WO2023184343A1

WO2023184343A1 - Apparatus and method for graceful cell shutdown

Info

Publication number: WO2023184343A1
Application number: PCT/CN2022/084422
Authority: WO
Inventors: ZhengLin NI; Han Zhang; Feng Li; Rongbin LI; Rong GE
Original assignee: Nokia Shanghai Bell Co., Ltd.; Nokia Solutions And Networks Oy
Priority date: 2022-03-31
Filing date: 2022-03-31
Publication date: 2023-10-05

Abstract

Various example embodiments relate to apparatuses and methods supporting graceful cell shutdown. A network device may receive first measurement reports from a plurality of terminal devices connected in a serving cell to the network device and determine a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports. The first measurement reports may indicate neighboring cells for the plurality of terminal devices. The network device may reduce downlink transmission power for the serving cell by the determined power reduction amount and handover the group of terminal devices to their neighboring cells. The determining, reducing and handing-over operations may be repeated until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.

Description

APPARATUS AND METHOD FOR GRACEFUL CELL SHUTDOWN

TECHNICAL FIELD

Various example embodiments described herein generally relate to communication technologies, and more particularly, to apparatuses and methods supporting graceful cell shutdown.

BACKGROUND

Certain abbreviations that may be found in the description and/or in the figures are herewith defined as follows:

ANR Automatic Neighbor Relationship

BTS Base Transceiver Station

CP Control Plane

HO Handover

KPI Key Performance Indicator

LTE Long Term Evolution

MP Management Plane

NR New Radio

OAM Operation Administration Maintenance

RSRP Reference Signal Received Power

RSRQ Reference Signal Received Quality

RRC Radio Resource Control

SINR Signal to Interference plus Noise Ratio

UE User Equipment

UP User Plane

In a communication network, a base station, also known as base transceiver station (BTS) , may deactivate one or more cells serviced by the base station for various reasons in many scenarios like power saving, cell blocking, site upgrade or the like. The deactivation of the one or more cells may be performed by means of a graceful cell shutdown procedure to avoid or minimize impact on ongoing services and improve end user experience.

SUMMARY

A brief summary of example embodiments is provided below to provide basic understanding of some aspects of various embodiments. It should be noted that this summary is not intended to identify key features of essential elements or define scopes of the embodiments, and its sole purpose is to introduce some concepts in a simplified form as a preamble for a more detailed description provided below.

In a first aspect, an example embodiment of a network device is provided. The network device may comprise at least one processor and at least one memory including computer program code. The at least one memory and the computer program code may be configured to, with the at least one processor, cause the network device at least to receive first measurement reports from a plurality of terminal devices connected in a serving cell to the network device and determine a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports. The first measurement reports may indicate neighboring cells for the plurality of terminal devices. The network device may reduce downlink transmission power for the serving cell by the determined power reduction amount and handover the group of terminal devices to their neighboring cells. The determining, reducing and handing-over operations may be repeated on remaining terminal devices of the plurality of terminal devices until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.

In a second aspect, an example embodiment of a method implemented at a network device is provided. The method may comprise receiving first measurement reports from a plurality of terminal devices connected in a serving cell to the network device and determining a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports. The first measurement reports may indicate neighboring cells for the plurality of terminal devices. The method may further comprise reducing downlink transmission power for the serving cell by the determined power reduction amount and handing over the group of terminal devices to their neighboring cells. The determining, reducing and handing-over operations may be repeated on remaining terminal devices of the plurality of terminal devices until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.

In the third aspect, an example embodiment of an apparatus is provided. The apparatus may comprise means for receiving, at a network device, first measurement reports from a plurality of terminal devices connected in a serving cell to the network device and means for determining a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports. The first measurement reports may indicate neighboring cells for the plurality of terminal devices. The apparatus may further comprise means for reducing downlink transmission power for the serving cell by the determined power reduction amount, means for handing over the group of terminal devices to their neighboring cells, and means for repeating the determining, reducing and handing-over operations on remaining terminal devices of the plurality of terminal devices until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.

In a fourth aspect, an example embodiment of a computer program product is provided. The computer program product may be embodied in at least one computer readable medium and comprise instructions, when executed by at least one processor of a network device, causing the network device at least to receive first measurement reports from a plurality of terminal devices connected in a serving cell to the network device and determine a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports. The first measurement reports may indicate neighboring cells for the plurality of terminal devices. The instructions may further cause the network device to reduce downlink transmission power for the serving cell by the determined power reduction amount, handover the group of terminal devices to their neighboring cells, and repeat the determining, reducing and handing-over operations on remaining terminal devices of the plurality of terminal devices until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.

Other features and advantages of the example embodiments of the present disclosure will also be apparent from the following description of specific embodiments when read in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of example embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Some example embodiments will now be described, by way of non-limiting examples, with reference to the accompanying drawings.

Fig. 1 is a schematic diagram illustrating a cellular communication network in which example embodiments of the present disclosure can be implemented.

Fig. 2 is a schematic graph illustrating power reduction based on configuration parameters.

Fig. 3 is a schematic graph illustrating dynamic power reduction according to an example embodiment of the present disclosure.

Fig. 4 is a flow chart illustrating a graceful shutdown procedure according to an example embodiment of the present disclosure.

Fig. 5 is a schematic graph illustrating a step rate curve as a function of the number of UEs scheduled for handover in a previous step.

Fig. 6 is a schematic diagram illustrating UEs ranks in a descending order of reference signal received power (RSRP) gap according to an example embodiment of the present disclosure.

Fig. 7 is a flow chart illustrating operations for determining a group of UEs to be handed over according to an example embodiment of the present disclosure.

Fig. 8 is a schematic message sequence chart illustrating a graceful shutdown procedure according to an example embodiment of the present disclosure.

Fig. 9 is a functional block diagram illustrating an apparatus for grace cell shutdown according to an example embodiment of the present disclosure.

Fig. 10 is a schematic structure block diagram illustrating devices in a communication system in which example embodiments of the present disclosure can be implemented.

Throughout the drawings, same or similar reference numbers indicate same or similar elements. A repetitive description on the same elements would be omitted.

DETAILED DESCRIPTION

Herein below, some example embodiments are described in detail with reference to the accompanying drawings. The following description includes specific details for the purpose of providing a thorough understanding of various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well known circuits, techniques and components are shown in block diagram form to avoid obscuring the described concepts and features.

As used herein, the term “network device” refers to any suitable devices or entities that can provide cells or coverage, through which terminal devices can access the network or receive services. The network device may be commonly referred to as a base transceiver station (BTS) , a base station (BS) , or some other suitable terminology. The term “base station” or “base transceiver station” used herein can represent a node B (NodeB or NB) , an evolved node B (eNodeB or eNB) , a next generation Node B (gNB) , or a next generation enhanced Node B (ng-eNB) . The base station may be embodied as a macro base station, a relay node, or a low power node such as a pico base station or a femto base station. The base station may also include or may be referred to as a RAN (radio access network) node, and may consist of several distributed network units, such as a central unit (CU) , one or more distributed units (DUs) , one or more remote radio heads (RRHs) or remote radio units (RRUs) . The number and functions of these distributed units depend on the selected split RAN architecture.

As used herein, the term “terminal device” or “user equipment” (UE) refers to any devices or entities that can wirelessly communicate with the network devices or with each other. Examples of the terminal device can include a mobile phone, a mobile terminal, a mobile station (MS) , a subscriber station, a portable subscriber station, an access terminal, a personal digital assistant (PDA) , a computer, a wearable device, an on-vehicle communication device, a machine type communication (MTC) device, a D2D communication device, a V2X communication device, a sensor and the like. The term “terminal device” can be used interchangeably with UE, a user terminal, a mobile terminal, a mobile station, or a wireless device.

