Abstract:
An enhanced uplink user equipment is in soft handover. A radio network controller selects a primary Node-B out of a plurality of Node-Bs supporting the soft handover. The radio network controller receiving successfully received enhanced uplink data packets from the plurality of Node-Bs. The radio network controller reordered the successfully received enhanced uplink data packets for in-sequence deliver. The primary Node-B sends specified scheduling information to the user equipment that the other Node-Bs does not transmit. At least the primary Node-B transmits acknowledgements and negative acknowledgements to the user equipment.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/962,720 filed Oct. 12, 2004 which claims the benefit of U.S. Provisional Application Ser. Nos. 60/578,674 filed Jun. 10, 2004; 60/520,692 filed Nov. 17, 2003; 60/519,990 filed Nov. 14, 2003; and 60/517,656 filed Nov. 5, 2003, which are incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention is related to a wireless communication system. More particularly, the present invention is related to a method and apparatus for coordinating Node-Bs and supporting enhanced uplink (EU) transmissions during handover.  
       BACKGROUND  
       [0003]     Many schemes have been proposed to improve coverage, throughput, and transmission latency for EU transmissions in third generation partnership project (3GPP). One of the developments is to move the functions for scheduling and assigning uplink (UL) physical channel resources from a radio network controller (RNC) to a Node-B. A Node-B can make more efficient decisions and manage UL radio resources on a short-term basis better than the RNC, even if the RNC retains overall control over Node-Bs. A similar approach has already been adopted in downlink for high speed data packet access (HSDPA) in both universal mobile telecommunication system (UMTS) frequency division duplex (FDD) and time division duplex (TDD) modes.  
         [0004]     It has also been recognized that performance is greatly enhanced with the use of medium access control (MAC) level automatic repeat request (ARQ) and hybrid ARQ (H-ARQ). Application of these techniques during soft handover provides additional significant benefits.  
         [0005]      FIG. 1  shows a conventional wireless multi-cell communication system  100  including a wireless transmit/receive unit (WTRU)  105 , a Node-B  110 , an RNC  115 , and at least two cells  120 A,  120 B. Each of the cells  120 A,  120 B, is served by the Node-B  110 . Node-B  110  is controlled by the RNC  115 . When a change in the cell offering the best radio conditions is determined between cells  120 A and  120 B, a handover process is initiated.  
         [0006]     An “intra-Node-B handover” occurs when a WTRU changes from one cell to another cell controlled by the same Node-B, as shown in  FIG. 1 . An “inter-Node-B handover” occurs when a WTRU changes from one cell to another cell controlled by a different Node-B. In the latter case, the Node-B that controls the cell before the handover is called a source Node-B, and the Node-B that controls the cell after the handover is called a target Node-B.  
         [0007]     During soft handover, a WTRU establishes a plurality of connections with a plurality of Node-Bs in an active set. In this situation, a problem may arise for scheduling and H-ARQ operation. A WTRU may receive conflicting EU transmission scheduling from more than one Node-B. It is also difficult for the WTRU to receive, decode and process H-ARQ positive and negative acknowledgements (ACKs/NACKs) generated by a plurality of Node-Bs. The soft buffer of an H-ARQ process in Node-Bs may be corrupted during soft handover.  
         [0008]     One method to support H-ARQ across multiple Node-Bs, when the WTRU is in soft handover, is to place the ACK/NACK generation function in the RNC, which derives a single ACK/NACK based on the results from the multiple Node-Bs. However, this approach presents a significant delay to the ACK/NACK process, which is highly undesirable for performance reasons.  
         [0009]     When a WTRU undergoes an inter-Node-B hard handover, there is a possibility that a source Node-B, which is a Node-B before hard handover is completed, may not successfully receive EU transmissions for data packets that have been NACKed prior to hard handover activation time. Other WTRUs competing for UL resources may not be provided with enough physical resources in the source cell. If data blocks that have been NACKed prior to the handover are retransmitted to the source Node-B before the handover activation timer expires, those data blocks can be combined with the previous data blocks for H-ARQ decoding. In this way, the decoding takes the advantage of previous, although failed, transmissions of those data blocks in the source cell. If data blocks that have been NACKed prior to the handover are not retransmitted to the source Node-B before the handover activation timer is expired, they have to be transmitted again in the target cell as new data blocks. In this case, the previous transmissions of those data blocks in the source cell are not utilized.  
