# Patent

## Title
Implicit DRX cycle length adjustment control in LTE—ACTIVE mode

## Abstract

A method for controlling discontinuous reception in a wireless transmit/receive unit includes defining a plurality of DRX levels, wherein each DRX level includes a respective DRX cycle length and transitioning between DRX levels based on a set of criteria. The transitioning may be triggered by implicit rules.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 12/022,233, filed Jan. 30, 2008; which claims the benefit of U.S. Provisional Patent Application No. 60/887,276, filed Jan. 30, 2007, the contents of which are incorporated by reference herein.


FIELD OF DISCLOSURE
The present invention is in the field of wireless communications.
BACKGROUND
A goal of the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) program is to develop new technology, new architecture and new methods for settings and configurations in wireless communication systems in order to improve spectral efficiency, reduce latency and better utilize the radio resource to bring faster user experiences and richer applications and services to users with lower costs.
In a typical LTE network, a wireless transmit/receive unit (WTRU) may operate in a number of modes. While in LTE_ACTIVE mode, the WTRU may operate in a discontinuous reception (DRX) mode. DRX mode allows the WTRU to operate in a low power, or sleep mode, for a preset time, and then switch to a full power, or awake, mode for another preset time in order to reduce battery consumption. The DRX cycle lengths are generally configured by the enhanced universal terrestrial radio access network (E-UTRAN) so that an enhanced Node B (eNB) and the WTRU are synchronized on a consistent sleep and wake-up cycle.
Live traffic situations and WTRU mobility may require frequent adjustment of the DRX cycle length in order to balance system performance, WTRU performance and WTRU power savings. However, relying only on WTRU/E-UTRAN signaling to make the fine DRX cycle adjustment may incur a heavy system and WTRU signaling load.
Implicit rules for DRX cycle length adjustment may be used for smooth LTE_ACTIVE DRX operations to reduce battery power consumption while not effecting system or WTRU performance issues. Implicit rules may assist the implicit DRX cycle length transitions between the WTRU and the E-UTRAN without using excessive explicit signaling.
SUMMARY
A method and apparatus are disclosed for controlling discontinuous reception in a WTRU. The method may include defining a plurality of DRX levels, wherein each DRX level includes a respective DRX cycle length, and transitioning between DRX levels based on a set of criteria. Transitioning may be triggered by implicit rules. Triggering may be invoked by a measurement event, a timer, a counter or a downlink command, for example. The transitions between DRX states may occur without explicit signaling.



BRIEF DESCRIPTION OF THE DRAWINGS
A more detailed understanding may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein:
FIG. 1 shows a wireless communications system in accordance with one embodiment;
FIG. 2 is a functional block diagram of a WTRU and an e Node B (eNB) in accordance with one embodiment; and
FIG. 3 is a state diagram of implicit DRX transition in accordance with one embodiment;
FIG. 4 is a signal flow diagram for implicit DRX transition in accordance with one embodiment;
FIG. 5 is a flow diagram for a method of implicit DRX signaling in accordance with one embodiment;
FIG. 6 is a flow diagram for a method of implicit DRX signaling in accordance with another embodiment;
FIG. 7 is a flow diagram for a method of implicit DRX signaling in accordance with an alternative embodiment; and
FIG. 8 is a flow diagram for a method of implicit DRX signaling in accordance with another alternative embodiment.



