Apparatuses And Methods Relating To The Reservation Of Reference Signals

  • Published: Oct 3, 2019
  • Earliest Priority: Mar 28 2018
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Apparatuses and Methods relating to the Reservation of Reference Signals

Field

This specification relates to the reservation of reference signals, for instance, within a wireless communication network.

Background

The use of unmanned aerial vehicles (UAVs) is becoming increasingly common. Indeed, the use cases for commercial UAVs are growing rapidly and include package delivery, search and rescue, monitoring of critical infrastructure, wildlife conservation, flying cameras, and surveillance. Many of these emerging use cases could benefit from connecting the UAVs to the cellular network as a user equipment (UE). Summary

In a first aspect, this specification describes an apparatus comprising means for performing: determining that at least one user equipment, UE, being served by a first base station satisfies a condition associated with causing reference signal contamination for one or more other base stations; responding to the determination by allocating one or more reference signals that are to be reserved for use by the at least one UE; and causing notification to at least the one or more other base stations of the identities of the one or more reference signals that are to be reserved.

Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is creating, is expected soon to be creating or has the potential to create reference signal contamination for one or more other base stations. Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is above, or is approaching, a threshold height.

Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is generating interference for the one or more other base stations that is above a threshold limit. Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is a potential source of unacceptably high interference for the one or more other base stations. Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is, or is carried on, an unmanned aerial vehicle. The one or more other base stations may be identified based on a location of the first base station or of the at least one UE. The one or more other base stations may be within a threshold distance of the location of the first base station or of the at least one UE.

The one or more other base stations may be identified based on a measurement report received from the at least one UE and/ or other information.

The apparatus may comprise means for causing notification to the one or more other base stations of: the identities of the one or more reference signals reserved for the at least one UE; and the identity of the at least one UE.

The means for causing notification may be configured to cause notification to the one or more other base stations of: the identities of plural UEs that have been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations; and the identities of the one or more reference signals reserved for the plural UEs.

The reference signals that are to be reserved for the at least one UE may be determined based on one or more of: a number of UEs that have been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations; a coherence time and/or a coherence bandwidth for the at least one UE; and requirements of other UEs, served by the first base station or the one or more other base stations which have not been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations. The apparatus may comprise means for performing: determining that the reservation of the one or more reference signals can be released; and notifying the one or more other base stations that the reservation of the reference signals can be released. Determining that the reservation of the one or more reference signals can be released may be based on determining that the at least one UE is no longer satisfying the condition associated with causing reference signal contamination for one or more other base stations.

The apparatus may be the first base station. In a second aspect apparatus, this specification describes apparatus comprising means for performing: receiving, at a base station, a notification of identities of one or more reference signals reserved for at least one user equipment, UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station; and reserving, by the base station in response to receipt of the notification, the one or more reference signals identified in the notification.

The notification may include the identity of the at least one UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station.

The apparatus may comprise means for determining whether to reserve the one or more reference signals identified in the notification. Determining whether to reserve the one or more reference signals identified in the notification may be based on one or more of: a number of reference signals currently being used by UEs served by the base station; and a benefit provided by reserving the one or more reference signals.

The apparatus may comprise means for performing: receiving, at the base station, a notification that the reservation of the one or more reference signals can be released; and releasing the reservation of the one or more reference signals in response to the notification that the reservation of the one or more reference signals can be released.

The apparatus may comprise means for causing notification, by the base station to at least one other base station, of the identities of the one or more reserved reference signals.

Notification, by the base station to at least one other base station, of the identities of one or more reserved reference signals may be caused in response to receipt of the notification of the identities of one or more reference signals reserved for at least one UE which has been determined as satisfying the condition associated with causing reference signal contamination for the base station.

In a third aspect, this specification describes a base station comprising the apparatus described in relation to either of the first and second aspects. In a fourth aspect this specification describes, a method comprising: determining that at least one user equipment, UE, being served by a first base station satisfies a condition associated with causing reference signal contamination for one or more other base stations; responding to the determination by allocating one or more reference signals that are to be reserved for use by the at least one UE; and causing notification to at least the one or more other base stations of the identities of the one or more reference signals that are to be reserved.

Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is creating, is expected soon to be creating or has the potential to create reference signal contamination for one or more other base stations. Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is above, or is approaching, a threshold height.

Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is generating interference for the one or more other base stations that is above a threshold limit. Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is a potential source of unacceptably high interference for the one or more other base stations.

Determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations may comprise determining that the at least one UE is, or is carried on, an unmanned aerial vehicle.

The one or more other base stations may be identified based on a location of the first base station or of the at least one UE. The one or more other base stations may be within a threshold distance of the location of the first base station or of the at least one UE.

The one or more other base stations may be identified based on a measurement report received from the at least one UE and/or other information.

The method may comprise causing notification to the one or more other base stations of: the identities of the one or more reference signals reserved for the at least one UE; and the identity of the at least one UE. Causing notification to at least the one or more other base stations may include causing notification to the one or more other base stations of: the identities of plural UEs that have been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations; and the identities of the one or more reference signals reserved for the plural UEs.

The reference signals that are to be reserved for the at least one UE may be determined based on one or more of: a number of UEs that have been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations; a coherence time and/or a coherence bandwidth for the at least one UE; and requirements of other UEs, served by the first base station or the one or more other base stations which have not been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations.

The method may further comprise: determining that the reservation of the one or more reference signals can be released; and notifying the one or more other base stations that the reservation of the reference signals can be released. Determining that the reservation of the one or more reference signals can be released may be based on determining that the at least one UE is no longer satisfying the condition associated with causing reference signal contamination for one or more other base stations.

In a fifth aspect, this specification describes a method comprising: receiving, at a base station, a notification of identities of one or more reference signals reserved for at least one user equipment, UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station; and reserving, by the base station in response to receipt of the notification, the one or more reference signals identified in the notification.

The notification may include the identity of the at least one UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station. The method may comprise determining whether to reserve the one or more reference signals identified in the notification. Determining whether to reserve the one or more reference signals identified in the notification may be based on one or more of: a number of reference signals currently being used by UEs served by the base station; and a benefit provided by reserving the one or more reference signals.

The method may comprise: receiving, at the base station, a notification that the reservation of the one or more reference signals can be released; and releasing the reservation of the one or more reference signals in response to the notification that the reservation of the one or more reference signals can be released.

The method may comprise causing notification, by the base station to at least one other base station, of the identities of the one or more reserved reference signals. The notification, by the base station to at least one other base station, of the identities of one or more reserved reference signals may be caused in response to receipt of the notification of the identities of one or more reference signals reserved for at least one UE which has been determined as satisfying the condition associated with causing reference signal

contamination for the base station.

In a sixth aspect, this specification describes computer-readable instructions which, when executing by one or more computing apparatus, causes the computing apparatus to perform a method according to either of the fourth and fifth aspects.