       SUMMARY  
       [0010]     An enhanced uplink user equipment is in soft handover. A radio network controller selects a primary Node-B out of a plurality of Node-Bs supporting the soft handover. The radio network controller receiving successfully received enhanced uplink data packets from the plurality of Node-Bs. The radio network controller reordered the successfully received enhanced uplink data packets for in-sequence deliver. The primary Node-B sends specified scheduling information to the user equipment that the other Node-Bs does not transmit. At least the primary Node-B transmits acknowledgements and negative acknowledgements to the user equipment. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein:  
         [0012]      FIG. 1  shows a conventional wireless communication system;  
         [0013]      FIG. 2  shows a system which uses a UL scheduler located in a primary Node-B during soft handover for EU in accordance with the present invention;  
         [0014]      FIG. 3  shows a system which uses an ACK/NACK generation function located in a primary Node-B during soft handover for EU in accordance with the present invention;  
         [0015]      FIG. 4  is a flowchart of a process including method steps for coordinating Node-Bs during soft handover in accordance with one embodiment of the present invention; and  
         [0016]      FIG. 5  is a flowchart of a process including method steps for prioritizing the transmission of NACKed data in a source Node-B before hard handover is completed in accordance with a separate embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     The present invention will be described with reference to the drawing figures wherein like numerals represent like elements throughout.  
         [0018]     Hereafter, the terminology “WTRU” includes but is not limited to a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment.  
         [0019]     When referred to hereafter, the terminology “Node-B” includes but is not limited to a base station, a site controller, an access point or any other type of interfacing device in a wireless environment.  
         [0020]     The present invention may be implemented in any type of wireless communication systems, such as UMTS-FDD, TDD, time division synchronous code division multiple access (TDSCDMA), code division multiple access 2000 (CDMA2000) (EV-DO and EV-DV) or any other type of wireless communication system.  
         [0021]     The features of the present invention may be incorporated into an IC or be configured in a circuit comprising a multitude of interconnecting components.  
         [0022]      FIG. 2  shows a wireless multi-cell communication system  200  which uses a UL scheduler located in a primary Node-B in accordance with the present invention. The wireless multi-cell communication system  200  includes a WTRU  205 , a plurality of Node-Bs  210  (i.e.,  210 A,  210 B), an RNC  215  and a plurality of cells  260  (i.e.,  260 A,  260 B,  260 C). Cells  260 A and  260 C are served by the Node-B  210 A. Cells  260 B are served by the Node-Bs  210 B. All of the Node-Bs  210  are controlled by the RNC  215 .  
         [0023]     During soft handover, the WTRU  205  establishes multiple connections with the Node-Bs  210  included in an active set. Each transmission from the WTRU  205  is processed independently at each of the Node-Bs  210 . One of the Node-Bs  210  in the active set is designated as a primary Node-B  210 A, and the other Node-Bs are designated as non-primary Node-Bs  210 B.  
         [0024]     As shown in  FIG. 2 , the primary Node-B  210 A includes a MAC entity  250 A including a UL scheduler  255 . Each of the non-primary Node-Bs  210 B also includes a MAC entity  250 B. Each of the MAC entities  250 A,  250 B, handles EU transmissions. The UL scheduler  255  in the MAC entity  250 A is responsible for scheduling the EU transmissions.  
         [0025]     In accordance with one embodiment of the present invention, the UL scheduler  255  is implemented only at the primary Node-B  210 A during soft handover. The WTRU  205  receives a UL transmission schedule only from the primary Node-B  210 A in a primary cell  260 A. However, the primary Node-B  210 A cannot send the scheduling information to the non-primary Node-Bs  210 B in every transmission time interval (TTI). In order to allow the primary Node-B  210 A to allocate resources for the WTRU  205  to transmit in cells controlled by the non-primary Node-Bs  210 B, those resources scheduled by the primary Node-B  250 A in a plurality of cells  260 B controlled by the non-primary Node-Bs  210 B cannot be assigned by the non-primary Node-Bs  210 B. Therefore, some physical resources common to all of the cells in the active EU subset should be assigned and reserved by a particular Node-B for the WTRU  205  during the soft handover, so that those resources can be used only by the primary Node-B  210 A.  