DETAILED DESCRIPTION
When referred to hereafter, the terminology “wireless transmit/receive unit (WTRU)” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, a cellular telephone, a personal digital assistant (PDA), a computer, or any other type of user device capable of operating in a wireless environment. When referred to hereafter, the terminology “base station” includes but is not limited to a Node-B, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment.
FIG. 1 shows a wireless communication system 100 in accordance with one embodiment. The system 100 includes a plurality of WTRUs 110 and an eNB 120. As shown in FIG. 1, the WTRUs 110 are in communication with the eNB 120. Although three WTRUs 110 and one eNB 120 are shown in FIG. 1, it should be noted that any combination of wireless and wired devices may be included in the wireless communication system 100. The eNB 120 and the WTRUs 110 may communicate while in DRX mode and may have coordinated DRX cycles.
FIG. 2 is a functional block diagram 200 of a WTRU 110 and the eNB 120 of the wireless communication system 100 of FIG. 1. As shown in FIG. 1, the WTRU 110 is in communication with the eNB 120. Both WTRU 110 and eNB 120 may operate in DRX mode.
In addition to the components that may be found in a typical WTRU, the WTRU 110 includes a processor 215, a receiver 216, a transmitter 217, and an antenna 218. The processor 215 may be configured to adjust DRX cycle length as necessary. The receiver 216 and the transmitter 217 are in communication with the processor 215. The antenna 218 is in communication with both the receiver 216 and the transmitter 217 to facilitate the transmission and reception of wireless data.
In addition to the components that may be found in a typical eNB 120, the eNB 120 includes a processor 225, a receiver 226, a transmitter 227, and an antenna 228. The processor 225 is configured to communicate with the receiver 226 and transmitter 227 to adjust DRX cycles as necessary. The receiver 226 and the transmitter 227 are in communication with the processor 225. The antenna 228 is in communication with both the receiver 226 and the transmitter 227 to facilitate the transmission and reception of wireless data.
In order to improve battery life, but not limit the eNB 120 and WTRU 110 performance, transitions between DRX cycle length states may be defined implicitly, rather than explicitly. The implicit rules may be implemented at the radio resource control (RRC) and the medium access control (MAC) levels while the WTRU 110 is in a LTE_ACTIVE DRX state.
Approximately half of WTRU 110 to eNB 120 interaction involves WTRU 110 requests and reports and eNB 120 responses while the WTRU 110 is in LTE_ACTIVE DRX mode. When the WTRU 110 measures a particular scenario, measurement events may be reported to the eNB 120, and the eNB 120 may respond to the situation by commanding the WTRU 110 to start a new service, mobility activity, and the like. If the downlink command transmission or reception is limited by a relatively long DRX cycle length, WTRU 110 and eNB 120 system performance during LTE_ACTIVE DRX mode may suffer. However, certain measurement events may make good candidates for the anticipated network downlink commands.
FIG. 3 shows an implicit DRX transition state machine 300 in accordance with one embodiment. The state machine 300, as well as associated transition mechanisms and parameter values, may be configured by the eNB (120 of FIG. 1). The state machine 300 may have a life span, also configured by the eNB 120. Each state may be applied at the WTRU (110 of FIG. 1) and at the eNB 120, so that operation is consistent and synchronized. At each defined and configured DRX state, a different DRX cycle length is associated with both the WTRU 110 and the eNB 120 operations.
The DRX cycle length transition rules may be based on WTRU 110 and eNB 120 experiences. Given a certain elapsed time, or a given set of measurement values, the WTRU 110 and the eNB 120 may learn and predict traffic patterns. These learned and predicted traffic patterns may be superimposed on a general model for a state machine, resulting in the DRX state machine 300 for a WTRU 110/eNB 120 system that permits implicit transition operation and consistent DRX actions for both the WTRU 110 and the eNB 120. The eNB 120 can prescribe DRX states for service and mobility conditions with the potential for continuous improvement and learned traffic patterns upon every invocation.
