WO2015079479A1

WO2015079479A1 - Method and apparatus for saving energy in base station with multiple antennas

Info

Publication number: WO2015079479A1
Application number: PCT/JP2013/007038
Authority: WO
Inventors: Boonsarn Pitakdumrongkija; Naoto Ishii
Original assignee: Nec Corporation
Priority date: 2013-11-29
Filing date: 2013-11-29
Publication date: 2015-06-04

Abstract

A method and device for saving energy while maintaining throughput in a geographical area served by a base station employing multiple antennas. A mobile communication system comprising at least one base station that can divide one geographical area into multiple cells by using an antenna array formed by multiple antennas, wherein, in the following order, at the said base station, determines new numbers of the said cells and the said antennas based on information indicating traffic load at current numbers of the said cells and the said antennas. When the determined new numbers of the said cells and the said antennas are different from the current numbers, at the said base station, determines and notifies UEs connecting to the current number of the said cells about parameters used in cell selection decision based on the determined new numbers of the said cells and the said antennas. Finally, at the said base station, changes the current numbers of the said cells and the said antennas to the determined new numbers.

Description

METHOD AND APPARATUS FOR SAVING ENERGY IN BASE STATION WITH MULTIPLE ANTENNAS

The present invention relates to a base station employing multiple antennas in a mobile communication system, and more particularly to a technique of saving energy while maintaining throughput in the geographical area served by the base station.

Background

In recent years, mobile traffic has been growing at an increasing rate due to wider use of smartphones and tablet PC (Personal Computer). This trend is expected to continue also in the future because of lower prices of smartphones and tablet PCs attracting more users to switch from conventional mobile phones. In order to cope with the growth in mobile traffic, mobile operators have to increase the capacity of their network.

One of the effective ways to increase the capacity is to divide one geographical area into multiple cells by using beamforming capability of a base station (BS) equipping with multiple antennas. Here, a cell is an area created by using multiple antennas of the BS to beamform radio wave energy to a certain direction, in order to provide radio accesses to users. Fig. 1 shows a BS dividing one geographical area into 4 cells by using an array of 10 antennas. By dividing one geographical area into multiple cells, the same radio resource in time and frequency domains can be simultaneously used by users locating in different cells to access the mobile communication network, and thus the capacity increases.

In more details, Fig. 2 shows a block diagram of a BS employing multiple antennas that can divide a geographical area into multiple cells. The BS comprises multiple schedulers. Each of the schedulers controls transmission of Cell-specific Reference Signal (CRS) and user data of an individual cell. The transmission signal of each individual cell is then mapped to the same antenna array by multiplying with a specific set of beamforming weights, in order to steer the radio wave energy to a specific direction. The antenna array also comprises Power Amplifier (PA) that amplifies the signal power.

When there is sufficiently large volume of traffic, the division of one geographical area into many multiple cells, as described previously, is cost-effective. This is because the creation of multiple cells incurs cost that is mostly attributed to power consumptions in the PAs and the schedulers, which usually reside in the baseband processing units of the BS. Therefore, larger number of cells incurs larger power consumption and cost.

In reality, the volume of traffic in the same geographical area usually varies with respect to time in a day due to movements and activities of the users. Therefore, during a low traffic period, creating large number of cells that exceeds the traffic demand is a waste of power consumption.

PTL1 and PTL2 are conventional arts for solving this problem. PTL1 discloses the procedures for reducing the number of active cells when the volume of the traffic becomes less than the threshold. The reduction of the number of active cells is achieved by turning off certain baseband processing units, PAs, and antennas associating with the cells to be deactivated, and re-configuring the remaining components to generate smaller number of cells that can provide a similar angular coverage to the state before reducing the number of active cells. The re-configuration of the remaining components includes adjusting beamforming weights of the remaining number of active antennas.

PTL2 also describes similar procedures as PTL1, with some additional procedures. After the numbers of active cells and antennas are reduced, and the new smaller number of cells is adjusted to provide sufficient angular coverage, the vertical tilt of the antenna array is adjusted so that the radio wave can propagate farther than before adjusting the tilt.

[PTL 1]US2011/0096687A1
[PTL 2]WO2009/011640A2

Although PTL1 can save energy when the traffic is low, the deactivation of the baseband processing units, PAs, and antennas causes the received CRS strength from the energy-saving BS as seen by the user equipments (UEs) to be weaker than before reducing the number of active cells and antennas. This is attributed to the reduction in beamforming gain. Also, this is attributed to he reduction in total transmit power per cell when there is a design constraint to maintain the same signal power per each multiplicative branch of beamforming weight regardless of the number of active antennas. This weaker CRS strength from the energy-saving BS can cause the currently connecting UEs to see an adjacent BS as a better radio access point, and therefore the UEs perform hand-over (HO) procedures to the adjacent BS. This will increase load to the adjacent BS and at the same time, under-utilize radio resource in the energy-saving BS. When the increased load to the adjacent BS exceeds the amount of radio resource available in the adjacent BS, the UEs handed over from the energy-saving BS will experience poorer service qualities and throughputs. Therefore, PTL1 causes the problem of unable to maintain throughput in the same geographical area before and after reducing numbers of active cells and antennas.

On the other hand, PTL2 further adjusts vertical tilt of the antenna array of the energy-saving BS. Although this results in the UEs seeing stronger CRS strength from the energy-saving BS than in the case of PTL1, it cannot completely neutralize the effects of reduced beamforming gain and reduced total transmit power per cell. This is because the additional CRS strength gain from adjusting vertical tilt is limited. Therefore, PTL2 also causes the same problem as PTL1.

The present invention has been accomplished in consideration of the above mentioned problem, and the objective thereof is, to provide method and apparatus that can realize energy saving in the BS with multiple antennas, while maintaining throughput in the same geographical area before and after the energy saving is executed at the BS.

The first aspect of the proposed invention for solving the above-mentioned problems, which is a method in a mobile communication system comprising at least one base station that can divide one geographical area into multiple cells by using an antenna array formed by multiple antennas, comprising: the said base station determining new numbers of the said cells and the said antennas based on information indicating traffic load at current numbers of the said cells and the said antennas, the said base station notifying user equipments connecting to the said current number of the said cells about parameters used in cell selection decision which are determined based on the said determined new numbers of the said cells and the said antennas, and the said base station changing the said current numbers of the said cells and the said antennas to the said determined new numbers.

