User Description36/1551-HSD 101 02/1 Uae A

User Description, WCDMA RAN HANDOVER© Ericsson Radio Systems AB 2000 - All Rights Reserved

The contents of this document are subject to revision without notice due to continued progress in methodology, design, and manufacturing.

Ericsson will have no liability for any error or damages of any kind resulting from the use of this document.

Contents

1 Introduction

2 Glossary 2.1 Concepts 2.2 Abbreviations and Acronyms

3 Capabilities 3.1 Macro-diversity gain 3.2 Interference 3.3 Seamless handover 3.4 Reliable handover 3.5 Capacity

4 Description 4.1 General 4.2 Soft and Softer Handover technical description

5 Performance Observability

6 Parameters 6.1 Main Controlling Parameters 6.2 Value ranges and defaults values

7 References

1   IntroductionIn a WCDMA system, handover functionality is performed through Soft and Softer Handover functions. These functions are used for mobility and capacity reasons with the purpose of

maintaining the connection quality on dedicated channels as the User Equipment (UE) moves around the area covered by different cells.

The aim of the Soft and Softer Handover function is to provide the ability to add and abandon radio links, with the same frequency, to serve a macro-diversity connection with an UE.

Soft and Softer Handover, as well as Power Control and Capacity Management, are essential interference-mitigating functions in WCDMA.

In soft handover, as shown in Figure 1, a mobile station is connected to more than one Radio Base Station simultaneously. Softer handover is a soft handover between two sectors of the same RBS as shown in Figure 2.

The major purpose of soft and softer handover is to provide a connection with sufficient signal-to-interference ratio, even at cell border, to reduce the interference in the network and to improve performance through macro-diversity. The most relevant aspect of soft and softer handover is the possibility for the UE to establish connections with more than one cell at the same time.

Figure 1   Soft handover

During softer handover (Figure 2) a UE is in the overlapping cell coverage area of two or more adjacent sectors of a base station. The communication between the UE and the RBS takes place concurrently through more than one radio link (one per sector separately).This implies the use of two separate channels with two codes in the downlink direction, so that the UE can distinguish between the signals. Signals from the different sources carrying the same data, are received in the UE and by means of RAKE receiver processing are combined, with maximal ratio combining, to increase the probability of the correct reception of data.

In order to allow the RAKE receiver to process signals with macro-diversity, different radio links must be synchronized. System Information or MEASUREMENT CONTROL message notifies the UE of the reference time difference between the radio links of different cells.

Figure 2   Softer handover

In the uplink an analogous process takes place. At the base station the signal received in each sector are routed to the same RAKE receiver for maximal ratio combining.

During soft handover the UE is in the overlapping regions of two or more different RBSs and can establish separate connections with each of them . The main difference between soft and softer handover is that, in the former, received data from the RBSs are routed to the RNC for selective combination.

Soft and softer handover can also occur simultaneously.

In any case a soft-softer handover gain can be obtained, allowing for an improvement in coverage and capacity of the WCDMA network and for more reliable connections.

The UE and the RNC are responsible for soft and softer handover evaluation. It's not possible for the RBS to take part in basic measurements and in decisions for Soft and Softer Handover.

Algorithm parameters and control messages are sent by the RNC to the UE, so the UE knows the cells that are actually connected (active set), the cells on which it has to perform measurements (measured cells = monitored set + active set), and the measurement evaluation criteria. When a typical event occurs, for example, a measured quantity exceeds a specific threshold, a MEASUREMENT REPORT (MR) is sent to the RNC. The RNC evaluates the report and decides whether to add, to remove, or to replace a radio link between the UE and any cells.

There are some parameters that affect the characteristic and functional behavior of the Soft and Softer Handover algorithm. Part of them are radio network parameters and allow the operator to manage the behaviour of the functions in the whole radio network. Others relate to quantities connected to individual cells and allow to manage such behaviour per cell. These parameters can be set using Radio Access Network Operation Support (RANOS) or Element Manager in the RNC.