FIG. 3 shows 3 defined DRX levels, 302, 304, 306 and an undefined DRX level 308. In DRX level 3 306, the WTRU 110 is operating in a normal DRX cycle. The actual length of the normal state may be defined by the eNB 120. DRX level 2 304 is a shorter cycle length than DRX level 3 306, and is associated with more frequent activity than normal. The eNB 120 may also define the cycle length for DRX level 2 304, and may also set a “resume” period. A resume period is a length of time in which there are no new transmissions and after which the WTRU 110 may return to DRX level 3 306 operation, unless the WTRU 110 is commanded to do otherwise.
DRX level 1 302 has the shortest DRX cycle length, and may be used by a WTRU 110 or eNB 120 to handle predicted immediate downlink commands and when uplink traffic patterns are recognized by the WTRU 110 and the eNB 120 as requiring immediate downlink action, such as during a handover event, for example.
A DRX level n 308 may be configured with longer DRX cycles than that for the DRX Level 3 306. The eNB 120 can redefine the DRX cycle lengths for each state at the end of the DRX configuration life span but may observe a DRX cycle length rule that lower level DRX states have shorter DRX lengths.
For a WTRU 110 at DRX level 3 306, a timer or counter trigger may be defined to trigger a transition to DRX Level 2 304 if the eNB 120 determines that the WTRU 110 should periodically transition to a “busy” cycle to check downlink data. This may be considered a trigger based on a measurement event. Another trigger based on a measurement event can also be defined to transition a WTRU 110 from DRX level-3 306 to DRX Level 1 when a traffic volume event on a certain radio bearer accumulating a larger amount of uplink data than a threshold is reported and an anticipated Radio Bearer (RB) Reconfiguration command is imminent.
If the WTRU 110 in DRX Level 1 302 state receives a RB Reconfiguration command, the current DRX Level 1 state is over. If the WTRU 110 at DRX Level 1 state 302 does not receive the anticipated command for the defined “resume period”, it can go back to its original DRX state and resume the power saving DRX cycle. Regular timers and counters may be used during a DRX mode to trigger the implicit DRX cycle length transition. The choice between the timers and counters and the values of the timers or counters may be based on learned traffic patterns and models with respect to the mobility and/or service state of the WTRU 110 at a particular time while the WTRU 110 is in LTE_ACTIVE DRX mode. The timer or counter triggers may be used as transition triggers to bring up the DRX cycle length as well as to bring down the DRX cycle length as the DRX state changes.
The eNB 120 may configure DRX parameters based on a network traffic monitoring operation and analysis. Several methods exist to select the parameter values, such as by including a default system value set that is defined for implicit DRX transition operation. Optionally, the parameters may be published in system information broadcasts, or they can be determined by the eNB 120 from time to time and loaded to a particular WTRU 110 via higher layer signaling before an intended DRX mode period.
Transitions between different states may be signaled in an information element (IE). An example of a skeleton for signaling an implicit DRX cycle transition is shown in Table 1. As shown in Table 1, the Implicit DRX Transition List is mandatory and is limited to a value indicating a maximum number of DRX states.
The DRX cycle length IE is mandatory, and is an integer. The trigger mechanisms are optional, and may be a trigger to move up a DRX state level, or move down a DRX state level. The Implicit DRX Transition configured life Span IE is mandatory, and sets the resume period for non-normal states. The Initial DRX state is optional, and may set the DRX state of the WTRU 110 at start-up.
To aid with easier DRX cycle length transition and maintain DRX cycle length synchronization between the WTRU 110 and the eNB 120, the DRX cycle length definition may be given as a function of the shortest DRX base number (L). Then various DRX length values may be:

