Patent Description:
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is in particular improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO systems, or just MIMO for short.

Massive MIMO, in which the number of antennas at the access node (AN) scales up by one or more orders of magnitude, has emerged as a new technology to replace conventional multi-user MIMO. This enhancement enables the system to exploit spatial multiplexing and serve a large number of user equipment (UEs) using the same time and frequency resources. In turn, this can improve the spectral efficiency by orders of magnitude. The success of massive MIMO technology to improve the system performance can be large when fully digital beamforming is adopted at the access node. Fully digital beamforming requires one radio frequency (RF) chain per antenna. In this respect, high-quality RF ends consume relatively high amounts of power, in addition to being expensive. Therefore, there might be some scenarios where the use of high-quality RF ends should be avoided. Further, when the number of antennas at the AN grows large, the digital beamforming complexity and RF chain power consumption also grow large which, in practice, limits the scaling.

Deploying a massive MIMO system with an arbitrarily large number of antennas thus introduces practical challenges to be resolved. These challenges make the achieved gains in the system performance when deploying massive MIMO systems to be far away from what theory suggests. This is mainly due to non-ideal effects of the hardware in the access node. In this respect, one of the main sources of distortion in a communication system in general, and in the access node in particular, is the non-linear behavior of amplifiers, such as power amplifiers (PAs), in the transceiver chains.

Digital predistortion (DPD) is known as an effective technique to mitigate the non-linear distortions in the output of amplifiers. In massive MIMO systems, due to the large number of antennas at the access node, adopting a MIMO DPD scheme commonly results in high complexity. In turn, this introduces challenges in the implementation, see for example <NPL>. Therefore, deploying per-antenna DPDs is a more practical technique for massive MIMO systems.

Based on the measurements in realistic environments which implements a massive MIMO system, the contribution of each antenna to the system capacity is not equal due to large-scale variations across the array of antennas. In this respect, as used throughout this document, large-scale variations in this context refer to variations across antennas within the array, not to variations across time. This implies that MIMO DPD should be used at the access node. However, MIMO DPDs are complex to implement and consume a high level of power when in operation. This is especially true when the number of antennas grows large.

Hence, there is still a need for improved configuration of DPDs intended for operation in a massive MIMO system.

<NPL> relates to access nodes comprising a MIMO digital DPD.

An object of embodiments herein is to distribute the coefficients of the DPDs among the per-antenna DPDs in a way that contributes to good, or even optimum, overall system performance.

According to a first aspect there is presented a method for DPD size determination of an access node. The access node is configured for operation in a digital massive MIMO system. The access node comprises a plurality of antennas, one radio chain per antenna, and one DPD per radio chain. The method is performed by the access node. The method comprises obtaining channel gain estimates per each of the plurality of antennas. The method comprises determining the size, in terms of number of coefficients, of each DPD according to a utility function that depends on the channel gain estimates per antenna. The method comprises allocating the determined number of coefficients to each DPD.

According to a second aspect there is presented an access node for DPD size determination of the access node. The access node is configured for operation in a digital massive MIMO system. The access node comprises a plurality of antennas, one radio chain per antenna, and one DPD per radio chain. The access node comprises processing circuitry. The processing circuitry is configured to cause the access node to obtain channel gain estimates per each of the plurality of antennas. The processing circuitry is configured to cause the access node to determine the size, in terms of number of coefficients, of each DPD according to a utility function that depends on the channel gain estimates per antenna. The processing circuitry is configured to cause the access node to allocate the determined number of coefficients to each DPD.

According to a third aspect there is presented an access node for DPD size determination of the access node. The access node is configured for operation in a digital massive MIMO system. The access node comprises a plurality of antennas, one radio chain per antenna, and one DPD per radio chain. The access node comprises an obtain module configured to obtain channel gain estimates per each of the plurality of antennas. The access node comprises a determine module configured to determine the size, in terms of number of coefficients, of each DPD according to a utility function that depends on the channel gain estimates per antenna. The access node comprises an allocate module configured to allocate the determined number of coefficients to each DPD.