In a seventh aspect, this specification describes an apparatus comprising at least one processor and at least one memoiy including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: determining that at least one user equipment, UE, being served by a first base station satisfies a condition associated with causing reference signal contamination for one or more other base stations; responding to the determination by allocating one or more reference signals that are to be reserved for use by the at least one UE; and causing notification to at least the one or more other base stations of the identities of the one or more reference signals that are to be reserved. The computer program code may be configured to cause the apparatus of the seventh aspect to perform any of the operations described with reference to the method of the fourth aspect.

In an eighth aspect, this specification describes apparatus comprising at least one processor, and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform: receiving, at a base station, a notification of identities of one or more reference signals reserved for at least one user equipment, UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station; and reserving, by the base station in response to receipt of the notification, the one or more reference signals identified in the notification. The computer program code may be configured to cause the apparatus of the eighth aspect to perform any of the operations described with reference to the method of the fifth aspect. In a ninth aspect, this specification describes a non-transitoiy computer readable medium comprising program instructions for causing an apparatus to perform at least the following: determining that at least one user equipment, UE, being served by a first base station satisfies a condition associated with causing reference signal contamination for one or more other base stations; responding to the determination by allocating one or more reference signals that are to be reserved for use by the at least one UE; and causing notification to at least the one or more other base stations of the identities of the one or more reference signals that are to be reserved. The computer program instructions may further be for causing the apparatus to perform any of the operations described with reference to the method of the fourth aspect.

In a tenth aspect, this specification describes a non-transitoiy computer readable medium comprising program instructions for causing an apparatus to perform at least the following: receiving, at a base station, a notification of identities of one or more reference signals reserved for at least one user equipment, UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station; and reserving, by the base station in response to receipt of the notification, the one or more reference signals identified in the notification. The computer program instructions may further be for causing the apparatus to perform any of the operations described with reference to the method of the fifth aspect.

Brief Description of the Figures

For better understanding of the present application, reference will now be made, by way of example only, to the accompanying drawings in which:

Fig. 1 is an example of a wireless communication network in which the reference signal contamination reduction methods described herein may be implemented;

Fig. 2 illustrates a flow of signalling throughout the wireless communication network;

Fig. 3A is a flowchart illustrating various operations which may be performed by a serving base station in order to reduce reference signal contamination;

Fig. 3B is a flowchart illustrating various operations which may be performed by a non- serving base station in order to reduce reference signal contamination;

Fig. 4 is another flowchart illustrating various operations which may be performed by a serving base station in order to reduce reference signal contamination;

Fig· 5 is an example schematic illustration of a configuration of the base stations depicted in Figs. 1 and 2; Fig. 6 is an illustration of a computer-readable medium upon which computer readable code may be stored; and

Fig. 7 is a graph illustrating the effects of reference signal contamination on data rates within a communication network.

Detailed Description

In the description and drawings, like reference numerals refer to like elements throughout. Fig. l is an example of a portion of cellular communications network l. The cellular communications network l comprises plural base stations to-i to io-h. Each of the base stations to-i to io-h has a coverage area and may serve user equipments (UEs) 12, 14, located within the coverage area. In some implementations, the cellular communications network 1 may be a long-term evolution (LTE) network (such as a 4G or 5G network). In such examples, the base stations 10-1 to 10-n may be referred to as eNBs.

Cellular communication networks 1 such as that of Fig. 1 utilise reference signals for estimating uplink (from UE to base station) channel quality. The channel quality may then be used by the base station for uplink frequency selective scheduling. In 4G and 5G, the reference signals may be referred to as sounding reference signals or pilot sequences.

Due to overhead constraints, only a limited number of reference signals are available for use and, as such, they are reused across the network 1. Reference signal contamination occurs when a reference signal that is sent by a certain UE to its serving base station collides with the same reference signal sent by another UE to a different base station. Reference signal contamination can severely degrade the performance of the cellular communication network.

As illustrated in Fig. 1, there may be two types of UE operating within the cellular communications network 1. More specifically, there may be UEs 12 of a first type, which have the potential to cause unacceptable levels of reference signal contamination for base stations other than the serving base station. There may also be a second type of UEs 14, which do not have the potential (or are less likely) to cause unacceptable levels of reference signal contamination for base stations other than the serving base station.

In the example of Fig. 1, the UE 12 of the first type is part of, or is carried by, a UAV 13. UEs 12 that are part of or are carried by a UAV 13 may be particularly capable of causing reference signal contamination since they are capable of flying above the heights of the base station antennas. By flying above the heights of the base station antennas, the UEs 12 are able to obtain a direct line-of-sight (LOS) to multiple different base stations, which significantly increases the likelihood of reference signal contamination. This is illustrated in Fig. 1.

UEs 12 of the first type may also be referred to as UAV-UEs 12. It should, however, be appreciated that the concepts described herein may not be limited to use with UAV-UEs 12, but may be utilised with any type of UE that has the potential to cause unacceptable reference signal contamination to surrounding base stations.

The second type of UEs 14 may be ground-based UEs (GUEs). These may include but are, of course, not limited to portable user devices of any type (such as smart phones, tablet computers and media players) and vehicle UEs.

In the example of Fig. 1, the base stations 10-1 to 10-n include multiple-input multiple- output (MIMO) antennas, which may be“massive MIMO” antennas. Massive MIMO may refer to systems with tens or hundreds of antennas. Massive MIMO networks may be particularly suited for serving UAV-UEs because they can better handle additional traffic generated and consumed by the UAV-UEs. Massive MIMO networks are also able to mitigate the strong downlink interference otherwise experienced by UAV-UEs through beamforming and a null-steering. However, massive MIMO networks may be particularly susceptible to reference signal contamination because they rely heavily on accurate channel state information (CSI) acquisition.

In one specific example, each of the massive MIMO base stations may include three sectors, each equipped with an 8 x 8 array of cross-polarised elements, which may serve a maximum of 8 users on each physical resource block. Pilot signal reuse 3 may be utilised, in which each pilot signal is orthogonal among the three sectors of the same base station but is reused among all base station sites. Fig. 7 is a graph showing the cumulative distribution function (CDF) versus bit rate for a massive MIMO cellular network which is in accordance with this specific example. The massive MIMO cellular network serves both UAV-UEs 12 and ground UEs 14 through spatial multiplexing, with a ratio of one active UAV-UE 12 for eveiy fourteen active GUEs 14. Fig. 7 shows five curves, which from left- to-right illustrate:

1) the downlink (DL) data rate for GUEs in a scenario including only GUEs and perfectly accurate channel state information; 2) the DL data rate for GUEs in a scenario including only GUEs, but including reference signal contamination from other GUEs;

3) the DL data rate for GUEs in a scenario including reference signal

contamination from the GUEs and UAV-UEs;

4) the DL data rate for UAV-UEs in a scenario which includes only UAV-UEs and assumes perfect CSI acquisition; and

5) the DL data rate for UAV-UEs in a scenario that includes reference signal contamination from GUEs and UAV-UEs. It is evident from Fig. 7 that reference signal contamination can significantly degrade the rate for both UAV-UEs 12 and GUEs 14 when served by massive MIMO base stations. Indeed, the median rates attained under reference signal contamination are reduced to nearly half of those achievable with perfect CSI acquisition. Moreover, Fig. 7 shows that a single UAV-UE 12 per cell is responsible for a significant fraction of the GUE rate degradation (3rd curve from the right). The effective reference signal contamination is significantly reduced when it only comes from other GUEs (2nd curve from the right). The presence of UAV-UEs 12 within a cellular network may be commonplace in the near future. For instance, let us imagine that someone has attached their non-optimised legacy device to a UAV-UE 12 with the intention of streaming video from the device. In such a scenario, the device may cause severe reference signal contamination to a plurality of neighbouring base stations, and may thus be a threat to the entire network.

