WO2010066063A1

WO2010066063A1 - Tdd intra frequency handover measurement enhancement

Info

Publication number: WO2010066063A1
Application number: PCT/CN2008/001990
Authority: WO
Inventors: Lin Dong; Jie Mao; Rui Tian
Original assignee: Telefonaktiebogalet L M Ericsson (Publ)
Priority date: 2008-12-10
Filing date: 2008-12-10
Publication date: 2010-06-17
Also published as: WO2010066063A8; CN102257850A; CN102257850B

Abstract

A method for performing handover measurement may comprise steps of calculating received signal code powers (RSCPs) of a serving cell and a target cell; determining if the RSCPs meet a trigger criteria; and comparing the RSCP of the serving cell with an absolute threshold. By comparing the RSCP of the serving cell with the absolute threshold, the UE may evaluate the communication quality with the current serving cell, making it possible to report to the RNC when the communication condition degrades rapidly without waiting for a fixed period as in the prior art.

Description

TDD intra frequency Handover measurement enhancement

TECHNICAL FIELD

The present invention relates generally to handover between frequency bands in cellular radio communications systems. The present invention relates in particular, but not exclusively, to TDD intra frequency handover between different cells in a 3 G (third generation) communication system, such as TD-SCDMA.

BACKGROUND

A Universal Mobile Telecommunications System (UMTS) network architecture as shown in FIG. 1 includes a core network (CN), a UMTS Terrestrial Radio Access Network (UTRAN) and a User Equipment (UE). There are two general interfaces in the system: the Iu interface between the UTRAN and the core network, and the radio interface Uu between the UTRAN and the UE. The UTRAN consists of several Radio Network Subsystems (RNSs), which are interconnected by the Iur interface. This interconnection allows core network independent procedures between different RNSs. The RNS is comprised of a Radio Network Controller (RNC) and several base stations (Node-Bs). The base stations are connected to the RNC by the Iub interface. One base station can serve one or multiple cells, and typically serves a plurality of UEs.

Each base station may communicate with a UE on a plurality of channels incommbination with various techniques such as intellegent antenas and RAKE diversity schemes. Each UE includes a transmitter, a receiver, a controller, and a user interface and is identified by a specific user equipment unit identifier.

In such cellular radio communication systems, a handover operation may be needed to allow an established radio connection to continue when the UE moves between cells. Often, a low signal strength or a poor signal quality indication means that the UE is near

_ i a border between the two cells. If the UE moves closer to a destination cell, handover of the radio connection to the destination cell usually results in an improved radio transmission and reception.

In the current UMTS TDD systems, the sole criterion for handover, whether intra- or inter-frequency, is the path loss difference between the current base station, (i.e. the "serving" base station), and a target base station. This situation is shown in FIG. 2.

The mobile user equipment (UE) is shown receiving signals from two base stations: 1) the serving base station BSl ; and 2) the target base station BS2. The UE receives the physical channel that carries the broadcast channel BCHl and 2 from both base stations BSl and BS2 and measures the signal strengthes of the channels BCHl , BCH2. When the BCH2 from the target cell is sufficiently stronger than the channel BCHl from the serving cell, the measurements are transmitted to the RNC, which then determines whether or not to initiate a handover.

The current machenism carried out by a prior art UMTS TDD communication system for determining whether or not to commence a handover can be explained with reference to Fig.3A. when a specific event fulfills a triger criteria, a "measurement report" message will be sent from the UE to its current serving RNC (Radio Network Controller). Generally such an event will be reported to the serving RNC when the measured P-CCPCH (Primary Common Control Physical CHannel) RSCP(Received Signal Code Power) is greater than the current P-CCPCH RSCP by a predertemined value, reffered to as a Hysteresis parameter in the TDD communication, which is a parameter set by the RNC through a "Measurement Control" message. The serving RNC may start a handover evaluation based on the the event report and the measuring result. However for example when the UE is rapidly moving at the border of two cells, the procedure can then be repeated, i.e. handed over back and forth, which is known as ping -pong handover. This is undesirable, as the call quality may be reduced and also the process is inefficient in terms of use of system resources.

Therefore to reduce the number of unnecessary handovers, the current 3 G handover algorithms employ hysteresis and the Time~to-Trigger mechanism to trade off between number of reported events and handover delay time. The value of the time-to-trigger is given to the UE in the Reporting criteria field of the Measurement Control message. The effect of the time-to-trigger is that the report is triggered only after the conditions for the event have existed for the specified time-to-trigger. That is to say a timer would start to run when the signal difference between two cells is equal to or greater than the Hysteresis value. An event would be reported only when a predetermined time period has lapsed after the measured RSCP of a targeted cell is greater than that of the serving cell by the hysteresis.

