WO2017151024A1

WO2017151024A1 - Network node and method for managing link adaption

Info

Publication number: WO2017151024A1
Application number: PCT/SE2016/050162
Authority: WO
Inventors: Patrik Rask
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2016-03-02
Filing date: 2016-03-02
Publication date: 2017-09-08

Abstract

A method performed by a network node (120), for managing Link Adaption (LA) for a UE (120) connected to the network node (110). A CQI comprising a measured SINR based on CRS has been reported to the network node by the UE (120). The network node (110) sends a transmission to the UE using a MCS, which is determined based on an estimated SINR comprising the measured SINR reported by the UE (120) and a first offset stored for a previous UE session. The network node (110) receives a confirmation message from the UE (120), comprising an ACK or NACK of receipt of the transmission. The network node (110) adjusts the estimated SINR based on the received ACK or NACK. The network node (110) determines a new MCS based on the adjusted estimated SINR value. The network node (110) sends a new transmission to the UE (120) using the determined new MCS. At the end of the UE session the network node (110) determines a second offset between an end value of the adjusted estimated SINR and the measured SINR value reported by the UE (120). The network node (110) then stores the determined second offset between the end value of the adjusted SINR and the SINR value reported by the UE (120).

Description

NETWORK NODE AND METHOD FOR MANAGING LINK ADAPTION

TECHNICAL FIELD

Embodiments herein relate to a network node and a method therein. In particular, it relates to a method for managing Link Adaption (LA).

BACKGROUND

Communication devices such as User Equipments (UEs) are enabled to

communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two UEs, between a UE and a regular telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.

UEs may further be referred to as wireless terminals, mobile terminals and/or mobile stations, mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless terminal or a server.

The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area is being served by a network node. A cell is the geographical area where radio coverage is provided by the network node, which area may also be referred to as a service area, a beam or a beam group.

The network node may further control several transmission points, e.g. having Radio Units (RRUs). A cell can thus comprise one or more network nodes each controlling one or more transmission/reception points. A transmission point, also referred to as a transmission/reception point, is an entity that transmits and/or receives radio signals. The entity has a position in space, e.g. an antenna. A network node is an entity that controls one or more transmission points. The network node may e.g. be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, or BTS

(Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. Further, each network node may support one or several communication

technologies. The network nodes communicate over the air interface operating on radio frequencies with the UEs within range of the network node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the UE to the base station.

In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks. In LTE the cellular communication network is also referred to as E-UTRAN.

An E-UTRAN cell is defined by certain signals which are broadcasted from the eNB. These signals contain information about the cell which can be used by UEs in order to connect to the network through the cell. The signals comprise reference and

synchronization signals which the UE uses to find frame timing and physical cell identification as well as system information which comprises parameters relevant for the whole cell.

A UE needing to connect to the network must thus first detect a suitable cell, as defined in 3GPP TS 36.304 v1 1.5.0. This is performed by measuring on received reference signals sent from different cells, also referred to as "listening" for a suitable cell. The suitable cell is commonly the cell with best quality of signal. Listening for a suitable cell may comprise searching for reference signals transmitted from the network node in an OFDM subframe. When a suitable cell is found the UE performs random access, according to a system information for the cell. This is done in order to transmit a Radio Resource Control (RRC) connection setup request to the network node. Assuming the random access procedure succeeds and the network node receives the request, the network node will either answer with an RRC connection setup message, which acknowledges the UE's request and tells it to move into RRC connected state, or an RRC connection reject, which tells the UE that it cannot connect to the cell. In RRC connected state the parameters necessary for communication between the network node and the UE are known to both entities and a data transfer between the two entities is enabled. The period when the UE is in RRC connected state may herein be referred to as a UE session.

When the UE is connected to the network node, the network node may perform Link Adaption (LA) for the DL channel to the UE. Link adaptation, which may also be referred to as Adaptive Modulation and Coding (AMC), is a term used in wireless communications to denote the matching of modulation, coding and

other signal and protocol parameters to the conditions on the radio link, such as e.g. a pathloss, an interference due to signals coming from other transmitters, a sensitivity of the receiver or an available transmitter power margin. For example, WiMAX uses a rate adaptation algorithm that adapts the modulation and coding scheme (MCS) according to the quality of the radio channel, and thus the bit rate and robustness of data transmission. The process of link adaptation is dynamic and the signal and protocol parameters change as the radio link conditions change. Such an adaption may for example take place every 2 ms.

Adaptive modulation systems invariably require some channel state information at the transmitter. The channel state information may in time division duplex systems be acquired by assuming the channel from the transmitter to the receiver is approximately the same as the channel from the receiver to the transmitter. Alternatively, the channel knowledge can also be directly measured at the receiver and fed back to the transmitter. Adaptive modulation systems improve rate of transmission, and/or bit error rates, by exploiting the channel state information that is present at the transmitter. Especially over fading channels which model wireless propagation environments, adaptive modulation systems exhibit great performance enhancements compared to systems that do not take channel state information into consideration at the transmitter.

The UE determines Channel State Information (CSI) in the DL based on

measurements of Cell specific Reference Signals (CRS) broadcasted by the network node and the UE reports the CSI to the network node.

