Patent Publication Number: US-2016226690-A1

Title: Methods and apparatus for uplink resource assignment

Description:
TECHNICAL FIELD 
     The disclosure relates to uplink resource assignment, and more specifically to a wireless terminal and a radio network node, as well as to methods for transmitting uplink data in response to a received assignment and for decoding the uplink data. 
     BACKGROUND 
     3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3 rd  Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB. A UE may more generally be referred to as a wireless device or a wireless terminal. 
       FIG. 1  illustrates a radio access network in an LTE system. An eNodeB  101   a  serves a UE  103  located within the eNodeB&#39;s geographical area of service or the cell  105   a . The eNodeB  101   a  is connected to a core network. The eNodeB  101   a  is also connected via an X2 interface to a neighboring eNodeB  101   b  serving another cell  105   b . LTE uses a scheduled Media Access Control (MAC) protocol, which implies that the UE radio resources in many ways are controlled by the eNodeB serving the UE. The usage of the radio resources can be optimized by the eNodeB if the UE&#39;s resources are known and fully controlled by a single eNodeB. 
     An uplink scheduler at the eNodeB determines dynamically, for each Transmission Time Interval (TTI) which UEs that are to transmit data and on which uplink resources. The shared resources controlled by the eNodeB scheduler are the time-frequency resource units. In LTE the smallest time-frequency resource unit that may be assigned is referred to as a physical resource block (PRB). In addition to assigning the time-frequency resources to the UE, the eNodeB scheduler is also responsible for controlling the transport format, also called transmission formats, that the UE should use, such as payload size, Modulation and Coding Scheme (MCS), rank and pre-coding matrix. The basis for uplink scheduling is scheduling grants or assignments sent by the eNodeB containing the scheduling decision and providing the UE with information about the resources and the associated transport format to use for the uplink channel. In addition to the dynamic scheduling described above, semi-persistent scheduling (SPS) is possible. With SPS, the UE gets a scheduling decision together with an indication that the decision applies to every n:th subframe until further notice. 
     Similar to the downlink case, reference signals for channel estimation are also needed for the LTE uplink to enable coherent demodulation of different uplink physical channels at the receiver side. These reference signals are more specifically referred to as Demodulation Reference Signals (DMRS). DMRS are time multiplexed with other uplink transmissions from the same UE. 
     A number of improvements have been introduced in LTE and is under development in order to better utilize the channel and propagation properties for an LTE heterogeneous network deployment. These improvements can be an important component in enabling a deployment where uplink and downlink transmission end up or originate at different locations. One example, illustrated in  FIG. 2 , is a deployment where two eNodeBs,  201  and  202 , connected by a non-ideal backhaul provide dual-connectivity. A UE  203  can be connected to the two eNodeBs,  201  and  202 , although the eNodeBs cannot quickly communicate with each other. Hence in this scenario the eNodeBs need to operate independently in the time frame of a TTI. This implies that the two eNodeBs,  201  and  202 , are jointly in control of the UE&#39;s  203  resources during the TTI time frame. 
     In some systems such as in LTE, the uplink assignment is made by the eNodeB. 
     However, it is the UE that is fully aware of its transmission conditions such as its uplink power budget, whether it needs to transmit to other eNodeBs, and its buffer status. The uplink assignment is therefore often suboptimal. In some cases, a suboptimal assignment can be accepted, as maintaining full control in the eNodeB provides coordination gains and other gains. However, in other cases, the losses from the suboptimal assignment are more severe, as in the dual-connectivity case described above. This suboptimal assignment is in that case due to the inability of the eNodeBs to communicate on a per TTI bases over the non-ideal backhaul. However, to solve the problem using a faster backhaul can in many cases be impossible or at least very expensive. 
     In other example scenarios, the impairments of the knowledge of the UE state can lead to similar suboptimal assignments. For example, the reporting delay from a UE can imply that the knowledge used by the eNodeB for making a scheduling decision is wrong. One example that is quite common is when an eNodeB sends an uplink resource assignment to the UE based upon an estimated amount of buffer data in the UE that is smaller than the actual amount of buffer data. In this case at least one extra uplink transmission will occur, resulting in an extra amount of overhead associated with the extra uplink transmission which also consumes valuable resources. 
     One possible solution to the above described problem of suboptimal assignments is to assign a too large radio resource to a UE to encompass any extra need of resources by the UE. However, this would lead to unnecessary interference as the UE is forced to transmit over the whole resource allocation of the assignment even if it would not be needed. 
     SUMMARY 
     It is therefore an object to address some of the problems outlined above, and to provide a solution making it possible to assign radio resources and transport formats adapted to the UE&#39;s situation thus avoiding suboptimal assignments. This object and others are achieved by the methods, the wireless terminal, and the radio network node according to the independent claims, and by the embodiments according to the dependent claims. 
