell border shifted original A - B cell border C- A cell border B - C cell border Figure 12 Merry-Go-Round Effect due to Offset Larger than Hysteresis Consider three cells, A, B and C. A mobile station entering into the black area will not be able to stay in a stable manner in any of the three cells. If the MS approaches the black area cell A, it will pass the C0A handover border and perform a handover to C. Once on C, the locating algorithm will notice that the MS is on the wrong side of the B0C handover border. Consequently there will be a handover to B. Now, since the A0B handover border has been shifted with an offset, the MS will be on the wrong side of that handover border. A handover back to cell A is then performed, and the merry-go-round is in motion. A practical example of how a sufficient level can give rise to a cell border is illustrated by Figure 13. In this figure three cells are shown, A, B and C. A and B have significantly higher EIRP than C, which might be a micro-cell. 42 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  47. 47. Technical Description effective sufficient level from C to B after modification cell ("transition") border between B and C L-L border between A and B cell ("transition") border between A and C sufficient level in C TROFFSET A B C Figure 13 Transition Borders The sufficient level in C acts as a cell border. Inside the sufficient level, C is ranked as an L cell. In this area cell C will always rank higher than A and B since the distance to C is shorter than to A and to B. Outside the sufficient level, C ranks as a K-cell and A and B as L- or K-cells. In either case, A and B rank higher than C. Thus, the cell border between C and any of the cells A and B will not have an L-L segment. (An L-L border is where neighbouring cells rank equal according to the L-criterion, see Figure 7). In Figure 13 it has also been indicated how the cell border (the "transition" border) can be displaced by applying a transition offset TROFFSET. If the transition offset is larger than the transition hysteresis however, instabilities may occur in the intersection area between the three cells. 3.2.4.6 Ericsson3 Overview The first three stages of the algorithm (output power correction, minimum signal strength evaluation and subtraction of signal strength penalties) are performed in exactly the same way as for Ericsson1. The ranking, however, is much simpler. Ericsson3 takes only signal strength into account and does not consider path loss. 43219/1553-HSC 103 12/21 Uen A | 2012-05-29
  48. 48. User Description, Locating Ranking In the ranking algorithm an offset value and a hysteresis value is used when ranking neighbouring cells. The offset value is used to displace the cell border as compared to the border strictly given by signal strength. The hysteresis reduces the risk for ping-pong handovers. For the effect of the offset and hysteresis, see Figure 4 and Figure 5. The offset value is given by parameter OFFSET. Parameter HYSTSEP specifies when the signal strength for serving cell is high or low. When the signal strength is high a larger hysteresis value can be allowed than when it is low, in order to reduce the number of handovers. Let HYST be the hysteresis value used in the ranking. Then HYST is set to LOHYST if downlink signal strength is less than HYSTSEP, else it is set to HIHYST. LOHYST and HIHYST are symmetrical cell-to-cell parameters and HYSTSEP is a cell parameter. OFFSET is an asymmetric cell-to-cell parameter. The ranking value is given by: RANK s = SS_DOWN s (22) RANK n = p_SS_DOWN n 0 OFFSET s,n 0 HYST s,n (23) where s denotes serving cell and n neighbouring cells. 3.2.5 Urgency Conditions Two criteria are used for urgency detection, excessive timing advance and bad quality. For connections using the codec type AMR Full Rate there are separate bad quality limits for the urgency detection. For more information about AMR see Reference [1]. AMR Wideband connections also have separate bad quality limits for urgency detection. For more information about AMR Wideband see Reference [21]. • Quality: 0 rxqual(uplink) > QLIMUL or 0 rxqual(downlink) > QLIMDL causes bad quality urgency. For AMR FR 0 rxqual(uplink) > QLIMULAFR or 0 rxqual(downlink) > QLIMDLAFR causes bad quality urgency for AMR FR connections. 44 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  49. 