The CRS are UE known symbols that are inserted in a Resource Element (RE) of a subframe of an Orthogonal Frequency-Division Multiplexing (OFDM) time and frequency grid and broadcasted by the network node. Each RE has an extension in the frequency domain corresponding to an OFDM sub carrier and an extension in the time-domain corresponding to an OFDM symbol interval.

The CRS are used by the UE for downlink channel estimation, i.e. for performing the measurements of CSI. Channel estimation is used for demodulation of downlink data both when the UE is in RRC connected state and is receiving user data and when the UE is in RRC idle state and is reading system information. Due to the latter use case, the CRSs must be transmitted even from cells which do not have any UEs in RRC connected state since the eNB cannot know whether a UE wants to access the network until it performs random access. Downlink cell specific reference signals are inserted within the first and third last OFDM symbol of each slot with a frequency domain spacing of six sub-carriers. A slot is a time period of the OFDM time and frequency grid, which is usually 0.5 ms long.

The UE indicates a Modulation and Coding Scheme (MCS) to the network node, for which MCS the UE would be able to demodulate and decode a transmitted downlink data with a predefined maximum Block Error Rate (BLER). The BLER is an indication of how successful a data transmission has been, i.e. the relationship between the number of erroneous blocks and the total number of received blocks in a transmission. In order for the network node to predict the downlink channel condition, a Channel Quality Indicator (CQI) feedback from the UE may be an input. The CSI may comprise CQI, a Precoding Matrix Indicator (PMI), which determines how the individual data streams are mapped to the antennas, and a Rank Indicator (Rl), which indicates the number of layers and the number of different signal streams transmitted in the downlink. The higher the CQI value reported by the UE is, the higher the modulation scheme and the higher the coding rate used by the network node will be. Thereby a higher efficiency of the channel may be achieved.

The CRSs may be shifted or unshifted in regards to each other. In a cell plan where unshifted CRS are used and the network nodes/cells in the communications network are time synchronized, the CRSs will align on top of each other for different cells. This has the advantage that the CRS from one cell do not interfere with data transmissions from another cell. However, the CRSs from different cells may interfere with each other. Since the CQI reporting from the UE comprises a Signal to Interference and Noise Ratio (SINR) determined based on the CRS, the UE may indicate a channel condition which is worse than the actual channel condition for PDSCH. This is e.g. the case in a scenario with light load, where the UE may report a channel condition which is estimated based on the interference caused by the CRS although the PDSCH is undisturbed. Thereby the network node may apply a very conservative MCS which limits the throughput in the PDSCH although this is not required by the PDSCH itself. To reduce the interference of reference signals between the cells, the position of the CRS may be shifted in frequency between the cells, which is also referred to as a shifted cell plan. The CRS may be shifted between 0 - 5 sub carriers, each sub carrier corresponding to a frequency shift of 15 kHz for LTE. The cell specific frequency shift can be derived from the physical Cell Identity (Cell ID) which is signaled to the UE by selection of appropriate Primary Synchronization Channel (PSCH) and Secondary Synchronization Channel (SSCH). Although this reduces the interference of reference symbols between cells, it has the problem that the reference symbols of one cell will disturb Physical Downlink Shared Channel (PDSCH) and Physical Downlink Control Channel (PDCCH) symbols of neighboring cells. Hence, in a cell plan where shifted CRS are used, the UE may report a better channel condition based on measured CRS than the actual conditions in the PDSCH and the PDCCH. This may e.g. be the case in scenarios with a high load in the cell. Based on the reported channel conditions from the UE the network node may apply a MCS which is too sensitive, which may lead to a high BLER and an increased number of dropped calls.

The above mentioned drawbacks may be overcome by estimating a SINR for the PDSCH, determining a MCS based on the estimated SINR and transmitting a message using the determined MCS to the UE. When the UE responds with an Acknowledgement (ACK) message the estimated SINR is increased and when the UE responds with a Non- Acknowledgement (NACK) the estimated SINR is decreased. Thereby, the network node may improve the MCS for each transmission to the UE to better suit the channel conditions for PDSCH for each UE session. A UE session shall herein be interpreted as the time a UE is in RRC connected state.

However, this solution is very time consuming since the network node has to perform a high number of iterations with the UE until an optimized MCS is found. In some cases the UE session may even be ended before the network node has found the suitable MCS for the channel conditions of the PDSCH. SUMMARY

It is therefore an object of embodiments herein to enhance the performance in a wireless communications network.

