Patent Publication Number: US-11647438-B2

Title: Method and apparatus for monitoring downlink channels of a plurality of cells and receiving data over a downlink channel

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/008,178, filed Aug. 31, 2020, which is a continuation of U.S. patent application Ser. No. 16/533,320 filed on Aug. 6, 2019, which issued as U.S. Pat. No. 10,764,803 on Sep. 1, 2020, which is a continuation of U.S. patent application Ser. No. 15/212,403, filed on Jul. 18, 2016, which issued as U.S. Pat. No. 10,390,279 on Aug. 20, 2019, which is a continuation of U.S. patent application Ser. No. 13/236,133 filed on Sep. 19, 2011, which issued as U.S. Pat. No. 9,438,381 on Sep. 6, 2016, which is a continuation of U.S. patent application Ser. No. 10/925,426, filed on Aug. 25, 2004, which issued as U.S. Pat. No. 8,023,463 on Sep. 20, 2011, which claims priority from U.S. Provisional Application Nos. 60/497,747 filed on Aug. 25, 2003; 60/507,554 filed on Oct. 1, 2003; 60/508,797 filed on Oct. 3, 2003; 60/520,207 filed on Nov. 14, 2003 and 60/585,174 filed on Jul. 2, 2004, which are incorporated by reference as if fully set forth. This application is related to U.S. patent application Ser. No. 15/212,402, filed Jul. 18, 2016, which issued as U.S. Pat. No. 10,251,106 on Apr. 2, 2019. This application is related to co-pending U.S. Patent Application entitled “Method and Apparatus for Transmitting Data Via a Plurality of Cells”, filed Nov. 16, 2020, and co-pending U.S. Patent Application entitled “Method and Apparatus for Transmitting Data Over a Downlink Channel of at Least One of a Plurality of Cells”, filed Nov. 16, 2020. 
    
    
     FIELD OF INVENTION 
     The present invention is related to a wireless communications. More particularly, the present invention is related to an enhanced uplink (EU) operation during a soft handover. 
     BACKGROUND 
     Cellular wireless communication networks are divided into a plurality of coverage regions. Each coverage region in the network is served by a Node-B. As a wireless transmit/receive unit (WTRU) travels, it may move from one coverage region to another in the network. 
     The WTRU is served by the designated Node-B for a particular coverage region. The regions covered by Node-Bs overlap each other, and at the boundary of the region a WTRU can establish connections with more than one Node-B. As the WTRU moves from one coverage region to another in the network, the WTRU goes through handover. Soft handover is widely used to ensure communication without interruption while roving around a plurality of cells. 
     Soft handover occurs when a WTRU is connected to two or more Node-Bs simultaneously, on the same frequency. In soft handover, all Node-Bs serving the WTRU process the received data, which is then routed to a radio network controller (RNC) for macro diversity combining. For simplicity, the RNC may use an error detection technique such as a Cyclic Redundancy Check (CRC) and may accept a packet that passes the CRC. 
     Softer handover is a special case of soft handover. When a WTRU is in softer handover, the WTRU is connected to two or more cells belonging to the same Node-B. In contrast to soft handover, in softer handover macro diversity with or without maximum ratio combining can be performed in the Node-B. 
     Automatic repeat request (ARQ) is a technique whereby the receiver requests a retransmission of packets by the transmitter if errors are detected. Hybrid ARQ (H-ARQ) is a technique whereby transmitted data blocks are encoded for partial error correction at the receiver, and only data blocks with uncorrected errors are retransmitted. In prior art, i.e. in high speed downlink packet access (HSDPA), the H-ARQ functionality is terminated and controlled by the Node-B, (a technique called Node-B-controlled H-ARQ), allowing for rapid transmissions and retransmissions of erroneously received packets. This feature was both highly desirable and practical because H-ARQ in HSDPA was not required for soft handover. This feature would be highly desirable for EU also, but problems exist because it is intended for EU (and H-ARQ) to operate during soft handover. 
