TECHNICAL FIELD

Embodiments presented herein relate to directional beams, and particularly to a method, a high-power network node, a computer program, and a computer program product for identifying directional beams.

BACKGROUND

In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.

For example, the large uptake of mobile broadband has resulted in that the traffic volume that needs to be handled by the communications networks has grown significantly. Therefore, mechanisms that allow cellular operators to manage their network more efficiently have been developed. One such mechanism is deployment of low-power network nodes (LPN) in the coverage area of a high-power network node (HPN). This type of deployment is called heterogeneous networks. Examples of LPNs are micro nodes, pico nodes, femto nodes, relay nodes, and remote radio head nodes.

Thus, in heterogeneous networks, in addition to the planned or regular placement of high-power network nodes, at least one low-power network node is deployed, as shown in the communications network 10a of Fig. la. The power transmitted by a low-power network node 12 is relatively small compared to that of the high-power network node 11, e.g., 2W as compared to 40 W for a typical high-power network node. The low-power network nodes may be deployed to eliminate coverage holes in homogeneous networks (using high-power network nodes only) and to off-load the high-power network nodes, thereby improving the capacity in hot-spot scenarios. Due to the lower transmit power and smaller physical size, a low-power network node can offer flexible site acquisitions. Deployed low-power network nodes in a heterogeneous network can have properties according to the following scenarios.

Firstly, according to one scenario each low-power network node has its own cell identity (scrambling code). Low-power network nodes and high-power network nodes thereby define different cells but they typically share the same frequency. This is referred to as co-channel deployment. Fig. lb schematically illustrates a typical heterogeneous network lob with co-channel deployment, where the cells Cell B and Cell C are created by low-power network nodes 12 in addition to Cell A being created by a high-power network node 11.

Individual cellsi6a, 16b are characterized by individual pilot signals, downlink and uplink control channels and data traffic channels.

Secondly, according to another scenario, the low-power network nodes have the same cell identities as the high-power network nodes. This is referred to as soft cell or combined cell. Fig. IC schematically illustrates a typical heterogeneous network 10c where the low-power network nodes 12 have cells 16b which are part of the cell 16a of the high-power network node 11, where this cell 16a thus is a soft cell or a combined cell. This scenario avoids frequent soft handovers, and hence higher layer signaling. In this deployment all the low-power network nodes may be coupled to a central node (in this case the high-power network node) via high speed data links.

Fig. 9 shows the average sector throughput in Mbps as a function of number of served wireless devices 15a, 15b, per high-power network node 11 with four low-power network nodes 12 with 37dBm and 3odBm power for Wideband Code Division Multiple Access (WCDMA). It can be seen that at high load co- channel deployment gives significant gains because more users are offloaded.

Fig. 10 shows the percentage of gain (with respect to a homogeneous network) achieved with co-channel deployment as a function of number of served wireless devices 15a, 15b per high-power network node 11. It can be observed that at low loads there is almost no gain and the gain increases as the load increases. The gain depends on the percentage of offloading. Since the low-power network nodes have less transmit power, the number of wireless devices 15c served by the low-power network nodes are less compared to the number of wireless devices 15a, 15b served by the high- power network node. The gains in heterogeneous networks can be improved if more wireless devices 15a, 15b are offloaded to the low-power network nodes. One mechanism to improve the overall system throughput is cell range expansion. In cell range expansion the wireless devices are offloaded to the low-power network nodes by increasing the cell individual offsets (CIO).

