Abstract:
Providing discontinuous reception (DRX) is disclosed. In DRX mode a wireless transmit/receive unit (WTRU) may periodically wake up, in relation to a DRX interval, to check for a paging message. The WTRU may reenter the DRX mode if there is no paging message. The WTRU may receive another specified DRX interval, in connection with a broadcast message, based on the activity of the WTRU. The another DRX interval may be increased as inactivity of the WTRU increases.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/785,491 filed Mar. 24, 2006, which is incorporated by reference as if fully set forth. 
     FIELD OF INVENTION 
     The present invention is related to wireless communication systems. More particularly, the present invention is related to a method and apparatus for maintaining uplink synchronization and reducing battery power consumption of a wireless transmit/receive unit (WTRU). 
     BACKGROUND 
     In a conventional third generation partnership project (3GPP) system, there are four non-idle radio resource control (RRC) states roughly corresponding to four levels of WTRU activity: a dedicated channel (DCH) cell level (Cell_DCH) state, a forward access channel (FACH) cell level (Cell_FACH) state, a paging channel (PCH) cell level (Cell_PCH) state, and a universal terrestrial radio access network (UTRAN) registration area (URA) PCH (URA_PCH) state. In a Cell_DCH state, a WTRU has a dedicated physical channel for data transport. In a Cell_FACH state, no dedicated channel is allocated to the WTRU, but the WTRU may use a random access channel (RACH) and a FACH channel for conveying and receiving signaling as well as user plane data. It is not efficient to send a large amount of user plane data in the Cell_FACH state. A Cell_PCH state reduces battery consumption by only listening to a PCH in a discontinuous reception (DRX) mode. As with the Cell_DCH and Cell_FACH states, the location of a WTRU in the Cell_PCH state is known at the cell level. A WTRU in the Cell_PCH state temporarily enters a Cell_FACH state when it relocates to a new cell in order to communicate its new location information. A URA_PCH state is similar to the Cell_PCH state, except that in the URA_PCH state the network is only informed when the WTRU moves to a new URA. When a WTRU changes cells, the WTRU generally stays in the same state. Currently, handovers in the Cell_DCH state are network-directed. 
     A WTRU that is in an active state has a non-access stratum (NAS) connectivity so that the WTRU may communicate to nodes in a core network. A WTRU in an active state also has an access stratum (AS) connectivity such that a radio bearer configuration, (e.g., WTRU capability exchange, ciphering, or the like), has been completed for the WTRU. 
     A WTRU in an idle state consumes less power and resources than a WTRU in a low-power active state. One important characteristic of a WTRU in an idle state is that the WTRU does not have to participate in an active mode handover. In other words, when a WTRU in an idle state moves from one cell to another, the WTRU does not configure radio bearers with the new cell if the WTRU remains in an idle state. 
     One of the goals in a next generation wireless communication system is maintaining an “always on” connectivity. However, for a battery-powered WTRU, battery power consumption is an issue. The “always on” connectivity is a desirable feature, but this tends to shorten the battery life. 
     Currently in 3GPP, a WTRU maintains uplink synchronization whenever it has a dedicated channel to a base station. The WTRU always maintains uplink synchronization in a Cell_DCH state. The WTRU also resynchronizes its uplink any time it has a new set of dedicated channels disjoint from its prior set. Maintaining uplink synchronization, (conventionally via RACH transmissions), is one of the sources for consuming the battery power of the WTRU. 
     Therefore, it would be desirable to provide a scheme for maintaining uplink synchronization efficiently and reducing battery power consumption while the WTRU is in an active state. 
