Window Parameter in Radio Heterogeneous

Communication Network Field of the invention

The present disclosure relates to a radio heterogeneous communication network and particularly to a method of optimizing a radio link monitoring window parameter in a radio heterogeneous communication network.

Background of the invention

Mobility improvements to an LTE Heterogeneous Network (HetNet) have been discussed in 3GPP. The handover of a user equipment moving at a high speed is a challenge to the HetNet. Among the mobility improvements to the RAN2 heterogeneous network, the Radio Link Failure (RLF) arising from mobility in the heterogeneous network is one of the important issues investigated in RAN2. As can be apparent from simulation, handover performance of the user equipment moving at a high speed deteriorates primarily due to a too small coverage area of a "low transmission-power node". Most of those handover failures eventually lead to a radio link failure. Due to the considerable overhead of a radio link failure (it will cause significant service interruption even if it may be recovered), it is desired to further reduce the probability that a radio link failure occurs and to reduce the total service interruption time if a radio link failure occurs.

The decision on an RLF for the user equipment highly depends upon the setting of a radio link monitoring window parameter. In a conventional macro cell system, the user equipment measures a Channel Quality Indicator (CQI) for monitoring a radio link. If the wideband CQI averaged over a long period is below a predetermined threshold, a radio link monitoring window (embodied as the parameter T30 in the existing standard) will be triggered while starting counting, and this indicates a poor condition of the radio link. Thus transmission of a measurement report and/or a handover command of the user equipment will be very likely to fail, thus resulting in a handover failure. The measurement report and the handover will be reattempted before T310 expires. If the averaged wideband CQI measurement result monitored over the radio link before T30 expires is above another predetermined threshold, the user equipment will stop T310 from counting and reset T310 to zero, and thus it is considered that the radio link has been resumed. If the measurement result monitored over the radio link is still below the another predetermined threshold until T310 expires, then the user equipment reports a radio link failure.

Indeed T310 is set to decide a radio link failure due to a blind spot of radio coverage. An eNB configures only one value of T310 for the user equipment as conventional setting of T310. However there is a complicated network condition in the heterogeneous network because a small cell, e.g., a pico cell and a femto cell, is introduced to alleviate the load of a macro cell or coverage of the macro cell is enhanced at the edge of the macro cell. Traditional setting of T310 is not applicable to various scenarios in the heterogeneous network.

Summary of the invention

In view of the foregoing, it will be advantageous if setting of a radio link monitoring window can be optimized for different possible scenarios in a heterogeneous network including a macro cell and a small cell.

According to a first aspect of the invention, there is provided a method of optimizing a radio link monitoring window parameter in a user equipment of a radio heterogeneous communication network, the radio heterogeneous communication network including a macro cell and a small cell, the method including the steps of: receiving a first radio link monitoring window parameter and a second radio link monitoring window parameter from an eNB of the macro cell, wherein the first parameter is below the second parameter; using the first parameter as the radio link monitoring window parameter; determining whether the user equipment is close to the small cell; and if a determination result is negative, then continuing the use of the first parameter as the radio link monitoring window parameter and determining again whether the user equipment is close to the small cell, or if the determination result is positive, then using the second parameter as the radio link monitoring window parameter.

Additionally if the user equipment is determined to be close to the small cell, then the method further includes: further determining whether the user equipment has been handed over to the small cell; and if a determination result is positive, then using a third parameter obtained from an eNB of the small cell as the radio link monitoring window parameter, wherein the third parameter is below the first parameter, or if the determination result is negative, then further determining whether the user equipment has left a coverage area of the small cell; and if a determination result is positive, then using the first parameter as the radio link monitoring window parameter and further determining whether the user equipment is close to the small cell, or if the determination result is negative, then continuing the use of the second parameter as the radio link monitoring window parameter.

In an embodiment of the invention, the first parameter and/or the second parameter are/is related to a motion speed of the user equipment. In another embodiment of the invention, the second parameter is related to a maximum service interruption time of a current service type of the user equipment. Additionally the third parameter is related to a motion speed of the user equipment and/or related to a maximum service interruption time and a radio link failure recovery time of a current service type of the user equipment.

