WO2016195548A1

WO2016195548A1 - Transmit power balancing of antenna branches in a multi-antenna system of a radio network node

Info

Publication number: WO2016195548A1
Application number: PCT/SE2015/050630
Authority: WO
Inventors: Arne Simonsson; Bo Hagerman; Henrik Asplund
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2015-06-01
Filing date: 2015-06-01
Publication date: 2016-12-08

Abstract

There is provided a method and a corresponding arrangement, apparatus and computer program for transmit power balancing of antenna branches in a multi- antenna system of a radio network node, the multi-antenna system having at least two antenna branches. The method comprises the steps of detecting a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch. The method further comprises the step of reallocating transmit power between the antenna branches based on the detected transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength. The arrangement and the computer program are configured to perform the method.

Description

TRANSMIT POWER BALANCING OF ANTENNA BRANCHES

IN A MULTI-ANTENNA SYSTEM OF A RADIO NETWORK NODE

TECHNICAL FIELD

The proposed technology generally relates to multi-antenna systems, and more specifically to a method for transmit power balancing of antenna branches in a multi- antenna system, an arrangement configured for transmit power balancing of antenna branches in a multi-antenna system, a radio network node comprising such an arrangement, and a corresponding computer program and computer-program product, as well as an apparatus for transmit power balancing of antenna branches in a multi- antenna system. BACKGROUND

Multi-antenna systems are commonly used to improve the robustness and performance in wireless communication networks. In particular, the capacity of a radio link can generally be increased using multiple transm it and/or receive antennas to exploit multipath propagation.

Many new products and/or systems are emerging for various applications, for example for small cell/indoor scenarios, such as Distributed Antenna Systems, DAS, Radio Dot Systems, RDS, and In-Building Systems, IBS and so forth.

The ever-increasing traffic demand requires a massive deployment of such new targeted solutions. In order to simplify production and reduce production costs, some of these products/systems may have less stringent requirements on e.g. Error Vector Magnitude, EVM, than regular macro base stations. To keep costs down, a larger variability on transmit power accuracy may also be accepted as a production requirement. Moreover antenna sizes may also be reduced to e.g. keep costs down. Such size reduction can lead to antenna diagram that differ between antenna branches. However, less stringent production requirements may lead to performance degradations, e.g. due to transmit power imbalances, especially for higher rank, i.e. rank > 2, transmissions, if not properly handled.

SUMMARY

It is desirable to provide transmit power balancing of antenna branches in a multi- antenna system.

It is an object to provide a method for transmit power balancing of antenna branches in a multi-antenna system.

It is also an object to provide an arrangement configured for transmit power balancing of antenna branches in a multi-antenna system.

Another object is to provide a radio network node that comprises such an arrangement. Yet another object is to provide a corresponding computer program and computer- program product.

It is also an object to provide an apparatus for transmit power balancing of antenna branches in a multi-antenna system.

These and other objects are met by embodiments of the proposed technology.

According to a first aspect there is provided a method for transmit power balancing of antenna branches in a multi-antenna system of a radio network node, the multi- antenna system having at least two antenna branches. The method comprises the step of determining a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch. The method also comprises the step of reallocating transmit power between the antenna branches based on the determining transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength. According to a second aspect there is provided an arrangement configured to transmit power balancing of antenna branches in a multi-antenna system of a radio network node, the multi-antenna system having at least two antenna branches. The arrangement is configured to determine a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch. The arrangement is further configured to reallocate transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength. According to a third aspect there is provided a .radio access point comprising an arrangement according to the second aspect.

According to a fourth aspect there is provided a computer program for performing, when executed by at least one processor, transmit power balancing of antenna branches in a multi-antenna system of a radio network node, the multi-antenna system having at least two antenna branches. The computer program comprising instructions, which when executed by the at least one processor, cause the at least one processor to determine a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch. The computer program also comprises instructions which when executed by the at least one processor, cause the at least one processor to reallocate transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength.

