WO2018067050A1

WO2018067050A1 - Spatial multiplexing transmission

Info

Publication number: WO2018067050A1
Application number: PCT/SE2016/050970
Authority: WO
Inventors: Arne Simonsson; Björn Halvarsson; Qiang Zhang; Peter ÖKVIST
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2016-10-07
Filing date: 2016-10-07
Publication date: 2018-04-12

Abstract

There is provided mechanisms for spatial multiplexing transmission from spatially separated transmission points in a network where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points. A method is performed by a network node. The method comprises determining channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device. The method comprises estimating channel rank for each of at least two sub-sets of the transmission points based on the channel conditions. The method comprises initiating the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank

Description

SPATIAL MULTIPLEXING TRANSMISSION

TECHNICAL FIELD

Embodiments presented herein relate to a method, a network node, a computer program, and a computer program product for spatial multiplexing transmission from spatially separated transmission points.

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, for wireless communications, the use of comparatively high frequency bands could result in comparatively small antenna sizes. Small antennas enable a comparatively large number of antennas to be used in radio transceiver units, both at the network side, such as in radio access nodes, and in served wireless devices, such as user equipment. Large number of antennas is foreseen to be used for beam forming by having closely spaced multiple antennas. On the network side it is expected to have a large number of antennas in transmission points of the radio access nodes, enabling both beam-forming and spatial multiplexing. In view of traditional cellular communication technologies, there is not necessarily a one-to-one

correspondence between one access point and one cell as several radio access nodes, or several transmission points for one or more radio access nodes together can form a cell. At comparatively high frequency bands the radio propagation conditions could require a dense site deployment (i.e., a tight deployment of access nodes) which in many cases results in more line-of-sight (LoS) conditions between access nodes and wireless devices. Distributed multiple input multiple output (MIMO) communications could be used as a mechanism to increase downlink (i.e., in the direction from access node to wireless device) transmission rank in LoS.

However, there is a need for improved mechanisms for communication using transmission points.

SUMMARY

An object of embodiments herein is to provide efficient mechanisms for spatial multiplexing transmission from spatially separated transmission points. According to a first aspect there is presented a method for spatial

multiplexing transmission from spatially separated transmission points in a network where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points. The method is performed by a network node. The method comprises determining channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device. The method comprises estimating channel rank for each of at least two sub-sets of the transmission points based on the channel conditions. The method comprises initiating the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank.

According to a second aspect there is presented a network node for spatial multiplexing transmission from spatially separated transmission points in a network where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points. The network node comprises processing circuitry. The processing circuitry is configured to cause the network node to determine channel conditions based on

measurements of received signal strength per antenna branch for an uplink signal received from a wireless device. The processing circuitry is configured to cause the network node to estimate channel rank for each of at least two sub-sets of the transmission points based on the channel conditions. The processing circuitry is configured to cause the network node to initiate the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank.

According to a third aspect there is presented a network node for spatial multiplexing transmission from spatially separated transmission points in a network where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points. The network node comprises processing circuitry and a storage medium storing

instructions that, when executed by the processing circuitry, causes the network node to perform operations, or steps. The operations, or steps, cause the network node to determine channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device. The operations, or steps, cause the network node to estimate channel rank for each of at least two sub-sets of the transmission points based on the channel conditions. The operations, or steps, cause the network node to initiate the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank.

According to a fourth aspect there is presented a network node for spatial multiplexing transmission from spatially separated transmission points in a network where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points. The network node comprises a determine module configured to determine channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device. The network node comprises an estimate module configured to estimate channel rank for each of at least two sub-sets of the transmission points based on the channel conditions. The network node comprises an initiate module configured to initiate the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank. According to a fifth aspect there is presented a computer program for spatial multiplexing transmission from spatially separated transmission points in a network where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points, the computer program comprising computer program code which, when run on a network node, causes the network node to perform a method according to the first aspect.

