WO2015198103A1

WO2015198103A1 - Rate scheduling coordination in a wireless cellular network

Info

Publication number: WO2015198103A1
Application number: PCT/IB2014/062689
Authority: WO
Inventors: Andres Reial; Sairamesh Nammi; Yi-Pin Eric Wang
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2014-06-27
Filing date: 2014-06-27
Publication date: 2015-12-30

Abstract

The proposed technology relates to methods and wireless network nodes for scheduling wireless communication devices in a wireless cellular network. For example, a method performed by a wireless network node (10) comprises the step (S10) of selecting for co-scheduling, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device (200) being served by a first wireless network node (20), and at least one second IC-capable wireless communication device (300) being served by a respective second wireless network node (30; 40). The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, where the rate metric is based on achievable interference cancellation efficiency of signals from the first wireless network node (20) causing interference at the at least one second wireless communication device (300). The achievable interference cancellation efficiency is determined based on link quality between the at least one second wireless communication device (300) and the first wireless network node(20), and scheduled transmission rate for the at least one first wireless communication device (200).

Description

RATE SCHEDULING COORDINATION IN A WIRELESS CELLULAR NETWORK

TECHNICAL FIELD The proposed technology generally relates to scheduling of wireless communication devices in a wireless cellular network, and in particular to rate scheduling coordination of wireless communication devices in a wireless cellular network.

BACKGROUND

In 3GPP Long Term Evolution (LTE) a scheduler controls assignment of resources among the users for both uplink and downlink and also determines the appropriate data rate to be used for each transmission. In most systems, the condition of the channel for each user is a consideration in determining the most efficient allocation of resources. For instance, a scheduler may be configured to give scheduling priority to user equipment (UEs) with the highest channel quality.

The quality of the signal received by a given user is dependent upon a number of factors, including the channel quality from the serving node, the level of interference from other cells and nodes, and the noise level. In order to optimize the overall capacity of the system, a scheduler will typically try to match the modulation, coding, and other signal/protocol parameters to the signal quality. For instance, when signal quality is low, a scheduler may reduce the coding rate or select a lower-order modulation scheme to increase tolerance to interference and raw bit error rates or to otherwise improve robustness.

According to the LTE standard, UEs may be configured to report Channel Quality Indicators (CQIs) to assist the scheduler. These CQI reports are derived from the downlink received signal quality and are often based on measurements of the downlink reference signals.

Despite the many advantages of existing LTE schemes and protocols, there exists a significant problem with inter-cell interference and a need to coordinate between cells in order to mitigate the negative effects of interference. The LTE standard is primarily designed to operate under the presumption that the entire spectrum is available in each cell; in other words that the same time-frequency resources may be used in neighbouring cells with limited interference. However, this is not always true in practice, particularly at the cell-edge. Transmissions intended for a first user in a first cell are often overheard by a second, unintended user in a second cell.

Thus, inter-cell interference presents a big performance issue for cell edge users. In a heterogeneous network (HetNet), the impact of inter-cell interference can be much higher due to large transmit power difference between a macro base station and a low power node (LPN). This is illustrated in Fig. 1 . The stripe-covered region in Fig. 1 is a region between an outer circle and an inner circle around an LPN. The inner circle represents an area where the received power from the LPN is higher than that from the macro base station. The outer circle represents an area where the path loss to the LPN is smaller than that to the macro base station. The stripe-covered area is often referred to as the imbalance area, which potentially could be an LPN range-expansion area because from the uplink perspectives the network still would like the terminal to be served by the LPN within this area. However, from the downlink perspectives, terminals at the outer edge of such an imbalance zone experience very large received power difference between the macro and LPN layers. For example, if the transmit power levels are 40 watt and 1 watt for the macro and LPN, respectively, such power difference can be as high as 16 dB. Thus, if a terminal in the range-expansion area is served by the LPN and at the same time, the macro cell is serving another terminal using the same radio resources, the LPN terminal is subject to very severe interference from the Macro base station.

In Long-Term Evolution (LTE) networks, inter-cell interference coordination (ICIC) is supported via the eNodeB-to-eNodeB X2 interface. Each cell can signal the high- power resource blocks in the frequency domain, or time domain, to its neighboring cells. This allows the neighboring cells to schedule cell-edge users avoiding these high-power resource blocks. Such a mechanism can be used to reduce the impact of inter-cell interference. In Ref. [3], Restricted Resource Subframes (RRS) for the macro cell were proposed to help improve the cancellation efficiency of macro interference at an LPN UE. The idea is to restrict the modulation order and number of High Speed Physical Downlink Shared Channel (HS-PDSCH) channelization codes in special RRS in the macro cell. For example, using Quadrature Phase Shift Keying (QPSK) instead of 16-QAM (Quadrature Amplitude Modulation) makes it easier for the LPN UE to detect the interfering signal more reliably. Once the interfering signal is detected more reliably, better cancellation efficiency can be achieved. Similarly, using 5 codes instead of 15 HS-PDSCH codes increases the power allocated to each macro HS-PDSCH symbol, resulting in higher detection reliability by the "victim" LPN UE (UE receiving signals intended for another UE) and somewhat improved cancellation efficiency.

Rate scheduling coordination has also been proposed where, instead of scheduling the macro UE according to its channel quality towards the macro cell, it is scheduled with a rate that is decodable at the LPN UE location to enable interference cancellation (IC) - typically a rate lower than allowed by conventional scheduling.

The use of ICIC or RRS may be seen as a trivial form of rate coordination, where however the scheduled macro rate is lowered or zeroed without regard to its instantaneous interference impact towards the LPN UE. Furthermore, both have a negative impact on the macro throughput.

When more dynamic rate scheduling coordination approaches are employed, the macro rate is lowered adaptively to enable a LPN rate increase. Unfortunately the macro average rate is decreased compared to the conventional scheduling solution and thus the sum rate of macro and LPN UEs does not improve in many scenarios.

There is thus a need for a rate scheduling coordination algorithm that increases interference cancellation efficiency at the LPN UE without significantly reducing the macro transmission rate. SUMMARY

It is an object to provide a method, a wireless network node and a computer program for scheduling wireless communication devices in a wireless communication network.

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

An aspect of the embodiments relates to a method performed by a wireless network node for scheduling wireless communication devices in a wireless cellular network. The method comprises the step of selecting, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co-scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, where the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device.