According to a fifth aspect there is provided a computer program product comprising a computer-readable medium having stored thereon a computer program of the third aspect. According to a sixth aspect there is provided an apparatus for transmit power balancing of antenna branches in a multi-antenna system having at least two antenna branches. The apparatus comprises a determining module for determining a detected transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch. The apparatus further comprises a reallocation module for reallocating transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength.

These aspects provide for a more balanced transmit power between the different antenna branches. A more balanced transmit power between the antenna branches will normally lead to a significant improvement with regard to capacity, maybe even reaching peak rates in throughput, and/or improved spatial multiplexing performance.

Other advantages will be appreciated when reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 is a schematic block diagram illustrating an example of a multi-antenna system of a radio access point according to an embodiment.

FIG. 2 is a schematic flow diagram illustrating an example of a method for transmit power balancing of antenna branches in a multi-antenna system.

FIG. 3 is a schematic curve diagram illustrating an example of the correlation between throughput and received power for higher and lower EVM, respectively. FIG. 4 is a schematic diagram illustrating an example of the spread in power difference between branches for three dot samples. FIG. 5 is a schematic diagram illustrating an example of the direction-dependency of the transmit power difference between antenna branches from a walk-around test.

FIG. 6 is a schematic diagram illustrating an example of a radio dot/access point with the reference angles used for the walk-around test of FIG. 6.

FIG. 7 is a schematic block diagram illustrating an example of an arrangement configured for transmit power balancing of antenna branches in a multi-antenna system. FIG. 8 is a schematic diagram illustrating an example of a computer implementation according to an embodiment.

FIG.9 is a schematic block diagram illustrating an apparatus according to a particular embodiment of the proposed technology.

DETAILED DESCRIPTION

Throughout the drawings, the same reference designations are used for similar or corresponding elements.

For a better understanding of the proposed technology, it may be useful to begin with a brief overview of a multi-antenna system of an access point with reference to FIG. 1 . The multi-antenna system 20 may be part of an access point 100 such as a base station or a radio dot. The multi-antenna system 20 normally comprises a number, N > 2, of antenna branches 10-1 , 10-N, each of which may include a transmission, TX, chain and power amplifier connected to a respective antenna. There is also shown a RX chain and signal strength measuring device, denoted SMD, connected to respectively antenna. The access point 100 may also include a transmission control unit 40, which in turn may have a power control unit 30 for controlling the transmit power of the antenna branches. The access point may be a base station or part thereof, such as a radio dot in a distributed base station system.

Ideally, the antenna branches are identical. However in practice there may be deviations from the ideal performance of the involved components. For example, to keep costs down in the production of a multi-antenna system, more or less stringent requirements, e.g. on EVM and/or transmit power accuracy, may be accepted. The antenna diagrams can also differ between antenna branches from both production cost reasons and to reduce the size of the antenna. Antenna impedance as well as impedance matching can vary from production and tolerance range of matching components. This can result in an imbalance in the transmitted power EIRP out from the antenna and power imbalance between antenna branches. Although the antenna branches are operated to transmit with full nominal power, there may thus still be a transmit power imbalance between branches, something which in turn may lead to a multi-antenna system performance that is far from optimal.

The imbalance between branches is true for large size macro antennas. For compact low cost antenna solutions, such as a radio DOT antenna, antenna branch differences may vary significantly in different directions, see i.e., FIG.5 which illustrate an example of the transmit power difference, TX power difference.