According to a fourth aspect there is presented a computer program for DPD size determination of an access node, the computer program comprising computer program code which, when run on the access node, causes the access node to perform a method according to the first aspect.

According to a fifth aspect there is presented a computer program product comprising a computer program according to the fourth 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, these aspects enable the DPD budget (i.e., the size, in terms of number of coefficients) to be used efficiently. In turn, this can enhance the system energy efficiency, and the overall system performance.

Advantageously, as a result of the proposed efficient DPD size selection, these aspects enable the gap between high-power consuming MIMO DPDs and low-performance per-antenna DPDs to be narrowed. The herein disclosed aspects thus provide an intermediate solution to DPDs in massive MIMO systems, between immensely complexity and high-performing MIMO DPDs and low-complexity but low-performing uniform per-antenna DPDs.

Advantageously, as a result of the proposed efficient DPD size selection, the resources for the DPDs can be used in the most efficient way, improving system performance at the same total level of DPD power consumption or, alternatively, maintaining system performance whilst reducing DPD power consumption, compared to standard and uniform per-antenna DPDs.

Advantageously, these aspects enable power and/or energy resources used for DPDs in massive MIMO systems to be used more efficiently. In this respect, the power consumed in the DPD block can be used efficiently and the system can decrease its overall power consumption and still have the same performance as in the case without DPD optimization. Further in this respect, the system capacity can be improved without increasing the DPD power consumption, which results in a higher system energy-efficiency.

Advantageously, the gain achieved by adapting the DPD size allows the use of less expensive RF hardware and reduce the system overall cost, which becomes more critical as massive MIMO systems grow.

<FIG> is a schematic diagram illustrating a communications network <NUM> where embodiments presented herein can be applied. The communications network <NUM> could be a third generation (<NUM>) telecommunications network, a fourth generation (<NUM>) telecommunications network, a fifth generation (<NUM>) telecommunications network, or any evolvement thereof, and support any 3GPP telecommunications standard, where applicable. The communications network <NUM> could alternatively be a non-cellular and/or a non-3GPP network, such as an IEEE <NUM> communications network, or any other wireless IEEE compliant communications network. In some examples, the communications network <NUM> is a massive MIMO system.

The communications network <NUM> comprises an access node <NUM> configured to provide network access to user equipment, as represented by user equipment <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-K, in a (radio) access network <NUM>, over wireless links <NUM>. The (radio) access network <NUM> is operatively connected to a core network <NUM>. The core network <NUM> is in turn operatively connected to a service network <NUM>, such as the Internet. The user equipment <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-K are thereby enabled to, via the access node <NUM>, access services of, and exchange data with, the service network <NUM>.

Examples of access nodes <NUM> are radio access network nodes, radio base stations, base transceiver stations, Node Bs, evolved Node Bs, gNBs, access points, and integrated access and backhaul nodes. Examples of user equipment <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-K are wireless devices, mobile stations, mobile phones, handsets, wireless local loop phones, smartphones, laptop computers, tablet computers, network equipped sensors, network equipped vehicles, and so-called Internet of Things devices.

As noted above, there is still a need for improved configuration of DPDs intended for operation in a massive MIMO system.

In this respect, using a per-antenna DPD at the access node <NUM> is known to be the practical choice for the massive MIMO systems. One issue with using a per-antenna DPD is that the per-antenna DPD sizes are selected uniformly, without taking the variation of the channel quality into account. Further, the power consumption of the DPD grows linearly with the DPD size, i.e., the total available number of coefficients used by the DPD. Dynamic model sizing algorithms have been introduced, see <NPL>. One aim of such dynamic model sizing algorithms is to change the DPD size of a transceiver when needed during the transmission and save the power. But the dynamic model sizing algorithms are limited to a single-input single-output (SISO) system and cannot be readily applied to a massive MIMO system where the DPD power consumption of all the antennas over the antenna array needs to be taken into account. With a difference in antennas contribution and limiting constraint on DPD power consumption, a new technique that improves the per-antenna DPD performance and the energy efficiency of the massive MIMO systems is therefore proposed and disclosed hereinafter.

