HK1180484B - Methods and arrangements in a telecommunication system - Google Patents

Description

Method and arrangement in a telecommunication system

Technical Field

The present invention relates to control of retransmissions in a user equipment supporting uplink spatial multiplexing.

Background

In data communication or data storage, it is customary to transmit or store data with redundancy in an encoded manner in order to improve the reliability with which the original message can be recreated. This process is commonly referred to as channel coding and the recovery process is commonly referred to as channel decoding. Even if in the following such a message is not strictly coded, we will refer to it as a codeword.

In communication systems such as the Long Term Evolution (LTE) system standardized by the third generation partnership project (3GPP), it is also common to combine several transmissions relating to the same codeword in different Transmission Time Intervals (TTIs) as needed to adaptively increase the level of redundancy to the transmission conditions. This can be done, for example, by repeating a shorter encoded or unencoded message one or several times. An alternative is to transmit in the first transmission attempt a part of the codeword containing sufficient information for correct decoding under favorable conditions. If not correctly received and decoded, additional portions of the codeword can be transmitted in subsequent attempts, after which the received portions of the codeword can be recombined at the receiver side, creating incremental redundancy for each retransmission. This can then help to ensure that sufficient, but not more than necessary, resources are used for the transmission of each message. For simplicity, we will refer to subsequent transmissions of the same codeword as a retransmission, even though it may not be the entire codeword that is being retransmitted. The information bits carried by the codeword will be referred to as a Transport Block (TB).

While waiting for the previous message to be decoded and possibly (partially) retransmitted, in order that the transmission of the subsequent codeword is not delayed, there are sets of buffers containing data of different codewords in parallel. In this way, while waiting for a message of the same transport block that was decoded by a previous transmission and correctly/incorrectly received is received (acknowledgement (ACK) or Negative Acknowledgement (NACK) message) on the transmitter side, it is possible to read other buffers for the (re-) transmission. These buffers are generally called hybrid automatic repeat request (hybrid ARQ or HARQ) buffers, and the process controlling each of them is called a HARQ process.

HARQ retransmissions are handled by the Medium Access Control (MAC) layer as part of layer 2 (L2) in the LTE protocol architecture. HARQ feedback, i.e., ACK or NACK indications, is signaled from the physical layer, also referred to as layer 1, to the MAC layer. Layer 2 uses this information for retransmission or new transmission during its data transfer.

Multiple antenna techniques can significantly increase the data rate and/or reliability of a wireless communication system. The performance is improved especially when both the transmitter and the receiver are equipped with multiple antennas. This results in multiple-input multiple-output (MIMO) communication channels, and such systems and/or related techniques are commonly referred to as MIMO techniques.

One MIMO technique is Spatial Multiplexing (SM) or single-user MIMO (SU-MIMO), in which one or several transport blocks associated with one particular user are mapped (usually linearly) to one or several layers of data simultaneously, which in turn may be mapped to different transmit antenna ports via a channel adaptive pre-encoder (also often a linear pre-coder). Currently for LTE, one or two codewords corresponding to one or two transport blocks are mapped to one or several layers of data. In this way, the spatial properties of the MIMO channel can be used to advantage to transmit more data related to the same user at the same time, thereby increasing user data throughput. Additional intermediate processing steps may also be present for various reasons.

In LTE release 10 (rel.10), the Uplink (UL), which is the communication link from user equipment to a base station or evolved nodeb (enb) in LTE terminology, is extended from supporting Single Input Single Output (SISO) to also supporting UL spatial multiplexing (UL-SM).

As in previous releases (Rel-8 and Rel-9), UL transmissions are triggered via an uplink transmission grant transmitted on the Physical Downlink Control Channel (PDCCH). However, the retransmission can be triggered by a full grant transmitted on the PDCCH, or by a negative determination indicating a decoding failure of a previous transmission attempt of the corresponding codeword on the Physical HARQ Indicator Channel (PHICH) if no PDCCH grant for the corresponding transport block is found. Since the PDCCH grant format allows a new transmission format (e.g., modulation constellation and code rate) to be specified, the former retransmission type is often referred to as adaptive retransmission. The latter type of retransmission is therefore referred to as a non-adaptive retransmission, since the PHICH only carries an indication of the ACK or NACK of the previous transmission and gives no other signalling possibilities to order the UE to use the new transport format.

