WO2023280420A1

WO2023280420A1 - Controlling ue rank reporting

Info

Publication number: WO2023280420A1
Application number: PCT/EP2021/069126
Authority: WO
Inventors: Petter ERSBO; David Astely; Antonios PITAROKOILIS
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2021-07-09
Filing date: 2021-07-09
Publication date: 2023-01-12

Abstract

The present disclosure relates to a method of a radio base station (101) of controlling rank reporting of a wireless communication device (103), and a radio base station (101) performing the method. The present disclosure further relates to a method of a wireless communication device (103) of reporting rank, and a wireless communication device (103) performing the method.In an aspect, a method of a radio base station (101) of controlling rank reporting of a wireless communication device (103) is provided. The method comprises determining (S101), from information indicating a property of interference that the wireless communication device (103) is subjected to, a rank indicator to be applied at the wireless communication device (103) upon determining channel state information, instructing (S102) the wireless communication device (103) to apply the determined rank indicator upon determining channel state information and receiving (S104), from the wireless communication device (103), the determined channel state information.

Description

CONTROLLING UE RANK REPORTING TECHNICAL FIELD

[0001] The present disclosure relates to a method of a radio base station of controlling rank reporting of a wireless communication device, and a radio base station performing the method. The present disclosure further relates to a method of a wireless communication device of reporting rank, and a wireless communication device performing the method.

BACKGROUND

[0002] To enable efficient downlink data transmission in a 5^th generation (5G) New Radio (NR) communication system, a radio base station (RBS) transmitting the downlink data requires channel state information (CSI). This information is generally obtained at the RBS by sending reference signals, such as a channel state information reference signal (CSI-RS), to a wireless communication device in reply to which the wireless communication device measures quality of a channel established with the RBS and includes the measured channel quality in a CSI report transmitted in uplink to the RBS.

[0003] Information comprised in the CSI report includes rank indicator (RI), precoding matrix indicator (PMI) and channel quality indicator (CQI). This information is used by the RBS to schedule downlink data transmission to the wireless communication device.

[0004] The RI is utilized by a wireless communication device being equipped with multiple receive (RX) antennas to recommend to the RBS on how many so-called layers the RBS is to transmit data to the wireless communication device, i.e. the more layers the higher the potential throughput.

[0005] The PMI specifies how beamforming is to be applied by the RBS and is dependent on the RI, while the CQI as the name implies generally specifies the quality of the channel and is dependent on both the RI and the PMI.

[0006] Now, if the wireless communication device overestimates the RI, which has as a further consequence that both the PMI and the CQI becomes incorrect, the RBS will (given that the RBS applies the RI, PMI and CQI reported by the wireless communication device) schedule the downlink transmission based on the overestimated - and potentially incorrect - RI and the resulting poorly estimated PMI and CQI. The RBS will thus transmit more data than the channel can support, resulting in data loss, which effectively reduces the throughput.

SUMMARY

[0007] One objective is to solve, or at least mitigate, this problem in the art and thus to provide a method of controlling rank reporting of a wireless communication.

[0008] This objective is attained in a first aspect by a method of a radio base station of controlling rank reporting of a wireless communication device. The method comprises determining, from information indicating a property of interference that the wireless communication device is subjected to, a rank indicator to be applied at the wireless communication device upon determining channel state information, instructing the wireless communication device to apply the determined rank indicator upon determining channel state information and receiving, from the wireless communication device, the determined channel state information.

[0009] This objective is attained in a second aspect by a radio base station configured to control rank reporting of a wireless communication device, said radio base station comprising a processing unit and a memory, said memory containing instructions executable by said processing unit, whereby the radio base station is operative to determine, from information indicating a property of interference that the wireless communication device is subjected to, a rank indicator to be applied at the wireless communication device upon determining channel state information, to instruct the wireless communication device to apply the determined rank indicator upon determining channel state information and to receive, from the wireless communication device, the determined channel state information.

[0010] This objective is attained in a third aspect by a method of a wireless communication device of reporting rank. The method comprises receiving an instruction from a radio base station to apply a rank indicator upon reporting channel state information, determining channel state information by applying the rank indicator received with the instruction and reporting the determined channel state information to the radio base station.