     In accordance with a first aspect, a method for uplink transmission performed in a wireless terminal served by a radio network node of a wireless communication system is provided. The method comprises receiving an assignment for an uplink transmission from the radio network node. The method also comprises determining alternative usages of the assignment based on the received assignment. Each alternative usage is associated with a different DMRS. The method further comprises selecting a usage among the alternative usages of the assignment, and applying the selected usage when transmitting uplink data to the radio network node. The method also comprises transmitting the DMRS associated with the selected usage. 
     In accordance with a second aspect, a method for decoding uplink data received from a wireless terminal is provided, wherein the method is performed in a radio network node of a wireless communication system serving the wireless terminal. The method comprises transmitting an assignment for an uplink transmission to the wireless terminal, and receiving a DMRS and uplink data from the wireless terminal in response to the assignment. The method also comprises correlating the received DMRS with at least one of a plurality of different DMRSs. Each different DMRS is associated with an alternative usage of the assignment. The method further comprises selecting a probable DMRS among the plurality of different DMRSs based on the correlation, and decoding the received uplink data using the alternative usage associated with the probable DMRS. 
     In accordance with a third aspect, a wireless terminal for uplink transmission configured to be served by a radio network node of a wireless communication system is provided. The wireless terminal comprises a processor and a memory. The memory contains instructions executable by said processor whereby the wireless terminal is operative to receive an assignment for an uplink transmission from the radio network node, and determine alternative usages of the assignment based on the received assignment. Each alternative usage is associated with a different DMRS. Further, said wireless terminal is operative to select a usage among the alternative usages of the assignment, apply the selected usage when transmitting uplink data to the radio network node, and transmit the DMRS associated with the selected usage. 
     In accordance with a fourth aspect, a radio network node of a wireless communication system configured to decode uplink data received from a wireless terminal served by the radio network node is provided. The radio network node comprises a processor and a memory, said memory containing instructions executable by said processor whereby the radio network node is operative to transmit an assignment for an uplink transmission to the wireless terminal, and receive a DMRS and uplink data from the wireless terminal in response to the assignment. Further, the radio network node is operative to correlate the received DMRS with at least one of a plurality of different DMRSs. Each different DMRS is associated with an alternative usage of the assignment. The radio network node is also operative to select a probable DMRS among the plurality of different DMRSs based on the correlation, and to decode the received uplink data using the alternative usage associated with the probable DMRS. 
     In accordance with a fifth aspect, a wireless terminal for uplink transmission configured to be served by a radio network node of a wireless communication system is provided. The wireless terminal comprises means adapted to receive an assignment for an uplink transmission from the radio network node via the receiver, and means adapted to determine alternative usages of the assignment based on the received assignment, each alternative usage being associated with a different demodulation reference signal. The wireless terminal further comprises means adapted to select a usage among the alternative usages of the assignment, means adapted to apply the selected usage when transmitting uplink data to the radio network node via the transmitter, and means adapted to transmit the demodulation reference signal associated with the selected usage via the transmitter. 
     In accordance with a sixth aspect, a radio network node of a wireless communication system configured to decode uplink data received from a wireless terminal served by the radio network node is provided. The radio network node comprises means adapted to transmit an assignment for an uplink transmission to the wireless terminal via the transmitter, and means adapted to receive a demodulation reference signal and uplink data from the wireless terminal via the receiver in response to the assignment. The radio network node also comprises means adapted to correlate the received demodulation reference signal with at least one of a plurality of different demodulation reference signals, each different demodulation reference signal being associated with an alternative usage of the assignment. The radio network node further comprises means adapted to select a probable demodulation reference signal among the plurality of different demodulation reference signals based on the correlation, and means adapted to decode the received uplink data using the alternative usage associated with the probable demodulation reference signal. 
     An advantage of embodiments is that it is possible to assign radio resources and transport formats to a UE that may be adapted to the actual need of the UE. This is beneficial as the usage of the assignment is flexible to changes in transmission conditions for the UE that are not known to the eNodeB when it sends its assignment. Suboptimal assignments are thus avoided. 
     Another advantage of embodiments is that the existing procedure for assigning resources and transport format to a UE may be used. Furthermore, when an alternative usage of the assignment is applied by the UE, this is signaled via a change related to the existing DMRS. 
     Still another advantage of embodiments is that an efficient usage of an assignment is made possible in a number of new use-cases such as Device-to-Device (D2D), self-backhauling and dual-connectivity. 
     A further advantage of embodiments is that performance in legacy deployments may be improved by allowing the system to handle uncertainties regarding UE resources and states, e.g. the uncertainty of the amount of data in the UE transmit buffer. 
     Other objects, advantages and features of embodiments will be explained in the following detailed description when considered in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an LTE radio access network. 