In LTE, UL synchronous HARQ is employed, which means that there is a fixed timing relationship between transmission and retransmission, so there is a direct mapping from TTI to HARQ process Identity (ID) and this information is not needed in the UL grant. The base station is thus able to grant UEUL retransmissions solely by means of the phich nack when there is limited PDCCH resources, which thus reduces the involvement of layer 2L 2 resources compared to grants received on PDCCH. The drawback is that new information about the transport format cannot thus be conveyed to the UE, such as link adaptation or frequency selective rescheduling. The reliability of the PHICH channel is also lower than that of the PDCCH grant.

However, in LTE downlink DL, asynchronous HARQ is employed and an explicit PDCCH assignment is needed to indicate that DL (re-) transmissions relate to a specific DL HARQ process. For DL spatial multiplexing, there is therefore always an assignment for retransmission of any codeword.

This means that for LTE, when DL spatial multiplexing is configured, the physical layer or layer 1L 1 of the UE reads the PDCCH for DL assignments and when a downlink assignment is detected, it will also detect whether the assignment is valid for one or two transport blocks. This means that if PDCCH signaling indicates no assignment for one of the transport blocks (e.g., TB 1), the UE will not read the Physical Downlink Shared Channel (PDSCH) for the data for this transport block. However, for TB2, it will read the PDSCH from the PDCCH to detect the corresponding codeword representing the data. The data is then forwarded to L2 or the Medium Access Control (MAC) layer and appropriate HARQ processes for decoding.

In the case of configuring UL-SM, for each TTI, the UE may be assigned a UL grant valid for one or two TBs. It is assumed that LI will detect whether grants are valid for one or two TBs based on explicit PDCCH signaling, similar to the way done for DL spatial multiplexing. The reason for disabling transport blocks may be that the UE buffer may be empty, or the MIMO channel may not be rich enough (rich) to carry multiple data layers.

It should be noted that for spatial multiplexing, the concept of a single grant valid for one or two transport blocks is actually equivalent to the concept of one or two grants valid for one transport block each. The differences are only semantic and may be used interchangeably hereafter.

Current 3gpp mac layer specification procedures for UL data transfer can only handle one UL grant (or lack of UL grant) per TTI, so some complexity can be expected when one transport block is assigned an UL grant and another transport block is not assigned an UL grant. Since in the current specification these two branches are mutually exclusive, it is more straightforward to handle each transport block separately, i.e. assuming that L2 receives the respective grant per transport block and each transport block is associated with a separate HARQ process. Thus, the grant reception process should iterate once for each grant associated with a certain TTI.

Assuming that the procedure is run separately for each transport block, different branches can be run for different cases of one transport block, e.g., TB1, with no UL grant and another transport block, e.g., TB2, with an UL grant.

Disclosure of Invention

Since layer 1L 1 only forwards grants and not lack of grants to layer 2L 2, information of only transport blocks with valid grants will be forwarded to L2 and no information is provided whether transport blocks without valid grants are scheduled or disabled. L2 will then initiate its data transfer process for each transport block. If a grant for a transport block is received, an adaptive retransmission or a new transmission is performed according to the grant. Otherwise, if a negative acknowledgement indication NACK is decoded for a previous transmission in the same HARQ process for the transport block, a non-adaptive retransmission is performed. If an ACK is indicated for a previously transmitted decoding acknowledgement in the same HARQ process for a transport block, no action is taken until an uplink grant for the transport block is received. Knowing how retransmission works in the UL, the lack of a valid UL grant for one of the transport blocks combined with the misinterpretation of the phiichack, such that the UE erroneously detects a NACK indicating retransmission, causes the UE to perform non-adaptive retransmission, which is an undesirable behavior. It can be assumed that the problem occurs when either of the two codewords is disabled.