Apparatuses, systems and methods described herein may serve to reduce the amount of reference signal contamination caused by UEs 12, such as airborne UEs (e.g. UAV-UEs), which are capable of causing unacceptable levels of reference signal contamination.

Various examples of the apparatuses, systems and methods may reduce the amount of reference signal contamination in a reference-signal-overhead efficient manner by dynamically reserving and releasing as required. In addition, examples of the approaches described herein may be utilised with existing network components (and may not require, for instance, additional signal processing capabilities which may not currently be available in existing network components).

Fig. 2 is a schematic illustration of various examples of approaches which may be utilised by the base stations 10-1 to 10-n within the network (or control apparatuses 60 associated with the base stations 10-1 to 10-n) to reduce the amount of reference signal

contamination caused by UEs 12, such as UAV-UEs. Figures 3 and 3A are flow charts illustrating examples of operations which may be performed by base stations (or their associated control apparatus 60) within the network l.

The situation depicted in Fig. 2 is similar to that shown in Fig. 1. In this case, the UAV-UE 12 is being served by a first of the base stations io-i. The first base station io-i may be referred to as the serving base station. The other base stations 10-2 to io-h may be referred to as neighbouring base stations. As will be discussed in more detail later, in some examples, the neighbouring base stations 10-2 to 10-n may include only base stations operated by the same network operator. In other examples, the neighbouring base stations may include base stations operated by at least two different operators. In examples in which the base stations are operated by at least two different operators, there may be a large degree of overlap in coverage areas provided by the base stations (for instance, two base stations operated by different operators may cover substantially the same area). Alternatively, the coverage areas of the base stations operated by different operators may not substantially overlap (although some degree of overlap is required in order for effective handover to be performed). Such an approach may be utilised when the distribution of base stations is intended to provide land mass coverage, instead of population coverage. The neighbouring base stations may not be required to directly neighbour the serving base station, but instead may be base stations that are within a particular area of the serving base station. Alternatively, the neighbouring base stations may be base stations that are within a particular area of the UAV-UE 12 and so are near enough to the UAV-UE 12 to communicate with it (and thereby to potentially suffer from reference signal

contamination caused by the UAV-UE 12).

In Fig. 2, the other base stations 10-2 to 10-n are only shown to be serving GUEs 14 (and not any UAV-UEs 12). However, as will be appreciated, the other base stations 10-2 to 10- n may also be serving UAV-UEs 12.

Turning now to Fig. 3A, which shows example operations which may be performed by the serving base station 10-1, in operation 3A-1, the serving base station 10-1 may determine that a UE 12 that it is serving has the potential to cause unacceptable reference signal contamination for one or more other base stations.

The determination of operation 3A-1 may be performed in any suitable manner. For instance, the UE 12 may identify itself as a UE 12 of the first type (that is, a UE, which has the potential to cause unacceptable reference signal contamination). For instance, some UAV-UEs 12 may identify themselves as such to their serving base stations 10-1.

In response to determining that a UE 12 being served by the serving base station has the potential to cause unacceptable reference signal contamination, the serving base station 10-1 may perform operation 3A-2, in which the serving base station 10-1 monitors the UE 12. In other examples, such as the legacy device scenario described above, the UE may not identify itself as a UE 12 of the first type. In such examples, operation 3A-1 may be omitted. As such, the serving base station 10-1 may instead monitor all UEs that it is serving.

Monitoring the UE 12 may include monitoring signals associated with the UE 12, which may indicate that the UE 12 is causing, or is expected soon to cause, reference signal contamination for neighbouring base stations 10-2 to 10-n. In some examples, the monitoring may include monitoring signals indicative of the height of the UE 12 and/or the level of interference caused by the UE 12 to neighbouring base stations.

The monitored signals may include any one or more of the following: Global Navigation Satellite System (GNSS) data indicative of a position of the UE, barometric pressure sensor data indicative of a height of the UE, measurement reports received from the UE

(such as reference signal received power (RSRP) reports, reference signal received quality (RSRQ) reports, received signal strength indicator (RSSI) reports), information from the core network, and any other indications received from the UE 12. For example, if it is determined by the base station (e.g. based on received measurement reports) that the UE 12 is receiving strong signals from a plurality of different base stations, the serving base station 10-1 may infer that the UE 12 is in line of sight with multiple base stations. This information may be indicative of height of the UE 12 and/or the level of interference caused by the UE 12 to those other base stations. In other examples, base stations may inform one another (directly or via the core network) regarding interference observed on particular physical resource blocks. In this way, if a serving base station receives information indicating that a neighbouring base station is observing strong interference on a certain physical resource block, the serving base station 10-1 may infer that one of the UEs 12 scheduled on this resource block is the source of interference observed by the neighbouring base station.

In operation 3A-3, the serving base station 10-1 determines whether the UE 12 satisfies a condition associated with causing reference signal contamination for one or more other base stations. The condition may be, for instance, that the UE 12 is creating, is expected soon to be creating or has the potential to create reference signal contamination for the one or more other base stations. This may be performed in any suitable way, based on the signals monitored in operation

3A-2. For instance, the serving base station may compare a particular property associated with the UE 12 to a threshold. Examples of this may include, but are of course not limited to, determining that the monitored UE 12 is above a threshold height (or distance from the ground) or determining that the UE 12 is generating interference for the other base stations 10-2 to 10-n that is above a threshold limit. If it is determined that the UE satisfies the condition associated with causing reference signal contamination for other base stations, the UE 12 may be termed a“threatening UE”.

The threshold height may be a height above which the UE 12 has a line of sight to the other base stations 10-2 to 10-n. Alternatively, the threshold height may be a height just below that at which the UE 12 has a line of sight to the other base stations 10-2 to 10-n (so the UE is expected soon to cause reference signal contamination to the other base stations). Similarly, the threshold interference may be set at the level which indicates that unacceptable reference signal contamination is already occurring or which indicates that unacceptable reference signal contamination is expected soon to occur.

In some specific examples, the serving base station 10-1 may ask the UE 12 how much signal power it is receiving from neighbouring base stations. The UE 12 may report this to the base station, for instance, in the form of RSRP measurement reports. If the UE 12 is receiving a signal power that is in excess of a threshold from one or more neighbouring base stations, the serving base station 10-1 may determine that the UE is likely to generate unacceptable reference signal contamination to said neighbouring base stations (due to uplink/downlink channel reciprocity). Therefore, the serving base station 10-1 may determine that the UE 12 satisfies the condition associated with causing reference signal contamination for one or more other base stations (so the UE 12 is a threatening one).