         FIG. 2  is a schematic illustration of a dual-connectivity deployment in LTE. 
         FIG. 3  is a signaling diagram illustrating embodiments of the invention. 
         FIGS. 4 a - b    are flowcharts illustrating the method in the wireless terminal according to embodiments. 
         FIG. 5  is a flowchart illustrating the method in the radio network node according to embodiments. 
         FIGS. 6 a - b    are block diagram schematically illustrating the wireless terminal and the radio network node according to embodiments. 
         FIGS. 7 a - c    schematically illustrate a D2D use case. 
         FIGS. 8 a - b    schematically illustrate a dual connectivity use case. 
     
    
    
     DETAILED DESCRIPTION 
     In the following, different aspects will be described in more detail with references to certain embodiments and to accompanying drawings. For purposes of explanation and not limitation, specific details are set forth, such as particular scenarios and techniques, in order to provide a thorough understanding of the different embodiments. However, other embodiments that depart from these specific details may also exist. 
     Embodiments are described in a non-limiting general context in relation to an example scenario in an E-UTRAN, where the radio network node responsible for scheduling of a wireless terminal is an eNodeB sending uplink assignments to a UE. However, it should be noted that the embodiments may be applied to any radio access network technology with uplink assignment procedures similar to those in an E-UTRAN. 
     In legacy networks the scheduler used for assignment of resources and transport formats have full control of the UEs&#39; resource allocation. However, in a backhaul application, a D2D, or a dual connectivity use case, the network does not know in advance what resources and transport formats the UE can use or wants to use to communicate with the network. The assignment sent to the UE may thus be suboptimal. This problem is addressed by a solution where alternative usages of the assignment are allowed. The advantage is that the UE may select among the alternative usages when transmitting in uplink so as to adapt to the current UE situation. This implies that the eNodeB and the UE have prior knowledge of possible alternative usages of an assignment. The UE may select an alternative usage of the assignment based on a number of aspects, such as the UE&#39;s capabilities, or transmission mode. Furthermore, the different alternative usages are associated to different DMRSs respectively, which allows an indication of the applied alternative usage for the uplink data transmission via the DMRS signaling. 
     According to embodiments of the invention, a radio resource R, can be assigned to a UE by an eNodeB, although the usage of this particular resource R is not necessarily predetermined at the time of assignment. In some cases, a UE may find the assigned radio resource inappropriate given its transmission situation. The UE may therefore select an alternative usage of the radio resource that better suits its needs. The alternative usages are determined by the UE based on the received assignment. The alternative usages may comprise alternative transmission parameters for different transport formats and/or alternative resource usages. They may for example correspond to alternative usages of the time/frequency resources assigned, to other MCS or pre-coding than assigned, and/or to less layers/another rank than assigned. Each alternative usage is associated with a respectively different DMRS. The selected alternative usage may thus be signaled to the eNodeB through the signaling of the DMRS associated with the selected alternative usage. 
     An embodiment of the invention is illustrated in the signaling diagram of  FIG. 3 . In S. 31 , an eNodeB  301  sends an assignment to a UE  303  for an uplink transmission. The assignment provides the UE with information about what time-frequency resources—denoted R in the diagram—that the UE may use for its transmission of uplink data. The UE may then in  310  determine alternative usages of the assignment. The UE may e.g. be configured such that it can use either 100% or 50% of the assigned resources R, i.e. the complete amount of assigned resources R, or half of the resources R. The alternative usage “50% of R” is denoted P 1  in the signaling diagram. Assuming that the UE may not need more resources than P 1  given the state it is in, the UE would thus in  320  select the usage “50% of R”, i.e. P 1 , for its uplink transmission. Each alternative usage of the assignment is associated with a specific DMRS. In one example embodiment, the usage “50% of R” is associated with a first cyclic shift (CS) of the DMRS, and the usage “100% of R” is associated with a second CS of DMRS. The UE  303  may thus transmit uplink data on the resources P 1  in S. 32 . In S. 33 , the UE  303  transmits the DMRS with the first CS that is associated with the usage of “50% of R” to the eNodeB  301 . The eNodeB  301  may in  330  correlate the received DMRS with the two possible DMRSs, i.e. the DMRS with the first CS and the DMRS with the second CS. In this way the eNodeB  301  may deduce a probable DMRS based on the correlation in  340 . In this example embodiment, the DMRS with the first CS will be deduced with high probability. As the DMRS with the first CS is associated with the usage “50% of R”, the eNodeB  301  will with a high probability be able to decode the received uplink data in  350  knowing that the uplink data is received on P 1 . It should be noted that the eNodeB cannot be 100% sure which alternative usage that is used By the UE. However, the correlation with the alternative DMRS will provide likeliness for each of the alternative usages. The eNodeB may select one or more alternative usages that are most likely used and may try to decode the data for more than one alternative usage. 
