Abstract:
A method of error correction during a soft handover process is disclosed wherein a radio network control node receives a plurality of versions of the same data block. Two or more of the received versions of the data block are combined to determine a substantially error corrected version of the data block. The resulting substantially error corrected version of the data block may then be forwarded to to other network nodes.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field of the Invention  
           [0002]    The present invention relates to error correction methods, and more particularly, to a method for error correction using packet combining techniques during a soft handover process.  
           [0003]    2. Description of Related Art  
           [0004]    Digital cellular mobile systems normally apply a combination of forward and backward error correction techniques to improve the bit error rate of radio channel transmissions over the air interface. Forward error correction (FEC) involves a process of adding redundant information within transmitted bit streams at the transmitter that is used by the receiver to correct transmission errors.  
           [0005]    The automatic repeat request (ARQ) technique is a form of backward error correction wherein the receiver requests the retransmission of packet data units (PDUs) that are incorrectly received from the transmitter. Packet combining techniques have been used within communication link protocols supporting automatic repeat request (ARQ). Data packets are transmitted and received sequentially within ARQ protocols. Once a packet is determined to be corrupt by the receiver, a retransmit request is sent back to the transmitter, requesting a retransmission of the corrupt packet. If the retransmitted packet is again determined to be corrupt by the receiver, the second packet may be combined with a stored version of the previously transmitted packet to correct any transmission errors. This technique is known as Type II Hybrid ARQ.  
           [0006]    The problem with these techniques and others presently utilized for error correction is the necessity of including additional overhead (information) on the radio air interface in order to correct the transmitted information. For example, the FEC techniques require the addition of redundant bit information within the transmitted bit stream, and the ARQ techniques involve the transmission of additional messages requesting the retransmission of previously transmitted information. Thus, each of these techniques utilize additional bandwidth which may be at a premium in certain systems. Thus, a need has arisen for an improved error correction scheme that does not require the utilization of additional overhead within the radio air interface or a scheme which may be used to improve existing FEC and ARQ techniques.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention overcomes the foregoing and other problems with a method of error correction during a soft handover process wherein a plurality of versions of a single data block are received at a radio network control node. At least two of the received versions of the data block are combined to create a substantially error corrected version of the data block for output to an attached MSC or other network node.  
           [0008]    In a first embodiment, a first version and a second version from the received plurality of versions of the data block are exclusive or-ed together to determine bit positions within the data block where potentially erroneous bits are present. Each bit combination for the bits in the plurality of erroneous bit positions are determined, and the combination of bit values determined to be a substantially error corrected version of the data block selected based upon an error detection determination for each combination of bit values.  
           [0009]    In a second embodiment, the plurality of versions of the data block are compared to determine which bit value occurs most frequently in each bit position of the data block. The bit values occurring a majority of the time for each bit position are selected, and the selected bit values are used to generate the substantially error corrected version of the data block. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    A more complete understanding of the method and apparatus of the present invention may be obtained by reference to the following Detailed Description when taken in conjunction with the accompanying Drawings wherein:  
         [0011]    [0011]FIG. 1 is a block diagram of the components involved in a soft handover within a CDMA cellular system;  
         [0012]    [0012]FIG. 2 illustrates a packet data unit (PDU) having quality information appended thereto;  
         [0013]    [0013]FIG. 3 is a flow diagram illustrating a first embodiment of the packet combining error technique of the present invention;  
         [0014]    [0014]FIG. 4 is an example of the process described in FIG. 3;  
         [0015]    [0015]FIG. 5 is a flow diagram illustrating an alternative embodiment of the packet combining error technique of the present invention; and  
         [0016]    [0016]FIG. 6 is an example of the process described in FIG. 5. 
     
    
     DETAILED DESCRIPTION  
       [0017]    Referring now to the drawings, and more particular to FIG. 1, there is illustrated a CDMA system using soft handover in which the soft handover packet combining technique of the present invention may be implemented. Soft handover involves a situation where a mobile station  10  communicating with a first base transceiver station  20   a  is handed off from the first base transceiver station  20   a  to another base transceiver station  20   b,    20   c.  During the soft handover process, rather than immediately ending a first radio air interface  15   a  between the mobile station  10  and the base transceiver station  20   a  upon the creation of a second radio air interface,  15   b  or  15   c,  the first air interface  15   a  is maintained with the presently connected base transceiver station  20   a  along with the air interface  15   b,    15   c  of a new base transceiver station  20   b,    20   c.    
         [0018]    The base transceiver stations  20  having air interfaces  15  with the mobile station  10  are interconnected with a radio network control node (RNC)  25  which controls the air interface connections  15  between the mobile station  10  and the base transceiver stations  20 . The RNC  25  includes a diversity handover unit  30  (DHO) for performing diversity combining techniques on the multiple data streams received from the base transceiver stations  20 . The diversity handover unit  30  compares multiple versions of a data packet  45  received from the base transceiver stations and selects the data packet (frame) having the best quality based upon quality information  40  attached to each packet at the base transceiver station  20 . The quality information  40  is attached to a data packet  45 , as is generally illustrated in FIG. 2, at the base transceiver station  20 . The quality information  40  may comprise an indication of interference level within the received data packet  45  or a cyclic redundancy check (CRC) test result. The data packet  45  will also include a CRC checksum  42 , which is used to determine the correctness of the packet. It should be realized that the order of the fields illustrated in the packet  45  is exemplary only and any order may be used. The selected data packet  45  is forwarded on to the mobile switching center (MSC)  35 . In alternative situations, for example, WCDMA, the selected data packet  45  may be further processed by higher protocol layers in the RNC before delivery to other nodes.  
