In the Third Generation Partnership Project (3GPP), discontinuous reception (DRX) is used between the network and a User Equipment (UE) to save the power of the UE. The UE may be configured by a radio resource control/media access control (RRC/MAC) with a DRX functionality that allows it to stop monitoring the packet data control channel (PDCCH) for a period of time (i.e., a sleep period). The DRX functionality consists of a long DRX cycle, a DRX inactivity timer, and a DRX retransmission timer. The DRX functionality optionally includes a short DRX cycle and a DRX short cycle timer, which are all defined in the 3GPP specification. The long DRX cycle provides a longer sleep period for the UE than does the short DRX cycle.
The active time is defined as the time that the UE is awake. When DRX is configured by a higher layer, this includes the on duration, which is the time that the UE is continuously monitoring the PDCCH while the DRX inactivity timer has not expired, and the time that the UE is continuously monitoring the PDCCH while any DRX retransmission timer is running.
As shown in FIG. 1, the DRX cycle specifies the periodic repetition of the on duration followed by a possible period of inactivity. The DRX inactivity timer specifies the number of consecutive transmission time intervals (TTIs) during which the UE monitors the PDCCH after successfully decoding a PDCCH transmission which indicates an initial uplink or downlink user data transmission for the UE. This DRX inactivity timer is restarted if a new PDCCH transmission is detected while the timer is still running. Expiration of the DRX inactivity timer indicates that a particular duration of inactivity has elapsed for receiving any PDCCH transmission. The DRX retransmission timer specifies the maximum number of consecutive TTIs the UE monitors the PDCCH when a downlink retransmission is expected by the UE. The DRX short cycle timer specifies a number of consecutive TTIs that the UE shall follow the short DRX cycle after the DRX inactivity timer has expired. The hybrid automatic repeat-request (HARQ) round-trip time (RTT) timer specifies the minimum amount of TTIs before a downlink HARQ retransmission is expected by the UE. The DRX on duration timer specifies a number of consecutive PDCCH subframes at the beginning of a DRX cycle and specifies the number of consecutive TTIs during which the UE monitors the PDCCH for possible allocations. The DRX on duration is a part of a DRX cycle.
As mentioned above, the UE may be configured by the RRC/MAC with a DRX functionality that allows it to stop monitoring PDCCH during some period of time. Regardless of whether the UE is monitoring PDCCH, the UE receives and transmits HARQ feedback when such is expected.
FIG. 2 illustrates an example DRX cycle timing diagram 200, as a demonstration of problems that may arise with interworking of the various DRX timers during the selection of a short DRX cycle or a long DRX cycle. As shown in FIG. 2, an initial DRX inactivity timer start 201 can occur during any subframe or TTI within the DRX on duration. The DRX inactivity timer duration 202 is fixed. However, due to the uncertainty of the first start of the first DRX inactivity timer 201, as well as the unknown number of DRX inactivity timer restarts 203 that may be required, the DRX inactivity timer expiration is uncertain to both the eNode-B and the UE. If the DRX short cycle timer is configured to start when the DRX inactivity timer expires, then the new DRX short cycle timer will start at an uncertain subframe or TTI. As a result, the eNode-B does not know with certainty when the short DRX cycle timer starts and the short DRX cycle may no longer be aligned with the long DRX cycle.