2   Glossary

2.1   Concepts

active radio link : One radio link in the active set.

active set : Set of radio links simultaneously involved in a specific communication service between a UE and a WCDMA RAN access point.

block combination

Two types of block combining are used by a macro-diversity connection:

Intra-RBS combining, where RBS receives symbols on each active radio link it controls and combines them in one RAKE receiver (the operation is activated and deactivated by virtue of softer handover)

Inter-RBS combining, where RNC receives blocks on each involved handover branch and combines them by selecting the best block (the operation is activated and deactivated by virtue of soft handover)

block splitting

Two types of block splitting are used by a macro-diversity connection:

Intra-RBS splitting, where RBS copies and channelizes blocks to each active radio link it controls (the operation is activated and deactivated by virtue of softer handover)

Inter-RBS splitting, where RNC copies and channelizes blocks to each RBS controlling one or more active radio links (the operation is activated and deactivated by virtue of soft handover)

cell: A cell is a geographical area (sector) that contains one primary cell carrier, with one primary downlink scrambling code through which an UE can identify it.

connected mode : The state of a UE when the RRC connection is established.

handover: A family of procedures that is able to add and remove one or several radio links between one UE and WCDMA RAN, when an RRC connection exists and the position of the UE is known on cell level in the WCDMA RAN.

handover branch : The Uu connection between the UE and the RBS for a specific cell.

handover leg : The union of all handover branches that belong to cells of a single RBS and that are combined within the RBS.

hard handover : A category of less seamless handover procedure where the old radio links in the UE are abandoned before the new radio links are established.

hysteresis window : A range of values between a maximum value used for entering the reporting range and a minimum value used for leaving the reporting range.

macro-diversity connection : A connection operating in a state in which a UE connection has radio links with more than one WCDMA RAN access point simultaneously. There is more than one radio link in the active set.

measurement control system information : all parameters and criteria used by UE to perform measurement. They are included both in System Information Block 11 and in MEASUREMENT CONTROL message.

monitored set : All cells that the UE controls by measurement but that are not active in the connection (all measured cells excluding the active set cells).

Radio Base Station: The RBS is the Ericsson implementation of the node designated Node B in 3GPP standards. Node B is a logical node responsible for radio transmission and reception in one or more cells to and from the UE. Node B terminates the Iub interface towards the RNC.

RAKE receiver : Correlation receiver that combines the demodulated and phase-adjusted symbols across all its active fingers, and presents them to the decoder for further processing.

radio link : A set of (radio) physical channels comprised of a point-to-point bidirectional communication path between a single UE and one single WCDMA RAN access point.

radio link addition : The procedure whereby a radio link is added to the active set.

radio link removal : The procedure whereby a radio link is removed from the active set.

radio link set : The set of radio links between a UE and WCDMA RAN, terminated in a certain Node B. For example, subsequent to a softer addition, the radio link set refers to the radio links in the active set that are combined and split by the same Node B.

RRC connection : A point-to-point bidirectional connection between RRC peer entities on the UE and the WCDMA RAN sides, respectively. A UE has either zero or one RRC connection.

seamless handover : A handover procedure that occurs without perceptible interruption to the radio connection.

Soft Handover : A family of procedures that may add and remove one or several radio links on the same frequency between one UE and WCDMA RAN when a RRC connection exists and the position of the UE is known on cell level in the WCDMA RAN.

Soft and Softer Handover : A WCDMA RAN function that performs mobility transitions by changing the active set. Every UE connected in CELL_DCH state has access to these procedures, including basic procedures soft addition, softer addition, soft removal, softer removal, and combinations of these. With this terminology, Soft and Softer Handover is a set of procedures rather than a state of a connection.

WCDMA access point: A conceptual point, within the WCDMA, that performs radio transmission and reception. A WCDMA access point is associated with one specific cell. It is the WCDMA-side end-point of a radio link.