The transmit power difference between antenna branches may lead to a large variation on received transmit power level for different directions and different UE positions around the radio DOT. This is illustrated in the transmit power difference distribution of FIG. 4 from a User Equipment, UE, as measured when walking around a radio DOT. In greater detail FIG. 4 illustrates a measured TX branch power difference from a walk around three different DOT samples. The power difference is also seen as the distribution per DOT sample in FIG. 4 from walk test around the DOTs. The power difference in different directions and areas around the DOT will result in ill-conditioned spatial multiplexing channel which in turn may reduce the rank-2 throughput. Reference [1 ] relates to a control method in a mobile communication system comprising an antenna group including a plurality of antennas, each transmitting different antenna identification information. The method involves transmitting identification information that allows each antenna to be uniquely identified and notifying the correspondence between each antenna and the identification information to a mobile station. The mobile station may then use the antenna identification information to distinguish among antennas. The mobile station may also transmit information about a specific antenna in association with the identification information. This may for example be PMI or CQI information about that specific antenna.

Reference [2] relates to techniques for enhancing uplink measurements for positioning by adaptively using multi-antenna systems. This is an entirely different technical application compared to the transmit power balancing of the proposed technology.

The proposed technology provides a solution for transmit power balancing of antenna branches in a multi-antenna system. There is in particular provided a solution for transmit power balancing of antenna branches for scenarios where the difference between the branches may reside from differences in the radio channels and the antenna gain. The proposed solution enables a quantitative determination of the transmit power differences and a reallocation of the transmit power among the antenna branches based on the determined transmit power difference.

FIG. 2 is a schematic flow diagram illustrating an embodiment of a method for transmit power balancing of antenna branches in a multi-antenna system. Illustrated is a method for transmit power balancing of antenna branches in a multi-antenna system of a radio network node, the multi-antenna system having at least two antenna branches. The method comprises the step of determining S1 a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch. The method also comprises the step of reallocating S2 transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength. In other words, the antenna branch transmit power difference is determined by utilizing measurements of the received signal strength in the uplink from transmissions from a UE. The outcome of these uplink measurements form the basis according to which the transmit power of the different branches may be adjusted in order to obtain a better transmit power balance between the antenna branch for transmissions in the downlink.

The proposed technology provides a particularly useful mechanism for balancing the transmit power utilized by different antenna branches when the transmit power imbalance between the antenna branches depends on e.g. radio channel differences and/or antenna diagram differences resulting in different antenna gain and not necessarily differences in the hardware. The mechanism is based on the assumption that there is a reciprocity between transmit power differences in the downlink and the transmit power difference on the uplink. That is, a difference in the uplink signal strength as recorded by different antenna branches may be used to determine the transmit power difference on the downlink for different antenna branches. The determined difference may then be used as a measure for reallocating transmit power resources between the antenna branches.

Power balancing according to the antenna gain mismatches creates increased probability for high rank transmission. In certain cases the full antenna mismatch cannot be compensated in all directions. However, a reduced transmitted power between antenna branches will still increase the possibility to transmit multiple data streams to, e.g. a UE over high quality channels. The transmit power is typically reallocated to provide a more balanced transmit power situation. A balanced transmit power between the antenna branches will normally lead to a significant improvement with regard to capacity, maybe even reaching peak rates in throughput, and/or improved spatial multiplexing performance.

According to a possible embodiment of the proposed technology there is provided a method wherein the step S2 of reallocating transmit power between the antenna branches is based on reducing the transmit power of a stronger antenna branch.

That is, the method determines the transmit power difference based on the measured uplink signal strength. Having determined the difference the method is able to reallocate the available transmit power resources in such a way that the strongest antenna branch, i.e. the antenna branch with the higher transmit power gets a reduced transmit power.

According to an alternative embodiment there is instead provided a method wherein the step S2 of reallocating transmit power between the antenna branches is based on increasing the transmit power of a weaker antenna branch.

If there are enough power resources available it might be possible to increase the transmit power of the weaker antenna branch in order to balance the relative transmit power differences. Transmission in good coverage deployments comprising, for example, individual DOT hardware samples will gain in performance by a reduction of the transmit power of the strongest antenna branch. The loss of received signal strength does not have any significant impact on coverage and the improved quality of the signal from the weaker antenna branch will instead improve the spatial multiplexing reception.

It is also possible reallocate transmission power in such a way that the strongest antenna branch is allowed a reduced transmit power while the weaker is allowed an increased transmit power.

