A method at a base station for performing data transmission to a user equipment for carrier aggregation of multiple carriers, the method comprising: transmitting at least one downlink scheduling grant, corresponding to data transmission to the user equipment in a downlink control region of at least one of a short Transmission Time Interval, TTI, on a first carrier and a long TTI on a second carrier; performing data transmission towards the user equipment in a data transmission region of a short TTI on the first carrier and in the data transmission region of a long TTI on the second carrier as indicated by the at least one downlink scheduling grant; and receiving Hybrid Automatic Repeat reQuest, HARQ, feedback corresponding to the data transmission in the same TTI where the data transmission is transmitted, wherein said transmitting at least one downlink scheduling grant comprises: transmitting, in the downlink control region of a short TTI on the first carrier, a downlink scheduling grant corresponding to data transmission on the second carrier, wherein the data transmission on the second carrier is performed in a long TTI, which is determined depending on a ratio of a length of the short TTI to a length of the long TTI and the short TTI where the downlink scheduling grant corresponding to the data transmission on the second carrier is transmitted, or wherein said transmitting at least one downlink scheduling grant comprises: transmitting, in the downlink control region of a long TTI on the second carrier, a downlink scheduling grant corresponding to data transmission on the first carrier, wherein the data transmission on the first carrier is performed in a short TTI which is determined depending on a ratio of a length of the long TTI to a length of the short TTI and the long TTI where the downlink scheduling grant corresponding to the data transmission on the first carrier is transmitted. transmitting at least one downlink scheduling grant, corresponding to data transmission to the user equipment in a downlink control region of at least one of a short Transmission Time Interval, TTI, on a first carrier and a long TTI on a second carrier; performing data transmission towards the user equipment in a data transmission region of a short TTI on the first carrier and in the data transmission region of a long TTI on the second carrier as indicated by the at least one downlink scheduling grant; and receiving Hybrid Automatic Repeat reQuest, HARQ, feedback corresponding to the data transmission in the same TTI where the data transmission is transmitted, wherein said transmitting at least one downlink scheduling grant comprises: transmitting, in the downlink control region of a short TTI on the first carrier, a downlink scheduling grant corresponding to data transmission on the second carrier, wherein the data transmission on the second carrier is performed in a long TTI, which is determined depending on a ratio of a length of the short TTI to a length of the long TTI and the short TTI where the downlink scheduling grant corresponding to the data transmission on the second carrier is transmitted, or transmitting, in the downlink control region of a short TTI on the first carrier, a downlink scheduling grant corresponding to data transmission on the second carrier, wherein the data transmission on the second carrier is performed in a long TTI, which is determined depending on a ratio of a length of the short TTI to a length of the long TTI and the short TTI where the downlink scheduling grant corresponding to the data transmission on the second carrier is transmitted, or wherein said transmitting at least one downlink scheduling grant comprises: transmitting, in the downlink control region of a long TTI on the second carrier, a downlink scheduling grant corresponding to data transmission on the first carrier, wherein the data transmission on the first carrier is performed in a short TTI which is determined depending on a ratio of a length of the long TTI to a length of the short TTI and the long TTI where the downlink scheduling grant corresponding to the data transmission on the first carrier is transmitted. transmitting, in the downlink control region of a long TTI on the second carrier, a downlink scheduling grant corresponding to data transmission on the first carrier, wherein the data transmission on the first carrier is performed in a short TTI which is determined depending on a ratio of a length of the long TTI to a length of the short TTI and the long TTI where the downlink scheduling grant corresponding to the data transmission on the first carrier is transmitted.