In response to a positive determination in operation 3A-3 (that is, a determination that the condition associated with causing reference signal contamination for one or more other base stations 10-2 to 10-n is satisfied), the serving base station 10-1 performs operation 3A-4. In response to a negative determination in operation 3A-3 (that is, a determination that the condition associated with causing reference signal contamination for one or more other base stations 10-2 to 10-n is not satisfied), the serving base station 10-1 continues to monitor properties associated with the UE 12.

In operation 3A-4, the serving base station 10-1 allocates one or more reference signals which are to be reserved for use by the UE 12. The allocation of reserved reference signals is determined dynamically, as needed.

As will be explained in more detail below, the identities of the reserved reference signals may be communicated to other base stations in the particular area. The other base stations may also then reserve those reference signals, such that they are not allocated for UEs 14 of the first type, such as GUEs.

Reserving reference signals may mitigate reference signal contamination and may thus provide better channel estimates and precoding gain. However, reserving reference signals consumes reference signal resources, which cannot be allocated to other users. As such, the allocation may be performed in a manner which seeks a fair and optimum performance for both UEs 12 of the first type (e.g. UAV-UEs) and UEs 14 of the second type (e.g. GUEs). For instance, the serving base station 10-1 may solve an optimization problem in order to determine the allocation. This may take a number of factors into account. For example, the optimization problem may take into account the number of threatening UEs 12 (for instance with respect to the number of available orthogonal pilot signals). The number of threatening UEs 12 may be the number determined by the serving base station 10-1 or may include the total number detected by the serving base station 10-1 and other base stations 10-2 to 10-n within a particular geographic area. For the latter case, as will be discussed below, the base stations within a particular geographic area may communicate to one another information regarding threatening UEs that have been detected. The optimisation problem may also take into account a channel coherence time for the threatening UEs, which is indicative of the frequency with which the channel needs to be estimated using reference signals. The channel coherence time (the time for which the channel conditions remain approximately constant) for the threatening UEs may be dependent on a number of factors including, but of course not limited to, the speed of the UE and the traffic dynamics. The coherence time for the threatening UEs may be taken into account because the more frequently a channel needs to be estimated using reference signals, the higher the overhead caused by reserving reference signals for threatening UAVs. In some examples, one or more of the coherence bandwidth and a priority of the threatening UE with respect to the non-threatening may additionally or alternatively be taken into account in the optimisation problem. In operation 3A-5, the serving base station 10-1 identifies one or more other base stations 10-2 to 10-n in the area to notify regarding the reserved reference signals. The other base stations may be identified in a number of different ways, examples of which are described below.

In some examples, the other base stations 10-2 to 10-n may be identified based on one or more measurement reports received from the threatening UE 12. The measurement reports may be indicative of the strength of signals received by the threatening UE 12 from base stations other than the serving base station. For instance, the measurement reports may include any one or more of reference signal received power (RSRP) measurements, received signal strength indicator (RSSI) measurements, and reference signal received quality (RSRQ) measurements. In such examples, the serving base station 10-1 may identify those other base stations for which the RSRP (or either of the RSSI and RSRQ) measurement received from the threatening UE 12 is above a certain threshold.

Alternatively, the serving base station 10-1 may identify those other base stations for which any RSRP (or either of the RSSI and RSRQ) measurement is received from the threatening UE 12 regardless of its value. Other examples of the way in which base stations are identified may include determining which base stations are within a predetermined distance of the serving base station 10-1 or determining which base stations are within a predetermined distance of the threatening UE 12. As will be appreciated, in the latter example, the base station will utilise knowledge of the location of the threatening UE 12. This knowledge may be obtained in any suitable way.

In operation 3A-6, having identified the other base stations, the serving base station 10-1 notifies the identified at least one other base station of the identities of the one or more reference signals that are to be reserved. The notification identifying the one or more reference signals that are to be reserved may, in some examples, further indicate the identity of the threatening UE 12. This notification from the serving base station 10-1 (which may be referred to as the first notification) is labelled in both Figs. 2 and 3A/3B using the reference“A”. In some examples, the first notification A may additionally include information identifying other reference signals that have previously been reserved in response to other base stations that have detected threatening UEs 12. The serving base station 10-1 may previously have been notified of these other reserved reference signals by other base stations. The serving base station 10-1 may keep a record of all reserved reference signals of which they are aware. The record may also include identities of the threatening UEs that the serving base station 10-1 has detected or about which the serving base station 10-1 has been notified. For instance, the first notification may include a table identifying the threatening UEs 12 and the corresponding reserved reference signals.

In the notification (and in the various other notifications described herein), the reference signals may be identified by identifying the time/frequency resources employed for the reference signal, as well as the corresponding orthogonal sequence that is to be employed. The sequence may be one that is selected from an available pool of orthogonal sequences.

In some examples, the first notification A may also include other information, beyond the information identifying the reference signals and the identity of the threatening UE(s). For example, the notification may include a measure of a severity of the interference generated by the threatening UE(s) 12. In some examples, a single measure of the severity may be included in the first notification (the same first notification being sent to each of the other base stations). In other examples, the measure of severity may be indicated separately for each base station to which the first notification is being sent. For instance, a different version of the first notification may be sent to each base station. Alternatively, the first notification may indicate plural measures of severity and may identify the base station to which each of the measure applies. The measure of the severity of interference may, for instance, be determined based on measurement reports received from the threatening UE. For instance, a higher value indicated by the measurement report may result in a higher measure of severity of interference. In some examples, the RSRP measurement may be utilised to determine the measure of severity of interference while, in other examples, any one or more of RSRP, RSSI and RSRQ may be utilised. As will be discussed in more detail below, other base stations may use this measure of severity of interference to decide whether or not to reserve the reference signal(s) identified in the notification.

In the example of FIG. 2, the serving base station 10-1 has sent the first notification A to three of its immediate neighbours 10-2, 10-3, 10-4 (that is neighbours having adjacent coverage areas), but has not sent the first notification to the other base station 10-n, which is not an immediate neighbour of the serving base station. In other examples, however, the serving base station 10-1 may send the first notification to any base stations that were identified in operation 3A-5, regardless of whether or not they are immediate neighbours of the serving base station 10-1.

As mentioned above, in some examples, the identified other base stations 10-2 to 10-n may include only base stations operated by the same network operator as the serving base station 10-1. In other examples, the neighbouring base stations may include base stations operated by at least two different operators. In examples in which the base stations are operated by at least two different operators, there may be a large degree of overlap in coverage areas provided by the base stations (for instance, two base stations operated by different operators may cover substantially the same area). Alternatively, the coverage areas of the base stations operated by different operators may not substantially overlap (although some degree of overlap is required in order for effective handover to be performed). The first notification A may be communicated over a one-to-one interface connection between base stations. For instance, the X2 or Xn interface might be employed. As will, of course, be appreciated, other communication channels/interfaces may instead be utilised for the delivery between base stations of information relating to reference signal contamination (such as, but not limited to the first notification A).