By way of example, a possible embodiment of the proposed technology provides a method wherein the transmit power difference is detected per UE and the transmit power is reallocated accordingly per UE. In other words, the transmit power reallocation may in particular embodiments be done per UE individually in order to adapt to the actual UE position and the path-direction from the antenna to the UE. This is specifically beneficial when the antenna diagrams differ between the antenna branches as exemplified in FIG. 5.

An optional embodiment of the proposed technology relates to a method wherein the reallocating step S2 is performed only for spatial multiplexing transmissions. By utilizing the proposed technology to improve the transmit power balance between different antenna branches for spatial multiplexing transmissions it will be possible to obtain a higher user throughput.

Having described certain embodiments of the proposed technology below there will be provided some detailed explanations of the proposed technology and some exemplary embodiments. The provided exemplary embodiments are merely intended to facilitate the understanding of the proposed technology.

Higher EVM results in a significant degradation in throughput performance of rank 2 transmissions when using the highest Modulation and Coding Scheme, MCS. This can be seen in FIG. 3, which is a schematic curve diagram illustrating an example of the correlation between throughput and received power for higher and lower EVM, respectively.

A spread in transmit power difference between antenna branches per DOT sample may degrade the rank 2 transmission performance from a certain DOT. See the average difference between DOT samples in the schematic curve diagram of FIG. 5. Note that within about 15 m distance from a DOT, the RSRP is typically above 100 dBm. A 15 m radius means a coverage area of about 700 m² which is the rule of thumb coverage area for DOT deployment. FIG. 5 is a schematic diagram illustrating an example of the direction-dependency of the transmit power difference between antenna branches from a walk-around test. FIG. 6 is a schematic diagram illustrating an example of a radio dot/access point with the reference angles used for the walk-around test of FIG. 6. The proposed method utilizes measurements on the uplink signal strength in order to determine a transmit power difference between antenna branches and reallocate the transmit power to ensure a more balanced transmit power distribution between the antenna branches. The method as such may be performed by an arrangement comprised in the radio access point containing the multi-antenna system, such as the radio access point illustrated in FIG.2. In such an embodiment the method may comprise the step of performing measurements on the uplink signal strength of received signals as detected by the various antenna branches. The measurements may be performed means of signal measurement devices, SMDs as illustrated in FIG.2. After having measured the signal strength the corresponding information is used to determine S1 the transmit power difference by assuming a reciprocity on the uplink and downlink. The determined transmit power difference is then utilized to reallocate S2 the transmit power between the antenna branches.

It is however also possible that the method is implemented and performed at some other remote arrangement relative the access point. In such a scenario the method takes as input the measured values and determines S1 the transmit power difference based on the input values. The determined difference is the used as a basis for reallocating S2 the transmit power difference. The particular reallocation pattern or scheme may then be provided to a power control unit controlling the power used by the various antenna branches.

As has been described earlier, a possible embodiment of the proposed technology provides a method wherein the transmit power difference is detected per UE and the transmit power is reallocated accordingly per UE. To further illustrate this consider a scenario where the small size antenna results in a direction dependent transmit power difference between antenna branches of 10 dB or more, as seen in FIG. 6. This difference will degrade the rank 2 transmission performance in a certain direction (direct or reflected). By compensating individually for each UE this directional difference can be mitigated. This directional antenna pattern difference is also equal for received signal as for transmitted signal. Measuring received signal on the antenna branches from a UE in a certain direction will enable one to determine the transmitted power difference towards that UE caused by the antenna diagram rather accurately. This enables a precise reallocation of transmitted power between the antenna branches reaching equal transmit power towards that UE. For a UE in another direction of the antenna a completely different reallocation should be done to compensate the antenna pattern as seen in FIG. 5. For example, two UEs in the direction 50° a way from right beneath the antenna in Fig. 7 (0=50°) are separated 120° in horizontal direction (Φ). One UE in the direction Φ = 0° and the other UE in the dire Φ = 120°. As can be seen in Fig. 6 the antenna gain difference between the antenna branches (P1 and P2) is around + 3 dB for the first UE while it is around -4 dB for the other UE. By measuring uplink received signal for both antenna branches this difference in antenna diagram can be detected. The corresponding difference in transmitted power between the antenna branches caused by antenna pattern is the same. The transmit power difference can be determined for both UEs to +3 dB and -4 dB respectively based on the received signal. The power is then be reallocated per UE by reducing the power for antenna P1 with 3 dB for the first UE and reducing the power for antenna P2 with 4 dB for the second UE.