Fig. 1 illustrates a schematic diagram of a wireless communication network 100 in which example embodiments of the present disclosure can be implemented. The wireless communication network 100 may be implemented as a cellular network such as a 4G LTE network, an LTE-A network, a 5G NR network or any 3GPP cellular network or system, which may employ one or more multiple access schemes capable of supporting communication with multiple users sharing available time and frequency resources. Examples of the multiple access schemes may include Time Division Multiple Access (TDMA) , Code Division Multiple Access (CDMA) , Time Division Synchronous Code Division Multiple Access (TD-SCDMA) , Frequency Division Multiple Access (FDMA) , Orthogonal Frequency Division Multiple Access (OFDMA) , Single Carrier Frequency Division Multiple Access (SC-FDMA) and the like. These multiple access schemes may be formulated in 4G Long Term Evolution (LTE) , 5G New Radio (NR) , or beyond 5G radio standards.

Referring to Fig. 1, the wireless communication network 100 may include a plurality of base stations 120 and a plurality of user equipments (UEs) 110 connected to the base stations 120. The plurality of base stations 120 may form a so-called random access network (RAN) which provides network access to the plurality of UEs 110. Each base station 120 may be configured with one or more transceivers that provide wireless signal coverage for one or more geographical areas known as cells 122. One or more UEs 110 may camp in one or more cells 122, establish radio resource control (RRC) connections with one or more base stations 120 servicing the one or more cells 122 and communicate with the connected base stations 120 on uplink and downlink channels. As an example, Fig. 1 shows three

UEs

110a, 110b, 110c and three

base stations

120a, 120b, 120c each supporting three

cells

122a, 122b, 122c. The cells 122 are shown to have a hexagon shape, but it would be appreciated that the cells 122 may have for example a circular shape, an ellipse shape or other shapes and neighboring cells may overlap with each other. Two neighboring cells 122, either supported by one base station or supported by two different base stations, may operate in the same or different frequencies. It would be appreciated that when the description herein indicates that a “cell” performs functions, a base station servicing the cell would perform the functions.

The base station 120 may be required to shutdown one or more cells 122 serviced by the base station 120 for various reasons. For example, the operator may need to regularly upgrade software/hardware of the base station 120 in order to introduce new functionalities or extend site capacity. The operator may also shutdown one or more cells 122 serviced by the base station 120 for maintenance, cell lock/block operations or the like. If the number of UEs 110 which need network service of the base station 120 is low at a particular time and area, it is also desirable to shutdown one or more cells or the whole base station 120 to save power. In addition, a specific cell may be temporarily unable to provide service to users in a service area due to unexpected equipment malfunction and it may cause auto recovery reset.

Ongoing service impact and procedure duration are two major factors which need to be considered in a cell shutdown solution design. Ongoing service (e.g. active connected UEs) amount varies quite a lot in different systems at different time, and the shutdown procedure duration will contribute to service downtime of the shutdown cell (s) . Without a good solution, either the ongoing services will be adversely impacted, or the shutdown procedure will be quite long. Therefore, it is beneficial to develop a graceful shutdown solution which enables a reliable and fast shutdown so as to improve end user experience, as well as guarantee the minimum service downtime.

Fig. 2 illustrates a schematic diagram of power reduction based on configuration parameters in a conventional graceful shutdown procedure. As shown in Fig. 2, the base station 120 may shutdown a cell 122 by reducing downlink transmission power for the cell 122 in a stepwise manner from an initial power level to a target power level. As the downlink transmission power decreases, signal quality of the cell 122 deteriorates and a handover procedure may be triggered to handover UEs 110 camped in the cell 122 to neighboring cells. After each power reduction step, the base station 120 waits for a predetermined delay interval before the next round of power reduction occurs. Each step has identical power reduction and delay interval. The power reduction step-length, the number of power reduction steps and the total shutdown window are configurable parameters which require manual configuration depending on the runtime cell service situation before the graceful shutdown procedure.

The graceful shutdown procedure shown in Fig. 2 has several drawbacks. The duration (total shutdown window) is quite long, which severely impacts the network Operation Administration Maintenance (OAM) Key Performance Indicator (KPI) , e.g., service downtime of activities such as site reset, upgrade, etc. The long duration cannot guarantee handover KPI. A determinative factor for configuring parameters of the graceful shutdown procedure is the number of UEs connected to the cell to be shutdown. The higher the number of UEs, the more steps and longer time window should be configured. The handover KPI fully relies on how well the parameters are configured by the operator. Without good knowledge and experience on the parameter configuration, the handover KPI will become bad even a long time window is configured. Complexity of configuring the shutdown parameters is high. As mentioned above, the parameter configuration is highly dependent on the ongoing service amount. The operator needs to know the number of UEs connected to the cell (s) /BTS to be shutdown. It is complex for the operator to calculate shutdown parameters based on the ongoing service amount before the cell shutdown operation, and the ongoing service amount varies quite a lot over time. In addition, the convention cell shutdown procedure causes a lot of call drops. For example, UEs close to the base station or the antenna of the base station may have sufficient signal quality when the cell power is reduced to the target level which may not be zero, and such UEs will be dropped for example when the operator resets the cell.

Example embodiments described herein may address one or more of the above problems by implementing self-adaptive power reduction together with a parallel proactive handover mechanism in a cell shutdown procedure. Fig. 3 schematically illustrates the self-adaptive power reduction process where the power reduction amount for each step can be dynamically adjusted. The power reduction amount can be continuously optimized for each step by adapting to real UEs’ signal conditions and feedback of previous power reduction steps, which will eliminate improper power reduction strategy e.g. the fixed power reduction amount being used continuously which will lead to poor KPI during the graceful cell shutdown procedure, or unnecessary long duration of the graceful shutdown procedure than actual needs.

Various example embodiments can utilize proactive handover, in addition to passive UE handover triggered in response to the power reduction, to accelerate the whole graceful cell shutdown procedure. The parallel passive and proactive handover can achieve the shortest cell shutdown window and minimize the service downtime. The delay interval after each power reduction step can reflect the real needs of the proactive handover procedure and blind waiting based on operator configured parameters can be avoided. The shutdown parameters can be automatically calculated at runtime in the graceful shutdown procedure, and manual configuration of the shutdown parameters is not needed, which makes the operator’s operation very simple. Furthermore, call drops for UEs close to the base station or the antenna of the base station can be avoided or minimized because the UEs can be proactively handed over to their neighboring cells.

Fig. 4 is a flow chart illustrating a graceful cell shutdown method 200 according to an example embodiment of the present disclosure. The method 200 may be performed for example at the base station 120 to shutdown a cell serviced by the base station 120. If two or more cells of the base station 120 need to be shutdown, the graceful shutdown method 200 can be performed in parallel for the two or more cells to avoid Ping Pong effect. For example, UEs connected to the first shutting down cell are handed over to a second shutting down cell. In some example embodiments, the base station 120 may include or be configured with a plurality of components, modules, means or elements to perform operations in the method 200, and the components, modules, means or elements may be implemented in various manners including but not limited to for example software, hardware, firmware or any combination thereof.

Referring to Fig. 4, the method 200 may begin at an operation 210 of triggering a graceful cell shutdown procedure at the base station 120. The base station 120 may trigger the graceful cell shutdown procedure for a cell 122 when for example the cell administrative state is changed to “shutting down” . The operator may change the cell administrative state of the cell 122 to “shutting down” via plan file download and activation. If more than one cell shall be shutdown with one plan file, the graceful shutdown procedure will be triggered and performed in parallel for the more than one cell. When the graceful shutdown procedure is trigged for the cell 122, the base station 120 may reject new connection and handover request to the cell 122. The base station 120 may also remove the cell 122 from a handover target cell list e.g. a neighbor relation table (NRT) of any other cells.

