Methods and devices for network energy saving in a wireless communication system considering distances between base stations

Abstract

The present disclosure relates to methods and devices for saving energy on a radio network access side of a wireless communication system.

Description

Methods and devices for network energy saving in a wireless communication system considering distances between base stationsTechnical field

[0001] The present disclosure relates to wireless communication systems and relates more specifically to methods and devices for saving energy on a radio access network side of the wireless communication system, a.k.a. Network Energy Saving (NES).Background

[0002] In order to limit energy consumption in Long Term Evolution (LTE), 5G or New Radio (NR) wireless communication systems, 3GPP (Third Partnership Project) is introducing NES functions in order to reduce energy consumption on the Radio Access Network (RAN) side of the wireless communication system.

[0003] For instance, in some cases, the RAN may comprise a macro-base station (BS) and a plurality of small BSs. By “macro” and “small”, it is meant that the macro-BS has a broader coverage than the respective coverages of the small BSs. Basically, the coverage of a BS, a.k.a. radio coverage, corresponds to the geographical area in which user equipment, UEs, can be serviced by the considered BS.

[0004] In the case where the small BSs, and their respective coverages, are located inside the coverage of the macro-BS, then the macro-BS can transmit some control messages on behalf of the small BSs. For instance, the control messages that can be transmitted by the macro-BS (sometimes referred to as “anchor cell”) for the small BSs (sometimes referred to as “non-anchor cells”) can include, in a 3GPP wireless communication system, e.g., System Information Block (SIB) messages, Synchronization Signal / physical broadcast channel Block (SSB) messages and paging messages.

[0005] Hence, these control messages, transmitted by the macro-BS, need not to be transmitted by the small BSs, which enables reducing the energy consumption of the small BSs. However, there is a need to enhance NES functions to further improve energy consumption.Summary

[0006] The present disclosure aims at improving the situation. In particular, the present disclosure aims at addressing at least some of the limitations of the prior art discussed above. In particular, the present disclosure aims at proposing a solution for enhancing NES functions in a wireless communication system.

[0007] For this purpose, the present disclosure proposes that a macro-BS of the wireless communication system, in charge of transmitting control messages for small-BSs in its coverage, can decide in some scenarios (i.e. when predetermined conditions are verified)to request one or more of the small BSs in its coverage to start transmitting all or part of these control messages, previously transmitted by the macro-BS, to UEs in the respective coverages of these one or more small BSs.

[0008] According to a first aspect, the present disclosure relates to a method for transmitting control messages in a wireless communication system, wherein the wireless communication system comprises a macro base station, BS, and a plurality of small BSs located in a coverage of the macro-BS, wherein the macro-BS transmits, for the plurality of small BSs, control messages to user equipment, UEs, in respective coverages of the plurality of small BSs, wherein the method is implemented by the macro-BS and comprises: selecting at least one small BS, among the plurality of small BSs, based on respective distances between the plurality of small BSs and the macro-BS, instructing the at least one selected small BS to start transmitting all or part of the control messages, previously transmitted by the macro-BS, to UEs in the coverage of said at least one selected small BS.

[0009] In some embodiments, the method according to the first aspect can further comprise one or more of the following optional features, considered either alone or in any technically possible combination.

[0010] In some embodiments of the method according to the first aspect, the at least one selected small BS includes the small BS which is the furthest or the closest from the macro- BS.

[0011] In some embodiments, the method according to the first aspect comprises receiving local load reports from the plurality of small BSs, wherein a local load report from a small BS is representative of a local load of said small BS due to the UEs in its coverage, and the selecting of at least one small BS among the plurality of small BSs is further based on the received local load reports.

[0012] In some embodiments of the method according to the first aspect, the local load report transmitted by a small BS is representative of the number of UEs located in the coverage of said small BS, and the at least one selected small BS includes the small BS which is the furthest or closest small BS from the macro-BS which has a number of UEs in its coverage that is greater than a threshold.

[0013] In some embodiments, the method according to the first aspect comprises receiving distance reports from the plurality of small BSs, wherein a distance report from a small BS is representative of a distance between said small BS and the macro-BS.

[0014] In some embodiments, the method according to the first aspect comprises receiving successive distance reports from a same small BS that is non-stationary.

[0015] In some embodiments, the method according to the first aspect comprises:receiving mobility status reports from the plurality of small BSs, wherein a mobility status report of a small BS is representative of whether the small BS is stationary or non-stationary, requesting small BSs which, according to the received mobility status reports, are non-stationary to transmit distance reports in a recurrent manner, preferably periodically.

[0016] In some embodiments of the method according to the first aspect, the macro-BS configures respective transmission periods of distance reports by the plurality of small BSs based on the received mobility status reports.

[0017] According to a second aspect, the present disclosure relates to a method for transmitting control messages in a wireless communication system, wherein the wireless communication system comprises a macro base station, BS, and a plurality of small BSs located in a coverage of the macro-BS, wherein the macro-BS transmits, for the plurality of small BSs, control messages to user equipment, UEs, in respective coverages of the plurality of small BSs, wherein the method is implemented by a small BS among the plurality of small BSs and comprises: receiving, from the macro-BS, an instruction to start transmitting all or part of the control messages, previously transmitted by the macro-BS, to UEs in the coverage of the small BS, transmitting all or part of the control messages, previously transmitted by the macro- BS, to UEs in the coverage of the small BS.

[0018] In some embodiments, the method according to the second aspect can further comprise one or more of the following optional features, considered either alone or in any technically possible combination.

[0019] In some embodiments, the method according to the second aspect comprises transmitting, by the small BS, a distance report to the macro-BS, wherein the distance report from a small BS is representative of a distance between said small BS and the macro-BS.

[0020] In some embodiments of the method according to the second aspect, the small BS being non-stationary, said small BS transmits distance reports in a recurrent manner, preferably periodically.

[0021] In some embodiments of the method according to the second aspect, the small BS transmits distance reports periodically, with a transmission period set by the macro-BS.

[0022] In some embodiments, the method according to the second aspect comprises: transmitting a local load report to the macro-BS, wherein the local load report is representative of a local load of the small BS due to the UEs in its coverage, and / ortransmitting a mobility status report to the macro-BS, wherein the mobility status report is representative of whether the small BS is stationary or non-stationary.

[0023] In some embodiments of the method according to the second aspect, the control messages transmitted by the macro-BS, for the plurality of small BSs, include system information block, SIB, messages, synchronization signal / physical broadcast channel block, SSB, messages and paging messages, and transmitting all or part of the control messages by a small BS corresponds to transmitting at least one among SIB messages, SSB messages and paging messages.

[0024] According to a third aspect, the present disclosure relates to a computer program product comprising instructions which, when executed by at least one processor, configure said at least one processor to carry out a method for transmitting control messages according to any one of the embodiments of the present disclosure. The computer program product can use any programming language, and can be in the form of source code, object code, or in any intermediate form between source code and object code, such as in a partially compiled form, or in any other desirable form.

