Method for determining handover criterion in a cellular wireless communication system

Abstract

The present application relates to a method for determining handover criterion in a cellular wireless communication system, said cellular wireless communication system employing handover procedures according to which mobile stations may be handed over from a cell to another cell; said method comprising the steps of: receiving at least one handover control criterion parameter, and determining at least one handover criterion based on said at least one handover control criterion parameter. Furthermore, the present application also relates to a method in a network control entity, a method in a network entity, a computer program, a computer program product, a network control entity device, a network entity device and a cellular communication system comprising such devices.

Claims

1. A method, comprising: obtaining, by a network entity in a cellular wireless communication system, parameters associated with a handover control criterion, wherein the handover control criterion is adopted for changing a handover trigger value, the handover trigger value is a cell specific offset that is associated with a threshold at which a cell initializes a handover preparation procedure, and the parameters comprise a threshold value used to define a maximum allowed frequency which indicates how often the handover trigger value is changed and to define a maximum value allowed for the handover trigger value to change; and changing, by the network entity, the handover trigger value according to the parameters.

2. The method according to claim 1, wherein the obtaining, by a network entity in a cellular wireless communication system, parameters associated with a handover control criterion comprises: receiving, by the network entity, the parameters from a network control entity.

3. The method according to claim 2, wherein the network control entity is at least one of the following: Operations and Maintenance (OAM) entity, Network Management System (NMS) entity and Element Management System (EMS) entity.

4. The method according to claim 2, wherein the network entity is integrated in a base station.

5. A network entity in a cellular wireless communication system, comprising a transceiver; and a processor, communicatively connected with the transceiver; wherein the processor is configured to obtain parameters associated with a handover control criterion, wherein the handover control criterion is adopted for changing a handover trigger value, the handover trigger value is a cell specific offset that associated with a threshold at which a cell initializes a handover preparation procedure, and the parameters comprise a threshold value used to define a maximum allowed frequency which indicates how often the handover trigger value is changed and to define a maximum value allowed for the handover trigger value to change; and change the handover trigger value according to the parameters.

6. The network entity according to claim 5, wherein the transceiver is configured to receive the parameters from a network control entity.

7. The network entity according to claim 5, wherein the network entity is integrated in a base station.

8. A cellular wireless communication system, comprising a network entity and a network control entity, wherein the network entity is configured to: receive, from the network control entity, parameters associated with a handover control criterion, wherein the handover control criterion is adopted for changing a handover trigger value, the handover trigger value is a cell specific offset that associated with a threshold at which a cell initializes a handover preparation procedure, and the parameters comprise a threshold value used to define a maximum allowed frequency which indicates how often the handover trigger value is changed and to define a maximum value allowed for the handover trigger value to change; and change the handover trigger value according to the parameters; and the network control entity is configured to: send the parameters to the network entity.

9. The system according to claim 8, wherein the network control entity is at least one of the following: Operations and Maintenance (OAM) entity, Network Management System (NMS) entity and Element Management System (EMS) entity.

10. The system according to claim 8, wherein the network entity is integrated in a base station.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The appended drawings are intended to clarify and explain different embodiments of the disclosure in which:

(2) FIG. 1 shows a typical cell scenario with cell measurement;

(3) FIG. 2 illustrates the relationship between OAM, MRO, HO algorithm and UE in a cellular communication system;

(4) FIG. 3 illustrates using HO trigger adjustment to control MRO from OAM;

(5) FIG. 4 illustrates using allowed adjustment range in the signal strength domain;

(6) FIG. 5 shows an example of system architecture; and

(7) FIG. 6 shows an example of system network entities.

DETAILED DESCRIPTION

(8) For the above reasons, the present application relates to a method for determining HO criterion in a cellular wireless communication system. The method comprises the steps of receiving a HO control criterion parameter, and determining at least one HO criterion based on the received HO control criterion parameter.

(9) The HO criterion is the criterion defining when the network entity shall decide to perform a HO for a certain mobile station (such as an UE) in the cellular system, and the HO control criterion parameter is a parameter defining the allowed HO criterion that can be used by the network entity. Hence, the network entity can choose to use any HO criterion as long as it fulfils the HO control criterion parameter. For example, a network control entity controlling a network entity sends a HO control criterion parameter to the network entity, and the network entity is free to define the actual HO criterion to be used as long as it is allowed according to the HO control criterion parameter sent by the network control entity.

(10) Preferably, the HO control criterion parameter is defined and provided by a one network control entity by means of transmission to a network entity which is configured to process mobility parameters for HO procedures for UEs in the system. Accordingly, the network entity may be configured to determine the HO criterion based on the received HO control criterion parameter, and using the HO criterion for HO execution decision.

(11) The network control entity is preferably any of OAM entity, NMS entity, EMS entity or any other suitable entity with the required capabilities. The network entity is preferably integrated in a base station (such as an eNB in a LTE system) or in a base station controller or in a Radio Network Controller (RNC). Further, mentioned base station or base station controller or RNC are configured to be responsible for initiating HO procedures in a cell associated with the same.