     In the example embodiment described with reference to  FIG. 3 , the alternative usages of the assignment are associated with different DMRS, differing with regards to the CS. However, other alternatives for the different DMRSs used to signal an alternative usage of the assignment are possible. Some alternative embodiments are described in the following three paragraphs. 
     DMRS Cyclic Shift Signaling 
     As already described above, it is possible to apply a number of different CSs to a DMRS which is based on a particular base sequence. The DMRS may thus carry additional information through the CS, and the CS may be used to signal what alternative usage of the assignment that the UE has selected for its uplink transmission. This solution is exemplified by the embodiment described above with reference to  FIG. 3 . Some or all of the possible CS of the DMRS may thus be used to signal a change in how the assigned radio resource R is used. However, it is of course also possible to signal a use of alternative MCS instead of use of alternative time-frequency resource with the CS of the DMRS, or any other variants of alternative usages of the assignment. 
     Power Sensing 
     A flexible solution is to allow the UE to change both the uplink data allocation and the DMRS allocation. Different DMRS allocations may be used to signal an alternative usage of the assignment. Each reduction in the number of used time-frequency resources or PRBs may e.g. map to a new DMRS allocation. When the UE selects an alternative usage for uplink data by selecting a lower number of PRBs than assigned, the following alternatives with regards to the DMRS allocation may be possible:
         DMRS is still sent over the whole assigned PRB allocation. This enables the eNodeB to better correlate with alternative DMRS, e.g. different CS DMRS.   DMRS is only sent over the PRBs of the alternative usage. Detection and correlation of DMRS will then be more sensitive to noise.       

     In the latter case of DMRS allocation, a matched filter may be used to detect the DMRS allocation. A channel estimate may be done and thereby also a power estimate for the channel estimate. For a reasonable Signal to Interference and Noise Ratio (SINR) this will give a good performance for detecting the DMRS allocation. The method is referred to as power sensing. This solution may in one embodiment be combined with the CS solution described in the previous section. The alternative usages of the transport format assigned, such as alternative usages of the MCS, may be signaled with different CS of the DMRS as described above. The DMRS may thus differ both with regards to the CS used and with regards to the allocation of the DMRS. 
     If the DMRS associated with different alternative usages of the assignment only differ with regards to the DMRS allocation used, power sensing may be used to detect the DMRS as described above. However, using only a power estimate could perform significantly worse than an embodiment using a known pre-agreed DMRS allocation for each resource reduction or alternative usage of the assignment. The power sensing is therefore combined with a decoding attempt of the uplink data and a Cyclic Redundancy Check (CRC) to check if the decoding attempts is successful or not. As power sensing is more sensitive to noise, the selection of a single alternative usage to continue with for decoding increases the likeliness of a failed decoding. Therefore, it may be better to blindly test a number of hypotheses regarding different DMRS allocations associated to different alternative usages of the assignment. If computing power at the receiver is not limited, a decoding and the CRC check may be performed for all of the hypotheses. In a worst case scenario of poor SINR, all of the alternative usages may need to be tested anyhow. To reduce the need for computing resources at the receiver, it may be preferred to use a limited number of alternative usages of the assignment in order to avoid too many tests. 
     Rank Selection 
     When the assignment transmitted by the eNodeB is for a multi-layer Single User-Multiple Input Multiple Output (SU-MIMO) transmission, one possibility to signal alternative usages of the assignment is to do it via a rank selection. The eNodeB may be able to detect on what layer that the DMRS is transmitted, which indicates a rank selection. Each layer may potentially be associated with different HARQ processes or the same HARQ process depending on the used and supported MIMO formats. Therefore, either a separate decoding and CRC check may be done per layer and associated DMRS, or the decoding and CRC check may be done jointly over multiple layers (multiple DMRS). In one embodiment, the UE may dynamically chose how many layers to use for its transmission depending on what alternative usages of the assignment that it has selected for its uplink transmission. It may thus be sufficient for the eNodeB to know the association between a rank selection and the different alternative usages of the assignment to deduce how to decode the uplink data. 
     Determining the Alternative Usages 
     As already described above, the alternative usages of an assignment are determined by the UE based on the received assignment. The alternative usages may comprise different transport formats and/or different time-frequency resource usages. In one example embodiment, the alternative usage is defined as a predetermined subset selection of the assigned radio resource R. The subset selection may be tabulated, or it may be calculated using a predetermined function. Assuming that the radio resource R={r_ 1 , r_ 2 , . . . , r_N} consists of N PRBs, one example of how to determine subset selections of the assigned resources R is to use the following pre-determined split function: 
         S =Round(( n−cs )/ n*N )  [1]
 
     where n is the total number of cyclic shifts used for DMRS, and cs is the index of the cyclic shift used for the DMRS. The subset selection P 1  of the resource R is then defined as: 
         P 1={ r _1, r _2, . . . , r _ S}.   [2]
 
     It is also possible to use the remaining subset P 2  of the PRBs determined by the function S in a similar manner, where P 2  is defined as: 
         P 2={ r _ {S+ 1}, r _ {S+ 2}, . . . , r _ N}.   [3]
 
     Another example of a direct mapping is to decide the set of PRBs to use as a function of the cyclic shift index of the DMRS, e.g. as every (cs+1)&#39;th PRB. 