49. Technical Description For AMR Wideband 0 rxqual(uplink) > QLIMULAWB or 0 rxqual(downlink) > QLIMDLAWB causes bad quality urgency for AMR Wideband connections. • Timing advance: 0 if OL/UL subcell is used and XRANGE =YES and channel in OL subcell then • if ta >= talimxol then TA urgency 0 else if ta >= TALIM then TA urgency All urgency limits are parameters specified per cell except talimxol that has a fixed value, see Reference [10], chapter Combined normal and extended range cells. The quantities tested against the parameters are the outputs from the respective filters. The information about excessive TA urgency is used in the organization phase (see Section 3.2.7 on page 51) to indicate when the cell needs to be abandoned urgently. The radio conditions that gave rise to the urgency condition may well continue to be present in the original cell. In addition, the original cell would be the best cell from a strictly signal strength or path loss point of view. In order to prevent an immediate handover back to the original cell after an urgency handover, the original cell is punished with a penalty during a certain time, see Section 3.2.4 on page 24. At bad quality urgency the following parameters are used: • Penalty value: PSSBQ • Penalty duration: PTIMBQ At excessive timing advance urgency the following parameters are used: • Penalty value: PSSTA • Penalty duration: PTIMTA BQ Urgency The main cause of bad quality, that is high values of rxqual , is co-channel interference. However, it can also appear as a result of for example adjacent channel interference, excessive time dispersion or low signal strength (the consequence of low signal strength is often co-channel or adjacent channel interference). In all cases, a change of cell may lead to an improvement of the connection quality. 45219/1553-HSC 103 12/21 Uen A | 2012-05-29
  50. 50. User Description, Locating However it is not allowed to perform a bad quality urgency handover to a worse cell from anywhere in the serving cell. This might cause a call to be connected to a cell that is far away from the correct cell according to the cell plan. Thereby the call may cause excessive uplink co-channel interference to another connection and, correspondingly, may experience excessive downlink co-channel interference. Therefore, when BQ urgency prevails, unsuitable neighbours are removed from the candidate list. The principle is to compare the signal strength of serving cell with the signal strength of the candidates and remove the candidates for which this difference is too large. The parameter BQOFFSET defines how far away from the nominal cell border an MS is allowed to be located (radiowise) in order to be eligible for bad quality handover. This parameter will be replaced with BQOFFSETAFR for connections using the codec type AMR FR. The region in serving cell A where it is possible to perform a bad quality handover to neighbouring cell B is henceforth called the “bad quality urgency region” for A with respect to B. The procedure for reducing the candidate list at BQ urgency is only applied to handover candidates with worse basic ranking: Ericsson1 Depending on the signal strength relative the sufficient level, for serving cell as well as the neighbour, the following formulas are applied: if K_RANK n 0 K_RANK s < - KHYST s,n 0 BQOFFSET s,n (24) then remove n. if L_RANK n 0 L_RANK s > LHYST s,n + BQOFFSET s,n (25) then remove n. Ericsson3 if RANK n 0 RANK s < 0 HYST s,n 0 BQOFFSET s,n (26) then remove n. For an explanation of HYST see Page 44. For Ericsson1, the candidate list reduction at BQ urgency results in a bad quality urgency region which at L-L and K-K border segments is given by BQOFFSET and the corresponding hysteresis. At transition border segments the bad quality urgency region equals the hysteresis corridor (given by TRHYST). Figure 14 shows the bad quality urgency region in a geographical plane and Figure 15 shows the same in a signal strength plane. 46 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  51. 51. Technical Description hysteresis corridor B A nominal cell border bad quality urgency region bad quality urgency handover prohibited BQOFFSETB,A Figure 14 Bad Quality Urgency Region, in Case of an Ms on the Cell Border The marked area to the right in Figure 14 is the area of the cell where bad quality urgency handover from cell A to cell B is prohibited. The region between this area and the cell border from cell A to cell B (the far side of the hysteresis corridor), is the bad quality urgency region for cell A towards cell B. Cells that do not meet with the signal strength criterion for urgency handover are removed from the basic ranking list. This means that the MS is outside the bad quality urgency region for those cells. 47219/1553-HSC 103 12/21 Uen A | 2012-05-29
  52. 52. User Description, Locating AB sufficient levels BQOFFSET B,A K-K border L-L border transition border (K-L border) bad quality urgency handover prohibited bad quality urgency region Figure 15 Bad Quality Urgency Region In Figure 15 it can be seen how BQOFFSET acts as a negative hysteresis. TA urgency Timing advance can be used as a measure of the base-to-mobile distance. TALIM can thus be used as a cell limit. An excessive TA urgency has none of the urgency region limitations that bad quality urgency has. If the cell to which the handover is made is co-sited with the original cell, the ta value will remain the same. Therefore, before a handover to a co-sited cell, a check is performed to see whether there immediately will be a TA urgency in the new cell. The current reported TA value is compared to TALIM in the neighbouring cell. If larger, a TA urgency will ensue. A co-sited neighbouring cell is removed if either one of the following three conditions are fulfilled: • Average TA exceeds MAXTA for the co-sited cell 48 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  53. 