The mechanism-a (long) DRX cycle in combination with the device remaining awake for some period after being scheduled-is sufficient for most scenarios. The value of the timer is preferably set to match the roundtrip time in the hybrid-ARQ protocol a roundtrip time that depends on the implementation. Therefore, the DRX functionality has a configurable timer, which is started after an erroneously received transport block and used to wake up the device receiver when it is likely for the gNB to schedule a retransmission. If the device has been scheduled a transmission in the downlink it could not decode, the typical situation is that the gNB retransmits the data shortly after the initial transmission.
Hybrid-ARQ retransmissions are asynchronous in both uplink and downlink. The BS agrees on individual DRX parameters if they are not in conflict with the higher layer requirements. Devices may request individual values by adding them into the request. DRX parameters should be requested by devices in RRC-related messages, for example, RRC_Reconfiguration and RRC_Connection requests. Therefore, the onDuration timer allows faster transition to the idle mode if a device is inactive, while the inactivity timer prevents oscillations between connected and idle states. If a control message arrives with a new DRX parameters allocation while onDuration or inactivity is running, both timers have to be set to zero and the next DRX cycle will apply new parameters. If no data in PDCCH is detected during the onDuration time, devices stop listening to the DL and power down their circuits until the end of the ongoing DRX cycle. The onDuration timer is used to indicate the time while devices should monitor all PDCCH opportunities within a single DRX cycle, that is, the time interval between two consequent onDuration periods. If no control message arrives while the inactivity timer is running, devices go idle upon its expiration. DRX defines inactivity timer to inform devices for how long they should continuously monitor PDCCH opportunities after the subframe where the P-RNTI in DCI has been successfully received and decoded. Another issue that DRX is aimed to deal with is a long time required for reporting out_of_sync status after the last data transmission in connected mode. Frequent transitions between idle and connected states should be avoided since they increase signaling overhead and energy consumption. However, if there is any upcoming data in the DL direction, the device has to obtain RRC_connection again in order to receive it. When the data report uploading is over, the device sends the RRC_connection Release or RRC_connection suspend message to stop UL synchronization and signaling in RAN and then goes to idle state. This mechanism addresses two main issues related to the device battery drain during the connected stage. Araniti, in LPWAN Technologies for IoT and M2M Applications, 2020 12.2.2 Discontinuous reception in connected mode
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