1. Field of the Invention
The present invention relates to a discontinuous reception mechanism and, more particularly, to a method for avoiding unnecessary excessive stay of short cycle in discontinuous reception mechanism.
2. Description of Related Art
There are only two different Radio Resource Control (RRC) states, RRC_IDLE and RRC_CONNECTED, in Evolved Universal Terrestrial Radio Access (E-UTRA), unlike the four different RRC connected states adopted in Universal Mobile Telecommunication System (UMTS). In order to achieve better user equipment (UE) power saving while preventing unnecessary UE state transitions between RRC_IDLE and RRC_CONNECTED, Discontinuous Reception (DRX) is introduced in E-UTRA. In E-UTRA, generally the UE has to always monitor the Physical Downlink Control Channel (PDCCH), which informs the UE of the resource allocation, and Hybrid ARQ information related to DL-SCH and also carries the uplink scheduling grant. The introduction of DRX functionality enables UE to stop monitoring PDCCH for uplink (UL) and downlink (DL) transmission during some period of time, so that the UE only needs to be “awake” during the “active-time” duration.
In the current DRX mechanism in E-UTRA (see “E-UTRA MAC protocol specification (Release 8); 3GPP TS 36.321 V8.1.0 (2008-03); 3rd Generation Partnership Project; Technical Specification Group Radio Access Network”, which is incorporated herein for reference), the UE behaviors are mainly determined by several relevant timers and/or whether some conditions are fulfilled. The timing to start and stop such timers, and corresponding UE behaviors upon the expiration of such timers are also defined.
In view of the DRX mechanism, the UE may be configured with a DRX functionality that allows it to stop monitoring PDCCH during some period of time. The DRX functionality consists of a long DRX cycle, a DRX inactivity timer, a short DRX cycle, a DRX short cycle Timer, and an on duration timer. Specifically, when a DRX cycle has been configured, the UE shall be operated according to the DRX functionality illustrated in the following Table 1.
TABLE 1- Whenever a new DRX cycle begins (long or short), the onduration timer is started. : :- If a DRX Command MAC control element is received: - stop the On Duration Timer; - stop the DRX Inactivity Timer.- If the on duration timer or DRX inactivity timer is running; or - an UL grant for a retransmission can occur; or  :  :   - UE shall monitor the PDCCH;   - if the PDCCH is successfully decoded:    - stop the DRX Short Cycle Timer if configured,    - if the PDCCH indicates a DL transmission:     - start the HARQ RTT Timer for the corresponding HARQ      process;     - stop the DRX Retransmission Timer for the corresponding      HARQ process.    - If the PDCCH indicates a new transmission (DL or UL); and     - this is the first received PDCCH addressed to the C-RNTI or      Temporary C-RNTI of the UE since successful reception of a      Random Access Response is indicated; or     - a Scheduling Request is pending; or     - the On Duration Timer is running; or     - the DRX Inactivity Timer is running:      - start or restart the DRX Inactivity Timer.     :     :- If the DRX Inactivity Timer expires: - if the short DRX cycle is configured:  - start DRX Short Cycle Timer;  - use the short DRX cycle; - else use the long DRX cycle.- If DRX Short Cycle Timer expire: - use the long DRX cycle.
The current DRX mechanism covers the UE behaviors upon the start, stop and expiration of DRX-related timers, and the occurrence of other events may affect or relate to DRX. But there are still some problems left either may lead to unwanted result, or not clearly specified.
Currently there are two different DRX cycles in E-UTRA DRX mechanism. As shown in FIG. 1, the indispensable long DRX cycle with which the on duration timer enables for every long period of time to provide an on duration period in which the UE is at active time to monitor PDCCH. As shown in FIG. 2, the optional short DRX cycle with which the on duration timer is on for every short period of time to provide an on duration period in which the UE is at active time to monitor PDCCH. If the short DRX cycle is configured, the switching between two cycle periods will be triggered upon the expiration of the DRX inactivity timer and the DRX short cycle timer. As shown in FIG. 3, the UE is at active time to monitor PDCCH when the DRX inactivity timer starts. When the DRX inactivity timer expires, the DRX short cycle timer is started, and short DRX cycle is used by the UE to be at active time to monitor PDCCH for every short period of time. When DRX short cycle timer expires, long DRX cycle is used by the UE to be at active time to monitor PDCCH for every long period of time.
The above procedure could work and the two DRX cycles would switch between each other when no other criterion would apply. But currently there are also other criterion will stop the DRX inactivity timer and DRX short cycle timer before they expire, e.g., the DRX inactivity timer will be immediately stopped “if a DRX command MAC control element is received”, and the DRX short cycle timer is stopped whenever “the PDCCH is successfully decoded”. As can be observed, it is possible now that the switching between two DRX cycles would be delayed for a long period of time, which may be unnecessary.
As shown in FIG. 4, if the DRX inactivity timer expires, the UE will use the short DRX cycle, and start the DRX short cycle timer. The DRX short cycle timer would be stopped when the PDCCH is successfully decoded. Notice that the UE is still using short DRX cycle although the DRX short cycle timer is stopped. If a DRX command Medium Access Control (MAC) control element (CE) is received before the DRX inactivity timer expires, the DRX inactivity timer is stopped, and the DRX short cycle timer won't be restarted again. The UE would stay in short DRX cycle until the expiration of the DRX short cycle timer, which may occur when the DRX inactivity timer starts due to the occurrence of a new data transmission and then expires to restart the DRX short cycle timer. Accordingly, the UE may stay in short DRX cycle for a long period of time if no new data transmission occurs between the network and the UE since the DRX inactivity timer would not be restarted. This could be a waste of UE power since the UE should better stay in long DRX cycle to avoid unnecessary wake-up in such situation.
Therefore, it is desirable to provide an improved method for avoiding unnecessary excessive stay of short cycle in discontinuous reception mechanism to mitigate and/or obviate the aforementioned problems.