[0011] This objective is attained in a fourth aspect by a wireless communication device configured to report rank, said wireless communication device comprising a processing unit and a memory, said memory containing instructions executable by said processing unit, whereby the wireless communication device is operative to receive an instruction from a radio base station to apply a rank indicator upon reporting channel state information, to determine channel state information by applying the rank indicator received with the instruction and to report the determined channel state information to the radio base station.

[0012] Advantageously, with embodiments disclosed herein, the radio base station is capable to attain more relevant and accurate CSI measurements. By selecting and recommending an adequate RI based on interference experienced at the wireless communication device, an aggressive - and undesired - increase in transmission layers and thus data transmission rate as a result of a high rank can be avoided in a scenario where the wireless communication device erroneously underestimates interference when performing CSI measurements. An aggressive increase of the transmission rate will likely lead to a large number of incorrectly received data transmissions and consequently to throughput degradation.

[0013] Hence, the radio base station will advantageously detect that it is likely that the interference measured by the wireless communication device is significantly lower than the interference that it actually is subjected to, and therefore provide rank indicator feedback so that the transmission becomes better adapted to actual interference conditions.

[0014] In an embodiment, the radio base station requests channel state information from the wireless communication device and receives the requested channel state information from the wireless communication device comprising at least a current rank indicator of the wireless communication device, wherein the determining, from information indicating at least one property of interference that the wireless communication device is subjected to, a rank indicator to be applied at the wireless communication device upon determining channel state information comprises determining that the current rank indicator is higher than what is motivated based on said information and that the rank indicator to be applied at the wireless communication device should be lower than the current rank indicator.

[0015] In an embodiment, the determining of a rank indicator comprises determining a plurality of rank indicators, one of which is selected by the wireless communication device and applied upon determining channel state information. [0016] In an embodiment, the channel state information is configured to comprise a PMI and a CQI.

[0017] In an embodiment, the information indicating a property of interference that the wireless communication device is subjected to includes one or more of block error rate (BLER) for the communication link with the wireless communication device, intra-cell or inter-cell interference experienced by the wireless communication device, path gain to a neighbouring radio base station, degree of back-off in outer loop link adaptation for communication with the device, cell specific reference signals, CRS, transmissions occurring in the serving or in neighbouring base radio base stations, low traffic load in cells served by neighbouring radio base stations, low traffic periods and/or low interference level periods identified from historic statistical traffic data of serving and/or neighbouring radio base stations.

[0018] Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which:

[0020] Figure 1 illustrates a wireless communications network in which embodiments may be implemented;

[0021] Figure 2 shows a signalling diagram illustrating a method performed by a radio base station of controlling rank reporting of a wireless communication device according to an embodiment;

[0022] Figure 3 shows a signalling diagram illustrating a method performed by a radio base station of controlling rank reporting of a wireless communication device according to a further embodiment; [0023] Figure 4 illustrates prior art throughout degradation and improvement provided according to an embodiment;

[0024] Figure 5 illustrates resource allocation in a slot for dynamic spectrum sharing;

[0025] Figure 6 illustrates resource allocation for dynamic spectrum sharing in a multicast-broadcast single-frequency network subframe;

[0026] Figure 7 illustrates a radio base station according to an embodiment; and

[0027] Figure 8 illustrates a wireless communication device configured to report rank according to an embodiment.

DETAILED DESCRIPTION

[0028] The aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown.

[0029] These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like numbers refer to like elements throughout the description.

[0030] Figure 1 illustrates a wireless communications network 100 in which embodiments may be implemented, which network comprises a radio base station (RBS) 101 equipped with an antenna system 102, and a group of wireless communication devices 103-107 served by the RBS 101. The wireless communication devices 103-107 are commonly referred to as User Equipment (UE). It is noted that in practice, an RBS may serve hundreds or even thousands of UEs in cell 108. Further shown is a neighbouring RBS 109 providing coverage in neighbouring cell no (comprising a number of not shown UEs).

[0031] In 3^rd generation (3G) Universal Mobile Telecommunications System (UMTS), the RBS is typically referred to as a NodeB, in 4^th generation (4G) Long Term Evolution (LTE), the RBS is typically referred to as an Evolved Node B (eNodeB), while in 5^th generation (5G) New Radio (NR), the RBS is typically referred to as a gNodeB (“Next Generation NodeB”) or gNB. [0032] The UEs may be embodied e.g. by smart phones, tablets, gaming consoles, connected vehicles, etc.

