Abstract:
A method and apparatus for supporting Adaptive Modulation and Coding (AM&amp;C) and Hybrid Automatic Repeat Request (H-ARQ) functions in a cellular system including at least one wireless transmit/receive unit (WTRU) and a Node B. A signal is received at the WTRU indicating a need for AM&amp;C and H-ARQ control signaling. An AM&amp;C and H-ARQ control signaling transport channel (TrCH) is dynamically allocated by informing a transport format selection process of the need for AM&amp;C and H-ARQ control signaling. The transport format is selected for required transmission time intervals (TTIs) which assign transport format combinations (TFCs) to establish the AM&amp;C and H-ARQ control signaling transport channel.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
   The application claims priority from Provisional Patent Application No. 60/379,830, filed May 10, 2002. 

   FIELD OF INVENTION 
   The present invention relates to the field of wireless communications. More specifically, the present invention relates to the reduction of uplink and downlink transmission errors in a third generation (3G) cellular system using Adaptive Modulation and Coding (AM&amp;C) and Hybrid Automatic Repeat Request (H-ARQ) techniques applied through the use of at least one transport channel (TrCH) 
   BACKGROUND 
   In conventional third generation (3G) cellular systems, AM&amp;C and HAR-Q techniques provide increased data rates and improved utilization of radio resources. Signaling between User Equipment (UE) and Base Stations (Node Bs) for the coordination of AM&amp;C and H-ARQ functions is provided by physical control channels, such as High Speed Data Packet Access (HSDPA) control channels, or the like. 
   The number of bits allocated for a physical channel message is generally predetermined and restricted. As a result, an error detection scheme, such as a cyclic redundancy check (CRC), is not performed on a physical channel for either uplink or downlink signaling and is not available for use. A primary problem with ensuring successful transmission within the Medium Access Control (MAC) layer is the inability to detect and realize errors when they occur. When transmission errors occur on physical control channels in either the uplink or the downlink, such as with AM&amp;C and H-ARQ control signaling, it is possible that transmission errors are not perceived by the physical layer (i.e. existing MAC layer data recovery mechanisms). Therefore, these MAC layer data recovery mechanisms are not initiated. 
   Accordingly, current 3G systems are not adequately configured to cope with the occurrence of errors on physical control channels. This results in a high probability of UE to Node B H-ARQ acknowledgements and channel quality indications for determination of AM&amp;C to be misinterpreted, which further results in a loss of data within the MAC layer and inefficient use of radio resources. 
   When a loss of data occurs in the MAC layer, the Radio Link Control (RLC) layer initiates mechanisms to recover the lost data. One disadvantage of relying on the RLC layer to recover the data is the latency of transmission, since the round-trip delay of the retransmission is significantly longer than that of transmissions at the H-ARQ level. This results in large data buffering requirements in the UE and a reduction in data throughput. 
   Accordingly, for efficient use of radio resources, for a higher user Quality of Service (QoS) and for the MAC to successfully recover failed transmissions, it is necessary to maintain low error rates on the AM&amp;C and H-ARQ control channels. The current use of physical channel control fields for AM&amp;C and H-ARQ signaling severely limits the ability to maintain low error rates. 
   One example of a type of failure which is not detected during the physical control signaling is the misinterpretation of generated acknowledgement (ACK) messages and negative acknowledgement (NACK) messages. An ACK message indicates a successful transmission of a data block, while a NACK message indicates a failed transmission of a data block. 
     FIG. 1  illustrates a prior art system  100  in which a transmitter  105  generates and sends a data block  110  comprised of one or more protocol data units (PDU) to a receiver  115 . The data block  110  is not received correctly by the receiver  115 . Although, the receiver  115  generates a NACK message  120 , this NACK message  120  is corrupted during transmission and is interpreted by the transmitter  105  as being an ACK message  122 . Thus, the transmitter  105  is never made aware that a transmission failure has occurred, and continues with another transmission without recovering and retransmitting the failed data block  110 . In this case of a NACK being misinterpreted as an ACK, recovery by the RLC is required. However, as aforementioned, the RLC takes a considerable amount of time to recover the data, which is undesirable. 