2.2   Abbreviations and Acronyms

AM Acknowledged ModeAS active setCPICH Common Pilot CHannel DCCH Dedicated Control CHannel (logical channel)

DCH Dedicated CHannel (transport channel)DL DownlinkDPDCH Dedicated Physical Data Channel (physical channel) DPCCH Dedicated Physical Control Channel (physical channel) DTCH Dedicated Traffic Channel (logical channel)IE Information ElementMR MEASUREMENT REPORTP-CPICH Primary Common Pilot CHannelRBS Radio Base StationRLS radio link setRNC Radio Network ControllerRRC Radio Resource ControlSIB System Information BlockTTT time-to-triggerUE User EquipmentUL UplinkUM Unacknowledged ModeUMTS Universal Mobile Telecommunications SystemWCDMA RAN Wideband Code Division Multiple Access Radio Access Network

3   CapabilitiesIn WCDMA RAN, only intra-RNC soft and softer handover can be performed. There are several reasons to handle UE mobility by allowing multiple radio links in the connection rather than one radio link at a time (achieved by hard handover, typical of TDMA and FDMA systems). These reasons are explained in the following paragraphs.

3.1   Macro-diversity gain

During soft handover the uplink transmission from the UE is received by two or more base stations. The probability of a correctly detected signal increases and macro-diversity gain can be obtained because there are at least two base stations attempting to detect the mobile transmission. In the downlink the signals from different RBSs are received by the UE by means of RAKE processing, which is very similar to multipath reception, and macro-diversity is obtained. The consistency of the gain depends on the multipath profile, mobile speed, receiver algorithms and base antenna configurations.

3.2   Interference

One benefit of having a connection in macro-diversity is that it decreases uplink interference. Without soft or softer handover a UE would penetrate from one cell into an adjacent cell without being power controlled by the latter. If hard handover is performed, the target cell suffers increased uplink interference caused by the approaching UE.

3.3   Seamless handover

There is no loss in or interruption of data flow during a soft or a softer handover, contrary to what occurs in traditional hard handover. This allows for better quality of service.

3.4   Reliable handover

Soft and softer handover allows to keep at least one radio link always active while handling mobility from one cell to a neighbouring cell. Complete loss of connection is less probable during soft or softer handover compared to hard handover.

3.5   Capacity

Macro-diversity that is achieved through Soft/Softer handover allows to obtain an uplink gain that permits to improve capacity for WCDMA network. Macro-diversity gain is important because at the edge of the coverage area the UE is likely transmitting on full power and the fast power control cannot fully compensate for the fast fading. However if macro-diversity gain is high enough, the UE can transmit at lower power and an increase in capacity can be obtained.

Nevertheless some drawbacks might arise from an erroneous use of Soft and Softer Handover. In the downlink each soft or softer handover connection increases the interference to the network. When the increased interference exceeds the diversity gain, the soft or softer handover does not provide any gain for system performance.

Besides in the downlink the soft or softer handover connections use more orthogonal codes than single-link connections. Consequently, soft and softer handover use more RBS resources in both the downlink and the uplink, require higher transmission capacity over the lub interface, and demand more RNC rescources

It is the task of radio network planning and optimization to keep the soft handover overhead below a desired threshold while still providing enough diversity in both uplink and downlink. To reach that goal an optimal size for the overlapping regions (Soft/Softer handover region) has to be found, as shown in Figure 3.

Limits have to be set on the number of Radio Links simultaneously allowed in the active set. The maximum number of Radio Links allowable is four, but an operator can choose to reduce this number.

Figure 3   trade-off between macro-diversity gain and waste of resources

4   Description

4.1   General

Soft and Softer Handover provides a category of handover procedure in which radio links are added and or removed from a macro-diversity connection on the same frequency (intra-frequency handover).

In the downlink operation of this connection, RNC creates more copies of user data blocks and channelizes them to each RBS in the active set (the set which the macro-diversity actually operates). In the case of softer handover, each RBS involved in the connection copies and channelizes the blocks further to each active radio link it handles. In both cases a block splitting is executed. The UE normally combines all different radio links in one RAKE receiver.