[0033] In the wireless communications network 100, the RBS 101 may be equipped with an antenna system 102 comprising a plurality of transmit (TX) antennas for downlink transmission of data to the UEs 103-107, while the UEs 103- 107 comprises multiple receive (RX) antennas for receiving the transmitted data.

Such a multi-antenna system is commonly referred to as a Multiple Input Multiple Output (MIMO) system.

[0034] For exemplifying purposes, the antenna system 102 of the RBS 101 utilizes up to 64 TX antennas upon transmitting data in the downlink, while the first UE 103 is equipped with up to 4 RX antennas 103a over which it receives the data sent by the RBS 101.

[0035] The RBS can use weights associated with the PMI as beamforming weights for the number of layers indicated by the RI.

[0036] In the following, the system will be described in a 5G context (even though the embodiments to be described herein is applicable to any multi-antenna communication system), and the RBS 101 will thus be referred to as a gNB.

[0037] As described, the gNB 101 performs control signalling to request channel state information (CSI) from each UE. This is then enabled by sending reference signals, such as a channel state information reference signal (CSI-RS), to a UE in reply to which the UE measures quality of a channel established with the gNB 101 and includes an indication of the measured channel quality in a CSI report transmitted in uplink to the gNB 101. Oftentimes, the CSI-RS is shared by many if not all UEs 103- 107 in the cell 108.

[0038] Information comprised in the CSI report includes rank indicator (RI), precoding matrix indicator (PMI) and channel quality indicator (CQI) and is used by the gNB 101 to schedule and adapt downlink data transmission to the UE. As is understood, the RI, PMI and CQI reported by the UEs are recommendations to the gNB 101, and the gNB 101 may apply different values of the RI, PMI and CQI than those reported by the UEs.

[0039] For instance, upon establishing a communication channel with the first UE 103, the gNB 101 requests a CSI report from the first UE 103, which replies by sending a CSI report including the RI (as well as the PMI, CQI and other information required by the gNB 101).

[0040] In this example, the recommended rank of the first UE 103 being equipped with four RX antennas 103a - as indicated by the RI - is either RANKi, RANK2, RANK3 or RANK 4 (i.e. either o, 1, 2 or 3, where the side being equipped with the fewest antennas stipulates the maximum rank, in this case the UE). Now, assuming that the first UE 103 measures the channel and interference level using the CSI-RS and concludes that RANK4 is motivated; the first UE 103 will then report RI=3.

[0041] It is noted that all four RX antennas typically are used for receiving data transmitted in the downlink by the gNB 101 regardless of reported rank. That is, even if RANKi would be recommended by the UE 103 and the gNB 101 accordingly will comply with the recommendation and transmit only a single layer of data with the 64 TX antennas, the UE 103 would still receive the data via its four RX antennas and combine the data appropriately for further processing.

[0042] The gNB 101 will in accordance with the reported RI transmit four layers of data in the downlink via all its TX antennas 102 for reception by the first UE 103 via all its RX antennas 103a.

[0043] As discussed, if the first UE 103 overestimates the RI - and reports RANK4 instead of a rank which would have been motivated in view of a more reliable estimate of the interference that the first UE 103 in fact is subjected to such as e.g. RANK2 - the gNB 101 will schedule data packets for downlink transmission over four layers (instead of over the motivated two layers) via its TX antennas 102. This may have as an affect that transmitted data is not correctly received at the first UE 103 due to interference among the signals being received via the RX antennas 103a, which may result in lower throughput and thus worse performance as compared to transmission over two layers. As previously mentioned, the PMI and the CQI are both further dependent on the RI, so any poorly estimated rank will further result in the gNB 101 making incorrect decisions based on the poorly estimated and reported PMI and CQI being dependent on the reported RI. Thus, not only will the number of transmitted layers be too high as stipulated by the RI reported by the first UE 103, but the beamforming based on the likewise incorrectly estimated reported PMI will also be inferior, as will the MCS selected based on the incorrectly estimated CQI. Also, if the gNB tries to override the RI and transmit with a lower rank, then the PMI and CQI are not valid.

[0044] Figure 2 shows a signalling diagram illustrating a method performed by the gNB 101 of controlling rank reporting of the UEs according to an embodiment, in this embodiment exemplified by the first UE 103.