At an operation 212, the base station 120 may configure one or more measurements for UEs 110 connected to the cell 122 to be shutdown. The base station 120 may send measurement configuration to the UEs 110 via one or more RRC reconfiguration requests, which may configure one or more event measurements for the UEs 110. In some example embodiments, the base station 120 may configure Event A4 measurement for the UEs 110 at the operation 212. Event A4 may be triggered when signal quality such as reference signal received power (RSRP) , reference signal received quality (RSRQ) or signal to interference plus noise ratio (SINR) of a neighboring cell becomes better than or equal to a threshold. At the operation 212, the base station 120 may configure the UEs 110 with a minimal threshold for the Event A4 measurement so that the base station 120 can obtain Event A4 measurement reports from as many UEs 110 as possible. An advantage is that the base station 120 can have a whole picture of the UEs 110 which potentially can be handed over out of the serving cell 122 and it is beneficial for further optimal planning of handover in batches. As an example, the minimal threshold of the signal quality represented by RSRP may have a value less than or equal to -100 dBm, preferably less than or equal to -110 dBm. On the other hand, the minimal threshold of the signal quality would be sufficient for a handover procedure. That is, the UEs 110 can be handed over successfully to the neighboring cell having the minimal threshold signal quality. In this regard, the minimal threshold for the Event A4 measurement report may have a value higher than or equal to -150 dBm, preferably higher than or equal to -140 dBm. In an example, the minimal threshold for the Event A4 measurement report may be set to around -120 dBm.

The base station 120 may configure the UEs 110 connected to the cell 122 to be shutdown to conduct the Event A4 measurement for inter-frequency and intra-frequency neighboring cells. The inter-frequency neighboring cell operates in different frequency than the serving cell 122 of the UEs 110, and the intra-frequency neighboring cell operates in the same frequency as the serving cell 122. The inter-frequency and intra-frequency neighboring cells may be provided in a neighboring cell list e.g. an automatic neighbor relation (ANR) based cell list for the cell 122. In a case where the whole base station 120 is to be shutdown, a neighboring cell supported by the base station 120 may be excluded/removed from the neighboring cell list for the cell 122 to be shutdown.

The base station 120 may also configure plural parameters for measurement reports triggered by the Event A4 measurement. For example, the base station 120 may configure a time-to-trigger parameter for the Event A4 measurement report. The time-to-trigger parameter specifies a time interval during which the Event A4 triggering condition is fulfilled before the Event A4 measurement report is transmitted. The time-to-trigger parameter may be appropriately set to make sure the measurement report is reliable and fast. A reporting interval parameter may be configured for periodic scheduling e.g. semi-persistent scheduling (SPS) of the measurement reports so that the reporting is updated time to time with an acceptable overhead. In addition, a report-on-leave parameter may be configured to enable a leaving report indicating that the triggering condition is not fulfilled any longer or a leaving condition is fulfilled. It would be appreciated that the base station 120 may also configure additional parameters such as measurement gaps, report amount and the like for the Event A4 measurement. With the configured parameters, the UEs 110 can perform the Event A4 measurement and periodically transmit measurement reports to the base station 120 when the triggering condition is fulfilled, i.e. the signal quality of at least one neighboring cell becomes better than or equal to the configured minimal threshold.

In some example embodiments, the base station 120 may further configure Event A3 measurement for the UEs 110 connected to the cell 122 at the operation 212. Event A3 may be triggered when the signal quality of a neighboring cell becomes better than the signal quality of the serving cell 122 by an amount higher than or equal to a threshold Dm. The base station 120 may configure an appropriate Event A3 threshold Dm for the UEs 110 to facilitate the Event A3 measurement reports. Unlike the Event A4 threshold which has an absolute value e.g. -120 dBm, the Event A3 threshold Dm may have a relative value e.g. 5 dB, 3 dB, 1 dB or other values. In some example embodiments, when the graceful cell shutdown procedure is triggered, the base station 120 may configure the UEs 110 with the Event A3 threshold Dm having a minimal value like 3 dB, 1 dB or zero to trigger Event A3 measurement reports earlier before the signal quality of the serving cell 122 becomes too bad and to guarantee a radio link with the UEs 110. Also, the Event A3 measurement may be configured for inter-frequency and intra-frequency neighboring cells, and relevant parameters such as time-to-trigger, report interval, report-on-leave and the like may be configured for the Event A3 measurement. In some example embodiments, the Event A3 measurement may be configured before the graceful cell shutdown procedure is triggered.

When the one or more measurements are configured for the UEs 110, the base station 120 may collect measurement reports from the UEs 110 at an operation 214. As mentioned above, the UEs 110 may measure signal quality like RSRP, RSRQ or SINR of the serving cell 122 and the neighboring cells and send measurement reports to the base station 120 when one or more of the configured events are triggered. The measurement report may include signal quality information of the serving cell 122 and signal quality information of at least one neighboring cell which meets the triggering condition of the configured event. In some example embodiments, the base station 120 may start a timer at the time of configuring the measurements to monitor for reception of the measurement reports.

If the base station 120 determines at an operation 216 that an Event A3 measurement report is received from a certain UE 110, the base station 120 may, in response to the received Event A3 measurement report, handover the UE 110 to a neighboring cell indicated in the Event A3 measurement report at an operation 218. If two or more neighboring cells are indicated in the Event A3 measurement report, the base station 120 may handover the UE 110 to its best neighboring cell having the best signal quality, or select a target neighboring cell from the two or more indicated neighboring cells for handover of the UE 110. The handover target cell may be selected in consideration of for example load of the two of more neighboring cells. The handover procedure triggered by the Event A3 may be performed in a legacy way and details thereof are omitted here.

If the base station 120 determines at the operation 216 that an Event A4 measurement report is received from a certain UE 110, the base station 120 may further determine if the Event A4 measurement report indicates an inter-frequency neighboring cell or an intra-frequency neighboring cell of the UE 110 at an operation 220. In a case where the UE 110 has an inter-frequency neighboring cell indicated in the Event A4 measurement report, the base station 120 may handover the UE 110 to the inter-frequency neighboring cell at an operation 222. If the UE 110 has both inter-frequency and intra-frequency neighboring cells indicated in the Event A4 measurement report, the base station 120 may determine the UE 110 has an inter-frequency neighboring cell at the operation 220 and handover the UE 110 to the inter-frequency neighboring cell at the operation 222. The UE 110 connected to the inter-frequency neighboring cell after the handover would have small or no interference with remaining UEs connected to the serving cell 122 because they operate in different frequencies, so the base station 120 can immediately initiate the handover procedure for the UEs having the inter-frequency neighboring cell at the operation 222. The inter-frequency handover procedure triggered by the Event A4 measurement report may be performed in a legacy way and details thereof are omitted here.

As mentioned above, the base station 120 may start a timer at the operation 212 and wait for a time period to receive measurement reports from the UEs 110. If the timer expiries or the base station 120 has received measurement reports from all the connected UEs 110 at an operation 224, the method 200 may proceed to a next operation 226. It would be appreciated that the base station 120 may receive further measurement reports after the operation 224 since the measurement reports are periodically transmitted from the UEs 110 as long as the event triggering condition is satisfied, and thus the

operations

218, 222 may be also performed after the operation 224 if relevant conditions are determined as in the

operations

216, 220.

At the operation 226, the base station 120 may determines a power reduction amount for a power reduction step and a group of UEs associated with the power reduction step based on the Event A4 measurement reports received from the UEs 110. Since the UEs 110 which have triggered the Event A3 measurement report or the inter-frequency Event A4 measurement report have been handed over out of the serving cell 122 at the

operations

218, 222, the group of UEs determined at the operation 226 are selected from the UEs 110 having triggered the intra-frequency Event A4 measurement report. In a particular case where there is no UE with available intra-frequency Event A4 measurement report, i.e., all the connected UEs 110 have triggered the Event A3 measurement report or the inter-frequency Event A4 measurement report and have been handed over out of the serving cell 122, the base station 120 can decide to reduce the downlink transmission power for the cell 122 directly to the target power level at the operation 226, and the operation of determining the group of UEs can be omitted.

As mentioned above with respect to Fig. 3, the downlink transmission power for the cell 122 to be shutdown may be decreased in a stepwise manner from the initial power to the target power, and the power reduction amount for each step may be dynamically determined adapting to for example the number of connected UEs, the signal quality of the connected UEs and feedback from previous power reduction steps. At the operation 226, the base station 120 may determine the power reduction amount at least based on the measurement reports received from the UEs 110 and optionally based on one or more additional considerations as discussed below.