         [0026]     The UL scheduler  255  located in the primary Node-B  210 A considers the interference level caused by the EU transmission at any cell  260 A,  260 B,  260 C, in the EU active subset to be below a predetermined maximum allowed interference level. Thus, the primary Node-B  250 A limits the transmit power level of the WTRU  205  such that the interference levels are also within the maximum allowed interference levels at other cells  260 B,  260 C. To achieve this, the RNC  215  needs to relay necessary information, such as transmission power level and interference level, of the cells  260 B controlled by the non-primary Node-Bs  210 B to the primary Node-B  210 A, which then uses the information to schedule the UL transmissions.  
         [0027]     The EU scheduling information is transmitted to the WTRU  205  only by the primary Node-B  210 A through the primary cell  260 A. During soft handover, the WTRU  205  receives EU scheduling information only in the primary cell  260 A, although the EU scheduling information is valid in all other cells  260 B,  260 C.  
         [0028]     In one embodiment, the primary Node-B  250 A is selected by either the RNC  215  or the WTRU  205 . The RNC  215  may choose a Node-B that has the highest percentage of correctly received data blocks during a predefined time window as a primary Node-B.  
         [0029]     In another embodiment, the RNC  215  generates statistics for each Node-B, such as a bit error rate (BER) or a frame error rate (FER), or the like, over a predetermined time period. Then, the RNC  215  may select a Node-B having the best performance to be the primary Node-B  210 A. The RNC  215  then notifies the WTRU  205  and all other Node-Bs about the primary Node-B  210 A via radio resource control (RRC) and Iub signaling, respectively.  
         [0030]     In another embodiment, the WTRU  102  may choose a Node-B  210  that has the best downlink pilot power, (i.e., best downlink path loss or highest code power), as a primary Node-B  210 A. The WTRU  205  measures the power of pilot signals received from all Node-Bs  210  and selects the Node-B  210  having the highest pilot power to be the primary Node-B  210 A. The WTRU  205  then notifies all other Node-Bs about the primary Node-B  210 A via fast physical layer signaling.  
         [0031]     The WTRU  205  may report the downlink pilot power of all cells  260  to the RNC  215 . The RNC  215  then chooses one Node-B  210  to be the primary Node-B  210   a  based on the combined uplink and downlink quality. The uplink quality of a cell  260  based on the percentage of correctly received data blocks, (or BER, FER, or the like), during a predefined time window, and the downlink quality of a cell  260  is based on the WTRU received downlink pilot power. Then, the RNC  215  notifies the WTRU  205  and all of the Node-Bs  210  about the primary Node-B  210 A via RRC and Iub signaling, respectively.  
         [0032]     The present invention is advantageous over prior art systems. Using the present invention, a WTRU does not receive conflicting scheduling of EU transmission from Node-Bs during soft handover. In addition, EU transmission is scheduled in consideration of an interference level and radio resources in cells controlled by non-primary Node-Bs. Signaling delay from the primary Node-B  210 A to the WTRU  205  is much lower as compared to signaling delay from the RNC  215  to the WTRU  205 .  
         [0033]     In a separate embodiment,  FIG. 3  shows a wireless multi-cell communication system  300 , similar to the system  200  shown in  FIG. 2 . As shown in  FIG. 3 , the primary Node-B  210 A includes a MAC entity  250 A including an ACK/NACK generator  305 . Only the primary Node-B  210 A has the ACK/NACK generator  305 . The primary Node-B  210 A may perform H-ARQ with incremental redundancy, or only ARQ without implementing incremental redundancy.  
         [0034]     Still referring to  FIG. 3 , the primary Node-B  210 A receives at least one data packet from the WTRU  205  through the primary cell  260 A and performs an error check on the data packet. Any error checking method, such as a cyclic redundancy check (CRC), may be utilized. If the primary Node-B  210 A correctly decodes the data packet, such as passing the CRC, the primary Node-B  210 A transmits an ACK to the WTRU  205  and also transmits the correctly decoded data packet to the RNC  215 . If the primary Node-B  210 A fails to correctly decode the data packet, the primary Node-B  210 A transmits a NACK to the WTRU  205 .  
         [0035]     The non-primary Node-Bs  210 B also perform an error check on the data packet. However, the non-primary Node-Bs  210 B do not send ACKs or NACKs to the WTRU  205 . Instead, the non-primary Node-Bs send successfully decoded data packets to the RNC  215 . During soft handover, only the primary Node-B  210 A generates H-ARQ (or ARQ), ACKs and NACKs, and controls retransmissions.  