The second aspect of the proposed invention for solving the above-mentioned problems, which is a base station in mobile communication system comprising at least one base station that can divide one geographical area into multiple cells by using an antenna array formed by multiple antennas, wherein the said base station comprising: a cell configuration determination unit to determine new numbers of the said cells and the said antennas based on information indicating traffic load at current numbers of the said cells and the said antennas, a cell selection control unit to notify user equipments connecting to the said current number of the said cells about parameters used in cell selection decision which are determined based on the said determined new numbers of the said cells and the said antennas, and a cell configuration execution unit to change the said current numbers of the said cells and the said antennas to the said determined new numbers.

As described above, according to several aspects of the proposed solution, it is possible to realize energy saving in the BS with multiple antennas when the traffic is sufficiently low, while maintaining throughput in the same geographical area before and after the energy saving is executed at the BS.

Fig. 1 is an example of a mobile communication system employing a conventional BS with multiple antennas. Fig. 2 is an example block diagram of a conventional BS with multiple antennas. Fig. 3 is an example of a mobile communication system which is used in common for all illustrative embodiments of the present invention. Fig. 4 is an example block diagram of an Energy-saving BS which is used in common for all illustrative embodiments of the present invention. Fig. 5 is an example block diagram of an Adjacent BS which is used in common for all illustrative embodiments of the present invention. Fig. 6 is an example block diagram of a UE which is used in common for all illustrative embodiments of the present invention. Fig. 7 is an example sequence diagram showing operations of the overall system according to the first illustrative embodiment. Fig. 8 is an example flow chart showing operations of the Energy-saving BS according to the first illustrative embodiment. Fig. 9 is an example of predefined possible configurations of active cells and antennas according to the first illustrative embodiment. Fig. 10 is an illustrative explanation about the function of A3 offset value according to the first illustrative embodiment. Fig. 11 is an example flow chart showing operations of the UE type A according to the first illustrative embodiment. Fig. 12 is an example flow chart showing operations of the UE type B according to the first illustrative embodiment. Fig. 13 is an illustrative example when the operations of the first illustrative embodiment are applied to a certain mobile communication system. Fig. 14 is an example sequence diagram showing operations of the overall system according to the second illustrative embodiment. Fig. 15 is an example flow chart showing operations of the Energy-saving BS according to the second illustrative embodiment. Fig. 16 is an example flow chart showing operations of the UE type B according to the second illustrative embodiment. Fig. 17 is an example sequence diagram showing operations of the overall system according to the third illustrative embodiment. Fig. 18 is an example flow chart showing operations of the Energy-saving BS according to the third illustrative embodiment. Fig. 19 is an example flow chart showing operations of the Adjacent BS according to the third illustrative embodiment. Fig. 20 is an illustrative explanation about the function of CIO offset value according to the third illustrative embodiment. Fig. 21 is an example flow chart showing operations of the UE of the Adjacent BS according to the third illustrative embodiment.

In the following, examples and embodiments of the present invention are described with reference to the drawings. For illustrating the present invention, an example of an application in a system according to 3GPP (3rd Generation Partnership Project) specifications for LTE (Long Term Evolution) is described. However, it is to be noted that embodiments of the present invention are not limited to an application in such a system or environment but are also applicable in other network systems, for example in networks according to other 3GPP specifications. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably
arranged network system.

First, a mobile communication system and devices which are used in common for illustrative embodiments of the present invention will be explained by making references to Fig. 3 to 6.

Fig. 3 shows an example mobile communication system that comprises a BS with multiple antennas of the present invention, hereafter denoted as Energy-saving BS (10), and an Adjacent BS (20). The Energy-saving BS (10) and the Adjacent BS (20) have accesses to Core Network (CN) (30) through BS-CN interfaces (50), which are equivalent to S1 interfaces in 3GPP LTE specifications. Also, the Energy-saving BS (10) and the Adjacent BS (20) can exchange information between each other through BS-BS interface (40), which is equivalent to X2 interface in 3GPP LTE specifications. The Energy-saving BS (10) provides radio access coverage in the Geographical area of the energy-saving BS (11) by using ten Antennas (101) at maximum to divide such geographical area into four cells; Cell-1 (60-1), Cell-2 (60-2), Cell-3 (60-3), and Cell-4 (60-4). UEs (70) that locate in those four cells select the corresponding cells for accessing the CN (30) through the Energy-saving BS (10). On the other hand, the Adjacent BS (20) provides radio access coverage in the Geographical area of the adjacent BS (21) by using a single Antenna (201) to create a single cell; Cell-5 (60-5). UEs (70) that locate in that single cell select the corresponding cell for accessing the CN (30) through the Adjacent BS (20).

Fig. 4 shows an example block diagram of the Energy-saving BS (10). In specific, Fig. 4 only shows parts that relate to radio transmission. Details of parts relating to radio reception are omitted here because they are obvious to a person skilled in the related art. Data targeting UEs (70) flow from the CN (30) into Network interface section (107). The Network interface section (107) then multiplexes the data into their intending cells, by inputting them in the corresponding Transmission data processors (105). Here, with reference to Fig. 3, it is assumed that there is maximum number of four cells, therefore there are Transmission data processors of Cell-1 (105-1) to Cell-4 (105-4). The Transmission data processor (105) operates according to instructions from Scheduler (104) belonging to the same cell. In specific, the Scheduler (104) first decides which data of which UEs to be transmitted using which time-frequency radio resource and what modulation-coding format, then instructs the Transmission data processor (105) to prepare the data accordingly. The Scheduler (104) also controls the multiplexing of time-frequency radio resource for transmitting data of UEs and CRS (106), which will be denoted collectively hereafter as transmission signal. The transmission signal of an individual cell is then mapped to multiple Antennas (101) by multiplying with the corresponding Beamforming weights (103). Here, with reference to Fig. 3, it is assumed that there is maximum number of ten antennas; from Antenna-1 (101-1) to Antenna-N=10 (101-N=10). The multiplicative results of transmission signals with beamforming weights of multiple cells targeting the same Antenna (101) are then combined and input into the corresponding PA (102) to amplifier the power, and transmit to the UEs (70).

