WO2016167696A1

WO2016167696A1 - Network node and method performed thereby for downlink interference mitigation in a cell of a serving rbs

Info

Publication number: WO2016167696A1
Application number: PCT/SE2015/050447
Authority: WO
Inventors: Tomas Lundborg; Fredric Kronestedt
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2015-04-17
Filing date: 2015-04-17
Publication date: 2016-10-20

Abstract

A network node and a method performed thereby are provided for downlink interference mitigation in a cell of a serving Radio Base Station, RBS, operable in a communication network employing at least two different Radio Access Technologies, RATs, i.e. a first RAT and a second RAT, the serving RBS employing the first RAT and serving wireless devices. The method comprises determining (110) potential interfering neighbouring RBS(s) employing the second RAT; and obtaining (120), from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT. The method further comprises obtaining (130), from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality; and employing (190) interference mitigation based on interference indications from neighbouring RBS(s) using the first and second RAT respectively and the obtained downlink measurement(s).

Description

NETWORK NODE AND METHOD PERFORMED THEREBY FOR DOWNLINK INTERFERENCE MITIGATION IN A CELL OF A SERVING RBS

Technical field

[0001 ] The present disclosure relates to wireless communication and in particular to interference mitigation in a cell of a serving Radio Base Station, RBS.

Background

[0002] Wireless communication is a technology that is ever evolving and usage thereof is ever increasing. Many different technologies have been developed during the years, wherein today wireless networks comprising more than one technology exist. Even if a wireless network may employ more than one

technology, the different technologies may be using same or adjacent frequency resources thereby possible causing interference, which generally is referred to as adjacent channel interference.

[0003] An example of a wireless communication technology is Global System for Mobile communication, GSM, which employs a Time Division Multiple Access, TDMA, technology. Code Division Multiple Access, CDMA, is another technology used. Another example is the Universal Mobile Telecommunications System, UMTS, which is based on GSM and uses Wideband CDMA, WCDMA technology. Still an example is Long Term Evolution, LTE, which employs Orthogonal

Frequency Division Multiplexing, OFDM.

[0004] When two technologies like WCDMA and LTE are deployed in the same band (adjacent frequencies) by an operator, adjacent frequency interference may occur due to a near-far situation. A wireless device communicating on a first technology, e.g. WCDMA, at the cell edge may be interfered by a neighbouring RBS of the other technology, e.g. LTE. This may result in dropped connection or bad performance since the wireless device cannot handover to the closest RBS when it is not capable of the second technology. Summary

[0005] The object is to obviate at least some of the problems outlined above. In particular, it is an object to provide a network node and a method performed thereby for downlink interference mitigation in a cell of a serving RBS. These objects and others may be obtained by providing an RBS and a method performed by an RBS according to the independent claims attached below.

[0006] According to an aspect a method performed by a network node for downlink interference mitigation in a cell of a serving RBS is provided. The RBS is operable in a communication network employing at least two different Radio Access Technologies, RATs, i.e. a first RAT and a second RAT, the serving RBS employing the first RAT and serving wireless devices. The method comprises determining potential interfering neighbouring RBS(s) employing the second RAT; and obtaining, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT. The method further comprises obtaining, from wireless device(s) communicating by means of the first RAT, downlink

measurement(s) relating to channel quality; and employing interference mitigation based on interference indications from neighbouring RBS(s) using the first and second RAT respectively and the obtained downlink measurement(s).

[0007] According to an aspect a network node for downlink interference mitigation in a cell of a serving RBS is provided. The RBS is operable in a communication network employing at least two different Radio Access

Technologies, RATs, i.e. a first RAT and a second RAT, the serving RBS employing the first RAT and serving wireless devices. The RBS is configured for determining potential interfering neighbouring RBS(s) employing the second RAT; and obtaining, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT. The RBS is further configured for obtaining, from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality; and employing interference mitigation based on interference indications from neighbouring RBS(s) using the first and second RAT respectively and the obtained downlink measurement(s).

[0008] The network node and the method performed by the network node may have several possible advantages. One possible advantage is that adjacent interference may be detected in multi-standard deployment for efficient coexistence between two technologies deployed in the same frequency band in the same area. The solution may associate two technologies with each other and provide an advantage for operating two systems from the same vendor in the same area.

Brief description of drawings

[0009] Embodiments will now be described in more detail in relation to the accompanying drawings, in which:

[00010] Figure 1 a is a flowchart of a method performed by a network node for downlink interference mitigation in a cell of a serving RBS, according to an exemplifying embodiment.

[0001 1 ] Figure 1 b is a flowchart of a method performed by a network node for downlink interference mitigation in a cell of a serving RBS, according to yet an exemplifying embodiment.

[00012] Figure 1 c is a flowchart of a method performed by a network node for downlink interference mitigation in a cell of a serving RBS, according to still an exemplifying embodiment.

[00013] Figure 1 d is a flowchart of a method performed by a network node for downlink interference mitigation in a cell of a serving RBS, according to a further exemplifying embodiment.

[00014] Figure 1 e is a flowchart of a method performed by a network node for downlink interference mitigation in a cell of a serving RBS, according to another exemplifying embodiment. [00015] Figure 1f is a flowchart of a method performed by a network node for downlink interference mitigation in a cell of a serving RBS, according to yet an exemplifying embodiment.

[00016] Figure 2a is an illustration of an interference situation for a wireless device in a mixed WCDMA and LTE heterogeneous network deployment.

[00017] Figure 2b is an illustration of an example of a measurement collection agent in a Self-Organising Network, SON, node.

[00018] Figure 2c is a flowchart illustrating an example of a method for determining adjacent channel interference level.

[00019] Figure 2d is a signalling diagram of an example of ordering periodic measurements.

[00020] Figure 2e is a block diagram illustrating an example of a decision function.

[00021 ] Figure 3 is a block diagram of a network node for downlink interference mitigation in a cell of a serving RBS, according to an exemplifying embodiment.

[00022] Figure 4 is a block diagram of a network node for downlink interference mitigation in a cell of a serving RBS, according to another exemplifying embodiment.

[00023] Figure 5 is a block diagram of an arrangement in a network node for downlink interference mitigation in a cell of a serving RBS, according to an exemplifying embodiment.

Detailed description

[00024] Briefly described, a network node and a method performed thereby for downlink interference mitigation in a cell of a serving RBS operable in a communication network employing at least two different Radio Access

Technologies, RATs, are provided. By obtaining measurements and interference indications of neighbouring RBSs employing the first RAT and obtaining measurements and interference indications of neighbouring RBSs employing the second RAT, the network node may determine if there is, or is a risk for, adjacent channel interference, whereby interference mitigation actions may be taken in order to improve the channel quality for wireless devices located in such an area where there is, or is a risk for, adjacent channel interference.