Prior to continuing with the operations of Fig. 3A, we will now turn to Fig. 3B to consider how the other base stations respond to receipt of the first notification A.

In operation 3B-1, the other base station (in Fig. 2 that is one of base stations 10-2, 10-3, 10-4) receives the first notification A.

In operation 3B-2, the other base station 10-2, 10-3, 10-4 determines whether or not to reserve the reference signals identified in the first notification. This determination may be made depending on whether reserving the reference signals causes a reference signal overhead that outweighs the capacity benefits provided by reference signal contamination relief. As discussed above, in some examples, the first notification A may include an indication of the level of severity of the interference caused by the threatening UE 12. This may be used in operation 2B-2 to determine whether or not to reserve the reference signals.

In some examples, base stations may employ look-up tables to determine whether or not to reserve the reference signals. For instance, the look up tables may allow the base station to determine whether to reserve the reference signals based on a number of factors which may include one or more of: the level of severity of interference, a number of threatening UEs and a number of UEs of the second type (non-threatening UEs) 14 being served by the base station. In some examples, the look up tables may be used to identify a throughput degradation which may result from the presence threatening UEs 12, which may then be used to determine whether or not to reserve the reference signals.

If it is determined to reserve the reference signals identified in the first notification, the other base station 10-2, 10-3, 10-4 reserves the identified reference signals for use by the threatening UE 12. In some examples, the other base station 10-2, 10-3, 10-4 may reserve the one or more reference signals for use by the specific threatening UE 12 identified in the first notification A. As such, the first notification A may be said to identify one or more reference signals that should be reserved by the recipient base station for a specific threatening UE 12. Similarly, in some examples, the first notification may identify plural threatening UEs and may identify a respective set of one or more reference signals that are to be reserved specifically for each of the threatening UEs. In this way, all of the base stations may reserve the same one or more reference signals for the same threatening UE. As mentioned above, reserving the reference signals means that they are not available for use by UEs 14 of the second type, such as GUEs.

In some examples, whether or not it has been determined to reserve the reference signals identified in the first notification A, the other base station may, in operation 3B-4, notify other base stations (for instance base stations that are further from the serving base station 10-1) of the identities of the reserved reference signals. This information may be provided in a second notification B (as illustrated in both Figs. 2 and 3B). In this way, information regarding the reserved reference signals may be disseminated over a larger area.

The second notification may include identities of all of the reserved reference signals of which the other base station 10-2 to 10-4 is aware. In this regard, each of the base stations in the network may be configured to keep a record of all of the reserved reference signals about which they have been notified by one or more other base stations. For instance, each of the base stations may store a table which contains identities of the threatening UEs 12 about which they have been notified as well as the corresponding reference signals reserved for those threatening UEs 12. This table may then be communicated as part of the second notification B. In other examples, the second notification B may include only the information received in the first notification A. For instance, the base station may simply forward the first notification A.

As with the first notification A, the second notification B may be transferred between the base stations via a one-to-one connection interface (such as the X2 or Xn interfaces).

Although it is not illustrated in either of Figs. 2 or 3B, base stations which receive the second notification may respond, in a manner that is similar to that described with reference to Fig. 3B.

Returning now to Fig. 3A, after sending the first notification A to the other base stations 10-2, 10-3, 10-4, the serving base station 10-1 may proceed to operation 3A-7 in which it monitors the threatening UE 12. This operation may be substantially as described with reference to operation 3A-2.

In operation 3A-8, the serving base station 10-1 determines whether the threatening UE 12 still satisfies the condition associated with causing reference signal contamination for one or more other base stations. This operation may be substantially as described with reference to operation 3A-3. For instance, the serving base station 10-1 may compare a particular property associated with the threatening UE 12 to a threshold and, based on this, may determine whether the threatening UE 12 still satisfies the condition. Similarly to as described above, examples of the condition may include the UE 12 being above a threshold height (or distance from the ground) or the UE 12 being determined as generating interference for the other base stations 10-2 to 10-n that is above a threshold limit. In such examples, if the height of the UE 12, or the interference generated by the UE 12, is determined no longer to be above (i.e. to be below) the threshold, a negative determination may be reached in operation 3A-8. Also, as will of course be appreciated, it may be determined that the previously-threatening UE 12 no longer satisfies the condition if it is determined that the UE 12 is no longer active.

If it is determined that the threatening UE 12 does still satisfy the condition associated with causing reference signal contamination for one or more other base stations, the serving base station 10-1 may continue to monitor the threatening UE 12 (in operation 3A- 7)· If it is determined that the threatening UE 12 does not still satisfy the condition associated with causing reference signal contamination for one or more other base stations, the serving base station 10-1 may proceed to operation 3A-9. In operation 3A-9, the serving base station 10-1 releases (or cancels) the reserved reference signals for the previously-threatening UE 12. By releasing the reserved reference signals, these reference signals may once again be used for UEs other than the previously-threatening UE 12. The releasing of the reserved reference signals may avoid the cost associated with an over-provisioning of reserved reference signal resources throughout the network 1.

The releasing of the reserved reference signals may include removing the identities of the reference signals reserved for the previously-threatening UE 12 (and the identity of the threatening UE, if appropriate) from the stored record. Although not illustrated in Fig. 3A, the serving base station 10-1 may also update the reference signals allocated to the UE

12 (which is no longer considered to be a threatening UE 12).

In operation 3A-10, the serving base station 10-1, notifies other base stations in the area that the reference signal reservation has been released. This notification may be referred to as a third notification C, which is illustrated by Figs. 2 and 3A/3B. The other base stations to which the third notification C is sent may be the same base stations to which the first notification A was communicated. The third notification C may be transferred between the base stations via a one-to-one connection interface (such as the X2 or Xn interfaces).

The third notification C may indicate the release of the previously-reserved reference signals in any suitable way. For instance, the third notification C may include an explicit indication of the previously-reserved reference signals that have been released. In some examples, the third notification C may also explicitly indicate the identity of the UE 12 that is no longer considered to be threatening. In other examples, the indication may be implicit. For instance, the third notification C may indicate (e.g. using the previously- mentioned table) all of the reserved reference signals about which the serving base station 10-1 is aware. However, in this case, the reference signals that were previously reserved for the previously-threatening UE 12 may not be included. The identity of the previously- threatening UE 12 may also be implicitly indicated in a similar manner.

After operation 3A-10, the serving base station may return to operation 3A-1. Returning now to Fig. 3B, in operation 3B-5, the other base station 10-2, 10-3, 10-4 receives the third notification C from the serving base station 10-1. In operation 3B-6, the other base station may respond to receipt of the third notification C by releasing the reference signals previously reserved for the previously-threatening UE 12.

On the basis of the third notification C, the other base station may additionally update its record of currently reserved reference signals and, in some examples, currently threatening UEs 12.

In operation 3B-7, the other base station may, similarly to operation 3B-4, notify other base stations (for instance base stations that are further from the serving base station 10- 1) that the previously-reserved reference signals are to be released. This information may be conveyed in a fourth notification D, and may include, either an explicit mention of the previously-reserved reference signals or information which implicitly identifies that the previously-reserved reference signals have been released (for instance, by their absence from a list of currently-reserved reference signals).