[0025] According to a fourth aspect, the present disclosure relates to a (non-transitory) computer-readable storage medium comprising instructions which, when executed by at least one processor, configure said at least one processor to carry out a method for transmitting control messages according to any one of the embodiments of the present disclosure.

[0026] According to a fifth aspect, the present disclosure relates to a base station comprising at least one memory and at least one processor configured to carry out one or more methods for transmitting control messages according to any one of the embodiments of the present disclosure.

[0027] According to a sixth aspect, the present disclosure relates to a wireless communication system comprising a macro-BS according to any one of the embodiments of the present disclosure and a plurality of small BSs according to any one of the embodiments of the present disclosure, wherein the plurality of small BSs are located in a coverage of the macro-BS.Brief description of figures

[0028] The invention will be better understood upon reading the following description, given as an example that is in no way limiting, and made in reference to the figures which show:Figure 1 : a schematic representation of an example of wireless communication system comprising a macro-BS and a plurality of small BSs,Figures 2 and 3: flow charts illustrating examples of methods for transmitting control messages implemented by a macro-BS and a small BS, respectively,Figures 4 and 5: flow charts illustrating other examples of methods for transmitting control messages implemented by a small-BS and a macro-BS, respectively, Figures 6 and 7: flow charts illustrating other examples of methods for transmitting control messages implemented by a macro-BS and a small BS, respectively, Figures 8 and 9: flow charts illustrating other examples of methods for transmitting control messages implemented by a macro-BS and a small BS, respectively, Figure 10: a schematic representation of an example of a base station suitable for implementing one or more methods for transmitting control messages.

[0029] In these figures, references identical from one figure to another designate identical or analogous elements. For reasons of clarity, the elements shown are not to scale, unless explicitly stated otherwise.Detailed description

[0030] The detailed description set forth below, with reference to the figures, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. For instance, although 3GPP terminology, from e.g., 5G NR, may be used in this disclosure to exemplify embodiments herein, this should not be seen as limiting the scope of the present disclosure.

[0031] Generally, all terms used herein are to be interpreted according to their ordinary meaning in the relevant technical field, unless a different meaning is clearly given and / or is implied from the context in which it is used. All references to a / an / the element, apparatus, component, means, step, etc. are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. Also, the order of steps of any methods disclosed herein, in particular in the figures, is provided only for illustration purposes and is not meant to limit the present disclosure which may be applied with the same steps executed in a different order and / or with all or part of the steps executed in parallel or jointly, unless a step is explicitly described as following or preceding another step and / or where it is implicit that a step must follow or precede another step. Also, in a figure, steps represented surrounded by a dashed line are to be considered as optional for the embodiment represented in this figure. Any feature of any of the embodiments disclosed herein may be applied to any other embodiment, wherever appropriate. Likewise, any advantage of any of the embodiments may apply to any other embodiments, and vice versa. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following description.

[0032] Figure 1 represents schematically an example of wireless communication system, which may be for example a 5G NR wireless communication system. More specifically, figure 1 represents a RAN of the wireless communication system, which is used exchange data with UEs (not represented in the figures) via radio signals. For example, the RAN may send data to the UEs (downlink, DL), for instance data received from a core network (CN, not represented in the figures). The RAN may also receive data from the UEs (uplink, UL), which data may be forwarded to the CN.

[0033] As illustrated by figure 1 , the RAN comprises several base stations, BSs. Each of these BSs may be referred to as NB, eNodeB (or eNB), gNodeB (or gNB), an access point or the like, depending on the wireless standard(s) implemented.

[0034] Each UE may be a cellular phone, a wireless modem, a wireless communication device, a handheld device, a laptop computer, or the like. Each UE may also be an Internet of Things (loT) device, like wireless camera, a smart sensor or smart meter, a vehicle, a global positioning system device, or any other device configured to communicate with a RAN of a wireless communication system.

[0035] In figure 1 , a BS, referred to as macro-BS 100 provides a broader coverage 101 (a.k.a. cell) than other BSs, referred to as small BSs 110. The small BSs 110, and their respective coverages 111 , are in the coverage 101 of the macro-BS 100. For instance, the smaller coverages 111 of the small BSs 110 may be due to the small BSs 110 using lower transmission powers than the macro-BS 100 and / or higher frequency carriers than the macro-BS 100.

[0036] As discussed above, the wireless communication system implements NES functions in order to reduce energy consumption of some of the BSs. In particular, since the coverages 111 of the small BSs 110 are included in the coverage 101 of the macro-BS 100, which implies that each UE located in the coverage 111 of a small BS 110 can receive radio signals from both this small BS 110 and the macro-BS 100, the macro-BS 100 is configured to transmit control messages, on behalf of the small BSs 110, to the UEs located in the respective coverages 111 of the small BSs 110. This reduces the energy consumption of the small BSs 110 since the energy required for transmitting these control messages is saved by said small BSs 110.

[0037] It should be noted that there may also be other small BSs in the coverage of the macro-BS 100, for which the macro-BS 100 is not responsible for transmitting control messages. Such small BSs, located in the coverage 101 of the macro-BS 100, for which the macro-BS 100 is not responsible for transmitting control messages, are out of scope of the present disclosure which relates to small BSs 110 which rely on the macro-BS 100 for transmitting control messages to UEs in their respective coverages 111.

[0038] In some examples, the control messages transmitted by the macro-BS 100, on behalf of the small BSs, may be of different types. For example, in a 3GPP wireless communication system, the control messages may include at least one of the following (as illustrated by figure 1 in a non-limitative manner):System Information Block (SIB) messages,Synchronization Signal / PBCH Block (SSB) messages (wherein PBCH stands for Physical Broadcast Channel), paging messages.

[0039] These control message types are considered known to the skilled person. For 3GPP wireless communication systems beyond LTE, definitions for these control message types may be found e.g., in the specification TS 38.300 V17.5.0.

[0040] In some embodiments, the macro-BS 100 transmits all SIB messages, SSB messages and paging messages on behalf of the small BSs 110 in its coverage, thereby saving energy at these small BSs 110.

[0041] As introduced above, the present disclosure proposes that the macro-BS 100, in charge of transmitting control messages for the small BSs 110 in its coverage 101 , can decide in some scenarios to request one or more of these small BSs 110 to start transmitting all or part of these control messages, previously transmitted by the macro-BS 100, to UEs in the respective coverages 111 of these one or more small BSs 110, at least temporarily. Such arrangements enable to achieve a tradeoff between energy saving at the small BSs 110 and energy saving at the macro-BS 100.

[0042] We now describe several examples of embodiments in which the macro-BS 100 may stop transmitting, at least temporarily, the control messages on behalf of one or more small BSs 110 in its coverage 101 , which take over responsibility for transmitting their own control messages which are no longer transmitted by the macro-BS 100.