[00025] Adjacent channel interference may also occur in the uplink, however it may be most problematic in the downlink since RBSs use high transmission power. A solution is to deploy the two technologies on every site in the network so that the single technology wireless device may make handover and be served from the same site that it is interfered from. This is however costly and not preferred by network operators. Another solution is to modify the cell plan changing e.g. transmission power, antenna type, antenna height or antenna direction, but then there is need to determine in what cells the problem occur. Radio resource management could also be used to mitigate adjacent channel interference, e.g. avoid schedule users on resources close to the adjacent frequency.

[00026] Embodiments herein relate to a method performed by a network node for downlink interference mitigation in a cell of a serving Radio Base Station, RBS, operable in a communication network employing at least two different Radio Access Technologies, RATs, i.e. a first RAT and a second RAT, the serving RBS employing the first RAT and serving wireless devices.

[00027] Embodiments of such a method will now be described with reference to figures 1a-1f. Figure 1a illustrates the method comprising determining 1 10 potential interfering neighbouring RBS(s) employing the second RAT; and obtaining 120, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT. The method further comprises obtaining 130, from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality; and employing 190 interference mitigation based on interference indications from neighbouring RBS(s) using the first and second RAT respectively and the obtained downlink measurement(s). [00028] The serving network node may have several neighbours, i.e.

neighbouring RBSs, of the same and/or different RATs. An RBS may be associated with one or more cells, wherein each cell is a coverage area of the RBS. Some neighbouring RBSs may be associated with cells that overlap with the cell or cells of the serving RBS. When two RBSs have overlapping cells, i.e.

overlapping coverage areas, the two RBSs may cause interference with each other in the overlapping area, or areas. Consequently, a wireless device being within or just at the edge of an overlapping area may be experiencing interference due to the two RBS providing radio coverage in the overlapping area. The serving RBS may also have neighbouring RBSs that do not have any overlapping cell or coverage area with the serving RBS.

[00029] Different neighbouring RBSs may also employ different RATs. Thus, there may be one or more neighbouring RBSs that employ the first RAT and one or more neighbouring RBSs that employ the second RAT. Out of these

neighbouring RBSs, some may have overlapping coverage areas with the serving RBS and some may not have overlapping coverage areas with the serving RBS. Those RBSs that do have overlapping coverage areas with the serving RBS may potentially interfere with the serving RBS.

[00030] Consequently, the network node determines 1 10 potential interfering neighbouring RBS(s) employing the second RAT. How this is done will be described in more detail below.

[00031 ] The serving RBS may serve different types of wireless devices, some that may communicate by means of both RATs, some that may communicate only by means of the first RAT, and some that may communicate only by means of the second RAT. The wireless devices that are limited to communicating by means of only one of the RATs cannot be handed over to the other RAT for obvious reasons.

[00032] The network node, when performing the method, obtains 120, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering RBS(s) employing the second RAT. [00033] Wireless device that supports both the first and the second RAT may measure signals transmitted from the serving RBS, the signals being related to the first RAT and also signals transmitted from neighbouring RBS(s), the signals being related to the first and/or the second RAT. The wireless devices may then send measurement reports to the serving RBS comprising information about the performed measurements. The measurements may comprise results of

measurements with regards to a plurality of signals from a plurality of RBSs which the respective wireless devices may receive. Thus, signals that are received by wireless devices from RBS(s) employing the second RAT are measured and reported to the serving RBS.

[00034] The network node then, when performing the method, obtains 130, from wireless device(s) communicating by means of the first RAT, downlink

measurement(s) relating to channel quality.

[00035] The wireless devices that are being served by the RBS, which wireless devices are communicating by means of the first RAT may receive downlink signals from the serving RBS. Depending on where in the cell those wireless devices are located, the signal quality may typically vary. There may be obstacles within the cell that creates a so-called shadow, but generally, wireless devices located relatively far from the serving RBS and thus relatively close to the cell border of the serving cell of the serving RBS are experiencing worse signal quality than those wireless devices that are located relatively close to the serving RBS. Also, wireless devices located relatively far from the serving RBS are more likely to be located close to or within an overlapping area of the serving cell and a cell from a neighbouring RBS. By serving cell means the cell of the serving RBS that the wireless device is being served by. As stated above, an RBS may have a plurality of cells, but generally, a wireless device is only being served by a so- called serving cell of the serving RBS.

[00036] The wireless devices communicating by means of the first RAT thus receives signals associated with the first RAT, and the wireless devices may perform different measurements on the received signal, e.g. to estimate a received signal quality, or channel quality. The wireless devices transmit measurement reports to the serving RBS informing the serving RBS about the current channel quality and radio conditions that the wireless devices are currently experiencing.

[00037] The network node may further receive interference indications from neighbouring RBS(s) employing the first and the second RAT. The network node may communicate directly (e.g. over an X2 interface) or indirectly (via intermediate nodes) with the neighbouring RBS(s) of the serving RBS. By means of this communication, the network node may obtain information from the neighbouring RBS(s) pertaining to different types of interference indications with may affect wireless devices of the serving RBS. Examples of interference indications will be given and discussed in more detail below.

[00038] The network node may then, determine which interference mitigation procedures, or actions, to engage/take in order to improve the interference situations for wireless devices based on interference indications from neighbouring RBS(s) employing the first and the second RAT and the obtained downlink measurement(s). The interference mitigation procedures, or actions, to

engage/take in order to improve the interference situations for wireless devices may be with respect to adjacent channel interference.

[00039] The method performed by the network node may have several possible advantages. One possible advantage is that adjacent interference may be detected in multi-standard deployment for efficient coexistence between two technologies deployed in the same frequency band in the same area. The solution may associate two technologies with each other and provide an advantage for operating two systems from the same vendor in the same area.

[00040] Determining 1 10 potential interfering neighbouring RBS(s) employing the second RAT may comprise receiving handover and/or mobility measurements from wireless devices capable of operating according to both the first and the second RAT and comparing signal strength of neighbour RBS(s) employing the second RAT and signal strength of neighbouring RBS(s) employing the first RAT or signal strength of the serving RBS. [00041 ] When wireless devices employing both RATs move around in the cell and find themselves in an area where there is an overlap between the serving cell of the serving RBS employing the first RAT and a cell of a neighbouring RBS employing the second RAT, those wireless devices may receive signals, data, pilot, or reference signals. The wireless devices may perform different types of measurements on the received channels. Two examples of measurements such a wireless device may perform are handover measurements and mobility

measurements. Mobility measurements may be performed by the wireless device when in idle mode and/or in connected mode. Handover measurements and mobility measurements may also be referred to as measurements for handover and cell reselection.

[00042] The handover measurements may be similar or different between different RATs, e.g. GSM, UMTS/WCDMA, and LTE. Also mobility measurements may be similar or different between different RATs. The manner in which these measurements are performed are out of the scope of this disclosure.

[00043] It shall be pointed out that measurements from a plurality of wireless devices received over a period of time may be used.

[00044] By means of these measurements, the network node may determine which neighbouring RBSs employing the second RAT that are potential interferers.