In the uplink operation of soft and softer handover connections, the UE transmits user data on one channel, while WCDMA RAN receives that channel on each radio link in the active set. In softer handover the RBS receives symbols on each active radio link it controls and combines them in one RAKE receiver. In soft handover the RNC receives blocks on each involved handover branch and combines them by selecting the best block. The best handover branch is determined by the DPDCH quality indication. In both cases a block combining is executed.

Soft and softer handover can take place in combination with each other.

Soft and softer handover in WCDMA RAN is event-based and assisted by event-triggered UE reporting. WCDMA RAN controls which events will trigger the UE to report and what it will include in the report. Any UE receives specific parameters, through which the measurement process is configured, from the Measurement Control System Information, which is included both in the System Information Block and in the MEASUREMENT CONTROL message sent by the RNC. Parameters and control messages can be controlled by the operator using RANOS or Element Manager.

4.2   Soft and Softer Handover technical description

4.2.1   Algorithm overview

The core part of the handover functions consist of measurements, performed by the UE, and handover evaluations, performed in the RNC, and can be summarized by the following steps :

The RNC knows which quantities have to be measured, the triggering thresholds, the events that have to trigger reports, the reporting criteria, the cells that have to be measured (handover monitored set and active set) and orders the UE to make measurements by sending a MEASUREMENT CONTROL message.

The UE receives, from Measurement Control System Information, radio network and cell parameters to be used in measurement evaluations

The UE performs measurements on the cells of the handover monitored set and active set.

The UE evaluates these measurements with respect to specific events. If the condition of an event is met, a MEASUREMENT REPORT is sent to the RNC.

Based on the received MEASUREMENT REPORT, the handover evaluation algorithm in RNC makes the proposal on which cells to add, delete, or replace in the active set.

RNC attempts to allocate and or deallocate resources according to the proposal. It attempts to allocate and or deallocate codes, radio links, and necessary transport connections. In the case of successful resources allocation and/or deallocation active set and monitored set are updated by ACTIVE SET UPDATE message and MEASUREMENT CONTROL message respectively. Soft or softer handover is then performed. In Figure 4 either one of dashed box must be considered for add or remove, while they both must be considered for replace.

In case execution attempts result in a failure to allocate resources successfully an exception procedure will be started. See paragraph 4.2.3 for details.

Figure 4 summarizes the steps described above

Figure 4  

Soft and Softer Handover can be divided into two separate functions : UE measurement evaluation and WCDMA RAN handover evaluation.

Figure 5   Soft and Softer Handover functions and algorithms

Figure 5 shows the main functions and algorithms involved in Soft and Softer Handover procedure. RANOS or Element Manager allows the operator to change algorithm parameters. Handover decisions go from the Handover Evaluation algorithm to the Soft/Softer Handover execution procedure; information about the outcome is sent back to the Handover evaluation. Soft/Softer HO executes the handover.

4.2.2   UE Activities

4.2.2.1   UE measurement evaluation algorithm

The UE receives radio network and cell parameters from System Information Block 11.The measurements to be performed by the UE are sent by the RNC in the Measurement Control System Information (Sistem Information Block 11 or MEASUREMENT CONTROL message). The object used for Soft and Softer Handover measurements is CPICH.

The quantity that is used for measurement evaluation can vary. The System, however, is configured to use CPICH_Ec/No for this purpose.

Filtering

The Measurement values are processed by a Layer 1 and a Layer 3 filter, in order to smooth out measurement noise. In addition, some fading components, with duration of about the same as the filter response time, are filtered out. Figure 6 gives an overview of the UE measurement model.

Figure 6   Filtering blocks

The impact of the filtering is that it adds delay but at the same time it makes the input to the HO evaluation more stable. The UE delay consists of two parts: the delay caused by the measurement filtering and the delay caused by UE event detection, that includes time-to-trigger (TTT) and measurement reporting delay (L3 filtering eval).

4.2.2.2   Triggering events

Soft and Softer Handover in WCDMA RAN is event-based, and assisted by event-triggered UE reporting. If the condition of an event is met, a MEASUREMENT REPORT is sent to RNC.