According to a first aspect of embodiments herein, the object is achieved by a method performed by a network node, for managing Link Adaption (LA) for a User Equipment (UE) connected to the network node. A Channel Quality Indicator (CQI) comprising a measured SINR based on Cell Reference Symbols (CRS) has been reported to the network node by the UE (120). The network node sends a transmission to the UE using a Modulation and Coding Scheme (MCS). The MCS is determined based on an estimated SINR comprising the measured SINR reported by the UE and a first offset stored for a previous UE session. The network node receives a confirmation message from the UE. The confirmation message comprises an ACKnowledgement (ACK) or a Non-Acknowledgement (NACK) of receipt of the transmission. The network node adjusts the estimated SINR. The adjusting comprises increasing the estimated SINR value for each transmission for which the network node has received an ACK from the UE and decreasing the estimated SINR value for each transmission for which the network node has received a NACK from the UE. The network node determines a new MCS based on the adjusted estimated SINR value. The network node then sends a new transmission to the UE using the determined new MCS. At the end of the UE session the network node determines a second offset between an end value of the adjusted estimated SINR and the measured SINR value reported by the UE. The network node then stores the determined second offset between the end value of the adjusted SINR and the SINR value reported by the UE. According to a second aspect of embodiments herein, the object is achieved by a network node for performing the method for managing Link Adaption (LA) for a User Equipment (UE) connected to the network node. A Channel Quality Indicator (CQI) comprising a measured SINR based on Cell Reference Symbols (CRS) has been reported to the network node by the UE. The network node is configured to send a transmission to the UE using a Modulation and Coding Scheme (MCS), which MCS is determined based on an estimated SINR comprising the measured SINR reported by the UE and a first offset stored for a previous UE session. The network node is configured to receive a confirmation message from the UE. The confirmation message comprises an ACKnowledgement (ACK) or a Non-Acknowledgement (NACK) of receipt of the transmission. The network node is configured to adjust the estimated SINR, wherein the network node is configured to adjust the estimated SINR by being configured to increase the estimated SINR value for each transmission for which the network node (110) has received an ACK from the UE and decrease the estimated SINR value for each transmission for which the network node has received a NACK from the UE. The network node is configured to determine a new MCS based on the adjusted estimated SINR value. The network node is configured to send a new transmission to the UE using the determined new MCS. The network node is further configured to, at end of the UE session, determine a second offset between an end value of the adjusted estimated SINR and the measured SINR value reported by the UE. The network node is further configured to store the determined second offset between the end value of the adjusted SINR and the SINR value reported by the UE.

By storing the end value of the determined offset at the end of a first UE session and applying this stored offset to the SINR value received from the UE in a second UE session, the network node will be able to apply a more suitable MCS already at the start of the second UE session, thereby enhancing the performance of transmissions from the network node to the UE by increasing the throughput and/or reducing the risk of dropped calls.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of embodiments herein are described in more detail with reference to the attached drawings in which:

Figure 1 is a schematic block diagram illustrating embodiments of a wireless

communications network.

Figure 2 is a schematic block diagram illustrating an OFDM subframe for a serving and a neighboring cell for an unshifted cell plan.

Figure 3 is a schematic block diagram illustrating an OFDM subframe for a serving and a neighboring cell for a shifted cell plan.

Figure 4 is a schematic block diagram illustrating embodiments of an OFDM subframe. Figure 5 is a flowchart depicting embodiments of a method in a network node.

Figure 6 is a schematic block diagram illustrating embodiments of a network node.

DETAILED DESCRIPTION

Figure 1 depicts an example of a wireless communications network 100

according to a first scenario in which embodiments herein may be implemented. The wireless communications network100 may be a wireless communication network such as an LTE, E-Utran, WCDMA, GSM network, any 3GPP cellular network, Wmax, or any cellular network or system. The communication network 100 comprises a Radio Access Network (RAN) and a Core Network (CN). The communication network 100 may use a number of different technologies, such as W-Fi, Long Term Evolution (LTE), LTE- Advanced, 5G, Wdeband Code Division Multiple Access (WCDMA), Global System for Mobile communications/Enhanced Data rate for GSM Evolution (GSM/EDGE), Worldwide Interoperability for Microwave Access (WMax), or Ultra Mobile Broadband (UMB), just to mention a few possible implementations. In the communication network 100, a UE 120 such as e.g. a wireless device, a mobile station, a non-access point (non-AP) STA, a STA, and/or a wireless terminals, communicate via one or more Access Networks (AN), e.g. RAN, to one or more CNs. It should be understood by those skilled in the art that "wireless device" is a non-limiting term which means any terminal, wireless

communication terminal, user equipment, Machine Type Communication (MTC) device, Device to Device (D2D) terminal, or node e.g. smart phone, laptop, mobile phone, sensor, relay, mobile tablets or even a base station communicating within a cell.

The RAN comprises a set of radio network nodes, such as radio network nodes 110, 111 each providing radio coverage over one or more geographical areas, such as a cell 130, 131 of a radio access technology (RAT), such as LTE, UMTS, Wi-Fi or similar. The radio network node 1 10, 1 11 may be a radio access network node such as radio network controller or an access point such as a wireless local area network (WLAN) access point or an Access Point Station (AP STA), an access controller, a base station, e.g. a radio base station such as a NodeB, an evolved Node B (eNB, eNodeB), a base transceiver station, Access Point Base Station, base station router, a transmission arrangement of a radio base station, a stand-alone access point or any other network unit capable of serving a wireless device within the cell, which may also be referred to as a service area, served by the radio network node 1 10, 1 11 depending e.g. on the first radio access technology and terminology used.

The UE 120 is located in the cell 130 of the network node 1 10, which is referred to as the serving cell, whereas the cells 131 of the network nodes 11 1 are referred to as neighboring cells. Although, the network node 110 in figure 1 is only depicted providing a serving cell 130, the network node 110 may further provide one or more neighboring cells to the serving cell. In the scenario described in figure 1 , there is a low load in the communications network 100, i.e. there is only a limited number of UEs 120 present in the serving cell 130. The UE 120 is located at a cell edge of the serving cell 130. When a cell plan with unshifted CRS is used, which is depicted in Figure 2, the UE 120 may report a low CQI based on the CRS, since the UE 120 at the cell edge will also hear interfering CRS from the neighboring cells which are located in the same resource element in a subframe as the CRS of the serving cell. As can be seen in fig. 2, the CRS of the neighboring cell 131 are located in the same Resource Elements (REs) 201 as the CRS in the serving cell. However, since there is no or only limited data traffic to other UEs in the neighboring cell 131 cell, the channel quality for PDSCH and PDCCH will be much better than the DL channel estimation performed by the UE based on the CRSs. Hence, a MCS based on the CQI from the UE will lead to a very conservative MCS for the DL channel in the serving cell 130, which unnecessarily reduces the performance of the channel.

