- One of the reasons we need to maintain time alignment in the uplink is to maintain orthogonality in the uplink.
- Secondly, RRC Connected state supports Discontinuous Reception (DRX) / Discontinuous Transmission (DTX). In uplink transmission sometimes the term discontinuous UL transmission is used which means UL time alignment (synchronization) has been lost. If a UE has been inactive for a certain period, it means that the UE has to re-synchronize with a Random Access attempt prior to any data transmission. This means that the procedures prior to commencing transmission differ depending on whether the UE has a synchronized uplink or not. The synchronization state is not captured in the RRC mode. Instead, the network configures the UE with a Time Alignment Timer that governs the status of the uplink time alignment.
- A synchronized uplink is also a prerequisite for downlink Hybrid Automatic Repeat Request (HARQ) operation, and this is why a re-synchronization of the uplink is needed also prior to any DL transmission.
The UL can also be kept synchronized even if the UE has nothing to send, e.g., by configuring the UE to send periodic buffer status reports during a period after discontinued transmission. However, there is of course a resource cost associated with such an implementation.