Fig. id schematically illustrates a communications network lod where a cell range expansion area 17 is identified. The communications network lod may have a co-channel deployment as in Fig. lb or as a soft cell or combined cell deployment as in Fig. ic. In the cell range expansion area, the strongest cell is the high-power network node. However if the low-power network node is less loaded than the high-power network node, wireless devices within the cell range expansion area can be served more often by the low-power network node, even though the throughput may be reduced due to the low-power network node not being the strongest cell. Since these wireless devices get scheduled more often when connected to the low-power network node, the overall throughput is higher. In the cell range expansion area the wireless devices are operatively connected to the low-power network node and experiences interference from the high-power network node. Fig. 11 shows the link throughput in Mbps as a function of lor/No in dB when a wireless device is operatively connected to the low-power network node with different interference values (Ioc) caused by the high-power network node. It can be observed that the wireless device performance is severely impacted when the dominant interferer power is 10 - 20 times that of received power from the low-power network node. It can be observed from above that there is performance degradation caused by the interference. The performance loss is in the range of 100% at high

geometries. Hence, even though the wireless device is offloaded to the low- power network node, the individual wireless device throughput is impacted in the cell range expansion area. Hence, there is still a need for mechanisms providing improved throughput in heterogeneous networks.

SUMMARY

An object of embodiments herein is to provide improved throughput in heterogeneous networks.

According to a first aspect there is presented a method for identifying directional beams. The method is performed by a high-power network node. The method comprises acquiring interference information regarding any interference caused by the high-power network node to at least one wireless device served by a low-power network node. The method comprises identifying directional beams to be used for transmitting signals to its own served at least one wireless device using the interference information.

Advantageously this provides improved throughput in heterogeneous networks. Advantageously this enables aggressive cell range expansion without any performance loss on the individual wireless device throughputs in the cell range expansion area. This in turn improves the wireless device throughput as well as provides improvement in the average sector capacity.

According to a second aspect there is presented a high-power network node for identifying directional beams. The high-power network node comprises a processing unit. The processing unit is configured to cause the high-power network node to acquire interference information regarding any interference caused by the high-power network node to at least one wireless device served by a low-power network node. The processing unit is configured to cause the high-power network node to identify directional beams to be used for transmitting signals to its own served at least one wireless device using the interference information.

According to a third aspect there is presented a computer program for identifying directional beams, the computer program comprising computer program code which, when run on a high-power network node, causes the high-power network node to perform a method according to the first aspect.

According to a fourth aspect there is presented a computer program product comprising a computer program according to the third aspect and a computer readable means on which the computer program is stored.

It is to be noted that any feature of the first, second, third and fourth aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of the first aspect may equally apply to the second, third, and/or fourth aspect, respectively, and vice versa. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

Figs, la, lb, IC, and id are schematic diagrams illustrating communication networks according to embodiments; Fig. 2a is a schematic diagram showing functional units of a high-power network node according to an embodiment;

Fig. 2b is a schematic diagram showing functional modules of a high-power network node according to an embodiment; Fig. 2c is a schematic diagram showing functional units of a communications interface of a high-power network node according to an embodiment;

Fig. 3 shows one example of a computer program product comprising computer readable means according to an embodiment; Figs. 4, 5, and 6 are flowcharts of methods according to embodiments; and

Fig. 7 is a signalling diagram for transmission of probe signals according to an embodiment;

Fig. 8 schematically illustrates subframes according to an embodiment; and Figs. 9, io, and n schematically illustrate simulation results. DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional. Fig. la is a schematic diagram illustrating a communications network 10a where embodiments presented herein can be applied. The communications network 10a comprises a high-power network node (HPN) 11, and a low- power network node (LPN) 12. The high-power network node 11 and the low- power network node 12 are operatively connected to a core network 13, which in turn is operatively connected to a service providing network 14. Wireless devices (WDs) 15a, 15b, 15c served by one of the high-power network node 11 and the low-power network node 12 in a cell 16a, 16b are thereby enabled to access data and services as provided by the service providing network 14. The following common terminologies are used in the embodiments and are elaborated below:

Network node: In some embodiments the non-limiting term network node is used and it refers to any type of network node serving wireless devices and/or being operatively connected to other network nodes or network elements. Examples of network nodes are high-power network nodes n and low-power network nodes 12. Each network node may be provided as a Node B, a base station (BS), a multi-standard radio (MSR) radio node such as an MSR BS, an e Node B, a network controller, a radio network controller (RNC), a base station controller, a relay, a donor node controlling relay, a base transceiver station (BTS), an access point (AP), a transmission point, a transmission node, a remote radio unit (RRU), a remote radio head (RRH), a node in distributed antenna system (DAS), etc.