The embodiments disclosed herein in particular relate to mechanisms for DPD size determination of the access node <NUM>. In order to obtain such mechanisms there is provided an access node <NUM>, a method performed by the access node <NUM>, a computer program product comprising code, for example in the form of a computer program, that when run on an access node <NUM>, causes the access node <NUM> to perform the method.

As noted above, in massive MIMO systems, the contribution of each antenna to the system capacity is not equal due to large-scale variations across the array of antennas. Only a portion of the antennas with strongest channels determines the system performance. Measurements in realistic environments with implemented massive MIMO systems have verified that even in wide-band multi-user scenarios, this property holds. In a fully digital massive MIMO system, there is one RF chain per antenna. This results in high power consumption in the analog front ends if all the RF chains have to meet the demanded linearity constraints. On the other hand, performing digital pre-distortion with the same number of coefficients in each DPD on all RF chains to mitigate non-linearity distortion, which is usually designed for the worst-case scenarios, also results in a large power consumption.

The downlink of a fully digital massive MIMO system where each antenna is connected to an RF chain (with non-ideal hardware components) is considered. <FIG> is a schematic block diagram of an access node <NUM> according to an embodiment. The access node <NUM> comprises a plurality of antennas <NUM>-<NUM>, <NUM>-<NUM>,. The access node <NUM> comprises one radio chain <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M per antenna <NUM>-<NUM>, <NUM>-<NUM>,. The access node <NUM> comprises one DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M per radio chain <NUM>-<NUM>, <NUM>-<NUM>,. The size of each DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M can be adaptively selected based on current channel state information. This allows individual adaptation per DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M of the level of pre-distortion based on current channel state information. The access node <NUM> further comprises one digital to analog (D/A) converter <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M per radio chain <NUM>-<NUM>, <NUM>-<NUM>,. The antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M are, via the radio chains <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, the D/A converters <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, and the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M operatively connected to a MIMO baseband signalling block <NUM>.

The power consumption of the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M has a linear relationship with the number of coefficients used in the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. This allows the DPD power consumption PDPD to be related to the total number of DPD coefficients across all RF chains, and thus all transmit branches, as PDPD = C · Ncoef, where Ncoef is the total number of DPD coefficients across all antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M and C is a proportionality constant. However, this relation does not require that the total number of DPD coefficients are equally divided among the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. Hence, for a given value of Ncoef (or equally, for a given value of PDPD) different number of coefficients could be allocated to the different DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M as long as the value of Ncoef (or equally, the value of PDPD) is not changed. In this respect, the size of each DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M is controlled by a DPD size optimizer block <NUM>. The input to DPD size optimizer block <NUM> is the channel gain estimates, which could be acquired in the uplink by receiving pilot signals from the UEs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-K, and the DPD power budget (as given by PDPD), which determines the total number of DPD coefficients that can be used across the antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M (according to the above relation, PDPD = C · Ncoef,). The DPD size optimizer block <NUM> is therefore configured to receive input from a channel state information (CSI) and DPD budget information block <NUM>. The DPD size optimizer block <NUM> is configured to optimize the per-antenna DPD sizes by exploiting CSI, in terms of the channel gain variation across the antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, to adapt the level of DPD compensation across the across the antennas <NUM>-<NUM>, <NUM>-<NUM>,.