Another aspect of the embodiments relates to a wireless network node configured to enable scheduling of wireless communication devices in a wireless cellular network. The wireless network node is configured to select, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co-scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, wherein the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device.

Yet another aspect of the embodiments relates to a computer program comprising instructions, which when executed by at least one processor, cause the processor or processors to select, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co- scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, where the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device.

Yet another aspect of the embodiments relates to a wireless network node for scheduling wireless communication devices in a wireless cellular network. The wireless network node comprises a selecting module configured to select, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co-scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, where the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device.

An advantage of the proposed technology is that increased interference cancellation efficiency is achieved together with improved transmission rates for the scheduled wireless devices.

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

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

Fig. 1 is an illustration of an example of inter-cell interference in a heterogeneous network.

Fig. 2 is a schematic illustration of a simulation scenario for analysis of network assisted interference cancellation.

Fig. 3 is a schematic illustration of some examples of paring arrangements according to some example embodiments. Fig. 4 is a schematic flow diagram illustrating an example of a method performed by a wireless network node for scheduling wireless communication devices in a wireless cellular network according to an embodiment.

Fig. 5 is a schematic diagram illustrating an example of a wireless network node configured to enable scheduling of wireless communication devices in a wireless cellular network according to an embodiment. Fig. 6 is a schematic block diagram illustrating an example of a wireless network node configured to enable scheduling of wireless communication devices in a wireless cellular network according to an alternative embodiment. DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements. When using the word "comprise" or "comprising" it shall be interpreted as non- limiting, i.e. meaning "consist at least of".

For a better understanding of the proposed technology, it may be useful to begin with a brief overview of some background technology, described with reference to some non-limiting examples.

As described in the background section, downlink data throughput in a wireless network may be limited by inter-cell interference. The effect of inter-cell interference is more pronounced for users at the edge of the serving cell or when a user is in the so-called low-power node (LPN) range expansion area, in which the received power level from the macro node may be higher than that from the serving LPN.

Rate scheduling coordination has been previously proposed where, instead of scheduling the macro UE according to its channel quality towards the macro cell, it is scheduled with a rate that is decodable at the LPN UE location to enable interference cancellation (IC) - typically a rate lower than allowed by conventional scheduling.

IC mitigation and NAIC

The impact of inter-cell interference depends closely on the interfered (or "victim") terminal's ability to mitigate interference. For example, the LPN UE may attempt to first decode the macro signal and cancel it before decoding its desired signal. A strong interference signal can thus be harmless provided that it can be decoded correctly and removed at a victim terminal. Network assisted interference cancellation (NAIC) is being considered in 3GPP for both LTE and High Speed Packet Access (HSPA). The concept is to have the network send assistance information to the UE to enable the UE to perform efficient interference cancellation. The concept was first described in Refs. [1 ] and [2]. While a less effective (pre-decoding) form of IC is possible without NAIC, the latter enables post-decoding IC which maximizes IC gains for given interferer transport format and reception conditions. To reap the benefits of NAIC, it is desired that the cancellation efficiency of the macro interference is maximized. Cancellation efficiency is defined as

where y is the macro interfering signal and y is the reconstructed signal at the victim UE after decoding and reconstruction. Thus if the reconstructed signal y has high fidelity and close to the actual interfering signal y , the victim UE can achieve cancellation efficiency close to 1 .

Approaching such high cancellation efficiencies and increased IC gains are possible if proper rate scheduling coordination is applied so that the transmission rate of the interferer Transport Format (TF) for the current Transmission Time Interval (TTI) is lowered so that the victim UE is able to decode the interfering transport block and regenerate and remove its contribution fully. Example of post-decoding IC and NAIC utility

Fig. 1 illustrates a scenario where NAIC could be beneficial. As shown, a wireless communication device 300, such as a UE, served by a wireless network node 30, such as an LPN, in the range expansion area (stripe covered area) experiences strong interference from another wireless network node 20, such as a macro node. In such a scenario, if the network provides certain information about the interference signal, e.g. UE ID, modulation format, transport block size, etc., to the victim UE, the victim UE may be able to cancel the interfering macro signal and boost its achievable data rate in the LPN downlink.

During 3^rd Generation Partnership Project Release 12 (3GPP Rel-12) study item Universal Mobile Telecommunications System (UMTS) Heterogeneous Networks, the performance of NAIC had been studied by various companies and the results are included in Ref. [3]. An exemplary performance as captured in [3] is described below. Fig. 2 shows a simplified heterogeneous networks model with one LPN and one dominant macro. This is a simplified layout of a heterogeneous network of 57 macro cells. The other macro and LPN cells are not shown, but their interference is captured as described below.

Twelve possible UE locations are created and shown in Fig. 2 (marked from L1 to L12). A hexagonal cell structure is assumed with inter-site distance (ISD) = 500 meters, and one macro is located at the origin O.

Received signal power levels at each of the 12 UE locations are shown in Table 1 , where macro 2 (not shown in Fig. 2) is the strongest received macro power level among those 56 macros not shown in Fig. 2 (lor is defined as the total transmit power at the eNodeB antenna connector and loc is defined as the total noise power at UE antenna connector). Interference from other macros and LPNs not listed in Table 1 is included in loc. According to Fig. 2 and Table 1 , UE at location L7 sees its received signal dominated by the macro, where UE at e.g. location L6 sees its received signal dominated by the LPN.

UE LPN lor / loc Macro lor / loc Macro2 lor/loc

Location [dB] [dB] [dB]

L1 5.2774 18.555 0.92192

L2 8.3307 18.003 0.66949

L3 12.144 17.59 1 .1988

L4 16.951 17.167 1 .6937

L5 23.603 16.737 2.1588

L6 34.812 16.302 2.5979

L7 -12.658 24.273 4.2725

L8 -10.256 15.356 1 .9603

L9 -20.806 6.9397 4.8632

L10 -18.964 15.547 2.6975

L1 1 -20.781 10.415 7.7891

L12 -28.1 1 1 3.8369 10.577

Table 1: Received signal powers at each UE location

Table 2 shows the gains of NAIC over blind IC in terms of LPN UE throughput. As shown, if the LPN UE is at location 1 , when the scheduled macro UE is at location L12, the LPN UE enjoys a high NAIC benefit (50% gain in UE throughput). On the other hand, when the scheduled macro UE is at location L7, the LPN UE sees a small NAIC benefit (5% in UE throughput). Thus, scheduling coordination may improve NAIC gains.