[00045] The method may further comprise, as illustrated in figure 1 b, estimating 140 adjacent channel interference from neighbouring RBS(s) employing the second RAT by dividing the signal strength of respective neighbouring RBS employing the second RAT by ACIR_e, where ACIR_e is Adjacent Channel

Interference Rejection ratio estimated from measurements and/or theoretical calculations.

[00046] The adjacent-channel interference which receiver A experiences from a transmitter B is the sum of the power that B emits into As channel— known as the "unwanted emission", and represented by the ACLR (Adjacent Channel Leakage Ratio) and the power that A picks up from B's channel, which is represented by the ACS (Adjacent Channel Selectivity). B emitting power into As channel is called adjacent-channel leakage (unwanted emissions).

[00047] The adjacent-channel interference may thus be estimated by dividing the signal strength of respective neighbouring RBS employing the second RAT

/ C C \ h_u Λ ί Ο ■ S^LTE neighbour

^LTEjieighbour) oy L l K_e . _{A IRe} ^■

[00048] Potential adjacent channel interference issues may occur when the estimated adjacent interference with regard to signals transmitted from

neighbouring RBSs employing the second RAT is larger or in the same order as the signal strengths of signals transmitted from neighbouring RBSs employing the first RAT.

[00049] The method may further comprise, as illustrated in figure 1c, determining 150 a correlation between interference indications of RBS(s) employing the first and second RAT and quality of signals received by wireless device(s) on a serving channel of the serving RBS , wherein the interference mitigation is further based on the correlation.

[00050] The network node may determine 150 the correlation between

interference indications of RBS(s) employing the first and second RAT respectively and quality of signals received by wireless device(s) on a serving channel of the serving RBS employing the first RAT. The serving channel is the frequency, frequencies or channel that is used between the serving RBS and a wireless device. The interference indications have been obtained as explained above, and examples and explanations of interference indications will be given below.

[00051 ] The correlation generates information about a relationship between the interference indications of RBS(s) employing the first and second RAT

respectively, and the quality of signals received by wireless device(s) on a serving channel of the serving RBS.

[00052] By performing the above described correlation, the network node may further determine the current interference situation with regard to how the potential neighbouring RBS(s) employing the second RAT and neighbouring RBS(s) employing the first RAT affect the signals transmitted to a wireless device when that wireless device is in an area of the serving cell of the serving RBS that overlaps with cell(s) of neighbouring RBS(s).

[00053] In an example, the method further comprises, as illustrated in figure 1d, disabling 160 inter-frequency handover for wireless devices communicating by means of the first RAT being involved in obtaining downlink measurements.

[00054] There may be neighbouring RBS(s) employing the first RAT and there may be neighbouring RBS(s) employing the second RAT. By disabling inter- frequency handover for wireless devices communicating by means of the first RAT being involved in obtaining downlink measurements, the network node may be provided with more measurements from wireless devices communicating by means of the first RAT.

[00055] These interference indications may be obtain e.g. by direct or indirect (via intermediate nodes) communication with those RBSs.

[00056] In yet an example, obtaining 120, 160 interference indications regarding the potential interfering neighbouring RBSs employing the second RAT and neighbouring RBSs employing the first RAT comprises requesting and receiving the indications directly from respective RBSs, and/or receiving the indications from any of: a Radio Network Controller, RNC; Operation Administration and

Maintenance, OAM, node; and an Operation Support System, OSS, node.

[00057] When the network node is to obtain the interference indications regarding the potential interfering neighbouring RBSs employing the second RAT and neighbouring RBSs employing the first RAT, the network node may send a request to those RBSs requesting the relevant information. The RBSs may then respond to the network node by sending the network node the requested information.

[00058] Alternatively, or additionally, some or all of the relevant information may be obtained by sending a request for the information to an RNC, OAM node or an OSS node. Generally, e.g. in WCDMA, an RNC is controlling at least one RBS, generally more than one RBS. The RNC may thus be in possession of the relevant information about the RBS or RBSs. Going up a step higher in hierarchy, the OAM system and/or the OSS system may receive the relevant information from either the RBSs themselves or from RNCs controlling the RBSs. Thus, the network node may request and receive the relevant information from a node in the OAM system or a node in the OSS system.

[00059] Interference indications of respective RBSs may refer to any of; traffic load; radio resource utilisation level; information indicating interference probability from potentially interfering RBSs; bitmap between scheduled Transmission Time Intervals, TTIs, in an RBS; and Relative Narrowband Transmission Power, RNTP.

[00060] There are many different examples of what may constitute an

interference indication. Some are given above. Traffic load and radio resource utilisation level may affect any interference caused by respective RBSs. For example, in case a neighbouring RBS has a relatively high traffic load and thus probably also a high resource utilisation level, that neighbouring RBS is more likely to cause interference to the serving RBSs in areas of overlapping cells than if that neighbouring RBS is experiencing a relatively low traffic load and thus probably also a low resource utilisation level. There may be other measurements that directly or indirectly relates to the interference transmitted by an RBS that may be used as interference indications; e.g. Inter-Cell Interference Coordination, ICIC.

[00061 ] Another example of interference indication is information indicating interference probability from potentially interfering RBSs. Based on e.g.

measurements during a relatively long period of time, a potentially interfering neighbouring RBSs may be more or less likely to cause interference or not to the serving RBS. Different times during the day may be associated with different likelihood or probability for that potentially interfering neighbouring RBSs to actually cause interference to the serving RBS.

[00062] Still an example of interference indication is bitmap between scheduled TTIs in an RBS, i.e. a neighbouring RBS. The bitmap includes information about when in what TTI the neighbouring RBS was transmitting hence potentially causing interference. At the same time, it gives information when neighbouring RBS was not transmitting, i.e. not risk for interference in a certain TTI.

[00063] Yet an example is RNTP. An RBS may provide this information to neighbouring RBSs, indicating the part of the bandwidth where it intends to limit the transmission power. A cell receiving the indication may schedule its downlink transmissions within this band, reducing the output power or completely freeing the resources on complementary parts of the spectrum. This information can be used to determine that there is potentially less interference caused as the transmission power is limited or vice versa.

[00064] The method may further comprise, as illustrated in figure 1e, determining 170 a relationship between the received interference indications with the obtained downlink measurement(s) comprising determining that: (1 ) the neighbouring RBS employing the first RAT is causing co-channel interference when the signal quality is low, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is low; (2) the neighbouring RBS employing the second RAT is causing adjacent channel interference when the signal quality is low, the load of neighbouring RBS employing the first RAT is low and the load of neighbouring RBS employing the second RAT is high; (3) there is no adjacent channel interference when the signal quality is high, the load of neighbouring RBS employing the first RAT is low and the load of neighbouring RBS employing the second RAT is high; and (4) there is no adjacent channel interference when the signal quality is high, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is high.