[0043] Examples considering a global overload criterion

[0044] Figure 2 represents a diagram showing steps of an exemplary embodiment of a method 20 for transmitting control messages, which is implemented by a macro-BS 100. Figure 3 represents a diagram showing corresponding steps of an exemplary embodiment of a method 30 for transmitting control messages, which is implemented by a small BS 110.

[0045] Initially, the macro-BS 100 is assumed to transmit control messages on behalf of the small BSs 110 in its coverage 101. These control messages include for instance SIB, SSB and paging messages.

[0046] As illustrated by figure 2, the method 20 for transmitting control messages comprises a step S20 of evaluating whether a global load of the macro-BS 100, due to the UEs and / orto the plurality of small BSs 110 in the coverage 101 of the macro-BS, verifies a predetermined global overload criterion.

[0047] By “global”, we mean at the level of the macro-BS 100, for the entire coverage 101 of said macro-BS 100. In turn, by “local”, when used hereinafter, we mean at the level of a small BS 110, for the corresponding coverage 111 of said small BS 110.

[0048] The global load of the macro-BS 100 may be any parameter representative of the amount of data that the macro-BS 100 may have to handle due to the UEs in its 101 and / or due to the small BSs 110 in its coverage 101. Basically, the evaluating step S20 aims at detecting situations in which transmitting control messages on behalf of the small BS 110 may become difficult and / or inefficient for the macro-BS 100.

[0049] For example, the global load may correspond to the number of small BSs 110 in its coverage, on behalf of which it is currently transmitting, or may have to transmit, control messages. In such a case, the global overload criterion may be considered verified for the macro-BS 100 if this number of small BSs 110 in its coverage becomes greater than a predetermined first threshold (for instance the first threshold may be equal to 7 and the global overload criterion is verified if the number of small BSs 110 is equal to 8 or greater than 8). According to another non-limitative example, the global load of the macro-BS 100 may correspond to the number of UEs in its coverage, or the number UEs in the respective coverages 111 of the small BS 110 on behalf of which the macro-BS 100 is currently transmitting, or may have to transmit, control messages. In such a case, the global overload criterion may be considered verified for the macro-BS 100 if this number of UEs becomes greater than a predetermined second threshold.

[0050] As illustrated by figure 2, if the global overload criterion is not verified (reference S20b in figure 2), then the macro-BS 100 continues to transmit the control messages, and the evaluating step S20 may be executed recurrently.

[0051] If the global overload criterion is verified (reference S20a in figure 2), then the method 20 for transmitting control messages comprises a step S21 whereby the macro-BS 100 instructs at least one small BS 110 to start transmitting all or part of the control messages, previously transmitted by the macro-BS 100, to UEs in the coverage of said at least one small BS 110. For that purpose, the macro-BS 100 transmits a corresponding message to the at least one small BS 110, for instance over an X2 interface.

[0052] This at least one small BS 110 may take over the responsibility for transmitting all control messages previously transmitted by the macro-BS 100, or only part of it. Hence, if the macro-BS 100 transmitted previously the SIB, SSB and paging messages, this at least one small BS 110 may have to transmit only one among the SIB, SSB and paging messages, or any combination thereof. Non-limitative examples on how the macro-BS 100 may indicateto a small BS 110 which control messages it takes responsibility for are discussed hereinbelow. In some examples, which maximize energy saving at the macro-BS 100, the at least one small BS 110 takes over the responsibility for transmitting all control messages previously transmitted by the macro-BS 100, i.e. , all SIB / SSB / paging messages in the non- limitative example considered herein. The macro-BS 100 may then stop transmitting, on behalf of this at least one small BS 110, the control messages that said at least one small BS 110 has taken responsibility for, thereby saving energy.

[0053] The at least one small BS 110, that the macro-BS 100 instructs to start transmitting all or part of the control messages it previously transmitted, may for instance be selected randomly among the plurality of small BS 110 in the coverage 101 of the macro-BS 100.

[0054] In other examples, and as illustrated in the non-limitative example of figure 2, the at least one small BS 110 may be selected based on one or more parameters. In the example illustrated by figure 2, the instructing step S21 comprises a step S22 of receiving local load reports from the plurality of small BSs 110 in its coverage 101.

[0055] It should be noted that the receiving step S22 may in some cases be executed independently from the outcome of the evaluating step S20, e.g., if the small BS 110 are configured to recurrently transmit local load reports without being prompted to do so. In other examples, which enable saving energy at the small BSs 110, the transmission of the local reports by the small BSs 110 is triggered by the fact that the global overload criterion is verified. In such a case, in response to the global overload criterion being verified, the macro-BS 100 may request (not shown in figure 2) the plurality of small BSs 110 to start transmitting local load reports, which are received by the macro-BS 100 during step S22.

[0056] As discussed above, by “local”, we mean at the level of a small BS 110, for the corresponding coverage 111 of said small BS 110.

[0057] The local load of a small BS 110 may be any parameter representative of the amount of data that the small BS 110 may have to handle due to the UEs in its coverage 111. According to an example, the local load of a small BS 110 may correspond to the number of UEs in its coverage 111 , that the small BS 110 services. According to another non- limitative example, the local load of a small BS 110 may correspond to the total traffic volume in this small BS 110.

[0058] In the non-limitative example of figure 2, the instructing step S21 comprises also a step S23 of selecting at least one small BS 110 among the plurality of small BSs 110, based on the received local load reports. For instance, the macro-BS 100 may select the small BS 110 that has reported the greatest local load, or a small BS 110 reporting a local load above a predetermined threshold. For example, if the local load of a small BS 110 corresponds to the number of UEs in its coverage, then the macro-BS 100 may select the small BSs 110having the greatest number of UEs in its coverage, or any small BS 110 having a number of UEs in its coverage 111 above a predetermined threshold, etc.

[0059] Selecting one more small BSs which report the greatest local loads may enable increasing the energy savings at the macro-BS 100. However, other selection strategies may also be considered in the present disclosure.

[0060] It should be noted that the selecting step S23 may also take into account other parameters for selecting at least one small BS 110. For instance, and as discussed hereinbelow, the macro-BS 100 may also take into account, during the selecting step S23, the respective distances between the small BSs 110 and the macro-BS 100, the respective mobility statuses of the small BSs 110, etc.

[0061] As illustrated by figure 2, the macro-BS 100 instructs (step S24) each selected small BS 110 to start transmitting all or part of the control messages, previously transmitted by the macro-BS 100, to UEs in their respective coverages 111 , as discussed above.