(12) Thereby a solution is provided by the disclosure where a network entity, such as a MRO, is controlled from a network control entity (e.g. a OAM) that is independent of implementation specific MRO and mobility algorithms. The advantage with such a solution is that by using the same HO control criterion parameter network entities implemented and configured in different ways may be controlled to behave in the same way.

(13) It shall be understood that some embodiments of the disclosure are also applicable to idle mode (cell reselection) as well as active mode (handover) mobility. Thus MRO may be controlled in how it adjusts idle mode cell reselection timing, either directly or indirectly according to control of the active mode mobility.

(14) Other considerations of the embodiments are e.g. how OAM controls MRO using trigger based HO in the signal strength domain, timing based HO according to timing parameters, or combinations thereof.

(15) With reference to FIG. 4, the present invention may according to a preferred embodiment be implemented in a LTE communication system as follows: 1. Use an existing OAM (i.e. a network control entity) mechanism to define a default mobility parameter setting(s); and 2. Define how much a MRO (i.e. a network entity) is allowed to modify the mobility parameters in terms of their impact on the HO trigger condition, either: a. In the signal strength domain, re-using an existing definition of “handover trigger” defined from 3GPP for mobility load balancing (please see below); b. In the time domain, using a new definition that applies to HO parameters that directly influence the time of the trigger point; or c. Using a combination of (a) and (b) above. 3. Letting the network entity (e.g. by an HO algorithm) decide how to perform the necessary adjustments of the algorithm behaviour and UE behaviour, e.g. through measurement configuration.

(16) HO trigger has been defined as: “is the cell specific offset that corresponds to the threshold at which a cell initialises the handover preparation procedure”, which therefore is expressed by the means of a difference (delta) between the current and the new values of the HO trigger. This definition may be used when defining how to control a network entity from a network control entity according to the disclosure.

(17) In the scope of SON requiring exchange of mobility parameters between eNBs, the need to exchange modifications of HO characteristics was agreed, but instead of controlling the measurement configuration parameters, a relative adjustment was used, referring to a more abstract “handover trigger”.

Signal Strength Domain Parameters

(18) According to an embodiment of the disclosure the HO criterion is a HO trigger value and preferably the HO control criterion parameter defines an allowed interval for the HO trigger value. Therefore, the HO control criterion parameter comprises at least one threshold value used to define the allowed interval. This is a signal strength domain parameter since it directly relates to measurements performed in the signal strength domain. There are also other parameters related to the time domain, for example requiring that a certain condition should be fulfilled during a certain time, which is explained below.

(19) Accordingly, the threshold value may be any in the group comprising: maximum value of a HO trigger, maximum change of a HO trigger, maximum difference between HO triggers for neighbouring cells, and maximum rate of change for a HO trigger according to another embodiment.

(20) The above mentioned three control parameters may be defined as depicted in FIG. 4: Max delta of HO trigger: this is the maximum allowed change of the HO trigger, from the default point of operation defined by the parameter values assigned by the OAM; Max difference HO trigger: this is the maximum difference between HO trigger used for different neighbour cells which is used to limit the individual difference between neighbours to avoid effects occurring when the offset to different neighbours are large; and Max change rate HO trigger: this parameter limits the rate of change and can therefore be used to either increase the stability of the algorithm or reduce the time it takes for the system to find it's optimum value. This parameter is used to control the stability and convergence of the HO algorithm.

(21) HO trigger is in many systems measured in dB, so this approach applies directly to HO parameters that impact the signal strength measurements, such as the CIO or hysteresis value (Hys). However, an implementation could adjust other parameters impacting the timing of the HO (such as the TTT) so that the HO takes place when the signal strength (or signal strength difference) has changed by a number of dB.

Time Domain Parameters

(22) The embodiments above relate to the signal strength aspect, but there is also a timing aspect which may be considered in this respect.

(23) Therefore, according to another embodiment of the disclosure, the HO control criterion parameter may comprise at least one timing parameter, which preferably is a timing threshold value defining an allowed interval for the HO criterion.

(24) The timing parameter may relate to any in the group comprising: maximum allowed relative change of HO timing, or maximum rate of change for HO timing. These parameters may be defined as: 1. Max delta of HO timing: this parameter defines, in relative terms, how much HO timing is allowed to change compared to a default point of operation defined by the parameter values assigned by the OAM; 2. Max difference HO timing: this parameter defines the maximum difference between HO timing used for different neighbour cells which is used to limit the individual difference between neighbours to avoid effects occurring when the offset to different neighbours are large; and 3. Max change rate HO timing: this parameter limits the rate of change and thereby can be used to either increase the stability of the HO algorithm or reduce the time it takes for the system to find it's optimum value.

(25) Examples of HO parameters that directly influence the timing are TTT and the RSRP measurement filter time constant.

(26) Further, as stated above one of the mobility parameters directly reflecting the time domain is the TTT parameter. The allowed range for this parameter is defined in TS 36.331 as: [0, 40, 64, 80, 100, 128, 160, 256, 320, 480, 512, 640, 1024, 1280, 2560, 5120] ms. Obviously, this range is not linear, and it may therefore be difficult to specify a suitable threshold for defining a valid range for this parameter.