According to a sixth aspect there is presented a computer program product comprising a computer program according to the fifth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

Advantageously this method and this network node provide efficient communication using transmission points.

Advantageously this method and this network node increase the user throughput and shorten the delay, which improves the experienced user quality and performance.

Advantageously this method and this network node enable advantages of distributed MIMO to be achieved without sacrificing capacity.

Advantageously this method and this network node enable configuring of multiple transmission points with adaptability to traffic variations. Advantageously this method and this network node enable energy saving by turning off transmission points not improving end-user throughput.

It is to be noted that any feature of the first, second, third, fourth, fifth and sixth aspects maybe applied to any other aspect, wherever appropriate.

Likewise, any advantage of the first aspect may equally apply to the second, third, fourth, fifth and/or sixth 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, l, 2, and 3 are schematic diagrams illustrating communication networks according to embodiments;

Figs. 4 and 5 are flowcharts of methods according to embodiments;

Fig. 6 is a schematic diagram showing functional units of a network node according to an embodiment;

Fig. 7 is a schematic diagram showing functional modules of a network node according to an embodiment; and

Fig. 8 shows one example of a computer program product comprising computer readable storage medium according to an embodiment. 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.

Figs. i(a), (b), (c) are schematic diagram illustrating communications networks 100a, 100b, 100c where embodiments presented herein can be applied. The communications networks 100a, 100b, looc comprise a radio access node 130. The radio access node 130 could be provided as a radio base station, a base transceiver station, a node B, an evolved node B, an access point, or an access node. The radio access node 130 provides network access to at least one wireless device 140a in beams 120a, 120b via transmission points 110a, nob (also known as remote radio units, or remote radio heads). The transmission points 110a, nob are spatially separated meaning that each of the transmission points 110a, 110b could be separately mountable and/or that antennas of one transmission point 110a are uncorrelated with antennas of another transmission point 110b. The communications networks 100a, 100b, 100c further comprise a network node 200. The network node 200 could be provided in, or co-located with, the radio access node 130. Further aspects of the network node 200 will be provided below.

With ordinary single point MIMO transmission (Fig. i(a)) the rank is often limited to 2 utilizing the polarization domain. For higher rank the wireless device 140a could have difficulties to separate the layers because the angle of arrival is the same (or at least similar enough) for the transmission layers. Dual (i.e. multiple) access point transmission (Fig. i(b)) does not either improve the rank because duplicated transmission of layers also arrives with the same angle of arrival, hence exploiting more or less the same radio channel. Transmitting different layers from different transmission points 110a, 110b (Fig. i(c)) creates an angle of arrival separation at the wireless device, which in turn enables receiver in the wireless device 140a to separate the transmitted layers from the different transmission points 110a, 110b.

To efficiently utilize higher order spatial multiplexing the difference of received power from transmission points 110a, 110b at the wireless device 140a should not be too large. Distributed MIMO can therefore be activated based on the path loss or signal strength difference between the transmission points iioa, nob, i.e. when the signal strength difference is below a certain threshold.

Under specific conditions, utilizing more than one transmission point noa, nob for distributed MIMO can be a waste of resources and there is not always a gain from distributed MIMO transmission. If there is a plurality of wireless devices 140a requiring transmission or reception of data in a region served by several radio access nodes 130 or transmission points 110a, 110b, an alternative to distributed MIMO and which could give higher capacity than distributed MIMO could be to transmit from one transmission point 110a to one wireless device 140a each. If the angle of separation the wireless device 140a sees between the transmission points 110a, 110b is too small, it can be better with ordinary dual access node transmission. If there is rich channel, such as indoors or from fagade reflections outdoors, high rank can be achieved with single access point transmission. When there are more than two transmission points 110a, 110b in LoS from the same wireless device 140a the optimal selection of which transmission points 110a, 110b to use is not straightforward.