[0062] In some examples, the macro-BS 100 may request the small BSs 110 to transmit local load reports recurrently. Hence, the macro-BS 100 receives successive local reports from the small BSs 110, which enables the macro-BS 100 to repeat the selecting step S23 in order to verify whether the at least one selected small BS 110 needs to be replaced by a different one small BS 110. Indeed, the number of UEs in the coverages 111 of the small BSs 110 may evolve over time, such that e.g., the small BS 110, among the plurality of small BSs 110, which reports the greatest local load may also change over time. Hence, in some cases, if the small BS 110 reporting the greatest local load changes, then the macro- BS 100 may for example: instruct the previously selected small BS 110 to stop transmitting the control messages it had taken responsibility for, which the macro-BS 100 restarts transmitting, instruct the newly selected small BS 110 to start transmitting all or part of the control messages, previously transmitted by the macro-BS 100, to UEs in its coverage 111 (which control messages the macro-BS 100 may stop to transmit).

[0063] For example, the small BSs 110 may be instructed to transmit local load reports periodically. The transmission period used for periodically transmitting local load reports may be predefined and / or may vary from one small BS 110 to another. In some examples, the respective transmission periods for the small BSs 110 may be set by the macro-BS 100. In such a case, the macro-BS 100 may set the same transmission period for all small BSs 110, or it may set possibly different transmission periods for different small BSs 110. In some examples, the macro-BS 100 may adjust the different transmission periods based on the received local load reports. For example, small BSs 110 reporting important local loadsmay be configured with greater transmission periods than small BSs 110 reporting low local loads. Alternatively, or in combination thereof, and as discussed hereinbelow, the macro- BS 100 may adjust the different transmission periods based on the respective distances between the small BSs 110 and the macro-BS 100, on the respective mobility statuses of the small BSs 110, etc.

[0064] As discussed above, figure 3 represents a diagram showing corresponding steps of an exemplary embodiment of a method 30 for transmitting control messages, which may be implemented by a small BS 110 when the macro-BS 100 implements the method 20 for transmitting control messages illustrated by figure 2.

[0065] As illustrated by figure 3, the method 30 for transmitting control messages comprises a step S30 of receiving an instruction (transmitted by the macro-BS 100 during step S21) to start transmitting all or part of the control messages, previously transmitted by the macro- BS, to UEs in the coverage of the small BS 110. In response to receiving such an instruction from the macro-BS 100, the small BS 110 starts transmitting, during a step S31 , the control messages it has taken responsibility for, previously transmitted by the macro-BS 100, to UEs in the coverage 111 of the small BS 110.

[0066] In some embodiments, and as illustrated by figure 3, the method 30 for transmitting control messages may comprise a step S32 of transmitting a local load report to the macro- BS 100 before receiving the instruction to start transmitting all or part of the control messages. As discussed above, the transmitting step S32 may, in some cases, be executed in response to receiving a corresponding request to do so from the macro-BS 100 (not represented in figure 3). As discussed above, the small BS 110 may transmit local load reports recurrently or, preferably, periodically, with a transmission period that may be set by the macro-BS 100.

[0067] It should be noted that the macro-BS 100 may return to transmitting the control messages to all small BSs 110 in its coverage, e.g., if the global load of the macro-BS 100 is reduced to an acceptable level. Hence, the macro-BS 100 may also execute a step (not represented in the figures) of evaluating whether the global load verifies a predetermined global normal load criterion (for instance if the number of small BSs 110 becomes lower than a predetermined threshold, preferably equal to or lower than the threshold considered for the global overload criterion). If the global normal load criterion is verified, i.e., if the global load is reduced to an acceptable level, then the macro-BS 100 may resume transmitting the control messages for all small BSs 110 and may instruct the small BSs 110 which were transmitting all or part of the control messages to stop doing so.

[0068] Examples considering a local overload criterion

[0069] Figure 4 represents a diagram showing steps of an exemplary embodiment of a method 40 for transmitting control messages, which is implemented by a small BS 110. Figure 5 represents a diagram showing corresponding steps of an exemplary embodiment of a method 50 for transmitting control messages, which is implemented by a macro-BS 100.

[0070] Initially, the macro-BS 100 is assumed to transmit control messages on behalf of the small BSs 110 in its coverage 101. These control messages include for instance SIB, SSB and paging messages.

[0071] As illustrated by figure 4, the method 40 for transmitting control messages comprises a step S40 of evaluating whether a local load of the small BS 110, due to the UEs in its coverage 111 , verifies a predetermined local overload criterion.

[0072] As discussed above, by “local”, we mean at the level of the small BS 110, for the corresponding coverage 111 of said small BS 110.

[0073] The local load of the small BS 110 may be any parameter representative of the amount of data that the small-BS 110 may have to handle due to the UEs in its coverage 111. Basically, the evaluating step S40 aims at detecting situations in which the contribution of the local load of the small BS 110 to the global load of the macro-BS 100, which transmits control messages for the considered small BS 110, becomes important.

[0074] For example, the local load of the small BS 110 may correspond to the number of UEs in its coverage 111 (for which the macro-BS 100 is currently transmitting, or may have to transmit, control messages). In such a case, the local overload criterion may be considered verified for the small BS 110 if this number of UEs in its coverage 111 becomes greater than a predetermined first threshold. Such a first threshold may be e.g., predefined, or set by the macro-BS 100, etc. According to another non-limitative example, the local load of a small BS 110 may correspond to the total traffic volume in this small BS 110, in which case the local overload criterion may be considered verified for the small BS 110 if this total traffic volume becomes too important (e.g., greater than a predetermined threshold).

[0075] As illustrated by figure 4, if the local overload criterion is not verified (reference S40b in figure 4), then the small BS 110 may re-execute the evaluating step S40 later.

[0076] If the local overload criterion is verified (reference S40a in figure 4), then the method 40 for transmitting control messages comprises a step S41 whereby the small BS 110 informs the macro-BS 100 that the local overload criterion is verified, by transmitting a corresponding message to the macro-BS 100, for instance over an X2 interface.

[0077] Once the macro-BS 100 has been informed that the local overload criterion is verified for the small BS 110, it may decide to request this small BS 110 to start transmitting all of part of the control messages for UEs in the coverage 111 of this small BS 110. In sucha case, the method 40 for transmitting control messages comprises a step S42 of receiving, by the small BS 110 and from the macro-BS 100, an instruction to start transmitting all or part of the control messages, previously transmitted by the macro-BS 100, to UEs in its coverage. The method 40 for transmitting control messages then comprises a step S43 whereby the small BS 110 starts to transmit these control messages to UEs in its coverage 111. As discussed above, the small BS 110 may take over the responsibility for transmitting all control messages previously transmitted by the macro-BS 100, or only part of it. Hence, if the macro-BS 100 transmitted previously the SIB, SSB and paging messages, this small BS 110 may have to transmit only one among the SIB, SSB and paging messages, or any combination thereof. Non-limitative examples on how the macro-BS 100 may indicate to a small BS 110 which control messages it takes responsibility for are discussed hereinbelow. In some examples, which maximize energy saving at the macro-BS 100, the small BS 110 takes over the responsibility for transmitting all control messages previously transmitted by the macro-BS 100, i.e., all SIB / SSB / paging messages in the non-limitative example considered herein. The macro-BS 100 may then stop transmitting, on behalf of this small BS 110, the control messages that said small BS 110 has taken responsibility for.