[0045] The gNB 101 determines in step S101, from information indicating at least one property of interference that the first UE 103 is subjected to, an RI to be applied at the first UE 103 in view of the interference information. The property of the interference may for instance be level of interference, type of interference, timing for the interference, etc.

[0046] For instance, the gNB 101 may measure, or receive from another entity such as a core network entity having access to the information, values of block error rate (BLER), which indicates amount of erroneously received data at the first UE 103 in respect of total of amount of data transmitted by the gNB 101 to the first UE 103. A high BLER indicates that the first UE 103 indeed is subjected to interference (even though the first UE 103 itself may measure a low interference and thus it is at risk of reporting too high RI).

[0047] In another example, the gNB 101 acquires information indicating path gain of the first UE 103 in the serving cell 108 and to neighbouring gNBs 109 and possibly gNB transmit power and duty cycle and the occurrence of any LTE CRS interference. If the path gain to the neighbouring gNB 109 is high relative to the path gain to the serving gNB 101 (such as over a threshold value) and the neighbouring gNB 109 causes LTE CRS interference, then the gNB 101 may determine that the RI of the first UE 103 should be restricted.

[0048] For instance, the gNB 101 may determine in step S101 that given the interference that the first UE 103 is indicated to experience based on the acquired information, RANKi or possibly RANK2 is recommended (but hence not RANK 3 or RANK 4). This may be performed even before the first UE 103 indeed has reported CSI.

[0049] In step S102, the gNB 101 instructs (using control signalling) the first UE 103 to apply the determined RI, i.e. giving the first UE 103 the recommendation to use either RANKi or RANK 2 (or either one of RANKi and RANK2). [0050] The first UE 103 will upon receiving the instruction to use RANK2 or RANKi apply any one of the two determined RIs in step S103 by estimating the PMI and the CQI using e.g. RANK2, and further adjust any other settings or estimate parameters at the first UE 103 which are related to the determined RI.

[0051] The first UE 103 will then in step S104 report CSI to the gNB 101, i.e. the PMI and CQI estimated using RANK2. In a scenario where the first UE 103 was given more than one RI option in step S102, the CSI report of the first UE 103 will also include the selected RI option, as the gNB 101 otherwise cannot know which one of the multiple RI options the first UE 103 chose in step S103.

[0052] For feedback-based precoding, transmission rank and precoder selection at the gNB 101 is generally chosen based on the RI and PMI reported by the respective UE 103-107. Modulation and coding scheme (MCS) is then selected based on the reported CQI. To control the transmission in order to attain a desired BLER, the MCS is then adjusted based on outer loop link adaptation. For example, the first UE 103 may report RI=i, PMI=i and CQI=io. The gNB 101 may then decide to transmit with RANK2 and the precoder corresponding to PMI 1 and choose an MCS setting of, say, “20”. Then, if the current average BLER is higher than desired, the MCS selection may be backed off, and an MCS setting of “19” is selected instead, and so on. In other words, if inaccurate CSI reports are supplied to the gNB 101, the gNB 101 needs to perform an inefficient process of elaborating different settings to arrive at a high-quality data transmission. If the CSI report is incorrect, the gNB 101 may have problems selecting beamforming weights.

[0053] Advantageously, the above described embodiment enables the gNB 101 to attain more relevant and accurate CSI measurements. By selecting and recommending an adequate RI based on interference experienced at the UE 103, an aggressive - and undesired - increase in transmission layers and thus data transmission rate as a result of a high rank can be avoided in a scenario where the first UE 101 erroneously underestimates interference when performing CSI measurements. An aggressive increase of the transmission rate will likely lead to a large number of incorrectly received data transmissions and consequently to throughput degradation.

[0054] Hence, the gNB 101 will advantageously detect that it is likely that the interference measured by the UE 103 is significantly lower than the interference caused to the data being transmitted by the gNB 101 and received by the UE 103, and therefore provide feedback so that the transmission becomes better adapted to actual interference conditions.

[0055] Figure 3 shows a signalling diagram illustrating a method performed by the gNB 101 of controlling rank reporting of the UEs according to a further embodiment.