Wireless device: In some embodiments the non-limiting term wireless devices used and it refers to any type of wireless device 15a, 15b, 15c communicating with a radio network node in a cellular or mobile

communication system. Examples of wireless devices are target devices, device-to-device communications enabled wireless devices, machine-type wireless devices, personal digital assistants, tablet computers, user

equipment, mobile terminals, smart phone, laptop embedded equipment (LEE), laptop mounted equipment (LME), universal serial bus (USB) dongles, etc.

Although the herein disclosed embodiments are described assuming one high-power network node 11 and one low-power network node 12, the herein disclosed embodiments are equally applicable to scenarios with more than one high-power network node 11 and more than one low-power network node 12. These embodiments are also applicable to homogeneous deployment scenarios. Further, as the skilled person understands, although only three wireless devices 15a, 15b, 15c are illustrated in Fig. la each network node 11, 12 may simultaneously server a plurality of such wireless devices 15a, 15b, 15c; the herein disclosed embodiments are not limited to a particular number of wireless devices.

As noted above, cell range expansion may offer benefits in terms of throughput, but may also result in interference for some wireless devices. For example, to serve its wireless device 15a, 15b the high-power network node 11 transmits signals in directional beams, two of which are illustrated at reference numerals 25a, 25b. As the skilled person understands, the high- power network node 11 is capable of transmitting signals in more than two directional beams, where each directional beam may point in a unique direction; the herein disclosed embodiments are not limited to a particular number of directional beams. For simplicity only two such directional beams are illustrated in the enclosed drawings. Depending on the direction of a directional beam 25a, 25b it may or may not cause interference to the wireless device 15c of the low-power network node 12. The risk of such interference increases when the wireless device 15c is located in the cell range expansion area 17. For example, in the illustrative example, directional beam 25a serving wireless device 15a may cause interference to wireless device 15c if wireless device 15a moves closer (in geographical sense) to wireless device 15c whilst still being served by the high-power network node 11. Further, as the skilled person understands, also the low-power network node 12 may serve its wireless devices 15c using one or more directional beams of its own (not illustrated).

At least some of the embodiments disclosed herein are directed to

mechanisms for enabling aggressive cell range expansion when at least the high-power network node 11 is configured for beamforming, for example by being equipped with an active array antenna system, for transmitting signals in directional beams 25a, 25b. The embodiments disclosed herein

particularly relate to mechanisms for identifying which directional beams to use (and when to use them). In order to obtain such mechanisms there is provided a high-power network node 11, a method performed by the high- power network node, a computer program comprising code, for example in the form of a computer program product, that when run on a processing unit of the high-power network node, causes the high-power network node to perform the method.

Fig. 2a schematically illustrates, in terms of a number of functional units, the components of a high-power network node n according to an embodiment. A processing unit 21 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate arrays (FPGA) etc., capable of executing software instructions stored in a computer program product 31 (as in Fig. 3), e.g. in the form of a storage medium 23. Thus the processing unit 21 is thereby arranged to execute methods as herein disclosed. The storage medium 23 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The high-power network node 11 may further comprise a communications interface 22 for communications with at least one low-power node 12, a core network 13, and at least wireless devices 15a, 15b of its own. As such the communications interface 22 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of antennas for wireless

communications and ports for wireline communications. The processing unit 21 controls the general operation of the high-power network node 11 e.g. by sending data and control signals to the communications interface 22 and the storage medium 23, by receiving data and reports from the communications interface 22, and by retrieving data and instructions from the storage medium 23. Other components, as well as the related functionality, of the high-power network node 11 are omitted in order not to obscure the concepts presented herein.