DRX-cycle-len=L×2n,  Equation (1)

where n=0, 1, 2 . . . such that the resulting DRX-cycle-len does not exceed a maximum DRX cycle length. The shortest DRX cycle length possible occurs when n=0, and is a fraction of a longer DRX cycle length.

The use of DRX cycle lengths that are multiples of each other reduces the probability that DRX periods may be mismatched and provides an efficient mechanism to resynchronize DRX periods between the WTRU 110 and eNB 120. When DRX periods are defined as multiples of each other, and when DRX periods become mismatched between the WTRU 110 and the eNB 120, each entity can determine the period of the other by increasing or decreasing the cycle length to determine the period being used by the other entity, and resynchronizing the entities accordingly.
Typically, a WTRU 110 in DRX Level 1 302 may count n times before it transits back to the original DRX state. The default may be given as: n=(Level-k DRX Cycle Length or original DRX cycle length)/Level-1 DRX Cycle Length; where Level-k cycle length is the length of the DRX cycle before the WTRU 110 enters DRX Level 1 302. Alternatively, the network may configure n for the “resume method”.











TABLE 1





Information






Element/


Type and
Semantics


Group name
Need
Multi
reference
description







Implicit DRX
MP
maxDRX




Transition

states


List

(TBD)


>DRX Cycle
MP

Integer


Length


(TBD)


>Trigger-UP-1
OP

Trigger
To next upper level





Mechanism
DRX State





A.B.C.D


>Trigger-UP-2
OP

Trigger
Used by Level-1 for





Mechanism
resume





A.B.C.D


>Trigger-Down-1
OP

Trigger
To next lower level





Mechanism
DRX state





A.B.C.D


>Trigger-Down-2
OP

Trigger
To Level-1 trigger





Mechanism





A.B.C.D


Implicit DRX
MP

TBD
Time in seconds


Transition


configured


life span


Initial DRX state
OP

TBD









Transitions from state to state may be initiated by a trigger. Table 2 shows an example of transition trigger IEs. Each of the IEs is mandatory, except for the resume period. The Transition Trigger is mandatory and is specified by the network if specified as shown in Table 1. The CHOICE mechanism allows the network to configure the WTRU 110 for implicit DRX operational triggers. The trigger Timer value may be in units of absolute time, LTE frames or transmission time intervals (TTIs) and is used to monitor or regulate ON and OFF periods for network signaling channel activities or data channel activities for the WTRU 110. The Counter values may be an integer value used to check the occurrences of certain trigger events. The measurement event may enumerate the event that causes the trigger. The resume period may be a time period given in seconds, DRX cycles, or some other value, that denotes the total time a WTRU 110 may remain in an elevated state without receiving a command to move back to normal state.











TABLE 2





Information






Element/


Type and
Semantics


Group name
Need
Multi
reference
description







Transition Trigger
MP





CHOICE
MP


mechanism


>Timer


>> Timer Value
MP

Integer TBD


>Counter


>> Counts
MP

Integer TBD


> Measurement


Event


>> measurement
MP

Enumerated


Event-Id


(TBD)


>resume period
CV-

TBD
Could be default in



Trigger-


Level-1 State.