[00065] This may be seen as a result of the above described correlations and what to do with the information that the network node has obtained. When(1 ) the signal quality of the serving channel of the serving RBS is low, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is low, then the neighbouring RBS employing the first RAT is causing co-channel interference with the serving RBS and there is no or limited adjacent channel interference. [00066] When the quality of the serving channel of the serving RBS is low, then the signals transmitted on the channel is more susceptible to interference. If then also the load of neighbouring RBS employing the first RAT is high, that

neighbouring RBS is more likely to cause interference to the signals transmitted on the serving channel of the serving RBS, thereby causing co-channel interference with the serving RBS. Since the load of neighbouring RBS employing the second RAT is low, that neighbouring RBS is less likely to cause interference to the signals transmitted on the serving channel of the serving RBS, wherein that neighbouring RBS is causing no or limited adjacent channel interference.

[00067] When (2) the signal quality is low, the load of neighbouring RBS employing the first RAT is low and the load of neighbouring RBS employing the second RAT is high, then there is no or limited co-channel interference but the neighbouring RBS employing the second RAT is causing adjacent channel interference.

[00068] As described above, when the quality of the serving channel of the serving RBS is low, then the signals transmitted on the channel is more

susceptible to interference. If then also the load of neighbouring RBS employing the first RAT is low, then that RBS is less likely to cause interference to the signals transmitted on the serving channel of the serving RBS. Hence there is no or limited co-channel interference. If, on top of this, the load of neighbouring RBS employing the second RAT is high, then that RBS is more likely to cause interference to the signals transmitted on the serving channel of the serving RBS, wherein that neighbouring RBS is causing adjacent channel interference.

[00069] When (3) the signal quality is high, the load of neighbouring RBS employing the first RAT is low and the load of neighbouring RBS employing the second RAT is high, then there is no or limited adjacent interference and no or limited co-channel interference.

[00070] When the quality of the serving channel of the serving RBS is high, then the signals transmitted on the channel is less susceptible to interference, even if there is high load or activity in neighbouring RBSs. Thus, even if the load of neighbouring RBS employing the second RAT is high, then there is no or limited adjacent interference. Since the load of neighbouring RBS employing the first RAT is low, there is no or limited co-channel interference.

[00071 ] When (4) the signal quality is high, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is high, then there is no adjacent channel interference and most likely also the co-channel interference is non-existent or low.

[00072] When the quality of the serving channel of the serving RBS is high, then the signals transmitted on the channel is less susceptible to interference.

Consequently, even if the load of neighbouring RBS employing the first RAT is high, transmissions from neighbouring RBS employing the first RAT are less likely to affect, and thereby cause interference to, transmitted on the serving channel of the serving RBS. The same reasoning applies for when the load of neighbouring RBS employing the second RAT is high. Thus, there is no or limited co-channel interference and no or limited adjacent channel interference.

[00073] As can be deduced from the examples above, the only situation where adjacent channel interference may be causing degradation of the quality of the serving channel is when the quality of the serving channel of the serving RBS is low and when, at the same time, the load of neighbouring RBS employing the second RAT is high.

[00074] In an example, the method further comprises, as illustrated in figure 1f, applying weights 175 to the determined relationships.

[00075] In order to further optimise or customise the interference mitigation, weights may be applied to the above described relationships. In this manner, certain factors may be made more important than others.

[00076] Merely as an example, weights can be applied to that the case that a neighbouring RBS employing the second RAT is considered "more important" than a neighbouring RBS employing the first RAT. [00077] In yet an example, the method further comprises taking radio resource utilisation of the serving RBS into account together with the determined

relationships when employing (190) interference mitigation.

[00078] By "radio resource utilisation of the serving RBS" is in an example meant a level of used resources of a radio channel of a certain frequency, e.g. the ratio of used codes in WCDMA, the average number of scheduled resource blocks over number of available resource blocks in LTE. A relatively high ratio of used codes or scheduled resource block, i.e. a relatively high load is associated with a general interference level on that radio channel. By "relatively" means that it may be defined by an operator, e.g. 50% may be chosen to indicate high load, or 55%, 60%, or 75%. The exact "value" of the level or ratio may be up to an operator to define.

[00079] Radio resource utilisation of the serving RBS information may be obtained from e.g. an RBS, an RNC, an OAM node, and/or an OSS node.

[00080] Employing 190 interference mitigation may comprise, when the neighbouring RBS employing the second RAT are causing adjacent channel interference, i.e. when it is determined that adjacent channel interference from the second RAT is likely to be degrading signal quality on the first RAT: (a) radio resource management; (b) inter-frequency handover; (c) changing antenna direction or antenna height; and (d) adjusting transmission power of serving RBS.

[00081 ] There are different examples of interference mitigation, and which one to select is out of the scope of this disclosure.

[00082] Radio resource management is an example, wherein resource mitigation comprises e.g. avoiding scheduling wireless devices on resources close to the adjacent frequency that is e.g. experiencing relatively much interference or when a load or activity is above a predefined threshold.

[00083] Another example is inter-frequency handover, wherein a wireless device being connected to, or served by, the serving RBS by means of a serving channel of a frequency that is experiencing adjacent channel interference, may be handed over to another frequency (i.e. another serving channel) of the same serving RBS, wherein the other frequency is not experiencing the same level of adjacent channel interference.

[00084] Still another example of interference mitigate is changing antenna direction or antenna height, wherein the antenna of the serving RBS may be e.g. tilted, raised, lowered or otherwise having its position or direction changed so that e.g. the channel quality of the serving channel is improved whereby the serving channel may become less susceptible to adjacent channel interference, and thus also co-channel interference. Yet another example is beamforming.

[00085] Yet an example of interference mitigate is adjusting transmission power of serving RBS. In case the quality of the serving channel is relatively low, or poor, an increase in transmission power of the serving RBS may improve the channel quality and thereby reduce impact from the adjacent channel interference.

[00086] The network node may be one of (i) the serving RBS; (ii) RNC; (iii) Base Station Controller, BSC; (iv) OAM node; (v) OSS node; Self Optimising Network, SON, node; or (vi) a logical node distributed in at least two physical nodes.

[00087] In an example, the network node is the serving RBS. In this example, the RBS, being the network node, may receive all measurements from wireless devices directly and request other information from e.g. the RNC, BSC, OAM node, OSS node or other nodes of the wireless communication network on which the serving RBS is operating.

[00088] In another example, the network node is an RNC or a BSC. Depending on the technology of the RAT, the RAT may comprise an RNC or a BSC. These are in control of, and/or in direct contact with, at least the serving RBS. The RNC or the BSC may request and receive information from the serving RBS regarding e.g. the measurement reports from wireless devices. The RNC or the BSC may request and receive other relevant information from other RNCs or BSCs as well as from e.g. an OAM node, an OSS node or other nodes of the wireless communication network on which the serving RBS and the RNC/BSC are operating.