For example, the number of UEs scheduled for handover (i.e., the previous group of UEs) and handover performance (e.g., handover success rate) of the scheduled UEs associated with the previous power reduction step may be considered in determination of the power reduction amount in the current step. If the handover success rate is above a certain threshold in the previous step, the power reduction amount for the current step may be increased to further speed up the shutdown procedure; otherwise if the handover success rate is below the threshold in the previous step, the power reduction amount for the current step can be decreased to improve the retainability KPI of the shutdown procedure. The number of UEs scheduled for handover in the previous step may be taken into account when determining whether the handover success rate is reliable for adjusting the power reduction amount in the current step. For example, if the scheduled UE number is small, the handover success rate tends to be unreliable for the power reduction amount adjustment.

In addition, in determining the power reduction amount, the base station 120 may also consider the handover processing capacity of the base station 120 to make sure that the determined group of UEs can be handled without time-out caused by the processing delay. In some example embodiments, the power reduction amount may be further limited within a predetermined range to avoid dramatic power change. If the determined power reduction amount is above the upper limit of the predetermined range or below the lower limit of the predetermined range, the base station 120 may apply the upper limit or the lower limit for the power reduction amount.

An example algorithm for determining the power reduction amount of each power reduction step will be described here in detail. However, it would be appreciated that the algorithm is described merely as an example, and the algorithm may be modified or other algorithms may be used. In the example algorithm, the power reduction amount Reduced_Power for each power reduction step may be calculated according to a formula (1) :

In the formula (1) , Default_Power denotes a default power reduction amount for the power reduction step and it may have a operator specified value e.g. 1 dB, 2 dB, 3 dB or other values. The operator may determine the default power reduction amount according to experiments to achieve fast and reliable power reduction. In some example embodiments, Default_Power may have a default value pre-configured at the base station 120. Adjust_Power denotes a power reduction adjustment amount for the power reduction step and it may be determined based on the number of UEs scheduled for handover and handover performance e.g. success rate of the scheduled UEs in the previous power reduction step. If the number of scheduled UEs is large and the handover success rate of the scheduled UEs is high in the previous power reduction step, the power reduction adjustment amount may have a large positive value to increase the power reduction amount in the current step. If the number of scheduled UEs is small or the handover success rate of the scheduled UEs is low in the previous power reduction step, the power reduction adjustment amount may have a relatively small value or even a negative value. The power reduction adjustment amount for the current step may also depend on the power reduction adjustment amount for the previous step. In an example, Adjust_Power may be calculated from a formula (2) :

Adjust_Power=Prev_Adjust_Power+Default_Adjust_Power*Adjust_Coef Formula (2)

In the formula (2) , Prev_Adjust_Power denotes the power reduction adjustment amount in the previous step and it may have a zero value when calculating Adjust_Power for the initial power reduction step. Default_Adjust_Power denotes a default power adjustment amount which may have an operator specified value e.g. 0.5 dB or other values. The operator may determine the default power adjustment amount in proportion with the default power reduction amount. The larger the default power reduction amount is, the larger the default power adjustment amount is. Adjust_Coef denotes an adjustment coefficient which may be determined for example according to a step rate parameter. The below table (1) shows an example relation between the adjustment coefficient and the step rate. For the initial power reduction step, Adjust_Coef and Adjust_Power may be zero.

Table 1

Adjust_Coef	Step_Rate
-1	0-0.2
-0.5	0.2-0.4
0	0.4-0.6
0.5	0.6-0.8
1	0.8-1.0

Step_Rate reflects handover performance and reliability in the previous step. In an example, Step_Rate may be calculated by a formula (3) :

where Prev_Success_Rate is the handover success rate in the previous step and it may be obtained from the number of UEs successfully handed over dividing the number of UEs scheduled for handover in the previous step. Prev_Scheduled_UE is the number of UEs scheduled for handover in the previous step, and Adj_Number is a predetermined value used to dilute the number of scheduled UEs and to fine-tune the value of Step_Rate. When Prev_Scheduled_UE has a relative small value, Prev_Success_Rate has low reliability and the calculated value of Step_Rate would be much lower than Prev_Success_Rate. When Prev_Scheduled_UE has a relative large value, Prev_Success_Rate has high reliability and the calculated value of Step_Rate would be close to Prev_Success_Rate. In an example, Adj_Number may have a value e.g. 10, 20, 30 or other values. Fig. 4 shows an example Step_Rate curve dependent on Prev_Scheduled_UE where Adj_Number is set to 20 and Prev_Success_Rate is set to 100%. When Prev_Scheduled_UE is 5, Step_Rate is around 0.5.

Referring back to the formula (1) , the maximum handover processing capability of the base station 120 is also considered in calculating the power reduction amount Reduced_Power for each power reduction step. MaxReducedPower_HOProcessingCapability is the maximum power reduction amount corresponding to the maximum handover processing capability of the base station 120 and it may be calculated from a formula (4) :

MaxReducedPowe_HOProcessingCapability=RSRP_Gap_List _{[Zero_Index]} -RSRP_Gap_List _{[max_Schedule_UE_Index]} Formula (4)

RSRP_Gap_List is a list of RSRP gap values of the UEs 110 in an order from high to low. A list index zero corresponds to a maximum RSRP gap value of the UEs 110, and a maximum list index corresponds to a minimum RSRP gap value of the UEs 110. Here RSRP is used as an example of signal quality of the UEs 110, but other parameters like RSRQ, SINR may also be used instead of RSRP. For the respective UEs 110, the RSRP gap is calculated as a difference between RSRP of the neighboring cell of the UE 110 and RSRP of the serving cell of the UE 110, i.e. RSRP _gap=RSRP _{NeighboringCell} –RSRP _ServingCell. The RSRP values of the neighboring cell and the serving cell of the UE 110 are available in the Event A4 measurement report received from the UE 110. If the Event A4 measurement report indicates two or more neighboring cells of the UE 110, the best neighboring cell may be used or the base station 120 may select one of the two or more neighboring cells for the RSRP gap calculation and subsequent handover of the UE 110.

Fig. 6 shows an example of ranking UEs from high to low by RSRP gap. The maximum RSRP gap value (RSRP gap list index =0) is below Dm (Event A3 threshold, 3 dB in the example) because UEs having the RSRP gap higher than or equal to the threshold Dm have triggered the Event A3 measurement reports and have been handed over out of the serving cell 122 in the operation 218. Considering handover processing capability of the base station 120, the maximum number of UEs scheduled for handover Max_Schedule_UE_Num may be determined according to a formula (5) :

Max_Schedule_UE_Num=Max_Schedule_UE_Index+1=Max_HO_UE_Per_Sec*UE_Signaling_Timeout*Ajd_Rate Formula (5)

In the formula (5) , Max_HO_UE_Per_Sec represents the maximum handover capacity per second of the base station 120 based on control plane (C-Plane) event handling performance and handover procedure message number. The maximum handover capacity per second may exclude potential Event A3 triggered handover processing. UE_Signaling_Timeout represents a timeout value of handover signaling procedure after which UE handover will fail and the UE will be returned to the RSRP gap list waiting for the next round of power reduction and handover procedure. Adj_Rate is an adjustment factor used to adjust the UE number for handover based on handover performance in the previous step and it can be obtained from a formula (6) :

In the formula (6) , Prev_Success_UE represents the number of UEs successfully handed over in the previous step, and Prev_Scheduled_UE represents the number of UEs scheduled for handover in the previous step. In some example embodiments, handover failure causes may be taken into consideration for determining the handover success rate in the previous step Prev_Success_Rate in the formulas (3) and (6) . For example, if the cause is related to air interface e.g. connection loss in the serving cell, the UE may be counted in determining the success rate. If handover is failed because a certain target neighboring cell is overloaded, it may not be considered in determining the success rate. The RSRP gap of UEs who have the overloaded target neighboring cell may be re-calculated with respect to other neighboring cells if they have measurements from other neighboring cells. The overloaded neighboring cell may be blocked from the handover target cell list for a certain time interval after which handover can be attempted again to the overloaded neighboring cell.