If no uplink grant is provided from the physical layer to higher layers, e.g., layer 2, for the HARQ process associated with the special subframe, HARQ feedback on the PHICH controls whether the HARQ process should perform non-adaptive retransmission in that subframe. When PDCCH indicates a grant for only one HARQ process (e.g., because one codeword corresponding to one transport block is disabled), control of another HARQ process is based on PHICH signaling that is less reliable than PDCCH signaling. In such a case, the UE can decode a NACK intended to be an ACK on the PHICH in error and initiate a non-adaptive retransmission for the transport block based on the erroneously decoded NACK.

Thus, given the two types of retransmissions in the UL, PDCCH grant triggered adaptive retransmission and phich nack triggered non-adaptive retransmission, it is possible that in UL spatial multiplexing mode, the UE is instructed to perform adaptive retransmission for one TB (as commanded by the PDCCH), but since L2 does not get explicit information about the other TB being suspended or disabled, it will process that TB as if it did not get a UL grant. For this TB, the UL data delivery procedure is run, so the UE can fail to decode the ACK on the PHICH and initiate a non-adaptive retransmission for this TB as described above, even though the base station may have explicitly indicated that it is not needed.

The base station always has to perform the same amount of PDCCH signaling regardless of whether it wants to schedule one or two transport blocks, and since adaptive retransmissions would give better performance, it is assumed that there is no situation where it would intentionally schedule only one transport block and want another transport block to perform non-adaptive retransmissions. Since PDCCH has a much lower error rate than PHICH, the solution is able to take advantage of this and allow PDCCH grant assignment to take precedence over PHICHA/N information, even if PDCCH indicates that a particular transport block is not assigned a grant.

Since it is assumed that L1 already knows from the PDCCH whether a transport block is disabled, the problem identified by the solution presented herein is that this information is not forwarded to L2, which may result in unnecessary non-adaptive retransmissions.

The present invention is therefore directed to preventing a UE from performing unexpected non-adaptive retransmissions for one or more transport blocks.

In an aspect of the present invention, a method for controlling retransmission in a user equipment supporting uplink spatial multiplexing is provided. The method comprises the following steps:

-detecting an uplink grant on a physical downlink control channel, the uplink grant being valid for at least one transport block;

-detecting that at least one transport block is disabled such that no grant is associated with at least one transport block; and

-interpreting at least one disabled transport block as an acknowledgement, ACK, corresponding to a previous transmission of said disabled transport block, regardless of which indication was received on a reception status feedback channel for said previous transmission.

The detecting step may in a particular embodiment be implemented at a first protocol layer, whereby the interpreting step comprises the first protocol layer conveying an indication of an acknowledgement, ACK, to a second protocol layer. In a particular embodiment, the indication comprises the step of setting an ACK/NACK flag to ACK. The acknowledgement may be used as an input in a HARQ process corresponding to the disabled transport block in an uplink data transfer process.

The first protocol layer may be a physical layer and the second protocol layer may be a higher protocol layer.

In another aspect of the present invention, an apparatus for controlling retransmission in a user equipment supporting uplink spatial multiplexing is provided. The apparatus includes a processing unit including circuitry configured to:

-detecting an uplink grant on a physical downlink control channel, the grant being valid for at least one transport block;

-detecting that at least one transport block is disabled such that no grant is associated with at least one transport block;

-interpreting at least one disabled transport block as an acknowledgement, ACK, corresponding to a previous transmission of said disabled transport block, regardless of which indication was received on a reception status feedback channel for said previous transmission.

Thus, in a particular embodiment, when L1 detects (based on PDCCH signaling or some other method) that a TB is disabled, it can set the a/N (ACK/NACK) bit to ACK regardless of the PHICH indication for this TB.

In this way, when one TB has a grant for adaptive retransmission and another TB is not, running the UL data transfer procedure, the non-granted TBs will not accidentally cause non-adaptive retransmission.

Other objects, advantages and novel features of the invention will become apparent from the following description when considered in conjunction with the drawings and the claims.