     One of the problems with Node-B-controlled H-ARQ in soft handover is the link imbalance. Since the associated uplink (UL) and downlink (DL) control signaling does not benefit from the soft handover gain, it might be error prone and require significant power offsets. In the DL direction, the WTRU may not be able to receive the acknowledge (ACK) or non-acknowledge (NACK) signals from all involved Node-Bs. In the UL, not all involved Node-Bs may be able to receive the associated control signaling from the WTRU, which may lead to soft buffer corruption. 
     A soft buffer is a buffer for implementing H-ARQ in a Node-B. Data packets received, but not acknowledged, by the Node-B are temporarily stored in the soft buffer for incremental combining. Therefore, a data packet transmitted, but not acknowledged previously, is combined with a retransmission of the same data packet transmitted in response to NACK signaling. Chase combining is a special case of an incremental combining. The soft buffer corruption causes misalignment of an H-ARQ protocol state among different Node-Bs and leads to loss of the soft handover gain. It would be desirable to achieve efficient H-ARQ operation without the problems associated with prior art systems. 
     Node-Bs can often make more efficient decisions and manage UL radio resources on a short-term basis better than an RNC, even if the RNC retains overall control over Node-Bs. In order for a Node-B to assign UL radio resources to WTRUs in EU operation, the Node-B must know several WTRU-specific parameters. Under the current 3GPP standard, only the RNC can know the WTRU-specific parameters by means of radio resource control (RRC) messages. Therefore, it is necessary to forward the information to the Node-B for proper scheduling of radio resources in EU transmissions. 
     An RNC maintains an active set of cells for each WTRU in soft handover. The RNC bases its decision to add to or remove cells from the WTRU&#39;s active set upon measurements provided by a WTRU and a Node-B and on management of available radio resources in each cell. Under the current 3GPP standards, the RNC applies RRC radio bearer (RB) control procedures to coordinate active set cells with the WTRU, and Node-B application part/radio network subsystem application part (NBAP/RNSAP) radio link procedures to coordinate active set cells with each Node-B. 
     During soft handover, some information should be communicated between network entities to support EU operation. The information includes, but is not limited to, information related to an active set, information regarding a Node-B that controls transmissions during soft handover, EU scheduling information during soft handover, and ACK/NACK status information during soft handover. The current 3GPP standards do not define specific protocols to transfer necessary information which are imperative in operation of EU during soft handover. Therefore, it is necessary to define a protocol for transferring WTRU-specific information and other EU related information among an RNC, a Node-B, and a WTRU so that a Node-B is enabled to schedule radio resources and EU connections are handed over properly during soft handover. 
     SUMMARY 
     The present invention is related to EU operation during a soft handover in a wireless communication system. The wireless communication system comprises a WTRU, at least two Node-Bs, and an RNC. In accordance with one embodiment of the present invention, for each WTRU one Node-B is designated as a primary Node-B and any other Node-B within the EU active set as a non-primary Node-B. The primary Node-B controls EU operation during soft handover including EU scheduling and H-ARQ. Soft buffer corruption is avoided by controlling H-ARQ during soft handover only by the primary Node-B. Alternatively, an RNC may control EU operation during soft handover including H-ARQ. In this case, an RNC generates final ACK/NACK decision based on the error check results of the Node-Bs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1 A and  1 B  are diagrams of the first embodiment of the present invention. 
         FIGS.  2 A and  2 B  are diagrams of the second embodiment of the present invention. 
         FIGS.  3 A and  3 B  are diagrams of the third embodiment of the present invention. 
         FIGS.  4 A and  4 B  are diagrams of the fourth embodiment of the present invention. 
         FIG.  5    is a diagram showing a streamlined connection between Node-Bs and an RNC in accordance with the present invention. 
         FIGS.  6  and  7    are diagrams of systems for transferring ACK/NACK signals in accordance with the present invention. 
         FIGS.  8 A and  8 B  are diagrams of a system and process for softer handover in accordance with the present invention. 
         FIG.  9    is a diagram for transferring WTRU-specific information among network entities in accordance with the present invention. 
         FIG.  10    is a diagram for transferring information during handover among network entities in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be described with reference to the drawing figures wherein like numerals represent like elements throughout. 