     SUMMARY 
     The present invention is related to a method and apparatus for maintaining uplink synchronization and reducing battery power consumption of a WTRU. A Node-B sends a polling message to a WTRU. The WTRU sends an uplink synchronization burst in response to the polling message without contention. The Node-B estimates an uplink timing shift based on the uplink synchronization burst and sends an uplink timing adjustment command to the WTRU without contention. The WTRU then adjusts uplink timing based on the uplink timing adjustment command. Alternatively, the Node-B may send a scheduling message for uplink synchronization to the WTRU. The WTRU may send the uplink synchronization burst based on the scheduling message. Alternatively, the WTRU may perform contention-based uplink synchronization after receiving a synchronization request from the Node-B. The WTRU may enter an idle state instead of performing a handover to a new cell when the WTRU moves to the new cell. A DRX interval for the WTRU may be set based on activity of the WTRU. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding of the invention may be had from the following description of a preferred embodiment, given by way of example and to be understood in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a signaling diagram of a process for maintaining uplink synchronization using a contention free procedure in accordance with one embodiment of the present invention; 
         FIG. 2  is a signaling diagram of a process for maintaining uplink synchronization using a contention free procedure in accordance with another embodiment of the present invention; 
         FIG. 3  is a signaling diagram of a process for uplink synchronization using a contention-based procedure in accordance with the present invention; and 
         FIG. 4  is a block diagram of a Node-B and a WTRU configured in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     When referred to hereafter, the terminology “WTRU” includes but is not limited to a user equipment (UE), a mobile station (STA), 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 “Node-B” includes but is not limited to a base station, a site controller, an access point (AP), or any other type of interfacing device capable of operating in a wireless environment. 
     The present invention is applicable to any wireless communication systems including, but not limited to, wideband code division multiple access (WCDMA) and long term evolution (LTE) of 3GPP cellular networks beyond 3GPP Release 7. 
       FIG. 1  is a signaling diagram of a process  100  for maintaining uplink synchronization using a contention-free procedure in accordance with one embodiment of the present invention. For uplink synchronization, a Node-B  152  sends a polling message to a WTRU  154  to request transmission of an uplink synchronization burst (step  102 ). The WTRU  154  may receive the polling message either during registration or via broadcasting after registration. The polling message indicates a specific time, (e.g., a system frame number or transmission time interval (TTI)), and/or resource for sending the uplink synchronization burst so that the specific WTRU may send the uplink synchronization burst without contending against other WTRUs. In response to the polling message, the WTRU  154  sends an uplink synchronization burst based on the parameters, (e.g., a specific time, a resource, or the like), included in the polling message (step  104 ). The Node-B  152  receives the uplink synchronization burst and estimates an uplink timing shift based on the uplink synchronization burst (step  106 ). The Node-B  152  sends an uplink timing adjustment command to the WTRU  154  (step  108 ). The WTRU  154  then adjusts uplink timing based on the uplink timing adjustment command (step  110 ). 
     The polling message may include uplink interference information so that the WTRU  154  may use the information in determining uplink transmit power for the uplink synchronization burst. Alternatively, the Node-B  152  may explicitly indicate an uplink transmit power for the uplink synchronization burst. The Node-B  152  may send the polling message via a downlink common control channel granting an access to an uplink shared channel for the uplink synchronization burst. 
     Alternatively, to save an additional power, the WTRU  154  may enter a DRX mode and wake at predetermined intervals for either paging or uplink shared channel allocation. If the WTRU  154  enters a DRX mode, the Node-B  152  does not need to send the polling messages to the WTRU  154  very often. The network configures a periodicity on how often the Node-B  152  should send the polling message to the WTRU  154 . This periodicity information can be sent to the WTRU  154  through a broadcast message. In this way, the WTRU  154  may only wake up at the moment when the polling message is expected. After listening to the polling message and perform the necessary uplink transmissions, the WTRU  154  reenters the DRX mode in order to save the battery power. 
     The polling message may address several WTRUs containing parameters for several polled WTRUs to send their uplink synchronization bursts. A polling rate may be different for each WTRU. The polling rate may be determined based on the estimated clock drift and/or mobility of the WTRUs. The polling rate may be adaptively changed by either the WTRU  154 , (via a request to the Node-B  152 ), or the Node-B  152 . The polling rate may be different for each RRC (or medium access control (MAC)) state of the WTRU  154 . The polling rate may increase over time, (e.g., exponentially), as the period of inactivity of the WTRU  154  increases. The Node-B  152  may use the results of the uplink synchronization as one factor in adaptively changing the polling rate for the WTRU  154 . An uplink channel allocation for the uplink synchronization burst provided by the polling message may be periodic or may optionally indicate duration of the uplink channel. 
     Since the WTRU  154  in the active state is already known to the Node-B  152  and the Node-B  152  can uniquely identify the WTRU  154  via the scheduled times for the WTRU  154 , the WTRU  154  may omit a cell ID or a WTRU ID, (e.g., a control radio network temporary identity (C-RNTI)), in the uplink synchronization burst. This will reduce an overhead. 