Drawings

The above and other objects, features and advantages of the present invention will be apparent from this detailed description as illustrated in the accompanying drawings.

FIG. 1 shows a flow chart illustrating an embodiment of the present invention.

Fig. 2a-2b show different scenarios for uplink spatial multiplexing.

Fig. 3 schematically shows an apparatus according to an embodiment of the invention.

Fig. 4 shows, in an alternative manner, a device according to an embodiment of the invention.

Detailed Description

In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular architectures, interfaces, techniques, etc. in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail.

It should be noted that although terminology from 3gpp lte has been used in this disclosure to illustrate the invention, this should not be taken as limiting the scope of the invention to only the above-described system. Other wireless systems including Wideband Code Division Multiple Access (WCDMA), WiMax, UMB, and GSM can also benefit from embodiments of the present invention.

It is also noted that terms such as base station and UE should be considered non-limiting and do not imply in particular a certain hierarchical relationship between the two; in general, a "base station" can be regarded as the apparatus 1, and a "UE" can be regarded as the apparatus 2, and the two apparatuses communicate with each other through a radio channel. In addition, in the following description of the embodiments of the present invention, a physical protocol layer will be referred to as layer 1, and a higher protocol layer will be referred to as layer 2. However, the present invention is not limited to layer 1 or layer 2.

In the following, embodiments of the invention are discussed in order to describe in detail suitable applications of the invention.

An illustration of a method according to a particular embodiment can be found in fig. 1 a. A UE in UL-SM mode, which is configured with N transport blocks such that N codewords can be spatially multiplexed, upon receiving a downlink subframe, reads the PDCCH, see step 101, and detects in step 102 a PDCCH message indicating at least one UL grant for at least one transport block for a particular TTI. If one grant for each configured transport block is detected for this TTI, N grants are forwarded to layer 2 for each transport block in step 104, wherein the process of layer 2 UL data transfer for each transport block is iterated or initiated, 105, thereby resulting in an adaptive retransmission or new codeword transmission according to the associated grant. In a particular embodiment, N = 2. However, N may also be a number greater than two.

If in step 103 it is detected that only K grants for N transport blocks are detected for a particular TTI, where 0< K < N, see step 103, e.g. only one grant associated with a single transport block is detected for a particular TTI, such as TB1 instead of TB2 (TB 1 and TB2 can of course be exchanged), the disabled transport block (i.e. the transport block for which no grant is detected) should be interpreted as: an acknowledgement ACK is received for a previous transmission corresponding to the disabled transport block. According to this particular embodiment, this is done so that layer 1 sets the associated ACK/NACK flag for the previous transmission to ACK, 106, regardless of the PHICH indication for the previous transmission, and forwards the available grant, e.g., for TB1, to layer 2, see step 107, which will iterate or initiate the layer 2 data transfer process for each transport block, see step 105. For transport blocks with valid grants, e.g., TB1, this will result in an adaptive retransmission or new codeword transmission according to the associated grant. For any transport block that does not have a grant, such as TB2, no non-adaptive retransmissions will occur because the a/N flag is set to ACK.

If no grant indicating an adaptive retransmission or a new transmission is detected upon reception of the downlink subframe and if the PHICH is decoded without a previously transmitted ACK for the corresponding transport block of any codeword, layer 1 sets the ACK/NACK flag to NACK for the corresponding transport block and forwards this ACK/NACK flag to L2, see step 108, which initiates a non-adaptive retransmission unless an expected or predetermined maximum number of transmissions has been made for the corresponding codeword, see step 105.

The above method has minimal impact on the 3GPP standard specifications. For each grant, the data transfer process at layer 2 is only initiated or iterated without modification. The absence of both grants still means that the ACK/NACK is read to determine if a non-adaptive retransmission should take place.

The exemplary method described above uses the convention of a separate grant for each transport block and a separate HARQ process for each transport block, but alternative methods can use the convention of a single grant addressing one or two transport blocks and one HARQ process to manage two codeword buffers. The actual results of both methods would be the same.