     FIG. 2  illustrates a prior art system  200  in which a transmitter  205  generates and sends a data block  210 A to a receiver  215 . The data block  210 A is received correctly by the receiver  215 . Although, the receiver  215  generates an ACK message  220 , this ACK message  220  is corrupted during transmission and is interpreted by the transmitter  205  as being a NACK message. Thus, the transmitter  205  interprets that a failure has occurred and unnecessarily retransmits the data block  210 B. In this case of an ACK being misinterpreted as a NACK, a failure is incorrectly indicated and an unnecessary retransmission is generated. This is an inefficient use of radio resources. 
     FIG. 3  illustrates a prior art system  300  in which a transmitter  305  generates and sends an allocation message  310  to a receiver  315 , which is not received by the receiver  315  due to the great amount of corruption and/or interference. As a result, the receiver  315  fails to send a message back to the transmitter  305 . In the interim, the transmitter  305  waits to receive a message back from the receiver  315 , (i.e., an ACK or NACK message). The transmitter  305  is unaware that the receiver  315  did not receive the allocation message  310 . Because the transmitter  315  is waiting to detect a message that never arrives, due to the great amount of corruption there is a high possibility that bit estimation and/or power threshold errors will cause noise  320  to be incorrectly be detected by transmitter  305  as being an ACK message, even though a failure exists. 
   Another example of a type of failure which is not detected during the physical control signaling is the misinterpretation of the channel quality indicator (CQI) which is sent as part of the AM&amp;C process, as shown in the system  400  of  FIG. 4 . The receiver  415  receives a plurality of PDUs  410  and generates a CQI as an indication of the quality of the data being received by the receiver  415 . The particular mechanism used to generate the CQI is outside the scope of the present invention and, accordingly, will not be described in detail herein. However, after the CQI  420  is generated it is transmitted from the receiver to the transmitter  405 , which utilizes the CQI during the AM&amp;C process to select the proper modulation and coding set (MCS). 
   As those of skill in the art would appreciate, if the CQI  420  is corrupted during the transmission of the CQI  420  from the receiver  415  to the transmitter  405 , an incorrect or corrupted CQI  422  will be received by the transmitter  405  and utilized during the AM&amp;C process. If the corrupted CQI  422  is incorrectly low, (erroneously indicating that channel conditions are poor), a more robust MCS than is necessary will be selected. This results in a waste of radio resources. Alternatively, if the corrupted CQI  422  is incorrectly high, (erroneously indicating that channel conditions are favorable), a less robust MCS than needed will be selected, subjecting the data transmissions to a high error rate. 
   These two types of failures, (i.e. H-ARQ signaling and AM&amp;C signaling), illustrate several of the many signaling errors that may occur between the Node B and a UE. 
   In 3G cellular systems, a wide range of services are provided; from high data rate services such as video and Internet downloads, to low data rate services such as speech. Referring to  FIG. 5 , a plurality of user services is shown as individual data streams. These, individual data streams (i.e., user services) are assigned to respective transport channels A,B,C whereby the data streams are coded and multiplexed. Each respective transport channel A, B, C is assigned a specific coding rate and a specific transmission time interval (TTI). The coding rate determines the number of transmitted bits of the physical layer and the TTI defines the delivery period of the block of data to be transmitted. For example, the TTI may be, for example, 10, 20, 40 or 80 ms. 
   Multiple transport channels A, B, C are multiplexed together into a coded composite transport channel (CCTrCh) consisting of a common set of physical channels. Since the CCTrCh is made up of a plurality of transport channels A, B, C it may have a plurality of different coding rates and different TTIs. 