     In general, it is possible to use any pre-determined mapping, e.g. from a CS to a specific sub-set of PRBs in R. The mapping could be specified using a function, a table or any other method defining a mapping. The mapping may not necessarily be a one-to-one mapping, as the selection of an alternative usage may be based also on other parameters. 
     The CS can also be used to signal an alternative usage of the assigned MCS. If the MCS provided in the assignment is denoted M, alternative usages of the assignment can be determined in analogy with the above described alternative resource usages based on a predetermined function. Some examples of how to determine the MCS usage are given in the following, where cs denotes the index of CS used for the DMRS:
         MCS=M if cs=0, MCS=M−1 if cs=1, MCS=M−2 if cs=2, etc.   MCS=M+1 if cs=1, MCS=M+2 if cs=2, etc.   Use the most robust MCS, or a configurable default MCS for a specific cs, for example cs&gt;1       

     In general, any pre-determined MCS is possible as alternative usage, and it may be defined relative to the assigned MCS. The used MCS can be seen as a function of the CS, or as a function of the assigned MCS. 
     The way to determine the allocation of PRBs and of MCS could also be done jointly, i.e. determining that both alternative MCS M and alternative PRBs P 1  should be used. As one example the allocation of PRBs could be reduced and the MCS could be increased to preserve the number of transmitted bits. It is also possible to use an increased MCS if the assignment of resources R is too small to empty the UE buffer. 
     Configuration of Alternative Usages and Associations with DMRS 
     As described above, the alternative usages may be determined by the UE based on the received assignment, and each alternative usage of the assignment is associated with a different DMRS. In one embodiment of the invention, the UE is configured with how to determine the alternative usages of the assignment, and with the associations between the alternative usages of the assignment and the different DMRSs. 
     Different signaling options for the configuration of the UE are possible. As different UEs may have different capabilities and also different needs, the eNodeB may in one embodiment acquire information regarding if the UE supports the described mechanisms of determining alternative usages of an assignment. It is only a UE that is capable of handling the alternative usages of an assignment that can make use of a signaled configuration. Furthermore, the configuration of alternative usages and the associations between alternative usages and different DMRS can be seen as an agreement between the eNodeB and UE. This agreement should be chosen such that it optimizes the behavior of the UE in a specific situation and state. Hereinafter, in the sections describing different use cases, it is described how the design of an agreement may vary depending on the use case. 
     In embodiments of the invention, the different DMRS alternatives may indicate different alternative usages of e.g. a resource assignment for different UEs. In one example, a first UE may operate in a D2D mode and a second UE may operate in a power limited mode. Hence, when the eNode receives an uplink transmission from the first of the two UEs, it is necessary for the eNodeB to know what agreement that applies for the first UE, such that the correct hypothesis regarding DMRS signaling and association with alternative usages is tested. This is enabled by the signaling options described hereinafter. 
     In a first embodiment, the alternative usages of an assignment may be configured by higher layers and may thus be signaled in a high layer configuration message, such as a Radio Resource Control (RRC) message or a broadcasted System Information message. The signaling of the configuration may be done using broadcast transmissions or jointly together with other signaling. As one example, a future configuration message for D2D operation specified to configure a D2D UE may also include a configuration of the alternative usages of an assignment and the corresponding associations with different DMRSs. Such signaling would thus reach all D2D UEs. 
     In a second embodiment, the configuration is done semi-statically by configuring the UE behavior using RRC reconfiguration messages, which thus reaches a specific UE. 
     In a third embodiments, one or more new Downlink Control Information (DCI) formats may be defined. A DCI format defines how an assignment is to be understood by a UE. A DCI message of a certain format may thus include information about the configuration of alternative usages. Either the DCI format itself or the content of the DCI message may carry the configuration information. It may be noted that such a new DCI format may anyhow be needed to support new services such as D2D and self-backhauling, and could therefor be designed to support configuration of alternative usages of an assignment. In this third embodiment, the configuration information is thus signaled per assignment. 
     In still another embodiment, multiple uplink DCIs valid for a same subframe may be used to signal the configuration. This may be done by sending multiple uplink DCIs in one subframe, using multiple of the Physical Downlink Control Channel (PDCCH) candidates. Another alternative is to use the SPS possibilities, where one or multiple SPS assignments are valid for a subframe. One or multiple DCIs can still be sent on PDCCH/EPDCCH where the terminal selects from all possible DCIs. 