According to the present invention, during a predefined period, Cell configuration determination unit (108) uses information from the Schedulers (104) and Beamforming weights (103) to first determines traffic load at current numbers of active cells and antennas. The traffic load can be a radio resource usage ratio, which the details of its determination will be shown later in this description. Then, the Cell configuration determination unit (108) determines new numbers of active cells and antennas, based on the determined traffic load at the current numbers of active cells and antennas in comparison to a predefined value of traffic load. The new numbers of active cells and antennas are determined to satisfy the condition that sums of cell throughputs in the Geographical area of the energy-saving BS (11) when using the current and the new numbers of active cells and antennas are equal. The details of such determination will be shown later in this description. The Cell configuration determination unit (108) then triggers the following operations in respective order at Cell selection control unit (109) and Cell configuration execution unit (110) when the determined new numbers of active cells and antennas are different from the current numbers.

The Cell selection control unit (109), after receiving the trigger from the Cell configuration determination unit (108), first determines parameters used in cell selection decision at the UEs (70) based on the determined new numbers of active cells and antennas. The parameters used in cell selection decision at the UEs (70) can be an A3 offset value according to 3GPP LTE specifications. The parameters are determined to compensate a change in Reference-Signal Received Power (RSRP) measurement result at the UEs (70) that would result from changing the current numbers of active cells and antennas to the new numbers. The details of such determination will be shown later in this description. Then, the Cell selection control unit (109) instructs the Schedulers (104) belonging to cells that would be active after the determined new numbers of active cells and antennas are applied to transmit the determined parameters to the UEs (70). The transmission of the determined parameters can be through System Information Block (SIB) and RRC (Radio Resource Control) Signaling according to 3GPP LTE specifications.

The Cell configuration execution unit (110), after the Cell selection control unit (109) transmitting the determined parameters used in cell selection decision to the UEs (70), changes the current numbers of active cells and antennas to the determined new numbers. In specific, the Cell configuration execution unit (110) deactivates or activates the Schedulers (104) and PAs (102) of the corresponding cells according to the determined new numbers of active cells and antennas. Also, the Cell configuration execution unit (110) applies the corresponding Beamforming weights (103) and instructs the Network interface section (107) to multiplex data from the CN (30) according to the determined new numbers of active cells and antennas.

In addition, according to another aspect of the present invention, the Cell selection control unit (109), before the Cell configuration execution unit (110) changing the current numbers of active cells and antennas to the determined new numbers, instructs the Schedulers (104) to transmit RRC Signaling to the UEs (70) for triggering HO procedures, so that all the UEs (70) would preemptively establish connections with the cells that are currently active and would continue to be active after changing the current numbers of active cells and antennas to the determined new numbers.

Furthermore, according to one another aspect of the present invention, the Cell selection control unit (109), after the Cell configuration execution unit (110) changing the current numbers of active cells and antennas to the determined new numbers, instructs the Network interface section (107) to notify the Adjacent BS (20) about the determined parameters used in cell selection decision at the UEs (70).

Fig. 5 shows an example block diagram of the Adjacent BS (20). In specific, Fig. 5 only shows parts that relate to radio transmission. Details of parts relating to radio reception are omitted here because they are obvious to a person skilled in the related art. Similar to the Energy-saving BS (10), Network interface section (206) of the Adjacent BS (20) receives data targeting UEs (70) from the CN (30) and then inputs the data to the intending cell through Transmission data processor (204). Here, with reference to Fig. 3, it is assumed that the Adjacent BS (20) creates only a single cell which is Cell-5 (60-5), therefore there is only Transmission data processor of Cell-5 (204-5). Also similar to the Energy-saving BS (10), Scheduler (203) first decides which data of which UEs to be transmitted using which time-frequency radio resource and what modulation-coding format, then instructs the Transmission data processor (204) to prepare the data accordingly. The Scheduler (203) also controls the multiplexing of time-frequency radio resource for transmitting data of UEs and CRS (205). The multiplexed data of UEs and CRS are then input into PA (202) to amplifier the power and then are transmitted to the UEs (70) through Antenna (201). Here, with reference to Fig. 3, it is assumed that there is only a single antenna at the Adjacent BS (20).

According to one aspect of the present invention, Inbound cell selection parameter processing unit (207) receives the determined parameters used in cell selection decision at the UEs (70) from the Energy-saving BS (10) through the Network interface section (206). Upon reception, the Inbound cell selection parameter processing unit (207) determines relative parameters indicating relationship between the Adjacent BS (20) and the Energy-saving BS (10) for using in cell selection decision at the UEs connecting to the Adjacent BS (20). The relative parameters can be a CIO offset value according to 3GPP LTE specifications, which the details of its determination will be shown later in this description. Then, the Inbound cell selection parameter processing unit (207) instructs the Scheduler (203) to transmit the determined relative parameters to the UEs connecting to the Adjacent BS (20). The transmission of the determined relative parameters can be through RRC Signaling according to 3GPP LTE specifications.

Fig. 6 shows an example block diagram of the UE (70). In specific, Fig. 6 only shows parts that relate to radio reception. Details of parts relating to radio transmission are omitted here because they are obvious to a person skilled in the related art. The multiplexed data targeting UE and CRS are arriving through Antenna (701) from either the Energy-saving BS (10) or the Adjacent BS (20). Upon arriving, the multiplexed data targeting UE and CRS are input into Low-Noise Amplifier (LNA) (702) to amplify the weak signal and reduce effect of noise in subsequent processes. Then, Reception controller (703) de-multiplexes the data targeting UE and CRS, and input them into Reception data processor (704) and RSRP measurement unit (705) respectively. The Reception data processor (704) performs demodulation-decoding in order to extract the original data. After the original data are obtained, the Reception controller (703) decides an appropriate action depending on the data type, for example when the data type is information intending for user, the Reception controller (703) stores such data in a user data reception buffer. On the other hand, when the data type is control signaling, the Reception controller (703) performs a specific action according to the control signaling. The RSRP measurement unit (705) uses the received CRS to calculate the RSRP. In specific, the RSRP measurement unit (705) can receive CRS of more than one cell not limiting only to the one from the serving cell, and can calculate the corresponding RSRP of each individual cell. Then, the calculated RSRPs are input into Cell selection evaluator (706) to evaluate condition to trigger cell selection or HO. When the condition is satisfied to perform cell selection or HO, the Cell selection evaluator (706) triggers Cell selection executor (707) to start such procedures.