From the formulas (5) and (6) , the maximum number of UEs scheduled for handover Max_Schedule_UE_Num may be calculated, thereby determining the UE index Max_Schedule_UE_Index in the RSRP gap list. Then the maximum power reduction amount MaxReducedPower_HOProcessingCapability depending on the maximum handover processing capability of the base station 120 may be calculated as a difference between the RSRP gap of the UE having the index zero and the RSRP gap of the UE having the index Max_Schedule_UE_Index, according to the formula (4) and the RSRP gap list. Fig. 6 shows an example of the max power reduction calculated from the formula (4) and the RSRP gap list. When the index Max_Schedule_UE_Index corresponding to the max handover processing capability of the base station 120 is determined, the max power reduction can be determined from the RSRP gap list using the index Max_Schedule_UE_Index.

Referring back to the formula (1) , the smaller one of Default_Power plus Adjust_Power and MaxReducedPower_HOProcessingCapability will be used as the calculated power reduction amount Reduced_Power so that it would not cause handover scheduling exceeding the processing capability of the base station 120. In the example shown in Fig. 6, the power reduction amount calculated from Default_Power plus Adjust_Power (e.g. 8 dB in the example) is smaller than the max power reduction (e.g. 12 dB in the example) corresponding to the max handover processing capability of the base station 120, and the value of Default_Power plus Adjust_Power is used as the power reduction amount calculated from the formula (1) . In addition, the operator may predetermine upper and lower limits for the power reduction amount or the upper and lower limits for the power reduction amount may be pre-configured at the base station 120. The upper limit is set to avoid dramatic power change, and the lower limit is set to ensure at least a small power reduction in each step and prevent the shutdown procedure ending up with an infinite loop. In an example, the upper and lower limits may be set depending on the default power reduction amount Default_Power. For example, the upper limit may be set to Default_Power*3, and the lower limit may be set to Default_Power/3. Other values for the upper and lower limits are also applicable. If the power reduction amount calculated from the formula (1) violates the upper limit or the lower limit, the base station 120 may apply the upper limit or the lower limit for the power reduction amount.

In the above example embodiments, the power reduction amount may be dynamically determined in consideration of a variety of factors including for example handover success rate, handover UE number, system processing capacity. The self-adaptive method for determining the power reduction amount can get the optimal power reduction in each step to guarantee that all potential UE handover can be fully processed in a timely manner, so call drops can be minimized, the whole shutdown process would have minimum impact on the ongoing services and can achieve the best balance between the process duration and handover KPI. In addition, the power reduction parameters are automatically calculated at runtime by the base station 120, the operator does not need to calculate and configure parameters like power reduction step amount and shutdown window. It makes the whole procedure very simple for the operator.

With continuous reference to Fig. 4, at the operation 226, a group of UEs is also determined for handover associated with the power reduction step. The group of UEs for handover may be determined based on the power reduction amount. Fig. 7 shows example operations of determining the group of UEs for handover in association with the power reduction step.

Referring to Fig. 7, at an operation 310, the base station 110 may calculate a signal quality gap between the neighboring cell signal quality and the serving cell signal quality for the respective UEs 110. If the measurement report received from the UE 110 indicates two or more neighboring cells, the base station 120 may use the best neighboring cell or one of the two or more neighboring cells selected for UE handover to calculate the signal gap for the UE 110. In some example embodiments, RSRP is used to represent the signal quality. Fig. 6 shows an example of a RSRP gap list calculated for the UEs 110. As mentioned above, the RSRP gap list is also used in determining the power reduction amount, and the operation 310 may re-use the RSRP gap list for determining the group of UEs.

At an operation 312, the base station 120 may determine a group of UEs of which the signal quality gap falls within a range from Dm to Dm-Dn. Dm is a threshold for a measurement event which can trigger an intra-frequency handover, e.g. the Event A3 threshold. If UE has RSRP from a neighboring cell Dm higher than RSRP from the serving cell, the UE will send the Event A3 measurement report to the base station 120 and the base station 120 will handover the UE to the neighboring cell in response to the Event A3 measurement report. Dn is the power reduction amount calculated for the current power reduction step. For example, if Dm is 3 dB and Dn is 8 dB, the base station 120 may determine the group of UEs of which the RSRP gap falls within the range from 3 dB to -5 dB. Since the power reduction amount Dn is determined in consideration of the handover processing capability of the base station 120, the group of UEs determined at the operation 312 would not exceed the handover processing capability of the base station 120.

Referring back to Fig. 4, after determining the power reduction amount and the group of UEs, the base station 120 may reduce the downlink transmission power for the cell 122 to be shutdown by the determined power reduction amount at an operation 228 and handover the group of UEs to their neighboring cells at an operation 230. The group of UEs may be handed over in a descending order of the signal quality gap (RSRP gap) . The operation 228 and the operation 230 may be performed in parallel with each other. It means that the base station 120 can proactively handover the group of UEs to their neighboring cells at the operation 230, and it does not need to wait for e.g. an Event A3 measurement report from the UEs after the downlink transmission power for the cell 122 is reduced and then handover the UEs in response to the Event A3 measurement report as in the legacy way. Here the handover triggered by the Event A3 measurement report may be referred to as passive handover.

By applying the proactive handover to the group of UEs, the example embodiments can shorten the time delay interval associated with the power reduction step because the delay interval can reflect the real handover needs. The delay interval associated with each power reduction step can be dynamically adapted to and fully spent on the handover handling, and a blind waiting period can be eliminated. Therefore, the duration of the whole shutdown procedure is reduced, which improves the service down time and energy efficiency.

It would be appreciated that the base station 120 may also receive new measurement reports from the UEs 110 during the time delay interval associated with the power reduction step. If the new measurement report received from a UE 110 is the Event A3 measurement report or the inter-frequency Event A4 measurement report, the base station 120 may handover the UE 110 to its neighboring cell as above mentioned with respect to the

operation

218 or 222, as long as handover of the UE 110 is not started. After successful handover, the UE 110 may be removed from the current group of UEs or from the waiting UE list (i.e., the RSRP gap list) . If the measurement report is the intra-frequency Event A4 measurement report and the handover of the UE is not started, the base station 120 may re-calculate the signal quality gap (e.g. RSRP gap) of the UE and update the signal quality gap list (e.g. RSRP gap list) . The base station 120 may further put the UE into the current group of UEs for handover if the UE is not included in the group but the updated RSRP gap of the UE falls within the RSRP gap range [Dm, Dm-Dn] corresponding to the group, or remove the UE from the current group of UEs for handover if the is included in the group but the updated RSRP gap of the UE is outside the RSRP gap range [Dm, Dm-Dn] corresponding to the group. If the base station 120 has started handover of the UE when it receives the new measurement report from the UE, the new measurement report may be ignored.

When the base station 120 has completed the handover of the group of UEs, the base station 120 may remove successful UEs from the signal quality gap list. If the downlink transmission power of the cell 122 under shutting-down is still higher than the target power and there are UEs left in the signal quality gap list, the base station 120 may repeat the operations 226-230 on the remaining UEs in the signal quality gap list. When repeating the operation 226, the base station 120 may update the signal quality gap list based on the power reduction performed at the operation 228 and/or new measurement reports received from the UEs 110 after the power reduction at the operation 228.

In some example embodiments, when the power reduction amount determined at the operation 226 is higher than or equal to a difference between the current power for the cell under shutting-down and the target power, the base station 120 knows this is the last power reduction step and it may determine the group of UEs for handover to include all remaining UEs with available measurement reports. It may further minimize call drops in the shutdown procedure. In some example embodiments, the base station 120 may adjust the power reduction amount such that the power of the cell under shutting-down may be reduced to the target level.

When the power of the cell under shutting-down is reduced to or below the target level or there is no UE with available measurement report left at the operation 232, the graceful cell shutdown procedure ends at an operation 234. If there is no UE with available measurement report left but the power of the cell is still higher than the target level, the base station 120 may directly reduce the power of the cell to the target level at the operation 234. When the power of the cell reaches the target level, the cell 122 can be totally shutdown, and the UEs without measurement reports and the UEs who failed the handover in the last step will be dropped.