CONTENT OF THE INVENTION

Although the introducing of "timer-to-trigger" will reduce lots of unnecessary handovers, it brings further problems. The designer must carefully choose the "timer-to-trigger" parameter. If the

"timer-to-trigger" parameter is set too short, it can not effecciently avoid unnecessary handovers, if the parameter is set too long, the communication system can not accommodate the rapidly signal degration on the current channel, because UE doesn't report to the base station until expriation of the time period set by the

"timer-to-trigger" parameter. This would make the handover happen too late or even lead to a droped call.

Therefore there is a need to provide a method for reporting to the base station before the power of the serving cell falls out of its tolerable scope. To solve the above problem in the prior art, the invention provides a method for performing handover measurement in TDD communication system comprising steps of: calculating received signal code powers (RSCPs) of a serving cell and a target cell; and determining if the RSCPs meet a trigger criteria; said method is characterized in that it further comprises step of comparing the RSCP of the serving cell with an absolute threshold.

By comparing the RSCP of the serving cell with the absolute threshold, the UE may evaluate the communication quality with the current serving cell, making it possible to report to the RNC when the communication condition degrades rapidly without waiting for a fixed period as in the prior art.

In addition, the RSCP value used here may be the one used in judging the satisfaction of the trigger criteria, therefore this method can be performed based on the current system resource of the UE and there is no need to perform additional measurements, thus improving the usage of the system resources.

It if preferred that the UE would send a measurement report to the RCN if the RSCP of the serving cell is below than the absolute threshold. Such a measurement report may comprise for example a I g event defined in 3GPP TS 25.331 v4.17.0. The Ig event is used to report TDD "change of best cell" to the RNC, and cause the RNC to start handover evaluation. The present invention shortens the measurement event Ig report time in case the current serving cell signal degraded rapidly and reduce the possibility of call drop.

Preferably, the trigger criteria is if the RSCP of the target cell is greater than the RSCP of the serving cell by a predetermined value, for example a hysteresis parameter, and the method further comprising steps of starting a timer for a predetermined time period when the trigger criteria is satisfied and sending the measurement report to the RNC of the serving cell after the predetermined time period lapsed. When the communication quality on the current channel is above the absolute channel, the present invention may effectively reduce the number of unnecessary handovers through the combination of the hysteresis parameter and a timer. Therefore the present invention has no negative impacts on the existing mechanisms.

It is preferred that the predetermined time period and the predetermined value and the absolute threshold are set by the RNC through a measurement control message. For example, the predetermined time period and the predetermined value may be the

"timer-to-trigger" and hysteresis parameters used in the event I g measurement. Therefore the present invention may be achieved by simply adding a new parameter, the absolute threshold, in RRC message "measurement control".

The invention also provides a device for performing handover measurement comprising: means for calculating received signal code powers (RSCPs) of a serving cell and a target cell; means for determining if the RSCPs meet a trigger criteria; and means for comparing the RSCP of the serving cell with an absolute threshold.

The present invention additionally provides a computer program product for carrying out the method as defined above. A computer program product may comprise a set of computer executable instructions stored on a data carrier, such as a memory stick. The set of computer executable instructions, which allow a general or specific purpose processor in the UE to carry out the method as defined above, may also be available for downloading from a remote server, for example via the WAP service.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Fig. 1 is diagram of example mobile communications system in which the present invention may be advantageously employed.

Fig. 2 is a prior art UE receiving the broadcast channels from two base stations.

Fig. 3A-C are diagrammatic views showing the handover measurement mechanisms emplyed in the prior art and the invention, respectively.

Fig. 4 is a flow chart illustrating an improved handover measurement method according to one embodiment of the present invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. The present invention is described in the non-limiting example context of a universal mobile telecommunications (UMTS) shown in Fig. 1.

Fig,3B is diagrammatic view showing the handover measurement mechanism employed in the prior art. After setting up a radio access bearer (RAB) to the UE, a radio network controller(RNC) sends a "measurement control" message to the UE to enable UE measurement. Then the UE will evaluate the primary common control physical channels' (P-CCPCHs') received signal code power (RSCP) of the current cell 1 and a target cell 2 against certain trigger condition, and send event Ig to the RNC when this trigger condition us fulfilled. In 3GPP spec 25.331 or CCSA RRC protocol, event Ig is used for reporting TDD "change of best cell". Normally it'll trigger intra frequency handover.