[0056] In this embodiment, a CSI report is requested by the gNB 101 in step Siooa, Thus, in first step Siooa, the gNB 101 acquires information of the first UE 103 comprising a current RI of the UE 103. As has been described, this maybe embodied by the gNB 101 sending by means of control signalling a request for CSI to the first UE 103 in the form of a CSI-RS, to which the first UE 103 replies in step Sioob by sending a CSI report comprising estimated RI, PMI and CQI. As is understood, steps Siooa and Sioob are steps also performed in the art upon performing the CSI reporting.

[0057] However, in this embodiment, the gNB 101 determines in step S103, from information indicating a property of interference that the first UE 103 is subjected to, whether the current rank of the first UE 103 as represented by the RI in CSI report is indicated to be too high and thus should be decreased to avoid packet loss at the first UE 103. In other words, the gNB 101 makes an assessment in step S103 if the current rank is higher than a value deemed motivated in view of the interference level information.

[0058] The information indicating a level of interference that the first UE 103 is subjected to may have been acquired by the gNB long before the CSI reporting of steps Siooa and Sioob is initiated.

[0059] In response to the determination of the gNB 101 in step S103 that the reported RI likely is too high in view of actual interference conditions, the gNB 101 sends an instruction in step S104 to the first UE 103 to lower the currently set rank.

[0060] As an example, if the first UE 103 reported RANK4 (i.e. RI=3) in step Sioob, the gNB 101 instructs the first UE 103 in step S102 to e.g. decrease the rank to RANK3, or gives the first UE 103 the option the select any one of RANK1-RANK3, or to just lower the RI. [0061] The first UE 103 will upon receiving the instruction thus decrease the maximum rank from e.g. RANK4 to RANK3 in step S103 and apply the lower rank. As discussed, the first UE 103 will estimate the PMI and the CQI, which both are dependent on the new lower rank and further adjust any other settings or parameters at the first UE 103 which are related to the lower rank.

[0062] The gNB 101 further receives updated CSI in step S104 as previously described.

[0063] Advantageously, the above described embodiment, enables the gNB 101 to attain more relevant and accurate CSI measurements. By restricting the set of allowed RIs, the aggressive - and undesired - increase in transmission rate as a result of a high rank can be avoided when the first UE 101 erroneously underestimates interference when performing CSI measurements/estimates. An aggressive increase of the transmission rate leads to, in case the estimated RI is too high, a large number of incorrectly received data transmissions and consequently to throughput degradation.

[0064] This is illustrated in Figure 4, where the solid curve illustrates prior art showing that degradation occurs at a throughput of about 17 Mpbs in an NR physical downlink shared channel (PDSCH) with a signal-to-noise ratio (SNR) of 10 dB and a signal-to-interference ratio (SIR) for some resource elements of -3 dB at the first UE 103.

[0065] As further shown in Figure 4, by restricting the set of allowable ranks at the UE, such degradation is avoided as shown by the dashed curve.

[0066] In a particular type of deployment known as dynamic spectrum sharing (DSS), LTE and NR co-exist on a single carrier. While NR is flexible and relatively lean with few always-on signals, LTE is less flexible and requires cell specific reference signal (CRS) transmission in all subframes (a subframe in LTE corresponds to a slot in NR). Further, LTE signals may cause interference to NR signals.

[0067] Figure 5 illustrates a possible resource allocation in a slot for DSS (with four-port CRS). NR PDSCH is rate-matched around LTE CRS, i.e. not transmitted on resource elements occupied by the serving cell’s CRS. [0068] In the DSS slot of Figure 5, in addition to already described abbreviations, LTE control signalling is denoted “LC”, while and NR control signalling is denoted “NC” and transferred demodulation reference signals are denoted “DMRS”.

[0069] Resource elements that may be affected by CRS interference are marked “X”. Empty resource elements and resource elements marked “X” can be used for LTE or NR data transmission.

[0070] To minimize the performance impact of DSS on LTE, the CSI-RS and CSI interference measurement (CSI-IM) signals may be configured in multicast- broadcast single-frequency network (MBSFN) subframes shared with other periodic NR signals such as e.g. synchronization signal blocks (SSB), system information blocks (SIB) and tracking reference signals (TRS). MBSFN subframes have fewer CRS than regular LTE subframes.

