A method of configuring an uplink multiple-input multiple-output, MIMO, transmission in which a primary stream and a secondary stream are spatially separated, said method being performed by a wireless user equipment, UE, and comprising: receiving, from a base station on an absolute grant channel, a grant for scheduled data and one or more grants for non-scheduled data for the uplink MIMO transmission, wherein said one or more grants for non-scheduled data are grants for one or more non-scheduled flows; said method being characterized by: determining whether allocation of data for the primary stream is power-limited, such that allocation of data is determined to be power-limited when the number of bits that the UE can support for the primary stream, based on an available transmit power at the UE on said stream, is less than the number of bits based on said scheduled and said non-scheduled grants, and is less than the number of bits of buffered data ready for transmission at the UE; and if said allocation of data is power-limited, determining whether the secondary stream is carrying a packet retransmission; and if the secondary stream is not carrying a packet retransmission, determining whether the primary stream transport block size, TBS, is less than a sum being determined based on all non-scheduled flows; and if said primary TBS is less than said sum, deciding to fall back from a rank 2 transmission in which the primary stream and the secondary stream are spatially separated to a rank 1 transmission in which only the primary stream is transmitted; wherein the sum is determined by: determining, for each non-scheduled flow, the minimum of: the number of bits corresponding to the grant for non-scheduled data on the non-scheduled flow, and the number of bits corresponding to a current buffer level for the non-scheduled flow; and adding together the determined minimums across all of the non-scheduled flows; wherein the primary TBS is determined as the number of bits that the UE can support for the primary stream, based on the available transmit power at the UE on said stream; wherein the number of bits based on said scheduled and said non-scheduled grants is determined by adding the number of bits based on said scheduled grant to the number of bits based on said non-scheduled grant. receiving, from a base station on an absolute grant channel, a grant for scheduled data and one or more grants for non-scheduled data for the uplink MIMO transmission, wherein said one or more grants for non-scheduled data are grants for one or more non-scheduled flows; said method being characterized by: determining whether allocation of data for the primary stream is power-limited, such that allocation of data is determined to be power-limited when the number of bits that the UE can support for the primary stream, based on an available transmit power at the UE on said stream, is less than the number of bits based on said scheduled and said non-scheduled grants, and is less than the number of bits of buffered data ready for transmission at the UE; and if said allocation of data is power-limited, determining whether the secondary stream is carrying a packet retransmission; and determining whether allocation of data for the primary stream is power-limited, such that allocation of data is determined to be power-limited when the number of bits that the UE can support for the primary stream, based on an available transmit power at the UE on said stream, is less than the number of bits based on said scheduled and said non-scheduled grants, and is less than the number of bits of buffered data ready for transmission at the UE; and if said allocation of data is power-limited, determining whether the secondary stream is carrying a packet retransmission; and if the secondary stream is not carrying a packet retransmission, determining whether the primary stream transport block size, TBS, is less than a sum being determined based on all non-scheduled flows; and if said primary TBS is less than said sum, deciding to fall back from a rank 2 transmission in which the primary stream and the secondary stream are spatially separated to a rank 1 transmission in which only the primary stream is transmitted; wherein the sum is determined by: determining, for each non-scheduled flow, the minimum of: the number of bits corresponding to the grant for non-scheduled data on the non-scheduled flow, and the number of bits corresponding to a current buffer level for the non-scheduled flow; and adding together the determined minimums across all of the non-scheduled flows; determining, for each non-scheduled flow, the minimum of: the number of bits corresponding to the grant for non-scheduled data on the non-scheduled flow, and the number of bits corresponding to a current buffer level for the non-scheduled flow; and adding together the determined minimums across all of the non-scheduled flows; wherein the primary TBS is determined as the number of bits that the UE can support for the primary stream, based on the available transmit power at the UE on said stream; wherein the number of bits based on said scheduled and said non-scheduled grants is determined by adding the number of bits based on said scheduled grant to the number of bits based on said non-scheduled grant.