In more detail, the position of the wireless device 140a in relation to the transmission points 110a, 110b providing network access to this wireless device 140a is one parameter for the achievable gain with distributed MIMO. If there are more transmission points 110a, 110b to choose among for distributed MIMO, the selection of the transmission points 110a, 110b giving largest angle of arrival could result in best performance for the wireless device 140a. This is illustrated in Fig. 2.

Fig. 2 is a schematic diagram illustrating a communications network lood where embodiments presented herein can be applied. The communications network lood is similar to the communications networks 100a, 100b, 100c and thus comprises a network node 200, a radio access node 130, and transmission points 110a, 110b, 110c providing network access in beams 120a, 120b, 120c to at least one wireless device 140a. In the illustrative example of Fig. 2, there are three possible transmission points 110a, nob, 110c to transmit distributed MIMO to the wireless device 140a. Transmitting one layer from transmission point 110a and one layer from transmission points 110c will result in a small angular difference a at the wireless device 140a between the two spatial multiplexing layers. With a small difference in angle of arrival, it could be more difficult to separate the two layers in the receiver of the wireless device 140a, resulting in less efficient spatial multiplexing, e.g. that a lower code-rate must be used to be able to decode the two layers. If instead one layer is transmitted from transmission point 110a and one layer from transmission point nob (or similarly, one layer from transmission point 110c and one layer from transmission point 110b), the angle of arrival difference φ (or φ+ a) is larger and higher code-rate can be used resulting in higher throughput.

The spatial multiplexing rank is also limited by the number of antennas at the wireless device 140a and it is therefore not always more beneficial to use all available transmission points 110a, 110b, 100c for transmission to the wireless device 140a. In the illustrative example of Fig. 2, If the wireless device 140a only has two antennas it could for this wireless device 140a not be possible to receive more than two spatial layers. It is not only the number of physical antennas that may limit the number of spatial layers. For example, even if there are more than two physical antennas the receiver of the wireless device 140a could be configured to combine the signals from two or more physical antennas, resulting in only two receiver branches. The wireless device 140a may also be according to a specified UE category (where UE is short for user equipment) with limited spatial multiplexing layer. There is thus no gain in transmitting from all three transmission points 110a, 110b, 100c with different MIMO data streams. It is therefore an advantage to select the best TPs utilizing the number of wireless device antennas to maximize spatial multiplexing capability. In a related fashion, the radio access node 130 could transmit the same MIMO data stream from multiple transmission points 110a, 110b, 100c to increase the received signal power, i.e. to improve the coverage of the transmission points 110a, 110b, 100c. The illustrative example of Fig. 2 is simplified by only illustrating one polarization direction, but the same principle applies for dual polarized antenna branches in all transmission points 110a, nob, 100c and antennas in the wireless device 140a. For dual polarized antenna branches two layers of spatial multiplexing can be transmitted from each transmission point 110a, 110b, 100c with orthogonal polarization and the wireless device 140a could still receive and separate the layers. But if the wireless device 140a has four antennas and all antennas in the wireless device 140a should be efficiently utilized for spatial multiplexing, a second transmission point transmitting additional two layers with as large angle of arrival separation as possible would provide higher throughput. Hence according to an embodiment the spatial multiplexing transmission is initiated such that different spatial multiplexing transmission layers are transmitted from different transmission points in the sub-set of transmission points having highest channel rank. One antenna branch is a physical or logical possible channel for one layer of spatial multiplexing transmission. An antenna branch may consist of several physical antennas. Several antenna branches can utilize the same physical antennas for example by pre-coding, e.g. all antennas can be mapped to all physical antennas in a transmission point 110a, 110b, 110c. Physical antennas can dynamically be mapped to different antenna branches. An antenna branch utilizing several antennas can be beam-formed. Antenna branches can be one to one mapped to antenna ports and identified by the wireless device 140a.