     When referred to hereafter, the terminology “WTRU” includes but is not limited to a user equipment, a mobile station, fixed or mobile subscriber unit, pager, or any other type of 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, site controller, access point or any other type of interfacing device in a wireless environment. 
       FIGS.  1 A and  1 B  are diagrams of a system  100  and a process  150  of a first embodiment of the present invention. The WTRU  102  establishes connections with at least two cells controlled by different Node-Bs  104   a ,  104   b  for soft handover. Data packets transmitted from the WTRU  102  are received and processed separately by at least two Node-Bs  104   a ,  104   b  during soft handover (step  152 ). 
     One Node-B in a group of Node-Bs in an “active set” is designated as a primary Node-B  104   a , while other Node-Bs in the active set are designated as non-primary Node-Bs  104   b . An RNC  106  or the WTRU  102  makes this decision (step  152 ). If it is decided by an RNC  106 , the RNC  106  informs all Node-Bs  104   a ,  104   b  and the WTRU  102 . If it is decided by a WTRU  102 , the WTRU  102  informs either all Node-Bs  104   a ,  104   b  or the RNC  106  which in turn informs all the Node-Bs  104   a ,  104   b.    
     In making a decision regarding the primary Node-B  104   a , the RNC  106  may use statistics, i.e. the number of successful decodings of particular WTRUs transmissions by each Node-B  104   a ,  104   b , to identify the Node-B  104   a ,  104   b  with the best UL performance. It is the performance of the best cell controlled by a Node-B that is evaluated, not the performance of all cells associated with a Node-B. The RNC  106  may also select the primary Node-B  104   a  by evaluating both UL performance as described above and DL performance as obtained from WTRU  102  measurements. The RNC  106  then notifies the Node-Bs  104   a ,  104   b  and the WTRU  102  regarding which one will be the primary Node-B  104   a  via Iub signaling and RRC signaling, respectively. The WTRU  102  may also be informed of the primary Node-B  104   a  by fast layer 1 signaling from Node-B. 
     The primary Node-B  104   a  employs incremental combining, while non-primary Node-Bs  104   b  may or may not use incremental combining. If the non-primary Node-Bs  104   b  do not use incremental combining, the non-primary Node-Bs  104   b  may use simple ARQ, and may always refresh their buffers and not perform any combining. This scheme eliminates the problem of soft buffer corruption in soft handover. If both a primary Node-B  104   a  and non-primary Node-Bs  104   b  perform incremental combining, soft buffer corruption may be eliminated with a new data indicator or a sequence number in physical control signaling sent by the WTRU  102  to inform Node-Bs  104   a ,  104   b  which data packet is being transmitted, and thereby the Node-Bs  104   a ,  104   b  can manage soft buffer without corruption. 
     All Node-Bs  104   a ,  104   b  in the active set receive a data packet from the WTRU  102  (step  154 ). Each Node-B  104   a ,  104   b  performs an error check on the data packet and generates an indication of success or failure in decoding the data packet (step  156 ). Determining whether a data packet is successfully received is performed via an error check procedure; such as implementing a cyclic redundancy check (CRC). The indication of success or failure in decoding the data packet by the Node-Bs can be configured in a variety of different forms, but will be referred to hereinafter as a CRC result, or an ACK/NACK, in all embodiments of the present invention. However, any type of error checking may be performed in accordance with the teachings of the present invention, and it should be understood that the term “CRC” or “ACK/NACK” is used only as an illustration, not as a limitation, of the present invention. 
     When a Node-B  104   a ,  104   b  correctly decodes the data packet as determined by the error check, the Node-B  104   a ,  104   b  transmits the data packet to the RNC  106 . If the primary Node-B  104   a  derives an ACK from the data packet, it transmits an ACK signal to the WTRU  102  and the RNC  106  without waiting for the CRC results from non-primary Node-Bs  104   b , and refreshes soft buffer (step  158 ). If the primary Node-B  104   a  derives a NACK from the data packet, it transmits a NACK to the RNC and waits for the final decision from the RNC or CRC results from non-primary Node-Bs forwarded through the RNC  106  (step  158 ). The primary Node-B  104   a  may set a timer as will be explained hereinafter. The primary Node-B  104   a  transmits an ACK/NACK signal to the WTRU  102  in accordance with the final decision made by the RNC  106  or CRC results forwarded from non-primary Node-Bs  104   b.    