According to the present invention, under the assumption that the UE (70) is connecting to the Energy-saving BS (10), the Reception controller (703) receives the determined parameter used in cell selection decision from the Energy-saving BS (10) through the Reception data processor (704). Upon reception of the determined parameter used in cell selection decision which is a type of control signaling, the Reception controller (703) instructs the Cell selection evaluator (706) to take into consideration the received parameter in evaluating condition to trigger cell selection or HO.

In addition, according to another aspect of the present invention, under the assumption that the UE (70) is connecting to the Adjacent BS (20), the Reception controller (703) receives from the Adjacent BS (20) the determined relative parameter indicating relationship between the Adjacent BS (20) and the Energy-saving BS (10) for using in cell selection decision through the Reception data processor (704). Upon reception of the determined relative parameter which is a type of control signaling, the Reception controller (703) instructs the Cell selection evaluator (706) to take into consideration the received relative parameter in evaluating condition to trigger cell selection or HO.

In the following, based on the previously described common mobile communication system and devices according to Fig. 3 to 6, the details specific to three illustrative embodiments of the present invention will be described in respective order.
(1. First illustrative embodiment)

According to the first illustrative embodiment, in summary, the Energy-saving BS (10) first determines the new numbers of active cells and antennas based on the traffic load during the predefined period at the current numbers of active cells and antennas. The new numbers of active cells and antennas are determined to satisfy the condition that sums of cell throughputs in the Geographical area of the energy-saving BS (11) when using the current and the new numbers of active cells and antennas are equal. When the determined new numbers of active cells and antennas are different from the current numbers, the Energy-saving BS (10) determines and notifies the UEs (70) connecting to the current number of active cells about the parameters used in cell selection decision at the UEs. The parameters are determined to compensate the change in RSRP measurement result at the UEs (70) that would result from changing the current numbers of active cells and antennas to the new numbers. Finally, the Energy-saving BS (10) changes the current numbers of active cells and antennas to the determined new numbers.

In the following, more details of the first illustrative embodiment will be described by making additional reference to Fig. 7 to 13. In specific, operations for the overall system comprising both the Energy-saving BS (10) and UEs (70), and operations specific to the individual components will be described first. Then, a specific example that employs such operations will be given, in order to illustrate an advantageous effect of the present invention.

(1.1 System operation)

Fig. 7 shows the operations for the overall system comprising both the Energy-saving BS (10) and the UEs (70) connecting to the Energy-saving BS. Here, the UEs are divided into two types due to their different operations, which will become clear later in the explanations. UE type A is a UE that initially connects to a cell that would still be active if a change happens to a cell configuration later. On the other hand, UE type B is a UE that initially connects to a cell that would not be active if a change happens to a cell configuration later.

At the beginning, the Energy-saving BS (10) transmits data to both UEs type A and B according to their initially connecting cells (operation S1101). After a predefined period, the Cell configuration determination unit (108) of the Energy-saving BS (10) calculates the radio resource usage ratio of all the currently active cells (operation S1102). The details of this calculation will be described in more details when explaining about individual operations of the Energy-saving BS. Then, based on the calculated radio resource usage ratio at the current numbers of active cells and antennas, the Cell configuration determination unit (108) of the Energy-saving BS (10) determines the new numbers of active cells and antennas, which produce the same sum of cell throughputs in the same geographical area as the current numbers (operation S1103). The details of this determination will be described in more details when explaining about individual operations of the Energy-saving BS. When the determined new numbers of active cells and antennas are the same as the current numbers, the Energy-saving BS (10) will not change numbers of active cells and antennas, and continue to transmit data to UEs type A and B through their initially connecting cells. On the other hand, when the determined new numbers of active cells and antennas are different from the current numbers, the Cell configuration determination unit (108) of the Energy-saving BS (10) will trigger the following operations denoted as operation S1104 to S1112 in the figure.

The Cell selection control unit (109) of the Energy-saving BS (10) determines the A3 offset values of to-be-active cells, that compensate change to RSRP observed by UE after the determined new numbers of active cells and antennas are applied (operation S1104). The details of this determination will be described in more details when explaining about individual operations of the Energy-saving BS. Then, the determined A3 offset values are notified to UE type A through RRC signaling (operation S1105). On the other hand, the UE type B receives A3 offset values through SIB (operation S1106). The UE type A can immediately apply the A3 offset values in evaluating the HO condition after it receives the notified A3 offset values (operation S1107).

Next, the Cell configuration execution unit (110) of the Energy-saving BS (10) changes the current numbers of active cells and antennas to the determined new numbers (operation S1108). When this happens, the UE type A can remain connect to the same cells before the change due to its prior consideration of the notified A3 offset values through RRC signaling. On the other hand, the UE type B will enter RRC_IDLE state because the former connecting cells suddenly disappear (operation S1109). After enter the RRC_IDLE state, the UE type B will search for new active cells and incorporate the A3 offset values received through SIB in evaluating cell selection condition (operation S1110). As a result, the UE type B will eventually establish connection with the new active cells by performing RRC connection setup (operation S1111). After this is completed, both UEs type A and B can receive data from the Energy-saving BS through the determined new active cells (operation S1112).

(1.2 Energy-saving BS operation)

Fig. 8 shows the operations of the Energy-saving BS (10). The Cell configuration determination unit (108) of the Energy-saving BS (10) regularly checks whether it is time to determine new numbers of active cells and antennas (operation S1201). The time to check can be predefined to be periodic with the period specified by the network operator. If it is not the time to determine the new numbers of active cells and antennas ("No" branch of operation S1201), the Cell configuration determination unit (108) of the Energy-saving BS (10) continues collecting information about radio resource usage of all currently active cells and stores it in a memory (operation S1202). On the other hand, if it is time to determine the new numbers of active cells and antennas ("Yes" branch of operation S1201), the Cell configuration determination unit (108) of the Energy-saving BS (10) determines the radio resource usage ratio of all currently active cells by reading from the memory collecting the information about the radio resource usage (operation S1203). The radio resource usage ratio of all currently active cells can be calculated as shown below.