Triggering events are based on UE measurements, and can be considered quantity events, where quantity refers to a certain criterion, e.g. the measured Ec/No on primary CPICH. Such an event can be, for example, that the measurement for a cell of the handover monitoring set exceeds a specific threshold.

The reporting range is the threshold for quantity events. It is a relative threshold, and is normally referred to the best primary CPICH in the active set.

For event 1a report from the UE will be sent to the RNC only if the measured quantity level enters the reporting range and doesn't go out of it for a period longer than a time-to-trigger.

For event 1b a report from the UE will be sent to the RNC only if the measured quantity level leaves the reporting range and doesn't return to it for a period longer than a time-to-trigger

Triggering events are summarized as follows :

Event 1a: A primary CPICH enters the reporting range

When event 1a has been ordered by WCDMA RAN in a MEASUREMENT CONTROL message, the UE sends a MEASUREMENT REPORT when a primary CPICH, of a cell belonging to the monitored set, enters the reporting range

This condition is defined by the following formula :

10Log(MNew)> W1a10Log[( i NA (Mi))/( MBest)]+10Log(MBest)-(R1a)

The variables in the formula are defined as follows:

MNew is the measurement result of the monitored cell entering the report range

Mi is the measurement result of a cell in the active setNA is the number of cells in the current active set MBest is the measurement result of the strongest cell in the active setW1a is used for tuning and it is sent from WCDMA RAN to the UE. The term weighted

by W1a in the formula is used to modify the threshold on the basis of measurement results from all cells in the active set. If the measurement quantity of the best cell is strongly predominant respect to other cells this term does not have a strong influence. Otherwise, the larger the number of cells that have a measurement value comparable with the best one, the higher the influence of this term. In the latter case the reporting range is reduced and event 1a is less probable

R1a is the relative threshold for reporting. Its value is set by the parameter REPORTINGRANGE1A. Using high values for R1a makes it easier to find a cell that satisfies the event 1a criteria

Through the parameter MAXACTIVESET the operator can set the maximum number of cells allowed in the active set.

Event 1b: A primary CPICH leaves the reporting range:

When this event is ordered by WCDMA RAN in a MEASUREMENT CONTROL message, the UE sends a MEASUREMENT REPORT when a primary CPICH, of a cell belonging to the active set, leaves the reporting range as defined by the following formula:

10Log(Mold)< W1b10Log[( i NA (Mi))/( MBest)]+10Log(MBest)-(R1b)

The variables in the formula are defined as follows:

MOld is the measurement result of the active cell leaving the reporting rangeMi is the measurement result of a cell in the active setNA is the number of cells in the current active set MBest is the measurement result of the strongest cell in the active setW1b is used for tuning and it is sent from WCDMA RAN to UE. The W1b parameter has

the same functions as W1a

R1b is the relative threshold for reporting. Its value is set by the parameter REPORTINGRANGE1B. Using high values for R1b makes it more difficult to find a cell that satisfies the event 1b criteria

Figure 7 shows event 1a and 1b when W1a = W1b=0. Through the parameters REPORTINGRANGE1A and REPORTINGRANGE1B a kind of hysteresis window can be achieved.

Figure 7   Reporting events 1a and 1b

Event 1c: A primary CPICH that is not included in the active set becomes

better than a primary CPICH in the active set

When event 1c has been ordered by WCDMA RAN in a MEASUREMENT CONTROL message, the UE sends a MEASUREMENT REPORT when a primary CPICH that is not included in the active set becomes better than a primary CPICH in the AS

In Figure 8 the cells belonging to the primary CPICH 1,2, and 3 are supposed to be in the active set, but the cell transmitting primary CPICH 4 is not yet in the active set. If a primary CPICH that is not included in the active set becomes better than a primary CPICH that is in the active set, and event 1c has been ordered by WCDMA RAN, this event triggers a report to be sent from the UE.

A cell replacement occurs only when the number of cells in the active set is strictly higher than MAXACTIVESET-1. Otherwise, adding a new radio link is preferred instead.