   For example, a first transport channel A may have a 20 ms TTI and a second transport channel B may have a 40 ms TTI. Accordingly, the formatting of the first transport channel A in the first 20 ms and the formatting of first transport channel A in the second 20 ms can change. In contrast, since the second transport channel B has a 40 ms TTI, the formatting, and hence the number of bits, are the same for each 20 ms period over the 40 ms TTI duration. It is important to note that all of the transport channels A, B, C are mapped to the CCTrCh on a TTI basis, using the smallest TTI within the CCTrCh. 
   Each individual data stream has an associated data rate, and each physical channel has an associated data rate. Although these data rates are related to each other, they are distinctly different data rates. 
   Once the smallest TTI within the CCTrCh has been established, it must be determined how many bits of data will be transmitted and which transport channels will be supported within a given TTI. This is determined by the formatting of the data. A Transport Format Combination (TFC) is applied to each CCTrCh based on the smallest TTI. This essentially specifies for each transport channel how much data is transmitted in a given TTI and which transport channels will coexist in the TTI. 
   A TFC Set (TFCS) is the set of all of the possible TFCs. If the propagation conditions do not permit all of the possible TFCs within the TFCS to be supported by the UE, a reduced set of TFCs which are supported by the UE is created. This reduced set is called a TFC subset. TFC selection is the process used to determine which data and how much data for each transport channel to map to the CCTrCh. A transport format combination indicator (TFCI) is an indicator of a particular TFC, and is transmitted to the receiver to inform the receiver which transport channels are active for the current frame. The receiver, based on the reception of the TFCIs, will be able to interpret which physical channels and which timeslots have been used. Accordingly, the TFCI is the vehicle which provides coordination between the transmitter and the receiver such that the receiver knows which physical transport channels have been used. 
   It would be desirable to minimize the need for RLC data recovery by maintaining low error rates on AM&amp;C and H-ARQ control channels, rather than physical channels. 
   SUMMARY 
   The present invention is method and apparatus for supporting Adaptive Modulation and Coding (AM&amp;C) and Hybrid Automatic Repeat Request (H-ARQ) functions in a wireless communication system. The AM&amp;C and H-ARQ control signaling is performed on a transport channel. This enables the control signaling to be positively acknowledged. The positive acknowledgment also enables feedback regarding the quality of transmissions. As a result, control signaling is much more robust, and other functions, such as the selection of a modulation and coding set during AM&amp;C, may be more accurately performed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a prior art wireless communication system wherein a NACK is corrupted during transmission. 
       FIG. 2  is a prior art wireless communication system wherein an ACK is corrupted during transmission. 
       FIG. 3  is a prior art wireless communication system wherein an allocator message is corrupted during transmission and an ACK or NACK is never sent. 
       FIG. 4  is a prior art wireless communication system wherein an ACK is corrupted during transmission. 
       FIG. 5  is a prior art block diagram showing individual data streams being combined into a physical channel. 
       FIG. 6  is a UMTS operating in accordance with a preferred embodiment of the present invention wherein control signaling is performed over one or more transport channels. 
       FIG. 7  is a system incorporating error detection therein for adjusting uplink and downlink control channel transmit power and coding. 
       FIG. 8  shows control channels with TTIs aligned with AM&amp;C/H-ARQ transmissions. 
       FIG. 9  shows control channel signaling for multiple AM&amp;C/H-ARQ transmissions. 
       FIG. 10  is a UMTS with a dedicated CCTrCH with UE-Node B AM&amp;C and H-ARQ signaling transport channel in accordance with an alternate embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. 
   In accordance with several aspects of the present invention, a wireless communication system: 1) provides deterministic error detection for control signaling used for AM&amp;C and H-ARQ processes by transmitting the control signaling over one or more transport channels; 2) uses CRC to determine control channel quality and correspondingly adjusts control channel attributes to achieve control channel quality targets; and 3) protects channel quality measurement and H-ARQ acknowledgements by implementing an error detection scheme in the control channels, such as with a CRC. 