...(Eq. 1)

The radio resource usage ratio of all currently active cells (Rcurrent) according to (Eq. 1) is then used for determining the new numbers of active cells and antennas. In specific, the Cell configuration determination unit (108) of the Energy-saving BS (10) determines the new numbers of active cells and antennas that produce the same sum of cell throughputs in the same geographical area as the current numbers (operation S1204). The new numbers of active cells and antennas can be chosen from a set of predefined possible configurations of active cells and antennas, for example, as shown in Fig. 9. Here, Fig. 9 shows that when the number of active antennas is determined to be 10, the Energy-saving BS (10) will create 4 active cells labeled Cell-1, Cell-2, Cell-3, and Cell-4. On the other hand, when the number of active antennas is determined to be 5, the Energy-saving BS (10) will create 2 active cells labeled Cell-1 and Cell-3, which provide the same angular coverage as when there are 4 active cells. Now, in order to determine which configuration in Fig. 9 would satisfy the condition that the sums of cell throughputs of the current and new numbers of active cells and antennas are equal, the following equations can be used.

(i) When there is a design constraint at the Energy-saving BS to maintain the same signal power per each multiplicative branch of beamforming weight regardless of the number of active antennas,

...(Eq.2)

(ii) When there is a design constraint at the Energy-saving BS to maintain the same sum of signal powers from multiplicative branches of beamforming weights corresponding to number of active antennas,

...(Eq. 3)

The desired radio resource usage ratio R_desired in (Eq. 2) and (Eq. 3) can be a predefined value by the operator, having a value between 0.0 and 1.0. Also, the average SINR at UE

in (Eq. 2) and (Eq. 3) can be readily calculated from the Channel-Quality Indicator (CQI) fed back by the UE to the Energy-saving BS during the data transmission process.

After the Cell configuration determination unit (108) of the Energy-saving BS (10) determining the new numbers of active cells and antennas, it makes a comparison with the current numbers (operation S1205). If the determined new numbers are the same as the current numbers ("Yes" branch of operation S1205), the Energy-saving BS (10) will not change numbers of active cells and antennas, and continue to transmit data to UEs with the current numbers of active cells and antennas. On the other hand, if the determined new numbers are different from the current numbers ("No" branch of operation S1205), the Cell configuration determination unit (108) will trigger the Cell selection control unit (109) to determine the A3 offset values of to-be-active cells, in order to compensate change to RSRP observed by UEs when the new determined numbers of active cells and antennas are applied (operation S1206). The illustrative explanation about the function of the determined A3 offset value is shown in Fig. 10. Here, when the Energy-saving BS decides to deactivate some number of antennas, the connecting UEs would observe lower RSRP from the serving-cell. By adding the A3 offset value that is determined to compensate the amount of serving-cell RSRP degradation, the UEs can use that offset RSRP value in evaluating the HO condition and as a result, maintain connection with the Energy-saving BS. In specific, the determination of A3 offset value can use the following equations.

...(Eq. 4)

...(Eq. 5)

After the Cell selection control unit (109) of the Energy-saving BS (10) determining the A3 offset values of to-be-active cells, it transmits those values to the connecting UEs through SIB and RRC signaling (operation S1207). Finally, the Cell configuration execution unit (110) of the Energy-saving BS (10) changes the current numbers of active cells and antennas to the determined new numbers by activating or de-activating cells based on the determined configuration (operation S1208).

(1.3 UE operation)

As previously explained during the description of the system operation, there are 2 types of UE. UE type A is a UE that initially connects to a cell that would still be active if a change happens to a cell configuration later. On the other hand, UE type B is a UE that initially connects to a cell that would not be active if a change happens to a cell configuration later. In the following, operations of different UE types will be explained.

Fig. 11 shows the operations of the UE type A. The Reception controller (703) of the UE (70) regularly checks whether the RRC signaling indicating the A3 offset value is received from the Energy-saving BS (10) (operation S1301). If the A3 offset value is received ("Yes" branch of operation S1301), the Reception controller (703) instructs the Cell selection evaluator (706) to update the stored A3 offset value to the latest one (operation S1302). On the other hand, if the A3 offset value is not received ("No" branch of operation S1301), the Reception controller (703) will not update the stored A3 offset value in the Cell selection evaluator (706). Next, the RSRP measurement unit (705) calculates RSRPs of all the detectable cells including the serving-cell and non-serving cells by using their corresponding CRSs (operation S1303). Then, the calculated RSRPs are input into the Cell selection evaluator (706), which will add the stored A3 offset value to the RSRP of the serving-cell (operation S1304). The Cell selection evaluator (706) makes a comparison between the RSRPs of the non-serving cells and the RSRP of the serving cell pluses the stored A3 offset value (operation S1305). If the RSRP of the serving cell pluses the stored A3 offset value is greater than the RSRPs of the non-serving cells ("Yes" branch of operation S1305), the Cell selection evaluator (706) will not trigger the HO procedures and the UE maintains connection with the current cell. On the other hand, if the RSRP of the serving cell pluses the stored A3 offset value is smaller than the RSRPs of the non-serving cells ("No" branch of operation S1305), the Cell selection evaluator (706) triggers the Cell selection executor (707) to initiate the HO procedures to the non-serving cell (operation S1306).

Fig. 12 shows the operations of the UE type B. The Reception controller (703) of the UE (70) regularly checks whether the UE has entered RRC_IDLE state (operation S1401). If the UE has not entered RRC_IDLE state ("No" branch of operation S1401), the Reception controller (703) continues to receive user data and CRS from the connecting cell. On the other hand, if the UE has entered RRC_IDLE state ("Yes" branch of operation S1401), the Reception controller (703) initiates the cell search procedures in order to synchronize with the nearby available cells (operation S1402). After the synchronization with the nearby available cells is completed, the Reception controller (703) reads the SIBs of the detectable cells and extracts the A3 offset value information from the cell that broadcasts it (operation S1403). Then, the Reception controller (703) stores the received A3 offset value of the cell that broadcasts it in the Cell selection evaluator (706) (operation S1404). The RSRP measurement unit (705) calculates the RSRPs of the detectable cells by using their corresponding CRSs and inputs the RSRPs into the Cell selection evaluator (706) to be added with the corresponding stored A3 offset value (operation S1405). Finally, the Cell selection evaluator (706) selects a cell with maximum RSRP pluses A3 offset value and triggers the Cell selection executor (707) to initiate RRC connection setup procedures with that cell (operation S1406).