     The different signaling embodiments described above may be used on their own or may be combined with each other in different ways. 
     Methods and Apparatus 
       FIG. 4 a    is a flowchart illustrating an embodiment of a method for uplink transmission performed in a wireless terminal served by a radio network node of a wireless communication system. As in the example embodiments described previously, the wireless terminal may be a UE, and the radio network node may be an eNodeB in LTE. The method comprises:
           410 : Receiving an assignment for an uplink transmission from the radio network node.     420 : Determining alternative usages of the assignment based on the received assignment. Each alternative usage is associated with a different DMRS. The alternative usages of the assignment may comprise alternative usages of assigned time-frequency resources, and/or alternative usages of assigned transmission formats. The alternative usages may be determined based on a function of the received assignment. One example of a function used to determine the alternative usages is given in [1] and [2] above. Furthermore, the different DMRSs associated with the alternative usages may differ with respect to at least one of: a cyclic shift of the DMRS, an allocation of the DMRS, and a rank selection determining on what layers the DMRS is transmitted.     430 : Selecting a usage among the alternative usages of the assignment. Selecting the usage may be based on at least one of: a capability of the wireless terminal, a transmission mode of the wireless terminal, a DCI format of the assignment, resources on which the assignment is received, and a rank granted in the assignment. As an example, an assignment received on a certain PRB set of the PDCCH may imply that the UE selects a certain alternative usage of the assignment, while another PRB set would imply another alternative usage.     440 : Applying the selected usage when transmitting uplink data to the radio network node.     450 : Transmitting the DMRS associated with the selected usage.       
       FIG. 4 b    is a flowchart illustrating another embodiment of the method in the wireless terminal. The method comprises in addition to the steps  410 - 450  described above:
           400 : Receiving configuration information from the radio network node configuring at least one of the following: how to determine the alternative usages of the assignment; and the associations between the alternative usages and the different DMRSs. The configuration information may be received in at least one of the following: a system information message, a RRC reconfiguration message, and a DCI message. A system information message may e.g. be broadcasted to all UEs in a cell, and a RRC reconfiguration message may adapt the configuration of alternative usages to a specific UE.       

       FIG. 5  is a flowchart illustrating an embodiment of a method for decoding uplink data received from a wireless terminal. The method is performed in a radio network node of a wireless communication system serving the wireless terminal. As in the example embodiments described previously, the wireless terminal may be a UE, and the radio network node may be an eNodeB in LTE. The method comprises:
           500 : The optional step of transmitting configuration information to the wireless terminal configuring at least one of the following: how to determine alternative usages of the assignment; and associations between the alternative usages and the different DMRSs. The configuration information may be transmitted in at least one of the following: a system information message, a RRC reconfiguration message, and a DCI message. A system information message may e.g. be broadcasted to all UEs in a cell, and a RRC reconfiguration message may adapt the configuration of alternative usages to a specific UE.     510 : Transmitting an assignment for an uplink transmission to the wireless terminal.     520 : Receiving a DMRS and uplink data from the wireless terminal in response to the assignment.     530 : Correlating the received DMRS with at least one of a plurality of different DMRSs. Each different DMRS is associated with an alternative usage of the assignment. The alternative usages of the assignment may comprise alternative usages of assigned time-frequency resources, and/or alternative usages of assigned transmission formats. Furthermore, the different DMRSs associated with the alternative usages may differ with respect to at least one of: a cyclic shift of the DMRS, an allocation of the DMRS, and a rank selection determining on what layers the DMRS is transmitted.     540 : Selecting a probable DMRS among the plurality of different DMRSs based on the correlation.     550 : Decoding the received uplink data using the alternative usage associated with the probable DMRS.       

     In embodiments of the invention, different DMRS hypothesizes need to be tested to find the most probable DMRS used by the UE, and to make a correct assumption regarding what alternative usage of the assignment that the UE has applied for the uplink data transmission. As described above in the section related to power sensing, the method may thus additionally comprise the steps of performing a CRC of the decoded uplink data. If the CRC indicates a correct decoding, nothing further needs to be done. However, if the CRC indicates an error in the decoded uplink data, the method also comprises selecting a new probable DMRS based on the correlation, and decoding the received uplink data using the alternative usage associated with the new probable DMRS. 