Figure 8   Reporting event 1c

Through the parameters MAXACTIVESET, REPORTINGRANGE1A and REPORTINGRANGE1B, the operator can achieve a good trade-off between soft and softer handover gain and waste of resources.

Event 1d: Change of best cell

If any of the primary CPICHs within the reporting range becomes better than the previously best primary CPICH then event 1d occurs , as shown in Figure 9, and a report is sent from the UE to RNC.

Figure 9   Reporting event 1d

4.2.2.3   Time-to-trigger

To limit the measurement signaling load, a time-to-trigger parameter may be connected with each reporting event.

The effect of this is that the report is triggered only after the conditions for the event have existed for a specified time interval, called time-to-trigger, as can be seen in Figure 10.

Figure 10   Time to trigger event 1b

4.2.2.4   Cell individual offset

For each cell that is monitored, an offset can be assigned through the cell parameter INDIVIDUALOFFSET. The offset can either be positive or negative.

The offset is added to the measurement quantity before the UE evaluates whether an event has occurred. The UE receives the cell individual offsets for each primary CPICH in the measurement object field of the MEASUREMENT CONTROL message. In the Figure 11 since an offset is added to primary CPICH_3, it is the dotted curve that is used to evaluate whether an event occurs.

A negative offset can be used to make it more difficult for a known congested cell to keep further resources for soft or softer handover, or to offload a particular cell.

As shown in figure 11, by applying a positive offset, the UE sends MEASUREMENT REPORTS as if the primary CPICH is offset x dB better than it really is. This could be useful if the operator knows that a specific cell is interesting to monitor more carefully, even though it is not so good for the moment.

Figure 11   Individual offset

4.2.2.5   Measurement reporting

Events 1a, 1b, and 1c trigger MEASUREMENT REPORTS according to specific reporting criteria. The UE transmits a MEASUREMENT REPORT message on the uplink DCCH when the reporting criteria are met for any ongoing measurements that are being performed in the UE.

There are two parts to the MEASUREMENT REPORT: event results and measured results. Event results must always be reported, while measured results need to be reported only in the case of events 1a and 1c. Event result contain identifications of the cells by primary scrambling code, so that WCDMA RAN knows which to add (event1a), remove (event1b), or replace (event1c). Measured results, on the other hand, is used to report measured quantities of cells. This is not needed for WCDMA RAN, either to initiate a change in the active set and in the resources allocated, or to handle any failures during such a change.

4.2.3   RNC activities

RNC receives the MEASUREMENT REPORT from the UE and evaluates which branches to add, remove, or replace. The handover procedures in the RNC are such that the active set always include at least one radio link. Soft and Softer Handover procedure do not permit the removal of the final radio link The Figure 12 outlines how the RNC handover function works.

Figure 12   Handover functions in RNC

Flow Chart description

Starting

The procedure starts after a connection has been setup. Each measurement activity may be started, modified or deleted by the RNC sending a MEASUREMENT CONTROL message. The MEASUREMENT CONTROL message is distributed in DCCH on DCH to the UE and contains the following information

Measurement type: Intra-frequency measurements.

Measurement identity number: A reference number used by the WCDMA RAN when modifying or releasing the measurement and by the UE in the MEASUREMENT REPORT.

Measurement command: There are three separate measurement commands

Setup: Sets up a new measurement.

Modify: Modifies a measurement, for example, in order to change the reporting criteria.

Release: Stops a measurement and clears related information.

Measurement object: CPICH

Measurement quantity: Ec/No.

Reporting quantities: The quantities the UE includes in the report, in addition to the quantities that it is mandatory to report for the specific event.

Measurement reporting criteria: The triggering for MEASUREMENT REPORT

Reporting mode: Specifies whether the UE will transmit the MEASUREMENT REPORT using acknowledged (AM) or unacknowledged (UM) data transfer of RLC. Ericsson supports acknowledged mode for reporting.

The following table is an example of a MEASUREMENT CONTROL message.