Fig. 2b schematically illustrates, in terms of a number of functional modules, the components of a high-power network node 11 according to an

embodiment. The high-power network node 11 of Fig. 2b comprises a number of functional modules; an acquire module 21a configured to perform below step S102, and an identify module 21b configured to perform below step S104. The high-power network node 11 of Fig. 2b may further comprises a number of optional functional modules, such as any of a transmit and/or receive module 21c configured to perform below steps Si02a, Si02b, S108, and an exchange module 2id configured to perform below step S106. The functionality of each functional module 2ia-2id will be further disclosed below in the context of which the functional modules 2ia-2id may be used. In general terms, each functional module 2ia-2id may be implemented in hardware or in software. Preferably, one or more or all functional modules 2ia-2id may be implemented by the processing unit 21, possibly in

cooperation with functional units 22 and/or 23. The processing unit 21 may thus be arranged to from the storage medium 23 fetch instructions as provided by a functional module 2ia-2id and to execute these instructions, thereby performing any steps as will be disclosed hereinafter.

Communications interfaces where radio frequency (RF) components such as power amplifiers (PA) and transceivers are integrated with an array of antennas elements is denoted an active-array-antenna system (AAS). Fig. 2c schematically illustrates, in terms of a number of functional units, the components of a communications interface 22 of a high-power network node 11 according to an embodiment. The communications interface 22 comprises a baseband processing unit 22b, a set of power amplifiers, one of which is identified at reference numeral 22b, and a set of antennas, one of which is identified at reference numeral 22c. In the communications interface 22 of Fig. 2c the PAs 22a and the antennas 22b are integrated as shown in dotted lines to form the AAS 22d. The set of power amplifiers and the set of antennas thus collectively form the AAS 22d. Also schematically illustrated are two directional beams 25a, 25b as being generated by the AAS 22d. An AAS 22d offer several benefits compared to traditional deployments with passive antennas connected to transceivers through feeder cables. By using an active antenna array, not only are cable losses reduced, leading to improved performance and reduced energy consumption, but also is the installation simplified and the required equipment space is reduced. There are many applications of Active antennas, for example cell specific beamforming, user specific beamforming, vertical sectorization, massive multiple input multiple output (MIMO) communications , elevation beamforming etc. may also be an enabler for further-advanced antenna concepts. Several techniques are possible, including dynamic terminal- specific down tilt, multi-user MIMO, and vertical sectorization. However, all these techniques will be useful in practice if proper specification of relevant RF and electro-magnetic compatibility (EMC) requirements are in place.

Fig. 3 shows one example of a computer program product 31 comprising computer readable means 33. On this computer readable means 33, a computer program 32 can be stored, which computer program 32 can cause the processing unit 21 and thereto operatively coupled entities and devices, such as the communications interface 22 and the storage medium 23, to execute methods according to embodiments described herein. The computer program 32 and/or computer program product 31 may thus provide means for performing any steps as herein disclosed.

In the example of Fig. 3, the computer program product 31 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 31 could also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 32 is here schematically shown as a track on the depicted optical disk, the computer program 32 can be stored in any way which is suitable for the computer program product 31.

Figs. 4 and 5 are flow chart illustrating embodiments of methods for identifying directional beams. The methods are performed by the high-power network node 11. The methods are advantageously provided as computer programs 32. Reference is now made to Fig. 4 illustrating a method for identifying directional beams as performed by the high-power network node 11 according to an embodiment.

When wireless devices 15c are in a cell range expansion area 17 and is served by the low-power network node 12, the high-power network node 12 should serve its own wireless devices 15a, 15b by transmitting directional beams 25a, 25b in a beam patterns which causes low interference to the wireless devices 15c served by the low-power network node 1, particularly those wireless devices in the cell range expansion area 17. Assume that the high-power network node 11 is an AAS capable radio access network node. This implies that the high-power network node 11 can serve multiple directional beams 25a, 25b through its cell (or coverage area) 16a. Assume further that the high-power network node 11 is capable of

transmitting AT multiple directional beams with M non-overlapping directional beams, where M≤N. The term non-overlapping is here to be understood as the directional beams being orthogonal or close to orthogonal. The directional beams are thus to be used by the high-power network node 11 for transmitting signals to its own served wireless devices 15a, 15b.