[0071] Figure 6 illustrates a possible CSI-RS and -IM configuration in an MBSFN subframe. Due to the absence of LTE data and CRS that could interfere with the resource elements used for CSI-RS and -IM, the CSI measurement may not reflect the transmission conditions for data. The CSI-RS and -IM could be configured in a non- MBSFN subframe, but then that slot can only be used for low rate LTE data transmission that can tolerate puncturing by CSI-RS and -IM as LTE data cannot be rate-matched around NR CSI-RS and -IM.

[0072] Thus, in the general case, the NR CSI measurement configuration may not capture interference from certain sources, due to the measurement resource allocation and the structure of the interference. This issue is common in DSS deployments, where the LTE CRS intercell interference impacts NR PDSCH reception but is difficult to measure.

[0073] If network load is low, the data interference - i.e. the interference to the empty resource elements in Figure 5 - may be very low, while there may be significant interference in the resource elements marked “X” due to CRS interference. Thus, the UEs 103-107 may determine from channel estimates based on CSI-RS and -IM that SINR is high and thus the interference low and as a result recommend a high RI to the gNB 101. To the contrary, the gNB 101 may conclude that one or more UEs in fact suffers from significant interference in the form of the CRS interference and thus instruct such UE to decrease the RI, i.e. to restrict the set of allowable ranks. [0074] At low load in the serving cell 108, for instance being below a load threshold, inaccurate interference measurement means that the estimated NR PDSCH SINR is higher than its true value. This leads to rank reporting from the NR UEs that is too aggressive, as discussed above. The selected rank however cannot be supported by the channel and the throughput is reduced. The throughput degradation is illustrated in Figure 4, where the throughput of the existing (legacy) solution is plotted. At higher load, data interference is more likely to impact the CSI reporting, and the problem with overly optimistic RIs becomes a bit less severe.

[0075] To minimize impact on LTE from DSS, the CSI measurements may be performed in MBSFN subframes, where there are no LTE data transmissions. This configuration for the CSI measurements means that the CSI measurement may be optimistic even with higher LTE load, as there are no LTE data transmissions that can interfere it, again resulting in overly aggressive rank reporting. This problem is especially pronounced in early DSS deployments, where the LTE load is high and the NR load low, as LTE then is the main source of interference.

[0076] As has been discussed hereinabove, it is an inefficient process to adjust RI, PMI and CQI if the UE reports a rank which is higher than what is motivated by the channel conditions. In an embodiment, the RI reported by a UE is used and the MCS selection is adjusted based on outer loop link adaption; it is not trivial to choose a RANKi precoder from e.g. a PMI indicating a RANK2 precoder.

[0077] In an embodiment, the gNB 101 acquires information indicating a property of interference that the first UE 103 is subjected to in that the gNB 101 is aware of recent mobility measurements reported from the first UE 103.

[0078] The interference property may be embodied in the form of high path gains to neighbouring gNBs supporting LTE or DSS, which commonly causes CRS interference as illustrated in Figure 5 to occur - resulting in inaccurate CSI measurements at the UE 103 - since the first UE 103 does not detect this type of interference.

[0079] As a result, the gNB 101 may conclude that a strong path gain relative to a neighbouring gNB indicates a too high RI reported by the first UE 103, since there is a risk that strong CRS interference is present but not captured by the CSI measurements of the UE 103. [0080] In a further embodiment, the gNB 101 restricts the rank reporting for the first UE 103 in DSS deployments if the back-off in the outer loop link adaption is higher than a given threshold value selected to give a desired trade-off between robustness and performance. A high back-off in the outer loop link adaption indicates that the CSI reports do not reflect the channel conditions well, and hence that the rank report may be optimistic. If separate outer loops are used per rank, rank reports may be restricted to only those ranks that have an outer loop back-off above some threshold.

[0081] In yet a further embodiment, the gNB 101 shares statistical data of its data traffic with neighbouring gNBs. The data is analysed so that low traffic periods and/or low interference level periods can be identified. During these periods it is likely that the interference estimates of the UEs are overoptimistic, as previously discussed with reference to low network load.

[0082] Further, known topology of the neighbouring gNBs can be exploited so that an a priori estimate of the interference level from always-on signals, i.e., LTE CRS signals, can be obtained. The combination of the traffic statistics with the gNB topology can provide an estimate of a transmission rate threshold due to intercell interference. When the CSI reports appear to be too aggressive so that the requested transmission rate exceeds or is very close to this rate threshold, the RI is restricted.