53. Technical Description (ta > = MAXTA n ). • Average TA is less than TALIM for serving cell and exceeds TALIM for the co-sited cell (ta < TALIM s and ta > = TALIM n ). • The co-sited cell is worse and it belongs to the same hierarchical layer as serving cell and the average TA exceeds TALIM for the co-sited cell (cell n is worse and LAYER n = LAYER s and ta > = TALIM n). Co-sited neighbours within the same layer are removed in order to reduce the risk for ping-pong handovers. A serving cell in another layer, however, might have new neighbours (for example within the same layer) with worse ranking than the new serving cell, but higher ranked than the old serving cell, that do not have TA urgency. The comparison and possible removal is repeated for every co-sited neighbour, as given by the parameter CS. After the TA urgency handover has been performed, the removed co-sited neighbouring cells are punished, in addition to the abandoned cell. 3.2.6 Auxiliary Radio Network Functions General Six auxiliary radio network functions are incorporated in the locating software: • Assignment to Other Cell, see Reference [2] • Hierarchical Cell Structures, see Reference [13] • Overlaid/Underlaid Subcells, see Reference [16] • Intra-cell Handover, see Reference [14] • Extended Range, see Reference [10] • Cell Load Sharing, see Reference [3] The complete features are described in detail in the references, together with the parameters that control them. Here are only given short notes about aspects of interest in conjunction with Locating. Assignment to Other Cell The normal locating algorithm is used in order to find the most suitable cell at call setup. This is possible since locating is started already at immediate 49219/1553-HSC 103 12/21 Uen A | 2012-05-29
  54. 54. User Description, Locating assignment, that is when a signalling channel is established between the BSC and the MS, which carries the measurement results from the BTS to the BSC. If a better cell than the serving cell (the one that provided the downlink idle connection) is found during the call setup signalling, that cell will be the first one in the locating candidate list at assignment. This is called "Assignment to better cell" (SDCCH in originating cell directly to TCH in target cell). At this stage of the locating evaluations, also cells worse than the serving cell can be found in the candidate list. At congestion in serving cell or in a better cell, the call can be set up in a worse cell. This is called "Assignment to worse cell". However, in the same manner as for bad quality urgency, cells at a large radio distance from the nominal cell border are not allowed to remain as candidates, see Figure 14. The controlling parameter in this case, defining the "assignment to worse cell region", is AWOFFSET, see Reference [2]. The reason for this extra signal strength criterion is the same as for the bad quality urgency region, that is not to cause, and not to become subject to, excessive interference. Hierarchical Cell Structures The HCS feature provides the possibility to give priority to cells that are not strongest but provide sufficient signal strength. The appropriate level of the sufficient signal strength mainly depends on co- and adjacent channel interference from the surrounding cells. With low interference it is the noise level, with a safety margin, that set the sufficient level. The priority of a cell is given by associating a layer to the cell. Each layer is also belonging to an HCS band. The lower the layer (and HCS band), the higher is the priority. The layer and band definition can be based on how much traffic the cells would capture with just basic ranking, how much traffic they at maximum can be dimensioned for, how much the cells interfere with the rest of the network etc. Up to eight layers may be defined using HCS. The layers are distributed in ascending order in up to eight HCS bands. A lower HCS band thus only includes lower layers compared to a higher HCS band. Cells in different bands should not interfere with each other. Therefore a lower layer cell can be prioritized in a larger area if it belongs to another band than the other reported cells. With HCS it is possible to utilize the network capacity better than with only basic ranking while at the same time avoiding quality problems due to interference or noise. The reduced HCS option means that no bands are available and all cells belong to layer 1, layer 2 or layer 3. For more details about HCS, see Reference [13]. Overlaid/Underlaid Subcells The Overlaid/Underlaid feature provides a way of increasing the traffic capacity in a cellular network without building new sites. Since an overlaid subcell serves a smaller area than the corresponding underlaid subcell, a smaller reuse distance can be used in the overlaid subcell than in the underlaid. The overlaid/underlaid evaluations (depending on ta , pathloss, distance to cell 50 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  55. 