Table 2: LPN UE throughput gain for the baseline NAIC over the blind IC (%) - each row stands for the LPN UE location and each column for the macro UE location Problems with existing solutions

The use of ICIC or RRS may be seen as a trivial form of rate coordination, where however the scheduled macro rate is not dynamically chosen based on the decodability at the LPN UE and the cancellation efficiency is not maximized. Furthermore, both have a negative impact on the macro throughput. In Ref. [4], it was shown that up to 67% loss in macro UE throughput could be a result of RRS. When more dynamic rate scheduling coordination approaches are employed by appropriately lowering the macro rate, the cancellation efficiency of the macro signal and the LPN UE Signal to Interference plus Noise Ratio (SINR) improvement is maximized. The LPN UE link performance may thus increase significantly. Unfortunately the macro average rate is decreased compared to the conventional scheduling solution that does not account for the interference impact. As a result, the increase of the LPN rate is offset by the decrease of the macro rate and the sum rate of macro and LPN UEs does not improve in many scenarios. As an example, in Ref. [5] the transmission data rate to a scheduled user in a given cell may be reduced to ensure effective interference cancellation performance at the receiver of a co-scheduled user in another cell. The rates of the co-scheduled users are selected in order to enable the UEs to completely cancel interference caused by the unintended signal.

There is thus a need for a rate scheduling coordination algorithm that increases interference cancellation efficiency at the LPN UE and increases the LPN average rate without significantly reducing the macro average rate. The present embodiments aim to provide an improved scheduling coordination in a wireless network by selecting LPN and macro UEs in a manner that achieves improved cancellation efficiency without having to lower the macro scheduled rate compared to the conventional scheduling. Fig. 4 is a schematic flow diagram illustrating an example of a method performed by a wireless network node for scheduling wireless communication devices in a wireless cellular network according to an embodiment. The method comprises the step S10 of selecting, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co- scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, where the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device.

In other words, in this embodiment the wireless communication devices are selected for co-scheduling in order to maximize the transmission rates for the co-scheduled wireless communication devices, without necessarily achieving complete cancellation of the interfering signal. As will be described in more detail below, the rate for the at least one second wireless communication device may for instance be calculated by taking into account the achievable interference cancellation efficiency, which may for instance be determined based on the scheduled transmission rate for the at least one first wireless communication device together with link quality information regarding link quality between the at least one second wireless communication device and the first wireless network node.

In a particular embodiment, the link quality information comprises channel quality indicator, CQI, information In some example embodiments, the rate metric may be calculated in one of the following manners:

sum of achievable rates of said selected wireless communication devices; sum of achievable rates of selected wireless communication devices from said first set of wireless communication devices;

sum of achievable rates of selected wireless communication devices from said at least one second set of wireless communication devices;

average of achievable rates of said selected wireless communication devices; average of achievable rates of selected wireless communication devices from said first set of wireless communication devices; or

average of achievable rates of selected wireless communication devices from said at least one second set of wireless communication devices.

In an example embodiment, as illustrated in Fig. 1 , the wireless cellular network is a heterogeneous network, and each of the respective second wireless network nodes 30; 40 is a low power node, LPN, and the first wireless network node 20 is a macro node. The concept of the invention also applies to other deployment scenarios. Thus, in other embodiments the LPN cell may then be interpreted more generally as a cell serving victim UEs and the macro cell as an interfering neighbour cell.

In one embodiment, the wireless communication devices are user equipment, UE. In one embodiment, the wireless cellular network is a Long Term Evolution, LTE, network.

This basic approach may not explicitly guarantee fair scheduling of all of the first wireless communication devices, since some wireless communication devices may have perpetually disadvantageous channel conditions due to their location. To ensure fairness, in one embodiment the rate metric is further based on duration of time that at least one of the selected wireless communication devices has been waiting in a scheduling queue at the first and second wireless network nodes. An example of an implementation of such an embodiment will be described further below.

In an alternative embodiment, the rate metric is further based on a proportional-fair criterion for the selected wireless communication devices. An example of an implementation of such an embodiment will be described further below. In a particular embodiment, the selecting is based on forming a group comprising one wireless communication device from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, at each scheduling instant.

In another particular embodiment, the selecting is based on forming a group comprising one wireless communication device from each of the at least one second set of wireless communication devices, and one out of several wireless communication devices from the first set of wireless communication devices, at each scheduling instant.

In yet another particular embodiment, the selecting is based on forming a group comprising one out of several wireless communication devices from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, at each scheduling instant.

In yet another particular embodiment, the selecting is based on forming several groups comprising one out of several wireless communication devices from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, to be used respectively at several scheduling instants. The above described embodiments of selection strategies are illustrated in Fig. 3, and will also be further exemplified below.

In the following, some non-limiting examples of illustrative embodiments are described.

Denote a set of macro cell UE indices (UE ID, Radio Network Temporary Identifier (RNTI), etc.) available for scheduling at time t (e.g. TTI or sub-frame) by

where ₀ is the number of available macro UEs. Denote a corresponding set of LPN cell UE indices by Denote the initial time of appearance of UE m^ ⁾ in its priority queue by t ⁾ . Consider a pairing hypothesis <n₀, where UEs with indices and are scheduled simultaneously. Let the available (maximum possible) scheduled rate at time t for the macro UE be R^ino, ^) and for the LPN UE in such pairing hypothesis R^ iri_Q, n^. The sum rate is then R_t (n₀, = fl_t ⁽⁰⁾ <n₀, + fl_t ⁽¹⁾ <n₀, n .

