WO2025058543A1

WO2025058543A1 - Techniques for passive intermodulation avoidance

Info

Publication number: WO2025058543A1
Application number: PCT/SE2023/050894
Authority: WO
Inventors: Roy NADDAF; Donald Stephen Staudte; Tobias FORSLÖW; Torbjörn WIGREN
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 2023-09-13
Filing date: 2023-09-13
Publication date: 2025-03-20
Anticipated expiration: 2026-03-13

Abstract

There is provided techniques for PIM avoidance. The method is performed by a controller. The method comprises obtaining a pattern for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers. All patterns extend over the same time units and frequency units and specify time-frequency resources of the transmission resources to be muted. In a union of all the patterns, there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted. The method comprises instructing a downlink scheduler to repeatedly apply the patterns, one per each of the at least two downlink carriers, during downlink transmission.

Description

TECHNIQUES FOR PASSIVE INTERMODULATION AVOIDANCE

TECHNICAL FIELD

Embodiments presented herein relate to a method, a controller, a computer program, and a computer program product for passive intermodulation (PIM) avoidance.

BACKGROUND

In general terms, PIM is caused by passive objects, such as filters, duplexers, connectors, antennas, as well as conductive and magnetic objects that may litter an antenna site. These objects may be exhibiting nonlinear behavior in the vicinity of radio signals. PIM can then cause an interference signal to be generated that can couple into a receiver and degrade the receiver’s sensitivity.

Depending on the location of the component that generates the PIM, the PIM is categorized as either internal or external. For example, PIM generated by the filters of the transmission (TX) radio chains in the antenna system at the cell site, loose cable connections, dirty connectors, poor performance duplexers, and aged antennas, is called internal PIM whereas PIM generated by a metal fence on the roof top of a building, a metal roof, or even a drainpipe, in vicinity of the cell site is called external PIM. External PIM thus refers to the case where the PIM occurs after the signals have left the transmitter antenna with the resultant intermodulation reflecting back into the receiver. PIM might cause a need for the transmission power of the cell site to be backed off in order to avoid PIM to affect the receiver (RX) radio chains in the antenna system of the cell site, thus compromising the network performance. For example, a 1-dB drop in uplink sensitivity caused by PIM might reduce coverage by as much as 11% in a macro network. Further, PIM is a super-linear effect, meaning that the PIM power can increase as a power of the downlink transmit power. For example, for third-degree PIM every decibel (dB) of transmit power increase could result in 3 dB of PIM power increase in the uplink.

To explain how PIM is generated at a high level, consider Fig. 1. Fig. 1 at (a) illustrates a communications network 100 where embodiments presented herein can be applied. The communications network 100 comprises an access network node 110, such as a radio access network node, radio base station, base transceiver station, node B (NB), evolved node B (eNB), gNB, access point, integrated access and access node, etc. configured to provide network access to user equipment 120a, 120b in carriers 120a, 120b. A PIM source 130 is located in the vicinity of the antenna system of the access network node 110. Fig. 1 at (b) shows a representation of the power spectral density (PSD) as a function of frequency. In the example of Fig. 1, downlink transmission in two frequency bands with carrier frequencies and f₂ mix nonlinearly in the PIM source 130. This results in new interference signals with a variety of center frequencies. Only one of these signals is shown in the power spectral density, centered at the frequency _PIM = 2/) - f₂. In this case, it is for illustrative purposes assumed that the linear combination 2f - f₂ happens to be located within the uplink band of the access network node no, manifesting itself as additional uplink interference. The uplink radio channel for the access network node no is marked at reference numeral 140, and the uplink frequency band for the access network node no is marked at reference numeral 150. The interference typically has a wider bandwidth than the downlink signals that caused this uplink PIM. This wider bandwidth is an effect of the nonlinear mixing.

Three types of common techniques to mitigate the impact of PIM, namely uplink PIM cancellation, downlink PIM avoidance by beamformed transmission, and general power reducing PIM avoidance, will be briefly summarized next.

PIM cancellation algorithms are designed to estimate parts of the complete PIM channel from downlink transmission to uplink reception. Given the estimated signal, and the downlink transmissions a predicted signal can be computed and subtracted from the received signal. In case the predicted signal is an accurate model of the received uplink PIM, a substantial reduction in the uplink PIM can be obtained. One common drawback of all PIM cancellation algorithms is the computational complexity. This computational complexity is much higher than that of conventional channel estimation since non-linear basis functions must be created. PIM cancellation also requires measurement of both downlink transmission and uplink PIM. Further, PIM requires access to the data streams of all aggressors. This might be problematic, especially when the aggressors are from different access network nodes, or radio units.