Fig. 8 shows a schematic message sequence chart of a graceful cell shutdown procedure according to an example embodiment of the present disclosure. The graceful cell shutdown procedure can be performed by the base station 120 servicing the cell to be shutdown and the UEs 110 connected to the cell to be shutdown. In some example embodiments, the base station 120 may be configured with a shutdown scheduler component, which may be a logic or physical entity, to implement at least one operation in the shutdown procedure. In some example embodiments, the shutdown scheduler may be implemented in the control plane (C-Plane) . Since details of the shutdown procedure have been discussed above with respect to Figs. 1-7, the operations shown in Fig. 8 will be described briefly and details thereof may refer to the above description.

Referring to Fig. 8, when the shutdown procedure is triggered for a cell 122 at an operation 410, the shutdown scheduler may send a request of rejecting new connections and handover to the cell 122 to the C-Plane at an operation 412, and send a request of blocking the cell 122 from a neighboring cell list of other cells at an operation 414 so the other cells do not need to measure signal quality of the cell 122.

At an operation 416, the C-Plane may send an RRC reconfiguration request to the UEs 110 connected to the cell 122. The RRC reconfiguration request may include measurement configuration for Event A4 measurement and optionally for Event A3 measurement. The measurement configuration may include a threshold to trigger Event A4 measurement report and optionally a threshold to trigger Event A3 measurement report. The C-Plane may also start a timer to monitor for receipt of measurement reports from the UEs 110 in response to the measurement configuration. The UEs 110 may apply the measurement configuration and send an RRC reconfiguration complete message to the C-Plane at an operation 418.

The UEs 110 measure signal quality e.g. RSRP of neighboring cells and send Event A4 measurement reports to the C-Plane of the base station 120 at an operation 420 when an Event A4 triggering condition is satisfied. The C-Plane may decode the Event A4 measurement reports and send the RSRP measurements of the UEs 110 to the shutdown scheduler at an operation 422.

If the shutdown scheduler determines at an operation 424 that a certain UE 110 has an inter-frequency neighboring cell with adequate RSRP, the shutdown scheduler may send a request of handing over the UE to the inter-frequency neighboring cell to the C-Plane at an operation 426. Then the C-Plane may initiate a handover procedure at an operation 428 to handover the UE to its inter-frequency neighboring cell.

When the C-Plane receives the Event A4 measurement reports from all connected UEs 110 or the timer for monitoring receipt of the measurement reports expires, the C-Plane may send an all measurement report received or timer expiry indication to the shutdown scheduler at an operation 430. In response to the indication, the shutdown scheduler stops waiting for the measurement reports and proceeds to a next step.

At an operation 432, the shutdown scheduler may create an RSRP gap list based on the RSRP measurements of the UEs 110 and determine a power reduction amount and a group of UEs for handover at least partially based on the RSRP gap list. Since UEs having the inter-frequency Event A4 measurement reports have been handed over out of the cell 122 at the operation 428, the RSRP gap list may be determined based on the intra-frequency Event A4 measurement reports. A variety of additional factors may also be considered in determining the power reduction amount and the group of UEs for each power reduction step, including for example handover processing capability, handover UE number and success rate in a previous power reduction step, and other factors.

Then the shutdown scheduler may request the C-Plane to handover the group of UEs to their neighboring cells at an operation 434 and request the management plane (M-Plane) and the user plane (U-Plane) to reduce the downlink transmission power of the cell 122 at an operation 436. The

operations

434 and 436 may be performed in parallel with each other.

At an operation 438, the C-Plane may handover the group of UEs to their neighboring cells. The handover of the group of UEs may be performed in a descending order of the RSRP gap.

The C-Plane may receive Event A3 and/or A4 measurement reports from the UEs 110 time to time at an operation 440. If the handover of a UE is already started when the C-Plane receives the measurement report from the UE, the measurement report may be ignored. If the handover of the UE is not started when the C-Plane receives the measurement report, the measurement report may be processed at the C-Plane. For example, if the received measurement report triggers the Event A3, the C-Plane may immediately initiate a handover procedure for the UE transmitting the measurement report at an operation 442. If the received measurement report triggers the Event A4, the C-Plane may send an RSRP measurement update to the shutdown scheduler at an operation 444.

If the RSRP measurement update is associated with an inter-frequency neighboring cell, the shutdown scheduler may request the C-Plane to handover the UE to its inter-frequency neighboring cell as in the operation 426. If the RSRP measurement update is associated with an intra-frequency neighboring cell, the shutdown scheduler may update the RSRP gap of the UE based on the RSRP measurement update at an operation 446.

The shutdown scheduler may decide to add the UE to the handover group or remove the UE from the handover group based on the updated RSRP gap of the UE. Then at an operation 448, the shutdown scheduler may send a handover add or cancel request to the C-Plane. In response to the request, the C-Plane may add the UE into the handover group or remove the UE from the handover group if the handover of the UE has not been started. At an operation 450, the C-Plane may send a handover add or cancel result message to the shutdown scheduler. In response to the handover add or cancel result, the shutdown scheduler may update the group of UEs for handover at an operation 452.

At an operation 454, the C-Plane may send a handover result indication to the shutdown scheduler. The handover result indication may indicate which UEs have been successfully handed over and/or which UEs failed in the handover. The shutdown scheduler may update the RSRP gap list based on the handover result indication, e.g. removing successful UEs from the RSRP gap list and keeping the failed UE in the RSRP gap list. The shutdown scheduler may also evaluate the handover of the group of UEs at an operation 456. For example, the shutdown scheduler may calculate a handover success rate of the group of UEs, which may be used in the next power reduction step.

The operations 432-456 may be repeated to reduce the downlink transmission power of the cell 122 in a stepwise manner, until there is no UE with measurement left or the target power is reached at an operation 458. If there is no UE with measurement left but the power for the cell 122 is still higher than the target power, the shutdown scheduler may request the M/U-Plane to reduce the power of the cell directly to the target power. Then the shutdown scheduler may request the M/U-Plane to totally shutdown the cell 122 at an operation 460.

Fig. 9 shows a functional block diagram illustrating an apparatus 500 for performing a graceful cell shutdown procedure according to an example embodiment. The apparatus 500 may be implemented for example at the base station 120 to perform the graceful cell shutdown procedure discussed above with respect to Figs. 1-8. Since the graceful cell shutdown procedure has been discussed above in detail, components of the apparatus 500 will be described briefly here and details thereof may refer to the above description.

Referring to Fig. 9, the apparatus 500 may include a first means 510 for configuring measurements for UEs 110 connected in a cell 122 to the base station 120 when a graceful shutdown procedure is triggered for the cell 122. In some example embodiments, the first means 510 may include a first sub-means 512 for configuring a first measurement (e.g. Event A4) for the UEs 110. For example, the first sub-means 512 may configure the UEs 110 with a first minimal threshold to trigger the first measurement report. The first measurement report may be triggered when signal quality of at least one neighboring cell is better than or equal to the first minimal threshold. In some example embodiments, the first minimal threshold may have a value less than or equal to -100 dBm and higher than or equal to -150 dBm, preferably less than or equal to -110 dBm and higher than or equal to -140 dBm. The first measurement report may indicate the signal quality of the at least one neighboring cell and signal quality of the serving cell. In some example embodiments, optionally, the first means 510 may further include a second sub-means 514 for configuring a second measurement (e.g. Event A3) for the UEs 110. For example, the second sub-means 514 may configure the UEs 110 with a second threshold to trigger the second measurement report. The second measurement report may be triggered when the signal quality of at least one neighboring cell is better than the signal quality of the serving cell by an amount higher than or equal to the second threshold.

The apparatus 500 may further include a second means 520 for receiving the first measurement reports from the UEs 110 connected in the cell 122 to the base station 120. The first measurement report may indicate the signal quality (e.g. RSRP) of at least one neighboring cell and the signal quality of the serving cell 122.

Optionally, the apparatus 500 may further include a third means 530 for handing over one or more UEs to their neighboring cells when the neighboring cells of the one or more UEs operate in different frequencies than the serving cell.