[0078] Figure 4 also illustrates a non-limitative example of how the macro-BS may resume transmitting control messages for the considered small BS 110. Basically, in this example, the macro-BS 100 may restart to transmit control messages once the local load of the small BS 110 is reduced to an acceptable level. However, other examples are possible. For instance, the macro-BS 100 may resume transmitting control messages for the considered small BS 110 after a predetermined timer has expired, after a predetermined number of control messages have been transmitted by the small BS 110, etc.

[0079] In the non-limitative example of figure 4, the method 40 for transmitting control messages comprises, after the small BS 110 has started transmitting control messages, a step S44 of evaluating whether the local load of the small-BS, due to the UEs in its coverage 111 , verifies a predetermined local normal load criterion. Basically, the evaluating step S44 aims at detecting situations in which the contribution of the local load of the small BS 110 to the global load of the macro-BS 100 resumes to an acceptable level enabling the macro- BS 100 to restart transmitting control messages on behalf of the considered small BS 110.

[0080] For example, if the local load of the small BS 110 corresponds to the number of UEs in its coverage 111 then the local normal load criterion is for instance verified if the number of UEs in the coverage of the small BS 110 becomes lower than a predetermined second threshold. This second threshold may be equal to the first threshold introduced above for the local overload criterion. In preferred examples, this second threshold is lower than the first threshold of the local overload criterion, to avoid constantly alternating between a localoverload and a local normal load if the number of UEs fluctuates around the first threshold. Such a second threshold may be e.g., predefined, or set by the macro-BS 100, etc.

[0081] As illustrated by figure 4, if the local normal load criterion is not verified (reference S44b in figure 4), then the small BS 110 may continue to transmit the control messages for which it took responsibility for.

[0082] If the local normal load criterion is verified (reference S44a in figure 4), then the method 40 for transmitting control messages comprises a step S45 whereby the small BS 110 informs the macro-BS 100 that the local normal load criterion is verified, by transmitting a corresponding message to the macro-BS 100, for instance over the X2 interface.

[0083] Once the macro-BS 100 has been informed that the local normal load criterion is verified for the small BS 110, it may decide to request this small BS 110 to stop transmitting the control messages for UEs in its coverage. In such a case, the method 40 for transmitting control messages comprises a step S46 of receiving, by the small BS 110 and from the macro-BS 100, an instruction to stop transmitting the control messages. The method 40 for transmitting control messages then comprises a step S47 whereby the small BS 110 interrupts the transmission of the control messages it previously took responsibility for, which the macro-BS 100 restarts transmitting on behalf of this small BS 110.

[0084] As discussed above, figure 5 represents a diagram showing corresponding steps of an exemplary embodiment of a method 50 for transmitting control messages, which may be implemented by the macro-BS 100 when a small BS 110 implements the method 40 for transmitting control messages illustrated by figure 4.

[0085] As illustrated by figure 5, the method 50 for transmitting control messages comprises a step S50 of receiving, from a small BS 110, an indication that a local load of this small BS 110, due to the UEs in its coverage 111 , verifies a local overload criterion.

[0086] Based on this indication received from the small BS 110, the macro-BS 100 may decide to stop transmitting all or part of the control messages on behalf of this small BS 110, in which case the method 50 for transmitting control messages comprises a step S51 of instructing the small BS 110 from which an indication has been received to start transmitting all or part of the control messages, previously transmitted by the macro-BS 100, to UEs in its coverage. The macro-BS 100 may then stop transmitting the corresponding control messages, which the small BS 110 has started transmitting, or is about to start transmitting.

[0087] In some examples, the macro-BS 100 may decide to automatically instruct a small BS 110, for which the local overload criterion is verified, to start transmitting its own control messages. However, in other examples, the macro-BS 100 may take into account other parameters to decide whether the small BS 110 should start transmitting all or part of its control messages. In the non-limitative example of figure 5, the method 50 for transmittingcontrol messages comprises a step S52 of evaluating, by the macro-BS 100, whether a global load verifies a predetermined global overload criterion. If the global overload criterion is verified (reference S52a in figure 5), then the macro-BS 100 instructs the small BS 110 to start transmitting all or part of its control messages. In turn, if the global overload criterion is not verified (reference S52b in figure 5), then the macro-BS 100 continues to transmit the control messages on behalf of the small BS 110. All that has been said hereinabove in relation with the global overload criterion (and with the global normal load criterion) applies similarly to the examples illustrated by figures 4 and 5.

[0088] As discussed above, the small BS 110 may also, in some examples, transmit an indication that a local normal load criterion is verified. In such cases, the method 50 for transmitting control messages may comprise, as illustrated by the example of figure 5, a step S53 of receiving such an indication that the local load of the small BS 110 verifies a local normal load criterion, and a further step S54 of instructing said small BS 110 to stop transmitting all or part of the control messages it previously took responsibility for (which control messages the macro-BS 100 then restarts transmitting).

[0089] Examples considering control message type selection

[0090] Figure 6 represents a diagram showing steps of an exemplary embodiment of a method 60 for transmitting control messages, which is implemented by a macro-BS 100. Figure 7 represents a diagram showing corresponding steps of an exemplary embodiment of a method 70 for transmitting control messages, which is implemented by a small BS 110.

[0091] Initially, the macro-BS 100 is assumed to transmit control messages on behalf of the small BSs 110 in its coverage 101.

[0092] As illustrated by figure 6, the method 60 for transmitting control messages comprises a step S60 of selecting a small BS 110, among the plurality of small BSs for which the macro-BS 100 transmits control messages. The macro-BS 100 may for instance select a small BS 110 randomly among the plurality of small BS 110 in the coverage 101 of the macro-BS 100. In other examples, the small BS 110 may be selected based on one or more parameters. For instance, the macro-BS 100 may use, during the selecting step S60, local load reports received from the small BSs 110, as discussed hereinabove. Other examples of parameters that can be taken into account, alternatively or in combination, during the selecting step S60, include e.g., the respective distances between the small BSs 110 and the macro-BS 100, the respective mobility statuses of the small BSs 110, etc.

[0093] In the example of figure 6, the method 60 for transmitting control messages comprises also a step S61 of selecting control message types, among the different control message types that the macro-BS 100 is currently transmitting for the selected small BS 110, which are to be transmitted by this selected small BS 110.

[0094] For example, the different possible control message types may correspond to i) SIB control messages, ii) SSB control messages and iii) paging control messages. Hence, the macro-BS 100 may select any one of these control message types, or any combination thereof. For example, the macro-BS 100 may select a single control message type, e.g., paging control messages. The macro-BS 100 may also select two control message types, e.g., both SIB and SSB control messages. The macro-BS 100 may also decide to select all different control message types, i.e. , all SIB / SSB / paging control messages.