Another embodiment is shown in fig. 1b, where instead of layer 1 setting ACKs for transport blocks with no valid grants to deliver to higher layers, said higher layers, e.g. layer 2, assume that for transport blocks for which grants are not forwarded by the physical layer to higher layers, acknowledgements have been received for transmission in the previous TTI. This assumption may be made, for example, by setting an ACK/NACK flag to ACK for any transport block with no valid grant before initiating the UL data transfer procedure, see step 106. This embodiment is shown in fig. 1b, where steps 101, 102, 105 and 108 are the same as those in fig. 1 a. In step 104b, the available grants are forwarded by layer 1 to layer 2. In step 106b, layer 2 assumes that any transport blocks that are not permitted to be forwarded from layer 1 are disabled. In a special embodiment, the mechanism in layer 2 sets the ACK/NACK flag to ACK in step 107b, regardless of what reception status feedback it receives from layer 1, i.e. ACK or NACK. In step 105, a UL data transfer procedure is then run for each transport block.

Still referring to fig. 1b, in another embodiment, layer 2 will perform the UL data transfer procedure only for transport blocks with associated grants after assuming in step 106b that no transport block forwarded from L1 is granted disabled, see step 109, which means that in this embodiment layer 2 will not read any ACK/NACK indications from layer 1. For transport blocks associated with a grant, this results in an adaptive retransmission or new codeword transmission according to the associated grant. For one or more transport blocks that do not have a grant, no retransmissions are initiated from L2. In such embodiments, the HARQ processes may inform each other whether a grant is received, and HARQ processes that do not receive a grant may suspend themselves if any other HARQ process has received a grant for that certain TTI. When no grant is detected, L1 runs a non-adaptive retransmission for the TB that detects NACK on PHICH, see step 108.

The application of an embodiment of the invention will also be illustrated with reference to fig. 2a and 2 b. Fig. 2a shows prior art cases 1-3 without the invention and fig. 2b shows cases 4 and 5 with the invention applied. In these cases, it is assumed that two transport blocks TB1 and TB2 can be spatially multiplexed.

Case 1

At time 1, the UE decodes the ACK for TB1, assuming that the ACK related to the earlier UL transmission in TB1 was signaled on PHICH. Meanwhile, the UL grant for the new transmission at time 1 is received on the PDCCH. Alternatively, the UE decodes the NACK assuming that the NACK is signaled and at the same time an adaptive retransmission of the failed codeword at time 2 is granted on the PDCCH. One of the same two alternatives occurs for TB 2.A transport format adaptation transmission (new transmission or retransmission) that depends on the grant at time 1 is then transmitted on the PUSCH at time 2. For TB1, one of the same alternatives occurs for time 3 and time 4 as for time 1 and time 2. However, the TB2 transmission at time 2 was acknowledged at time 3, but for some reason no new transmission was scheduled for TB2, e.g., the UE buffer may be empty, or the MIMO channel is considered to be insufficiently rich, multiple layers cannot be reserved, or due to other scheduling decisions. Thus, at time 4, there is a new transmission or retransmission of TB1 but no transmission/retransmission of TB2, according to the PDCCH grant of TB1 at time 3.

Case 2

The same alternative occurs for time 1 and time 2 as for case 1 above. However, in this case, the transmission of none of the TBs results in a successful reception and is negatively acknowledged at time 3. However, there is no new grant, e.g., there may not be enough PDCCH resources to command adaptive retransmission of the two codewords, so the UE interprets the NACK as performing non-adaptive retransmission at time 4.