The illustrative example of Fig. 2 is simplified by not considering any reflection paths. Absence of reflections might occur in an outdoor open area in LoS, but in many cases such as indoor environments, there are strong reflection paths. In such case more than two layers can be efficiently transmitted from a single transmission point. If one of the transmission points 110a, 110b, 100c has enough transmission antenna branches and significantly lower path loss to the wireless device 140a, it could be more efficient to transmit all layers from that transmission point than to use more transmission points that could result in lower received signal strength at the wireless device 140a for some layers.

Further, the use of transmission points 110a, nob, 100c could consider if there are any other wireless devices with data to be transmitted to. This is illustrated in Fig. 3.

Fig. 3 is a schematic diagram illustrating a communications network looe where embodiments presented herein can be applied. The communications network looe is similar to the communications networks 100a, 100b, 100c, lood and thus comprises a network node 200, a radio access node 130, and transmission points 110a, nob providing network access in beams 120a, 120b, 120c, i2od to two wireless devices 140a, 140b.

In the illustrative example of Fig. 3 there are two transmission points 110a, 110b and two wireless devices 140a, 140b with two antenna branches each (in one polarization). If single transmission point transmission is used in LoS conditions, spatial multiplexing can be limited to rank-i (rank-2 for dual polarization transmission). If distributed MIMO using both transmission points 110a, 110b is considered, the rank can be doubled. But if both wireless devices 140a, 140b have data in buffer to be received from the network it is in most cases in total more efficient to transmit from one transmission point 110a, 110b to one wireless device 140a, 140b each. It has often a cost to use another transmission point for distributed MIMO. In some cases where both wireless devices 140a, 140b have similar path loss to both transmission points 110a, 110b, it can still be most efficient to transmit distributed MIMO to both wireless devices 140a, 140b using one antenna (per polarization) in each transmission point 110a, 110b.

In view of the above it could thus be cumbersome to evaluate the selection of transmission of spatial multiplexing rank as well as the number of transmission points 110a, 110b, 110c to use.

The embodiments disclosed herein relate to mechanisms for spatial multiplexing transmission from spatially separated transmission points. In order to obtain such mechanisms there is provided a network node 200, a method performed by the network node 200, a computer program product comprising code, for example in the form of a computer program, that when run on a network node 200, causes the network node 200 to perform the method.

Figs. 4 and 5 are flow charts illustrating embodiments of methods for spatial multiplexing transmission from spatially separated transmission points. The methods are performed by the network node 200. The methods are advantageously provided as computer programs 820. Reference is now made to Fig. 4 illustrating a method for spatial multiplexing transmission from spatially separated transmission points as performed by the network node 200 according to an embodiment. Transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points. In general terms, the selection of which transmission points to use for transmission to a wireless device 140a is dependent on channel conditions, including reflections, and the interaction with the available antennas. Hence, the network node 200 is configured to perform step S102:

S102: The network node 200 determines channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device 140a, 140b.

The channel conditions are by the network node 200 used to estimates channel rank. Channel ranks are estimated for sub-sets of the transmission points. In this respect, each sub-set of sub-sets of transmission points comprises at least one of the transmission points 110a, nob, 110c. Hence, the network node 200 is configured to perform step S104:

S104: The network node 200 estimates channel rank for each of at least two sub-sets of the transmission points based on the channel conditions. The sub-set of transmission points to use is then defined based on highest channel rank. Hence, the network node 200 is configured to perform step S108:

S108: The network node 200 initiates the spatial multiplexing transmission to the wireless device 140a, 140b from the sub-set of transmission points having highest channel rank.

According to an embodiment the channel conditions comprise amplitude and phase of the uplink signal.

According to an embodiment the uplink signal is an uplink sounding reference signal.

As disclosed above, according to an embodiment different spatial

multiplexing layers are transmitted from different transmission points 110a, 110b, 110c.

According to embodiments, based on uplink sounding measurements, singular values of the channel matrix for each sub-set of transmission points and antenna branches are measured. The transmission points and antenna branches with lowest singular value ratio can then be selected for downlink MIMO transmission. This can be combined with the total load of the transmission points only to activate distributed MIMO when the total capacity not is degraded.