     Alternatively, the Node-B  152  may include a short, (preferably random), identifier, tag or a sequence number in the polling message, and the WTRU  154  may use the same short identifier, tag or sequence number in the uplink synchronization burst. Since this identifier, tag, or sequence number is smaller than other forms of identification, (e.g., a cell ID or a C-RNTI), the overhead is reduced. 
       FIG. 2  is a signaling diagram of a process  200  for maintaining uplink synchronization using a contention-free procedure in accordance with another embodiment of the present invention. A Node-B  252  generates a schedule for uplink synchronization for a WTRU  254  and sends a scheduling message for uplink synchronization to the WTRU  254  (step  202 ). The scheduling message may include a schedule for several WTRUs. Uplink synchronization is performed at predetermined times using a predetermined resource specified in the scheduling message. The Node-B  252  may signal the resource for uplink synchronization to the WTRU  254  prior to the scheduled synchronization time. The scheduling message may include uplink interference information or uplink transmit power information. The uplink transmit power information may be for a group of WTRUs if they are in a similar situation. Alternatively, the uplink transmit power information may be for each WTRU or may just be used as a reference. The scheduling message may be transmitted via a downlink common control channel granting an access to an uplink shared channel for the synchronization burst. 
     The WTRU  254  sends an uplink synchronization burst based on the scheduling message (step  204 ). The WTRU  254  may optionally indicate the next synchronization time in the uplink synchronization burst, (i.e., the synchronization burst payload may include a field indicating the next synchronization time). This synchronization time may be viewed as a recommendation by the Node-B  252 , and the Node-B  252  may modify the schedule or the recommendation by sending a signal via a downlink signaling channel, (e.g., a shared control channel). The WTRU  254  may also send a scheduling request informing the amount of data waiting for transmission in the WTRU  254 . The WTRU  254  may also send measurement results such as a channel quality indicator (CQI). 
     The Node-B  252  estimates an uplink timing shift based on the uplink synchronization burst (step  206 ). The Node-B  252  sends an uplink timing adjustment command to the WTRU  254  (step  208 ). The WTRU  254  then adjusts uplink timing based on the uplink timing adjustment command (step  210 ). 
     Since the WTRU  254  in an active state is already known to the Node-B  252  and the Node-B  252  can uniquely identify the WTRU  254  via the scheduled times for the WTRU  254 , the WTRU  254  may omit a cell ID or a WTRU ID, (e.g., a C-RNTI), in the uplink synchronization burst. This will reduce an overhead. 
     Alternatively, the Node-B  252  may include a short, (preferably random), identifier, tag or a sequence number in the scheduling message, and the WTRU  254  may use the same short identifier, tag or sequence number in the uplink synchronization burst. Since this identifier, tag, or sequence number is smaller than other forms of identification, (e.g., a cell ID or a C-RNTI), the overhead is reduced. 
       FIG. 3  is a signaling diagram of a process  300  for uplink synchronization using a contention-based procedure in accordance with the present invention. A Node-B  352  sends a synchronization request message to a WTRU  354  instructing or recommending the WTRU  354  to perform an uplink synchronization procedure during the WTRU  354  is in an active state (step  302 ). The synchronization request message may address multiple WTRUs. The synchronization request message may include a specific time and/or resource for the WTRU to send the synchronization burst. The synchronization request message may include uplink interference information or uplink transmit power information. The synchronization request message may be transmitted via a downlink common control channel granting an access to an uplink shared channel for the synchronization burst. 
     In response, the WTRU  354  performs the conventional contention-based procedure for uplink synchronization. The WTRU  354  sends an uplink transmission, (e.g., an RACH preamble), to the Node-B  352 , (e.g., via an RACH), using a contention-based mechanism, (e.g., a slotted Aloha mechanism) (step  304 ). Either non-synchronized or synchronized RACH can be used for this uplink transmission, which is indicated either through an RRC signaling or by the synchronization request message from the Node-B  352 . The Node-B  352  receives the uplink transmission and estimates an uplink timing shift based on the uplink transmission (step  306 ). The Node-B  352  sends an uplink timing adjustment command to the WTRU  354  (step  308 ). The WTRU  354  then adjusts uplink timing based on the uplink timing adjustment command (step  310 ). 