The apparatus 500 may further include a fourth means 540 for determining a power reduction amount and a group of UEs associated with the power reduction amount at least partially based on the first measurement reports. In some example embodiments, the fourth means 540 may include a first sub-means 542 for determining the power reduction amount and a second sub-means 544 for determining the group of UEs associated with the power reduction amount. The first sub-means 542 may determine the power reduction amount at least partially based on a default power reduction amount and a power adjustment amount. The power adjustment amount may be calculated at least partially based on a previous power adjustment amount and previous handover performance (e.g., handover success rate) . The first sub-means 542 may further determine the power reduction amount in consideration of a handover processing capability of the base station 120. In some example embodiments, when the determined power reduction amount violates an upper limit or a lower limit of a predetermined power reduction range, the first sub-means 542 may apply the upper limit or the lower limit of the predetermined power reduction range as the power reduction amount.

The second sub-means 544 for determining the group of UEs associated with the power reduction amount may include a first module 545 for calculating a signal quality gap between the neighboring cell and the serving cell of the UEs based on the first measurement reports received from the UEs, and a second module 547 for determining the group of UEs of which the signal quality gap is within a range from Dm to Dm-Dn where Dm is a threshold to trigger an intra-frequency handover (e.g., the threshold to trigger the second measurement report, which in turn triggers the intra-frequency handover) and Dn is the determined power reduction amount. In a case where the downlink transmission power for the serving cell 122 is higher than a predetermined target power level of the cell shutdown procedure by an amount less than or equal to the determined power reduction amount, the second module 547 may determine the group of UEs to include all the UEs with available measurement reports. In some example embodiments, the second sub-means 544 may further include a third module 549 for updating the signal quality gap of the UEs according to the previously reduced downlink transmission power for the serving cell 122 and/or a new first measurement report before the handing-over of the UEs starts. In some example embodiments, when the signal quality gap of a UE is updated based on a new first measurement report received from the UE, the third module 549 may further update the group of UEs for handover by adding the UE into the group or removing the UE from the group based on the updated signal quality of the UE if the handover of the UE is not started.

The apparatus 500 may further include a fifth means 550 for reducing downlink transmission power for the serving cell 122 by the determined power reduction amount and a sixth means 560 for handing over the group of UEs to their neighboring cells. In some example embodiments, the handing-over of the group of UEs may be performed in parallel with the reducing of the downlink transmission power for the serving cell 122. The fifth means 550 may handover the group of UEs to their neighboring cells in a descending order of the signal quality gap of the UEs.

Optionally, the apparatus 500 may further include a seventh means 570 for receiving from a UE the second measurement report indicating the signal quality of at least one neighboring cell and the signal quality of the serving cell of the UE and an eighth means 580 for handing over the UE to its neighboring cell in response to the received second measurement report.

The apparatus 500 may further include a ninth means 590 for determining if the downlink transmission power for the serving cell 122 is lower than or equal to a predetermined target level or the plurality of UEs are handed over to their neighboring cells. If the downlink transmission power for the serving cell 122 is higher than the predetermined target level and one or more of the UEs with available measurements still connect to the serving cell 122, the ninth means 590 may instruct the fourth means 540, the fifth means 550 and the sixth means 560 to perform their functions until the power for the serving cell 122 reaches the predetermined target level or there is no UE with available measurement connected to the serving cell 122.

Fig. 10 illustrates a block diagram of an example communication system 600 in which embodiments of the present disclosure can be implemented. As shown in Fig. 10, the communication system 600 may comprise a terminal device 610 which may be implemented as the UE 110 discussed above, and a network device 620 which may be implemented as any one of the base stations 120 discussed above. Although Fig. 10 shows one terminal device 610, it would be appreciated that the communication system 600 may comprise a plurality of terminal devices 610 connected in a serving cell to the network device 620.

Referring to Fig. 10, the terminal device 610 may comprise one or more processors 611, one or more memories 612 and one or more transceivers 613 interconnected through one or more buses 614. The one or more buses 614 may be address, data, or control buses, and may include any interconnection mechanism such as series of lines on a motherboard or integrated circuit, fiber, optics or other optical communication equipment, and the like. Each of the one or more transceivers 613 may comprise a receiver and a transmitter, which are connected to one or more antennas 616. The terminal device 610 may wirelessly communicate with the network device 620 through the one or more antennas 616. The one or more memories 612 may include computer program code 615. The one or more memories 612 and the computer program code 615 may be configured to, when executed by the one or more processors 611, cause the terminal device 610 to perform operations and procedures relating to the UE 110 as described above.

The network device 620 may comprise one or more processors 621, one or more memories 622, one or more transceivers 623 and one or more network interfaces 627 interconnected through one or more buses 624. The one or more buses 624 may be address, data, or control buses, and may include any interconnection mechanism such as a series of lines on a motherboard or integrated circuit, fiber, optics or other optical communication equipment, and the like. Each of the one or more transceivers 623 may comprise a receiver and a transmitter, which are connected to one or more antennas 626. The network device 620 may operate as a base station for the terminal device 610 and wirelessly communicate with terminal device 610 through the one or more antennas 626. The one or more network interfaces 627 may provide wired or wireless communication links through which the network device 620 may communicate with other network devices, entities, elements or functions. The one or more memories 622 may include computer program code 625. The one or more memories 622 and the computer program code 625 may be configured to, when executed by the one or more processors 621, cause the network device 620 to perform operations and procedures relating to any one of the base stations 120.

The one or

more processors

611, 621 discussed above may be of any appropriate type that is suitable for the local technical network, and may include one or more of general purpose processors, special purpose processor, microprocessors, a digital signal processor (DSP) , one or more processors in a processor based multi-core processor architecture, as well as dedicated processors such as those developed based on Field Programmable Gate Array (FPGA) and Application Specific Integrated Circuit (ASIC) . The one or

more processors

611, 621 may be configured to control other elements of the UE/network device and operate in cooperation with them to implement the procedures discussed above.

The one or

more memories

612, 622 may include at least one storage medium in various forms, such as a volatile memory and/or a non-volatile memory. The volatile memory may include but not limited to for example a random access memory (RAM) or a cache. The non-volatile memory may include but not limited to for example a read only memory (ROM) , a hard disk, a flash memory, and the like. Further, the one or

more memories

612, 622 may include but not limited to an electric, a magnetic, an optical, an electromagnetic, an infrared, or a semiconductor system, apparatus, or device or any combination of the above.

It would be understood that blocks in the drawings may be implemented in various manners, including software, hardware, firmware, or any combination thereof. In some embodiments, one or more blocks may be implemented using software and/or firmware, for example, machine-executable instructions stored in the storage medium. In addition to or instead of machine-executable instructions, parts or all of the blocks in the drawings may be implemented, at least in part, by one or more hardware logic components. For example, and without limitation, illustrative types of hardware logic components that can be used include Field-Programmable Gate Arrays (FPGAs) , Application-Specific Integrated Circuits (ASICs) , Application-Specific Standard Products (ASSPs) , System-on-Chip systems (SOCs) , Complex Programmable Logic Devices (CPLDs) , etc.

Some example embodiments further provide computer program code or instructions which, when executed by one or more processors, may cause a device or apparatus to perform the procedures described above. The computer program code for carrying out procedures of the example embodiments may be written in any combination of one or more programming languages. The computer program code may be provided to one or more processors or controllers of a general purpose computer, special purpose computer, or other programmable data processing apparatus, such that the program code, when executed by the processor or controller, cause the functions/operations specified in the flowcharts and/or block diagrams to be implemented. The program code may execute entirely on a machine, partly on the machine, as a stand-alone software package, partly on the machine and partly on a remote machine or entirely on the remote machine or server.

Some example embodiments further provide a computer program product or a computer readable medium having the computer program code or instructions stored therein. The computer readable medium may be any tangible medium that may contain, or store a program for use by or in connection with an instruction execution system, apparatus, or device. The machine readable medium may be a machine readable signal medium or a machine readable storage medium. A machine readable medium may include but is not limited to an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples of the machine readable storage medium would include an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM) , a read-only memory (ROM) , an erasable programmable read-only memory (EPROM or Flash memory) , an optical fiber, a portable compact disc read-only memory (CD-ROM) , an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.