According to a second aspect, there is provided a method of optimizing a radio link monitoring window parameter in a user equipment of a radio heterogeneous communication network, the radio heterogeneous communication network including a macro cell and a small cell, the method including the steps of: receiving a first radio link monitoring window parameter from an eNB of the macro cell as the radio link monitoring window parameter; and determining whether the user equipment is close to the small cell, and if a determination result is positive, then sending a first request message to the eNB of the macro cell to request the eNB of the macro cell to send a second radio link monitoring window parameter to the user equipment as the radio link monitoring window parameter, wherein the first parameter is below the second parameter.

According to a third aspect, there is provided a method of optimizing a radio link monitoring window parameter in an eNB of a small cell of a radio heterogeneous communication network, the radio heterogeneous communication network including a macro cell and the small cell, the method including the steps of: configuring a third radio link monitoring window parameter for a user equipment, wherein the third radio link monitoring window parameter is below a first radio link monitoring window parameter and a second radio link monitoring window parameter configured by an eNB of the macro cell for the user equipment; and sending the third radio link monitoring window parameter to the user equipment.

According to a fourth aspect, there is provided a method of optimizing a radio link monitoring window parameter in an eNB of a macro cell of a radio heterogeneous communication network, the radio heterogeneous communication network including the macro cell and a small cell, the method including the steps of: configuring a first radio link monitoring window parameter and a second radio link monitoring window parameter for a user equipment, wherein the first parameter is below the second parameter; and sending the first parameter and the second parameter to the user equipment.

According to a fifth aspect, there is provided a method of optimizing a radio link monitoring window parameter in an eNB of a macro cell of a radio heterogeneous communication network, the radio heterogeneous communication network including the macro cell and a small cell, the method including the steps of: configuring a first radio link monitoring window parameter for a user equipment; sending the first parameter to the user equipment as the radio link monitoring window parameter; configuring a second radio link monitoring window parameter for the user equipment upon reception of a first request message from the user equipment, wherein the first parameter is below the second parameter; and sending the second parameter to the user equipment as the radio link monitoring window parameter.

If ABS is not available in the small cell, then the user equipment connected to the macro cell will be subject to significant interference from the small cell when the user equipment passing the small cell is disallowed for a handover to the small cell or the handover fails. Thereafter a series of procedures have to be performed, for example, to report a failure of a radio link with the macro cell and to attempt to establish a connection with the small cell, but the user equipment will leave the small cell very soon and finally establish a connection again with the macro cell. Thus these procedures will waste considerable signalings. With the solution of the invention, the procedures can be avoided and signalings between the eNBs or between the eNB and the user equipment can be reduced.

With the solution of the invention, different time values of radio link invalidation can be set for different scenarios in a heterogeneous network to thereby improve mobility performance, including a lowered failure of a radio link failure and a shortened service interruption time and also a reduced signaling overhead for guaranteed mobility, avoidance of some unnecessary flows and reduced signalings between the eNBs or between the eNB and the user equipment.

The respective aspects of the invention will become apparent from the following description of particular embodiments. Brief description of drawings

Other features, objects and advantages of the invention will become more apparent upon review of the following detailed description of non-limiting embodiments with reference to the drawings in which:

Fig.l illustrates a network topology diagram of a radio heterogeneous communication network;

Fig.2 illustrates a flow chart of a method according to an embodiment of the invention;

Fig.3 illustrates a flow chart of a method according to another embodiment of the invention; and

Fig.4 illustrates a flow chart of a method according to still another embodiment of the invention.

Identical or like reference numerals denote identical or like devices (modules) or steps throughout the different figures in the drawings. Detailed description of embodiments

Reference will be made to the drawings which are a part of the invention in the following detailed description of preferred embodiments. By way of an example, the drawings illustrate particular embodiments in which the invention can be embodied. The example embodiments are not intended to exhaust all the embodiments of the invention. As can be appreciate, other embodiments can be employed and structural or logical modifications can be made without departing from the scope of the invention. Thus the following detailed description is not intended to be limiting, and the scope of the invention will be defined by the appended claims

Fig.l illustrates a network topology diagram of a radio heterogeneous communication network. In the embodiment illustrated in Fig.l , a small cell is embodied as a pico cell, but those skilled in the art can appreciate that the small cell can also be a femto cell.