         [0019]    The present invention includes packet combining functionality logic 50 within the diversity handover unit  30  that utilizes packet combining techniques on the received data frames to carry out improved error correction. Several versions of a same data packet  45  are received in parallel at the DHO  30  from the base transceiver stations  20  during a soft handover process. The versions may be the same or different depending on errors introduced during the transmission process. The diversity handover unit  30  determines if all CRC checksum calculations have failed for each of the received data packets, and if so, uses a defined process to combine two or more of the received data packets to provide a data packet having all bit information in the correct form. This process may utilize any of a number of procedures, two embodiments of which are described in detail below with respect to FIGS. 3 and 4.  
         [0020]    Referring now to FIG. 3, there is illustrated a flow diagram describing a first embodiment of the process which may be performed by the packet combining functionality logic 50 for correcting errors within received signals during a soft handover using packet combining techniques. This functionality is located after the channel decoding processes have been performed. Initially, a plurality of data packets  45  are received at step  55  at the diversity unit from the plurality of base transceiver stations  20 . The versions may be the same or different depending on errors introduced during the transmission process. However, all packets are encoded in the same manner. The diversity handover unit  30  first makes a comparison at inquiry step  60  of all of the quality information  40  appended to the plurality of data packets  45  to determine if any data packet is error free based on the attached quality information  40 . If a packet is indicated as being error free at inquiry step  60 , the error free packet is forwarded at step  90  to the MSC  35 . If none of the received data packets  45  are error free, the diversity handover unit  30  selects at step  65  the two best data packets  45  based upon the appended quality information  40 . The corresponding bit positions of each of the selected data packets  45  are exclusive or-ed together at step  70  to determine bit positions which are not identical (these positions will be indicated by a value “one”) and thus contain potential errors. This process is graphically illustrated in FIG. 4.  
         [0021]    The process illustrated in steps  75  through  80  successively inverts the indicated erroneous bits to all possible combinations to determine the correct bit information for the data packet  45 . This process involves establishing a first combination of bits at step  75 , recalculating the CRC for the data packet  45  based upon the changed bit position or positions, and determining at inquiry step  85  whether the data packet contains errors. If the data packet  45  contains errors, control passes back to step  75  and a next combination of bits is processed. Once the correct combination of bits is found (i.e., substantially error free), the correct packet  45  is identified and forwarded at step  90  to the mobile switching center  35 . In alternative situations, for example, WCDMA, the selected data packet  45  may be further processed by higher protocol layers in the RNC before delivery to other nodes. The process of FIG. 4 may be interrupted at any time as the maximum process of time is reached. When this occurs, no correct version is found and a packet having the best quality information is forwarded. The reason for this limitation is that the number of CRC calculations increases exponentially with the number of erroneous bit positions. It may be necessary to set an upper limit to the number of bit inversions and CRC calculations that can be performed without heeding the delay requirements for a packet  
         [0022]    An alternative embodiment of the process performed by the packet combining functionality logic 50 is illustrated in FIG. 5. In this embodiment, three or more versions of a same data packet  45  are received at step  95  by the diversity handover unit  30  from a number of base transceiver stations  20 . Inquiry step  100  determines if an error-free data packet was provided by any of the base transceiver stations  20  by examining the quality information  40  attached to the data packets  45 . If so, the error-free data packet  45  is identified at step  115  and forwarded to the MSC  35 . If no error-free packets  45  are detected, each bit position of the received data packets are compared to each other at step  105 , and the majority bit value determined and selected for each bit position. Thus, if three data packets were received and within the first bit position two of the packets  45  indicated a value of “one” and one of the packets indicated a value of “zero”, the value of “one” would be selected for the first bit position. This process is repeated for each subsequent bit position of the data packet  45  until a new substantially error free packet is generated. This process is graphically illustrated in FIG. 6. Once a new data packet  45  is determined in this manner, a CRC calculation may be made at step  110  if this functionality is available at the RNC node  25  to check the new packet for errors. However, this step is optional. Once the majority determinations have been completed, the corrected data packet is selected and forwarded to the mobile switching center  35  at step  115 .  
         [0023]    Utilizing the above described packet combining functionality method and apparatus, an error correction technique utilizing packet combining may be performed without requiring the addition of any overhead within the air interface between the base transceiver stations and the mobile station  10  within, for example, a CDMA system. This provides for more efficient utilization of the system resources and faster operation.  
         [0024]    The previous description is of a preferred embodiment for implementing the invention, and the scope of the invention should not necessarily be limited by this description. The scope of the present invention is instead defined by the following claims.