The directional beams should be selected so as to cause little interference to other wireless devices, particularly to wireless devices 13b served by a low- power network node 12. The high-power network node 11 therefore needs to acquire information about which directional beams are causing more interference to wireless devices 15 operatively connected to other network nodes, such as low-power network nodes 12. Hence, the high-power network node 11 is configured to, in a step S102, acquire interference information regarding any interference caused by the high-power network node 11 to at least one wireless device 15c served by a low-power network node 12.

Based on the interference information the high-power network node 11 then selects which beams to use. Particularly the high-power network node 11 is configured to, in a step S104, identify directional beams to be used for transmitting signals to its own served at least one wireless device 15a, 15b using the interference information.

The high-power network node 11 may thereby perform beamforming which avoids interference, or at least causes only little interference, to other wireless devices 15c. As the skilled person understands, the level of interference for example depends on the relative locations of the wireless devices 15a, 15b served by the high-power network node 11 and the wireless devices 15c served by the low-power network node 12; if the wireless devices 15a, 15b served by the high-power network node 11 and the wireless devices 15c served by the low-power network node 12 are geographically close, some interference may be unavoidable.

In this respect, the identified directional beams may be identified to cause interference below a threshold to the at least one wireless device 15c served by the low-power network node 12. Additionally and/or alternatively, the identified directional beams may be identified to enable signal reception above a threshold at its own at least one served wireless device 15. Hence, the directional beams may be selected to cause little, or no, interference at other wireless devices 15c whilst enabling the wireless devices 15a, 15b of the high- power network node 11 itself to receive signals at a sufficiently high quality level.

As noted above, the high-power network node 11 and the low-power network node 12 may be deployed in a heterogeneous network 10b with co-channel deployment, as in Fig. lb and/or in a heterogeneous network 10c with soft cell or combined cell deployment, as in Fig. lc. As further noted above, the wireless devices 15c may be wireless devices in a cell range expansion area 17 of the low-power network node 12, independently if the network has a co- channel deployment, a soft cell or combined cell deployment.

Reference is now made to Fig. 5 illustrating methods for identifying directional beams as performed by the high-power network node 11 according to further embodiments. As explained, the high-power network node needs to acquire interference information to identify which of its directional beams are useful for its own served wireless devices 15a, 15b, and which directional beams are causing interference to other wireless devices 15c. There may be different ways for the high-power network node 11 to acquire this interference information.

Embodiments relating thereto will now be described in turn.

For the purpose of acquiring information about which directional beams are causing interference to the wireless devices 15c in the neighbor cell 16a, and, optionally, which directional beams are useful for the wireless devices 15a, 15b in its own cell 16a, the high-power network node 11 may send probe signals. Hence, according to an embodiment the high-power network node 11 is configured to, in an optional step Si02a, transmit probe signals using directional beams. According to this embodiment the high-power network node 11 is further configured to, in an optional step Si02b, receive at least one response to the probe signals. The interference information comprises the at least one response.

The probe signals may be sequentially transmitted in directional beams specified by a beam pattern. To create different directional beams, the probe signals may therefore be multiplied with different directional beam weights. There may be different kinds of probe signals. For example, channel state information reference signals (CSI-RS) can be used for this purpose. There may be different kinds of responses to the probe signals. For example, the at least one response may be indicative of channel measurements, such as channel quality indicator (CQI) measurements. There may be different ways for the high-power network node 11 to receive the at least one response to the probe signals. For example, the at least one response may be received from wireless devices 15a, 15b, 15c receiving the probe signals. For example, the at least one response may by the high-power network node 11 be received using a control channel (such as on the Physical Uplink Control Channel, PUCCH) or using a data traffic channel carrying control information (such as on the Physical Uplink Shared Channel, PUSCH). Alternatively, the at least one response is received from the low- power network node 12 or from another network node, such as a centralized node, a core network node, etc. The at least one response may then be received using x2 signalling and/or Si signalling, for example if the at least one response is received from a core network node.

