iers would create some limitations for normal HARQ operation. The uncertainty may arise due to the LBT operation needed to acquire the channel or because the maximum transmission duration for LAA is exceeded. Fig. 8.37 illustrates an example in which the maximum transmission duration of an LAA cell has been reached before the third retransmission of a HARQ process can be performed. There are two ways to address this issue. One approach is to keep the HARQ retransmissions on the same LAA cell or to ensure that the HARQ process is completed within the maximum transmission duration of an LAA cell as shown in Fig. 8.37. A new HARQ process is started when the LAA cell reacquires the channel after an LBT operation. Alternatively, a retransmission can be delayed until the LAA cell acquires the channel again. The RLC retransmission may be invoked if HARQ transmission is not successfully completed. Another alternative would be to move HARQ retransmissions to another cell (note that each carrier in the carrier aggrega- tion framework is referred to as a cell). The retransmission may also be performed via another cell which would be either the PCell or another SCell. This would change the base- line LTE HARQ protocol as it may be linked to two or more cells. In synchronous HARQ, the UE identifies the HARQ process that is associated with the cur- rent transmission time interval (TTI). Depending on whether spatial multiplexing is used, each TTI has one or two associated HARQ processes, and for the identified HARQ 3rd Transmission Alternative 2 Primary cell 2nd Transmission Alternative 1 3rd Transmission 1st Transmission LAA cell 1 2nd Transmission 1st Transmission New data Figure 8.37 Illustration of HARQ issue in LAA cell [9,12]. Operation in Unlicensed and Shared Spectrum 903 processes, the UE will perform a (re)transmission. The association between TTIs and HARQ processes relies on uplink HARQ being synchronous and is derived from the timing relation. If there has been an initial transmission in a certain subframe for a ce