[00089] In yet an example, the network node is an OAM node. Generally, the OAM system, or subsystem handles, or is responsible for, processes, activities, tools, standards etc. involved with operating, administering, managing and maintaining the wireless communication network in which the serving RBS is operating.

[00090] The OAM node may communicate with the serving RBS, neighbouring RBSs of any RAT belonging to the same wireless network, and optionally also any possible RNC and/or BSC if any. The OAM node may collect all necessary information from relevant nodes and RBSs in order to perform the method as described above.

[00091 ] In a further example, the network node is an OSS node. Generally, the OSS system, or subsystem is a set of programs that help a communications service provider monitor, control, analyse and manage wireless communication network in which the serving RBS is operating. Like the OAM node, the OSS node may collect all necessary information from relevant nodes and RBSs in order to perform the method as described above.

[00092] Alternatively, the network node is a logical node distributed in at least two physical nodes. Parts of the network node may be distributed in e.g. at least two of the serving RBS, an RNC or BSC, an OAM node, and an OSS node. In this manner, not one physical node comprises the network node, but rather the functionality of the network node is distributed in two at least two physical nodes that thus cooperates in order to realise the network node and perform the method.

[00093] In an example, the network node is a SON node. With radio networks like those used for LTE and other cellular technologies becoming more complex, network planning needs to be made easier: planning, configuration, management, optimisation and healing all need to be automated to bring improvements. As a result the concept of self-optimising, or self-organising, networks, SON is growing in interest and use. With the networks themselves being able to monitor performance, they may optimise themselves to be able to provide the optimum performance. By using self-optimising networks, SON technology, networks are able to organise and optimise their performance. Operators may then benefit from significant improvements in terms of both capital expenditure and later operational expenditure. Thus, the network node may be implemented in a SON node.

[00094] Below, different exemplifying embodiments will be described, wherein the first RAT is WCDMA and the second RAT is LTE. It is to be emphasised that this is merely an example of the two different RATs and that the first and the second RAT may be of any other combination of technologies.

[00095] Figure 2a is an illustration of an interference situation for a wireless device in a mixed WCDMA and LTE heterogeneous network deployment.

[00096] The serving RBS employing WCDMA may estimate potential interfering neighbouring RBSs employing LTE, also referred to as LTE-RBSs. Handover measurements (event triggered) from a plurality of WCDMA wireless devices may be analysed. Signal strength of neighbouring LTE-RBSs may be compared to both signal strengths the serving RBS employing WCDMA and signal strengths of neighbouring WCDMA-RBSs to find potential adjacent interfering RBS(s).

[00097] Adjacent channel interference may be estimated as SSit_e_neighbo ACIR_e where ACIRe is adjacent channel interference rejection ratio, estimated from measurements and/or theoretical calculations, SSi_te_neighbor is the signal strength of neighbouring LTE-RBS(s).

[00098] Potential adjacent channel interference issues may occur when the estimated adjacent channel interference is larger or in same order as the signal strengths of neighbouring WCDMA-RBSs.

[00099] The network node may further collect information about downlink interference with respect to the serving WCDMA-RBS. Measurements from wireless devices communicating using WCDMA may be collected for the serving WCDM-RBS. It may be measurements of channel quality indication used for modulation and coding selection or downlink signal strength measurements of neighbouring WCDM-RBSs. Wireless devices communicating using WCDMA may be exposed to adjacent interference from neighbouring LTE-RBSs and cannot handover to the LTE cell to avoid interference unless they support LTE. During interference collection, WCDMA inter-frequency handover may be disabled to further improve the interference detection.

[000100] Further, interference indications may be requested from neighbouring WCDMA-RBSs and potential adjacent interfering LTE-RBSs. Indications may e.g. be traffic load (activity) information indicating interference probability from the potentially interfering cells/RBSs, bitmap of scheduled TTIs in a cell or RNTP message used between LTE-RBSs (or cells) to indicate downlink interference.

[000101 ] The measured interference may be correlated with interference indications. Examples are:

Low quality / high interference with high activity in neighbour WCDMA RBS(s)/cells and low activity in LTE RBS(s)/cells -^(indicates that) WCDMA neighbours are co- channel interfering.

Low quality / high interference with low activity in neighbour WCDMA RBS(s)/cells and high activity in LTE RBS(s)/cells -^(indicates that) LTE cells are adjacent channel interfering.

High quality / low interference with low activity in neighbour WCDMA RBS(s) cells and high activity in LTE RBS(s)/cells indicates no adjacent interference.

High quality / low interference with high activity in neighbour WCDMA RBS(s) / cells and LTE RBS(s)/cells -^(indicates) no adjacent interference.

[000102] The correlation may also be made at different traffic load levels as well as analysing the change on quality/interference when load changes. For example: high quality / low interference changes to low quality / high interference when load increases in LTE RBS(s)/cells and WCDMA activity is "unchanged". This information may be used for determining at what load level X the adjacent channel interference becomes severe. For example, when load > X% radio resource utilisation in an LTE RBS/cell, then adjacent channel interference is noticeable. If load is <X% utilisation, adjacent channel interference is less problematic. See figure 2e

[000103] As described above, examples of interference mitigation are several ways of modifying the cell plan, e.g. changing transmission power, antenna type, antenna height or antenna direction for the victim (WCDMA) and/or aggressor (LTE) RBSs/cells. Radio resource management could also be used to mitigate adjacent channel interference in aggressor cells, e.g. avoid schedule users on resources close to the adjacent frequency when radio resource utilisation > X%. Another example may be to initiate preventive inter-frequency handovers (before normal thresholds are met) in victim cells or RBS. Generally, the victim RBS (cell) is the serving RBS.

[000104] Measurements may be managed by a controlling entity, e.g. a "SON controller". The controller then manages measurements from RBSs/cells in the wireless network, and calculates the adjacent channel interference estimate for each RBS/cell. The controller may be centrally located e.g. together with core network equipment, or implemented distributed in e.g. Radio Access Network, RAN, nodes, see figure 2b.

[000105] Figure 2c illustrates an exemplifying embodiment of a method for determining adjacent interference level.

[000106] To start with, some criteria may determine that adjacent channel interference shall be investigated for a serving RBS, or a cell of the serving RBS - this could e.g. be scheduled for all RBSs/cells in a suspected problem area, for all cells in the network, or triggered on poor quality conditions. Measurements may then be collected for individual wireless devices until these either hand over out of cell, i.e. to another cell and/or RBS, or if the amount of measurements collected for the RBS/cell (by several wireless devices) is deemed sufficient for the analysis.

[000107] Measurements of the adjacent channel may be performed as periodic inter-frequency measurements according to the normal Radio Resource Control, RRC, procedure (figure 2d). To minimise the service impact for the measuring wireless device (which needs to enter compressed mode during the measurements), the measurement cycle may be kept at a quite low frequency (e.g. measuring once every 10 seconds). Together with the Received Signal Strength Indicator, RSSI, measured on the adjacent channel(s), own channel active set signal quality may also be reported.