We can now express some of the possible scheduling embodiments according to the invention. A: Single-pair search over two sets

In one general embodiment, a UE pair is selected out of M₁≥ 1 LPN UEs and ₀ > 1 macro UEs available, where M₁ + M₀ > 2. At time t, the pair {n₀*, n{) may be scheduled such that

(n₀*, n ) = arg max R_t (n₀, n^ , n₀ G n_x G

As mentioned above, this basic approach does not explicitly guarantee fair scheduling of all macro UEs; some UEs may have perpetually disadvantageous channel conditions due to their location. To ensure fairness, a penalty function ⁾ may in one embodiment be added to the rate metric that penalizes long waiting time in the priority queue:

{n₀*, n ) = arg max p^ (t - t^p^ ft - t^R_t <n₀, n^ , n₀ G t/[⁰⁾ n_x G t/[^1} The penalty function is preferably a monotonically non-decreasing function and may be UE-specific, to account e.g. for different subscription types or Quality of Service (QoS) requirements. In an alternative embodiment, a proportional-fair criterion can be used, for example the sum proportional fairness metric as illustrated below:

_{/ * * l r}(l)

<n₀, rii ^Ut

Here ^J(n₀) and ^n-J are the average throughputs, measured at time t, of UEs with indices and m^, respectively. The average throughputs may be averaged over the last W time instances (e.g. TTI), where W is a parameter used by the scheduler. For example if l V=10, the actual data rates for UEs n₀ and n during the last 10 TTI's are used to obtain

The actual throughput A _k n₀) is 0, if UE n₀ is not scheduled in the macro cell during time instant t-k, and equal to R^ n₀, if UE n₀ is scheduled during time instant t-k. Similarly, the actual throughput

is 0, if UE n_x is not scheduled during time instant t-k, and equal to R^_k(n₀, if UE n is scheduled during time instant t-k.

In this approach, all (or many) combinations of macro and LPN UE selections are evaluated to identify the best-performing combination. The complexity of this general approach is 0 M_QM₁). For the next scheduling time t + 1, the UE sets

and may be formed by removing indices n₀ ^* and n{ for the sets t/[⁰⁾ and respectively and adding any new UEs present in the priority queues. B: Single-pair search over a single set

In related embodiments, one of the UE groups may be eliminated and a fixed UE from that cell selected instead, e.g. the UE that would have been scheduled according to a state-of-the-art scheduling approach at time t.

B(macro): If a fixed LPN UE is selected {M₁ = 1), the solution is found as (n₀ ^*, m^) where n₀ ^* = arg max p^(t - t^R_t <n₀, m^) , n_Q G t/[⁰⁾

Typical scheduling patterns from this scheduling approach are that if the macro cell Channel Quality Indicator (CQI) with respect to the LPN UE is high, a low-to-medium- rate macro UE is selected for pairing, since full decoding of the interferer is possible. If the macro CQI is low, a high-rate macro UE is selected, since the interferer cannot be decoded at any rate.

This approach is also highly compatible with the motivation behind HetNet deployments in the first place, which is off-loading the macro. In such scenarios, there are typically many UEs waiting to be served in the macro cell and the size of the set u ⁰⁾ is large.

B(LPN): If instead a fixed macro UE is selected ( ₀ = 1), the solution is

n{) with

The complexity of these approaches is linear in O( ₀) and 0 M₁) respectively. C: Multi-pair search over two sets

In another class of embodiments, multiple pairs (number of pairs M = M₀ = M₁ ) are formed out of several pending LPN UEs and several pending macro UEs.

All pairing combinations to be applied at times t, t + 1, t + M - 1 are evaluated (complexity ( !)) and the combination resulting in the largest sum rate

∑fc=i Rt+fc-i(¾fc> ⁿi,k) is selected for scheduling.

The pairing arrangements according to the different embodiments are illustrated in Fig. 3.

Pairwise rate estimation

In one embodiment, for each UE pair (ηο, η^, the pairwise rate is some combination of macro and LPN rates that may be decoded simultaneously at the macro and LPN UEs respectively. The details of how the rate metric is formed depend on the rate optimization criterion. Some examples are:

Macro rate optimization

The maximum macro UE rate, unconstrained by IC considerations at the victim UE is selected:

where F1 is the conventional SINR-to-rate mapping table in the scheduler. Based on the macro rate and the macro node-to-LPN UE channel quality, the resulting cancellation efficiency is estimated, e.g. by fetching from a pre-computed look-up table (LUT): p = F2 (K_t ^c ₀, n₁>, S/.VK⁽⁰⁾(n₁)) Finally, based on the estimated cancellation efficiency, the achievable LPN rate is estimated. The post-IC SINR estimate for the victim UE is obtained by removing the corresponding fraction of macro interference in the serving-cell SINR computation. In one embodiment, it is obtained as:

(l-p)l(o)_+I(other) where /⁽⁰⁾ is the interference level from the macro node experienced by the LPN UE n_x. The LPN UE rate is determined as:

LPN rate optimization

The macro UE rate is chosen so that it may be fully decoded at the LPN UE (and of course at the macro UE):

The post-IC SINR estimate for the victim UE is obtained by fully removing the macro interference in the serving-cell SINR computation. In one embodiment, it is obtained as

_{5/jV ?}(l,post-/C) _(ni)

j (o ither) Finally, the LPN UE rate is again determined as

= Fl (5/N ?(¹'P°^st-^/c)(n₁)) Sum rate optimization

The sum rate may be maximized by the macro-optimized rate combination above, the LPN-optimized rate combination above, or by some rate combination where the macro rate lies between the two bounds. In one embodiment, the sum rate is optimized by the following sequence:

1 . Compute macro rates for the macro-optimized and LPN-optimized designs above

2. For each macro rate R^in^ n^) in the defined macro rate range, compute the

LPN rate R^in^ n^ according to the description in the macro-optimized design.

3. Select the rate combination that maximizes the sum rate R^in^ nj) +

some mechanisms for providing the network with CQI information for estimating the IC impact may be for example legacy or new modes of CQI reporting, or standard UE measurement reports, such as for example Received Signal Code Power (RSCP), Received Signal Strength Indicator (RSSI), or SINR.

Example embodiment

We illustrate the invention using the example according to the embodiment B(macro) above.

Consider a network scenario according to Fig. 1 at time t when the macro cell has 20 UEs in its priority queue t/[⁰⁾ = k =

1 ... 20. Let the LPN have 3 active UEs, out of which UE has been selected for scheduling at time t. According to the invention, the network will then select one out of the 20 macro UEs to be scheduled together with the LPN UE τη so as to maximize the sum rate, subject to the scheduling delay penalty term. Let's assume that a super- linear but sub-quadratic penalty function is applied, common to all macro UEs: ^(At) = At¹-⁵. We also assume that the LPN interference to macro UEs is negligible and the macro rate does not depend on the simultaneously scheduled LPN rate. Here, At = t - as used below, and ¾ .is known to the scheduler at run time as the time instant the UE first appeared in the priority queue.