In a further scenario which is depicted in Figure 3, a cell plan using shifted CRS may be used in the communications network 100. AS can be seen in Fig. 3, the CRS of the neighboring cell is not located in the same RE 120 as the CRS of the serving cell, hence the CRS of the serving cell 130 are not interfered as much as the CRS of the serving cell in the unshifted cell plan depicted in Fig. 2. In the shifted CRS scenario, the UE 120 may report a better channel condition based on measured CRS than the actual conditions in the PDSCH and the PDCCH, since the CRS for the neighboring cells 131 will interfere with PDSCH and PDCCH of the serving cell instead of the CRS. This may further be aggravated in scenarios with a high loads in the cells. Based on the reported channel conditions from the UE 120 the network node may apply a MCS which is to aggressive, which may lead to a high BLER and an increased number of dropped calls.

Figure 4 shows an exemplary downlink OFDM time and frequency grid, which is also referred to as an OFDM subframe. Each subframe comprises two slots. Each slot comprising a number of resource elements (RE) 201 extending both in the time domain (x-axis) and in the frequency domain (z-axis). Each RE's 201 extension in the frequency domain is referred to as a sub-carrier whereas the extension in the time domain is referred to as an OFDM symbol. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, wherein each radio frame comprises ten equally- sized subframes. Furthermore, the resource allocation in LTE is typically described in terms of Physical Resource Blocks (PRBs), comprising a plurality of REs. A resource block corresponds to one slot in the time domain and 12 contiguous subcarriers in the frequency domain.

Downlink and uplink transmissions are dynamically scheduled, i.e. in each subframe the network node 130 transmits control information about to or from which UE 120 data is transmitted and upon which resource blocks the data is transmitted. The control information may comprise system information, paging messages and/or random access response messages. The control information for a given UE 120 is transmitted using one or multiple Physical Downlink Control Channels (PDCCH). Control information of a PDCCH is transmitted in the control region of each subframe. Figure 2 shows an exemplary size of a normal control region of three OFDM symbols allocated for control signaling, for example the PDCCH. The size of the control region may however be dynamically adjusted according to the current traffic situation. In the example shown in the figure only the first OFDM symbol out of the three possible is used for control signalling. Typically the control region may comprise many PDCCHs carrying control information to multiple UEs 120 simultaneously. The REs used for control signaling are indicated with wave-formed lines and REs used for CRS are indicated with diagonal lines. The CRS are used by the UE 120 for downlink channel estimation. Channel estimation is used for determining the demodulation of downlink data both when the UE 120 is in RRC connected state and is receiving user data and when the UE 120 is in RRC idle state and is reading system information. Downlink CRS are inserted within the first and third last OFDM symbol of each slot with a frequency domain spacing of six sub- carriers.

The subframe also comprises data symbols used for transmitting user data between the network node 110 and the UE 120. The data symbols are situated in the region following the control region, which is also referred to as the data region.

Example of embodiments of a method in the network node 110 for managing Link Adaption (LA) for a UE 120, will now be described with reference to a flowchart depicted in Figure 5. A Channel Quality Indicator (CQI) comprising a measured SINR based on Cell Reference Symbols (CRS) has been reported to the network node by the UE 120, which is connected to a cell of the network node 1 10.

The method comprises the following actions, which actions may be taken in any suitable order.

Action 501

The network node 1 10 sends a transmission to the UE 120 using a Modulation and Coding Scheme (MCS). The MCS has been determined based on an estimated SINR, which estimated SINR comprises the measured SINR reported by the UE 120 and a first offset stored for a previous UE session. The offset referred to herein may be a difference between an estimated SINR at the end of the previous UE session and a measured SINR reported by the UE 120 in the previous UE session. The estimated SINR value may e.g. be a sum of the measured SINR value received from the UE 120 and the first offset.

In some embodiments herein the network node 110 may have stored different first offsets for different types of UEs. In such an embodiment, the network node determines the MCS based on the offset stored for the type of UE 120 currently connected to the network node 110.

Action 502

The network node 110 receives a confirmation message from the UE 120, which confirmation message comprises an ACKnowledgement (ACK) or a Non- Acknowledgement (NACK) of receipt of the transmission transmitted by the network node 1 10.

Action 503

The network node 1 10 adjusts the estimated SINR. The adjusting may be performed for a DL channel of the network node 110. The adjusting is based on the received confirmation message. When the network node 110 has received an ACK message as response to a transmission, the network node increases the estimated SINR value. When the network node 1 10 has received a NACK message as response to a transmission, the network node decreases the estimated SINR value for each

transmission for which the network node 110 has received a NACK from the UE 120.