As illustrated in Fig. l, the radio heterogeneous communication network 100 includes a macro cell 110, a pico cell 120 and a user equipment 130 connected to the macro cell 110. The pico cell 120 in the heterogeneous network is configured to alleviate the load of the macro cell 110, and it is possible to arrange the pico cell 120 at the edge of the macro cell 110 to thereby enhance the coverage of the macro cell 110. However simulation results show a higher probability that a radio link failure occurs while the user equipment moving at a high speed traverses the pico cell.

For example, in the embodiment illustrated in Fig. l , the user equipment 130 traverses the pico cell 120 while moving in the direction from A to D along the dotted line, where the point A represents the location where the user equipment 130 discovers the pico cell, the point B represents a location in the pico cell, the point C represents the edge of the pico cell, and the point D represents the location where the user equipment is connected again with the macro cell.

In embodiments of the invention, different radio link monitoring window parameters are adopted for the user equipment in different mobility scenarios in the heterogeneous network. A radio link monitoring window parameter will be represented as T310 in the following description. Different mobility scenarios will be introduced below for such a motion route of the user equipment.

Scenario 1 : Macro cell to macro cell

In a normal scenario, the user equipment is connected with the macro cell, and no handover or only a macro-to-macro handover is expected, and a normal value of T310 in a conventional macro cell system will be adopted.

Scenario 2: Macro cell to pico cell

(1) The case where the user equipment moving at high speed is disallowed for a handover to the pico cell

The user equipment 130 connected to the macro cell enters the pico cell, and at the point B, for example, it will suffer interference from the pico cell if there is no handover to the small cell, and the user equipment 130 will suffer an RLF and possibly trigger a reestablishment procedure. Firstly the user equipment will make cell selection to locate an appropriate cell. In this case, since the pico cell 120 has a strong signal, the pico cell 120 is very likely to be selected, and since the user equipment 130 is moving at a high speed, the user equipment is very likely to move to the edge of the pico cell 120, e.g., the point C, during a random access procedure. The use equipment 130 is disconnected from the macro cell 130, and a Radio Resource Control (RRC) connection establishment procedure will fail. Thereafter the user equipment 130 reenters the macro cell and is connected again with the macro cell at the point D.

Thus in an embodiment of the invention, the radio link monitoring window parameter is extended to a larger value in this case to thereby ensure the user equipment 130 to be kept connecting with the macro cell 110.

(2) The case where the user equipment moving at a high speed is allowed for a handover to the pico cell (2. a) The user equipment hands over in the pico cell successfully and hands over out of the pico cell unsuccessfully

If the user equipment can hand over in the pico cell and finish the handover, then the user equipment will release its connection with the macro cell. Thus the user equipment will not use T310 of the macro cell any longer and possibly receive T310 configured by the pico cell. When the user equipment hands over out from the pico cell to the macro cell, there is a very high probability of the handover failure as known from the simulation results. If the handover fails, then the user equipment is unlikely to resume its radio connection with the pico cell and thus has to re-access the macro cell again as quickly as possible. Thus in this case, T310 shall be set back to the short value after the user equipment is successfully handed over to the pico cell.

(2.b) The user equipment hands over in the pico cell unsuccessfully and thereafter leaves a coverage area of the pico cell.

In this case, the user equipment 130 moves at a high speed and is very likely to traverse the pico cell 120 in a very short time. When the user equipment 130 moves into the pico cell 120, e.g., to the point B, it will suffer interference from the pico cell 120 and thus fail to hand over into the pico cell 120.