[0083] Figure 7 illustrates an RBS 101 configured to control rank reporting of a UE according to an embodiment, where the steps of the method performed by the RBS 101 in practice are performed by a processing unit 111 embodied in the form of one or more microprocessors arranged to execute a computer program 112 downloaded to a storage medium 113 associated with the microprocessor, such as a Random Access Memory (RAM), a Flash memory or a hard disk drive. The processing unit 111 is arranged to cause the RBS 101 to carry out the method according to embodiments when the appropriate computer program 112 comprising computer- executable instructions is downloaded to the storage medium 113 and executed by the processing unit 111. The storage medium 113 may also be a computer program product comprising the computer program 112. Alternatively, the computer program 112 may be transferred to the storage medium 113 by means of a suitable computer program product, such as a Digital Versatile Disc (DVD) or a memory stick. As a further alternative, the computer program 112 may be downloaded to the storage medium 113 over a network. The processing unit 111 may alternatively be embodied in the form of a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a complex programmable logic device (CPLD), etc. The RBS 101 further comprises a communication interface 114 (wired and/or wireless) over which the RBS 101 is configured to transmit and receive data.

[0084] Figure 8 illustrates an UE 103 configured to report rank according to an embodiment, where the steps of the method performed by the UE 103 in practice are performed by a processing unit 211 embodied in the form of one or more microprocessors arranged to execute a computer program 212 downloaded to a storage medium 213 associated with the microprocessor, such as a RAM, a Flash memory or a hard disk drive. The processing unit 211 is arranged to cause the UE 103 to carry out the method according to embodiments when the appropriate computer program 212 comprising computer-executable instructions is downloaded to the storage medium 213 and executed by the processing unit 211. The storage medium 213 may also be a computer program product comprising the computer program 212. Alternatively, the computer program 212 maybe transferred to the storage medium 113 by means of a suitable computer program product, such as a DVD or a memory stick. As a further alternative, the computer program 212 maybe downloaded to the storage medium 213 over a network. The processing unit 211 may alternatively be embodied in the form of a DSP, an ASIC, an FPGA, a CPLD, etc. The UE 103 further comprises a communication interface 214 over which the UE 103 is configured to transmit and receive data.

[0085] The aspects of the present disclosure have mainly been described above with reference to a few embodiments and examples thereof. 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 invention, as defined by the appended patent claims.

[0086] Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

Claims

1. A method of a radio base station (101) of controlling rank reporting of a wireless communication device (103), comprising: determining (S101), from information indicating a property of interference that the wireless communication device (103) is subjected to, a rank indicator to be applied at the wireless communication device (103) upon determining channel state information; instructing (S102) the wireless communication device (103) to apply the determined rank indicator upon determining channel state information; and receiving (S104), from the wireless communication device (103), the determined channel state information.

2. The method of claim 1, further comprising: requesting (Siooa) channel state information from the wireless communication device (103); and receiving (Sioob) the requested channel state information from the wireless communication device (103) comprising at least a current rank indicator of the wireless communication device (103); wherein the determining (S101), from information indicating at least one property of interference that the wireless communication device (103) is subjected to, a rank indicator to be applied at the wireless communication device (103) upon determining channel state information comprises: determining that the current rank indicator is higher than what is motivated based on said information and that the rank indicator to be applied at the wireless communication device (103) should be lower than the current rank indicator.

3. The method of claims 1 or 2, wherein the determining (S101) of a rank indicator comprises determining a plurality of rank indicators, one of which is selected by the wireless communication device (103) and applied upon determining channel state information.

4. The method of any one of the preceding claims, wherein the channel state information is configured to comprise a precoding matrix indicator, PMI, and a channel quality indicator, CQI.

5. The method of any one of the preceding claims, wherein the information indicating a property of interference that the wireless communication device (103) is subjected to includes one or more of block error rate, BLER, for the communication link with the wireless communication device (103), intra-cell or inter-cell interference experienced by the wireless communication device (103), path gain to a neighbouring radio base station, degree of back-off in outer loop link adaptation for communication with the device (103) , cell specific reference signals, CRS, transmissions occurring in the serving or in neighbouring base radio base stations, low traffic load in cells served by neighbouring radio base stations (108), low traffic periods and/or low interference level periods identified from historic statistical traffic data of serving and/ or neighbouring radio base stations.