Figure 4 is a flowchart illustrating an example of an alternative approach which may be taken by the serving base station 10-1 in order to reduce reference signal contamination in the communication 1. In operation 4-1, the serving base station 10-1 determines that a UE 12 has the potential to cause unacceptable reference signal contamination for other base stations. This may be based, for instance, on an indication received from the UE 12. Such indications may include, but are not limited to, an indication that the UE 12 is a UAV-UE. In this example, the determination that the UE 12 has the potential to cause unacceptable reference signal contamination for the base stations may be considered to be a determination that the UE satisfies the condition associated with causing reference signal contamination for other base stations. Put another way, in this example, the condition that must be satisfied is simply that the UE 12 has the potential to cause unacceptable reference signal

contamination for other base stations.

In response to determining that the UE 12 satisfies the condition associated with causing reference signal contamination for other base stations, the serving base station may proceed to determine the reference signals that are to be reserved for the UE 12. This may be performed as described with reference to operation 3A-4 in Fig. 3A.

In operation 4-3, the base station 10-1 may identify other base stations to notify regarding the presence of the potentially threatening UE 12. This may be performed similarly to as described above with reference to operation 3A-5. In operation 4-4, the serving base station 10-1 notifies those other base stations. This may be performed using the first notification A, which may be substantially as described with reference to Figs. 3A and 3B. Although not illustrated in Fig. 4, the base stations which receive the first notification A may respond in the manner described with reference to Fig. 3B.

Next, in operation 4-5, the serving base station 10-1 monitors the potentially threatening UE 12 and determines whether it is still active. In response to determining that the potentially threatening UE 12 is still active, the serving base station 10-1 continues to monitor it. In response to determining that the potentially threatening UE 12 is not still active, the base station 10-1 proceeds to operation 4-6.

Operation 4-6 may be substantially as described with reference to operation 3A-9 of Fig. 3A. In operation 4-7, the serving base station 10-1 notifies the other base stations that the previously-reserved reference signals have been released. This notification may be the same as the third notification C described with reference to Figs. 3A and 3B. Again, although not illustrated in Fig. 4, the base stations which receive the third notification C may respond in the manner described in Fig. 3B. After operation 4-7, the serving base station 10-1 may return to operation 4-1.

As will be appreciated, the approach illustrated in Fig. 4 may only be appropriate for use with UEs that are configured to explicitly indicate that they have the potential to cause unacceptable levels of reference signal contamination for other base stations. For UEs 12 that do not provide such an explicit indication, the approach shown in Fig. 3A may be more appropriate. In addition, the approach of Fig. 3A may result in a lower reference signal overhead, than does the approach of Fig. 4. This is because reference signals may only be reserved when it is determined that the UE is already creating, or is expected soon to be creating, unacceptable reference signal contamination. In contrast, in the approach of Fig. 4, reference signals will be reserved for potentially threatening UEs 12, even when those UEs are not actually creating or expected soon to create reference signal contamination. As will also be appreciated, in some situations, a (potentially) threatening UE 12 may be handed over between two base stations, while it is still considered to be (potentially) threatening. In such examples, the identity of the serving base station changes and so operations 3A-7 to 3A-10 (or, in Figure 4, operations 4-5 to 4-7) may be performed by a different base station to that which performed operations 3A-1 to 3A-6 (or operations 4-5 to 4-7). Put another way, the base station to which the threatening UE 12 is handed over may take over the monitoring of the threatening UE and the subsequent determination as to whether the threatening UE still satisfies the condition/is active.

Figures 2, 3A and 3B, and 4 have been described in the context of a single threatening UE 12 being detected within a particular base station coverage area. However, as will be appreciated, there may be multiple threatening UEs within a particular coverage area.

The serving base station 10-1 may notify the other base stations regarding the detection of the threatening UEs 12 and the corresponding reserved reference signals on a one-by-one basis as they are detected. Alternatively, the serving base station 10-1 may wait for a particular period after detecting a first threatening UE 12 and, if a second threatening UE 12 is detected within the time period, the serving base station 10-1 may include the identities of the reference signals reserved for each of the first and second UEs 12 in a single notification. Put another way, the first notification A may include a batch of reference signals that have been reserved for plural detected threatening UEs. In this way, the amount of signalling between the base stations in the network 1 may be reduced.

It should be understood that the order of operations depicted in the flow charts of Figs. 3A, 3B and 4 may not be the only order in which the operations can be performed.

Indeed, various operations depicted in the flow charts may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described operations may be omitted or may be combined. For instance, as a non- limiting example, operation 3A-1 may be omitted from the method set out in Fig. 3A. Also, operations 3A-4 and 3A-5 may be performed in a reversed order and/or

concurrently. This is also true of operations 4-to 4-3 of Fig. 4. Similarly, in Fig. 3B, operations 3B-4 and 3B-7 may be omitted.

Centrally-Controlled Approach

In the examples described above, the serving base station 10-1 is responsible for reserving the reference signals for use by a UE that has been identified as satisfying the condition associated with causing reference signal contamination for other base stations (a threatening UE), and also for notifying the other base stations regarding the reserved reference signals. However, in other example implementations, each base station may report back to a central control apparatus when it detects that it is serving a threatening UE. This may be detected, for instance, as described with reference to operations 3A-1, 3A-2 and 3A-3. The report may include, for instance, the identity of the threatening UE, the identity of any other base stations with which the threatening UE may interfere and, in some examples, a measure of the severity of the interference. The central control apparatus may be configured to respond to receipt of this report by allocating one or more reference signals which are to be reserved for use by the

threatening UE. This may be performed similarly to operation 3A-4. The central control apparatus may then identify the other base stations (similarly to as described in operation 3A-5). The central control apparatus then notifies the serving base station and the one or more other base stations of the reference signals reserved for the threatening UE. This may be similar to operation 3A-6. The serving base station and the one or more other base stations may respond to the notification in a manner that is similar to that described with reference to operation 3B-2. The serving base station 10-1 continues to monitor the threatening UE (similarly to 3A-7). In response to determining that the UE is no longer threatening (similarly to 3A-8), the serving base station 10-1 may report this to the central control apparatus. The central control apparatus may then instruct the serving base station 10-1 and the one or more other base stations to release the reserved one or more reference signals. The serving base station and the one or more other base stations may respond to the instruction in a manner that is similar to that described with reference to operation 3B-6.

Since central coordination within multiple operators may be unfeasible, the base station- controlled approach described above with reference to Figs. 1 to 4 may be more suitable than is this centrally-controlled approach for situations in which the base stations are operated by multiple different operators. In addition, the base station controlled approach may result in less traffic being generated.

Example Physical Configurations of Computing Apparatuses Figure 5 is a schematic illustration of an example configuration of the base stations 10-1 to 10-n (generally referenced using numeral 10) described with reference to Figures 1 to 4, which may be configured to mitigate reference signal contamination within the network. The base station 10 comprises a control apparatus 50. The control apparatus 50 may be configured to process signals received from UEs 12, 14 via the antenna array 501 (which may, in some examples, be a massive MIMO antenna array) and a radio frequency interface 503. The control apparatus 50 may comprise processing apparatus 502 and memory 504. Computer-readable code 504-2A may be stored on the memoiy 504 and may, when executed by the processing apparatus 502, cause the control apparatus 50 to perform any of the operations described with reference to Figures 1 to 4.