In brief, and as will be disclosed in more detail next with reference to <FIG>, the access node <NUM> first obtains an estimate of the channel from each antenna <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M to the UEs <NUM>-<NUM>, <NUM>-<NUM>,. In this respect, although channel gain estimates are disclosed below as estimates of the channel, the herein disclosed embodiments are equally applicable also to other types estimates of the channel, such as channel phase estimates, CSI (which can contain information reflecting both channel gain estimates and channel phase estimates as well as other channel parameters), etc. A utility function is then selected according to which the per-antenna optimization of the DPD size is performed. This utility function depends on the channel gain estimates. High mobility of the UEs <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-K might cause their channel conditions to change fast over time. Therefore, the utility function is selected to only depend on large-scale fading of the channel. The utility function might further depend on other parameters, such as hardware quality, and the total amount of available coefficients to be allocated to the DPD. The access node <NUM> can then determine the optimum values of the per-antenna DPD sizes to optimize the utility function for the given constraints and apply the results to the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M before performing any transmission.

<FIG> is a flowchart illustrating embodiments of methods for DPD size determination of the access node <NUM>. The methods are performed by the access node <NUM>. The methods are advantageously provided as computer programs <NUM>.

The method is based on having access to information of channel gain estimates per antenna <NUM>-<NUM>, <NUM>-<NUM>,. Hence, the access node <NUM> is configured to perform step S102:
S102: The access node <NUM> obtains channel gain estimates per each of the plurality of antennas <NUM>-<NUM>, <NUM>-<NUM>,.

Step <NUM> might be implemented in, or performed by, the CSI and DPD budget information block <NUM>. The size of each of the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M is then determined as a function of the channel gain estimates per antenna <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, as in step S104:
S104: The access node <NUM> determines the size, in terms of number of coefficients, of each DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M according to a utility function. The utility function depends on the channel gain estimates per antenna <NUM>-<NUM>, <NUM>-<NUM>,.

Step <NUM> might be implemented in, or performed by, the DPD size optimizer block <NUM>. Information about how many coefficients to use is then communicated to the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, as in step S106:
S106: The access node <NUM> allocates the determined number of coefficients to each DPD <NUM>-<NUM>, <NUM>-<NUM>,.

Step <NUM> might be implemented in, or performed by, the DPD size optimizer block <NUM>.

Enabling the DPD size, i.e. the number of coefficients per each DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, to be adapted can help the massive MIMO system to adapt to large-scale fluctuations in channel gains and signal power. The available DPD coefficients can be distributed across the antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M based on their large-scale fading channel conditions, as indicated by the channel gain estimates per each of the plurality of antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, to ensure that the antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M with more contribution to the system experience less hardware distortion.

Embodiments relating to further details of DPD size determination of the access node <NUM> as performed by the access node <NUM> will now be disclosed.

As disclosed above, the available DPD coefficients can be distributed across the antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M based on their large-scale fading channel conditions. Particularly, in some embodiments, the channel gain estimates represent moving average channel conditions.

There may be different actions taken by the access node <NUM> once having allocated the determined number of coefficients to each DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, as in step S106.

In general terms, each of the coefficients has a value. However, it might be difficult to determine, assign, or select, the values of the coefficients for the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M before it is known how many coefficients that will be allocated to each DPD <NUM>-<NUM>, <NUM>-<NUM>,. Therefore, in some aspects, once the determined number of coefficients has been allocated to each DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, as in step S106, the actual values of the coefficients can be selected. Hence, in some embodiments, the access node <NUM> is configured to perform (optional) step S108:
S108: The access node <NUM> selects values of the coefficients as determined for each DPD <NUM>-<NUM>, <NUM>-<NUM>,.

Step <NUM> might be implemented in, or performed by, the DPD size optimizer block <NUM>. Alternatively, step S106 is implemented in, or performed by, each of the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, possibly in collaboration with the DPD size optimizer block <NUM>.

Once the values of coefficients have been determined for each DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M can be used for pre-distorting a signal to be transmitted by the access node <NUM>. Hence, in some embodiments, the access node <NUM> is configured to perform (optional) step S110:
S110: The access node <NUM> transmits a downlink signal from the access node <NUM>. The downlink signal is pre-distorted by the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M according to the selected values of the coefficients.