UP-2


Default is to stay n






Level-1 cycles so the






total length is






equivalent to its






original DRX state






DRX length









FIG. 4 is a signal flow diagram for implicit DRX transition 400 in accordance with one embodiment. A WTRU 402 may receive an RRC message or an IE 406 from the E-UTRAN 404 that triggers the WTRU 402 to enter DRX mode. The WTRU 402 may enter DRX mode 408 at a default level which may be a normal cycle length DRX level 3 (306 of FIG. 3). Both the WTRU 402 and the E-UTRAN 404 enter DRX mode (408, 410 respectively). The WTRU 402 may receive another RRC message or IE 412 that triggers the WTRU 402 to enter a faster DRX cycle mode (DRX level 1 302 of FIG. 3). The WTRU 402 and the E-UTRAN 404 enter the DRX level 1 (414, 416 respectively). A WTRU timer 418, synchronized with an E-UTRAN timer (not shown), expires. As the timers are synchronized, no notice of timer expiration is required. The expiration of the timer 418 triggers the WTRU 402 and the E-UTRAN 404 to return to normal DRX level. The WTRU 402 returns 422 to DRX level-3 306 at the same time that the E-UTRAN 404 returns 424 to DRX level-3 306.
FIG. 5 is a flow diagram of a method of implicit signaling 500 in accordance with one embodiment. At step 502 the WTRU is in normal operating mode, or Level-3. At step 504, the WTRU checks to see if a timer has timed-out, or a trigger has been received that would force the WTRU to move to another DRX state. If no, at step 506, the WTRU remains in normal state. If the WTRU detects a time out signal or a trigger at step 504, at step 508, the WTRU determines if it should move to DRX Level 1 or DRX level 2. If the WTRU determines that the trigger is a level-2 trigger, at step 510 the WTRU moves to DRX Level 2. At step 512, the WTRU determines that the resume period has ended, and returns to DRX level-3. If, however, the WTRU, at step 508, determines that it received a level 1 trigger, at step 514, the WTRU goes into a DRX level 1. At step, 516, the WTRU determines if it has received a Radio Bearer Reconfiguration message. If not, the WTRU, at step 518, waits for the resume period to end and returns to normal operation at step 522. If, however, at step 516, the WTRU receives a radio bearer reconfigure message, at step 520, the WTRU returns to normal DRX cycle operation.
FIG. 6 is a flow diagram of an implicit DRX method 600 in accordance with another embodiment. At step 602, the WTRU is in normal or DRX Level-3 mode. At step 604, the WTRU conducts a traffic volume measurement. At step 606, the WTRU compares the traffic volume measurement with a threshold. If the volume is below the threshold, at step 608, the WTRU takes no action and remains in DRX Level-3 mode. However, if, at step 606, the WTRU determines that the traffic is above a threshold, at step 610, the WTRU changes mode to a shorter DRX cycle. Based on the traffic, the new DRX mode may be DRX level-2 or DRX level-1. At step 612, the WTRU determines if a command or message has been received. If yes, at step 614, the WTRU returns to Level-3 mode. If not, at step 616 the WTRU waits the resume period before returning to level-3 mode at step 618. Optionally, the E-UTRAN may determine the traffic volume measurement reporting threshold level for DRX state transition triggering. Once the defined traffic volume measurement event occurs, the DRX state transition is triggered.
While in LTE_ACTIVE DRX mode, a WTRU may perform traffic volume measurements for uplink traffic. The E-UTRAN may configure the WTRU to report the events on threshold crossing. Based on learned traffic patterns, the E-UTRAN determines that there is a large volume change, which may means that an RB addition, an RB reconfiguration or an RB release command is imminent. Therefore, the traffic volume event reports may be used as implicit DRX transition triggers. For example, a large volume change may be used to trigger the WTRU into the shortest DRX cycle (DRX level 1, 302 of FIG. 3, for example) in order to receive the network command The network, when receiving the predetermined measurement event, may determine the WTRU's DRX state via implicit DRX transition rules and either sends the anticipated command to the WTRU or wait for the WTRU to return to its previous DRX state with the specified “resume period”.
By way of another example, the WTRU, while in LTE_ACTIVE mode, may use configured handover measurements. Certain measurement event reports may indicate that a handover (HO) command is imminent for intra-frequency, inter-frequency or an inter-radio access technology (RAT) handover. Depending on handover measurement events, certain other measurement events may act as triggers for DRX transition control. FIG. 7 is a flow diagram of a method of implicit DRX signaling 700 in accordance with an alternative embodiment. At step 702, the WTRU is in normal DRX level 3 state. At step 704, the WTRU determines that a serving cell measurement is below a threshold. The WTRU may then determine that an intra-frequency measurement is high 706, meaning that an intra-frequency neighbor is measuring as the best cell. Alternatively, the WTRU may determine that an inter-frequency band measures to be the best 708. As another alternative, the WTRU may determine that a non-LTE system measures the best 710.
At step 712, the WTRU, due to the measurements, may anticipate a handover command. At step 714, the WTRU reports the measurement event. This may invoke, at step 716, an implicit DRX transition trigger that causes the WTRU to go to a Level-1 DRX state in order to receive the possible handover command from the network. At step 718, the WTRU receives the handover command. At step 720, the WTRU transitions back to its original DRX state.
FIG. 8 is a flow diagram of a method of implicit DRX cycle signaling 800 in accordance with yet another embodiment. At step 802, the WTRU is level-1 mode. At step 804, the WTRU begins to monitor a Level 1/Level 2 control channel to intercept anticipated downlink commands. At step 806, the WTRU determines if an anticipated network command is received. If received, at step 808, the WTRU will follow the command to end the DRX mode or will receive instruction on the next DRX activity with the command. If the command is not received, at step 810, the WTRU transitions back to its original DRX state before entering the Level-1 state.
Although the features and elements are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements or in various combinations with or without other features and elements. The methods or flow charts provided may be implemented in a computer program, software, or firmware tangibly embodied in a computer-readable storage medium for execution by a general purpose computer or a processor. Examples of computer-readable storage mediums include a read only memory (ROM), a random access memory (RAM), a register, cache memory, semiconductor memory devices, magnetic media such as internal hard disks and removable disks, magneto-optical media, and optical media such as CD-ROM disks, and digital versatile disks (DVDs).
Suitable processors include, by way of example, a general purpose processor, a special purpose processor, a conventional processor, a digital signal processor (DSP), a plurality of microprocessors, one or more microprocessors in association with a DSP core, a controller, a microcontroller, Application Specific Integrated Circuits (ASICs), Field Programmable Gate Arrays (FPGAs) circuits, any other type of integrated circuit (IC), and/or a state machine.
A processor in association with software may be used to implement a radio frequency transceiver for use in a wireless transmit receive unit (WTRU), user equipment (UE), terminal, base station, radio network controller (RNC), or any host computer. The WTRU may be used in conjunction with modules, implemented in hardware and/or software, such as a camera, a video camera module, a videophone, a speakerphone, a vibration device, a speaker, a microphone, a television transceiver, a hands free headset, a keyboard, a Bluetooth® module, a frequency modulated (FM) radio unit, a liquid crystal display (LCD) display unit, an organic light-emitting diode (OLED) display unit, a digital music player, a media player, a video game player module, an Internet browser, and/or any wireless local area network (WLAN) module.