PIM avoidance by beamformed transmission aims at avoiding beams of an advanced antenna system used for transmission to be pointed in the direction of the PIM source. This might require estimation of the location of the PIM source, which might not be knowns in advance. Therefore, techniques have been proposed to estimate a subspace of the beam space generated by the antenna array system where the PIM source is located. Generation of beams in this subspace is then avoided for downlink transmissions. This technique is only applicable for advanced antenna systems, and thus for (radio) access network nodes equipped with large antenna arrays. Further, this technique limits the number of possible directions in which beams can be used for downlink transmission. Hence, a drawback is that the coverage as well as the throughput is reduced, especially in the direction towards the PIM source. PIM avoidance in general is applicable to any type of access network node. Once the frequency bands in which downlink transmissions are used that cause the uplink PIM are identified, the uplink PIM can be reduced by at least periodically avoiding transmission in these identified frequency bands. PIM avoidance can thereby reduce the uplink PIM depending on how much of the frequency bands that can be muted. For network deployments a set of restrictions is defined on how much muting in downlink transmissions is allowed, as an upper limit on how much downlink throughput and capacity can be sacrificed. The implication is that the muting might be implemented sporadically and non-continuously, essentially adding more variations to the uplink signal to interference and noise ratio (SINR). Variations in the SINR must be considered carefully when used in uplink link adaptation, since the uplink PIM is mixed with the interference from other user equipment 120a, 120b. Muting carriers identified as aggressors over some time intervals, such as one or more transmission time intervals (TTIs) reduces the PIM levels during these time intervals, but the benefits of this reduction may not be reached because the link adaptation is not as fast.

Hence, there is still a need for improved PIM mitigation techniques.

SUMMARY

An object of embodiments herein is to provide techniques for efficient PIM avoidance that does not suffer from the issues noted above, or where the above noted issues at least have been mitigated or reduced. According to a first aspect there is presented a method for PIM avoidance. The method is performed by a controller. The method comprises obtaining a pattern for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers. All patterns extend over the same time units and frequency units and specify time-frequency resources of the transmission resources to be muted. In a union of all the patterns, there is at least one timefrequency resource that for each of the time units is muted or at least one timefrequency resource that for each of the frequency units is muted. The method comprises instructing a downlink scheduler to repeatedly apply the patterns, one per each of the at least two downlink carriers, during downlink transmission.

According to a second aspect there is presented a controller for PIM avoidance. The controller comprises processing circuitry. The processing circuitry is configured to cause the controller to obtain a pattern for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers. All patterns extend over the same time units and frequency units and specify timefrequency resources of the transmission resources to be muted. In a union of all the patterns, there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted. The processing circuitry is configured to cause the controller to instruct a downlink scheduler to repeatedly apply the patterns, one per each of the at least two downlink carriers, during downlink transmission.

According to a third aspect there is presented a controller for PIM avoidance. The controller comprises an obtain module configured to obtain a pattern for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers. All patterns extend over the same time units and frequency units and specify time-frequency resources of the transmission resources to be muted. In a union of all the patterns, there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted. The controller comprises an instruct module configured to instruct a downlink scheduler to repeatedly apply the patterns, one per each of the at least two downlink carriers, during downlink transmission. According to a fourth aspect there is presented a computer program for PIM avoidance. The computer program comprises computer code which, when run on processing circuitry of a controller, causes the controller to perform actions. One action comprises the controller to obtain a pattern for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers. All patterns extend over the same time units and frequency units and specify time-frequency resources of the transmission resources to be muted. In a union of all the patterns, there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted. One action comprises the controller to instruct a downlink scheduler to repeatedly apply the patterns, one per each of the at least two downlink carriers, during downlink transmission.

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 provide efficient PIM avoidance that does not suffer from the issues noted above.

Advantageously, these aspects will not require runtime communication for the PIM avoidance to work. Neither is runtime communication required between the carriers causing the PIM, nor between the victims of the PIM and the carriers.

Advantageously, these aspects enable efficient power pooling to be performed among the downlink carriers.