As described with reference to Fig.3 A, to reduce the number of unnecessary handovers this precedure always employs two parameters: the hysteresis parameter and the "timer-to-trigger" parameter, both set by the RNC through a "measurement control" message. The I g event criteria in Fig.3B is:

1) If equationl fulfilled, start the "timer-to-trigger"; and

2) Only after "timer-to-trigger" expired, I g event will be reported. Equation 1 is defined as follow:

10LogMtar - Hl g > lOLogMcurr (Eq.1),

wherein Mtar is the current P-CCPCH RSCP of the currently evaluated cell 2 expressed in mW, HIg is the hysteresis parameter for the event Ig, and Mcurr is the current P-CCPCH RSCP of the current serving cell expressed in mW.

It can be understood that the I g event will be triggered only after the "timer-to-trigger" expires. Although this may reduce the unnecessary handovers, it will cause other problems, for example when the communication conditions of the serving cell degrades rapidly. As shown in Fig.3B the UE begins to counts the timer-to-trigger at tl when the P-CCPCH RSCPs of cell 1 and cell 2 fulfils equation 1. According to the prior art, the UE would report event I g at t2, when the timer-to-trigger expires. The RNC would accordingly begins to perform handovers in response to the I g event received from the UE. However, under some situations, for example when the communication link between UE and Cell 1 are obscured by some high building, the communication quality of cell 1 degrades rapidly after tl . The user of UE would thus endure a very poor communication quality during the time period between tl and t2, even be intterupted due to the rapid degradation.

Therefore it is needed to shorten the report delay when the communication quality degrade rapidly. Fig.3C is diagrammatic view showing the handover measurement mechanism in accordance with the present invention. An absolute threshold THa is introduced in the embodiment of Fig.3C. In addition to judging if the RSCPs of Cell 1 and 2 meet equation 1 and, if yes, waiting for the time period set by the "timer-to-trigger" parameter, the UE according to the present invention would also compare the P-CCPCH RSCP of the serving cell, Cell 1. with the absolute threshold. When the RSCP of the serving cell is below this absolute threshold, it means the communication quality between UE and the current serving cell is not sufficient to maintain a satisfying service. Then the UE would report the I g event to the RNC at t2' , rather than waiting until time point t2 determined by the "timer-to-trigger" parameter. The handover measurement mechanism according to the present invention shortens the measurement event I g report time in case the current serving cell signal degraded rapidly and reduce the possibility of call drop.

The RSCP value used here may be the one used in judging the satisfaction of the trigger criteria, therefore this method can be performed based on the current system resource of the UE and there is no need to perform additional measurements, thus improving the usage of the system resources. In addition, the absolute threshold may also be set by the RNC through the same "measurement control" message as the "timer-to-trigger" and hysteresis parameters. Beneficially, the absolute threshold can be varied from one cell to another cell or from one RAB type to another RAB type. That is, an individual threshold can be estabilished for each cell or RAB type.

Fig, 4 is a flow chart illustrating an improved handover measurement method according to one embodiment of the present invention. The method 400 start with step 402 in which the UE recieves a "measurement control" message. This message comprises a hysteresis parameter HI g, a "timer-to-trigger" parameter and an absolute threshold. At step 404, the UE initialize its internal components based on these parameters by setting a counter according to the value of "timer-to-trigger" parameter. Then it calculates the received signal code power Mcurr on the primary common control physical channel of cell 1 (step 406) and the received signal code power Mtar on the primary common control physical channel of cell 2 (step 408). At step 410, the UE determines if the relationship between these two received signal code powers satisfies equation 1 as mentioned above, i.e. if the received signal code power of the target cell Cell 2 is bigger than the received signal power of the current serving cell Cell 1 by the pretermined value HI g.

If the determination result in step 410 is positive, then the method proceeds to step 412, the counter begins to derement and counts the time period set by the "timer-to-trigger" parameter. In step 414, the UE dtetermines if the "timer-to-trigger" has expired since the relation between the received signal code powers of the two cells meets equation 1 by determining if the counter is bigger than 0. If the determination result in step 414 is negative, the method turns to step 418 to determine if the communication quality between the UE and the current serving cell has degraded too rapidly by comparing the received signal code power with the absolute threshold. If the received signal code power of the serving cell is above the absolute threshold, then the method goes to step 406 and repeats the steps 406-414.