     Non-primary Node-Bs  104   b  transmit a data packet to the RNC  106  only if they derive an ACK from the data packet (step  158 ). The RNC  106  makes an ACK/NACK decision (step  160 ). If the RNC  106  receives at least one ACK from Node-Bs  104   a ,  104   b , the RNC  106  makes an ACK decision, and if the RNC  106  receives no ACK from Node-Bs  104   a ,  104   b  within a predetermined time period, the RNC  106  makes a NACK decision. The RNC  106  transmits an ACK/NACK decision to the primary Node-B  104   a . The RNC  106  may not send an ACK decision to the primary Node-B  104   a  when the primary Node-B  104   a  derives an ACK. The RNC  106  optionally transmits the ACK/NACK decision to the non-primary Node-Bs  104   b  for soft buffer management depending on the scheme of incremental combining at the non-primary Node-Bs  104   b.    
     It is optional for the RNC  106  to use the packets delivered from non-primary Node-Bs  104   b . If the RNC  106  uses the packet from non-primary Node-Bs  104   b , a media access control (MAC) function of the RNC  106  performs an in-sequence delivery mechanism over all the received packets from all the involved Node-Bs  104   a ,  104   b . If a Radio Link Control (RLC) layer realized an out of sequence transmission it assumes data is lost and requests retransmission. If the RNC  106  does not use the packets from non-primary Node-Bs  104   b , the RNC  106  processes only the packets received from the primary Node-B  104   a . The RNC  106  extracts and enters the data packet into the MAC level reordering buffer. After the RNC MAC performs the re-sequencing process, it sends the data to the RLC layer. Missed packets are identified and notified to the WTRU  102  through RLC messaging. 
     Optionally, a streamlined connection may be implemented in transmission of the result of the error check between Node-Bs and an RNC. A fast streamlined connection is explained with reference to  FIG.  5   . The streamlined connection is dedicated to fast signaling between the RNC  506  and the Node-Bs  504   a ,  504   b , and eliminates a long delay between the RNC  506  and Node-Bs  504   a ,  504  b. A high speed streamlined connection  510   a ,  510   b  is established between Node-Bs  504   a ,  504   b  and an RNC  506 . CRC results from Node-Bs  504   a ,  504   b  to the RNC  506  and an ACK/NACK decisions from the RNC  506  to Node-Bs  504   a ,  504   b  are transmitted via the streamlined connections  510   a ,  510   b . No direct physical link is required between Node-Bs  504   a ,  504   b . Rather, a logical channel between Node-Bs  504   a ,  504   b  is required. The RNC  506  coordinates establishing the logical channel. 
     A fast streamlined connection  510   a ,  510   b  may be implemented in accordance with two alternatives. In accordance with the first alternative, two logical channels are established between an RNC  506  and two Node-Bs  504   a ,  504   b , respectively. The RNC  506  receives H-ARQ signaling  510   b  from one Node-B  504   b  and processes it before forwarding it  510   a  to another Node-B  504   a . The RNC  506  recognizes the status of H-ARQ process of each Node-B  504   a ,  504   b  by processing the signaling. As explained above, the CRC results are processed by the RNC  506  and the RNC  506  makes a final ACK/NACK decision and transmits the ACK/NACK decision to the Node-Bs  504   a ,  504   b.    
     Upon reception of the first ACK from any Node-B  504   a ,  504   b  in the RNC  506 , the RNC  506  transmits ACK decision to all Node-Bs  504   a ,  504   b . In the case that all Node-Bs  504   a ,  504   b  derive a NACK, it takes some time to wait for all Node-Bs  504   a ,  504   b  to provide the CRC results. Therefore, optionally the RNC  506  may set a timer waiting for all Node-B&#39;s responses and if the timer expires the RNC  506  transmits a NACK to all Node-Bs  504   a ,  504   b.    