[0095] The selection of the control message types may use any suitable selection strategy. For example, the number of different control message types selected may depend on the amount of energy saving desired (i.e., the macro-BS 100 may increase the number of different control message types selected to increase energy savings).

[0096] In the example of figure 6, the method 60 for transmitting control messages comprises a step S62 of transmitting to the selected small BS 110 an indication of the selected control message types, previously transmitted by the macro-BS 100, that the selected small BS 110 is to start transmitting to UEs in its coverage 111. This indication is for instance transmitted to the selected small BS 110 over an X2 interface.

[0097] The selected small BS 110 then takes over the responsibility for transmitting all control message types mentioned in the indication received from the macro-BS 100, which were previously transmitted by the macro-BS 100. The macro-BS 100 may then stop transmitting, on behalf of this selected small BS 110, the control message types that the selected small BS 110 has taken responsibility for. For instance, for SIB, SSB and paging control message types, if the indication transmitted to the selected small BS 110 mentions only the SIB control message type, then the macro-BS 100 continues to transmit SSB and paging messages. If the indication transmitted to the selected small BS 110 mentions all SIB / SSB / paging control message types, then the macro-BS 100 may stop transmitting control messages on behalf of the selected small BS 110.

[0098] It should be noted that the indication transmitted to the selected small BS 110 may directly indicate the control message types that the selected small BS 110 is to start transmitting, or indirectly by indicating the control message types that the macro-BS 100 will continue to transmit. In the latter case, the selected small BS 110 starts transmitting the control message types that are not explicitly mentioned in the indication received. In the following, we consider in a non-limitative manner the case where the indication mentions explicitly the control message types that the selected small BS 110 is to start transmitting.

[0099] In some examples, the indication of the control message types to be transmitted may be provided as a bit vector. For instance, the bit vector may comprise one bit per control message. In the example considered herein of SIB, SSB and paging control message types,it is therefore possible to use three bits. With three bits and three different control message types, it is therefore possible to indicate any single control message type and any combination thereof (and even no control message type at all if e.g., the selected small BS 110 is to stop transmitting control messages). It is also possible to use more bits or even fewer bits. For instance, with SIB, SSB and paging control message types, it is possible to use only two bits indicating either which single control message type, among the SIB, SSB and paging control message types, is to be transmitted by the small BS, or that the small BS is to transmit all SIB, SSB and paging control message types. For example, it is possible to use a 2-bits bit vector as follows: a bit vector set to ‘00’ means that the selected small BS 110 is to start transmitting SSB messages, a bit vector set to ‘0T means that the selected small BS 110 is to start transmitting SIB messages, a bit vector set to ‘10’ means that the selected small BS 110 is to start transmitting paging messages, a bit vector set to ‘1 T means that the selected small BS 110 is to start transmitting all SIB / SSB / paging messages.

[0100] In some examples, the macro-BS 100 may decide to automatically select a small BS 110 that is to start transmitting. However, in other examples, the macro-BS 100 may take into account other parameters to decide whether a small BS 110 should be selected to start transmitting all or part of its control messages. As discussed above in relation with figure 4 and 5, the selecting step S60 may be executed in response to receiving from a small BS 110 an indication that a local overload criterion is verified.

[0101] In the non-limitative example of figure 6, the selecting step S60 is executed in response to a global overload criterion being verified, and the method 60 for transmitting control messages comprises a step S63 of evaluating whether a global load of the macro- BS 100 verifies a predetermined global overload criterion. If the global overload criterion is verified (reference S63a in figure 6), then the selecting steps S60 and S61 are executed by the macro-BS 100. In turn, if the global overload criterion is not verified (reference S63b in figure 6), then the selecting steps S60 and S61 are not executed. All that has been said hereinabove in relation with the global overload criterion (and with the global normal load criterion) applies similarly to the examples illustrated by figures 6 and 7.

[0102] As discussed above, figure 7 represents a diagram showing corresponding steps of an exemplary embodiment of a method 70 for transmitting control messages, which may be implemented by a small BS 110 when the macro-BS 100 implements the method 60 for transmitting control messages illustrated by figure 6.

[0103] As illustrated by figure 7, the method 70 for transmitting control messages comprises a step S70 of receiving, from the macro-BS 100, an indication (for example as a bit vector) of which control message types, among e.g., the SIB, SSB and paging control message types previously transmitted by the macro-BS 100, that the small BS 110 is to start transmitting to UEs in its coverage 111 . In response to receiving such an indication from the macro-BS 100, the small BS 110 starts transmitting the control message types indicated by the macro-BS 100 to UEs in its coverage 111 , during step S71.

[0104] Examples considering distance and / or mobility status

[0105] Figure 8 represents a diagram showing steps of an exemplary embodiment of a method 80 for transmitting control messages, which is implemented by a macro-BS 100. Figure 9 represents a diagram showing corresponding steps of an exemplary embodiment of a method 90 for transmitting control messages, which is implemented by a small BS 110.

[0106] Initially, the macro-BS 100 is assumed to transmit control messages on behalf of the small BSs 110 in its coverage 101. These control messages include for instance SIB, SSB and paging messages. The examples represented in figures 8 and 9 focus on how the macro-BS 100 may select a small BS 110 that is to start transmitting control messages.

[0107] As illustrated by figure 8, the method 80 for transmitting control messages comprises a step S80 of selecting at least one small BS 110, among the plurality of small BSs 110, that is to start transmitting all or part of the control messages previously transmitted by the macro-BS 100. In the example illustrated by the figure 8, the selection is based on respective distances between the plurality of small BSs 110 and the macro-BS 100.

[0108] Indeed, the distance to the macro-BS 100 is a parameter that may be of interest in the context of NES functions.

[0109] For example, if the macro-BS 100 uses the same transmission power for transmitting control messages for all small BSs 110, then this transmission power may be uselessly high for a small BS 110 close to the macro-BS 100. In such a case, it may be more efficient to select the small BS 110 that is the closest to the macro-BS 100, since in such case the control messages can be transmitted with a lower transmission power if transmitted by the closest small BS 110 than if transmitted by the macro-BS 100, or to select small BSs 110 which have distances lower than a predetermined threshold (i.e., which are close to the macro-BS 100).

[0110] According to another example, if the macro-BS 100 adapts the transmission power to the recipient UEs, then it will use a higher transmission power for UEs in the coverage 111 of a small BS 110 that is far from the macro-BS 100 than for UEs in the coverage 111 of a small BS 110 that is close to the macro-BS 100. In such a case, it may be more efficient to select the small BS 110 that is the furthest from the macro-BS 100, since in such casethe control messages can be transmitted with a lower transmission power if transmitted by the furthest small BS 110 than if transmitted by the macro-BS 100, or to select the small BSs 110 which have distances greater than a predetermined threshold (i.e., which are far from the macro-BS 100).