For all k = 1 ... 20, the delay-adjusted sum rate is computed,

p£>(t - tS»)¾ <n„, m<"> = (t - O' 'inf > + ««<»f , m'¹'))

The different rates are determined by the transport block length in the chosen modulation and coding scheme (MCS), i.e. to get a certain rate, the scheduler selects the appropriate transport format/MCS.

Denote the macro UEs' CQI with respect to the macro cell by The CQI

may be obtained e.g. from Uplink Control Info (UCI) in LTE. Let the LPN UE πιψ CQI with respect to the LPN cell be SINR^^m^^ and with respect to the macro cell The last CQI quantity is not required in legacy implementations. For

the current embodiment, if we don't assume new CQI signalling, it may be estimated e.g. from the average SINR that the LPN UE has reported with respect to the macro cell as part of its mobility measurements. The rate terms of interest are then fetched from a pre-computed lookup table L as

and

where SINR^w'^mod(m^ in turn is estimated based on the SINR without cancellation and the anticipated cancellation efficiency when the macro rate is i?_t ⁰⁾<n ⁰⁾ ): i?_t ⁽⁰⁾<n )) )

The LUT L is part of legacy scheduler design where the MCS (transport format) is selected as a function of the CQI (essentially SINR) report.

F(-) and ?(^■) may be implemented as lookup tables or closed-form expressions.

The macro UE k^* maximizing the sum rate metric is selected to be scheduled together with the LPN UE The LPN UE then performs IC of the macro cell signal scheduled for UE k^* as part of the process of demodulating and decoding its signal from the LPN

Other comments The victim UE is assumed to employ an IC receiver. Preferably post-decoding IC is used to maximize performance, and some form of network assistance or related signalling is provided for the UE to obtain interfering signal transport format information required for decoding. However, the invention is also applicable to scenarios where the victim UE applies pre-decoding IC. The IC type is reflected in the IC efficiency lookup table (LUT), containing the β values.

The idea of the invention may be naturally applied to scenarios with more than one LPN cell per macro cell. The configuration B(LPN) with a fixed macro UE is particularly amenable to such scenarios, since the search for a proper LPN UE pairing may be done on a per-LPN cell basis by searching over each of the K LPN cells and the sets

Scheduling and CQI information communication between the cells may be implemented using e.g. the X2 interface or a proprietary interface in a main-remote configuration.

The invention has been illustrated using the example of a heterogeneous network where a macro cell is the dominant interferer towards one or more LPN cells. The concept of the invention also applies to other deployment scenarios. The LPN cell may then be interpreted more generally as a cell serving victim UEs and the macro cell as an interfering neighbour cell. As used herein, the non-limiting terms "User Equipment" and "wireless communication device" may refer to a mobile phone, a cellular phone, a Personal Digital Assistant, PDA, equipped with radio communication capabilities, a smart phone, a laptop or Personal Computer, PC, equipped with an internal or external mobile broadband modem, a tablet PC with radio communication capabilities, a target device, a device to device UE, a machine type UE or UE capable of machine to machine communication, i PAD, customer premises equipment, CPE, laptop embedded equipment, LEE, laptop mounted equipment, LME, USB dongle, a portable electronic radio communication device, a sensor device equipped with radio communication capabilities or the like. In particular, the term "UE" and the term "wireless communication device" should be interpreted as non-limiting terms comprising any type of wireless device communicating with a wireless network node in a cellular or mobile communication system or any device equipped with radio circuitry for wireless communication according to any relevant standard for communication within a cellular or mobile communication system.

As used herein, the non-limiting term "wireless network node" may refer to base stations, network control nodes such as network controllers, radio network controllers, base station controllers, and the like. In particular, the term "base station" may encompass different types of radio base stations including standardized base stations such as Node Bs, or evolved Node Bs, eNBs, and also macro/micro/pico radio base stations, home base stations, also known as femto base stations, relay nodes, repeaters, radio access points, base transceiver stations, BTSs, and even radio control nodes controlling one or more Remote Radio Units, RRUs, or the like. It will be appreciated that the methods and devices described herein can be combined and re-arranged in a variety of ways. For example, embodiments may be implemented in hardware, or in software for execution by suitable processing circuitry, or a combination thereof.

The steps, functions, procedures, modules and/or blocks described herein may be implemented in hardware using any conventional technology, such as discrete circuit or integrated circuit technology, including both general-purpose electronic circuitry and application-specific circuitry.

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

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

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

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

In an example of an implementation, at least some of the steps, functions, procedures, modules and/or blocks described herein are implemented in a computer program, which is loaded into the memory for execution by processing circuitry including one or more processors. The processor(s) and memory are interconnected to each other to enable normal software execution. An optional input/output device may also be interconnected to the processor(s) and/or the memory to enable input and/or output of relevant data such as input parameter(s) and/or resulting output parameter(s).

The term 'processor' should be interpreted in a general sense as any system or device capable of executing program code or computer program instructions to perform a particular processing, determining or computing task.

The processing circuitry including one or more processors is thus configured to perform, when executing the computer program, well-defined processing tasks such as those described above.

The processing circuitry does not have to be dedicated to only execute the above- described steps, functions, procedure and/or blocks, but may also execute other tasks. The embodiments herein may thus be implemented through one or more processors, such as a processor in the wireless network node depicted in Fig. 5, together with computer program code for performing the functions and actions of the embodiments herein. According to an embodiment, a wireless network node 10 is configured to enable scheduling of wireless communication devices in a wireless cellular network. The wireless network node is configured to select, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co-scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, wherein the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device. In a particular embodiment, the link quality information comprises channel quality indicator, CQI, information

In some example embodiments, the rate metric may be calculated in one of the following manners:

In an example embodiment, as illustrated in Fig. 1 , the wireless cellular network is a heterogeneous network, and each of the respective second wireless network nodes 30; 40 is a low power node, LPN, and the first wireless network node 20 is a macro node.

In one embodiment, the wireless cellular network is a Long Term Evolution, LTE, network. To ensure fair scheduling, in one embodiment the rate metric is further based on duration of time that at least one of the selected wireless communication devices has been waiting in a scheduling queue at the first and second wireless network nodes. An example of an implementation of such an embodiment has been described above.