Table 1    MEASUREMENT CONTROL message

M. Type M. Id M. Command

M. Object M. Quantity Rep Quan.

M. Rep. criteria

Rep. mode

Intra freq. 1101 Setup

Modify

Release

CPICH Ec/No   Event triggered

AM

Receiving a MEASUREMENT REPORT

To reply to a MEASUREMENT CONTROL message received from RNC, the UE sends a MEASUREMENT REPORT message. Since more parallel measurements are possible, controlled and reported independently each other, a measurement identity number is included in the MEASUREMENT CONTROL message and it is associated to each measurement. The same identity number is included in the MEASUREMENT REPORT message related to the MEASUREMENT CONTROL message.

When any MEASUREMENT REPORT is received, it is buffered. Last received MEASUREMENT REPORT overwrites last buffered MEASUREMENT REPORT (no queue). The system waits until a new MEASUREMENT REPORT is received before proceeding to MEASUREMENT REPORT evaluation.

MEASUREMENT REPORT Evaluation

The handover evaluation algorithm (HO_eval) evaluates the event of the MEASUREMENT REPORT and declares the cells to add or delete if the following criteria are satisfied. Then It sends its proposal to Soft and Softer Handover function.

Evaluation of MEASUREMENT REPORTS triggered on event 1a (A primary CPICH enters the reporting range)

In the case of a MEASUREMENT REPORT being triggered on event 1a, including time-to-trigger, HO_Eval carries out the following step:

If the reported cell is not in the active set and if 'number of cells in active set' < 'maximum number of cells in active set', the cell is proposed to be added to the active set. Otherwise, there is no action.

Evaluation of MEASUREMENT REPORTS triggered on event 1b (A primary CPICH leaves the reporting range)

In the case of a MEASUREMENT REPORT being triggered on event 1b, including time-to-trigger, HO_Eval carries out the following step:

If the reported cell is in the active set, the cell is proposed to be deleted from the active set. Otherwise, there is no action.

Evaluation of MEASUREMENT REPORTS triggered on event 1c (A primary CPICH, that is not included in the active set, becomes better than a primary CPICH in the active set)

In the case of a MEASUREMENT REPORT being triggered on event 1c, including time-to-trigger, HO_Eval carries out the following step:

If 'number of cells in active set' = 'maximum number of cells in active set', the cell is proposed to replace the worst cell in the active set. Otherwise, there is no action.

Execution

The proposal goes to Soft/Softer HO function procedure. The execution proposal is performed by Soft/Softer Handover function procedure.

This function attempts to allocate and or deallocate resources according to the proposal. It attempts to allocate and or deallocate codes, radio links, and necessary transport connections. Any execution sequence involving allocation of new resources, go first to the Capacity Management algorithms to request for admission. If the admission is granted, than the handover is performed; otherwise an exception handling procedure is started.

Evaluation conclusion

In the evaluation conclusion the handover evaluation algorithm updates function variables for cells in the active set and also updates the size of the active set. It retrieves the sets of neighbouring cells for each cell in the active set and takes the exclusive union of the sets, so that any multiple occurrences of a neighbouring cell are removed. It also calculates the monitored set by removing any occurrences of cells that are in the active set. It decides which cells will be added and which will be removed from the monitored set by comparing the calculated monitored set with the monitored set currently stored to be used by the UE (function variable). It ensures that the maximum size of the monitored set is not exceeded taking into account that, according 3GPP specifications, the maximum number of measured cells (monitored cells + active cells) is 32. If there are more than 32 cells to measure on, a random truncation of neighbouring cells will occur. Finally, the algorithm updates the monitored set by executing dedicated

MEASUREMENT CONTROL message, with modify command in the measurement command field towards the UE.

After the evaluation conclusion, there are two possible scenarios:

Case A

Cell addition, deletion or, replacement: if allocation or deallocation of resources succeed, the active set is updated and a radio link is added, removed, or replaced.

Exception: exceptions happen when Admission Control refuse the admission for the new cell and the CPICH_Ec/No (new cell) is lower than a specific CPICH_Ec/No value. In this case the UE is not dropped.