By way of example, in an indoor deployment of a RDS system with two antenna branches, the signal strength is typically high and thereby likely a high fraction of rank 2 transmission opportunities. In a conventional system, the peak rate may however still not be reached but is limited by high EVM in combination with transmit power imbalance. In case of transmit power imbalance between the antenna branches, the transmit power may be reallocated between the branches, e.g. by reducing the power of the strongest antenna branch. The balancing may be applied only for spatial multiplexing keeping the full power advantage for transmit diversity with single stream transmissions. In particular, reducing the power level for a transmitter will in turn alleviate and reduce EVM as an additional benefit. When operating at full nominal power, the working conditions are closer to the non-linear limiting part of the amplifier thus creating higher EVM, whereas lower power levels imply more linear working conditions of the amplifier thus creating lower EVM. According to a particular example of the proposed technology there is provided a method where the uplink received signal strength from the UE is measured per antenna branch. During ordinary circumstances each antenna branch is able to perform its own measurement and hence all antenna branches may perform a measurement on its particular received uplink signal strength. Based on the measured or detected received uplink signal strength, the method may proceed and perform a reallocation of the transmit power between the antenna branches. The downlink transmission may thus be performed with, for example, a power reduction on the strongest received uplink branch where the value of the power reduction corresponds to the difference in uplink received signal strength. In this way the transmit power may be balanced between the antenna branches. An alternative reallocation comprises to increase the power of the weakest antenna branch instead of decreasing the power of the strongest antenna branch.

To further illustrate the method consider the simplified case where two antenna branches are used. In such a scenario the first antenna branch may measure the received uplink signal strength to be Pi and the second antenna branch may measure the received uplink signal strength to be P₂. These measurements enables a determination of the power difference between the branches. It may in particular cases be given by P= abs (Pi - P₂). Here "abs" denotes the absolute value of the expression and it is included to ensure that the corresponding value is positive. By invoking that there is a reciprocity between the transmitted power and the received power it may be concluded that there is a transmit power imbalance, at least to first order, between the antenna branches given by P= abs (k (Pi - P₂)), where k is some coefficient >0. It may in particular examples be a coefficient that is smaller or equal to 1. The determined transmit power difference value may, according to the proposed technology, be used to reallocate transmit power resources between the two antenna branches. According to one particular example the transmit power may be reallocated by decreasing the transmit power of the antenna branch that measured the highest uplink received signal strength with a value abs (k (Pi - P₂)), alternatively the antenna branch that measured the lowest uplink received signal may have its transmit power strength increased with a value given by abs (k (Pi - P2)). Other possibilities are also possible such as providing a certain fraction of abs (k (Pi - P₂)) to one antenna branch and another fraction to the second antenna branch in such a way that the transmit power difference gets reduce. Note moreover, that the value of k in particular embodiments may be set to 1 .

The transmit power reallocation procedure according to the proposed technology may be applied on rank 2 transmissions by using full power for rank 1 transmission to thereby maintain transmit diversity quality. In case there are more antenna branches than layers of spatial multiplexing the power reallocation may be applied between groups of antennas allocated to different spatial multiplexing layers.

According to a particular exemplary embodiment a reallocation of the transmit power between antenna branches may comprise a power reduction that corresponds to the received power difference. That is, if the detected power difference between the antenna branches is P, then a power reduction of P may be performed on the strongest branch. Another possible embodiment instead provides a reduction of k x P, where k is a coefficient taking values between 0 and 1 . If k < 1 this will correspond to a part compensation where there is still an antenna branch having a stronger transmit power.