Case 3

Again, the same two alternatives occur for time 1 and time 2, as for case 1 above. In this case, the transmission of one of the TBs at time 2 is unsuccessful. Currently, only one of the transport blocks (such as TB 1) receives the UL grant on the PDCCH at time 3. If this TB is successfully decoded at time 2, a new transmission at time 4 is triggered by the grant, or if the previous transmission of the corresponding TB failed, resulting in an adaptive retransmission grant, an adaptive retransmission at time 4 is triggered. However, another TB (referred to as TB2 in fig. 2 a) that receives a NACK without grant would perform a non-adaptive retransmission. Now suppose that the purpose of one grant for TB1 only is that we want to disable another TB (TB 2), e.g., due to poor channel conditions, and suspend retransmissions, e.g., until more favorable channel conditions apply, it is not possible to distinguish between the two cases, and TB2 would undesirably make an L2-initiated non-adaptive retransmission. It should also be noted that the information in the grant for TB1, e.g., precoder rank, may also subsequently conflict with the non-adaptive transmission format for TB2 retransmission.

Case 4

Now, assuming that the ACK for TB2 was misinterpreted as a NACK in case 1 above, a non-adaptive retransmission of the corresponding codeword would be triggered in error according to the current criteria.

Case 5

The solution for error cases 3 and 4 according to embodiments of the present invention is to interpret the disabling of TBs as ACKs to higher layers, which in this example means letting grants valid for a single TB always mean ACKs for TBs with no valid grants, regardless of the PHICH indication, as in this case. This means that case 3 cannot be used to trigger a non-adaptive retransmission at the same time as an adaptive retransmission or a new transmission. Instead, the adaptive retransmission is used together with the adaptive retransmission or new transmission of another TB. The risk of accidental non-adaptive retransmissions due to misinterpretation of the PHICH is avoided. The overhead of explicit grants on the PDCCH already when used for one TB to use grants for another TB as well is very limited or non-existent. In addition, for adaptive retransmissions, the performance is better than for non-adaptive retransmissions.

The two lowermost graphs in case 5 show how the situation of one TB to be retransmitted due to unsuccessful transmission while signaling an adaptive retransmission for the other TB or a new transmission is made, see case 3 in fig. 2. a. With the present invention, since a single grant on PDCCH implies ACK for TB2 regardless of PHICH reception, the PHICH and content on PDCCH as indicated in case 3 would be the disabling of TB2, therefore the penultimate subgraph is dashed in fig. 2 b. To enable retransmission of failed TBs, we also explicitly grant another TB, thus obtaining adaptive retransmission (since PDCCH load is the same for either single TB grant or both TB grants). Thus, because PDCCH takes precedence over PHICH, the content of the transmission on PHICH does not matter (in principle, PHICH need not be transmitted), a decoding ACK failure will be interpreted as a NACK, and even the decoded ACK (whether erroneous or not) will be ignored in favor of the PDCCH grant for adaptive re-transmission.

Thus, embodiments of the present invention make the communication system more stable by preventing accidental non-adaptive retransmissions with virtually no cost in implementation.

Fig. 3 schematically shows an arrangement 300 in a user equipment according to the present invention, the arrangement comprising a receiving unit 310 configured for e.g. reading PDCCH and PHICH. The apparatus 300 further comprises a processing unit 320 configured to: detecting 330 a grant on the PDCCH valid for at least one transport block; detecting 340 that at least one transport block is disabled such that no grant is associated with the at least one transport block; and interpreting 350 the at least one disabled transport block as receipt of an acknowledgement message ACK, regardless of an indication on a receipt status feedback channel (e.g., PHICH, etc.) used for the transport block. The apparatus 300 also comprises a transmitting unit 360 configured to send information. It will be appreciated that the processing unit 340 can be one or more suitably programmed electronic processors or circuits, and that the receiving unit 310 and the transmitting unit 360 process signals appropriate for the particular communication system, such as LTE channels and signals.

Fig. 4 schematically shows the device 300 in an alternative manner. The apparatus 400 comprises an input unit 410 and an output unit 420 and a processing unit 430, which may be a single unit or a plurality of units. The apparatus 400 also includes at least one computer program product 440 in the form of a non-volatile computer-readable medium, such as EEPROM, flash memory, and disk drives. The computer program product comprises a computer program 450 comprising program instructions which, when executed, cause the processing unit 430 to perform the steps of the process described above in connection with fig. 1a-b and 3.