In this case, in an embodiment of the invention, a longer value of T310 is adopted as the radio link monitoring window parameter, and if the user equipment attempts to hand over into the pico cell 120 but fails, then it is still possibly kept connected with the macro cell. Even if the radio connection with the macro cell is very poor due to interference of the macro cell, no RLF will occur before T310 expires. If the user equipment traverses the pico cell before T310 expires, then the radio connection with the macro cell will be resumed and T310 will be set to the normal value.

Scenario 3: Pico cell to macro cell

For this case, in an embodiment of the invention, T310 is set to a short value because the quality of the radio connection with the pico cell is very likely to deteriorate in a short time and unlikely to resume in a short time.

Scenario 4: Pico cell to pico cell

For this case, in an embodiment of the invention, T310 is set to a short value because the user equipment is unlikely to get back to the original serving pico cell in a short time. T310 shall be set the same as in the pico cell and the macro cell.

Fig.2 illustrates a flow chart of a method according to an embodiment of the invention.

In the step S201 of the method, the eNB of the macro cell 110 configures a first radio link monitoring window parameter and a second radio link monitoring window parameter for the user equipment 130, where the first parameter is below the second parameter.

In an embodiment of the invention, the first parameter and/or the second parameter is related to the motion speed of the user equipment. For example, the higher the motion speed of the user equipment is, the smaller the first parameter and/or the second parameter will be. In another embodiment of the invention, the second parameter is related to the maximum service interruption time of the current service type of the user equipment. For example, the second parameter is below the allowable maximum service interruption time.

In the step S201 of the method, the eNB of the macro cell 110 sends the first parameter and the second parameter to the user equipment 130.

In the step S203 of the method, the user equipment 130 uses the first parameter as a radio link monitoring window parameter. In the step S204 of the method, the user equipment 130 determines whether it is close to the pico cell, and if the determination result is negative, then the flow goes back to the step S203 of the method to continue the use of the first parameter as the radio link monitoring window parameter and performs the step S204 of the method again. If the determination result is positive, then the second parameter is used as the radio link monitoring window parameter.

Specifically in the step S203 of the method, the user equipment 130 determines whether it is close to the pico cell by determining whether the pico cell is identified successfully and a handover condition is satisfied.

In an embodiment of the invention, after the user equipment 130 determines that it is close to the pico cell 120, the second parameter is used as the radio link monitoring window parameter, and it is further determined in the step 206 of the method whether the user equipment 130 has been handed over to the pico cell 120. If the determination result is positive, then a third parameter obtained from the eNB of the pico cell is used as the radio link monitoring window parameter in the step S209 of the method, where the third parameter is below the first parameter. Here those skilled in the art can appreciate the eNB of the pico cell configures third radio link monitoring window parameter for the user equipment 130 in the step S207 of the method, where the third radio link monitoring window parameter is below the first radio link monitoring window parameter and the second radio link monitoring window parameter configured by the eNB of the macro cell for the user equipment. In the step S208 of the method, the eNB of the pico cell sends the third radio link monitoring window parameter to the use equipment.

In an embodiment of the invention, the third parameter is related to the motion speed of the user equipment and/or related to the maximum service interruption time and the radio link failure recovery time of the current service type of the user equipment. For example, the higher the motion speed of the user equipment is, the smaller the third parameter will be. In another example, the sum of the third parameter and the radio link failure recovery time is small than or equal to the allowable maximum service interruption time.

If the user equipment is not handed over to the pico cell 120, for example, the user equipment moving at a high speed is disallowed for a handover from the macro cell to the pico cell or the handover of the user equipment from the macro cell to the pico cell fails as described previously, then it is further determined in the step S210 of the method whether the user equipment has left the pico cell. If the determination result is positive, then the flow goes back to the step S203 of the method to continue the use of the first parameter as the radio link monitoring window parameter and further performs the step S204 of the method; or if the determination result is positive, then the use of the second parameter as the radio link monitoring window parameter is continued.

Specifically in the step S210 of the method, it is determined whether the channel quality indicator of the macro cell is above a predetermined threshold, and if the channel quality indicator is above the predetermined threshold, then it is determined that the user equipment has left the coverage area of the pico cell; or if the channel quality indicator is below the predetermined threshold, then it is determined that the user equipment has not left the coverage area of the pico cell.