In some embodiments herein the size of the increase and/or decrease of the SINR value is dependent on the number of confirmation messages received from the UE 120 during the UE session. This may also be referred to as being dependent on the number of iterations of the method performed. The size of the increased and/or decrease of the SINR value, i.e. the size of the dB value which the SINR is adjusted with, may be reduced from a predefined start value for each received response message.

The estimated SINR may be increased and/or decreased stepwise. Stepwise shall herein be interpreted as the network node adjusting the estimated SINR value one step for each received ACK or NACK from the UE 120. In some embodiments herein the stepwise increase and/or decrease, i.e. the size of each step, is larger in the beginning of a UE session and reduces with each iteration of the method, i.e. with each new estimation of the SINR. Hence, the size of the step is a function of the number of iterations of the method, i.e. the number of re-estimations of the SINR performed due to a received response message from the UE 120, during the UE session. The size of the stepwise increase is hence dependent on the number of received confirmation messages received from the UE 120 during a UE session.

By having the size of the increase and/or decrease being dependent on the number of received confirmation messages received from the UE 120, the estimated SINR value will more rapidly reach a region corresponding to an optimized MCS for the channel when a new UE session is started. This is especially beneficial in a scenario where the actual channel condition is worse than the estimated SINR would predict, i.e. in scenarios where a too aggressive, which may also be referred to as too sensitive, MCS is applied, since the MCS thereby quickly can be changed to a more conservative, which may also be referred to as a more stable, MCS. Such scenarios may e.g. be a scenario with a shifted CRSs plan and a high load in the cell or in case the added offset is to large. By quickly applying a more conservative MCS, the risk of dropped calls is reduced. However, also a scenario where a too conservative MCS is applied in the beginning of a session benefits from the steps being larger in the beginning of the UE session, since the throughput may be increased more rapidly which increases the performance of the communications network. In some embodiments herein, the size of the increase decreases from a predefined start value for each received response message. The size of the step may e.g. be linearly dependent on the number of iterations of the method. The step size may e.g. be 1 dB in the beginning of the UE session, i.e. the predefined start value may be 1 dB and then gradually decrease towards an end value for each new iteration, which end value may be of different size for an increase in SINR value compared to a decrease in SINR value. In a further embodiment herein, the size of the step may decrease exponentially with the number of iterations of the method, i.e. with the number of received response messages.

In some embodiments herein, decreasing the SINR value may be faster than increasing the SINR value, e.g. by the step of decreasing the SINR value being larger than the step of increasing the SINR value. Thereby, the network node 1 10 will quickly decrease the estimated SINR value in case the UE 120 responds with a NACK, which in turn leads to the network node 1 10 applying a more conservative MCS for the next transmission to the UE 120. Thereby the risk of dropped calls due to a too aggressive MCS may be reduced. In further embodiments herein, decrease of the SINR value may be in the range of 2 to 10 times faster than the increase of the SINR value. This may e.g. be achieved by the step of decreasing the SINR value being in the range of 2 to 10 times larger than the step of increasing the SINR value.

The SINR value may e.g. be increased in steps of 0,01 dB as response to an ACK, whereas the SINR value may be reduced in steps of 0,1 dB in response to a NACK.

These values may also be suitable as end values for the embodiment where the size of the step is dependent on the number of iterations of the method.

In a scenario where the estimated SINR comprises a stored first offset which leads to a MCS which is too aggressive for the current channel conditions and the network node 1 10 has received negative response messages, such as e.g. NACKs, for a series of successive transmissions, the network node 110 may adjust the estimated SINR value to correspond with the measured SINR value received from the UE 120. Hence, the stored first offset is removed from the estimated SINR value and a new SINR value is estimated with the measured SINR value received from the UE 120 as a starting point. Thereby, a more conservative MCS may be quickly applied for the channel which reduces the risk of dropped calls. In some embodiments herein, instead of only removing the offset from the estimated SINR the network node 110 may add a negative offset to the estimated SI NR. This will lead to a very conservative MCS, which ensures that the UE session, such as a call will not be dropped. In some embodiments herein, the offset may be removed and/or the negative offset may be applied after e.g. more than 4 successive negative response messages.

Action 504

The network node 1 10 further determines a new MCS corresponding to the adjusted estimated SINR.

In some embodiments the MCS may be determined based on a CQI to MCS mapping as shown in Table 1. The CQI index is determined based on the SINR. In the embodiments described herein, the MCS is determined based on a CQI index

corresponding to the estimated SINR.

Table 1

The CQI index represents a degree of quality of the channel, wherein CQI index 0 indicates the worst quality and 15 indicates the best quality. Higher modulation schemes are used when the quality is better, since these improve the spectrum efficiency of the channel. The higher modulation schemes are however not suitable for lower quality channels, since the data transmitted with these modulation schemes are more sensitive to interference and noise.

The code rate indicates the amount of redundancy in the data transmitted. A lower code rate corresponds to a higher redundancy. As the channel quality gets better, a lower amount of redundancy is required to ensure transmission. Hence, the code rate may be increased, which improves the performance of the communications network since a bandwidth of the channel may be more efficiently used. Action 505

The network node 1 10 sends a new transmission to the UE 120 using the determined new MCS.

Action 506

At the end of the UE session the network node 110 determines a second offset between an end value of the adjusted estimated SINR and the measured SINR value reported by the UE 120 in the beginning of the session.