Advantageously, according to these aspects, the proposed PIM avoidance will not affect the uplink link adaptation as the PIM improvements are occurring all the time, and not only within some subframes or symbols or other limited time interval.

Advantageously, according to these aspects, a degree greedy algorithm can be utilized for the PIM avoidance, allowing static as well as dynamic selection of the most efficient carriers for achieving the PIM avoidance. Advantageously, these aspects enable flexibility in individual configuration of carrierspecific thresholds for the muting.

Advantageously, these aspects will not affect the uplink link adaptation as PIM levels variations are reduced to a configured limit. Advantageously, these aspects enable power savings when more packages are sent over fewer subframes.

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, module, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, 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: Fig. 1 is a schematic diagram illustrating a communications network according to embodiments;

Fig. 2 is a flowchart of methods according to embodiments;

Figs. 3, 4, 5, and 6 schematically illustrate different patterns with time-frequency resources of transmission resources to be muted according to embodiments;

_1 Fig. 7 schematically illustrates as a function of x for different values of n according to an embodiment;

Fig. 8 is a schematic diagram showing structural units of a controller according to an embodiment; Fig. 9 is a schematic diagram showing functional modules of a controller according to an embodiment; and

Fig. to 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. As noted above, there is still a need for improved PIM mitigation techniques.

The embodiments disclosed herein therefore relate to techniques for PIM avoidance. In order to obtain such techniques, there is provided a controller 800, 900, a method performed by the controller 800, 900, a computer program product comprising code, for example in the form of a computer program, that when run on a controller 800, 900, causes the controller 800, 900 to perform the method.

At least some of the herein disclosed embodiments are based on controlling the downlink scheduler of the downlink carriers to follow predefined patterns so that high power from all downlink carriers identified as PIM aggressors will not occur at the same time and/or frequency (depending on what type of pattern is used). Fig. 2 is a flowchart illustrating embodiments of methods for PIM avoidance. The methods are performed by the controller 800, 900. The methods are advantageously provided as computer programs 1020.

The method is based on controlling a downlink scheduler of downlink carriers (that represent PIM aggressors) to follow muting patterns with certain properties. Accordingly, high power from all carriers will not occur at the same time or frequency. In the time domain this is achieved by there always being at least one time-frequency resource that for each of the time units is muted. In the frequency domain this is achieved by there always being at least time-frequency time unit that for each of the frequency units is muted. In this respect, it is envisioned that multiple downlink schedulers might be involved in the scheduling of the downlink carriers.

Hence, any reference to a/the downlink scheduler should be construed as referring to at least one downlink scheduler.

S102: The controller 800, 900 obtains a pattern 300a: 600c for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers. All patterns 300a: 600c extend over the same time units 11: t4 and frequency units f1:f4. The patterns 300a:600c specify time-frequency resources of the transmission resources to be muted. In a union of all the patterns 300a: 600c, there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted. In this way, the above properties are satisfied.

S104: The controller 800, 900 instructs the downlink scheduler to repeatedly apply the patterns 300a:600c, one per each of the at least two downlink carriers, during downlink transmission.

For illustration, and without limitations, the power of a dual aggressor PIM product (i.e., a scenario where the PIM is caused by two downlink carriers) can be expressed by the following equation

where | h_PIM |² is the PIM channel power, where subscript 1 corresponds to downlink carrier 1 and subscript 2 corresponds to downlink carrier 2, where P₁ is the transmit power of the first downlink carrier, P₂ is the transmit power of the second downlink carrier, where n_r ≥ 1 is the exponent of downlink carrier 1, and where n₂ ≥ 1 is the exponent of downlink carrier 2. Furthermore, n₁ + n₂ is the PIM degree or as it is often denoted, the PIM order. The extension to more than two downlink carriers is straight forward. The above equation shows that for the PIM power to be at its maximum, the transmission powers of both downlink carriers must take the maximum value at the same time and frequency unit. Therefore, in case the maximum allowed transmission powers are alternating in a synchronized way so that at any time unit, or frequency unit, at least one of the downlink carriers has a low power, then the PIM power will be significantly reduced. This is achieved by means of using patterns 300a:600c satisfying the above disclosed properties.

Embodiments relating to further details of PIM avoidance as performed by the controller 800, 900 will now be disclosed with continued reference to Fig. 2.