[0111] The distances between the small BSs 110 and the macro-BS 100 may be determined by using any suitable method. In some examples, the distances may be determined by the macro-BS 100 without relying on specific data received from the small BS 110. For instance, the reception power of radio signals received from the small BSs 110 may be used to sort the small BSs 110 from the closest (highest reception power) to the furthest (lowest reception power). However, in other examples, the macro-BS 100 may use specific data received from the small BSs 110 to sort the small BSs 110 from the closest to the furthest. This is the case in the non-limitative example illustrated by figure 8, in which the small BSs 110 transmit distance reports which are received by the macro-BS 100 during a step S82. Basically, a distance report from a small BS 110 is representative of a distance between said small BS 110 and the macro-BS 100 and corresponds to any information that enables the macro-BS 100 to sort the small BSs 110 according to their distances to the macro-BS 100. For instance, the distance report from a small BS 110 may directly include an estimate of the distance between this small BS 110 and the macro-BS 100, as estimated by e.g., the small BS 110. According to other non-limitative examples, the distance report from a small BS 110 may include any information that enables the macro-BS 100 to estimate the distance to this small BS 110. For example, such a distance report may include an estimated position of the small BS 110 (for instance provided by a GPS module of the small BS 110), that the macro-BS 100 may compare to its own estimated position (for instance provided by a GPS module of the macro-BS 100) to estimate the distance to this small BS 110.

[0112] The distances received in the distance reports, or derived from information included in the received distance reports, are then used during the selecting step S80 to select the at least one small BS 110 that is to start transmitting its own control messages.

[0113] It should be noted that, for a small BS 110 that is stationary (i.e., immobile) with respect to the macro-BS 100, it is sufficient for the macro-BS 100 to receive a single distance report, since the distance between a stationary small BS 110 and the macro-BS 100 is not expected to vary over time. However, if a small BS 110 is non-stationary (i.e., mobile) with respect to the macro-BS 100, then such a non-stationary small BS 110 may, in some examples, transmit successive distance reports to the macro-BS 100, in a recurrent manner, to let the macro-BS 100 know how the distance to this non-stationary small BS 110 varies over time. Such recurrent distance reports may be transmitted more frequently by asmall BS 110 moving fast than by a small BS 110 moving slowly with respect to the macro- BS 100. In some examples, the selecting step S80 may be executed each time it is determined that the sorting of the small BSs 110 according to their distances to the macro- BS 100 may have changed.

[0114] In some examples, the small BSs 110 may decide on their own whether they should transmit distance reports recurrently. In other examples, the macro-BS 100 may decide which small BSs 110 are to transmit recurrent distance reports. This is the case in the non- limitative example of figure 8, in which the method 80 for transmitting control messages comprises a step S83 of receiving, by the macro-BS 100, mobility status reports from the plurality of small BSs, and a step S84 of instructing small BSs 110 which, according to the received mobility status reports, are non-stationary to transmit distance reports in a recurrent manner, preferably periodically.

[0115] Basically, a mobility status report of a small BS 110 is representative of whether the small BS is stationary (i.e. , immobile) or non-stationary (i.e., mobile) with respect to the macro-BS 100. In a simple form, the mobility status report may consist in two possible states, i.e., stationary or non-stationary. In other examples, the mobility status report may provide, for a non-stationary small BS 110, an indication of the moving speed of the small BS 110, e.g., fast or slowly moving. In the latter case, the macro-BS 100 may further adjust the respective transmission periods of distance reports by non-stationary small BS 110. For example, the macro-BS 100 may configure a small BS 110 moving fast with a shorter transmission period (i.e., more frequent transmissions) than a small BS 110 moving slowly.

[0116] As illustrated by figure 8, the method 80 for transmitting control messages comprises a step S81 whereby the macro-BS 100 instructs the at least one selected small BS 110 to start transmitting all or part of the control messages, previously transmitted by the macro- BS 100, to UEs in the coverage of said at least one small BS 110. For that purpose, the macro-BS 100 transmits a corresponding message to the at least one selected small BS 110, for instance over an X2 interface. As discussed above, this at least one selected small BS 110 may take over the responsibility for transmitting all control messages previously transmitted by the macro-BS 100, or only part of it. The macro-BS 100 may then stop transmitting, on behalf of this at least one small BS 110, the control messages that said at least one selected small BS 110 has taken responsibility for, thereby saving energy.

[0117] In some examples, the selecting step S80 may consider only the respective distances between the small BSs 110 and the macro-BS 100.

[0118] However, the selecting step S80 may also take into account other parameters for selecting at least one small BS 110. For example, the macro-BS 100 may also take into account, during the selecting step S80, the respective local loads of the small BSs 110 inaddition to the respective distances between the small BSs 110 and the macro-BS 100. This is the case in the non-limitative example illustrated by figure 8, in which the method 80 for transmitting control messages comprises a step S85 of receiving local load reports from the plurality of small BSs 110, which are used during the selecting step S80 in addition to the respective distances between the small BSs 110 and the macro-BS 100.

[0119] All that has been said hereinabove in relation with the local load applies similarly to the examples illustrated by figures 8 and 9. For example, the local load of a small BS 110 may correspond to the number of UEs in the coverage 111 of this small BS 110. In such a case, the macro-BS 100 may for example select at least the furthest small BS 110 from the macro-BS (or the closest, as discussed above) which has a number of UEs in its coverage that is greater than a predetermined threshold. Of course, it is also possible to consider other selection strategies during the selecting step S80.

[0120] As discussed above, figure 9 represents a diagram showing corresponding steps of an exemplary embodiment of a method 90 for transmitting control messages, which may be implemented by a small BS 110 when the macro-BS 100 implements the method 80 for transmitting control messages illustrated by figure 8.

[0121] As illustrated by figure 9, the method 90 for transmitting control messages comprises a step S90 of receiving an instruction (transmitted by the macro-BS 100 during step S81) to start transmitting all or part of the control messages, previously transmitted by the macro- BS 100, to UEs in the coverage of the small BS 110. In response to receiving such an instruction from the macro-BS 100, the small BS 110 starts transmitting, during a step S91 , the control messages it has taken responsibility for, previously transmitted by the macro- BS 100, to UEs in the coverage 111 of the small BS 110.

[0122] In some embodiments, and as illustrated in the non-limitative example of figure 9, the method 90 for transmitting control messages may comprise a step S92 of transmitting a distance report to the macro-BS 100 before receiving the instruction to start transmitting all or part of the control messages. As discussed above, the small BS 110 may transmit a single distance report (if e.g., stationary) or successive distance reports (if e.g., non- stationary). In the latter case, a small BS 110 may transmit the distance reports periodically, with a transmission period be set by the macro-BS 100.

[0123] In some embodiments, and as illustrated in the example of figure 9, the method 90 for transmitting control messages may comprise: a step S95 of transmitting a local load report to the macro-BS 100 (which may be transmitted recurrently to let the macro-BS 100 know how the local load of the small BS 110 varies over time), and / ora step S93 of transmitting a mobility status report to the macro-BS 100 (which may be transmitted recurrently to let the macro-BS 100 know how the mobility status of the small BS 110 varies over time) which may be, in some examples, accompanied by a step S94 of receiving instructions regarding distance reports (e.g., recurrent or not, transmission period, etc.).