In some examples, the control apparatus 50 may also be configured to control the radio frequency interface 503 to generate suitable RF signals for transmission to the UEs 12, 14 via the antenna array.

The base station 10 may also comprise a second interface 509 via which it can

communicate with the other parts of the network, such as other base stations. For instance, the second interface 509 may be a one-to-one communication interface, such as an X2 or Xn interface.

Although the configuration of the UE 12, 14, is not illustrated in the Figs., it should be appreciated that the UE 12, 14, may be of any configuration that is suitable for

communicating with the wireless communication network 1 described herein.

As will be appreciated, the base station and associated apparatuses illustratively depicted in Figure 5 and described above with reference to Figures 1 to 4 may comprise further components which may or may not be directly involved with processes and operations in respect of which this application is focussed.

Some further details of components and features of the above-described apparatus 10, 50 and alternatives for them will now be described.

The control apparatus 50 described above may comprise processing apparatus 501, 402 communicatively coupled with memoiy 504. The memoiy 504 has computer readable instructions 504-2A stored thereon which, when executed by the processing apparatus 502 causes the control apparatus 50 to cause performance of various ones of the operations described with reference to Figures 1 to 4. The control apparatus 50 may, in some instances, be referred to as simply“apparatus”.

The processing apparatus 502 may be of any suitable composition and may include one or more processors 502A of any suitable type or suitable combination of types. Indeed, the term“processing apparatus” should be understood to encompass computers having differing architectures such as single/multi-processor architectures and

sequencers/parallel architectures. For example, the processing apparatus 502 may be a programmable processor that interprets computer program instructions 504-2A and processes data. The processing apparatus 502 may include plural programmable processors. Alternatively, the processing apparatus 502 may be, for example,

programmable hardware with embedded firmware. The processing apparatus 502 may alternatively or additionally include one or more specialised circuit such as field programmable gate arrays FPGA, Application Specific Integrated Circuits (ASICs), signal processing devices etc. In some instances, processing apparatus 502 may be referred to as computing apparatus or processing means.

The processing apparatus 502 is coupled to the memory 504 and is operable to read/write data to/from the memoiy 504. The memoiy 504 may comprise a single memoiy unit or a plurality of memoiy units, upon which the computer readable instructions (or code) 504- 2A is stored. For example, the memoiy 504 may comprise both volatile memoiy 504-1 and non-volatile memory 504-2. In such examples, the computer readable

instructions/program code 504-2A may be stored in the non-volatile memoiy 504-2 and may be executed by the processing apparatus 502 using the volatile memory 504-1 for temporary storage of data or data and instructions. Examples of volatile memory include RAM, DRAM, and SDRAM etc. Examples of non-volatile memory include ROM, PROM, EEPROM, flash memory, optical storage, magnetic storage, etc.

The memoiy 504 may be referred to as one or more non-transitoiy computer readable memoiy medium or one or more storage devices. Further, the term‘memory’, in addition to covering memory comprising both one or more non-volatile memory and one or more volatile memory, may also cover one or more volatile memories only, one or more non- volatile memories only. In the context of this document, a“memoiy” or“computer- readable medium” may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer. The computer readable instructions/program code 504-2A may be pre-programmed into the control apparatus 50. Alternatively, the computer readable instructions 504-2A may arrive at the control apparatus 50 via an electromagnetic carrier signal or may be copied from a physical entity 60 such as a computer program product, a memory device or a record medium such as a CD-ROM or DVD, an example of which is illustrated in Figure 6. The computer readable instructions 504-2A may provide the logic and routines that enables the base stations 10 to perform the functionality described above. The

combination of computer-readable instructions stored on memory (of any of the types described above) may be referred to as a computer program product. In general, references to computer program, instructions, code etc. should be understood to express software for a programmable processor firmware such as the programmable content of a hardware device as instructions for a processor or configured or configuration settings for a fixed function device, gate array, programmable logic device, etc. The control apparatus 50 may, in some example implementations, be referred to as control circuitiy. As used in this application, the term“circuitry” may refer to one or more or all of the following:

(a) hardware-only circuit implementations (such as implementations in only analog and/or digital circuitry) and

(b) combinations of hardware circuits and software, such as (as applicable):

(i) a combination of analog and/or digital hardware circuit(s) with

software/firmware and

(ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a based station, to perform various functions) and

(c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation. This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device. Although various aspects of the methods and apparatuses described herein are set out in the independent claims, other aspects may comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims.

It is also noted herein that while the above describes various examples, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims. The extent of protection is defined by the following claims, with due account being taken of any element which is equivalent to an element specified in the claims.

Claims

1. An apparatus comprising means for performing:

determining that at least one user equipment, UE, being served by a first base station satisfies a condition associated with causing reference signal contamination for one or more other base stations;

responding to the determination by allocating one or more reference signals that are to be reserved for use by the at least one UE; and

causing notification to at least the one or more other base stations of the identities of the one or more reference signals that are to be reserved.

2. The apparatus of claim 1, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is creating, is expected soon to be creating or has the potential to create reference signal contamination for one or more other base stations.

3. The apparatus of claim 1 or claim 2, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is above, or is approaching, a threshold height.

4. The apparatus of claim 1 or claim 2, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is generating interference for the one or more other base stations that is above a threshold limit.

5. The apparatus of claim 1 or claim 2, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is a potential source of unacceptably high interference for the one or more other base stations.

6. The apparatus of claim 1 or claim 2, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is, or is carried on, an unmanned aerial vehicle.

7. The apparatus of any one of the preceding claims, wherein the one or more other base stations are identified based on a location of the first base station or of the at least one UE. 8. The apparatus of claim 7, wherein the one or more other base stations are within a threshold distance of the location of the first base station or of the at least one UE.

9. The apparatus of any one of the preceding claims, wherein the one or more other base stations are identified based on a measurement report received from the at least one UE and/ or other information.

10. The apparatus of any one of the preceding claims, comprising means for causing notification to the one or more other base stations of:

the identities of the one or more reference signals reserved for the at least one UE; and

the identity of the at least one UE.

11. The apparatus of claim 10, wherein the means for causing notification is configured to cause notification to the one or more other base stations of:

the identities of plural UEs that have been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations; and

the identities of the one or more reference signals reserved for the plural UEs. 12. The apparatus of any one of the preceding claims, wherein the reference signals that are to be reserved for the at least one UE are determined based on one or more of: a number of UEs that have been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations;

a coherence time and/or a coherence bandwidth for the at least one UE; and requirements of other UEs, served by the first base station or the one or more other base stations, which have not been determined as satisfying the condition, associated with causing reference signal contamination for one or more other base stations. 13. The apparatus of any one of the preceding claims, comprising means for performing: determining that the reservation of the one or more reference signals can be released; and

notifying the one or more other base stations that the reservation of the reference signals can be released.