There may be different types of time-frequency resources. In some embodiments, the time-frequency resources are physical resource blocks (PRBs). Further, in some embodiments, operation is performed per subframe, TTI, or per Orthogonal Frequency Division Multiplexing (OFDM) symbol and thus each of the patterns 300a: 600c spans one subframe, one transmission time interval, or one OFDM symbol.

There could be different ways in which the patterns are repeatedly applied. For example, the patterns could be always on, periodically on, or selectively on, for example depending on the traffic situation, or external process. Such an external process could be a feedback control process, a predetermined periodical sequence, or a random process.

For example, in some aspects, the patterns are applied only if the requested traffic is above some allowed traffic threshold level. That is, in some embodiments, the downlink scheduler is instructed to only repeatedly apply the patterns 300a:600c when requested traffic of the downlink transmission is above a traffic threshold level. There could be different ways to define the traffic threshold level. In some aspects, the union of all the patterns represent the traffic threshold level, and where the muting is applied only if the evaluation of the requested traffic indicates that the requested traffic is above the traffic threshold level. Hence, in some embodiments, the traffic threshold level is defined by the union of all the patterns 300a:600c.

Further aspects of the patterns will be disclosed next with parallel references to Figs.

3, 4, 5, and 6 which schematically illustrate different patterns with time-frequency resources of transmission resources to be muted according to embodiments. In each pattern is indicated the time-frequency resource available for scheduling and the time-frequency resource not available for scheduling (i.e., where muting is to be applied). For ease of illustration, each of the patterns 300a:600c is composed of 16 time-frequency resources provided in time-frequency grids extending over four time units tl:t4 and four frequency units fl:fq. The extension to larger time-frequency grids is straight forward. Figs. 3 and 4 provide examples of patterns to be applied at two downlink carriers whereas Figs. 5 and 6 provide examples of patterns to be applied at three downlink carriers. Taking Fig. 3 as an example, pattern 300a is to be applied for a first downlink carrier and pattern 300b is to be applied at the same time for a second downlink carrier. Taking Fig. 6 as an example, pattern 600a is to be applied for a first downlink carrier, pattern 600b is to be applied for a second downlink carrier, and pattern 600c is to be applied at the same time for a third downlink carrier. The extension to more downlink carriers is straight forward.

In some aspects, unsymmetric patterns, with respect to how many time-frequency resources that are kept from being muted, can be used. An example of this is illustrated in Fig. 3 where the pattern 300a as to be applied for a first downlink carrier includes six time-frequency resources not available for scheduling whereas the pattern 300b as to be applied for a second downlink carrier includes only four timefrequency resources not available for scheduling. In the pattern 300a, these six timefrequency resources not available for scheduling are distributed over two time units (and thus cover three frequency units each). The four time-frequency resources not available for scheduling in the pattern 300b are also distributed over two time units (and thus cover two frequency units each). Therefore, in some embodiments, each of the patterns 300a, 300b includes a unique number of time-frequency resources to be muted. Such patterns could be especially helpful for unequal load control and unequal power saving purposes.

However, also symmetric patterns can be used. An example of this is illustrated in Fig. 4 where the pattern 400a as to be applied for a first downlink carrier as well as the pattern 400b as to be applied for a second downlink carrier include four timefrequency resources not available for scheduling. Therefore, in other embodiments, all of the patterns 400a, 400b include the same number of time-frequency resources to be muted. Such patterns could be especially helpful for equal load control and equal power saving purposes.

As disclosed above, the patterns are so designed that high power from all downlink carriers identified as PIM aggressors will not occur at the same time and/or frequency, depending on what type of pattern is used.

Fig. 3 provides an example of patterns 300a, 300b where high power from all downlink carriers identified as PIM aggressors will not occur at the same time. That is, in some aspects, per time unit, a respective minimum amount of time-frequency resources is kept from being muted in each of the patterns 300a: 600c. In some embodiments, this minimum amount of time-frequency resources differs between at least two of the at least two downlink carriers. In other embodiments, this minimum amount of time-frequency resources is the same for all of the at least two downlink carriers. As the patterns 300a, 300b are applied, per time unit, there will always be two or three time-frequency resources that are muted and hence not available for scheduling; at time ti the time-frequency resources at frequencies f3 and fq for the second downlink carrier (for which pattern 300b is applied) will be muted, at time t2 the time-frequency resources at frequencies f2, f3 and fq for the first downlink carrier (for which pattern 300a is applied) will be muted, and so on. That is, maximum power will never be applied simultaneously to the first downlink carrier and the second downlink carrier.