The proposed technology also provides an arrangement 10 configured to transmit power balancing of antenna branches in a multi-antenna system of a radio network node, the multi-antenna system having at least two antenna branches. The arrangement 10 is configured to determine a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch. The arrangement 10 is also configured to reallocate transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength. According to a possible embodiment of the proposed technology there is provided an arrangement 10 wherein the arrangement 10 is configured to reallocate transmit power between the antenna branches by reducing the transmit power of a stronger antenna branch.

Another possible example of an embodiment provides an arrangement 10 wherein the arrangement 10 is configured to reallocate transmit power between the antenna branches by increasing the transmit power of a weaker antenna branch. By way of example, the proposed technology provides an embodiment of an arrangement 10, wherein the arrangement 10 is configured to collect information representative of spatial multiplexing performance of the multi-antenna system.

FIG. 7 is a schematic block diagram illustrating an example of an arrangement comprising a processor and an associated memory. Illustrated is an arrangement 10, that comprises at least one processor 210 and a memory 220, the memory 220 comprising instructions executable by the at least one processor 210, whereby the at least one processor 210 is operative to perform transmit power balancing of antenna branches.

Optionally, the arrangement may also include a communication circuit. The communication circuit may include functions for wired and/or wireless communication with other devices and/or network nodes in the network. In a particular example, the communication circuit may be based on radio circuitry for communication with one or more other nodes, including transmitting and/or receiving information. The communication circuit may be interconnected to the processor and/or memory. The schematic block diagram of FIG.7 also provides an illustration of an arrangement 10, wherein the arrangement 10 comprises communication circuitry 230 configured to perform the transmissions with at least two different transmit power configurations.

The proposed technology also provides a radio access point 100 that comprises an arrangement 10 according to the earlier described embodiments. The proposed technology may be applied to a user terminal such as a wireless device. As used herein, the non-limiting terms "User Equipment" and "wireless device" may refer to a mobile phone, a cellular phone, a Personal Digital Assistant, PDA, equipped with radio communication capabilities, a smart phone, a laptop or Personal Computer, PC, equipped with an internal or external mobile broadband modem, a tablet PC with radio communication capabilities, a target device, a device to device UE, a machine type UE or UE capable of machine to machine communication, iPAD, customer premises equipment, CPE, laptop embedded equipment, LEE, laptop mounted equipment, LME, USB dongle, a portable electronic radio communication device, a sensor device equipped with radio communication capabilities or the like. In particular, the term "UE" and the term "wireless device" should be interpreted as non-limiting terms comprising any type of wireless device communicating with a radio network node in a cellular or mobile communication system or any device equipped with radio circuitry for wireless communication according to any relevant standard for communication within a cellular or mobile communication system. As used herein, the non-limiting term "radio network node" may refer to base stations, network control nodes such as network controllers, radio network controllers, base station controllers, and the like. In particular, the term "base station" may encompass different types of radio base stations including standardized base stations such as Node Bs, or evolved Node Bs, eNBs, and also macro/micro/pico radio base stations, home base stations, also known as femto base stations, relay nodes, repeaters, radio access points, base transceiver stations, BTSs, and even radio control nodes controlling one or more Remote Radio Units, RRUs, or the like.

It will be appreciated that the methods and devices described herein can be combined and re-arranged in a variety of ways.

For example, embodiments may be implemented in hardware, or in software for execution by suitable processing circuitry, or a combination thereof. The steps, functions, procedures, modules and/or blocks described herein may be implemented in hardware using any conventional technology, such as discrete circuit or integrated circuit technology, including both general-purpose electronic circuitry and application-specific circuitry.

Particular examples include one or more suitably configured digital signal processors and other known electronic circuits, e.g. discrete logic gates interconnected to perform a specialized function, or Application Specific Integrated Circuits, ASICs.