(1.4 Example)

In order to clearly demonstrate the advantageous effect of the first illustrative embodiment, a simple example that utilizes the previously described operations to de-activate certain numbers of active cells and antennas will be used as shown in Fig. 13.

Initially, the Energy-saving BS (10) divides the Geographical area of the Energy-saving BS (11) into 4 cells (Cell-1, Cell-2, Cell-3, and Cell-4) by using 10 antennas. The UEs (70) can access the CN (30) through their corresponding cells.

At Step-1, the radio resource usage ratio of all currently active cells is calculated by using operation S1203 described previously, and is found to have the value of X.

At Step-2, using operation S1204 and predefined possible configurations of active cells and antennas as shown in Fig. 9, the radio resource usage ratio of all currently active cells having the value of X is determined to be sufficient lower than the predefined value of radio resource usage ratio that can maintain the same sum of cell throughputs in the same geographical area when the new numbers of active cells and antennas are 2 and 5, respectively.

At Step-3, using operation S1206, the A3 offset values of to-be-active cells, which are Cell-1 and Cell-3, are determined to compensate changes in RSRP observed at UE when the new determined numbers of active cells and antennas are applied. Then, the determined A3 offset values are transmitted to the UEs (70) through SIB and RRC signaling.

At Step-4, using operation S1208, the Energy-saving BS (10) applies the new determined numbers of active cells and antennas by de-activating Cell-2 and Cell-4, and five antennas. With the addition of the A3 offset value to the RSRP observed at UE, the combined coverage of Cell-1 and Cell-3 can be made equivalent to that before de-activating Cell-2 and Cell-4.

At Step-5, the UEs which initially connect to Cell-1 and Cell-3 update HO condition and maintain connections with the same cells according to operation S1305.

Finally, at Step-6, the UEs which initially connect to Cell-2 and Cell-4 perform cell selection procedures and establish connections with Cell-1 and Cell-3 according to operation S1406.

(1.5 Advantageous effect)

Based on the example of the first illustrative embodiment, it can be concluded that certain numbers of cells and antennas can be de-activated when the traffic is sufficiently low without changing the UE's perception of the Energy-saving BS's coverage. As a result, all the UEs remain connecting to the Energy-saving BS and the reduced amount of radio resource is effectively utilized. Therefore, the first illustrative embodiment of the present invention can realize energy saving in the BS with multiple antennas, while maintaining throughput in the same geographical area before and after the energy saving is executed at the BS.

(2. Second illustrative embodiment)

According to the second illustrative embodiment, in summary, in addition to the operations described in the first illustrative embodiment, the Energy-saving BS (10) before changing the current numbers of active cells and antennas to the new numbers changes connections of the UEs (70) so that all the UEs (70) preemptively have connections with the cells that are currently active and will continue to be active.

In the following, more details of the second illustrative embodiment will be described by making additional reference to Fig. 14 to 16.

(2.1 System operation)

Fig. 14 shows the operations for the overall system comprising both the Energy-saving BS (10) and the UEs (70) connecting to the Energy-saving BS. Again, the UEs are divided into UE type A and B with the same definitions as previously described.

Since the operations up to the point of determining A3 offset values of to-be-active cells in order to compensate change in RSRP observed at the UE are the same as the first illustrative embodiment (operation S1101 to S1104), the description of such operations will be omitted for conciseness.

After the A3 offset values of to-be-active cells are determined, the Cell selection control unit (109) of the Energy-saving BS (10) transmits the offset values to the UE type A by using RRC signaling (operation S1105). Then, the Cell selection control unit (109) initiates HO procedures with the UE type B, in order to change connection of the UE type B to the to-be-active cell and also give UE type B the determined A3 offset value of that cell (operation S2101). After both the UE type A and B receiving the determined A3 offset value, both can readily incorporate the offset value in evaluating the HO condition (operation S1107). Then, the Cell configuration execution unit (110) of the Energy-saving BS (10) changes the current numbers of active cells and antennas to the new determined numbers (operation S1108). Since both UE type A and B already incorporate the received A3 offset value in evaluating HO condition, they can maintain their connections with their respective cells, and can readily receive data transmission from the Energy-saving BS (operation S1112).

(2.2 Energy-saving BS operation)

Fig. 15 shows the operations of the Energy-saving BS (10). Since the operations up to the point of determining A3 offset values of to-be-active cells in order to compensate change in RSRP observed at the UE are the same as the first illustrative embodiment (operation S1201 to S1206), the description of such operations will be omitted for conciseness.

After the A3 offset values of to-be-active cells are determined, the Cell selection control unit (109) of the Energy-saving BS (10) transmits the offset values to the UE type A by using RRC signaling (operation S2201). Then, the Cell selection control unit (109) initiates HO procedures with the UE type B, in order to change connection of the UE type B to the to-be-active cell and also give UE type B the determined A3 offset value of that cell (operation S2202). Finally, the Cell configuration execution unit (110) of the Energy-saving BS (10) changes the current numbers of active cells and antennas to the new determined numbers by activating or de-activating cells and antennas (operation S1208).

(2.3 UE operation)

Operations of the UE type A are the same as the first illustrative embodiment shown in Fig. 11 (operation S1301 to S1306), therefore their description is omitted for conciseness.