Fig. q provides an example of patterns qooa, qoob where high power from all downlink carriers identified as PIM aggressors will not occur at the same frequency. That is, in some aspects, per frequency unit, a respective minimum amount of timefrequency resources is kept from being muted in each of the patterns 300a: 600c. In some embodiments, this minimum amount of time-frequency resources differs between at least two of the at least two downlink carriers. In other embodiments, this minimum amount of time-frequency resources is the same for all of the at least two downlink carriers. Thereby, as the patterns 400a, 400b are applied, each frequency fi:fq will be muted once and hence not available for scheduling; frequencies fl and f3 for the second downlink carrier (for which pattern qoob is applied) as well as frequencies f2 and fq for the first downlink carrier (for which pattern qooa is applied) will all be muted at time t3. The above principles apply also to situations where there are more than two downlink carriers. This is illustrated in the examples in Figs. 5 and 6.

Fig. 5 provides an example where pattern 500a is to be applied for a first downlink carrier, pattern 500b is to be applied for a second downlink carrier, and pattern 500c is to be applied at the same time for a third downlink carrier. In this example, the patterns 500a and 500c both result in muting at times t2 and t4, whereas the pattern 500b results in muting at times ti and t3- However, the muting thresholds for the patterns 500a and 500c are different; the pattern 500c yield more aggressive muting since three time-frequency resources are not available for scheduling at times t2 and tq whereas only two time-frequency resources are not available for scheduling at times t2 and t4 according to the pattern 500a.

As noted above, Fig. 6 provides an example where pattern 600a is to be applied for a first downlink carrier, pattern 600b is to be applied for a second downlink carrier, and pattern 600c is to be applied at the same time for a third downlink carrier. In this example, the pattern 600c does not include any time-frequency resource that are not available for scheduling. That is, all time-frequency resource that are available for scheduling and hence the transmission power of the third downlink carrier is thus unaffected. This could be the case where either the third downlink carrier has important information (relative to the information to be transmitted in the first and second downlink carriers) to transmit or where the third downlink carrier does not affect the PIM (or at leas has a very low PIM degree). However, since the pattern 600a includes time-frequency resource that are not available for scheduling at times t2 and t4, and pattern 600b includes time-frequency resource that are not available for scheduling at times ti and t3, application of the patterns 600a, 600b, 600c to the downlink carriers still enable high power from all downlink carriers identified as PIM aggressors to not occur at the same time.

As noted above, there could be different levels of muting in the different patterns. It is therefore understood that the patterns 300a:600c illustrated in in Figs. 3 to 6 are only some examples of patterns that satisfy the above disclosed properties. In general terms, the amount of muting, and hence how many time-frequency resource that are not available for scheduling in total per pattern and/or per time unit per pattern, can vary from pattern to pattern (for example as for the patterns 300a, 300b), and/or over time. In this respect, the accepted PIM level can be controlled by configuring the muting thresholds (with respect to how many time-frequency resource that are not available for scheduling in total per pattern and/or per time unit per pattern) for maximum scheduling for each downlink carrier to provides a limited variation of PIM levels over time.

In particular, in some embodiments the controller 800, 900 is configured to perform step S102-2 as part of step S102 to configure the muting thresholds, i.e., to configure the minimum amount of time-frequency resources that are not to be available (per time and/ or frequency unit) for scheduling for each downlink carrier.

S102-2: The controller 800, 900 determines the respective minimum amount of time-frequency resources for each of the patterns 300a:600c.

In particular, in some embodiments the controller 800, 900 is configured to perform step S102-4 as part of step S102 to configure the total amount of time-frequency resources that are not to be available (per time and/or frequency unit) for scheduling for each downlink carrier.

S102-4: The controller 800, 900 determines the total amount of time-frequency resources of the transmission resources to be muted in each of the patterns 300a:600c.

In cases where multiple downlink carriers, or at least more than two downlink carriers, are contributing to the PIM, the patterns can be designed in a tailored way to meet the demands of each situation. In this respect, the design of the patterns might be impacted by relationships between aggressors (in terms of downlink carriers) and victim cell(s) as well as the severity of the PIM source(s). In particular, in some embodiments, how many of the time-frequency resources of the transmission resources to be muted in each of the patterns 300a:600c is determined based on relationships between the at least two downlink carriers and a victim of the PIM as well as an amount of PIM generated by the at least two downlink carriers.