Although the subject matter has been described in a language that is specific to structural features and/or method actions, it is to be understood the subject matter defined in the appended claims is not limited to the specific features or actions described above. On the contrary, the above-described specific features and actions are disclosed as an example of implementing the claims.

Claims

A network device comprising:

at least one processor; and

at least one memory including computer program code, the at least one memory and the computer program code being configured to, with the at least one processor, cause the network device at least to:

receive first measurement reports from a plurality of terminal devices connected in a serving cell to the network device, the first measurement reports indicating neighboring cells for the plurality of terminal devices;

determine a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports;

reduce downlink transmission power for the serving cell by the determined power reduction amount;

handover the group of terminal devices to their neighboring cells; and

repeat the determining, reducing and handing-over operations on remaining terminal devices of the plurality of terminal devices until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.
The network device of Claim 1 wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the network device at least to:

handover at least one terminal device to its neighboring cell before determining the group of terminal devices, when the neighboring cell of the at least one terminal device operates in different frequency than the serving cell.
The network device of Claim 1 wherein the power reduction amount is determined at least partially based on a default power reduction amount and a power adjustment amount, and the power adjustment amount is calculated at least partially based on a previous power adjustment amount and previous handover performance.
The network device of Claim 3 wherein the power reduction amount is further determined in consideration of a handover processing capability of the network device.
The network device of Claim 3 wherein in response to the determined power reduction amount violating an upper limit or a lower limit of a predetermined power reduction range, the upper limit or the lower limit of the predetermined power reduction range is applied as the power reduction amount.
The network device of Claim 1 wherein determining the group of terminal devices comprises:

calculating, for the respective terminal devices, a signal quality gap between the neighboring cell and the serving cell based on the first measurement reports; and

determining the group of terminal devices of which the signal quality gap is within a range from Dm to Dm-Dn where Dm is a threshold to trigger an intra-frequency handover and Dn is the determined power reduction amount.
The network device of Claim 6 wherein the signal quality gap calculated for the terminal device is updated according to the previously reduced downlink transmission power for the serving cell and/or a new first measurement report received from the terminal device, before the handing-over of the terminal device starts.
The network device of Claim 6 wherein the group of terminal devices are determined to comprise the terminal devices with available measurement reports when the downlink transmission power for the serving cell is higher than the predetermined target level by an amount less than or equal to the determined power reduction amount.
The network device of Claim 6 wherein the group of terminal devices are handed over to their neighboring cells in a descending order of the signal quality gap.
The network device of Claim 1 wherein the handing-over of the group of terminal devices is performed in parallel with the reducing of the downlink transmission power for the serving cell.
The network device of Claim 1 wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the network device at least to:

configure terminal devices connected in the serving cell to the network device with a first minimal threshold to trigger the first measurement reports, wherein the first measurement report is triggered when signal quality of at least one neighboring cell is better than or equal to the first minimal threshold, and the first measurement report indicates the signal quality of the at least one neighboring cell and signal quality of the serving cell.
The network device of Claim 11 wherein the first minimal threshold has a value less than or equal to -100 dBm and higher than or equal to -150 dBm, preferably less than or equal to -110 dBm and higher than or equal to -140 dBm.
The network device of Claim 11 wherein the at least one memory and the computer program code are further configured to, with the at least one processor, cause the network device at least to:

configure the terminal devices connected in the serving cell to the network device with a second threshold to trigger a second measurement report, wherein the second measurement report is triggered when the signal quality of at least one neighboring cell is better than the signal quality of the serving cell by an amount higher than or equal to the second threshold;

receive the second measurement report from a terminal device, the second measurement report indicating the signal quality of the at least one neighboring cell and the signal quality of the serving cell; and

handover the terminal device in response to the received second measurement report.
A method implemented at a network device comprising:

receiving first measurement reports from a plurality of terminal devices connected in a serving cell to the network device, the first measurement reports indicating neighboring cells for the plurality of terminal devices;

determining a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports;

reducing downlink transmission power for the serving cell by the determined power reduction amount;

handing over the group of terminal devices to their neighboring cells; and

repeating the determining, reducing and handing-over operations on remaining terminal devices of the plurality of terminal devices until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.
The method of Claim 14 further comprising:

handing over at least one terminal device to its neighboring cell before determining the group of terminal devices, when the neighboring cell of the at least one terminal device operates in different frequency than the serving cell.
The method of Claim 14 wherein the power reduction amount is determined at least partially based on a default power reduction amount and a power adjustment amount, and the power adjustment amount is calculated at least partially based on a previous power adjustment amount and previous handover performance.
The method of Claim 16 wherein the power reduction amount is further determined in consideration of a handover processing capability of the network device.
The method of Claim 16 wherein in response to the determined power reduction amount violating an upper limit or a lower limit of a predetermined power reduction range, the upper limit or the lower limit of the predetermined power reduction range is applied as the power reduction amount.
The method of Claim 14 wherein determining the group of terminal devices comprises:

calculating, for the respective terminal devices, a signal quality gap between the neighboring cell and the serving cell based on the first measurement reports; and

determining the group of terminal devices of which the signal quality gap is within a range from Dm to Dm-Dn where Dm is a threshold to trigger an intra-frequency handover and Dn is the determined power reduction amount.
The method of Claim 19 wherein the signal quality gap calculated for the terminal device is updated according to the previously reduced downlink transmission power for the serving cell and/or a new first measurement report received from the terminal device, before the handing-over of the terminal device starts.
The network device of Claim 19 wherein the group of terminal devices are determined to comprise the terminal devices with available measurement reports when the downlink transmission power for the serving cell is higher than the predetermined target level by an amount less than or equal to the determined power reduction amount.
The method of Claim 19 wherein the group of terminal devices are handed over to their neighboring cells in a descending order of the signal quality gap.
The method of Claim 14 wherein the handing-over of the group of terminal devices is performed in parallel with the reducing of the downlink transmission power for the serving cell.
The method of Claim 14 further comprising:

configuring terminal devices connected in the serving cell to the network device with a first minimal threshold to trigger the first measurement reports, wherein the first measurement report is triggered when signal quality of at least one neighboring cell is better than or equal to the first minimal threshold, and the first measurement report indicates the signal quality of the at least one neighboring cell and signal quality of the serving cell.
The method of Claim 24 wherein the first minimal threshold has a value less than or equal to -100 dBm and higher than or equal to -150 dBm, preferably less than or equal to -110 dBm and higher than or equal to -140 dBm.
The method of Claim 24 further comprising:

configuring the terminal devices connected in the serving cell to the network device with a second threshold to trigger a second measurement report, wherein the second measurement report is triggered when the signal quality of at least one neighboring cell is better than the signal quality of the serving cell by an amount higher than or equal to the second threshold;

receiving the second measurement report from a terminal device, the second measurement report indicating the signal quality of the at least one neighboring cell and the signal quality of the serving cell; and

handing over the terminal device in response to the received second measurement report.
An apparatus comprising:

means for receiving, at a network device, first measurement reports from a plurality of terminal devices connected in a serving cell to the network device, the first measurement reports indicating neighboring cells for the plurality of terminal devices;

means for determining a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports;

means for reducing downlink transmission power for the serving cell by the determined power reduction amount;

means for handing over the group of terminal devices to their neighboring cells; and

means for repeating the determining, reducing and handing-over operations on remaining terminal devices of the plurality of terminal devices until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.
A computer program product embodied in at least one computer readable medium and comprising instructions, when executed by at least one processor of a network device, causing the network device at least to:

receive first measurement reports from a plurality of terminal devices connected in a serving cell to the network device, the first measurement reports indicating neighboring cells for the plurality of terminal devices;

determine a power reduction amount and a group of terminal devices associated with the power reduction amount at least partially based on the first measurement reports;

reduce downlink transmission power for the serving cell by the determined power reduction amount;

handover the group of terminal devices to their neighboring cells; and

repeat the determining, reducing and handing-over operations on remaining terminal devices of the plurality of terminal devices until the downlink transmission power for the serving cell is lower than or equal to a predetermined target level or the plurality of terminal devices are handed over to their neighboring cells.