Step <NUM> might be implemented in, or performed by, the MIMO baseband signalling block <NUM>.

Further aspects of how the DPD sizes might be determined will now be disclosed.

In some aspects, the DPD sizes are selected to maximize the utility function for given constraints. In particular, in some embodiments, the sizes of all the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M are collectively determined to maximize the utility function. In some aspects, the utility function is constrained by the total number of available coefficients for all the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. That is, in some embodiments, the utility function is constrained by a maximum total number of coefficients collectively allocatable to all DPDs <NUM>-<NUM>, <NUM>-<NUM>,. In some embodiments, the maximum total number of coefficients is dependent on a total available power budget for the DPDs <NUM>-<NUM>, <NUM>-<NUM>,.

Further aspects of the utility function will now be disclosed.

As disclosed above, the utility function might further depend on other parameters, such as hardware quality. That is, in some embodiments, the utility function further depends on properties of hardware components per radio chain <NUM>-<NUM>, <NUM>-<NUM>,. Some examples, of the hardware components are PAs and D/A converters <NUM>-<NUM>, <NUM>-<NUM>,.

There could be different ways to select the utility function. In some embodiments, the utility function is derived from an optimization criterion. There could be different examples of such optimization criteria. In some non-limiting examples, the optimization criterion pertains to any of: maximization of downlink capacity of the access node <NUM>, minimization of power consumption of the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M for a given system performance of the access node <NUM>, maximization of system performance of the access node <NUM> for a given power consumption of the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. That is, given an optimization criterion (such as minimal DPD power consumption for a given system performance or maximum system performance for a limited DPD power consumption) the individual DPD sizes can be optimized based on the current channel state information.

In some aspects, the utility function is an objective function. In other aspects, the utility function can be regarded as a reward function, a profit function, or, a fitness function.

One non-limiting way to implement the utility function for per-antenna DPD size determination will be disclosed next for completeness of this disclosure.

Consider that an access node <NUM> with M antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M is to, over a radio propagation channel represented by a channel vector h, serve a UE <NUM>-<NUM> having a single-antenna in the presence of hardware impairment at both the access node <NUM> and the UE <NUM>-<NUM>. The received baseband signal y at the UE <NUM>-<NUM> side can be expressed as: <MAT> where s is the transmit signal at the access node <NUM> with a covariance matrix W such that tr(W) = PT, where tr(W) is the trace of W, where PT is the transmission power, where dAN and dUE are the residual hardware impairment at the access node and UE side, respectively, and where ω~CN(o, σ<NUM>) represents additive noise at the UE side. To quantify the dependency of the capacity on the per-antenna DPD sizes, the residual hardware impairment terms can be modelled as follows: <MAT> where <MAT> and <MAT> where <MAT>.

In the above expression, κAN is a measure of hardware impairment variance if no digital pre-distortion is applied, Wmm is the m:th diagonal element in the matrix W, <MAT> is the DPD size allocated to the m:th antenna branch, and κUE is a measure of the hardware impairment variance at the UE side.

An upper bound C̃UB on the capacity when the channel distribution is h~CN(o, R) can be found as: <MAT> where <MAT> where <MAT>.

The capacity upper bound C̃UB is thus a function of the large-scale fading components of the channel and the DPD sizes at each antenna <NUM>-<NUM>, <NUM>-<NUM>,. Therefore, this capacity can be selected as the optimization criterion for the utility function. For given channel state information and a constraint on the total number of the per-antenna DPD coefficients, the below optimization problem can thus be solved to find the optimal per-antenna DPD sizes <MAT>: <MAT> subject to: <MAT> <MAT> <MAT>.