     The Node-B  352  may include a short, (preferably random), identifier, tag or a sequence number in the uplink synchronization request message, and the WTRU  354  may use the same short identifier, tag or sequence number in the uplink synchronization burst. 
     The Node-B  352  may designate a frame, a sub-frame or a timeslot in which the uplink synchronization procedure (or random access procedures) should be performed while the WTRU  354  is in an active state. The designated frame, sub-frame or timeslot is different than the frames, sub-frames or timeslots that are used to perform the uplink synchronization procedure (or random access procedures) during the WTRU  354  is in an idle state, (i.e. different than the RACH timeslots). The designation of the frame, sub-frame or timeslot may be performed via prior signaling, (e.g., broadcast messages), or via pre-configuration. The Node-B  352  may provide different service levels or meet the different performance requirements or targets for WTRUs in an active state as opposed to WTRUs in an idle state. When a WTRU  354  is in an active state, in order to support the active traffic, more tightly maintained uplink synchronization is required. Therefore, the WTRU  354  may need to send uplink synchronization transmission more frequently compared to an idle state which requires less tight uplink synchronization since there is no active traffic going on. 
     In all the above embodiments, the Node-B may include a flag in the polling message, the scheduling message or the synchronization request message to indicate whether it is mandatory or optional that the WTRU performs the procedure for uplink synchronization. Commanding the WTRU to perform uplink synchronization, (i.e., setting the flag to “mandatory”), is useful when the Node-B needs to send packets to the WTRU, (e.g., high speed downlink packet access (HSDPA)), since the WTRU needs to be uplink synchronized in order to send a hybrid automatic repeat request (H-ARQ) positive feedback. Preferably, the flag is included within the polling message, the scheduling message or the synchronization request message. If the flag indicates the uplink synchronization is optional, the WTRU may or may not perform the uplink synchronization procedure. 
       FIG. 4  is a block diagram of a Node-B  400  and a WTRU  450  configured in accordance with the present invention. The Node-B  400  includes an uplink synchronization controller  402  and a transceiver  404 . The WTRU  450  includes a transceiver  452  and an uplink synchronization controller  454 . The uplink synchronization controller  402  generates a polling message, a scheduling message or a synchronization request message for the WTRU  450 . The transceiver  404  transmits the polling message, the scheduling message or the synchronization request message to the WTRU  450 . The transceiver  452  of the WTRU  450  receives the polling message, the scheduling message or the synchronization request message, and sends an uplink synchronization burst based on the polling message, the scheduling message or the synchronization request message to the Node-B  400 . 
     The uplink synchronization controller  402  estimates an uplink timing shift based on an uplink synchronization burst transmitted by the WTRU  450 , and generates an uplink timing adjustment command. The transceiver  404  then sends the uplink timing adjustment command to the WTRU  450 . The uplink synchronization controller  454  of the WTRU  450  then adjusts uplink timing based on the uplink timing adjustment command. 
     In accordance with another embodiment of the present invention, a WTRU may use cell reselection as a trigger to go from a low-power active state to an idle state. When a WTRU, through its cell search and cell reselection procedures, determines that the WTRU should move to a new cell, the WTRU may enter an idle state, instead of performing a handover and radio bearer reconfiguration. In this manner, the WTRU may conserve power by avoiding the control signaling associated with the handover and radio bearer reconfiguration. 
     In accordance with yet another embodiment of the present invention, a DRX interval, (i.e., the gap between the WTRU&#39;s wake-up time intervals for reception), may be configured adaptively according to a service level, (i.e., activity of the WTRU). The DRX interval is increased as the period of inactivity of the WTRU increases subject to a predetermined maximum value. The DRX interval may be increased exponentially. Preferably, the network determines the DRX interval and signals it to the WTRU. 
     Alternatively, the WTRU may inform the Node-B whether the WTRU is currently powered by a battery or a constant power supply, so that the DRX interval is set accordingly. The WTRU may inform the Node-B of its currently remaining battery capacity and other characteristics, (such as consumed power in transmitting data), that may assist the Node-B in computing the estimated battery life. The Node-B then sets up power saving policies, (e.g., DRX interval), for the WTRU based on the information. 
     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention. The methods or flow charts provided in the present invention 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.