By determining the end value of the adjusted SINR, the network node may estimate a load in neighboring cells. A high end value indicates a low load in the neighboring cells, since the measured SINR reported worse channel conditions than the actual channel conditions in the cell.

In some further embodiments herein the network node 110 may determine a statistical measure of offsets for a plurality of UEs 120 previously connected to the serving cell 130. The statistical measure may e.g. be a mean value of the offsets for the plurality of UEs 120 previously connected to the cell.

In some embodiments herein the network node 1 10 may determine different statistical measures of offsets for different types of UEs 120. A type may be one or more UEs 120 having a certain characteristic in common, such as e.g. functionalities, capabilities, brand, type of chipset, timing alignment, i.e. the distance from the network node, path loss and/or other measurements of the channel. The UEs 120 having the same characteristics may be grouped and an offset may be determined for each group of UEs.

In some embodiments, the offset may also be determined based on characteristics of a previous connection of a UE 120, i.e. the offset used in a previous session for the UE 120 may be used as the offset for the estimated SINR for the UE 120 during the current UE session.

Action 507

The network node 1 10 stores the determined second offset between the end value of the adjusted SINR and the SINR value reported by the UE 120. At the end of each UE session the second offset value is stored for later use in a following UE sessions. In the following UE session, the first offset may be a previously stored second offset.

The first offset may be stored as a statistical measure of second offsets for a plurality of previously connected UEs 120. In some embodiments the statistical measure of the second offsets may be a mean value of the second offsets for a plurality of previous UE sessions, i.e. UE sessions for UEs 120 previously connected to the network node 1 10.

In a further embodiment herein, different first offsets may be stored for different types of UEs 120. The UEs 120 may be grouped based on e.g. functionalities,

capabilities, brands, type of chipset, timing alignment, i.e. the distance from the network node, and/or other measurements of the channel, and a first offset may be stored for each group of UEs.

The second offset between the end value of the adjusted SINR and the reported SINR value from the UE 120 may further be stored by mapping the offset to a "Path loss/Timing alignmenf-table. The "Path loss/Timing alignmenf-table discloses the path loss of the UE 120, which indicates the reduction in power density of the signal, on a first axis of the table and the timing alignment, which indicates the distance from the UE 120 to the network, on a second axis of the table. Thereby, a suitable start value for the first offset for a UE 120 connecting to a cell of the network node 110 may be determined from the "Path loss/Timing alignmenf-table comprising mapped second offsets, based on a determined path loss and timing alignment of the UE 120 connecting to the network node 1 10.

The first offset may e.g. be a statistical measure of second offsets for a plurality of previous UE sessions. The statistical measure may e.g. be a mean value of the second offsets for the plurality of previous UE sessions.

In some embodiments herein the offset may be dependent on the type of UE 120 connected. The UEs may be grouped based on e.g. functionalities, capabilities, brands, type of chipset, timing alignment, i.e. the distance from the network node, and/or other measurements of the channel, and an offset may be determined for each group of UEs. When a UE 120 transmits a measured SINR to the network node 110, the network node 1 10 may in action 501 estimate a SIN R by adding the offset stored for that UE type to the transmitted measured SI NR. The network node 110 may then use a MCS corresponding to the estimated SINR to transmit the transmission.

When the network node has sent a new transmission to the UE 120 according to action 505, the network node 110 will receive a confirmation message according to action 502 from the UE 120 for the new transmission. Hence, the actions 502 to 505 may be are repeated for each new transmission from the network node 1 10 to the UE 120 during the UE session. Thereby, an MCS suitable for the current channel is determined by iteration. By estimating a SINR by applying the stored first offset to the measured SINR received from the UE 120 and determining a MCS corresponding to the adjusted estimated SINR, a more suitable MCS than the one corresponding to the measured SINR may be provided for the channel, such as the DL channel, already at the start of a UE session. Thereby, the number of iterations of the method required to identify the suitable MCS is reduced, which enhances the performance of transmissions from the network node to the UE by increasing the throughput and/or reducing the risk of dropped calls. A suitable MCS may e.g. be a MCS for which the BLER reaches a target value. In some embodiments the target value may e.g. be a BLER of 10%.

To perform the method actions for managing Link Adaption (LA) for a UE120 connected to the network node 1 10, described above in relation to Figure 5, the network node 1 10 may comprise the following arrangement depicted in Figure 6. As mentioned above the network node 110 operates one or more cells 130, 131. A Channel Quality Indicator (CQI) comprising a measured SINR based on CRS has been reported to the network node by the UE 120. Dashed lines of a box in Figure 3 indicate that this module is optional.

The network node 1 10 comprises a radio circuitry 601 to communicate with the UEs 120 and a processing unit 602.

The network node 1 10 is configured to, e.g. by means of a sending module 603 and/or the radio circuitry 601 being configured to, send a transmission to the UE 120 using a MCS, which MCS is determined based on an estimated SINR comprising the measured SINR reported by the UE 120 and a first offset stored for a previous UE session.

The network node 1 10 is configured to, e.g. by means of a receiving module 604 and/or the radio circuitry 601 being configured to, receive a confirmation message from the UE 120. The confirmation message comprises an ACKnowledgement (ACK) or a Non- Acknowledgement (NACK) of receipt of the transmission transmitted by the network node 1 10.