## Claims

1. A method for controlling discontinuous reception (DRX) in a wireless transmit/receive unit (WTRU), the method comprising:
the WTRU receiving a DRX configuration comprising a plurality of DRX cycle lengths;
the WTRU performing DRX using a shorter DRX cycle length of the plurality of DRX cycle lengths;
the WTRU setting a WTRU timer upon initiating DRX operation using the shorter DRX cycle length, wherein the WTRU timer is associated with a length of time during which the WTRU performs DRX using the shorter DRX cycle length prior to transitioning to a longer DRX cycle length of the plurality of DRX cycle lengths;
the WTRU determining that the WTRU timer has expired; and
the WTRU transitioning from DRX operation using the shorter DRX cycle length to DRX operation using the longer DRX cycle length in response to determining that the WTRU timer has expired.

2. The method as in claim 1, wherein the WTRU timer is synchronized with a base station timer.

3. The method as in claim 1, wherein the longer DRX cycle length is a multiple of the shorter DRX cycle length.

4. The method as in claim 1, wherein each of the plurality of DRX cycle lengths is a function of a shortest DRX cycle length.

5. The method as in claim 4, wherein the shorter DRX cycle length is the shortest DRX cycle length of the plurality of DRX cycle lengths.

6. The method as in claim 1, further comprising the WTRU performing DRX using the longer DRX cycle length prior to the WTRU performing DRX using the shorter DRX cycle length.

7. The method as in claim 1, wherein the longer DRX cycle length is a default DRX cycle length.

8. The method as in claim 1, wherein transitioning between DRX cycle lengths is implemented at a Medium Access Control (MAC) level.

9. The method as in claim 1, further comprising the WTRU resetting the WTRU timer upon transmission activity.

10. The method as in claim 1, wherein a value for the WTRU timer is received from an evolved Node-B (eNB) via higher layer signaling.