Thus far it has been assumed that the total number of DPD coefficients that can be used across the antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M has a fixed value Ncoef. However, as the DPD power consumption PDPD might be varying over time, this also allows the total number of DPD coefficients that can be used across the antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-Mto vary over time. Hence, in some embodiments, the maximum total number of coefficients varies over time. In this respect, the DPD sizes can be updated in real-time using a dynamic DPD model that adapts the DPD sizes from previous time. In particular, in some embodiments, the sizes of the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M are, according to a dynamic model, adaptively determined over time based on previously determined sizes of the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. As illustrated in the above example, this is possible with a low complexity optimization problem to be solved at the access node <NUM>. Real-time dynamic DPD sizing methods that can converge very fast when the DPD size change over time can be applied.

In some aspects, in multi-user scenario the utility function can be made dependent on the downlink capacity of each served user equipment <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-K. In particular, in some embodiments, the access node <NUM> in step S102 obtains channel gain estimates per each of the plurality of antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M for a plurality of user equipment <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-K served by the access node <NUM>, and the utility function depends on downlink capacity for each of the user equipment <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-K. In further detail, the above analysis can be implemented for an optimization problem for a multi-user scenario. This can be done by finding the downlink capacity of each user equipment <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-K after applying the per-antenna DPDs with the same model described in the above example, selecting a function of the capacities, e.g. summation or the minimum of them, and solving the optimization problem to find the optimum per-antenna DPD sizes.

One particular embodiment of a method for DPD size determination of the access node <NUM> based on at least some of the above embodiments, aspects, and examples, will now be disclosed with reference to the flowchart of <FIG>.

Determining Channel Properties: The access node <NUM> obtains channel gain estimates per each of the plurality of antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M from each antenna <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M to the UEs <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-K. The access node <NUM> stores the obtained channel gain estimates in a buffer. The needed data for performing the estimation can be achieved from measurements in the uplink on pilot signals transmitted by the UEs <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-K to the access node <NUM>.

Determining Budget: The access node <NUM> determines the total available power budget that can be used in all the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. As the DPD power consumption grows linearly with the DPD size (i.e., the number of coefficients per DPD), this power budget can be translated into the total number of available DPD coefficients across all antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M that can be allocated to the DPDs <NUM>-<NUM>, <NUM>-<NUM>,.

Optimizing Per-Antenna DPD Sizes: The access node <NUM> selects a utility function according to which the DPD sizes are to be optimized. The selected utility function can be different depending on the application and other network design criteria, but has a dependency on channel characteristics (as defined by the channel gain estimates) and, possibly, the per-antenna DPD sizes (number of coefficients). One such option for the utility function is to consider the downlink capacity. After selecting the utility function, the access node <NUM> determines the size of each DPD <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M according to the utility function and the DPD power budget constraints.

Performing Transmission: The access node <NUM> allocates the determined number of coefficients to each DPD <NUM>-<NUM>, <NUM>-<NUM>,. The access node <NUM> also selects values of the coefficients as determined for each DPD <NUM>-<NUM>, <NUM>-<NUM>,. Further, the access node <NUM> transmit a downlink signal. The downlink signal is pre-distorted by the DPDs <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M according to the selected values of the coefficients.

Next it will with reference to <FIG> be shown how the herein disclosed embodiments can be used in a massive MIMO system to achieve the above-mentioned advantages. <FIG> shows simulation results in terms of a numerical example of the system performance for the proposed method for per-antenna DPD size selection. In particular, in <FIG> is shown the achievable gain of adopting the proposed method in comparison to the case of uniform DPD size selection in terms of spectral efficiency upper bound (UB) as a function of number of antennas at the access node <NUM>. To take into account the varying channel gain over the antennas <NUM>-<NUM>, <NUM>-<NUM>,. , <NUM>-M, it is assumed that <NUM>% of the antennas experience a gain which is 10dB higher than the rest of the antennas. A Rayleigh fading model is used to generate the channel realizations and the average DPD size per antenna is set to <NUM>.