Embodiments relating to further details of spatial multiplexing transmission from spatially separated transmission points as performed by the network node 200 will now be disclosed.

Reference is now made to Fig. 5 illustrating methods for spatial multiplexing transmission from spatially separated transmission points as performed by the network node 200 according to further embodiments. It is assumed that steps S102, S104, S108 are performed as described above with reference to Fig. 4 and a thus repeated description thereof is therefore omitted There may be different ways to estimate the channel rank. For example, the channel rank could be estimated based eigenvalues, singular values, condition numbers, and/or ratios of singular values. The singular values λί for 1=1, 2..., N of the MIMO channel matrix, where N is the number of singular values for the MIMO channel matrix, represent a measure for MIMO channel condition and spatial multiplexing capacity. One measure of channel rank capacity for rank i is the singular value ratio sun defined as:

A low singular value ratio indicates good conditions for spatial multiplexing transmission for rank i. Condition number is the maximum singular value ratio (SVTN) indicating the condition to utilize all antenna branches for spatial multiplexing. Hence, according to an embodiment the network node 200 is configured to estimates the channel rank by performing steps Si04a and Si04b: Si04a: The network node 200 determines singular values of a channel matrix defined by the channel conditions.

Si04b: The network node 200 estimates the channel rank from a ratio of the singular values. A respective ratio is determined for each of the at least two sub-sets of transmission points. The singular value ratio can thus be estimated for all antenna branches and transmission points that are considered for distributed MIMO transmission.

With reference again to Fig. 2, the singular value ratio for two alternative distributed MIMO cases in Fig. 2 will indicate the best choice of transmission points for transmission to the wireless device 140a. The singular value ratio for a set of antenna branches in transmission point 110a and transmission point 110b will be lower than the singular value ratio for a set of antenna branches in transmission point 110a and transmission point 110c.

If there is a rich indoor channel the singular value ratio estimated for antenna branches in a single transmission point will be low. If there is a significant path loss difference from two transmission points to the wireless device 140a, 140b the singular value ratio will be high. Thus the choice of which

transmission points to use for distributed MIMO, or if distributed MIMO shall be used or not, can be based on singular value ratio measures. As described above, polarized transmission can be suited for spatial multiplexing also with a single transmission point. The singular value ratio svr₂ is often low for single transmission point transmission indicating good rank-2 conditions. Assuming the polarization isolation is very good it is possible to measure only on one polarization and select transmission points for distributed MIMO based on that measure for both polarizations. The measure useful for selection of distributed MIMO transmission is svr₃ or higher. The singular value ratios svr₃ and svr₄ measures will be lower for a transmission point 130a and transmission point 130b based measure than for a transmission point 130a and transmission point 130c measure in the illustrative example of Fig. 2.

Each transmission point could comprise at least two antenna branches. A further channel rank could then be estimated for different sub-sets of antenna branches and the spatial multiplexing be transmitted for the sub-set of antennas with highest channel rank. Hence, according to an embodiment each of the transmission points 110a, 110b, 110c comprises at least two of the antenna branches and the network node 200 is configured to perform step S104:

S104C: The network node 200 estimates a further channel rank for different sub-sets of the antenna branches at one or more of the transmission points 110a, 110b, 110c. The spatial multiplexing transmission is then in step S108 initiated to the wireless device 140a, 140b from the sub-set of antenna branches having highest channel rank. This further channel rank could be based on the channel conditions.

According to an embodiment, when the further channel rank is below a channel rank threshold value, the sub-set of transmission points for which the spatial multiplexing transmission is initiated from comprises at least two transmission points.

As described above, one cost for distributed MIMO is the use of more transmission points 110a, nob, 110c, which in turn limits the capacity. In low load situations this may be more or less for free but at high load multi-user multiplexing allocating one transmission point 110a, nob, 110c to each wireless device 140a, 140b often yields higher capacity. So there is a tradeoff between user quality and capacity when selecting which transmission points 110a, 110b, 110c that shall be used for distributed MIMO. In a further embodiment, to limit the number of used transmission points 110a, 110b, 110c and antennas for transmission towards one single wireless device 140a thresholds can be used. For example, distributed MIMO can be activated only when the singular value ratio is below a certain threshold.