55. Technical Description border and load) may result in a recommendation to change subcell from the one currently used. For details, see Reference [16]. Intra-cell Handover The Intra-cell Handover feature provides a way of improving the speech quality during a conversation. This is accomplished by changing the channel within the (sub-)cell when there is bad quality although the signal strength is high. For details, see Reference [14]. For Half Rate connections it is possible to improve the speech quality using Intra-cell Handover due to channel rate change from HR to FR, for more information see Reference [5]. An intra-cell handover can also be initiated when the channel is on the BCCH channel group and the MS is outside the borders of the BCCH channel group, see Reference [19]. Extended Range Extended Range allows the usage of cells with a radius of maximum 121 km (in contrast to the normal 35 km). For extended range cells, the value range of those parameters that concern ta related quantities is larger. For details, see Reference [10]. Cell Load Sharing Cell Load Sharing (CLS) makes it possible to cope with high load peaks in a cell. If the load is above a certain threshold in a cell, all active connections close to the border to cells with low load (below a certain threshold) become load sharing handover candidates. This is achieved by recalculating the ranking values for the suitable neighbouring cells, using a reduced hysteresis value. The hysteresis reduction applies to all hysteresis (KHYST, LHYST and TRHYST for Ericsson1, HYST for Ericsson3). It is applied in a gradual manner over a certain time, starting from zero to a specified percentage of the normal hysteresis value. The result is that a worse neighbour may eventually change status and become better, thus initiating a load sharing handover. For details, see Reference [3]. 3.2.7 Organizing the List After urgency condition evaluation, basic ranking and penalty evaluation the cells are ordered in a basic ranking list either according to Ericsson 1 or according to Ericsson 3. The cells are then divided into three categories, better cell, worse cell and serving cell (S). Next step is to evaluate the radio network 51219/1553-HSC 103 12/21 Uen A | 2012-05-29
  56. 56. User Description, Locating functions, Cell Load Sharing, Overlaid/Underlaid and Intra-cell Handover. Evaluation of an auxiliary function is only performed if the function is active. After the auxiliary function evaluation the cells are organized in a candidate list. Non valid neighbours are removed from the list and HCS evaluation is performed, if HCS is used. The cells in the candidate list are then divided into a new set of categories, serving cell (S), above S and below S. Finally, the order of the candidate list and cause codes are decided. The final cell candidate list can contain seven candidates: maximum six (or less for Multi-RAT MSs see Section 3.1.8 on page 9) neighbouring cell candidates and one serving cell candidate. The main flow after basic ranking is shown in Figure 16 below. Intra Cell Handover Evaluation Prioritize Higher Layer HCS Band Evaluation Categorization 2 Order the Candidates Prepare Candidate List Fast Moving Mobile NO YES Removal of Candidates Overlaid/Underlaid Subcell Evaluation Categorization 1 Cell Load Sharing Evaluation Auxiliary feature, evaluation only performed when used Part of algorithm which is always performed Figure 16 The Main Flow In Locating After Basic Ranking Has Been Performed. 3.2.7.1 Categories 1 The candidates in the ranking list are divided into categories. The signal strength is used to set the categories for the neighbouring cells. The existing categories are: 52 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  57. 57. Technical Description • Better cell: Cell with a higher ranking value than serving cell in the candidate list. • Worse cell: Cell with a lower ranking value than serving cell in the candidate list. • Serving cell: The cell serving the connection. These categories are later used in evaluations and selection of cause values. 3.2.7.2 Cell Load Sharing Evaluation The cell load sharing evaluation is performed after the normal ranking procedure. The evaluation is only performed if cell load sharing is active for the cell, if no urgency conditions apply and if the channel is a TCH where the channel mode is “speech/data”. A channel in a multislot configuration is not evaluated.. Only candidates that are worse, internal, exists in the same cell layer as serving cell and accepts 'handover due to cell load sharing' being performed to them are involved in the evaluation. The evaluation is then performed as a new ranking with reduced hysteresis areas between serving cell and the neighbouring cells. For detailed information see Reference [3]. 