(27) One possible solution is however that the OAM defines the starting point (default value) and then the OAM specify the valid range by indicating how many steps or an index value in the defined range the network entity is allowed to adjust the TTT parameter. Hence, instead of signalling the allowed range in terms of time, e.g. in ms, the OAM define it as steps in the allowed range or allowed index positions in the list of valid values. In summary, the timing parameter indicates number of allowed steps or an index value defining an allowed interval based on a predefined interval for the HO criterion, and preferably, the predefined interval is an interval defined for a measurement control parameter, such as a TTT parameter. The advantage of referring to the pre-defined allowed range of TTT is that the system can better control the non-linear range of the TTT parameter. If a linear scale is used, there is no point in differentiating between values of for example [320, 350, 370, . . . 480], since the values that are allowed to be used in this range is anyway limited to only 320 or 480.

Combinations of Signal Strength Domain and Time Domain Parameters

(28) The impact of the timing of the HO trigger point from the adjustments permitted in the signal strength domain is dependent upon the expected variation of signal strength with time. If serving and neighbouring signal strengths change very rapidly at the cell edge during a HO then changes in, for example, changing parameters in the signal strength domain will have a very minor impact on when the HO decision is taken when looking at the actual geographic position for the UE when the decision is taken. This argues for a use of the time domain ranges, as described above. An alternative approach would be to allow all HO parameters to be managed by controlling the change of trigger timing. Thus, the eNB would be free to change both signal domain and time domain parameters such that the timing of the HO stayed within specified bounds.

(29) One solution for this would be that the eNB continuously stores reported mobility measurements to get a view on the typical radio conditions in the cell and creates a statistical model of the relationship between the time and signal strength domain.

(30) In accordance with example embodiments of the disclosure, it is possible to control the behaviour of a network entity (MRO) from a control network entity (OAM). Furthermore, it is also possible to use the same range to achieve the same control affect independent of vendor specific MRO and mobility algorithms.

(31) In previous sections, the general term OAM have been used as the network control entity controlling the network entity (eNB). In LTE, the OAM functionality for controlling the eNB can be performed by the Element Management Systems (EMS) or the Network Management System (NMS). FIG. 5 illustrates some interfaces for a radio base station (such as eNB) to other network nodes in the system. X2 is the interface used in LTE for connection of a radio base station to one or more other radio base stations. I.sub.tf-S is the interface towards the EMS and i.sub.tf-N is the interface between the EMS and the NMS. It has also been suggested to enable the direct connection from NMS to the eNB using the i.sub.tf-N interface.

(32) FIG. 6 illustrates an example central network control entity, C, such as an OAM entity, NMS entity or EMS entity, or other network entity other than a base station, and an example base station, BS. The central network control entity and the base station entity are capable of communicating across an interface, I. The example central network control entity comprises processing equipment, □, capable of controlling a base station HO criterion and sending circuitry, TX, capable of sending one or more HO control criterion parameters, such as threshold values. The central network control entity is generally connected over at least one interface to a plurality of elements of which it provides control. The example base station comprises receiving circuitry, RX, capable of receiving one or more HO control criterion parameters, such as the one or more HO control criterion parameters provided by the sender, TX, and processing equipment of determining a HO criterion on the basis of the respective received one or more HO control criterion parameters, such as not exceeding a threshold value, incremental or relative change not exceeding a threshold value, or time-dynamics or change-rate not exceeding a particular threshold value.

(33) Furthermore, the present application relates to a method in a network control entity for a cellular wireless communication system. The method comprises the steps of: defining at least one handover control criterion parameter; and providing the at least one handover control criterion parameter to a network entity.

(34) The present application also relates to a complementary method in a network entity configured to process mobility parameters in a cellular wireless communication system. The method comprises the steps of: receiving at least one handover control criterion parameter; and determining at least one handover criterion based on the at least one handover control criterion parameter.

(35) According to an embodiment, the least one handover control criterion parameter is received from a network control entity which is configured to define the at least one handover control criterion parameter. Preferably, the network entity is a MRO entity which may be integrated in a base station or in a base station controller or in a RNC.

(36) Moreover, the present application also relates to a network control entity device and a network entity device corresponding to the above methods.

(37) It is realised by the skilled person that the mentioned devices may be modified, mutatis mutandis, according to different embodiments of the methods in the description. The present application further relates to a cellular wireless communication system comprising at least one network control entity and/or at least one network entity as defined above.

(38) Furthermore, as understood by the person skilled in the art, a method according to the present application may also be implemented in a computer program, having code means, which when run in a computer causes the computer to execute the steps of the method. The computer program is included in a computer readable medium of a computer program product. The computer readable medium may consist of essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive.

(39) Finally, it should be understood that the present application is not limited to the embodiments described above, but also relates to and incorporates all embodiments within the scope of the appended independent claims.