For example, the load of the radio access node 130 can be measured, and transmission points could be excluded from the sub-set of transmission points for which channel rank is estimated based on the load being above or below a threshold. Hence, according to an embodiment the transmission points 110a, 110b, 100c are associated with a radio access node 130 having a load parameter with a value, and those transmission points for which the load parameter value fulfils a threshold criterion are excluded from the determination of channel rank. The load parameter could be based on the amount of data to be transmitted by the radio access node 130. How many of the transmission points to be in each of the sub-sets of transmission points could be dependent on the load. The load parameter could be based on the number of wireless devices 140a, 140b served by the radio access node 130. For each of the served wireless devices 140a, 140b one of the transmission points 110a, 110b, 110c can be excluded from each of the sub-sets of transmission points. The load parameter could further be based on the system resource usage; the system resource could be defined in terms of subframes or parts of the system bandwidth that are not used by distributed MIMO transmission. If the system resource usage for non-distributed MIMO l6 transmission is above certain threshold, the transmission point should be excluded from the sub-sets of transmission points.

Further received signal strength can be measured at each transmission point 110a, 110b, 110c and spatial transmission multiplexing on more than one transmission point 110a, 110b, 110c may then only be considered if the difference between signal strength is below a certain threshold. Hence, according to an embodiment the network node 200 is configured to perform step S106:

S106: The network node 200 determines a difference between the

measurements of received signal strength per antenna branch and

measurements of received signal strength per antenna branch for a further uplink signal received from the wireless device 140a, 140b. The sub-set of transmission points for which the spatial multiplexing transmission is initiated from comprises at least two transmission points only when this difference is smaller than a threshold value.

The threshold value can be load dependent, for example based on the number of wireless devices 140a, 140b served by the radio access node 130, channel utilization, or data buffer size in the transmission points 110a, 110b, 110c.

In an embodiment the total capacity can be estimated for different

alternatives. The spatial multiplexing capacity gain from distributed MIMO can be estimated and compared with the single transmission point

alternative. For example, if svr₄ is below a certain threshold value the distributed MIMO gain is estimated to 2 times the single transmission point alternative, assuming rank-4 can be transmitted instead of rank-2. This can then be compared with the alternative to use the transmission point transmitting data to another wireless device 140b.

There are different types of networks 100a, 100b, 100c, lood, lood, looe. According to an embodiment the network 100a, 100b, 100c, lood, lood, looe is a time division duplex (TDD) network. For TDD the channel conditions are reciprocal and the singular values for the downlink MIMO channel can be measured on uplink transmissions. If the same antennas are used in uplink as in downlink this also measures the channel including the antenna configuration. According to another embodiment the network 100a, 100b, 100c, lood, lood, looe is a frequency division duplex (FDD) network. The herein disclosed embodiments can be combined with other mechanisms and measures for MIMO transmission. For example, the singular value ratio could be combined with signal strength, path loss or signal to noise plus interference ratio (SINR). This can be combined either as multi-dimensional functions or thresholds. The quality of the radio link can also be taken into account and combined with the MIMO gain estimating the throughput for each alternative for example with mutual information.

Fig. 6 schematically illustrates, in terms of a number of functional units, the components of a network node 200 according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), etc., capable of executing software instructions stored in a computer program product 810 (as in Fig. 8), e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry 210 is configured to cause the network node 200 to perform a set of operations, or steps, S102-S108, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 maybe configured to retrieve the set of operations from the storage medium 230 to cause the network node 200 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of l8 magnetic memory, optical memory, solid state memory or even remotely mounted memory. The network node 200 may further comprise a

communications interface 220 at least configured for communications with the radio access node 130 and the transmission points 110a, 110b, 110c. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the network node 200 e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related

functionality, of the network node 200 are omitted in order not to obscure the concepts presented herein.