[0124] Figure 10 represents schematically an example of architecture for a BS 120, which architecture can be used for either a macro-BS 100 or a small BS 110, or both. Hence, the architecture of the BS 120 in figure 10 is suitable to implement any method for transmitting control messages discussed in the present disclosure.

[0125] As illustrated by figure 10, the BS 120 comprises one or more processors 121 (which may belong to a same computer or to different computers) and one or more memories 122 (which may belong to a same computer or to different computers). The one or more processors 121 may include for instance a central processing unit (CPU), a digital signal processor (DSP), a field-programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc. The one or more memories 122 may include any type of computer readable volatile and non-volatile memories (magnetic hard disk, solid-state disk, optical disk, electronic memory, etc.). The one or more memories may store a computer program product 123, in the form of a set of program-code instructions to be executed by the one or more processors in order to implement all or part of the steps of a method for transmitting control messages according to any one of the embodiments disclosed herein.

[0126] As illustrated by figure 10, the BS 120 comprises also a wireless communication unit124, configured to exchange data with UEs using radio signals. The wireless communication unit 124 may for instance be a 3G, 4G, 5G, NR, WiFi, Wimax, etc. transceiver or the like.

[0127] As illustrated by figure 10, the BS 120 comprises also a network communication unit125, configured to exchange data with other base stations of the RAN. The network communication unit 125 is used to exchange data between a macro-BS 100 and a small BS 110, for instance via the X2 interface. The network communication unit 125 may also be used to exchange data with CN. The network communication unit 125 may support one or more suitable communication protocols, which may be wired (including optical) and / or wireless.

Claims

Claims1. A method (80) for transmitting control messages in a wireless communication system, wherein the wireless communication system comprises a macro base station, BS, and a plurality of small BSs located in a coverage of the macro-BS, wherein the macro-BS (100) transmits, for the plurality of small BSs, control messages to user equipment, UEs, in respective coverages of the plurality of small BSs (110), wherein the method is implemented by the macro-BS and comprises: selecting (S80) at least one small BS, among the plurality of small BSs, based on respective distances between the plurality of small BSs and the macro-BS, instructing (S81) the at least one selected small BS to start transmitting all or part of the control messages, previously transmitted by the macro-BS, to UEs in the coverage of said at least one selected small BS.

2. The method (80) according to claim 1 , wherein the at least one selected small BS includes the small BS which is the furthest or the closest from the macro-BS.

3. The method according to claim 1 , comprising receiving (S85) local load reports from the plurality of small BSs, wherein a local load report from a small BS is representative of a local load of said small BS due to the UEs in its coverage, and wherein the selecting (S80) of at least one small BS among the plurality of small BSs is further based on the received local load reports.

4. The method (80) according to claim 3, wherein the local load report transmitted by a small BS is representative of the number of UEs located in the coverage of said small BS, and wherein the at least one selected small BS includes the small BS which is the furthest or closest small BS from the macro-BS which has a number of UEs in its coverage that is greater than a threshold.

5. The method (80) according to any one of the preceding claims, comprising receiving (S82) distance reports from the plurality of small BSs, wherein a distance report from a small BS is representative of a distance between said small BS and the macro-BS.

6. The method (80) according to claim 5, comprising receiving successive distance reports from a same small BS that is non-stationary.

7. The method (80) according to any one of claims 5 to 6, comprising: receiving (S83) mobility status reports from the plurality of small BSs, wherein a mobility status report of a small BS is representative of whether the small BS is stationary or non-stationary, requesting (S84) small BSs which, according to the received mobility status reports, are non-stationary to transmit distance reports in a recurrent manner, preferably periodically.

8. The method (80) according to claim 7, wherein the macro-BS configures respective transmission periods of distance reports by the plurality of small BSs based on the received mobility status reports.

9. A method (90) for transmitting control messages in a wireless communication system, wherein the wireless communication system comprises a macro base station, BS, and a plurality of small BSs located in a coverage of the macro-BS, wherein the macro-BS (100) transmits, for the plurality of small BSs, control messages to user equipment, UEs, in respective coverages of the plurality of small BSs (110), wherein the method is implemented by a small BS among the plurality of small BSs and comprises: receiving (S90), from the macro-BS, an instruction to start transmitting all or part of the control messages, previously transmitted by the macro-BS, to UEs in the coverage of the small BS, transmitting (S91) all or part of the control messages, previously transmitted by the macro-BS, to UEs in the coverage of the small BS.

10. The method (90) according to claim 9, comprising transmitting (S92), by the small BS, a distance report to the macro-BS, wherein the distance report from a small BS is representative of a distance between said small BS and the macro-BS.

11. The method (90) according to claim 10, wherein, the small BS being non-stationary, said small BS transmits distance reports in a recurrent manner, preferably periodically.

12. The method (90) according to claim 11 , wherein the small BS transmits distance reports periodically, with a transmission period set by the macro-BS.

13. The method (90) according to any one of claims 9 to 12, comprising: transmitting (S95) a local load report to the macro-BS, wherein the local load report is representative of a local load of the small BS due to the UEs in its coverage, and / or transmitting (S93) a mobility status report to the macro-BS, wherein the mobility status report is representative of whether the small BS is stationary or non- stationary.

14. The method (90) according to any one of the preceding claims, wherein the control messages transmitted by the macro-BS, for the plurality of small BSs, include system information block, SIB, messages, synchronization signal / physical broadcast channel block, SSB, messages and paging messages, and transmitting all or part of the control messages by a small BS corresponds to transmitting at least one among SIB messages, SSB messages and paging messages.

15. A computer program product comprising instructions which, when executed by at least one processor, configure said at least one processor to carry out a method (80)according to any one of claims 1 to 8 and / or a method (90) according to any one of claims 9 to 14.

16. A computer-readable storage medium comprising instructions which, when executed by at least one processor, configure said at least one processor to carry out a method (80) according to any one of claims 1 to 8 and / or a method (90) according to any one of claims 9 to 14.

17. A base station, BS, (120) comprising at least one memory (122) and at least one processor (121) configured to carry out a method (80) according to any one of claims 1 to 8 and / or a method (90) according to any one of claims 9 to 14.

18. Wireless communication system comprising a macro base station, BS, and a plurality of small BSs located in a coverage of the macro-BS, wherein the macro-BS (100) transmits control messages, for the plurality of small BSs, to user equipment, UEs, in respective coverages of the plurality of small BSs (110), wherein: the macro-BS comprises at least one memory and at least one processor configured to carry out a method (80) according to any one of claims 1 to 8, each small BS comprises at least one memory and at least one processor configured to carry out a method (90) according to any one of claims 9 to 14.

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