Patent Publication Number: US-2004042426-A1

Title: Communication device and method for frame reconstruction among soft handoff legs

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates generally to wireless communication and, in particular, to frame reconstruction among soft handoff legs.  
       BACKGROUND OF THE INVENTION  
       [0002] To provide continuous communication in wireless communication systems, such as cellular communication systems, a mobile unit must be handed off from one coverage area to a neighboring coverage area as the mobile unit travels across coverage area boundaries. There are several types of handoff arrangements. Hard handoffs typically involve communication between a mobile unit and two base sites such that the mobile unit only communicates with one base site at a time. In contrast, soft handoffs typically involve simultaneous communication between the mobile unit and multiple base sites, while softer handoffs involve communication between the mobile unit and multiple base transceivers, some of which support individual sectors at the same base site.  
       [0003] In code division multiple access (CDMA) systems, during a soft handoff (or a softer handoff), mobile unit transmissions are received by multiple base transceivers. Furthermore, each of these base transceivers transmits the same information to the mobile unit but spread with independent spreading sequences. Thus, during a soft handoff, there are multiple receive legs (at both the mobile unit and in the fixed infrastructure) from which the received information can be obtained. In other words, each transmitted frame is received by each individual soft handoff leg. These individually received frames (i.e., leg frames) each correspond to the same transmitted frame. From these corresponding leg frames, a reconstructed frame representing the originally transmitted information is typically generated.  
       [0004] Within cellular communication systems, voice quality is often perceived by the user as the most important attribute to any call. Cellular providers along with equipment manufacturers continuously strive to improve voice quality within cellular communication systems. Usually a higher quality voice channel requires more Radio Frequency (RF) capacity, thereby limiting the total number of services a system can simultaneously provide. Thus, system capacity often must be traded-off to increase quality. For example, in CDMA systems, the transmit power can be boosted or the transmitted information can be encoded with greater redundancy to reduce the number of air frames that are lost or that must be retransmitted. However, techniques such as these reduce the RF capacity of CDMA systems. In contrast, techniques for increasing the reliability of information as it is received that do not require additional RF capacity are particularly desirable. Therefore, a need exists for a device and method of frame reconstruction among soft handoff legs that improves the reliability of received information without reducing RF capacity. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0005]FIG. 1 is a block diagram depiction of a communication system in accordance with an embodiment of the present invention.  
     [0006]FIG. 2 is a logic flow diagram of steps executed in accordance with an embodiment of the present invention.  
     [0007]FIG. 3 is an illustration of a frame reconstruction from soft handoff leg frames in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
     [0008] The present invention addresses the need for a device and method of frame reconstruction among soft handoff legs that improves the reliability of received information without reducing RF capacity. Given received frames from each leg of a soft handoff, a reconstructed frame is produced. Subframes with passing inner frame quality indicators are selected for the reconstructed frame. For portions of the reconstructed frame that do not have corresponding portions in the leg frames with passing inner frame quality indicators, a bit-wise majority rule is applied to select values.  
     [0009] The disclosed embodiments can be more fully understood with reference to FIGS.  1 - 3 . FIG. 1 is a block diagram depiction of a communication system  100  in accordance with a first embodiment of the present invention. Communication system  100  is a well-known Code Division Multiple Access (CDMA) system, specifically a CDMA 2000 system, which is based on the Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) standard IS-2000, suitably modified to implement the present invention. However, the present invention is not limited to a particular wireless technology. For example, the present invention may be applied whenever the same transmitted data is transmitted or received by multiple devices thereby creating multiple receive frames that need to be combined into a single receive frame. Therefore, in alternate embodiments, communication system  100  may utilize other communication system protocols such as, but not limited to, UMTS, 1× EVDV, and 1× EVDO.  
     [0010] The first embodiment of the present invention includes radio access network (RAN)  120  and remote units, such as mobile station (MS)  110 . However, the present invention is not limited to remote units that are mobile. For example, a remote unit may comprise a desktop computer wirelessly connected to the radio access network.  
     [0011] Those skilled in the art will recognize that FIG. 1 does not depict all of the network equipment necessary for system  100  to operate but only those logical entities particularly relevant to the description of embodiments of the present invention. For example, RAN  120  comprises well-known entities such as transceivers  121 - 123 , and frame constructor  124 . Those skilled in the art are aware of the many ways each of these entities can be implemented and/or purchased from wireless communications companies such as “MOTOROLA.” Frame constructors, for example, typically comprise components such as processors, memory, and/or logic circuitry designed to implement algorithms that have been expressed as computer instructions and/or in circuitry. Given an algorithm or a logic flow, those skilled in the art are aware of the many design and development techniques available to implement a frame constructor that performs the logic.  