     In accordance with the second alternative, a single logical channel between two Node-Bs  504   a ,  504   b  via an RNC  506  is established. The RNC  506  receives CRC results from one Node-B  504   b  and just forwards it to another Node-B  504   a  without processing it. This process is fast since the signaling is just routed between Node-Bs  504   a ,  504   b  without processing at the RNC  506 . Therefore, it avoids the processing delay and protocol delay at the RNC  506 . Each Node-B  504   a ,  504   b  derives the final ACK/NACK decision based on the collected CRC results from all the involved Node-Bs  504   a ,  504   b  in the active set. If there is at least one ACK from any Node-B, a final ACK decision will be made at each Node-B  504   a ,  504   b . Otherwise, a final decision of NACK will be made by the Node-B  504   a ,  504   b . As stated above, each Node-B  504   a ,  504   b  generates an ACK decision upon reception of the first ACK from any Node-B  504   a ,  504   b . The Node-Bs  504   a ,  504   b  may set a timer waiting for an ACK from other Node-Bs  504   a ,  504   b , and if the Node-Bs  504   a ,  504   b  do not receive any ACK before the expiration of the timer, the Node-Bs  504   a ,  504   b  generates a NACK decision. 
     The streamlined connection  510   a ,  510   b  between Node-Bs  504   a ,  504   b  and an RNC  506  may be implemented in any embodiment of the present invention described herein. 
     With reference to  FIGS.  6  and  7   , signaling of the ACK/NACK decision between an RNC and Node-Bs is explained.  FIG.  6    shows a system  600  whereby a non-primary Node-B  604   b  has the same controlling RNC (CRNC)  606  as the primary Node-B  604   a . In this case, the CRNC  606  sends an asynchronous ACK to the WTRU  602  via the primary Node-B  604   a.    
       FIG.  7    shows a system  700  whereby a non-primary Node-B  704   b  has a different CRNC  706   b  from a CRNC  706   a  of the primary Node-B  704   a . In this case, a serving RNC (SRNC)  707  sends an asynchronous ACK to the WTRU  702  via the primary Node-B  704   a.    
       FIGS.  2 A and  2 B  are diagrams of a system  200  and a process  250  of the second embodiment of the present invention. In this second embodiment, incremental combining is performed in each Node-B  204   a ,  204   b  whereby each Node-B  204   a ,  204   b  combines a previous transmission of a data packet with a retransmission of the same data packet with or without increased redundancy from the WTRU  202 . 
     A WTRU  202  establishes connections with at least two cells controlled by different Node-Bs  204   a ,  204   b  for soft handover, and data packets transmitted from the WTRU  202  are received and processed separately by the Node-Bs  204   a ,  204   b  (step  252 ). Each Node-B  204   a ,  204   b  performs an error check on the data packet and generates a CRC result (step  254 ). Each Node-B  204   a ,  204   b  transmits the CRC result to the RNC  206 . Simultaneously, each Node-B  2041 ,  204   b  transmits the CRC result to the WTRU  202  as well (step  256 ). The WTRU  202  makes a determination regarding whether there is at least one ACK received from Node-Bs  204   a ,  204   b  (step  258 ). The WTRU  202  may receive both ACK and NACK signals from the Node-Bs  204   a ,  204   b . If the WTRU  202  receives no ACK, it schedules retransmission of the data packet (step  264 ). The Node-Bs  204   a ,  204   b  perform incremental combining of the retransmission with the previous transmission. If the WTRU  202  receives at least one ACK from any Node-B  204   a ,  204   b , the WTRU  202  transmits the next data packet (step  262 ). 