         [0036]     The MAC layer WTRU identities received by the non-primary Node-Bs  210 B may be used for routing of successfully received transmissions in a universal terrestrial radio access network (UTRAN). Since the non-primary Node-Bs  210 B are not aware of which WTRUs have been scheduled for EU transmission by the primary Node-B  210 A, the non-primary Node-Bs  210 B may rely on in-band MAC layer signaling of the WTRU ID to route correctly received transmissions to the correct RNC radio link. Even though the primary Node-B  210 A may be aware of which WTRU is scheduled, the same method may be implemented by the primary Node-B  210 A.  
         [0037]     Preferably, the primary Node-B  210 A may use soft combining to process transmissions, while the non-primary Node-Bs  210 B may process each transmission without soft combining. If the primary Node-B sends a NACK to the WTRU  205 , the NACKed data packet is stored in a buffer of the primary Node-B  210 A, and the NACKed data packet is combined with a retransmitted data packet. In contrast, the non-primary Node-Bs  210 B do not store the NACKed data packets. This eliminates the problem of soft buffer corruption between the Node-Bs  210 , and the complexities of multiple independent ACKs and/or NACKs.  
         [0038]     When an incremental combining process is implemented, measures should be taken to avoid soft buffer corruption. Sequence information or a new data indicator is required to enable a Node-B  210  to detect that the WTRU  205  is no longer repeating data for a particular WTRU H-ARQ process, but instead is sending new data. This is specifically required because the Node-B  210  has no other way to learn that a new transmission has started. Alternatively, the non-primary Node-Bs  210 B may simply perform an ARQ, without using an incremental combining process. This eliminates the soft buffer corruption problem.  
         [0039]     In the case where non-primary Node-Bs  210 B perform simple ARQ without incremental combining, the WTRU  205  must transmit self-decodable data packets to ensure that all of the Node-Bs  210  may decode transmissions, regardless of the result of earlier transmissions. Preferably, the H-ARQ functionality is terminated at the Node-Bs  210 . Each of the Node-Bs  210  sends to the RNC  215  successfully decoded data packets with explicit identification of transmission, such as a transmission sequence number (TSN). The RNC  215  may optionally use data packets delivered from the non-primary Node-Bs  210 B. A MAC entity  310 , located in the RNC  215 , is used to implement an in-sequence delivery process for delivering data to higher layers over all of the packets received from the Node-Bs  210 . After the RNC MAC entity  310  has completed its re-ordering process, it sends the data to a radio link control (RLC) (not shown). Missed packets are identified at the RNC  215  and the WTRU  205  is informed through RLC messaging.  
         [0040]     Alternatively, EU transmissions may identify WTRU ID, H-ARQ process, transmission sequence and/or new data indication (NDI) to allow for soft combining in the non-primary Node-B&#39;s  210 B. If this method is used to allow soft combining in the non-primary Node-Bs  210 B, the primary Node-B  210 A may not have to rely on scheduling and H-ARQ ACK/NACK decisions to determine when combining should be performed.  
         [0041]     There are two options for the transmission of ACK/NACK messages. The first option is a synchronous transmission. The ACK/NACK messages are transmitted after a unique time delay with respect to the corresponding uplink transmission or the EU channel allocation message. The second option is an asynchronous transmission. There is no unique delay between the transmission of ACK/NACK messages and the corresponding uplink transmission or the EU channel allocation message. Explicit information in the ACK/NACK message identifies the corresponding uplink transmission to enable the WTRU  205  to make the correct association between the ACK/NACK message and the transmission. This association is made by either identifying the H-ARQ process number and/or a unique sequence number, such as a TSN with each ACK/NACK feedback message to the WTRU  205 .  
         [0042]     In a separate embodiment, preferably implemented for the asynchronous ACK/NACK feedback case, the non-primary Node-Bs  210 B may provide H-ARQ ACK/NACK results to the primary Node-B  210 A in order to avoid unnecessary retransmissions for transmissions that are not correctly received by the primary Node-B  210 A, but are correctly received by the non-primary Node-Bs  210 B. A non-primary Node-B  210 B does not directly send an ACK or NACK message to the WTRU  205 . The non-primary Node-Bs  210 B sends ACK/NACK or CRC results to the RNC  215 . Then, the RNC  215  sends ACK or CRC results to the primary Node-B  210 A.  
         [0043]     In order to speed up H-ARQ processing, the first ACK message from any non-primary Node-B  210 B received by the RNC is preferably immediately forwarded to the primary Node-B  210 A. The primary Node-B  210 A also immediately generates an ACK message if the transmission is received correctly in the primary Node-B  210 A without waiting for feedback from the non-primary Node-Bs  210 B. The primary Node-B  210 A also generates an ACK message immediately upon reception of a forwarded ACK message from the RNC, even if other ACK messages may be forwarded. Since an ACK is generated if any of the paths are successful, an ACK can be generated as soon as the first successful transmission is found.  