<FIG> schematically illustrates, in terms of a number of functional units, the components of an access node <NUM> according to an embodiment. Processing circuitry <NUM> 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 <NUM> (as in <FIG>), e.g. in the form of a storage medium <NUM>. The processing circuitry <NUM> may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry <NUM> is configured to cause the access node <NUM> to perform a set of operations, or steps, as disclosed above. For example, the storage medium <NUM> may store the set of operations, and the processing circuitry <NUM> may be configured to retrieve the set of operations from the storage medium <NUM> to cause the access node <NUM> to perform the set of operations.

Thus the processing circuitry <NUM> is thereby arranged to execute methods as herein disclosed. The access node <NUM> may further comprise a communications interface <NUM> at least configured for communications with other entities, functions, nodes, and devices of the communications network <NUM> as well as entities, functions, nodes, and devices served by the communications network <NUM>. As such the communications interface <NUM> may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry <NUM> controls the general operation of the access node <NUM> e.g. by sending data and control signals to the communications interface <NUM> and the storage medium <NUM>, by receiving data and reports from the communications interface <NUM>, and by retrieving data and instructions from the storage medium <NUM>. Other components, as well as the related functionality, of the access node <NUM> are omitted in order not to obscure the concepts presented herein.

<FIG> schematically illustrates, in terms of a number of functional modules, the components of an access node <NUM> according to an embodiment. The access node <NUM> of <FIG> comprises a number of functional modules; an obtain module 210a configured to perform step S102, a determine module 210b configured to perform step S106, and an allocate module 210c configured to perform step S106. The access node <NUM> of <FIG> may further comprise a number of optional functional modules, such as any of a select module 210d configured to perform step S <NUM> and a transmit module 210e configured to perform step S210. In general terms, each functional module 210a:210e may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium <NUM> which when run on the processing circuitry makes the access node <NUM> perform the corresponding steps mentioned above in conjunction with <FIG>. 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 210a:210e may be implemented by the processing circuitry <NUM>, possibly in cooperation with the communications interface <NUM> and/or the storage medium <NUM>. The processing circuitry <NUM> may thus be configured to from the storage medium <NUM> fetch instructions as provided by a functional module 210a:210e and to execute these instructions, thereby performing any steps as disclosed herein.

The access node <NUM> may be provided as a standalone device or as a part of at least one further device. For example, the access node <NUM> may be provided in a node of the radio access network or in a node of the core network. Alternatively, functionality of the access node <NUM> may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part (such as the radio access network or the core network) or may be spread between at least two such network parts. In general terms, instructions that are required to be performed in real time may be performed in a device, or node, operatively closer to the cell than instructions that are not required to be performed in real time. Thus, a first portion of the instructions performed by the access node <NUM> may be executed in a first device, and a second portion of the of the instructions performed by the access node <NUM> may be 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 access node <NUM> may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by an access node <NUM> residing in a cloud computational environment. Therefore, although a single processing circuitry <NUM> is illustrated in <FIG> the processing circuitry <NUM> may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 210a:210e of <FIG> and the computer program <NUM> of <FIG>.

Claim 1:
An access node (<NUM>) for digital pre-distorter, DPD, size determination of the access node (<NUM>), the access node (<NUM>) being configured for operation in a digital massive multiple-input multiple-output, MIMO, system (<NUM>), the access node (<NUM>) comprising a plurality of antennas (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M), one radio chain (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M) per antenna (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M), and one DPD (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M) per radio chain (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M), the access node (<NUM>) comprising processing circuitry (<NUM>), the processing circuitry (<NUM>) being configured to cause the access node (<NUM>) to:
obtain channel gain estimates per each of the plurality of antennas (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M);
determine the size, in terms of number of coefficients, of each DPD (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M) according to a utility function that depends on the channel gain estimates per antenna (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M); and
allocate the determined number of coefficients to each DPD (<NUM>-<NUM>, <NUM>-<NUM>, ..., <NUM>-M).