The network node 110 is configured to, e.g. by means of an adjusting module 605 and/or the processing unit 602 being configured to, adjust the estimated SINR for a 5 channel. The network node 1 10 may further be configured to, e.g. by means of the adjusting module 605 and/or the processing unit 602 being configured to, adjust the SINR by increasing the estimated SINR value for each transmission for which the network node 110 has received an ACK from the UE 120 and decreasing the estimated SINR value for each transmission for which the network node 1 10 has received a NACK from the UE 10 120.

In some embodiments herein, the network node 110 may further be configured to, e.g. by means of the adjusting module 605 and/or the processing unit 602, adjust the size of the increase and/or decrease of the SINR value dependent on the number of received confirmation messages from the UE 120.

15 In some embodiments herein, the network node 110 may further be configured to, e.g. by means of the adjusting module 605 and/or the processing unit 602, increase and/or decrease the SINR value with linearly dependent on the number of received response messages from the UE 120.

In some embodiments herein, the network node 110 may be configured to, e.g. by 20 means of the adjusting module 605 and/or the processing unit 602, increase and/or decrease the SINR value exponentially dependent on the number of received response messages from the UE 120.

In some embodiments herein, the network node 110 may be configured to, e.g. by means of the adjusting module 605 and/or the processing unit 602, decrease the SINR 25 value faster than it is configured to increase the SINR value.

In some embodiments herein, the network node 110 may further be configured to, e.g. by means of the adjusting module 605 and/or the processing unit 602, decrease the SINR value in the range of 2 to 10 times faster than it is configured to increase the SINR value.

30 In some embodiments herein, when the network node 110 has received a number of successive negative response messages, the network node 1 10 may further be configured to, e.g. by means of the adjusting module 605 and/or the processing unit 602, remove the stored first offset from the estimated SINR value and to select an estimated SINR value corresponding to the measured SINR value received from the UE 120.

35 The network node 1 10 is configured to, e.g. by means of a determining module 606 and/or the processing unit 602 being configured to, determine a new MCS based on the adjusted estimated SINR value.

The network node 1 10 is configured to, e.g. by means of the sending module 603 and/or the radio circuitry 601 being configured to, send a transmission to the UE 120 using the determined new MCS.

The network node 1 10 is configured to, e.g. by means of the determining module

606 and/or the processing unit 602 being configured to, determine a second offset between an end value of the adjusted estimated SINR and the measured SINR value reported by the UE 120 at the end of the UE session.

The network node 1 10 may further be configured to, e.g. by means of the

determining module 606 and/or the processing unit 602 being configured to, determine the first offset as a statistical measure of the second offsets for a plurality of previous UE sessions. The network node 110 may be configured to determine the statistical measure of second offsets as a mean value of the second offsets for a plurality of previous UE sessions.

The network node 1 10 is configured to, e.g. by means of a memory 607 and/or the processing unit 602 being configured to, store the determined second offset between the end value of the adjusted SINR and the SINR value reported by the UE 120.

The network node 1 10 may further be configured to, e.g. by means of a memory

607 and/or the processing unit 602 being configured to, store the first offset as a statistical measure of the second offsets for a plurality of previous UE sessions.

The network node 1 10 may further be configured to, e.g. by means of the

determining module 606 and/or the processing unit 602 being configured to, determine different first offsets for different types of UEs 120 based on stored second offsets for each type of UE 120.

The embodiments herein for managing LA for a UE120 connected to the network node 1 10, may be implemented through one or more processors, such as the processing unit 602 in the network node 1 10 depicted in Figure 6, together with computer program code for performing the functions and actions of the embodiments herein. The program code mentioned above may also be provided as a computer program product, for instance in the form of a data carrier carrying computer program code for performing the

embodiments herein when being loaded into the in the network node 1 10. One such carrier may be in the form of a CD ROM disc. It is however feasible with other data carriers such as a memory stick. The computer program code may furthermore be provided as pure program code on a server and downloaded to the network node 1 10.

The network node 110 may further comprise a memory 607 comprising one or more memory units. The memory 607 may be arranged to be used to store obtained information, measurements, data, configurations, schedulings, and applications to perform the methods herein when being executed in the network node 1 10.

Those skilled in the art will also appreciate that the sending module 603, the receiving module 604, the adjusting module 605, and the determining module 606, described above may refer to a combination of analog and digital circuits, and/or one or more processors configured with software and/or firmware, e.g. stored in the memory 607, that when executed by the one or more processors such as the processing unit 602 as described above. One or more of these processors, as well as the other digital hardware, may be included in a single Application-Specific Integrated Circuitry (ASIC), or several processors and various digital hardware may be distributed among several separate components, whether individually packaged or assembled into a system-on-a-chip (SoC).

When using the word "comprise" or "comprising" it shall be interpreted as non- limiting, i.e. meaning "consist at least of".

The embodiments herein are not limited to the above described preferred embodiments. Various alternatives, modifications and equivalents may be used.