[000108] The different actions described above are relatively slow in the sense that statistics over many wireless devices over a long period is used for finding the potentially interfering LTE-RBSs/cells. As part of estimating neighbouring potential adjacent channel interfering LTE-RBSs, the most likely interfering WCDMA neighbouring RBSs/cells may be determined from concurrent handover

measurements. The actions taken above may be executed again when new sites (RBSs) are added to the network or other changes have occurred.

[000109] Determining RBS causing potential adjacent interference may alternatively be made from LTE-RBSs/cells in similar way. Adjacent channel interference may be estimated from neighbour measurement statistics from

WCDMA capable wireless devices in the LTE-RBS/cell. The WCDMA potential affected RBS/cell is then determined by comparing signal strength of LTE serving (compensated with ACLR) and WCDMA neighbour RBSs/cells.

[0001 10] Embodiments herein also relate to a network node for downlink interference mitigation in a cell of a RBS, operable in a communication network employing at least two different RATs, i.e. a first RAT and a second RAT, wherein the serving RBS employs the first RAT and serves wireless devices. The network node has the same technical features, object and advantages as the method performed by the network node described above. The network node will only be described in brief in order to avoid unnecessary repetition.

[0001 1 1 ] Embodiments of such a network node will be described in brief with reference to figures 3 and 4. Figures 3 and 4 illustrate the network node 300, 400 being configured for determining potential interfering neighbouring RBS(s) employing the second RAT; and obtaining, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT, together with location related information of respective wireless device(s). Figures 3 and 4 illustrate further the network node 300, 400 being configured for obtaining, from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality, together with location related information of respective wireless device(s); and employing interference mitigation based on the obtained interference indications from neighbouring RBS(s) employing the first and the second RAT and the obtained downlink measurement(s).

[0001 12] The network node 300, 400 may be realised or implemented in various different ways. A first exemplifying implementation or realisation is illustrated in figure 3. Figure 3 illustrates the network node 300 comprising a processor 321 and memory 322, the memory comprising instructions, e.g. by means of a computer program 323, which when executed by the processor 321 causes the network node 300 to determine potential interfering neighbouring RBS(s) employing the second RAT; and to obtain, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT. The memory further comprises instructions, which when executed by the processor 321 causes the network node 300 to obtain, from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality; and to employ interference mitigation based on interference indications from neighbouring RBS(s) employing the first and the second RAT and the obtained downlink measurement(s).

[0001 13] Figure 3 also illustrates the network node 300 comprising a memory 310. It shall be pointed out that figure 3 is merely an exemplifying illustration and memory 310 may be optional, be a part of the memory 322 or be a further memory of the network node 300. The memory may for example comprise information relating to the network node 300, to statistics of operation of the network node 300, just to give a couple of illustrating examples. Figure 3 further illustrates the network node 300 comprising processing means 320, which comprises the memory 322 and the processor 321 . Still further, figure 3 illustrates the network node 300 comprising a communication unit 330. The communication unit 330 may comprise an interface through which the network node 300 communicates with other nodes or entities of the communication network as well as other

communication units. Figure 3 also illustrates the network node 300 comprising further functionality 340. The further functionality 340 may comprise hardware of software necessary for the network node 300 to perform different tasks that are not disclosed herein. Merely as an illustrative example, the further functionality 340 may comprise, or realise, a scheduler for scheduling transmissions to and/or from wireless devices.

[0001 14] An alternative exemplifying implementation of the network node 300, 400 is illustrated in figure 4. Figure 4 illustrates the network node 400 comprising a determining unit 403 for determining potential interfering neighbouring RBS(s) employing the second RAT. The network node 400 further comprises an obtaining unit 404 for obtaining, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT; and for obtaining, from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality. The network node 400 further comprises an

interference mitigation unit 405 for employing interference mitigation based on interference indications from neighbouring RBS(s) employing the first and the second RAT and the obtained downlink measurement(s).

[0001 15] In figure 4, the network node 400 is also illustrated comprising a communication unit 401 . Through this unit, the network node 400 is adapted to communicate with other nodes and/or entities in the wireless communication network. The communication unit 401 may comprise more than one receiving arrangement. For example, the communication unit 401 may be connected to both a wire and an antenna, by means of which the network node 400 is enabled to communicate with other nodes and/or entities in the wireless communication network. Similarly, the communication unit 401 may comprise more than one transmitting arrangement, which in turn is connected to both a wire and an antenna, by means of which the network node 400 is enabled to communicate with other nodes and/or entities in the wireless communication network. The network node 400 further comprises a memory 402 for storing data. Further, the network node 400 may comprise a control or processing unit (not shown) which in turn is connected to the different units 403-405. It shall be pointed out that this is merely an illustrative example and the network node 400 may comprise more, less or other units or modules which execute the functions of the network node 400 in the same manner as the units illustrated in figure 4.

[0001 16] It should be noted that figure 4 merely illustrates various functional units in the network node 400 in a logical sense. The functions in practice may be implemented using any suitable software and hardware means/circuits etc. Thus, the embodiments are generally not limited to the shown structures of the network node 400 and the functional units. Hence, the previously described exemplary embodiments may be realised in many ways. For example, one embodiment includes a computer-readable medium having instructions stored thereon that are executable by the control or processing unit for executing the method steps in the network node 400. The instructions executable by the computing system and stored on the computer-readable medium perform the method steps of the network node 400 as set forth in the claims.

[0001 17] The network node has the same possible advantages as the method performed by the network node. One possible advantage is that adjacent interference may be detected in multi-standard deployment for efficient

coexistence between two technologies deployed in the same frequency band in the same area. The solution may associate two technologies with each other and provide an advantage for operating two systems from the same vendor in the same area.

[0001 18] According to an embodiment, the network node 300, 400 is configured for determining potential interfering neighbouring RBS(s) employing the second RAT by receiving handover and/or mobility measurements from wireless devices capable of operating according to both the first and the second RAT and comparing signal strength of neighbour RBS(s) employing the second RAT and signal strength of neighbouring RBS(s) employing the first RAT or signal strength of the serving RBS.

[0001 19] According to still an embodiment, the network node 300, 400 is configured for estimating adjacent channel interference from neighbouring RBS(s) employing the second RAT by dividing the signal strength of respective

neighbouring RBS employing the second RAT by ACIR_e, where ACIR_e is

Adjacent Channel Interference Rejection ratio estimated from measurements and/or theoretical calculations.

[000120] According to yet an embodiment, the network node 300, 400 is configured for determining a correlation between interference indications of RBS(s) employing the first and second RAT respectively and quality of signals received by wireless device(s) on a serving channel of the serving RBS together with neighbouring RBS(s) employing the first RAT, wherein the interference mitigation is further based on the correlation.

[000121 ] According to another embodiment, the network node 300, 400 is configured for disabling inter-frequency handover for wireless devices

communicating by means of the first RAT being involved in obtaining downlink measurements.