     An embodiment of a wireless terminal  650  for uplink transmission configured to be served by a radio network node  610  of a wireless communication system, is schematically illustrated in the block diagram in  FIG. 6 a   . The wireless terminal  650  comprises a processor  651 , a memory  652 , a receiver  653 , and a transmitter  654 . The memory  652  contains instructions executable by said processor  651 , whereby the wireless terminal  650  is operative to receive an assignment for an uplink transmission from the radio network node  610  via the receiver  653 , and determine alternative usages of the assignment based on the received assignment. Each alternative usage is associated with a different DMRS. The wireless terminal  650  is further operative to select a usage among the alternative usages of the assignment, apply the selected usage when transmitting uplink data to the radio network node via the transmitter, and transmit the DMRS associated with the selected usage via the transmitter  654 . The alternative usages of the assignment may comprise at least one of: alternative usages of assigned time-frequency resources, and alternative usages of assigned transmission formats. Furthermore, the memory  652  may contain instructions executable by said processor  651  whereby the wireless terminal is further operative to determine the alternative usages based on a function of the received assignment. The different DMRSs associated with the alternative usages may differ with respect to at least one of: a cyclic shift of the DMRS, an allocation of the DMRS, and a rank selection determining on what layers the DMRS is transmitted. 
     In embodiments, the memory  652  may contain instructions executable by said processor  651  whereby said wireless terminal is further operative to receive configuration information from the radio network node via the receiver  653 . The configuration information may configure at least one of the following: how to determine the alternative usages of the assignment; and the associations between the alternative usages and the different demodulation reference signals. 
     In another embodiment, the memory  652  may contain instructions executable by the processor  651  whereby the wireless terminal is further operative to receive the configuration information in at least one of the following: a system information message, a RRC reconfiguration message, and a DCI message. 
     In another embodiment, the memory  652  may contain instructions executable by the processor  651  whereby the wireless terminal is further operative to select the usage based on at least one of: a capability of the wireless terminal, a transmission mode of the wireless terminal, a downlink control information format of the assignment, resources on which the assignment is received, and a rank granted in the assignment. 
     An embodiment of a radio network node  610  of a wireless communication system configured to decode uplink data received from a wireless terminal  650  served by the radio network node, is also schematically illustrated in the block diagram in  FIG. 6 a   . The radio network node comprises a processor  611 , a memory  612 , a transmitter  613 , and a receiver  614 . The memory contains instructions executable by the processor whereby the radio network node is operative to transmit an assignment for an uplink transmission to the wireless terminal via the transmitter  613 , receive a DMRS and uplink data from the wireless terminal via the receiver  614  in response to the assignment, and correlate the received DMRS with at least one of a plurality of different DMRSs. Each different DMRS is associated with an alternative usage of the assignment. The alternative usage of the assignment may comprise at least one of: alternative usages of assigned time-frequency resources, and alternative usages of assigned transmission formats. Furthermore, the different DMRSs associated with the alternative usages may differ with respect to at least one of: a cyclic shift of the DMRS, an allocation of the DMRS, and a rank selection determining on what layers the DMRS is transmitted. The memory also contains instructions executable by the processor whereby the radio network node is operative to select a probable DMRS among the plurality of different DMRSs based on the correlation, and decode the received uplink data using the alternative usage associated with the probable DMRS. 
     In one embodiment, the memory  612  may contain instructions executable by said processor  611  whereby the radio network node is further operative to perform a CRC of the decoded uplink data. If the CRC indicates an error in the decoded uplink data, the radio network node is further operative to select a new probable DMRS based on the correlation, and decode the uplink data using the alternative usage associated with the new probable DMRS. 
     In another embodiment, the memory  612  may contain instructions executable by said processor  611  whereby the radio network node is further operative to transmit configuration information via the transmitter  613  to the wireless terminal  650  configuring at least one of the following: how to determine the alternative usage of the assignment; and the associations between the alternative usages and the different DMRSs. Further, the radio network node may be operative to transmit the configuration information in at least one of the following: a system information message, a RRC reconfiguration message, a DCI message. 
     In an alternative way to describe the embodiment in  FIG. 6 a   , illustrated in  FIG. 6 b   , the wireless terminal  650  comprises means  661  adapted to receive an assignment for an uplink transmission from the radio network node via the receiver, and means  662  adapted to determine alternative usages of the assignment based on the received assignment. Each alternative usage is associated with a different DMRS. The wireless terminal  650  also comprises means  663  adapted to select a usage among the alternative usages of the assignment, means  664  adapted to apply the selected usage when transmitting uplink data to the radio network node via the transmitter, and means  665  adapted to transmit the DMRS associated with the selected usage via the transmitter. The radio network node  610  comprises means  621  adapted to transmit an assignment for an uplink transmission to the wireless terminal via the transmitter, and means  622  adapted to receive a DMRS and uplink data from the wireless terminal via the receiver in response to the assignment. The radio network node  610  also comprises means  623  adapted to correlate the received DMRS with at least one of a plurality of different DMRSs, each different DMRS being associated with an alternative usage of the assignment. The radio network node  610  further comprises means  624  adapted to select a probable DMRS among the plurality of different DMRSs based on the correlation, and means  625  adapted to decode the received uplink data using the alternative usage associated with the probable DMRS. The means described above are functional units which may be implemented in hardware, software, firmware or any combination thereof. In one embodiment, the means are implemented as a computer program running on a processor. 