Fig. 16 shows the operations of the UE type B. The Reception controller (703) of the UE type B regularly checks whether the HO command has been received (operation S2301). If no HO command is received ("No" branch of operation S2301), the UE type B continues its connection with the current cells. On the other hand, if HO command is received ("Yes" branch of operation S2301), the UE type B performs HO procedures to the specified cell according to the command (operation S2302). Then, after the HO is completed, the Reception controller (703) checks whether the A3 offset value of the new connecting cell is received (operation S2303). If the A3 offset value is not received ("No" branch of operation S2303), the Reception controller (703) will not update the A3 offset value stored in the Cell selection evaluator (706), and continue to trigger the RSRP measurement unit (705) to calculate RSRPs of the nearby cells (operation S1303). On the other hand, if the A3 offset value is received ("Yes" branch of operation S2303), the Reception controller (703) will update the A3 offset value stored in the Cell selection evaluator (706) to the latest received value (operation S1302). Then, the Reception controller triggers the RSRP measurement unit (705) to calculate RSRPs of the nearby cells (operation S1303).

Since the operations of UE type B from this point forward are the same with the operations of UE type A of the first illustrative embodiment when it calculates RSRPs and evaluating HO condition (operation S1303 to S1306), the description of such operations will be omitted for conciseness.

(2.4 Advantageous effect)

Based on the description of the second illustrative embodiment, it can be concluded that the UE type B is not required to entering RRC_IDLE state and performing cell search procedures when its initially connecting cell is de-activated. As a result, the data transmission between the Energy-saving BS and UE type B will not be disrupted even when the Energy-saving BS applies the new determined numbers of active cells and antennas. Therefore, the second illustrative embodiment of the present invention can, in addition to the advantageous effect provided by the first illustrative embodiment, guarantee continuous data transmission between the Energy-saving BS and all the UEs before and after the change in numbers of active cells and antennas.

(3. Third illustrative embodiment)

According to the third illustrative embodiment, in summary, in addition to the operations described in either the first or second illustrative embodiment, the Energy-saving BS (10) after changing the current numbers of active cells and antennas to the new numbers notifies the Adjacent BS (20) about the determined parameter used in cell selection decision at the UE connecting to the Energy-saving BS. The Adjacent BS (10) then determines and notifies the UE connecting to the Adjacent BS about a relative parameter indicating relationship between the Energy-saving BS (10) and the said Adjacent BS (20) for using in cell selection decision, based on the received parameter from the Energy-saving BS (10).

In the following, more details of the third illustrative embodiment will be described by making additional reference to Fig. 17 to 21.

(3.1 System operation)

Fig. 17 shows the operations for the overall system comprising the Energy-saving BS (10), the UEs connecting to the Energy-saving BS, the Adjacent BS (20), and the UEs connecting to the Adjacent BS. Again, the UEs connecting to the Energy-saving BS are divided into UE type A and B with the same definitions as previously described.

In Fig. 17, it is assumed that the Energy-saving BS (10), the UE type A, and the UE type B operates according to the operations described in the first illustrative embodiment. It is also assumed, for the conciseness of the explanation, that the Energy-saving BS (10) has already determined the new numbers of active cells and antennas, therefore it transmits the necessary A3 offset values to the UE type A and B, and finally changes the configuration of active cells and antennas to the determined configuration (operation S1104 to S1112).

After the Energy-saving BS (10) changing the numbers of active cells and antennas to the new determined numbers, the Cell selection control unit (109) of the Energy-saving BS (10) transmits the determined A3 offset values to the Adjacent BS (20) (operation S3101). Then, the Inbound cell selection parameter processing unit (207) of the Adjacent BS (20) determines the CIO offset values with respect to the cells of the Energy-saving BS (10), based on the received A3 offset values (operation S3102). The details of this determination will be described in more details when explaining about individual operations of the Adjacent BS. After that, the determined CIO offset values are transmitted to the UE of the Adjacent BS through RRC signaling (operation S3103). Finally, the Reception controller (703) of the UE of the Adjacent BS instructs the Cell selection evaluator (706) to consider the latest received CIO offset value in evaluating HO condition (operation S3104).

(3.2 Energy-saving BS operation)

Fig. 18 shows the operations of the Energy-saving BS (10). Since the operations up to the point of activating or de-activating cells and antennas according to the determined new numbers of active cells and antennas are the same as the first illustrative embodiment (operation S1201 to S1208), the description of such operations will be omitted for conciseness.

After the Energy-saving BS (10) changing the numbers of active cells and antennas to the new determined numbers, the Cell selection control unit (109) of the Energy-saving BS (10) transmits the determined A3 offset values to the Adjacent BS (20) (operation S3201). The transmission of the determined A3 offset values can use the BS-BS interface (40) between the Energy-saving BS (10) and the Adjacent BS (20).

(3.3 Adjacent BS operation)

Fig. 19 shows the operations of the Adjacent BS (20). The Inbound cell selection parameter processing unit (207) of the Adjacent BS (20) regularly checks whether the A3 offset value from the Energy-saving BS (10) is received (operation S3301). If no A3 offset value is received ("No" branch of operation S3301), the Adjacent BS (20) returns to its regular operations. On the other hand, if A3 offset value is received ("Yes" branch of operation S3301), the Inbound cell selection parameter processing unit (207) of the Adjacent BS (20) determines the CIO offset value with respect to the cell of the Energy-saving BS (10), based on the received A3 offset value (operation S3302). The illustrative explanation about the function of the CIO offset value is shown in Fig. 20. Here, when the Energy-saving BS decides to de-activate some number of antennas, the UE of the Adjacent BS would observe lower RSRP from the Energy-saving BS, which is the non-serving cell. By adding the CIO offset value to the RSRP from the Energy-saving BS, the UE of the Adjacent BS can use that offset RSRP value in evaluating the HO condition and as a result, maintain the same perception of the Energy-saving BS's coverage as before some antennas are de-activated. Therefore, in this context, the Adjacent BS (20) can set the CIO offset value to be equal to the received A3 offset value from the Energy-saving BS (10).

After the Adjacent BS (20) determining the CIO offset value, the Inbound cell selection processing unit (207) instructs the Scheduler (203) to transmit the determined offset value to the UE of the Adjacent BS through RRC signaling (operation S3303).

(3.4 UE operation)

Operations of the UE type A are the same as the first illustrative embodiment shown in Fig. 11 (operation S1301 to S1306), therefore their description is omitted for conciseness.

Also, operations of the UE type B are the same as the first illustrative embodiment shown in Fig. 12 (operation S1401 to S1406), therefore their description is omitted for conciseness.