[000122] According to still an embodiment, the network node 300, 400 is configured for obtaining (120, 160) interference indications regarding the potential interfering neighbouring RBSs employing the second RAT and neighbouring RBSs employing the first RAT by requesting and receiving the indications directly from respective RBSs, and/or receiving the indications from any of: an RNC; an OAM node; and an OSS node.

[000123] According to yet an embodiment, the interference indications of respective RBSs refer to any of; traffic load; radio resource utilisation level;

information indicating interference probability from potentially interfering RBSs; bitmap between scheduled TTIs in an RBS; and RNTP. [000124] According to another embodiment, the network node 300, 400 is configured for determining a relationship between the received interference indications with the obtained downlink measurement(s) by determining that: (1 ) the neighbouring RBS employing the first RAT is causing co-channel interference when the signal quality is low, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is low; (2) the neighbouring RBS employing the second RAT is causing adjacent channel interference when the signal quality is low, the load of neighbouring RBS

employing the first RAT is low and the load of neighbouring RBS employing the second RAT is high; (3) there is no adjacent channel interference when the signal quality is high, the load of neighbouring RBS employing the first RAT is low and the load of neighbouring RBS employing the second RAT is high; and (4) there is no adjacent channel interference when the signal quality is high, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is high.

[000125] According to still an embodiment, the network node 300, 400 is configured for applying weights to the determined relationships.

[000126] According to yet an embodiment, the network node 300, 400 is configured for taking radio resource utilisation of the serving RBS into account together with the determined relationships when employing interference mitigation.

[000127] According to another embodiment, the network node 300, 400 is configured for employing interference mitigation by, when the neighbouring RBS employing the second RAT are causing adjacent channel interference: (a) radio resource management; (b) inter-frequency handover; (c) changing antenna direction or antenna height; and (d) adjusting transmission power of serving RBS.

[000128] The network node may be one of (i) the serving RBS; (ii) RNC; (iii) Base Station Controller, BSC; (iv) OAM node; (v) OSS node; Self Optimising Network, SON, node; or (vi) a logical node distributed in at least two physical nodes. [000129] Figure 5 schematically shows an embodiment of an arrangement 500 in a network node 400. Comprised in the arrangement 500 in the network node 400 are here a processing unit 506, e.g. with a Digital Signal Processor, DSP. The processing unit 506 may be a single unit or a plurality of units to perform different actions of procedures described herein. The arrangement 500 of the network node 400 may also comprise an input unit 502 for receiving signals from other entities, and an output unit 504 for providing signal(s) to other entities. The input unit and the output unit may be arranged as an integrated entity or as illustrated in the example of figure 4, as one or more interfaces 401 .

[000130] Furthermore, the arrangement 500 in the network node 400 comprises at least one computer program product 508 in the form of a non-volatile memory, e.g. an Electrically Erasable Programmable Read-Only Memory, EEPROM, a flash memory and a hard drive. The computer program product 508 comprises a computer program 510, which comprises code means, which when executed in the processing unit 506 in the arrangement 500 in the network node 400 causes the network node to perform the actions e.g. of the procedure described earlier in conjunction with figure 1a.

[000131 ] The computer program 510 may be configured as a computer program code structured in computer program modules 510a-510e. Hence, in an

exemplifying embodiment, the code means in the computer program of the arrangement 500 in the network node 400 comprises a determining unit, or module, for determining potential interfering neighbouring RBS(s) employing the second RAT. The computer program further comprises an obtaining unit, or module, for obtaining, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT; and for obtaining, from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality. Still further, the computer program further comprises an employing unit, or module, for employing interference mitigation based on interference indications from neighbouring RBS(s) employing the first and second RAT respectively with the obtained downlink measurement(s). [000132] The computer program modules could essentially perform the actions of the flow illustrated in figures 1a-1f, to emulate the network node 400. In other words, when the different computer program modules are executed in the processing unit 506, they may correspond to the units 403-405 of figure 4.

[000133] Although the code means in the embodiments disclosed above in conjunction with figure 4 are implemented as computer program modules which when executed in the respective processing unit causes the network node to perform the actions described above in the conjunction with figures mentioned above, at least one of the code means may in alternative embodiments be implemented at least partly as hardware circuits.

[000134] The processor may be a single Central Processing Unit, CPU, but could also comprise two or more processing units. For example, the processor may include general purpose microprocessors; instruction set processors and/or related chips sets and/or special purpose microprocessors such as Application Specific Integrated Circuits, ASICs. The processor may also comprise board memory for caching purposes. The computer program may be carried by a computer program product connected to the processor. The computer program product may comprise a computer readable medium on which the computer program is stored. For example, the computer program product may be a flash memory, a Random-Access Memory RAM, Read-Only Memory, ROM, or an EEPROM, and the computer program modules described above could in alternative embodiments be distributed on different computer program products in the form of memories within the network node.

[000135] It is to be understood that the choice of interacting units, as well as the naming of the units within this disclosure are only for exemplifying purpose, and nodes suitable to execute any of the methods described above may be configured in a plurality of alternative ways in order to be able to execute the suggested procedure actions.

[000136] It should also be noted that the units described in this disclosure are to be regarded as logical entities and not with necessity as separate physical entities. [000137] While the embodiments have been described in terms of several embodiments, it is contemplated that alternatives, modifications, permutations and equivalents thereof will become apparent upon reading of the specifications and study of the drawings. It is therefore intended that the following appended claims include such alternatives, modifications, permutations and equivalents as fall within the scope of the embodiments and defined by the pending claims.

Claims

1 . A method (100) performed by a network node for downlink interference mitigation in a cell of a serving Radio Base Station, RBS, operable in a

communication network employing at least two different Radio Access

Technologies, RATs, i.e. a first RAT and a second RAT, the serving RBS employing the first RAT and serving wireless devices, the method comprising:

- determining (1 10) potential interfering neighbouring RBS(s) employing the second RAT,

- obtaining (120), from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT,

- obtaining (130), from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality, and

- employing (190) interference mitigation based on interference indications from neighbouring RBS(s) employing the first and second RAT respectively and the obtained downlink measurement(s).

2. A method (100) according to claim 1 , wherein determining (1 10) potential interfering neighbouring RBS(s) employing the second RAT comprises receiving handover and/or mobility measurements from wireless devices capable of operating according to both the first and the second RAT and comparing signal strength of neighbour RBS(s) employing the second RAT and signal strength of neighbouring RBS(s) employing the first RAT or signal strength of the serving RBS.

3. A method (100) according to claim 2, further comprising estimating (140) adjacent channel interference from neighbouring RBS(s) employing the second RAT by dividing the signal strength of respective neighbouring RBS employing the second RAT by ACIR_e, where ACIR_e is Adjacent Channel Interference Rejection ratio estimated from measurements and/or theoretical calculations.