Fig. 7 schematically illustrates, in terms of a number of functional modules, the components of a network node 200 according to an embodiment. The network node 200 of Fig. 7 comprises a number of functional modules; a determine module 210a configured to perform step S102, an estimate module 210b configured to perform step S104, and an initiate module 2iog configured to perform step S108. The network node 200 of Fig. 7 may further comprises a number of optional functional modules, such as any of a determine module 210c configured to perform step Si04a, an estimate module 2iod configured to perform step Si04b, an estimate module 2ioe configured to perform step S104C, and a determine module 2iof configured to perform step S106. In general terms, each functional module 2ioa-2iog may in one embodiment be implemented only in hardware or and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 230 which when run on the processing circuitry makes the network node 200 perform the corresponding steps mentioned above in conjunction with Fig 7. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 2ioa-2iog maybe implemented by the processing circuitry 210, possibly in cooperation with functional units 220 and/or 230. The processing circuitry 210 may thus be configured to from the storage medium 230 fetch instructions as provided by a functional module 2ioa-2iog and to execute these instructions, thereby performing any steps as disclosed herein.

The network node 200 maybe provided as a standalone device or as a part of at least one further device. For example, the network node 200 maybe provided in, or co-located with, the radio access node 130. Hence, according to aspects there is provided a radio access node 130 comprising a network node 200 as herein disclosed.

Alternatively, functionality of the network node 200 maybe distributed between at least two devices, or nodes. Thus, a first portion of the

instructions performed by the network node 200 may be executed in a first device, and a second portion of the of the instructions performed by the network node 200 maybe executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the network node 200 maybe executed.

Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a network node 200 residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig. 6 the processing circuitry 210 maybe distributed among a plurality of devices, or nodes. The same applies to the functional modules 2ioa-2iog of Fig. 7 and the computer program 820 of Fig. 8 (see below).

Fig. 8 shows one example of a computer program product 810 comprising computer readable storage medium 830. On this computer readable storage medium 830, a computer program 820 can be stored, which computer program 820 can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 820 and/or computer program product 810 may thus provide means for performing any steps as herein disclosed. In the example of Fig. 8, the computer program product 810 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 810 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 820 is here schematically shown as a track on the depicted optical disk, the computer program 820 can be stored in any way which is suitable for the computer program product 810.

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.

Claims

1. A method for spatial multiplexing transmission from spatially separated transmission points (iioa, nob, HOC) in a network (looa, loob, looc, lood, looe) where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points, the method being performed by a network node (200), the method comprising:

determining (S102) channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device (140a, 140b);

estimating (S104) channel rank for each of at least two sub-sets of the transmission points based on the channel conditions; and

initiating (S108) the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank.

2. The method according to claim 1, wherein the channel conditions comprise amplitude and phase of the uplink signal.

3. The method according to any of the preceding claims wherein the channel rank is estimated based on at least one of eigenvalues, singular values, condition numbers, and ratios of singular values.

4. The method according to claim 2 or 3, wherein estimating the channel rank comprises:

determining (Si04a) singular values of a channel matrix defined by the channel conditions; and

estimating (Si04b) the channel rank from a ratio of the singular values, wherein a respective ratio is determined for each of the at least two sub-sets of transmission points.

5. The method according to any of the preceding claims, wherein the spatial multiplexing transmission is initiated such that different spatial multiplexing transmission layers are transmitted from different transmission points in the sub-set of transmission points having highest channel rank.

6. The method according to any of the preceding claims, wherein the uplink signal is an uplink sounding reference signal.

7. The method according to any of the preceding claims, wherein each of the transmission points comprises at least two of the antenna branches, the method further comprising:

estimating (S104C) a further channel rank for different sub-sets of the antenna branches at one or more of the transmission points, and

wherein the spatial multiplexing transmission is initiated to the wireless device from the sub-set of antenna branches having highest channel rank.