14. The apparatus of claim 13, wherein determining that the reservation of the one or more reference signals can be released is based on determining that the at least one UE is no longer satisfying the condition associated with causing reference signal contamination for one or more other base stations.

15. The apparatus of any one of claims 1 to 14, wherein the apparatus is the first base station.

16. An apparatus comprising means for performing:

receiving, at a base station, a notification of identities of one or more reference signals reserved for at least one user equipment, UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station; and

reserving, by the base station in response to receipt of the notification, the one or more reference signals identified in the notification.

17. The apparatus of claim 16, wherein the notification includes the identity of the at least one UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station.

18. The apparatus of claim 16 or claim 17, comprising:

means for determining whether to reserve the one or more reference signals identified in the notification. 19. The apparatus of claim 18, wherein determining whether to reserve the one or more reference signals identified in the notification is based on one or more of:

a number of reference signals currently being used by UEs served by the base station; and

a benefit provided by reserving the one or more reference signals.

20. The apparatus of any one of claims 16 to 19, comprising means for performing: receiving, at the base station, a notification that the reservation of the one or more reference signals can be released; and

releasing the reservation of the one or more reference signals in response to the notification that the reservation of the one or more reference signals can be released.

21. The apparatus of any one of claims 16 to 20, comprising:

means for causing notification, by the base station to at least one other base station, of the identities of the one or more reserved reference signals. 22. The apparatus of claim 21, wherein notification, by the base station to at least one other base station, of the identities of one or more reserved reference signals is caused in response to receipt of the notification of the identities of one or more reference signals reserved for at least one UE which has been determined as satisfying the condition associated with causing reference signal contamination for the base station.

23. A base station comprising the apparatus of any one of the preceding claims.

24. A method comprising:

determining that at least one user equipment, UE, being served by a first base station satisfies a condition associated with causing reference signal contamination for one or more other base stations;

responding to the determination by allocating one or more reference signals that are to be reserved for use by the at least one UE; and

causing notification to at least the one or more other base stations of the identities of the one or more reference signals that are to be reserved.

25. The method of claim 24, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is creating, is expected soon to be creating or has the potential to create reference signal contamination for one or more other base stations.

26. The method of claim 24 or claim 25, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is above, or is approaching, a threshold height.

27. The method of claim 24 or claim 25, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is generating interference for the one or more other base stations that is above a threshold limit.

28. The method of claim 24 or claim 25, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is a potential source of unacceptably high interference for the one or more other base stations.

29. The method of claim 24 or claim 25, wherein determining that the at least one UE satisfies the condition associated with causing reference signal contamination for one or more other base stations comprises determining that the at least one UE is, or is carried on, an unmanned aerial vehicle.

30. The method of any one of claims 24 to 29, wherein the one or more other base stations are identified based on a location of the first base station or of the at least one UE.

31. The method of claim 30, wherein the one or more other base stations are within a threshold distance of the location of the first base station or of the at least one UE.

32. The method of any one of claims 24 to 31, wherein the one or more other base stations are identified based on a measurement report received from the at least one UE and/or other information.

33. The method of any one of claims 24 to 32, comprising causing notification to the one or more other base stations of:

the identities of the one or more reference signals reserved for the at least one UE; and

the identity of the at least one UE.

34. The method of claim 33, wherein causing notification to the one or more other base stations comprises causing notification to the one or more other base stations of: the identities of plural UEs that have been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations; and

the identities of the one or more reference signals reserved for the plural UEs.

35. The method of any one of claims 24 to 34, wherein the reference signals that are to be reserved for the at least one UE are determined based on one or more of:

a number of UEs that have been determined as satisfying the condition associated with causing reference signal contamination for one or more other base stations;

a coherence time and/or a coherence bandwidth for the at least one UE; and requirements of other UEs, served by the first base station or the one or more other base stations, which have not been determined as satisfying the condition, associated with causing reference signal contamination for one or more other base stations.

36. The method of any one of claims 24 to 35, comprising:

determining that the reservation of the one or more reference signals can be released; and

notifying the one or more other base stations that the reservation of the reference signals can be released.

37. The method of claim 36, wherein determining that the reservation of the one or more reference signals can be released is based on determining that the at least one UE is no longer satisfying the condition associated with causing reference signal contamination for one or more other base stations.

38. A method comprising:

receiving, at a base station, a notification of identities of one or more reference signals reserved for at least one user equipment, UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station; and

reserving, by the base station in response to receipt of the notification, the one or more reference signals identified in the notification.

39. The method of claim 38, wherein the notification includes the identity of the at least one UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station. 40. The method of claim 38 or claim 39, comprising:

determining whether to reserve the one or more reference signals identified in the notification.

41. The method of claim 40, wherein determining whether to reserve the one or more reference signals identified in the notification is based on one or more of:

a number of reference signals currently being used by UEs served by the base station; and

a benefit provided by reserving the one or more reference signals.

42. The method of any one of claims 38 to 41, comprising:

receiving, at the base station, a notification that the reservation of the one or more reference signals can be released; and

releasing the reservation of the one or more reference signals in response to the notification that the reservation of the one or more reference signals can be released.

43. The method of any one of claims 38 to 42, comprising:

causing notification, by the base station to at least one other base station, of the identities of the one or more reserved reference signals.

44. The method of claim 43, wherein notification, by the base station to at least one other base station, of the identities of one or more reserved reference signals is caused in response to receipt of the notification of the identities of one or more reference signals reserved for at least one UE which has been determined as satisfying the condition associated with causing reference signal contamination for the base station.

45. Computer-readable instructions, which when executing by one or more computing apparatus, causes the computing apparatus to perform a method according to any one of claims 24 to 44.

46. An apparatus comprising:

at least one processor; and

at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform:

determining that at least one user equipment, UE, being served by a first base station satisfies a condition associated with causing reference signal contamination for one or more other base stations;

responding to the determination by allocating one or more reference signals that are to be reserved for use by the at least one UE; and causing notification to at least the one or more other base stations of the identities of the one or more reference signals that are to be reserved.

47. An apparatus comprising:

at least one processor; and

at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to perform:

receiving, at a base station, a notification of identities of one or more reference signals reserved for at least one user equipment, UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station; and

reserving, by the base station in response to receipt of the notification, the one or more reference signals identified in the notification.

48. A non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the following:

determining that at least one user equipment, UE, being served by a first base station satisfies a condition associated with causing reference signal contamination for one or more other base stations;

responding to the determination by allocating one or more reference signals that are to be reserved for use by the at least one UE; and

causing notification to at least the one or more other base stations of the identities of the one or more reference signals that are to be reserved.

49. A non-transitory computer readable medium comprising program instructions for causing an apparatus to perform at least the following:

receiving, at a base station, a notification of identities of one or more reference signals reserved for at least one user equipment, UE, which has been determined as satisfying the condition associated with causing reference signal contamination for the base station; and

reserving, by the base station in response to receipt of the notification, the one or more reference signals identified in the notification.


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