However if the result from step 414 is negative, which means the "timer-to-trigger" has expired since equation 1 holds valid, or if the received signal code power of the current serving cell is below the absolute threshold to indicate that the communication quality on the current communicating channel is not enough for satisfying service, then the method goes to step 416.

At step 416, the UE will report the I g event to the RNC to initiate a handover evaluation. Then the method ends at step 420. If the determination result in step 410 is negative, the method also proceeds to step 420 and end the measurement on current target cell. Although the method is illustrated with an example of the current serving cell and one target cell, the skilled in the art may understand that there may be multiple cells evaluated in this precedure. The RSCPs of these multiple cells may be evaluated one by one to reduce the hardware complexity, or concurrently to reduce the measurement time, depending on the specific system requirements. In addition, it may be perferably that these multiple cells are sorted according to their received signal code powers or other communication quality parameters, before being evaluated against the received signal code power of the serving cell. The one with the highest received signal code power will get a highest priority to be evaluated first. In this way, the method will have higher probability to find the best cell, and to change to the best candidate cell when the communication quality on the current cell is not sufficient. While a series of steps has been described with regard to Fig. 4, the order of these acts may be modified in other implementations consistent with the principles of the invention. Further, non-dependent acts may be performed in parallel. For example, the step 41 8 of comparing the RSCP of the serving cell with the absolute threshold may be put in different position from step 406 to step 414 and can also achieve the object of reporting the I g event before the "timer-to-trigger" expires. The foregoing description of preferred embodiments of the invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed.

Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention.

It will be apparent to one of ordinary skill in the art that aspects of the invention, as described above, may be implemented in many different forms of software, firmware, and hardware in the implementations illustrated in the figures. The actual software code or specialized control hardware used to implement aspects consistent with the principles of the invention is not limiting of the invention. Thus, the operation and behavior of the aspects were described without reference to the specific software code — it being understood that one of ordinary skill in the art would be able to design software and control hardware to implement the aspects based on the description herein.

Further, certain portions of the invention may be implemented as "logic" that performs one or more functions. This logic may include hardware, such as hardwired logic, an application specific integrated circuit, a field programmable gate array, a microprocessor, software, or a combination of hardware and software.

It should be emphasized that the term "comprises/comprising" when used in this specification and/or claims is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

No element, act, or instruction used in the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article "a" is intended to include one or more items. Where only one item is intended, the term "one" or similar language is used.

Claims

1. A method for performing handover measurement comprising steps of: calculating received signal code powers (RSCPs) of a serving cell and a target cell(406,408); and determining if the RSCPs meet a trigger criteria (410); said method is characterized in that it further comprises step of comparing the RSCP of the serving cell with an absolute threshold(418).

2. A method as claimed in claim 1 , further comprising the step of sending a measurement report to RNC of the serving cell immediately if the RSCP of the serving cell is below the absolute threshold (416).

3. A method as claimed in claim 1 , wherein the trigger criteria is if the RSCP of the target cell is greater than the RSCP of the serving cell by a predetermined value.

4. A method as claimed in claim 3, further comprising steps of: starting a timer for a predetermined time period when the trigger criteria is satisfied (412); and sending the measurement report to the RNC of the serving cell after the predetermined time period lapsed (416).

5. A method as claimed in claim 4, wherein the predetermined time period, the predetermined value and the absolute threshold are set by the RNC through a measurement control message.

6. A method as claimed in claim 2, 4 or 5, wherein said measurement report comprises a Ig event to inform the RNC of the measurement result.

7. A device for performing handover measurement comprising: means for calculating received signal code powers (RSCPs) of a serving cell and a target cell (406,408); means for determining if the RSCPs meet a trigger criteria (410); and means for comparing the RSCP of the serving cell with an absolute threshold (418).

8. A device as claimed in claim 7, further comprising means for sending a measurement report to RNC of the serving cell immediately if the RSCP of the serving cell is below the absolute threshold (416).

9. A device as claimed in claim 7, wherein the trigger criteria is if the RSCP of the target cell is greater than the RSCP of the serving cell by a predetermined value.

10. A device as claimed in claim 9, further comprising: a timer for counting a predetermined time period when the trigger criteria is satisfied (412); and means for sending the measurement report to the RNC of the serving cell after the predetermined time period lapsed (416).

11. A device as claimed in claim 10, wherein the predetermined time period, the predetermined value and the absolute threshold are set by the RNC through a measurement control message.

12. A device as claimed in claim 8, 10 or 1 1 , wherein said measurement report comprises Ig event to inform the RNC.

13. A computer program product for carrying out the method according to one of claims 1 -6.