In more detail, the wireless devices 15a, 15b which are operatively connected to the high-power network node 11 responds to the probe signals by sending channel quality information on an uplink feedback channel. From these measurements, the high-power network node 11 can identify which

directional beams are best suitable particular wireless devices 15a, 15b in its own cell 16a. Further, the wireless devices 15c operatively connected to the low-power network node 12 may also been impacted due to multiple beam transmissions from high-power network node 11 and hence cause

interference to the wireless devices 15c. The wireless devices 15c may report channel quality information measurements to the low-power network node 12.

Hence the low-power network node 12 may determine under which time intervals the channel quality information is impacted and may pass this information to the high-power network node 11, as in step Si02b. In Long- Term Evolution (LTE) networks the low-power network node 12 may pass this information through a high speed link (for example via the X2 signaling), while in High Speed Packet Access (HSPA) networks, the low-power network node 12 may pass this information to a Radio Network Controller (RNC), and the RNC may forward the information to the high-power network node 11, as in step Si02b.

Fig. 7 shows as example of sequential probing, where the high-power network node 11 transmits probe signals (through multiple antenna elements/antenna ports) sequentially with different beam patterns at times Τι, T2, ... ΊΜ. The high-power network node 11 transmits a first probe signal at time Ti, a second probe signal at T2, etc., and an :th probe signal at time ΊΜ. The wireless devices 15a, 15b, 15c respond to these probe signals by transmitting respective probe responses, such as CQI reports, in an uplink feedback channel. Based on the CQIs received for the respective probe the high-power network node 11 then identifies which directional beams to be used for transmitting signals to its own served wireless devices 15a, 15b, as in step S104.

During transmission of the probe signals from the high-power network node 11 the wireless devices 16b served by the low-power network node 12 are assumed to experience interference. This interference changes with the different beam patterns (probe signals). For example, assume that CQ1-U, is the channel quality reported by the wireless devices 16b served by the low- power network node 12 when there is no probe signal transmitted from the high-power network node 11. Further, let CQI-i, CQI-2, CQI- be the channel quality indicators reported by the wireless devices 16b served by the low-power network node 12 during the probing periods for each of the probe signals. Then the low-power network node 12 can identify which directional beams of the high-power network node 11 that might be causing interference if CQI-i7- CQI-j > V, for j = 1, 2, M, where Vis a threshold. The low-power network node 12 can pass this information (i.e., which directional beams are causing interference in its cell) to the high-power network node 11, as in step Si02b. In this respect, it is thus to be understood that the low-power network node 12 does not have to be slave-like; rather the low-power network node 12 could actively participate in the negotiation with the high-power network node 11 regarding which directional beams to use.

Once the high-power network node 11 has acquire information about the directional beams which are interfering to the wireless devices 15c of the low- power network node 12, the high-power network node 11 may perform resource sharing with the low-power network node 12. There may be different ways for the high-power network node 11 to perform resource sharing with the low-power network node 12. One example of resource sharing involves the use of the transmission time intervals (TTIs) that affected by the interference from the high-power network node 11. Further examples of resource sharing will be disclosed further below. There may be different degrees of negotiation between the high-power network node 11 and the low-power network node 12. According to an embodiment the high-power network node 11 is configured to, in an optional step sio6, exchange scheduling information with the low-power network node 12 regarding the identified directional beams. There may be different types of scheduling information to be exchanged. The type of scheduling information to be exchanged may depend on the degree of negotiation between the high-power network node 11 and the low-power network node 12. According to a first embodiment, the high-power network node 11 informs the low-power network node 12 of the TTIs numbers in a frame during which the high-power network node 11 intends to use the directional beams.