Alternatively, at least some of the steps, functions, procedures, modules and/or blocks described herein may be implemented in software such as a computer program for execution by suitable processing circuitry such as one or more processors or processing units.

Examples of processing circuitry includes, but is not limited to, one or more microprocessors, one or more Digital Signal Processors, DSPs, one or more Central Processing Units, CPUs, video acceleration hardware, and/or any suitable programmable logic circuitry such as one or more Field Programmable Gate Arrays, FPGAs, or one or more Programmable Logic Controllers, PLCs.

It should also be understood that it may be possible to re-use the general processing capabilities of any conventional device or unit in which the proposed technology is implemented. It may also be possible to re-use existing software, e.g. by reprogramming of the existing software or by adding new software components.

In this particular example, at least some of the steps, functions, procedures, modules and/or blocks described herein are implemented in a computer program, which is loaded into the memory for execution by processing circuitry including one or more processors. The processor(s) and memory are interconnected to each other to enable normal software execution. An optional input/output device may also be interconnected to the processor(s) and/or the memory to enable input and/or output of relevant data such as input parameter(s) and/or resulting output parameter(s). The term 'processor' should be interpreted in a general sense as any system or device capable of executing program code or computer program instructions to perform a particular processing, determining or computing task.

The processing circuitry including one or more processors is thus configured to perform, when executing the computer program, well-defined processing tasks such as those described herein. The processing circuitry does not have to be dedicated to only execute the above- described steps, functions, procedure and/or blocks, but may also execute other tasks.

According to a particular embodiment, there is thus provided a computer program 225; for performing, when executed by at least one processor 210, transmit power balancing of antenna branches in a multi-antenna system of a radio network node, the multi-antenna system having at least two antenna branches. The computer program 225 comprising instructions, which when executed by the at least one processor 210, cause the at least one processor 210 to:

• determine a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch;

• reallocate transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength.

The proposed technology also provides a carrier comprising the computer program, wherein the carrier is one of an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or a computer-readable storage medium.

By way of example, the software or computer program may be realized as a computer program product, which is normally carried or stored on a computer-readable medium, in particular a non-volatile medium. The computer-readable medium may include one or more removable or non-removable memory devices including, but not limited to a Read-Only Memory, ROM, a Random Access Memory, RAM, a Compact Disc, CD, a Digital Versatile Disc, DVD, a Blu-ray disc, a Universal Serial Bus, USB, memory, a Hard Disk Drive, HDD, storage device, a flash memory, a magnetic tape, or any other conventional memory device. The computer program may thus be loaded into the operating memory of a computer or equivalent processing device for execution by the processing circuitry thereof.

Hence the proposed technology also provides a computer program product comprising a computer-readable medium 220 having stored thereon a computer program 225 according to the proposed technology. FIG.8 is a schematic block diagram illustrating how such a computer program product comprising the computer program 225 may be used in an arrangement 10 for transmit power balancing. The flow diagram or diagrams presented herein may therefore be regarded as a computer flow diagram or diagrams, when performed by one or more processors. A corresponding apparatus may be defined as a group of function modules, where each step performed by the processor corresponds to a function module. In this case, the function modules are implemented as a computer program running on the processor. Hence, the arrangement may alternatively be defined as a group of function modules, where the function modules are implemented as a computer program running on at least one processor.

The computer program residing in memory may thus be organized as appropriate function modules configured to perform, when executed by the processor, at least part of the steps and/or tasks described herein. An example of such function modules is illustrated in FIG. 9.

FIG. 9 is a schematic block diagram illustrating an example of an apparatus comprising a group of function modules. More specifically is shown an apparatus 100 for transmit power balancing of antenna branches in a multi-antenna system having at least two antenna branches. The apparatus comprises a determining module 15 for determining a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch. The apparatus further comprises a reallocation 25 module for reallocating transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength.