     The RNC  206  also makes an ACK/NACK decision based on collected ACK/NACK signals from the Node-Bs  204   a ,  204   b  (step  260 ). The RNC  206  generates and transmits an ACK decision (step  268 ) if the RNC  206  receives at least one ACK from the Node-Bs  204   a ,  204   b . Otherwise, the RNC  206  generates and transmits a NACK decision to the Node-Bs  204   a ,  204   b  (step  270 ). The ACK/NACK decision is transmitted to the Node-Bs  204   a ,  204   b . Each Node-B  204   a ,  204   b  refreshes its soft buffer once it receives ACK decision from the RNC  206  (step  272 ). With this scheme, soft buffer corruption is eliminated. 
       FIGS.  3 A and  3 B  are diagrams of a system  300  and a process  350  of a third embodiment of the present invention. The WTRU  302  establishes at least two connections with cells controlled by different Node-Bs  304   a ,  304   b  for soft handover. Data packets transmitted from the WTRU  302  are received and processed separately by at least two Node-Bs  304   a ,  304   b  during soft handover (step  352 ). Each Node-B  304   a ,  304   b  performs an error check on the data packet and generates an ACK/NACK result based on the error check on the received data packet (step  354 ). A Node-B coordinator  308  is provided to coordinate among Node-Bs  304   a ,  304   b , and between Node-Bs  304   a ,  304   b  and the RNC  306 . Each Node-B  304   a ,  604   b  sends the ACK/NACK result to the Node-B coordinator  308  (step  356 ). In this embodiment, a final decision on whether an ACK or a NACK is transmitted to the WTRU  302  is made by the Node-B coordinator  308 . It is determined whether any of the involved Node-Bs  304   a ,  304   b  generates an ACK as a result of the error check (step  358 ). If so, the Node-B coordinator  308  commands each of all the involved Node-Bs  304   a ,  304   b  to flush out the corresponding soft buffer and to prepare for a new transmission, regardless of the result of the error check derived at each Node-B  304   a ,  304   b  (step  360 ). In response, each Node-B  304   a ,  304   b  sends an ACK to the WTRU  302  and refreshes its soft buffer (step  362 ). 
     If the results of the error check from all Node-Bs  304   a ,  304   b  fail, (i.e. all of the Node-Bs  304   a ,  304   b  generate NACKs) or a response timer Node-B coordinator expires, the Node-B coordinator  308  informs all of the Node-Bs  304   a ,  304   b  that they failed to successfully decode the transmitted data packet and that they should prepare for retransmission of the data packet (step  364 ). In response, the Node-Bs  304   a ,  304   b  send an NACK to the WTRU  302  (step  366 ). 
       FIGS.  4 A and  4 B  are diagrams of a system  400  and a process  450  of a fourth embodiment of the present invention. For soft handover, a WTRU  402  establishes a separate connection with at least two cells controlled by different Node-Bs  404   a ,  404   b  in an active set. Data packets transmitted from the WTRU  402  are received and processed separately by the Node-Bs  404   a ,  404   b  during soft handover (step  452 ). Each Node-B  404   a ,  404   b  performs an error check on the received data packets and generates an indication of success or failure in decoding the data packet (step  454 ). 
     Each Node-B  404   a ,  404   b  transmits a CRC result to the RNC  406  (step  456 ). If the Node-B  404   a ,  404   b  succeeds in decoding the data packet, the Node-B  404   a ,  404   b  sends an ACK to the RNC  406  along with the data packet. If the Node-B  404   a ,  404   b  fails in decoding the data packet, the Node-B  404   a ,  404   b  sends a NACK to the RNC  406 . An ACK and NACK may be sent with each data block within Iubdur frame protocols between Node-Bs  404   a ,  404   b  and the RNC  406 . The RNC  406  makes a final ACK/NACK decision regarding the transmission of the data packet from the error check results conducted by the Node-Bs  404   a ,  404   b  (step  458 ). The RNC  406  makes an ACK decision if the RNC  406  receives at least one ACK from the Node-Bs  404   a ,  404   b . Otherwise the RNC  406  makes a NACK decision. The ACK or NACK decision made by the RNC  406  is then transmitted back to the Node-Bs  404   a ,  404   b  at steps  460  and  464 , respectively. Each Node-B  404   a ,  404   b  clears its buffer upon receipt of the ACK decision from the RNC  406 . All Node-Bs  404   a ,  404   b  transmit the same ACK or NACK signal made by the RNC  406  to the WTRU  402  regardless of the CRC result that each Node-B  404   a ,  404   b  individually derived from the data packet (steps  462  and  466 ). In this case, the WTRU  402  may apply maximum ratio combining (MRC) to the received ACK/NACK feedback signals from the Node-Bs  404   a ,  404   b.    