According to a second embodiment the high-power network node 11 informs the low-power network node 12 which subframes it does not use the interfering directional beams. Any of these embodiments reduce, or may even nullify, the interference caused by the high-power network node 11 to the wireless devices 15c of the low-power network node 12.

Hence, according to a first embodiment the scheduling information identifies a first beam pattern according to which the high-power network node 11 is to transmit signals to its own served wireless devices 15a, 15b. For example, the scheduling information may then identify at least one of symbol, time slot, TTI, numbers in a frame during which the high-power network node 11 is to use the first beam pattern, subframes during which the high-power network node 11 is to use the first beam pattern, and frames during which the high- power network node 11 is to use the first beam pattern

Hence, according to a second embodiment the scheduling information identifies a second beam pattern according to which the low-power network node 12 is to transmit signals to its own served wireless devices 15c. For example, the scheduling information may then identify at least one of symbol, time slot, TTI, numbers in a frame during which the high-power network node 11 is not to use the second beam pattern, subframes during l8 which the high-power network node 11 is to not to use the second beam pattern, and frames during which the high-power network node 11 is not to use the second beam pattern.

Reference is now made to Fig. 8 schematically illustrating an example of transmission of directional beams over 10 subframes based on beam restriction from the high-power network node 11. Assume, for illustrative purposes and without limitations, that the high-power network node 11 has five directional beams (i.e., that =s); say beam 1, beam 2, beam 3, beam 4, and beams. Assume further, for illustrative purposes and without limitations, that beam 1 and beam 3 causes interference to the wireless devices 15c served by the low-power network node 12. Then at subframe beam restriction can be applied as shown in Fig. 8, where the high-power network node 11 in subframes 1, 5, and 10 transmits beams 2 and/or 4 and/or 5, and in the remaining subframes 2, 3, 4, 6. 7, 8, and 9 transmits beams 1 and/or 2 and/or 3 and/or 4 and/or 5.

That is, during those subframes (TTIs) marked black the high-power network node 11 serves wireless devices 15a, 15b only on directional beams 2 and/or 4 and/or 5. These directional beams are non-interring to the other wireless devices 15c. During the remaining subframes (TTIs), the high-power network node 11 serves the wireless devices 15a, 15b with directional beams 1 and/or 2 and/or 3 and/or 4 and/or 5. That is, the directional beams are not restricted in these TTIs. At the low-power network node 12, the wireless devices 15c which are in the vicinity of the high-power network node 11 can be served in those subframes (TTIs) which are marked as black, i.e., TTIs 1, 5 and 10. There may be different ways for the high-power network node 11 to act once it has identified the directional beams as in step S104. According to one embodiment the high-power network node 11 is configured to, in a step S108, transmit signals to its owned served wireless devices 15a, 15b using the identified directional beams. The high-power network node 11 may thereby utilize beamforming, as defined by the identified directional beams, for transmitting (and receiving) data to (from) its owned served wireless devices 15a, 15b.

One particular embodiment for identifying directional beams as performed by the high-power network node 11 based on at least some of above disclosed embodiments will now be disclosed with reference to the flowchart of Fig. 6.

S202: The high-power network node 11 transmits probe signals to identify which directional beams are suitable for individual wireless devices 15a, 15b in its own cell 16a and which directional beams cause interference to wireless devices 15c served by the low-power network node 12. One way to implement step S202 is to perform steps S102, Si02a.

S204: The high-power network node 11 acquires information about directional beams which by the wireless devices 15c served by another network node are considered as causing interference. According to the present embodiment this information is acquired from the low-power network node 12. One way to implement step S204 is to perform step S102, Si02b.

S206: The high-power network node 11 identifies directional beams for transmitting signals and exchange information about beam restrictions, such as scheduling information relating to when in time particular directional beams are to be used, with the low-power network node 12. One way to implement step S206 is to perform steps S104, S106.

S208: The high-power network node 11 transmits data to its own served wireless devices 15a, 15b using directional beams according to a beam restriction pattern coordinated with the low-power network node 12 in step S206. One way to implement step S208 is to perform step S108.

The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.