     [0012] Typically, RAN transceivers are components of RAN base transceiver stations (BTSs), which interface with devices such as base site controllers (BSCs), frame selection and distribution units (SDUs) mobile switching centers/virtual location registers (MSCNLR), home location registers (HLR), etc. In a first embodiment of the present invention, a known CDMA 2000 RAN is adapted using known telecommunications design and development techniques to implement the logic of the present invention. The result is RAN  120 , which performs the method described with respect to FIG. 2. Those skilled in the art will recognize that the present invention may be implemented in and across various physical components of RAN  120 . For example, frame constructor  124  may be implemented in a base site or an SDU.  
     [0013] RAN  120  and MS  110  communicate via CDMA 2000 air interface resources  101 - 104 . In the first embodiment of the present invention, MS  110  comprises transmitter  111 , receiver  112 , processor  113 , and frame constructor  114 . Processor  113  includes components such as memory, programming, and microprocessor devices. Frame constructor  114  is physically implemented by the components that makeup processor  113 . Transmitters, receivers, processors, and frame constructors as used in CDMA MSs are common and well known in the art. In a first embodiment of the present invention, a known CDMA 2000 MS is adapted using known telecommunications design and development techniques to implement the logic of the present invention. The result is MS  110 , which also performs the method described with respect to FIG. 2.  
     [0014] When MS  110  is involved in a soft/softer handoff, it transmits uplink frames via wireless link  101 . A transceiver in each of the base sites and/or individual sectors involved in the handoff (i.e., transceivers  121 - 123 ) receives the transmitted frames. Similarly, transceivers  121 - 123  transmit downlink frames via wireless links  102 - 104 , respectively, and receiver  112  receives each of the transmitted frames from each of the transceivers. Thus, there are three leg frames received by RAN  120  for each uplink frame transmitted, and three leg frames received by MS  110  for each downlink frame sent to MS  110 . Frame constructors  114  and  124  forward a single frame corresponding to the originally transmitted frame using each of the received leg frames. Frame constructors  114  and  124  do this according to logic flow  200 .  
     [0015]FIG. 2 is a logic flow diagram of steps executed in accordance with a first embodiment of the present invention. Logic flow  200  is more clearly described with reference to FIG. 3, illustrating received leg frames  300 - 303  and reconstructed frame  304 , which is forwarded. Logic flow  200  begins ( 202 ) when leg frames  300 - 303 , for example, are received ( 204 ). Each leg frame  300 - 303  includes an outer frame quality indicator (FQI) that indicates a frame quality of the frame as a whole. Although any FQI may be used, the first embodiment utilizes cyclical redundancy checking (CRC). Thus, the outer FQIs are outer (CRCs)  310 - 313 . Similarly, each leg frame  300 - 303  includes multiple inner frame quality indicators (FQI) that each indicate a frame quality of a portion of each leg frame  300 - 303 . Again, although any FQI may be used, the first embodiment utilizes CRCs. Thus, inner CRCs  330 - 333  and  350 - 353  each indicate the quality of subframes  340 - 343  and  360 - 363 , respectively.  
     [0016] As leg frames  300 - 303  are received ( 204 ), well-known receive quality metrics, such as the symbol error rate (SER) and total metric (TM), are determined for each leg frame. As illustrated in FIG. 3 leg frame  303  is considered to have the worst receive quality metric (e.g., the worst TM and/or SER). Outer CRCs  310 - 313  and inner CRCs  330 - 333  and  350 - 353  are also checked. However, as indicated by the slashes in FIG. 3, outer CRCs  310 - 313  and inner CRCs  330 - 333 ,  350 , and  351  all fail. Since some CRCs did pass ( 206  and  207 ), correlating the leg frames is not necessary. If none of the leg frame CRCs had passed then the leg frames would be correlated ( 208 ) to determine whether a frame was actually transmitted, i.e., a non-DTX frame. When the correlation indicates that the leg frames comprise transmitted information, the frame is counted ( 210 ) as an outer loop power control erasure, since none of the leg frame outer CRCs passed.  
     [0017] Even though a transmitted frame may be considered an erasure, a frame can still be reconstructed from the received leg frames, in accordance with the present invention. Clearly, if a reliable frame can be reconstructed from an erased frame, then quality is improved without using any additional RF capacity. The composition of reconstructed frame  304  is determined as follows in accordance with the first embodiment of the present invention.  