6. A method of a wireless communication device (103) of reporting rank, comprising: receiving (S102) an instruction from a radio base station (101) to apply a rank indicator upon reporting channel state information; determining (S103) channel state information by applying the rank indicator received with the instruction; and reporting (S104) the determined channel state information to the radio base station (101).

7. The method of claim 6, wherein the receiving (S102) of an instruction comprises receiving a plurality of rank indicators, one of which is selected and applied upon determining channel state information.

8. The method of claims 6 or 7, wherein the reported channel state information is configured to comprise a precoding matrix indicator, PMI, a channel quality indicator, CQI.

9. A computer program (112) comprising computer-executable instructions for causing a radio base station (101) to perform steps recited in any one of claims 1-5 when the computer-executable instructions are executed on a processing unit (111) included in the radio base station (101).

10. A computer program product comprising a computer readable medium (113), the computer readable medium having the computer program (112) according to claim 9 embodied thereon.

11. A computer program (212) comprising computer-executable instructions for causing a wireless communication device (103) to perform steps recited in any one of claims 6-8 when the computer-executable instructions are executed on a processing unit 2111) included in the wireless communication device (103).

12. A computer program product comprising a computer readable medium (213), the computer readable medium having the computer program (212) according to claim 11 embodied thereon.

13. A radio base station (101) configured to control rank reporting of a wireless communication device (103), said radio base station (101) comprising a processing unit (111) and a memory (113), said memory containing instructions (112) executable by said processing unit (111), whereby the radio base station (101) is operative to: determine, from information indicating a property of interference that the wireless communication device (103) is subjected to, a rank indicator to be applied at the wireless communication device (103) upon determining channel state information; instruct the wireless communication device (103) to apply the determined rank indicator upon determining channel state information; and to receive, from the wireless communication device (103), the determined channel state information.

14. The radio base station (101) of claim 13, further being operative to: request channel state information from the wireless communication device

(103); and receive the requested channel state information from the wireless communication device (103) comprising at least a current rank indicator of the wireless communication device (103); further being operative to, when determining, from information indicating at least one property of interference that the wireless communication device (103) is subjected to, a rank indicator to be applied at the wireless communication device (103) upon determining channel state information: determine that the current rank indicator is higher than what is motivated based on said information and that the rank indicator to be applied at the wireless communication device (103) should be lower than the current rank indicator.

15. The radio base station (101) of claims 13 or 14, wherein the determining of a rank indicator comprises determining a plurality of rank indicators, one of which is selected by the wireless communication device (103) and applied upon determining channel state information.

16. The radio base station (101) of any one of claims 13-15, wherein the channel state information is configured to comprise a precoding matrix indicator, PMI, and a channel quality indicator, CQI.

17. The radio base station (101) of any one of claims 13-16, wherein the information indicating a property of interference that the wireless communication device (103) is subjected to includes one or more of block error rate, BLER, for the communication link with the wireless communication device (103), intra-cell or inter cell interference experienced by the wireless communication device (103), path gain to a neighbouring radio base station, degree of back-off in outer loop link adaptation for communication with the device (103) , cell specific reference signals, CRS, transmissions occurring in the serving or in neighbouring base radio base stations, low traffic load in cells served by neighbouring radio base stations (108), low traffic periods and/or low interference level periods identified from historic statistical traffic data of serving and/or neighbouring radio base stations.

18. A wireless communication device (103) configured to report rank, said wireless communication device (103) comprising a processing unit (211) and a memory (213), said memory containing instructions (212) executable by said processing unit (211), whereby the wireless communication device (103) is operative to: receive an instruction from a radio base station (101) to apply a rank indicator upon reporting channel state information; determine channel state information by applying the rank indicator received with the instruction; and to report the determined channel state information to the radio base station (101).

19. The wireless communication device (103) of claim 18, wherein the receiving (S102) of an instruction comprises receiving a plurality of rank indicators, one of which is selected and applied upon determining channel state information.

20. The wireless communication device (103) of claims 18 or 19, wherein the reported channel state information is configured to comprise a precoding matrix indicator, PMI, a channel quality indicator, CQI.