4. A method (100) according to any of claims 1 -3, further comprising determining (150) a correlation between interference indications of RBS(s) employing the first and second RAT respectively and quality of signals received by wireless device(s) on a serving channel of the serving RBS together with neighbouring RBS(s) employing the first RAT, wherein the interference mitigation is further based on the correlation.

5. A method (100) according to any of claims 1 -4, further comprising disabling (160) inter-frequency handover for wireless devices communicating by means of the first RAT being involved in obtaining downlink measurements.

6. A method (100) according to any of claims 1 -5, wherein obtaining (120, 160) interference indications regarding the potential interfering neighbouring RBSs employing the second RAT and neighbouring RBSs employing the first RAT comprises requesting and receiving the indications directly from respective RBSs, and/or receiving the indications from any of: a Radio Network Controller, RNC; Operation Administration and Maintenance, OAM, node; and an Operation

Support System, OSS, node.

7. A method (100) according to claim 6, wherein the interference indications of respective RBSs refer to any of; traffic load; radio resource utilisation level; information indicating interference probability from potentially interfering RBSs; bitmap between scheduled Transmission Time Intervals, TTIs, in an RBS; and Relative Narrowband Transmission Power, RNTP.

8. A method (100) according to any of claims 1 -7, further comprising determining (170) a relationship between the received interference indications with the obtained downlink measurement(s) comprising determining that: (1 ) the neighbouring RBS employing the first RAT is causing co-channel interference when the signal quality is low, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is low; (2) the neighbouring RBS employing the second RAT is causing adjacent channel interference when the signal quality is low, the load of neighbouring RBS

employing the first RAT is low and the load of neighbouring RBS employing the second RAT is high,; (3) there is no adjacent channel interference when the signal quality is high, the load of neighbouring RBS employing the first RAT is low and the load of neighbouring RBS employing the second RAT is high, ; and (4) there is no adjacent channel interference when the signal quality is high, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is high.

9. A method (100) according to claim 8, further comprising applying weights (175) to the determined relationships.

10. A method (100) according to any of claims 1 -9, further comprising taking radio resource utilisation of the serving RBS into account together with the determined relationships when employing (190) interference mitigation.

1 1 . A method (100) according to any of claims 1 -10, wherein employing (190) interference mitigation comprises, when the neighbouring RBS employing the second RAT are causing adjacent channel interference: (a) radio resource management; (b) inter-frequency handover; (c) changing antenna direction or antenna height; and (d) adjusting transmission power of serving RBS.

12. A method (100) according to any of claims 1 -1 1 , wherein the network node is one of (i) the serving RBS; (ii) RNC; (iii) Base Station Controller, BSC; (iv) OAM node; (v) OSS node; Self Optimising Network, SON, node; or (vi) a logical node distributed in at least two physical nodes.

13. A network node (300, 400) for downlink interference mitigation in a cell of a serving Radio Base Station, RBS, operable in a communication network employing at least two different Radio Access Technologies, RATs, i.e. a first RAT and a second RAT, the serving RBS employing the first RAT and serving wireless devices, the network node (300, 400) being configured for:

- determining potential interfering neighbouring RBS(s) employing the second RAT, - obtaining, from wireless device(s) supporting both the first and the second RAT, downlink signal measurements indicating potential interfering neighbouring RBSs employing the second RAT,

- obtaining, from wireless device(s) communicating by means of the first RAT, downlink measurement(s) relating to channel quality, and

- employing interference mitigation based on interference indications from neighbouring RBS(s) employing the first and second RAT respectively and the obtained downlink measurement(s).

14. A network node (300, 400) according to claim 13, being configured for determining potential interfering neighbouring RBS(s) employing the second RAT by receiving handover and/or mobility measurements from wireless devices capable of operating according to both the first and the second RAT and comparing signal strength of neighbour RBS(s) employing the second RAT and signal strength of the serving RBS.

15. A network node (300, 400) according to claim 14, further being configured for estimating adjacent channel interference from neighbouring RBS(s) employing the second RAT by dividing the signal strength of respective

neighbouring RBS employing the second RAT by ACIR_e, where ACIR_e is Adjacent Channel Interference Rejection ratio estimated from measurements and/or theoretical calculations.

16. A network node (300, 400) according to any of claims 13-15, further being configured for determining a correlation between interference indications of RBS(s) employing the first and second RAT respectively and quality of signals received by wireless device(s) on a serving channel of the serving RBS together with neighbouring RBS(s) employing the first RAT, wherein the interference mitigation is further based on the correlation.

17. A network node (300, 400) according to any of claims 13-16, further being configured for disabling inter-frequency handover for wireless devices communicating by means of the first RAT being involved in obtaining downlink measurements.

18. A network node (300, 400) according to any of claims 13-17, further being configured for obtaining (120, 160) interference indications regarding the potential interfering neighbouring RBSs employing the second RAT and

neighbouring RBSs employing the first RAT by requesting and receiving the indications directly from respective RBSs, and/or receiving the indications from any of: a Radio Network Controller, RNC; Operation Administration and

Maintenance, OAM, node; and an Operation Support System, OSS, node.

19. A network node (300, 400) according to claim 18, wherein the

interference indications of respective RBSs refer to any of; traffic load; radio resource utilisation level; information indicating interference probability from potentially interfering RBSs; bitmap between scheduled Transmission Time Intervals, TTIs, in an RBS; and Relative Narrowband Transmission Power, RNTP.

20. A network node (300, 400) according to any of claims 13-19, further being configured for determining a relationship between the received interference indications with the obtained downlink measurement(s) by determining that: (1 ) the neighbouring RBS employing the first RAT is causing co-channel interference when the signal quality is low, the load of neighbouring RBS employing the first RAT is high and the load of neighbouring RBS employing the second RAT is low; (2) the neighbouring RBS employing the second RAT is causing adjacent channel interference when the signal quality is low, the load of neighbouring RBS

21 . A network node (300, 400) according to claim 20, further being

configured for applying weights to the determined relationships.

22. A network node (300, 400) according to any of claims 13-21 , further being configured for taking radio resource utilisation of the serving RBS into account together with the determined relationships when employing interference mitigation.

23. A network node (300, 400) according to any of claims 13-22, further being configured for employing interference mitigation by, when the neighbouring RBS employing the second RAT are causing adjacent channel interference: (a) radio resource management; (b) inter-frequency handover; (c) changing antenna direction or antenna height; and (d) adjusting transmission power of serving RBS.

24. A network node (300, 400) according to any of claims 13-23, wherein the network node is one of (i) the serving RBS; (ii) RNC; (iii) Base Station Controller, BSC; (iv) OAM node; (v) OSS node; Self Optimising Network, SON, node; or (vi) a logical node distributed in at least two physical nodes.

25. A Computer program (510), comprising computer readable code means, which when run in a processing unit (506) comprised in an arrangement (500) in a network node (400) according to claims 13-24 causes the network node (400) to perform the corresponding method according to any of claims 1 -12.

26. A Computer program product (508) comprising the computer program (510) according to claim 25.