Therefore, the above embodiments should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

A method performed by a network node (110), for managing Link Adaption, LA, for a User Equipment, UE, (120) connected to the network node (110), wherein a Channel Quality Indicator, CQI, comprising a measured SINR based on Cell Reference Symbols, CRS, has been reported to the network node by the UE (120), wherein the method comprises:

- sending (501) a transmission to the UE (120) using a Modulation and Coding Scheme, MCS, which MCS is determined based on an estimated SINR comprising the measured SINR reported by the UE (120) and a first offset stored for a previous UE session,

- receiving (502) a confirmation message from the UE (120), which

confirmation message comprises an ACKnowledgement, ACK, or a Non-Acknowledgement, NACK, of receipt of the transmission,

- adjusting (503) the estimated SINR, wherein the adjusting comprises:

o increasing the estimated SINR value for each transmission for which the network node (110) has received an ACK from the UE (120), and

o decreasing the estimated SINR value for each transmission for which the network node (1 10) has received a NACK from the UE (120);

- determining (504) a new MCS based on the adjusted estimated SINR value,

- sending (505) a new transmission to the UE (120) using the determined new MCS;

at end of UE session:

- determining (506) a second offset between an end value of the

adjusted estimated SINR and the measured SINR value reported by the UE (120),

- storing (507) the determined second offset between the end value of the adjusted SINR and the SINR value reported by the UE (120).

2. The method according to claim 1 , wherein the stored first offset is a statistical measure of second offsets for a plurality of previous UE sessions.

3. The method according to claim 2, wherein the statistical measure of the second offset is a mean value of the second offsets for a plurality of previous UE sessions.

The method according to any of the claims 1 to 3, wherein different first offsets may be determined for different types of UEs (120) based on stored second offsets for each type of UE (120).

The method according to any of the previous claims, wherein the size of the increase and/or decrease is dependent on the number of received confirmation messages received from the UE 120 during a UE session.

The method according to claim 5, wherein, when the SINR is decreased, the size decreases from a predefined start value for each received response message.

The method according to claim 5 or 6, wherein the size of the increase and/or decrease is linearly dependent on the number of received confirmation messages.

The method according to any of the claims 5 to 7, wherein the start value for the increase and/or decrease is 1 dB.

The method according to any of the previous claims, wherein the decrease of the SINR value is faster than the increase of the SINR value.

10. The method according to claim 9, wherein the decrease of the SINR value is in the range of 2 to 10 times faster than the increase of the SINR value.

1 1. The method according to any of the previous claims, wherein, when the network node (1 10) has received a number of successive negative response messages, the network node removes the stored first offset from the estimated SINR value and performs the method steps (501)-(505) based on an estimated SINR value corresponding to the measured SINR value received from the UE (120).

12. A network node (1 10), for managing Link Adaption, LA, for a User Equipment, UE, (120) connected to the network node (110), wherein a Channel Quality Indicator, CQI, comprising a measured SINR based on Cell Reference Symbols, CRS, has been reported to the network node by the UE (120), wherein the network node (1 10) is configured to: - send a transmission to the UE (120) using a Modulation and Coding Scheme, MCS, which MCS is determined based on an estimated SINR comprising the measured SINR reported by the UE (120) and a first offset stored for a previous UE session,

- receive a confirmation message from the UE (120), which confirmation message comprises an ACKnowledgement, ACK, or a Non- Acknowledgement, NACK, of receipt of the transmission,

- adjust the estimated SINR, wherein the network node (110) is configured to adjust the estimated SINR by being configured to:

o increase the estimated SINR value for each transmission for which the network node (110) has received an ACK from the UE (120), and

o decrease the estimated SINR value for each transmission for which the network node (110) has received a NACK from the UE (120);

- determine a new MCS based on the adjusted estimated SINR value,

- send a new transmission to the UE (120) using the determined new MCS;

at end of UE session:

- determine a second offset between an end value of the adjusted

estimated SINR and the measured SINR value reported by the UE (120),

- store the determined second offset between the end value of the

adjusted SINR and the SINR value reported by the UE (120).

13. The network node (110) according to claim 12, wherein the network node (110) is configured to store the first offset as a statistical measure of second offsets for a plurality of previous UE sessions.

14. The network node (1 10) according to claim 13, wherein the statistical measure of the second offsets is a mean value of the second offsets for a plurality of previous UE sessions.

15. The network node (1 10) according to any of the claims 12 to 14, wherein the

network node (1 10) may be configured to determine different first offsets for different types of UEs (120) based on stored second offsets for each type of UE (120).

16. The network node (110) according to any of the previous claims 1 1 to 15, wherein the network node (110) is configured to adjust the size of the increase and/or decrease dependent on the number of received confirmation messages received from the UE (120) during a UE session.

17. The network node according to claim 16, wherein, when the SINR is decreased, the network node (1 10) is configured to reduce the size of the decrease from a predefined start value for each received response message.

18. The network node according to claim 16 or 17, wherein the network node (1 10) is configured to adjust the size of the increase and/or decrease linearly dependent on the number of received confirmation messages from the UE (120).

19. The network node (1 10) according to any of the claims 12 to 18, wherein the

network node (1 10) is configured to decrease the SINR value faster than it is configured to increase the SINR value.

20. The network node (1 10) according to claim 19, wherein the network node (1 10) is configured to decrease the SINR value in the range of 2 to 10 times faster it is configured to increase the SINR value.

21. The network node (110) according to any of the previous claims 12 to 20, wherein, when the network node (1 10) has received a number of successive negative response messages, the network node (1 10) is configured to remove the stored first offset from the estimated SINR value and to select an estimated SINR value corresponding to the measured SINR value received from the UE (120).