     The soft buffer in each Node-B  404   a ,  404   b  is managed according to the ACK/NACK decision made by the RNC  406 , regardless of the associated error check result derived by the Node-Bs  404   a ,  404   b . Consequently, the fourth embodiment of the present invention allows the RNC  406  to align the soft buffer status in each Node-B  404   a ,  404   b . Additionally, the WTRU  402  can benefit from the soft handover gain for the ACK/NACK signaling, since identical ACK/NACK signaling is transmitted by all Node-Bs  404   a ,  404   b . As such, the WTRU  402  may perform macro diversity combining (maximum ratio combining) for ACK/NACK signaling, since the ACK/NACK signals transmitted back to the WTRU  402  from all the involved Node-Bs  404   a ,  404   b  are identical. 
     A fifth embodiment of the present invention will be explained with reference to  FIG.  2 A . The fifth embodiment is similar to the second embodiment, except that Node-Bs  204   a ,  204   b  do not perform incremental combining during soft handover. A WTRU  202  establishes connections with at least two cells controlled by different Node-Bs  204   a ,  204   b  for soft handover. Data packets transmitted from the WTRU  202  are received and processed separately by at least two Node-Bs  204   a ,  204   b  during soft handover. Each Node-B  204   a ,  204   b  performs an error check on the data packet and transmits an ACK/NACK signal to the WTRU  202 . The Node-Bs  204   a ,  204   b  send ACKs along with an identification of transmission to an RNC  206 . The WTRU  202  sends a sequence of data packets and simultaneously looks at the MAC level for an ACK from any Node-B  204   a ,  204   b  when it is in soft handover, and only from the current Node-B when it is not in soft handover. This method causes retransmission when either the time-out threshold is exceeded for an ACK or an out-of-sequence is reported by all cells. Alternatively, this embodiment may be implemented with respect to other embodiments including the first embodiment shown in  FIG.  1 A . 
       FIGS.  8 A and  8 B  are diagrams of a system  800  and a process  850  for softer handover in accordance with the present invention. During softer handover, the WTRU  802  establishes connections with more than one cell  808  which are controlled by the same Node-B  804  (step  852 ). EU transmissions from the WTRU  802  are processed by each cell  808  independently (step  854 ), and each cell  808  transmissions received from the WTRU  802  are processed by the Node-B  804  controlling these cells (step  856 ). There are two alternatives with respect to incremental combining of transmissions transmitted from the WTRU  802 . 
     In accordance with the first alternative, the Node-B  804  receives data packets from all the involved cells  808  and combines them using a technique, such as maximum ratio combining, before performing error check on the data packet. The resulting combined data packet is error checked at the Node-B  804 . 
     In accordance with the second alternative, each cell  808  processes the data packet individually determining error check on the data packet received from the WTRU  802 . The Node-B  804  accepts the data packet that the error check has passed in any of the cells  808  within the active set. 
     In downlink, the Node-B  804  sends messages including ACK/NACK to the WTRU  802  via all the involved cells  808  (step  858 ). The WTRU  802  needs to monitor all channels, preferably shared channels, from the involved cells  808  to detect downlink messages. The number of shared channels that the WTRU  802  should monitor from each cell  808  may be limited, such as up to 4 channels. 