Fig. 21 shows the operations of the UE of the Adjacent BS. The Reception controller (703) of the UE (70) regularly checks whether the RRC signaling indicating the CIO offset value is received from the Adjacent BS (20) (operation S3401). If the CIO offset value is received ("Yes" branch of operation S3401), the Reception controller (703) instructs the Cell selection evaluator (706) to update the stored CIO offset value to the latest one (operation S3402). On the other hand, if the CIO offset value is not received ("No" branch of operation S3401), the Reception controller (703) will not update the stored CIO offset value in the Cell selection evaluator (706). Next, the RSRP measurement unit (705) calculates RSRPs of all the detectable cells including the serving-cell and non-serving cells by using their corresponding CRSs (operation S3403). Then, the calculated RSRPs are input into the Cell selection evaluator (706), which will add the stored CIO offset value to the RSRP of the non-serving-cell (operation S3404). The Cell selection evaluator (706) makes a comparison between the RSRPs of the serving cell and the RSRP of the non-serving cell pluses the stored CIO offset value (operation S3405). If the RSRP of the serving cell is greater than the RSRP of the non-serving cell pluses the CIO offset value ("Yes" branch of operation S3405), the Cell selection evaluator (706) will not trigger the HO procedures and the UE maintains connection with the serving-cell. On the other hand, if the RSRP of the serving cell is smaller than the RSRP of the non-serving cell pluses the CIO offset value ("No" branch of operation S3405), the Cell selection evaluator (706) triggers the Cell selection executor (707) to initiate the HO procedures to the non-serving cell (operation S3406).

(3.5 Advantageous effect)

Based on the description of the third illustrative embodiment, it can be concluded that the UE of the Adjacent BS can maintain the same perception of the Energy-saving BS's coverage as before some antennas are de-activated, because it considers the determined CIO offset value in evaluating the HO condition. As a result, when the UE of the Adjacent BS moves out of the Geographical area of the Adjacent BS (21) into the Geographical area of the Energy-saving BS (11), it will trigger HO procedures to switch connection to the Energy-saving BS (10). Therefore, the third illustrative embodiment of the present invention can, in addition to the advantageous effects provided by the first and second illustrative embodiments, maintain perceptions of both UE connecting to the Energy-saving BS and UE connecting to the Adjacent BS regarding the Energy-saving BS's coverage before and after the Energy-saving BS changing the numbers of active cells and antennas.

Claims

A method in a mobile communication system comprising at least one base station that can divide one geographical area into multiple radio access areas, hereafter denoted as cells, by using an antenna array formed by multiple antennas, comprising:

the said base station determining new numbers of the said cells and the said antennas based on information indicating traffic load at current numbers of the said cells and the said antennas,

the said base station notifying user equipments connecting to the said current number of the said cells about parameters used in cell selection decision which are determined based on the determined new numbers of the said cells and the said antennas, and

the said base station changing the said current numbers of the said cells and the said antennas to the said determined new numbers.
The method of Claim 1, wherein the said parameters used in cell selection decision are determined to compensate a change in received signal power at the said user equipment that would result from changing the said current numbers of the said cells and the said antennas to the said new numbers.
The method of Claim 2, wherein the said new numbers of the said cells and the said antennas are determined to satisfy the condition that sums of cell throughputs in the said geographical area when using the said current and the said new numbers of the said cells and the said antennas are equal.
The method of Claim 3, wherein the said sum of cell throughputs when using the said current numbers of the said cells and the said antennas is calculated from values of the said current numbers of the said cells and the said antennas, and the said information indicating traffic load at the said current numbers of the said cells and the said antennas, and

the said sum of cell throughputs when using the said new numbers of the said cells and the said antennas is calculated from values of the said new numbers of the said cells and the said antennas, and a predefined value of traffic load.
The method of Claim 4, wherein the said base station performs determination and notification to the said user equipments about the said parameters used in cell selection decision when the said determined new numbers of the said cells and the said antennas are different from the said current numbers.
The method of Claim 5, further comprising:

changing connections of the said user equipments so that all of the said user equipments have connections with cells that are currently active and will continue to be active after changing the said current numbers of the said cells and the said antennas to the said new numbers, before changing the said current numbers of the said cells and the said antennas to the said new numbers.
The method of Claim 5 or 6, further comprising:

the said base station notifying adjacent base stations about the said parameters used in cell selection decision, after changing the said current numbers of the said cells and the said antennas to the said new numbers, and

the said adjacent base stations notifying user equipments connecting to the said adjacent base stations about relative parameters indicating relationship between the said base station and the said adjacent base stations for using in cell selection decision, based on the said received parameters from the said base station.
A base station in mobile communication system comprising at least one base station that can divide one geographical area into multiple radio access areas, hereafter denoted as cells, by using an antenna array formed by multiple antennas, wherein the said base station comprising:

a cell configuration determination unit to determine new numbers of the said cells and the said antennas based on information indicating traffic load at current numbers of the said cells and the said antennas,

a cell selection control unit to notify user equipments connecting to the said current number of the said cells about parameters used in cell selection decision which are determined based on the determined new numbers of the said cells and the said antennas, and

a cell configuration execution unit to change the said current numbers of the said cells and the said antennas to the said determined new numbers.
A mobile communication system comprising at least one base station that can divide one geographical area into multiple radio access areas, hereafter denoted as cells, by using an antenna array formed by multiple antennas, wherein the said base station comprising:

determining new numbers of the said cells and the said antennas based on information indicating traffic load at current numbers of the said cells and the said antennas,

notifying user equipments connecting to the said current number of the said cells about parameters used in cell selection decision which are determined based on the said determined new numbers of the said cells and the said antennas, and

changing the said current numbers of the said cells and the said antennas to the said determined new numbers.
A recording medium stored a program for a base station in mobile communication system comprising at least one base station that can divide one geographical area into multiple radio access areas, hereafter denoted as cells, by using an antenna array formed by multiple antennas, the said program causing a computer to execute the processes of:

determining new numbers of the said cells and the said antennas based on information indicating traffic load at current numbers of the said cells and the said antennas,

notifying user equipments connecting to the said current number of the said cells about parameters used in cell selection decision which are determined based on the said determined new numbers of the said cells and the said antennas, and

changing the said current numbers of the said cells and the said antennas to the said determined new numbers.