3.2.7.3 Overlaid and Underlaid Subcell Evaluation The evaluation is only performed if the serving cell has an overlaid and underlaid subcell structure. An evaluation is performed to determine if a subcell change (overlaid to underlaid or underlaid to overlaid) is desired. This is based on the downlink signal strength, timing advance measurements from the serving cell, distance to cell border criteria and the traffic load in the cell. For detailed information see Reference [16]. 3.2.7.4 Intra-Cell Handover Evaluation The Intra-cell Handover evaluation is not performed at assignment and otherwise only performed if intra-cell handover is allowed for the current subcell. The criterion for determining if an intra-cell handover is desired is based on uplink and downlink quality and signal strength measurements from the serving cell. The criterion is fulfilled when the quality is worse than could be expected from the signal strength level. There are parameters to prevent too many consecutive intra-cell handovers in a sequence during a connection. If the MS is located in the overlaid subcell and the number of allowed consecutive intra-cell handovers have been executed, Locating proposes a subcell change from overlaid to underlaid subcell to prevent the MS from 53219/1553-HSC 103 12/21 Uen A | 2012-05-29
  58. 58. User Description, Locating remaining on connections with bad quality. A timer is used to prevent an immediate return back to the overlaid subcell. For detailed information see Reference [14]. Intra-cell Handover is also used at dynamic mode adaptation to improve the quality for HR connections by changing the mode from HR to FR. For more information about dynamic mode adaptation see Reference [5]. An intra-cell handover can also be initiated when the channel is on the BCCH channel group and the MS is outside the borders of the BCCH channel group, see Reference [19]. 3.2.7.5 Removal of Candidates Depending on different conditions and parameters candidates might be removed from the list of possible candidates. The general rule when removing candidates from the list is that for a good radio connection the requirements on the candidates are high. When the radio connection deteriorates, the requirements on the candidates is lowered. In an urgency situation, for example, it is important to make sure that there are alternative candidates in the list to make a handover to. The reasons for removal of candidates from the list are: • The frequency band of the candidate not supported by the mobile. • Evaluation of BSC parameters and timers controlling handover, that is if it is allowed to make handover on SDCCH etc. The evaluated parameters are IBHOSICH, SCHO, ASSOC, IBHOASS, TALLOC and TURGEN. • Evaluation of co-sited neighbours. Three checks are performed: 1 If average timing advance exceeds the MAXTA value for the co-sited cell. 2 If average timing advance is less than the TALIM value for serving cell and greater than or equal to the TALIM value for the co-sited cell. 3 If the cell is worse and in the same cell layer as serving cell and the average timing advance exceeds the TALIM value for the co-sited cell. If any of the three statements is true the evaluated cell is removed from the candidate list. • Evaluation and removal of candidates if the MS is classified as a fast moving mobile (if this feature is used). 3.2.7.6 Handling of Fast Moving Mobile Handling of fast moving mobile is only performed if the feature is active for the cell. When a fast moving mobile has been identified a different sorting (opposed 54 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  59. 59. Technical Description to HCS Band evaluation) is performed. The candidate list is rearranged to prioritize candidates with the same system type as the serving cell and candidates with higher ranking value in higher cell layers. Example, for a MS identified as fast moving MS in cell layer 1 (three cell layers 1 to 3 with same system type) a candidate with higher ranking value in cell layer 2 or cell layer 3 is given higher priority. If the MS is in cell layer 2, or in cell layer 3, the candidate with higher ranking value in cell layer 3 is given higher priority. For detailed information see Reference [13]. 3.2.7.7 HCS Band Evaluation To be a candidate for HCS Band Evaluation the signal strength for a cell must exceed the HCS Band Threshold. If a cell does not fulfil this criterion it is added to the candidate list after the HCS Band Evaluation. A number of conditions are evaluated to see if the candidate could be chosen as a HCS candidate. The cells that are prioritized as HCS candidates are sorted in ascending layer-number order, that is, all in layer 1, all in layer 2, ... . The remaining cells, both evaluated and non evaluated, are added sorted according to basic ranking. This is done, provided that HCS handovers out from serving cell is allowed and that the prioritized neighbouring cells also are accepting HCS handovers in to them. If it is allowed or not is depending on the traffic load in serving cell and the traffic load in the neighbouring cells. For detailed information see Reference [13]. 3.2.7.8 Categories 2 The cells in the candidate list are then organized in the following categories: • Above S: Neighbouring cell above serving cell in the candidate list • Below S: Neighbouring cell below serving cell in the candidate list • S: Serving cell 3.2.7.9 Ordering the Candidate List The ordering of the candidate list modifies the candidate list depending on the current conditions. How the candidate list is built up at all possible combinations of different conditions is defined in Table 5 below. Concepts in the table: Case: A number of different condition flags is checked to determine in what order the different categories are to be chosen to create the candidate list. Categories : see Categories 2 above. 55219/1553-HSC 103 12/21 Uen A | 2012-05-29