Even in complex scenarios, where two or more victim cells share one or more aggressor, it will be possible to allocate patterns to all downlink carriers involved in order to minimize the PIM levels. In this respect, in some embodiments, how many of (and/or how often) the time-frequency resources of the transmission resources to be muted in each of the patterns 300a: 600c (as in step S102-2 and/or as in step S102-4) is determined according to a greedy algorithm.

In further detail, assume that the PIM is generated via the model:

where P_m,1 (t) and P_m,2 (t) are the downlink transmission powers of two carriers acting as aggressors, with PIM exponents n

₁ and n₂, respectively, and where h_p(t) is the fading aggregated PIM channel gain. The PIM degree in this case is hence n = n₁ + n₂. Assume further that the objective is to find out how the muting patterns are to be selected for the carriers, as disclosed above. Towards that end, let the muting be defined by the muting factors q₁ and q₂, where 0 < q_1, q₂ ≤ 1 . Assuming full buffer traffic, this means that the muted PIM power can be expressed as

The assumption on full buffer traffic will be relaxed below.

The muting is applied to reduce PIM from down to

Here δ_H and δ_H are user selected constants.

As disclosed above the reduction of the PIM is to be obtained whilst maximizing the downlink throughput. The downlink throughput is quantified by

Merging these facts leads to the following maximization problem

The above optimization problem is well structured. Solving the main constraint equation for q₂ and inserting the result in V(q₁, q₂) results in the loss function

Any extreme point fulfills

Another differentiation renders

Hence, the only possible internal extreme point is a (local) minimum point. It follows that the maximum solution must be obtained at the boundary of the remaining constraint set

To analyze the problem further, the constraint set needs to be investigated. To find the optimum all possibilities need to be separately evaluated, followed by selection of the best boundary solution. The constraint set has 4 sides and 4 corners and each case needs to be evaluated. Case 1,

This gives

Case 2,

This requires that

and gives

Case 3,

This gives

Case 4

This requires and gives

Case 5

This requires that A= 1 and gives

This requires that and gives

This requires that and gives

This requires that

There are now three intervals to consider. To do so, consider Fig. 7. The figure illustrates as a function of x for n = {2, 3, 4} and x in (0,1]. The figure thus

illustrates that is increasing in (0,1], and

Therefore, to understand the way an algorithm for should be designed to determine how many of the time-frequency resources of the transmission resources to be muted in each of the patterns 300a: 600c, assume that n₁ > n₂.

In case A= 1, only case 5 applies and

In case Δ< 1 but large so none of q₁ or q₂ is at its lower limit, then case 1 or 3 applies and due to property 2 above This implies that

That is, that some of the time-frequency resources of carrier 1 (with largest exponent in the present example) are to be muted. This continues to be the case until q₁ hits the lower limit. remains to be the case. Then

which is case 7. Hence and cannot decrease.

Then, for gives that

This situation continues until no further muting is possible. Then case 8 is reached.

It can now be observed that this leads to a greedy algorithm for muting, with greediness in the PIM exponent value. This greedy algorithm can be expressed as follows:

Step 1: Sort the PIM exponents in descending orders.

Step 2: Pick the highest and second highest ones and compute the PIM reduction obtained for the selected pattern. Set the number of carriers to 2.

Step 3: If enough muting, or no more carriers to add, then stop.

Step 4: If not enough muting, increase the number of carriers with 1. Step 5: Add the remaining additional carrier with the highest remaining exponent, and select a pattern for the present number of carriers.

Step 6: Go to step 3.

Therefore, when the PIM as caused by each of the at least two downlink carriers is represented by a respective PIM exponent, according to the greedy algorithm, the downlink carrier with highest PIM exponent is first selected to have some of its timefrequency resources muted.

The greedy algorithm can be applied at configuration time, or be re-run according to the present PIM level, for example as the PIM level varies over time. Fig. 8 schematically illustrates, in terms of a number of structural units, the components of a controller 800 according to an embodiment. Processing circuitry 810 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 1010 (as in Fig. 10), e.g. in the form of a storage medium 830. The processing circuitry 810 may further be provided as at least one application specific integrated circuit (ASIC), or field programmable gate array (FPGA).