In Case A the UE retains its connection

Case B

No UE Response: in the case of an execution attempt resulting in a failure due to no UE reponse, for example owing to expiry of Radio Resource Control timer supervising response, WCDMA RAN terminates the function and requests release of the UE connection.

Congestion Evaluation: this condition is reached when a failure to successfully allocate resource occurs and the function evaluates if the UE is about to be cause of a congestion situation in the target cell. This can happen when it is not possible to allocate a RL on the new cell and the CPICH_Ec/No (new cell) value is higher than a specific CPICH_Ec/No value. In this case the UE is considered a risk of congestion (high interference) for the target cell because its power is high, owing to the distance from connected cell,and cannot be power controlled by the target cell because of lack of resources.

Measurement Control Failure: if WCDMA RAN instructs the UE to perform a measurement that is not supported by the UE or if the MEASUREMENT CONTROL message contains a protocol error, the UE transmits a MEASUREMENT CONTROL FAILURE to the RNC.

In Case B WCDMA RAN stops the function and requests release of the UE connection.

4.2.4   Relations to other features

Power Control

During Soft and Softer Handover, a special algorithm for power control is used. When adding a radio link, power control algorithms are requested to determine which power parameters should be used for the radio link. For further details, refer to [1].

Capacity Management

Capacity Management allows usage of the resources when possible, and limits usage when necessary. Capacity Management has therefore to be informed about changes in the active set. In addition Soft and Softer Handover function has to request admission when adding a new radio link. For details refer to [2]

5   Performance ObservabilityStatistics for Soft and Softer Handover include counter calculating (reported per cell) [3]:

1. The average number of UEs with X radio link set and Y radio links in the active set.

2. Percentage of UEs with 1, 2, 3, or 4 radio link sets.

3. Percentage of UEs with 1, 2, 3, or 4 radio links.

This information can be used to determine the average number of cells the UEs are connected to during soft and softer handover.

Performance recording can be obtained for soft and softer handover to get information such as the following:

1. The number of successful/failed handovers additions per cell and event over a specified time period as a function of the measurement quantity.

2. The frequency of handover reports from the UE per event as a function of the measurement quantity.

If the number of failed handover attempts per cell and event, in relation to number of total handover attempts per cell and event, exceeds a predetermined threshold within a predetermined time period, an alarm may be sent to the operator, who may change some thresholds.

6   Parameters

6.1   Main Controlling Parameters

INDIVIDUALOFFSET controls the offset per cell. The offset is added to the measured quantity before the UE evaluates whether an intra-frequency event has occurred. Improper use of non-default values may result in instability and unequal cell borders. It is a cell parameter.

MAXACTIVESET sets the maximum number of cells allowed in the active set. It is used in WCDMA RAN evaluations of event 1a and 1c. If the number of cells in the active set is equal to MAXACTIVESET , handover evaluation alghorithm proposes to replace a cell; otherwise it proposes to add a cell. It is a Radio Network parameter.

REPORTINGRANGE1A sets the relative threshold referred to the best CPICH used for addition-window in evaluation criteria for event 1a. It is a Radio Network parameter.

REPORTINGRANGE1B controls the relative threshold referred to the best CPICH used for drop-window in evaluation criteria for event 1b. It is configured through the RNC and is a Radio Network parameter.

6.2   Value ranges and defaults values

Table 2   

Parameter name Default value Recommended value Value range Unit

INDIVIDUALOFFSET 0   -100 to 100

-100: -10 dB

-95: -9,5 dB

...

100: 10 dB

deci dB

MAXACTIVESET 3   1 to 4 cells

REPORTINGRANGE1A 30   0 to 145 step 5

0: 0,0

5: 0,5

...

145: 14,5

decidB

REPORTINGRANGE1B 50   0 to 145 step 5

0: 0,0

5: 0,5

...

145: 14,5

decidB

7   References1. User Description, WCDMA RAN Power Control, 35/1551 HSD 101 02/1 Uae 2. User Description, WCDMA RAN Capacity Management, 34/1551 HSD 101 02/1 Uae 3. User Description, WCDMA RAN Performance Management, 44/1551 HSD 101 02/1Uae