Fig.3 illustrates a flow chart of a method according to another embodiment of the invention. In this embodiment of the invention, both the eNB of the pico cell and the eNB of the macro cell configure the foregoing three radio link monitoring window parameters for the user equipment. Thus it is not necessary to distinguish the configurations of the radio link monitoring window parameters by the sizes of the cells, so the system can be configured conveniently and the standardized implementation will be easy.

Specifically in the step S301 and the step S307 of the method, both the eNB of the macro cell and the eNB of the pico cell configure the foregoing three radio link monitoring window parameters for the user equipment, and in the step S302 and the step S308, the macro cell and the pico cell send the foregoing three radio link monitoring window parameters respectively to the user equipment. In the step S303 of the method, the user equipment 130 takes the smallest one of the three parameters as the third parameter, the largest one as the second parameter and the remaining one as the first parameter. The remaining steps of the method are similar to the description of the embodiment illustrated in Fig.2, and a repeated description thereof will be omitted here. For example, the user equipment 130 selects the smallest third parameter from the three parameters as a radio link monitoring window parameter after being handed over to the pico cell 120.

Those skilled in the art can appreciate that the eNB of the macro cell configuring and sending the third parameter for the user equipment and the eNB of the pico cell configuring the first parameter and the second parameter for the user equipment will not be necessary for an implementation of the invention.

Fig.4 illustrates a flow chart of a method according to still another embodiment of the invention.

The difference from the embodiment illustrated in Fig.2 is that the eNB of the macro cell and the eNB of the pico cell reconfigure a radio link monitoring window parameter for the user equipment in response to a request of the user equipment instead of initially sending all the available radio link monitoring window parameters to the user equipment.

Specifically the eNB of the macro cell configures only a first parameter for the user equipment in the step S401 of the method and sends the first parameter to the user equipment 130 in the step S402 of the method. If the user equipment determines that the user equipment 103 is close to the pico cell 120 in the step S404 of the method, then it sends a first request message to the eNB of the macro cell in the step S405 of the method. The eNB of the macro cell configures a second parameter for the user equipment in the step S406 of the method upon reception of the first request message, where the first parameter is below the second parameter, and thereafter sends the second parameter to the user equipment in the step S407 of the method.

Those skilled in the art can appreciate that if the user equipment 130 detects that the user equipment is not handed over to the pico cell in the step S409 of the method, then the user equipment 130 further determines whether the user equipment has left the pico cell similarly to Fig.2. If the determination result is positive, then it sends a second request message to the eNB of the macro cell to request the eNB of the macro cell to reconfigure the radio link monitoring window parameter for the user equipment 130. The eNB of the macro cell sends the first parameter to the user equipment 130 upon reception of the second request message. The user equipment 130 goes back to the step S403 of the method and further performs the step S404. If it is determined that the user equipment has not left the pico cell, then the use of the second parameter as the radio link monitoring window parameter is continued. A repeated description of the steps in the embodiment illustrated in Fig.3 similar to those in Fig.2 will be omitted here.

With the solution of the invention, different time values of radio link invalidation can be set for different scenarios in a heterogeneous network to thereby improve mobility performance, including a lowered failure of a radio link failure and a shortened service interruption time and also a reduced signaling overhead for guaranteed mobility, avoidance of some unnecessary flows and reduced signaling between the eNBs or between the eNB and the user equipment.

Those skilled in the art shall appreciate that the invention apparently will not be limited to the details of the foregoing exemplary embodiments and can be embodied in other specific forms without departing from the spirit or essence of the invention. Accordingly the embodiments shall be construed anyway to be exemplary and non-limiting. Moreover apparently the term "comprising" will not preclude another element(s) or step(s), and the term "a" or "an" will not preclude plural. A plurality of elements stated in an apparatus claim can alternatively be embodied as a single element. The terms "first", "second", etc., are intended to designate a name but not to suggest any specific order.