Particularly, the processing circuitry 810 is configured to cause the controller 800 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 830 may store the set of operations, and the processing circuitry 810 may be configured to retrieve the set of operations from the storage medium 830 to cause the controller 800 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

Thus the processing circuitry 810 is thereby arranged to execute methods as herein disclosed. The storage medium 830 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The controller 800 may further comprise a communications (comm.) interface 820 at least configured for communications with other entities, functions, and devices. As such the communications interface 820 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 810 controls the general operation of the controller 800 e.g. by sending data and control signals to the communications interface 820 and the storage medium 830, by receiving data and reports from the communications interface 820, and by retrieving data and instructions from the storage medium 830. Other components, as well as the related functionality, of the controller 800 are omitted in order not to obscure the concepts presented herein.

Fig. 9 schematically illustrates, in terms of a number of functional modules, the components of a controller 900 according to an embodiment. The controller 900 of Fig. 9 comprises a number of functional modules; an obtain module 910 configured to perform step S102, and an instruct module 940 configured to perform step S104. The controller 900 of Fig. 9 may further comprise a number of optional functional modules, such as any of a (first) determine module 920 configured to perform step S102-2, and a (second) determine module 930 configured to perform step S102-4. In general terms, each functional module 910:940 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 830 which when run on the processing circuitry makes the controller 800, 900 perform the corresponding steps mentioned above in conjunction with Fig 9. 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 910:940 maybe implemented by the processing circuitry 810, possibly in cooperation with the communications interface 820 and/or the storage medium 830. The processing circuitry 810 may thus be configured to from the storage medium 830 fetch instructions as provided by a functional module 910:940 and to execute these instructions, thereby performing any steps as disclosed herein.

The controller 800, 900 may be provided as a standalone device or as a part of at least one further device. For example, the controller 800, 900 may be provided in a node of the radio access network or in a node of the core network. Alternatively, functionality of the controller 800, 900 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 controller Soo, 900 may be executed in a first device, and a second portion of the of the instructions performed by the controller 8oo, 900 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 controller 800, 900 may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a controller 800, 900 residing in a cloud computational environment. Therefore, although a single processing circuitry 810 is illustrated in Fig. 8 the processing circuitry 810 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 910:940 of Fig. 9 and the computer program 1020 of Fig. 10.

Some (radio) access network architectures define network nodes (or gNBs) comprising multiple component parts or nodes: a central unit (CU), one or more distributed units (DUs), and one or more radio units (RUs). The protocol layer stack of the network node is divided between the CU, the DUs and the RUs, with one or more lower layers of the stack implemented in the RUs, and one or more higher layers of the stack implemented in the CU and/or DUs. The CU is coupled to the DUs via a fronthaul higher layer split (HLS) network; the CU/DUs are connected to the RUs via a fronthaul lower-layer split (LLS) network. The DU may be combined with the CU in some embodiments, where a combined DU/CU may be referred to as a CU or simply a baseband unit. A communication link for communication of user data messages or packets between the RU and the baseband unit, CU, or DU is referred to as a fronthaul network or interface. Messages or packets may be transmitted from the network node in the downlink (i.e., from the CU to the RU) or received by the network node in the uplink (i.e., from the RU to the CU).

Fig. 10 shows one example of a computer program product 1010 comprising computer readable storage medium 1030. On this computer readable storage medium 1030, a computer program 1020 can be stored, which computer program 1020 can cause the processing circuitry 810 and thereto operatively coupled entities and devices, such as the communications interface 820 and the storage medium 830, to execute methods according to embodiments described herein. The computer program 1020 and/or computer program product 1010 may thus provide means for performing any steps as herein disclosed. In the example of Fig. 10, the computer program product 1010 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 1010 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 1020 is here schematically shown as a track on the depicted optical disk, the computer program 1020 can be stored in any way which is suitable for the computer program product 1010.

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 passive intermodulation, PIM, avoidance, the method being performed by a controller (800, 900), the method comprising: obtaining (S102) a pattern (300a: 600c) for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers, wherein all patterns (300a: 600c) extend over the same time units (t1:t4) and frequency units (f1:f4) and specify time-frequency resources of the transmission resources to be muted, and wherein, in a union of all the patterns (300a: 600c), there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted; and instructing (S104) a downlink scheduler to repeatedly apply the patterns (300a:600c), one per each of the at least two downlink carriers, during downlink transmission.