11. A wireless transmit receive unit (WTRU) comprising a processor, the processor configured to:
receive a discontinuous reception (DRX) configuration comprising a plurality of DRX cycle lengths;
perform DRX using a shorter DRX cycle length of the plurality of DRX cycle lengths;
set a WTRU timer upon initiating DRX operation using the shorter DRX cycle length, wherein the WTRU timer is associated with a length of time during which the WTRU is configured to operate using the shorter DRX cycle length, and if no new transmissions have been received during the length of time the WTRU is configured to transition to a longer DRX cycle length of the plurality of DRX cycle lengths;
determine that the WTRU timer has expired; and
transition the WTRU from DRX operation using the shorter DRX cycle length to DRX operation using the longer DRX cycle length in response to determining that the WTRU timer has expired.

12. The WTRU as in claim 11, wherein the WTRU timer is synchronized with a base station timer.

13. The WTRU as in claim 11, wherein the processor is further configured to perform DRX using the longer DRX cycle length prior to performing DRX using the shorter DRX cycle length.

14. The WTRU as in claim 11, wherein each of the plurality of DRX cycle lengths is a multiple of the shorter DRX cycle length.

15. The WTRU as in claim 14, wherein the shorter DRX cycle length is a shortest DRX cycle length of the plurality of DRX cycle lengths.

16. The WTRU as in claim 11, wherein the WTRU is configured to implement transitioning between DRX cycle lengths at a Medium Access Control (MAC) level.

17. The WTRU as in claim 11, wherein the processor is further configured to reset the WTRU timer upon transmission activity.

18. The WTRU as in claim 11, wherein the processor is configured to receive a value for the WTRU timer from an evolved Node-B (eNB) via higher layer signaling.

19. An evolved Node B (eNB) comprising a processor configured to:
send a discontinuous reception (DRX) configuration to a wireless transmit/receive unit (WTRU), the DRX configuration comprising a plurality of DRX cycle lengths;
determine that the WTRU has initiated DRX operation using a shorter DRX cycle length of the plurality of DRX cycle lengths;
set a base station timer upon determining that the WTRU has initiated DRX operation using the shorter DRX cycle length, the base station timer being synchronized with a WTRU timer used for DRX cycle length transitions at the WTRU and being associated with a length of time during which the WTRU performs DRX using the shorter DRX cycle length;
determine that the base station timer has expired; and
determine that the WTRU has transitioned from performing DRX using the shorter DRX cycle length to performing DRX using a longer DRX cycle length of the plurality of DRX cycle lengths in response to determining that the base station timer has expired.

20. The eNB as in claim 19, wherein the eNB does not send an explicit indication to the WTRU that instructs the WTRU to transition from the shorter DRX cycle length to the longer DRX cycle length.

21. The method as in claim 1, wherein the WTRU transitions from operation using the shorter DRX cycle length to operation using the longer DRX cycle length of the plurality of DRX cycle lengths without receiving an explicit indication from an evolved Node-B (eNB) that instructs the WTRU to transition from the shorter DRX cycle length to the longer DRX cycle length.

22. The WTRU as in claim 11, wherein the processor is configured to transition the WTRU from operation using the shorter DRX cycle length to operation using the longer DRX cycle length of the plurality of DRX cycle lengths without receiving an explicit indication from an evolved Node-B (eNB) that instructs the WTRU to transition from the shorter DRX cycle length to the longer DRX cycle length.

23. The method as in claim 1, wherein expiration of the WTRU timer is an implicit transition trigger to transition the WTRU from performing DRX using the shorter DRX cycle length to performing DRX using the longer DRX cycle length if no new transmissions have been received.

24. The WTRU as in claim 11, wherein expiration of the WTRU timer is an implicit transition trigger to transition the WTRU from performing DRX using the shorter DRX cycle length to performing DRX using the longer DRX cycle length if no new transmissions have been received.

25. The method as in claim 1, wherein the WTRU timer is implemented as a counter.

26. The WTRU as in claim 11, wherein the WTRU is configured to implement the WTRU timer using a counter.