The program instructions or code means in the computer program 450 advantageously comprise a module 450a for detecting an uplink grant for at least one transport block, a module 450b for detecting that at least one transport block is disabled, and a module 450c for interpreting the at least one disabled transport block as a receipt of an acknowledgement message ACK. The program 450 can thus be implemented as computer program code constituted by computer program modules. The modules referred to above substantially perform the steps performed by the processing unit of fig. 3. In other words, the different modules correspond to the steps of the arrangements shown in fig. 1a-b and 3 when they are run on a processing unit.

Although the program 450 in the embodiment illustrated by fig. 4 can be implemented as computer program modules which, when run on a processing unit, cause the processing unit to perform the steps described above in connection with the figures mentioned above, one or more code means 450 can in alternative embodiments be implemented at least partly as hardware circuits.

The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The described embodiments are to be considered in all respects only as illustrative and not restrictive.

Claims (12)

1. A method for controlling retransmission in a user equipment supporting uplink spatial multiplexing, the method comprising the steps of:

-detecting an uplink grant on a physical downlink control channel, the uplink grant being valid for at least one transport block;

-detecting that at least one transport block is disabled such that no grant is associated with the at least one disabled transport block;

the method is characterized by further comprising the steps of:

-interpreting the at least one disabled transport block as a previously transmitted acknowledgement, ACK, corresponding to the disabled transport block, regardless of whether the previously transmitted ACK or negative acknowledgement, NACK, indication is received on a reception status feedback channel.

2. The method according to claim 1, wherein all detection steps are implemented at a first protocol layer, whereby said interpreting step comprises said first protocol layer conveying an indication of an acknowledgement, ACK, to a second protocol layer.

3. The method according to claim 2, wherein said indication comprises the step of setting an ACK/NACK flag to ACK.

4. The method of claim 1, the interpreting step comprising, upon receiving from a first protocol layer one or more grants valid for fewer transport blocks than transport blocks capable of spatial multiplexing, a second protocol layer assuming that an Acknowledgement (ACK) has been received for a previous transmission of a transport block for which no grant has been forwarded from the first protocol layer to the second protocol layer.

5. The method according to any of claims 2-4, wherein the first protocol layer is a physical layer and the second protocol layer is a higher protocol layer than the physical layer.

6. The method according to any of claims 1-4, wherein the acknowledgement is used as an input in a HARQ process corresponding to the disabled transport block in an uplink data transfer process.

7. An apparatus for controlling retransmissions in a user equipment supporting uplink spatial multiplexing, the apparatus comprising a processing unit comprising processing circuitry configured to:

-detecting an uplink grant on a physical downlink control channel, the grant being valid for at least one transport block;

-detecting that at least one transport block is disabled such that no grant is associated with the at least one disabled transport block; wherein the processing circuit is further configured to:

-interpreting the at least one disabled transport block as a previously transmitted acknowledgement, ACK, corresponding to the disabled transport block, regardless of whether the previously transmitted ACK or negative acknowledgement, NACK, indication is received on a reception status feedback channel.

8. The apparatus of claim 7, wherein the processing unit further comprises processing circuitry configured to convey an indication of an acknowledgement, ACK, from the first protocol layer to the second protocol layer.

9. The apparatus of claim 8, wherein the processing unit further comprises processing circuitry configured to set an ACK/NACK flag to ACK.

10. The apparatus of claim 7, the processing unit further comprising processing circuitry configured to, upon receiving from a first protocol layer one or more grants valid for fewer transport blocks than transport blocks capable of spatial multiplexing, assume at a second protocol layer that an acknowledgement, ACK, has been received for a previous transmission corresponding to a transport block for which no grant has been forwarded from the first protocol layer to a higher layer.

11. The apparatus according to any of claims 8-10, wherein the first protocol layer is a physical layer and the second protocol layer is a higher protocol layer than the physical layer.

12. The apparatus according to any of claims 7-10, wherein the processing unit comprises processing circuitry configured to use the acknowledgement as an input in a HARQ process corresponding to the disabled transport block in an uplink data transfer process.