2. The method according to claim 1, wherein each of the patterns (300a: 600c) includes a unique number of time-frequency resources to be muted.

3. The method according to claim 1 or 2, wherein, per time unit, a respective minimum amount of time-frequency resources is kept from being muted in each of the patterns (300a: 600c), and wherein the minimum amount differs between at least two of the at least two downlink carriers.

4. The method according to any of claims 1 to 3, wherein, per frequency unit, a respective minimum amount of time-frequency resources is kept from being muted in each of the patterns (300a: 600c), and wherein the minimum amount differs between at least two of the at least two downlink carriers.

5. The method according to claim 3 or 4, wherein the method further comprises: determining (S102-2) said respective minimum amount for each of the patterns (300a:600c).

6. The method according to any of claims 1 to 5, wherein the method further comprises: determining (S 102-4) total amount of time-frequency resources of the transmission resources to be muted in each of the patterns (300a: 600c).

7. The method according to claim 6, wherein how many of the time-frequency resources of the transmission resources to be muted in each of the patterns (300a:600c) is determined based on relationships between the at least two downlink carriers and a victim of the PIM as well as an amount of PIM generated by the at least two downlink carriers.

8. The method according to claim 6 or 7, wherein how many of the time-frequency resources of the transmission resources to be muted in each of the patterns (300a:600c) is determined according to a greedy algorithm.

9. The method according to claim 8, wherein the PIM as caused by each of the at least two downlink carriers is represented by a respective PIM exponent, and wherein, according to the greedy algorithm, the downlink carrier with highest PIM exponent is first selected to have some of its time-frequency resources muted.

10. The method according to any of claims 1 to 9, wherein the time-frequency resources are physical resource blocks.

11. The method according to any of claims 1 to 10, wherein each of the patterns (300a:600c) spans one subframe, one transmission time interval, or one OFDM symbol.

12. The method according to any of claims 1 to 11, wherein the downlink scheduler is instructed to only repeatedly apply the patterns (300a: 600c) when requested traffic of the downlink transmission is above a traffic threshold level.

13. The method according to claim 12, wherein the traffic threshold level is defined by the union of all the patterns (300a:600c).

14. A controller (800) for passive intermodulation, PIM, avoidance, the controller (800) comprising processing circuitry (810), the processing circuitry being configured to cause the controller (800) to: obtain a pattern (300a:600c) for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers, wherein all patterns (300a: 600c) extend over the same time units (tl: t4) and frequency units (fl:f4) and specify time-frequency resources of the transmission resources to be muted, and wherein, in a union of all the patterns (300a: 600c), there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted; and instruct a downlink scheduler to repeatedly apply the patterns (300a: 600c), one per each of the at least two downlink carriers, during downlink transmission.

15. A controller (900) for passive intermodulation, PIM, avoidance, the controller (900) comprising: an obtain module (910) configured to obtain a pattern (300a: 600c) for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers, wherein all patterns (300a: 600c) extend over the same time units (t1:t4) and frequency units (t1:t4) and specify time-frequency resources of the transmission resources to be muted, and wherein, in a union of all the patterns (300a:600c), there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted; and an instruct module (940) configured to instruct a downlink scheduler to repeatedly apply the patterns (300a:600c), one per each of the at least two downlink carriers, during downlink transmission.

16. The controller (800, 900) according to claim 14 or 15, further being configured to perform the method according to any of claims 2 to 13.

17. A computer program (1020) for passive intermodulation, PIM, avoidance, the computer program comprising computer code which, when run on processing circuitry (810) of a controller (800), causes the controller (800) to: obtain (S102) a pattern (300a: 600c) for each of at least two downlink carriers for selective muting of transmission resources on the at least two downlink carriers, wherein all patterns (300a: 600c) extend over the same time units (tl:t4) and frequency units (fl:f4) and specify time-frequency resources of the transmission resources to be muted, and wherein, in a union of all the patterns (300a: 600c), there is at least one time-frequency resource that for each of the time units is muted or at least one time-frequency resource that for each of the frequency units is muted; and instruct (S104) a downlink scheduler to repeatedly apply the patterns (300a:600c), one per each of the at least two downlink carriers, during downlink transmission.

18. A computer program product (1010) comprising a computer program (1020) according to claim 17, and a computer readable storage medium (1030) on which the computer program is stored.