8. The method according to claim 7, wherein the further channel rank is based on the channel conditions.

9. The method according to claim 7 or 8, wherein, when the further channel rank is below a channel rank threshold value, the sub-set of transmission points for which the spatial multiplexing transmission is initiated from comprises at least two transmission points.

10. The method according to any of the preceding claims, further

comprising:

determining (S106) a difference between the measurements of received signal strength per antenna branch and measurements of received signal strength per antenna branch for a further uplink signal received from the wireless device, and

wherein the sub-set of transmission points for which the spatial multiplexing transmission is initiated from comprises at least two

transmission points only when said difference is smaller than a threshold value.

11. The method according to any of the preceding claims, wherein the transmission points are associated with a radio access node (130), and wherein the threshold value is based on at least one of number of wireless devices served by the radio access node, data buffer size of the transmission points, and channel utilization.

12. The method according to any of the preceding claims, wherein the transmission points are associated with a radio access node (130) having a load parameter with a value, and wherein those transmission points for which the load parameter value fulfils a threshold criterion are excluded from the determination of channel rank.

13. The method according to claim 12, wherein the load parameter is based on number of wireless devices served by the radio access node.

14. The method according to claim 13, wherein for each of the served wireless devices one of the transmission points is excluded from each of the sub-sets of transmission points.

15. The method according to claim 11, wherein the load parameter is based on amount of data to be transmitted by the radio access node.

16. The method according to claim 14, wherein how many of the

transmission points to be in each of the sub-sets of transmission points is dependent on the load.

17. The method according to any of the preceding claims, wherein the network (100a, 100b, 100c, lood, lood, looe) is a time division duplex, TDD, network.

18. A network node (200) for spatial multiplexing transmission from spatially separated transmission points (110a, nob, 110c) in a network (100a, 100b, 100c, lood, looe) where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points, the network node (200) comprising processing circuitry (210), the processing circuitry being configured to cause the network node (200) to: determine channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device (140a, 140b);

estimate channel rank for each of at least two sub-sets of the

transmission points based on the channel conditions; and initiate the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank.

19. A network node (200) for spatial multiplexing transmission from spatially separated transmission points (110a, nob, 110c) in a network (100a, 100b, 100c, lood, looe) where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points, the network node (200) comprising:

processing circuitry (210); and

a storage medium (230) storing instructions that, when executed by the processing circuitry (210), causes the network node (200) to:

determine channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device (140a, 140b);

estimate channel rank for each of at least two sub-sets of the transmission points based on the channel conditions; and

initiate the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank.

20. A network node (200) for spatial multiplexing transmission from spatially separated transmission points (110a, 110b, 110c) in a network (100a, 100b, 100c, lood, looe) where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points, the network node (200) comprising:

a determine module (210a) configured to determine channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device (140a, 140b);

an estimate module (210b) configured to estimate channel rank for each of at least two sub-sets of the transmission points based on the channel conditions; and

an initiate module (2iog) configured to initiate the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank.

21. A radio access node (130) comprising a network node (200) according to any of claims 18, 19, or 20.

22. A computer program (820) for spatial multiplexing transmission from spatially separated transmission points (110a, nob, 110c) in a network (100a, 100b, 100c, lood, looe) where transmit antennas are distributed between antenna branches on at least two of the spatially separated transmission points, the computer program comprising computer code which, when run on processing circuitry (210) of a network node (200), causes the network node (200) to:

determine (S102) channel conditions based on measurements of received signal strength per antenna branch for an uplink signal received from a wireless device (140a, 140b);

estimate (S104) channel rank for each of at least two sub-sets of the transmission points based on the channel conditions; and

initiate (S108) the spatial multiplexing transmission to the wireless device from the sub-set of transmission points having highest channel rank.

23. A computer program product (810) comprising a computer program (820) according to claim 22, and a computer readable storage medium (830) on which the computer program is stored.