         [0044]     Alternatively, in order to simplify the design of the ACK/NACK generator  205 , only a subset of the generating nodes may be used. For example, ACKs may be generated only at the RNC, or at the RNC and the primary Node-B  210 A.  
         [0045]     When the WTRU  205  sends an uplink transmission, for each H-ARQ process the WTRU  205  waits at least the time required for the primary Node-B  210 A to send ACK/NACK feedback. For each H-ARQ process, if an ACK is received by the WTRU  205 , the WTRU  205  may send new data in the next available or assigned opportunity.  
         [0046]     A NACK message can only originate in the RNC  215  since it is the only node that has all of the information necessary in the soft handover to determine that there have been no successful receptions at any Node-B  210 . The RNC  215  generates a NACK command if the RNC  215  receives no ACK from the Node-Bs  210  within a predetermined time interval. The RNC  215  forwards the NACK message to the WTRU  205  via the primary Node-B  210 A.  
         [0047]     It is also possible that this procedure can be implemented without an explicit NACK command. In this case, the lack of ACK reception within a particular period of time is considered the same as an explicit NACK command at either the primary Node-B  210 A and/or the WTRU  205 .  
         [0048]      FIG. 4  is a flowchart of a process  400  including method steps for coordinating Node-Bs during soft handover in accordance with one embodiment of the present invention. In step  405 , the RNC  215  makes a decision to initiate an inter-Node-B soft handover. In step  410 , the WTRU  205  establishes connections with at least two Node-Bs  210  in an active set. In step  415 , one of the Node-Bs  210  in the active set is designated as a primary Node-B  210 A and the one or more Node-B(s)  210  remaining in the active set are designated as a non-primary Node-Bs  210 B. In step  420 , the primary Node-B  210 A controls UL transmissions during soft handover by performing EU scheduling and H-ARQ operations.  
         [0049]      FIG. 5  is a flowchart of a process  500  including method steps for prioritizing the transmission of NACKed data in a source Node-B before hard handover is completed in accordance with a separate embodiment of the present invention. In step  505 , the RNC  215  makes a decision to initiate a hard handover for a WTRU  205  connected to a source Node-B  210 . In step  510 , the RNC  215  informs the source Node-B  210  when the WTRU  205  will stop transmission and reception in the source cell  260 . In step  515 , the RNC  215  sends an activation timer to the source Node-B  210  to set the time for handover.  
         [0050]     Still referring to  FIG. 5 , if the source Node-B  210  determines that there are data packets that were previously NACKed, as many previously NACKed data packets as possible should be retransmitted before the handover activation timer expires. Otherwise, the system may lose the benefit of incrementally combining the previous transmission with the retransmission. Therefore, the source Node-B scheduler  255  takes the handover activation time into account when it schedules the data packets that have been NACKed. If there is not enough radio resource for the source Node-B  210  to schedule transmission of all the NACKed data packets in time, the source Node-B  210  should manage to schedule transmission of as many NACKed data packets as possible.  
         [0051]     Still referring to  FIG. 5 , in order to transmit as many NACKed data packets as possible before the activation timer expires, the source Node-B  210  adjusts the priority of transmissions (step  525 ) and, in step  530 , the source node-B  210  adjusts the MCS of the transmissions (step  530 ). Higher priority of scheduling is given to the data packets that have been NACKed. If the radio resources are sufficient, a more robust MCS may be used to increase the probability of successful transmissions from the WTRU  205  to the source Node-B  210 . In step  535 , the handover is completed at the expiration of the activation timer.  
         [0052]     In order for the WTRU  205  to understand that the scheduled uplink transmission is intended for data blocks with previous transmission failures, the source Node-B  210  uplink scheduler  255  may specify that the scheduled UL transmission is intended for the data blocks that were previously NACKed. This may be implemented by including H-ARQ process identification in the UL scheduling information that is sent from the source Node-B  210  to the WTRU  205 . By receiving the scheduling information from the source Node-B  210 , the WTRU  205  knows that the scheduled transmission is for specific data associated with HARQ process identification sent together with the scheduling information.  
         [0053]     While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in forms and details may be made therein without departing from the scope of the invention as described above.