  60. 60. User Description, Locating Conditions : see Table 4 below. The TRUE, FALSE and DON'T CARE states of the indications listed in Table 5, are given by 1, 0 and - respectively. Table 4 Indications Used to Organize the Candidate List Indication Description 1 Assignment request arrived 2 AW state 3 Excessive TA urgency detected 4 Bad Quality urgency detected 5 Overlaid/underlaid subcell change requested or Intra-cell handover requested Table 5 Ordering of Cell Categories for All Existing Cases Case 1 2 3 4 5 Categories in sorted order 1 0 - 0 0 0 Above S 2 0 - 0 0 1 Above S S 3 0 - 0 1 0 Above S Below S 4 0 - 0 1 1 Above S S Below S 5 0 - 1 - 0 Above S Below S 6 0 - 1 - 1 Above S Below S S 7 1 0 0 0 - Above S S 8 1 - 0 1 0 Above S Below S S 9 1 - 0 1 1 Above S S Below S 10 1 - 1 - - Above S Below S S 11 1 1 0 0 - Above S S Below S For a more detailed explanation of Table 5 , see Appendix B on Section 7 on page 105. 3.2.7.10 List Preparation The only thing remaining before the candidate list is ready to be sent is to set the assignment/handover cause value. Counters in the Ericsson GSM STS subsystem (Statistics and Traffic Measurement Subsystem), see Reference [18] are increment for each handover, indicating the cause for it. Appendix A on Section 6 on page 99 specifies how the cause value is selected. 56 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  61. 61. Technical Description Figure 17 below summarizes how the final candidate list is organized logically. Cell A Cell B Cell C Cell D Serving cell Cell E Cell F Cell D Cell C Cell E Serving cell Cell A Cell B Cell F Basic candidate list Cell load sharing Ordering into categories Recalculation with reduced hysteresis Hierarchical cell structures Locating indications Handover candidate list in ranking order Example : Cell D, CellC, Cell E, Serving cell Assignment request arrived AW state Excessive TA urgency detected Bad Quality urqency detected Overlaid/underlaid subcell change or Intra-cell handover requested L-cells K-cells Better cells Worse cells Band evaluation Layer evaluation Above serving cell Below serving cell Load evaluation Figure 17 Overview of the Compilation of the Final Handover Candidate List 57219/1553-HSC 103 12/21 Uen A | 2012-05-29
  62. 62. User Description, Locating 3.2.8 Sending the List The resulting candidate list forms the basis for cell selection. If the result is an empty list, then there simply is no option better than remaining on the current channel. The first cell in the candidate list is ideally the cell to which the MS should be connected. If there are no channels available in that cell, an attempt to allocate a channel in the next cell in the candidate list is made, etc. 3.2.9 Allocation Reply General The reply from the Channel Allocation to the locating individual contains information of the result of the Channel Allocation. This result can be either a success or a failure. The failure can be either due to congestion or to a signalling failure. The result determines a number of actions, such as setting of penalties and enabling of certain timers. Detailed information regarding Channel Allocation is found in Reference [4]. Success A successful allocation means that an available radio resource was found in one of the cell candidates, and that the transfer of the connection to the new channel was successful. If the handover was due to an urgency condition, a penalty has to be calculated in order not to cause immediate handover back to the old serving cell, and this penalty has to be transferred to the new locating individual. Penalty Lists The penalties for a connection are organized in two penalty lists: • The locating penalty list, containing penalties associated with handover failure and the two types of urgency condition • The temporary penalty list, containing penalties associated with fast moving mobiles in HCS, see Reference [13]. The locating penalty list can contain a maximum of three cells at any time for a connection. If a fourth cell is punished, the oldest penalty of the first three is deleted. Each punished neighbouring cell in this list can only be punished with one of the three types of penalty at a time. If a neighbouring cell has been punished with for example a handover failure penalty and the same cell receives a TA urgency, the largest of the two penalties is applied with the time giving the longest remaining duration. 58 219/1553-HSC 103 12/21 Uen A | 2012-05-29