     [0018] Inner CRCs are present ( 212 ) in leg frames  300 - 303  and inner CRCs  352  and  353  have passed, indicating that leg frame portions  362  and  363  are of acceptable quality. Thus, subframes  362  and  363  presumably contain the same information, and are selected for reconstructed frame  304  as subframe  364 . However, it is possible that subframes  362  and  363  do not contain the same information. This could occur in the case where a CRC passes falsely. Therefore, the subframe with a passing CRC from the leg frame with the best receive quality metric may be chosen. Since leg frame  303  has the worst receive quality metric, subframe  362  is selected for subframe  364  rather than subframe  363 .  
     [0019] For portions of the leg frames for which there is no inner CRC or all inner CRCs have failed, one of two approaches is taken in the first embodiment to recover the transmitted information. When less than three leg frames ( 214 ) have been received, information is selected ( 220 ) from the leg frame with the better receive quality metric. For example, if only leg frames  302  and  303  had been received, information for subframe  324  and  344  would be selected from subframes  322  and  342 , respectively, rather than from subframes  323  and  343 , since leg frame  302  has the better receive quality metric.  
     [0020] When three or more leg frames have been received, as illustrated in FIG. 3, a bit-wise majority rule is applied. In order to apply this majority rule, however, an odd number of leg frames is needed for consideration. Thus, when an even number have been received ( 216 ), the leg frame having the worst receive quality metric is discarded ( 222 ), i.e., not considered. Leg frame  303  is thus not considered when applying the bit-wise majority rule.  
     [0021] Instead, for each bit in reconstructed frame  304  that corresponds to a bit in each of the leg frames  300 - 302  for which there is no passing inner CRC, the bit value represented by the majority of the corresponding bits is selected ( 224 ). For example, the first bits of subframes  320 - 322  correspond to the first bit of subframe  324  of the reconstructed frame  304 . Applying the bit-wise majority rule results in a “0” for the first bit of subframe  324 , since the first bits of subframes  320 - 322  are “0”, “1”, and “0”, respectively. Again, this rule is applied to select values for each bit in reconstructed frame  304  that corresponds to a bit in each of the leg frames  300 - 302  for which there is no passing inner CRC. Thus, subframes  324  and  344  of reconstructed frame  304  are determined in this manner. The bit values shown in subframes  324  and  344  are consistent with the application of the bit-wise majority rule to subframes  320 - 322  and  340 - 342  of leg frames  300 - 302 . Once completed, reconstructed frame  304  is then forwarded ( 226 ) for further communication processing.  
     [0022] Reconstructed frame  304  represents a best determination of the information originally transmitted. By embodying the present invention, such as described above, the reliability of received information can be improved without reducing RF capacity. Reconstructed frames can be created from the received leg frames even when the frames would otherwise be discarded as erasures.  
     [0023] In the foregoing specification, the present invention has been described with reference to specific embodiments. However, one of ordinary skill in the art will appreciate that various modifications and changes may be made without departing from the spirit and scope of the present invention as set forth in the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. In addition, those of ordinary skill in the art will appreciate that the elements in the drawings are illustrated for simplicity and clarity. For example, the dimensions of some of the elements in the drawings may be exaggerated relative to other elements to help improve an understanding of the various embodiments of the present invention.  
     [0024] Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments of the present invention. However, the benefits, advantages, solutions to problems, and any element(s) that may cause or result in such benefits, advantages, or solutions, or cause such benefits, advantages, or solutions to become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims.  
     [0025] As used herein and in the appended claims, the term “comprises,” “comprising,” or any other variation thereof is intended to refer to a non-exclusive inclusion, such that a process, method, article of manufacture, or apparatus that comprises a list of elements does not include only those elements in the list, but may include other elements not expressly listed or inherent to such process, method, article of manufacture, or apparatus. The terms “a” or “an,” as used herein, are defined as one or more than one. The term “plurality,” as used herein, is defined as two or more than two. The term “another,” as used herein, is defined as at least a second or more. The terms “including” and/or “having,” as used herein, are defined as comprising (i.e., open language). The term “coupled,” as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically or electrically. The term “program” (or “programming”), as used herein, is defined as a sequence of instructions designed for execution on a computer system. A program, programming, or computer program, may include a subroutine, a function, a procedure, an object method, an object implementation, an executable application, an applet, a servlet, a source code, an object code, a shared library/dynamic load library and/or other sequence of instructions designed for execution on a computer system.