     One of the cells  808  may be designated as a primary cell  808   a , while other cells are designated as non-primary cells  808   b . The primary cell  808   a  sends a message on any of the downlink shared channels allocated to the WTRU  802 . The message carries a shared channel indicator for non-primary cells  808   b . The non-primary cells  808   b  send messages on the channel indicated by the shared channel indicator. In order to implement this scheme, there is a timing offset between the transmission of the shared channel indicator from the primary cell  808   a  and the transmission of messages from non-primary cells  808   b . The WTRU  802  first monitors all shared channels from the primary cell  808   a . Once the WTRU  802  detects that one of the shared channels carry messages to the WTRU  802 , the WTRU  802  reads shared channel indicator along with the downlink messages from the primary cell  808   a . Then, the WTRU  802  receives messages from the non-primary cells  808   b  indicated by the shared channel indicator. With this scheme, it is possible to lower the number of channels that the WTRU  802  should monitor. The WTRU  802  then combines the messages received from all the involved cells  808  using a technique, such as maximum ratio combining. 
     Alternatively, for the DL, only the primary cell  808   a  may transmit messages to the WTRU  802 . The Node-B  804  transmits downlink messages via the primary cell  808   a , while all non-primary cells  808   b  switch off the downlink signaling to the WTRU  802 . With this scheme, the WTRU  802  receive processing is simplified and downlink interference is reduced. 
       FIG.  9    is a diagram of a system  900  for transferring WTRU-specific information to support EU operation  912  in accordance with the present invention. Initially, an RNC  906  obtains WTRU-specific information from a WTRU  902  using RRC messaging  908  at the initial connection. Then, the WTRU-specific information is forwarded from the RNC  906  to a Node-B  904  to be used in scheduling EU transmissions for the WTRU  902 . The transfer of the information from the RNC  906  to the Node-B  904  is via an Iub interface  910 , and an Iur interface if an SRNC is not the same as a CRNC. A new signaling mechanism may be utilized to transfer the information from the RNC  906  to the Node-B  904 , or alternatively, the existing mechanisms over Iur and Iub interfaces may be modified in order for the RNC  906  to forward relevant WTRU-specific information to the Node-B  904 . 
       FIG.  10    is a diagram of a system  1000  for transferring information among network entities during soft handover in accordance with the present invention. During EU operation, if a WTRU  1002  needs to change the serving cell or the serving Node-B, a softer or soft handover procedure is initiated. Hereinafter, for simplicity, the present invention will be explained only with respect to a soft handover. During soft handover, some information should be communicated between network entities to support EU operation. The information includes, but is not limited to, information related to an active set, information regarding a primary Node-B if the system so designates, EU scheduling/rate information, and ACK/NACK status information. 
     An RNC  1006  maintains an active set of cells for handover. The RNC  1006  selects and removes cells in the active set based on measurements reported from Node-Bs  1004   a ,  1004   b  and the WTRU  1002  and on available radio resources. Once the RNC  1006  selects cells for the active set, the RNC  1006  sends messages to the Node-Bs  1004   a ,  1004   b  and the WTRU  1002  to inform the selected cells for the active set to support soft handover for EU. The RNC  1006  also sends messages to update the active set each time the RNC  1006  adds or removes a cell in the active set. The messages may be transmitted using existing RRC and NBAP/RNSAP active set management procedures or new procedures. 
     Either the RNC  1006  or the Node-Bs  1004   a ,  1004   b  and the WTRU  1002  may designate one Node-B as a primary Node-B  1004   a  and other Node-Bs in the active set as non-primary Node-Bs  1004   b  during soft handover. The selection of the primary Node-B  1004   a  is based on UL performance measured and reported by each Node-B  1004   a ,  1004   b  and/or DL performance measured and reported by the WTRU  1002 . 
     During soft handover, only the primary Node-B  1004   a  performs scheduling and assigning radio resources to the WTRU  1002 . The primary Node-B  1004   a  informs the RNC  1006  of scheduled EU transmissions via Iub NBAP signaling or within the EU frame protocol. The RNC  1006  then informs non-primary Node-Bs  1004   b  of the allocation of radio resources for EU and routing of received data. This is also signaled over NBAP or within the EU frame protocol. Alternatively, non-primary Node-Bs  1004   b  may be informed by Iub NBAP procedures of sets of EU physical channels for the period each cell is within the active subset. Each non-primary Node-B  1004   b  within the active set continuously receives these channels independent of radio resources allocation scheduled by the primary Node-B  1004   a.    
     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.