  63. 63. Technical Description Locating penalties can be set only when the result of an allocation reply has been received. Any locating penalties set at that occasion or penalties that might have been set at previous occasions but not yet expired, must be transferred to the new locating individual. The second list, called the temporary penalty list, is intended to be used to prevent a mobile moving fast passing a microcell from making a handover to the microcell. The list has one entry for each possible neighbour. If a cell is punished when a previous penalty for the same cell still is in effect, the new punishment will replace the old one. A cell can be included in the two lists at the same time. In this case the two punishments are added. Abandoned cell is punished last if both abandoned cell and co-sited neighbouring cells are going to be punished at the same time. Inter-BSC Handover at Urgency If the connection is handed over to a cell in another BSC, the locating penalty list can not be transferred. Instead, the inter-BSC handover command transfers a handover cause value to the new BSC (not to be confused with the cause values attached to the cell candidates in the cell candidate list, see Section 3.2.7 on page 51 and Appendix A on Section 6 on page 99). The cause value is interpreted by the receiving BSC as "urgency handover". TA urgency as well as bad quality urgency can be recognized. The receiving BSC carries out the punishment of the cell in the old BSC that was abandoned. However, it uses the penalty parameter value and penalty time of the new cell. For CME 20 systems, this action can only be performed if the receiving BSC is of R5 or later status. For compatibility reasons, the handling in a non-Ericsson manufacture, candidates in other BSCs (non-Ericsson) at urgency handover is retained. The handling consists of removing candidates that are located in the region where immediate handover back to the old serving cell would occur. In effect, this handling allows inter-BSC urgency handover only if the MS is inside the hysteresis corridor, see Figure 11. Parameter EXTPEN is used for external neighbouring cells, and can be used to indicate whether the cause value can be received and correctly interpreted by the BSC controlling that cell. When locating encounters the value OFF for that external neighbouring cell at urgency, no penalty list is sent. The value ON activates the penalty handling that consists of sending a handover cause value to the target BSC. Congestion At congestion in all cells in the candidate list, the connection remains on the channel it currently has. A new attempt will be made (that is a new candidate list may be sent) within a certain time. At a normal handover attempt, this time is given by the timer parameter TALLOC. At an urgency handover attempt, it is given by the timer parameter TURGEN. 59219/1553-HSC 103 12/21 Uen A | 2012-05-29
  64. 64. User Description, Locating After the setting of one of these timers, the locating cycle is closed. Locating evaluations are resumed by filtering when new measurements arrive. Enabling one of the timers does not inhibit locating totally (as is the case with TINIT, see Section 3.2.2 on page 15). TALLOC inhibits handover unless an urgency condition has been detected. TURGEN inhibits handover unless a better cell is found in the ranking. Signalling Failure If signalling failure occurs at handover, and if the connection can not be re-established on the old channel, the connection is lost. If it can be re-established on the old channel by the MS itself, the cell to which the handover failed is punished. The following parameters are used: Penalty value: PSSHF Penalty duration: PTIMHF Except for the fact that a neighbouring cell is punished instead of own cell, as is the case for urgency handover penalty, the handover failure penalty is handled in the same manner as the other types of locating penalty. In case that the feature enhanced handover success rate is activated and the reestablish of the old channel has been performed by the system, the cell to witch he handover failed (indicated by Handover Failure) is not punished. Signalling failure at subcell change or intra-cell handover does not lead to punishment. Subsequent to setting the penalty, the locating cycle is closed. Locating evaluations are resumed by filtering when new measurements have arrived. Enhanced Handover Success Rate The Enhanced Handover Sucess Rate feature, containing two functionalities, is used to enhance the downlink FACCH performance. For more information see User Description, Handover and Signalling Robustness. Repeated Downlink FACCH In order to increase the possibilityies of the MS to successfully decode the FACCH messages, all Layer 3 downlink FACCH messages are repeated as described in Reference [20]. 60 219/1553-HSC 103 12/21 Uen A | 2012-05-29

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