In an alternative embodiment, the rate metric is further based on a proportional-fair criterion for the selected wireless communication devices. An example of an implementation of such an embodiment has been described above. In a particular embodiment, the wireless network node is configured to select the wireless communication devices based on forming a group comprising one wireless communication device from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, at each scheduling instant.

In another particular embodiment, the wireless network node is configured to select the wireless communication devices based on forming a group comprising one wireless communication device from each of the at least one second set of wireless communication devices, and one out of several wireless communication devices from the first set of wireless communication devices, at each scheduling instant.

In yet another particular embodiment, the wireless network node is configured to select the wireless communication devices based on forming a group comprising one out of several wireless communication devices from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, at each scheduling instant.

In yet another particular embodiment, the wireless network node is configured to select the wireless communication devices based on forming several groups comprising one out of several wireless communication devices from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, to be used respectively at several scheduling instants. Fig. 5 is a schematic diagram illustrating an example of a wireless network node 10 operative to enable scheduling of wireless communication devices in a wireless cellular network according to an embodiment. In this example, the wireless network node 10 basically comprises a processor 1 1 , an associated memory 12 and optional communication circuitry 13. The optional communication circuitry 13 is adapted for wireless and/or wired communication with one or more other nodes, including transmitting and/or receiving information. As indicated in the specific example of Fig. 5, the wireless network node 10 comprises a processor 1 1 and a memory 12, wherein the memory 12 comprises instructions executable by the processor 1 1 to perform operations of the wireless network node 10. Thus, in this example embodiment the processor is operative to select, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co-scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, wherein the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device. As indicated in Fig. 5, the wireless network node 10 may also include communication circuitry 13 for communication with one or more other nodes, including transmitting and/or receiving information. Thus, in a particular embodiment the wireless network node 10 comprises communication circuitry 13 configured to receive link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node. As described above, at least some of the steps, functions, procedures, modules and/or blocks described above may be implemented in software such as a computer program for execution by suitable processing circuitry including one or more processing units.

According to an embodiment, a computer program comprises instructions, which when executed by at least one processor, cause the processor(s) to select, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co-scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, wherein the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device.

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

By way of example, the software or computer program may be realized as a computer program product, which is normally carried or stored on a computer-readable medium, in particular a non-volatile medium. The computer-readable medium may include one or more removable or non-removable memory devices including, but not limited to a Readonly Memory (ROM), a Random Access Memory (RAM), a Compact Disc (CD), a Digital Versatile Disc (DVD), a Blueray disc, a Universal Serial Bus (USB) memory, a Hard Disk Drive (HDD) storage device, a flash memory, a magnetic tape, or any other conventional memory device. The computer program may thus be loaded into the operating memory of a computer or equivalent processing device for execution by the processing circuitry thereof. The flow diagram or diagrams presented above may be regarded as a computer flow diagram or diagrams, when performed by one or more processors. A corresponding wireless network node may be defined as a group of function modules, where each step performed by the processor corresponds to a function module. In this case, the function modules are implemented as a computer program running on the processor. Hence, the wireless network node may alternatively be defined as a group of function modules, where the function modules are implemented as a computer program running on at least one processor.

Hence, the computer program residing in memory may be organized as appropriate function modules configured to perform, when executed by the processor, at least part of the steps and/or tasks described herein. An example of such function modules is illustrated in Fig. 6. Fig. 6 is a schematic block diagram illustrating an example of a wireless network node for enabling scheduling of wireless communication devices in a wireless cellular network. The wireless network node in this example comprises a selecting module 100 configured to select, at each scheduling instant, and based on scheduled transmission rates for the wireless communication devices, at least one first wireless communication device from a first set of wireless communication devices being served by a first wireless network node in the wireless cellular network, and at least one second wireless communication device from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node in the wireless cellular network, and each of the second wireless communication devices being capable of interference cancellation, where the selected wireless communication devices are to be co- scheduled at each respective scheduling instant. The wireless communication devices are selected such that a rate metric for the selected wireless communication devices is maximized over one or more scheduling instants, wherein the rate metric is a combination of achievable rates for the selected wireless communication devices, the achievable rates being based on achievable interference cancellation efficiency of signals from the first wireless network node causing interference at the at least one second wireless communication device. The achievable interference cancellation efficiency is determined based on link quality information from the at least one second wireless communication device representing link quality between the at least one second wireless communication device and the first wireless network node, and scheduled transmission rate for the at least one first wireless communication device. In a particular embodiment, the link quality information comprises channel quality indicator, CQI, information.

sum of achievable rates of said selected wireless communication devices; sum of achievable rates of selected wireless communication devices from said first set of wireless communication devices; sum of achievable rates of selected wireless communication devices from said at least one second set of wireless communication devices;

In one embodiment, the wireless cellular network is a Long Term Evolution, LTE, network.

To ensure fair scheduling, in one embodiment the rate metric is further based on duration of time that at least one of the selected wireless communication devices has been waiting in a scheduling queue at the first and second wireless network nodes. An example of an implementation of such an embodiment has been described above.

In an alternative embodiment, the rate metric is further based on a proportional-fair criterion for the selected wireless communication devices. An example of an implementation of such an embodiment has been described above.

In a particular embodiment, the selecting module 100 is configured to select the wireless communication devices based on forming a group comprising one wireless communication device from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, at each scheduling instant.

In another particular embodiment, the selecting module 100 is configured to select the wireless communication devices based on forming a group comprising one wireless communication device from each of the at least one second set of wireless communication devices, and one out of several wireless communication devices from the first set of wireless communication devices, at each scheduling instant. In yet another particular embodiment, the selecting module 100 is configured to select the wireless communication devices based on forming a group comprising one out of several wireless communication devices from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, at each scheduling instant.

In yet another particular embodiment, the selecting module 100 is configured to select the wireless communication devices based on forming several groups comprising one out of several wireless communication devices from the first set of wireless communication devices, and one out of several wireless communication devices from each of the at least one second set of wireless communication devices, to be used respectively at several scheduling instants.

As described above, there is a need for a rate scheduling coordination algorithm that increases interference cancellation efficiency at the LPN UE and increases the LPN average rate without significantly reducing the macro average rate.

The present embodiments aim to provide an improved scheduling coordination in a wireless network by selecting LPN and macro UEs in a manner that achieves improved cancellation efficiency without having to lower the macro scheduled rate compared to the conventional scheduling.