Alternatively it is possibly to realize the modules in FIG. 9 predominantly by hardware modules, or alternatively by hardware, with suitable interconnections between relevant modules. Particular examples include one or more suitably configured digital signal processors and other known electronic circuits, e.g. discrete logic gates interconnected to perform a specialized function, and/or Application Specific Integrated Circuits, ASICs, as previously mentioned. Other examples of usable hardware include input/output, I/O, circuitry and/or circuitry for receiving and/or sending signals. The extent of software versus hardware is purely implementation selection.

The embodiments described above are merely given as examples, and it should be understood that the proposed technology is not limited thereto. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the present scope as defined by the appended claims. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible.

REFERENCES

[1] EP 2164186

[2] WO 2014/027941

Claims

1 . A method for transmit power balancing of antenna branches in a multi- antenna system of a radio network node, the multi-antenna system having at least two antenna branches, wherein said method comprises the steps of:

determining (S1 ) a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch;

reallocating (S2) transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength.

2. The method of claim 1 , wherein the step (S2) of reallocating transmit power between the antenna branches is based on reducing the transmit power of a stronger antenna branch.

3. The method of claim 1 or 2, wherein the step (S2) of reallocating transmit power between the antenna branches is based on increasing the transmit power of a weaker antenna branch.

4. The method of any of the claims 1 to 3, wherein the transmit power difference is detected per UE and the transmit power is reallocated accordingly per UE.

5. The method of any of the claims 1 to 4, wherein the reallocating step (S2) is performed only for spatial multiplexing transmissions

6. An arrangement (10) configured to transmit power balancing of antenna branches in a multi-antenna system of a radio network node, the multi-antenna system having at least two antenna branches, wherein:

the arrangement (10) is configured to determine a transmit power difference between at least two of the antenna branches based on measurements of uplink received signal strength per antenna branch; and the arrangement (10) is configured to reallocate transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength.

7. The arrangement of claim 6, wherein the arrangement (10) is configured to reallocate transmit power between the antenna branches based on reducing the transmit power of a stronger antenna branch.

8. The arrangement according to any of the claims 6 or 7, wherein the arrangement (10) is configured to reallocate transmit power between the antenna branches based on increasing the transmit power of a weaker antenna branch.

9. The arrangement (10) according to any of the claims 6 - 8, wherein the arrangement (10) is configured to collect information representative of spatial multiplexing performance of the multi-antenna system.

10. The arrangement of any of the claims 6 to 8, wherein the arrangement (10) comprises at least one processor (210) and a memory (220), said memory (220) comprising instructions executable by the at least one processor (210), whereby the at least one processor (210) is operative to perform transmit power balancing of antenna branches.

1 1 . The arrangement of any of the claims 6 to 10, wherein the arrangement (10) comprises communication circuitry (230) configured to perform the transmissions with at least two different transmit power configurations.

12. A radio access point (100) comprising an arrangement (10) of any of the claims 6 to 1 1 .

13. A computer program (225) for performing, when executed by at least one processor (210), transmit power balancing of antenna branches in a multi-antenna system of a radio network node, the multi-antenna system having at least two antenna branches, said computer program (225) comprising instructions, which when executed by the at least one processor (210), cause the at least one processor (210) to:

determine a transmit power difference between at least two of the 5 antenna branches based on measurements of uplink received signal strength per antenna branch;

reallocate transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal 10 strength.

14. A computer program product comprising a computer-readable medium (240) having stored thereon a computer program (225) of claim 13.

15 15. An apparatus (100) for transmit power balancing of antenna branches in a multi-antenna system having at least two antenna branches, wherein said apparatus comprises:

a determining module (15) for determining a transmit power difference between at least two of the antenna branches based on measurements of uplink 20 received signal strength per antenna branch;

a reallocation module (25) module for reallocating transmit power between the antenna branches based on the determined transmit power difference by performing a transmit power adjustment of a selected antenna branch corresponding to a difference in uplink received signal strength.

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