     Use Cases 
     D2D 
     In a D2D deployment, a UE  703  is assigned resources R by an eNodeB  701 . The resources R has to be shared for the communication with the network and the D2D communication. The UE is responsible for the usage of the assigned resources R for the D2D communication. Two D2D scenarios are possible, illustrated in  FIG. 7 a    and  FIG. 7 b    respectively. In  FIG. 7 a    the D2D communication takes place between two other UEs  705   a  and  705   b  than the UE  703  that receives the assignment. The D2D communication between the two other UEs  705   a  and  705   b  uses some part P 1  of the radio resources R. The remaining resources P 2  may thus be used for the communication between the UE  703  receiving the assignment and the eNodeB  701 . 
     In  FIG. 7 b   , the D2D communication takes place between the UE  703  receiving the assignment and another UE  705   c , on some part P 1  of the radio resource R. And again, the remaining resources P 2  can be used for communication between the UE  703  receiving the assignment and the eNodeB  701 . 
     This use-case could also be a applicable for self-backhauling applications. 
       FIG. 7 c    is a signaling diagram illustrating the method of assigning resources in a D2D use case. The eNodeB  701  transmits an assignment of resources R to the UE  703 . The UE  703  determines what alternative usages of the assignment that are possible, and determines that P 1  should be used for the D2D communication and P 2  for the communication with the eNodeB  701 . The D2D communication is scheduled on P 1  in  710 . In S. 72  the assignment is sent for the D2D, and D2D communication may be performed on P 1  in  720 . In S. 73  the UE  703  transmits uplink data on P 2  and also signals the selected alternative usage via the DMRS signaling. 
     Dual Connectivity 
     A dual connectivity use case is illustrated in  FIGS. 8 a  and 8 b   . A UE  803  is assigned to two eNodeBs  801   a  and  801   b . The scheduling made by the first eNodeB  801   a  is unknown to the second eNodeB  801   b . Hence, in case the UE  803  implements a single Power Amplifier (PA), and the scheduling from the first eNodeB  801   a  and the scheduling from the second eNodeB  801   b  are both valid for the same TTI, a UE may be forced to drop one transmission. Such a solution limits the benefits with dual connectivity strongly as this, for example, means that the possibility for simultaneous downlink/uplink transmissions is restricted. Therefore embodiments of the current invention address a solution making simultaneous uplink transmission possible for the first and the second eNodeB  801   a  and  801   b .  FIG. 8 a    is a signaling diagram illustrating the signaling for such a use case. The first eNodeB  801   a  sends an assignment for resources RA, and the second eNodeB  801   b  sends an assignment for resources RB for the same TTI. If the assigned resources RA and RB are over-lapping or the UE is power limited and no power reduction is allowed, the UE may use the possibility of alternative usages of assigned resources to solve the situation. According to embodiments of the invention, the UE  803  may determine an alternative usage of the assignments, such that resources P 1  which is a subset of the resources RA are used for the communication with the first eNodeB  801   a , and resources P 2  which is a subset of the resources RB are used for the communication with the second eNodeB  801   b , which is also illustrated in  FIG. 8   b.    
     Power Limited UE 
     If a UE is assigned more resources than it has the power to use, the UE may reduce the power on the allocation in accordance to an alternative usage of the assignment. The lowered power setting would result in a drop in SINR. To maintain a target block-error rate, the UE may thus also need to change MCS, pre-coding and/or rank. The UE may in one example select an alternative usage comprising another MCS than assigned. The alternative usage could be signaled as described above using DMRS signaling. Any of the different alternatives of DMRS signaling may be used in this case. 
     The alteration of e.g. MCS or rank may also be predetermined. In one example, if the UE use a power setting that is 3 dB lower than what is assumed by the eNodeB, an MCS corresponding to a 3 dB lower SINR would be used and thus signaled. Alternatively one layer could be dropped to indicate the alternative usage. 
     Latency Reduction and Buffer Uncertainty 
     In many use cases the amount of buffered data in the UE is uncertain. This may be due to a number of factors, such as, a long reporting delay compared to the packet inter-arrival time. In many scenarios, latency sensitive services can benefit from getting a larger allocation to make sure that the UE can empty its buffer with the allocated uplink resources. The flexible allocation of uplink resource according to embodiments of the invention may also make it possible to allocate a larger amount of uplink resources to UEs, where some of the resources may not be used, without any large performance down-side. 
     The above mentioned and described embodiments are only given as examples and should not be limiting. Other solutions, uses, objectives, and functions within the scope of the accompanying patent claims may be possible.