In some embodiments, at each scheduling instant, a LPN UE and a macro UE is scheduled so as to create UE pairs maximizing a per-pair scheduled rate metric (e.g. sum rate, LPN rate or macro rate), taking into account the IC efficiency at the LPN UE for the different rate and channel conditions combinations. The rate metric may also account for individual UE priorities, accumulated scheduling delays, etc.

Various selection approaches are possible, for example: • Pairing a fixed LPN UE to one of several pending macro UEs,

• Pairing a fixed macro UE to one of several pending LPN UEs,

• Pairing one of several pending LPN UEs to one of several pending macro UEs, · Forming multiple pairs out of several pending LPN UEs and several pending macro UEs.

Variants of the invention are also applicable to handling more than one LPN cells per macro cell, i.e. creating groups of more than 2 UEs.

An advantage of the disclosed embodiments is that rate scheduling coordination and LPN and/or sum rate improvement is achieved without lowering the macro UE rate at each scheduling instant. Since the rate remains the same as without coordination, rate scheduling coordination may be used to improve overall NW capacity.

In contrast, with ICIC and RRS, macro rate is lowered or zeroed without regard to its instantaneous interference impact towards the LPN UE. In conventional rate scheduling, the macro rate is lowered adaptively to enable a LPN rate increase. The invention has been illustrated using the example of a heterogeneous network where a macro cell is the dominant interferer towards one or more LPN cells. The concept of the invention also applies to other deployment scenarios. The LPN cell may then be interpreted more generally as a cell serving victim UEs and the macro cell as an interfering neighbour cell.

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

[1 ] WO2013/1 33751 A3

[2] WO2013/1 33747 A3

[3] TR 25.800, "Technical Report: Study on UMTS heterogeneous networks", v12.1 .0, Dec. 2013

[4] R1 -135782 "Link Performance of RRS in HetNet", Qualcomm, November 2013.

[5] US2014/0135028 A1

Claims

1 . A method performed by a wireless network node (10) for scheduling wireless communication devices (200; 300) in a wireless cellular network, wherein said method comprises:

selecting (S10), at each scheduling instant, and based on scheduled transmission rates for said wireless communication devices (200; 300), at least one first wireless communication device (200) from a first set of wireless communication devices being served by a first wireless network node (20) in said wireless cellular network, and at least one second wireless communication device (300) from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node (30; 40) in said wireless cellular network, and each of said second wireless communication devices (300) being capable of interference cancellation, wherein said selected wireless communication devices are to be co-scheduled at each respective scheduling instant,

such that a rate metric for said selected wireless communication devices is maximized over one or more scheduling instants,

wherein said rate metric is a combination of achievable rates for said selected wireless communication devices, said achievable rates being based on achievable interference cancellation efficiency of signals from said first wireless network node (20) causing interference at said at least one second wireless communication device (300), wherein said achievable interference cancellation efficiency is determined based on link quality information from said at least one second wireless communication device (300) representing link quality between said at least one second wireless communication device (300) and said first wireless network node (20), and scheduled transmission rate for said at least one first wireless communication device (200).

2. The method according to claim 1 , wherein said link quality information comprises channel quality indicator, CQI, information.

3. The method according to claim 1 or 2, wherein said rate metric is calculated as one of:

4. The method according to any of the claims 1 to 3, wherein said wireless cellular network is a heterogeneous network, and wherein each of said respective second wireless network nodes (30; 40) is a low power node, LPN, and said first wireless network node (20) is a macro node.

5. The method according to any of the claims 1 to 4, wherein said rate metric is further based on duration of time that at least one of said selected wireless communication devices has been waiting in a scheduling queue at said first and second wireless network nodes.

6. The method according to any of the claims 1 to 4, wherein said rate metric is further based on a proportional-fair criterion for said selected wireless communication devices.

7. The method according to any of the claims 1 to 6, wherein said selecting is based on forming a group comprising one wireless communication device from said first set of wireless communication devices, and one out of several wireless communication devices from each of said at least one second set of wireless communication devices, at each scheduling instant.

8. The method according to any of the claims 1 to 6, wherein said selecting is based on forming a group comprising one wireless communication device from each of said at least one second set of wireless communication devices, and one out of several wireless communication devices from said first set of wireless communication devices,

5 at each scheduling instant.

9. The method according to any of the claims 1 to 6, wherein said selecting is based on forming a group comprising one out of several wireless communication devices from said first set of wireless communication devices, and one out of several wireless

10 communication devices from each of said at least one second set of wireless communication devices, at each scheduling instant.

10. The method according to any of the claims 1 to 6, wherein said selecting is based on forming several groups comprising one out of several wireless communication

15 devices from said first set of wireless communication devices, and one out of several wireless communication devices from each of said at least one second set of wireless communication devices, to be used respectively at several scheduling instants.

1 1 . The method according to any of the claims 1 to 10, wherein said wireless 20 communication devices (200; 300) are user equipment, UE.

12. The method according to any of the claims 1 to 1 1 , wherein said wireless cellular network is a Long Term Evolution, LTE, network.

25 13. A wireless network node (10) configured to enable scheduling of wireless communication devices (200; 300) in a wireless cellular network, wherein said wireless network node (10) is configured to:

select, at each scheduling instant, and based on scheduled transmission rates for said wireless communication devices (200; 300), at least one first wireless

30 communication device (200) from a first set of wireless communication devices being served by a first wireless network node (20) in said wireless cellular network, and at least one second wireless communication device (300) from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node (30; 40) in said wireless cellular network, and each of said second wireless communication devices (300) being capable of interference cancellation, wherein said selected wireless communication devices are to be co-scheduled at each respective scheduling instant, such that a rate metric for said selected wireless communication devices is maximized over one or more scheduling instants,

14. The wireless network node (10) of claim 13, wherein said link quality information comprises channel quality indicator, CQI, information.

15. The wireless network node (10) of claim 13 or 14, wherein said rate metric is calculated as one of:

16. The wireless network node (10) of any of the claims 13 to 15, wherein said wireless cellular network is a heterogeneous network, and wherein each of said respective second wireless network nodes (30; 40) is a low power node, LPN, and said first wireless network node (20) is a macro node.

5

17. The wireless network node (10) according to any of the claims 13 to 16, wherein said rate metric is further based on duration of time that at least one of said selected wireless communication devices has been waiting in a scheduling queue at said first and second wireless network nodes.

10

18. The wireless network node (10) according to any of the claims 13 to 16, wherein said rate metric is further based on a proportional-fair criterion for said selected wireless communication devices.

15 19. The wireless network node (10) of any of the claims 13 to 18, wherein said wireless network node (10) is configured to select said wireless communication devices based on forming a group comprising one wireless communication device from said first set of wireless communication devices, and one out of several wireless communication devices from each of said at least one second set of wireless communication devices,

20 at each scheduling instant.

20. The wireless network node (10) of any of the claims 13 to 18, wherein said wireless network node (10) is configured to select said wireless communication devices based on forming a group comprising one wireless communication device from each of said

25 at least one second set of wireless communication devices, and one out of several wireless communication devices from said first set of wireless communication devices, at each scheduling instant.

21 . The wireless network node (10) of any of the claims 13 to 18, wherein said wireless 30 network node (10) is configured to select said wireless communication devices based on forming a group comprising one out of several wireless communication devices from said first set of wireless communication devices, and one out of several wireless communication devices from each of said at least one second set of wireless communication devices, at each scheduling instant.

22. The wireless network node (10) of any of the claims 13 to 18, wherein said wireless network node (10) is configured to select said wireless communication devices based on forming several groups comprising one out of several wireless communication devices from said first set of wireless communication devices, and one out of several wireless communication devices from each of said at least one second set of wireless communication devices, to be used respectively at several scheduling instants.

23. The wireless network node (10) of any of the claims 13 to 22, wherein said wireless cellular network is a Long Term Evolution, LTE, network.

24. The wireless network node (10) of any of the claims 13 to 23, wherein said wireless network node (10) comprises a processor (1 1 ) and a memory (12), said memory (12) comprising instructions executable by said processor (1 1 ), whereby said processor (1 1 ) is operative to:

select, at each scheduling instant, and based on scheduled transmission rates for said wireless communication devices (200; 300), at least one first wireless communication device (200) from a first set of wireless communication devices being served by a first wireless network node (20) in said wireless cellular network, and at least one second wireless communication device (300) from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node (30; 40) in said wireless cellular network, and each of said second wireless communication devices (300) being capable of interference cancellation, wherein said selected wireless communication devices are to be co-scheduled at each respective scheduling instant, such that a rate metric for said selected wireless communication devices is maximized over one or more scheduling instants,

25. The wireless network node (10) of any of the claims 13 to 24, wherein said wireless network node (10) comprises communication circuitry (13) configured to receive said link quality information from said at least one second wireless communication device (300) representing link quality between said at least one second wireless communication device (300) and said first wireless network node (20).

26. A computer program comprising instructions, which when executed by at least one processor, cause the processor or processors to:

27. A carrier comprising the computer program of claim 26, wherein the carrier is one of an electronic signal, an optical signal, an electromagnetic signal, a magnetic signal, an electric signal, a radio signal, a microwave signal, or a computer-readable storage medium.

28. A wireless network node (10) for enabling scheduling of wireless communication devices (200; 300) in a wireless cellular network, wherein said wireless network node comprises:

a selecting module (100) configured to select, at each scheduling instant, and based on scheduled transmission rates for said wireless communication devices (200; 300), at least one first wireless communication device (200) from a first set of wireless communication devices being served by a first wireless network node (20) in said wireless cellular network, and at least one second wireless communication device (300) from at least one second set of wireless communication devices, each second set of wireless communication devices being served by a respective second wireless network node (30; 40) in said wireless cellular network, and each of said second wireless communication devices (300) being capable of interference cancellation, wherein said selected wireless communication devices are to be co-scheduled at each respective scheduling instant, such that a rate metric for said selected wireless communication devices is maximized over one or more scheduling instants,

29. The wireless network node (10) of claim 28, wherein said link quality information 5 comprises channel quality indicator, CQI, information.

30. The wireless network node (10) of claim 28 or 29, wherein said rate metric is calculated as one of:

sum of achievable rates of said selected wireless communication devices; 10 sum of achievable rates of selected wireless communication devices from said first set of wireless communication devices;

average of achievable rates of said selected wireless communication devices; 15 average of achievable rates of selected wireless communication devices from said first set of wireless communication devices; or

20 31 . The wireless network node (10) of any of the claims 28 to 30, wherein said wireless cellular network is a heterogeneous network, and wherein each of said respective second wireless network nodes (30; 40) is a low power node, LPN, and said first wireless network node (20) is a macro node.

25 32. The wireless network node (10) according to any of the claims 28 to 31 , wherein said rate metric is further based on duration of time that at least one of said selected wireless communication devices has been waiting in a scheduling queue at said first and second wireless network nodes.

30 33. The wireless network node (10) according to any of the claims 28 to 31 , wherein said rate metric is further based on a proportional-fair criterion for said selected wireless communication devices.

34. The wireless network node (10) of any of the claims 28 to 33, wherein said selecting module (100) is configured to select said wireless communication devices based on forming a group comprising one wireless communication device from said first set of wireless communication devices, and one out of several wireless

5 communication devices from each of said at least one second set of wireless communication devices, at each scheduling instant.

35. The wireless network node (10) of any of the claims 28 to 33, wherein said selecting module (100) is configured to select said wireless communication devices

10 based on forming a group comprising one wireless communication device from each of said at least one second set of wireless communication devices, and one out of several wireless communication devices from said first set of wireless communication devices, at each scheduling instant.

15 36. The wireless network node (10) of any of the claims 28 to 33, wherein said selecting module (100) is configured to select said wireless communication devices based on forming a group comprising one out of several wireless communication devices from said first set of wireless communication devices, and one out of several wireless communication devices from each of said at least one second set of wireless

20 communication devices, at each scheduling instant.

37. The wireless network node (10) of any of the claims 28 to 33, wherein said selecting module (100) is configured to select said wireless communication devices based on forming several groups comprising one out of several wireless 25 communication devices from said first set of wireless communication devices, and one out of several wireless communication devices from each of said at least one second set of wireless communication devices, to be used respectively at several scheduling instants.

30 38. The wireless network node (10) of any of the claims 28 to 37, wherein said wireless cellular network is a Long Term Evolution, LTE, network.