   It should be understood by those of skill in the art that the foregoing description is applicable to both uplink and downlink applications. Additionally, although the present invention will be described with reference to the use of transport channels, it is applicable to other control signaling channels for support of AM&amp;C and H-ARQ techniques which implement error detection and potentially variable signaling fields. 
     FIG. 6  shows a Universal Mobile Telecommunications System (UMTS)  600  made in accordance with the present invention. The UMTS includes a wireless transmit/receive unit (WTRU)  605  and a Node B  610 . The WTRU  605  includes a MAC entity  615  and a physical entity  616 . The Node B  610  includes a MAC entity  620  and a physical entity  621 . The MAC entity  615  in the WTRU  605  is the peer entity to the MAC entity  620  in the Node B  610 . Likewise, the physical entity  616  in the WTRU  605  is the peer entity to the physical entity  621  in the Node B  610 . As shown in  FIG. 6 , although a physical channel  623  with physical control signaling fields exists between physical entities  616 ,  621 , in accordance with the present invention the AM&amp;C and the H-ARQ control signaling is performed for uplink and/or downlink operation on one or more transport channels  625  which are terminated in the MAC entities  615 ,  620 . 
   Performing the AM&amp;C and H-ARQ signaling over one or more transport channels  625  provides much greater flexibility than the prior art scheme of performing the AM&amp;C and H-ARQ signaling with physical channel control fields. As is well understood by those of skill in the art, physical channels comprise strictly defined fields that do not vary between transmissions. In contrast, transport channels are dynamically configured and include an error detection capability. Transport channels comprise a transport format set. In each TTI, a different transport format set may be selected and each transport format represents a different payload size. Variable control signaling may be supported. Accordingly, the transport channel may be tailored to each cell, each user, each signaling transmission or each release of the system. 
   Utilizing the AM&amp;C and H-ARQ signaling over one or more transport channels  625  provides several advantages over prior art systems. First, introduction of an error detection capability within the H-ARQ process results in a substantial reduction of ACK/NACK misinterpretations since a verification of the ACK/NACK is provided. This will avoid the three scenarios described with reference to  FIGS. 1-3  and the problems resulting therefrom; such as inefficient utilization of the radio resources due to an ACK being misinterpreted as a NACK, and delay of the transmission of data during RLC data recovery as a result of an NACK being misinterpreted as an ACK. 
   Second, introduction of an error detection capability in the AM&amp;C process results in a substantial reduction of CQI misinterpretations due to corruption since a verification of the CQI is provided. This will avoid the scenario described with reference to  FIG. 4 . Since a correct CQI is critical to the selection of the proper MCS, in accordance with the present invention when the CQI is verified using an error tracking capability the integrity of the CQI will be maintained. As a result, the proper CQI value will be used during the AM&amp;C process and the correct MCS will be selected. This results in a much more efficient utilization of the radio resources. Utilizing transport channels for control signaling enhances the ability to determine a proper MCS set. Since CQI errors are known, MCS selection is more accurate and physical resources are better utilized with reduced failures of user data transmissions. 
   Third, using transport channels in accordance with the present invention permits the quality of a control channel to be tracked, and the power and coding of the control channel, (either uplink or downlink), to be adjusted. The CRC provides a channel quality indication for the uplink and downlink control channels. The system can monitor the amount of errors that are received and, based upon those errors, adjust the transmit power and coding rate of the control channels to achieve proper AM&amp;C and H-ARQ operation. 
     FIG. 7  shows a system  700  including a WTRU  705  in communication with a Universal Terrestrial Radio Access Network (UTRAN)  710 . Introduction of an error detection capability provides an indication of the number of errors that are occurring on the control channel during the control signaling, such as with a block error rate (BLER). The quality of the control channel can be adjusted according to the difference between the received BLER and the setting of a BLER target. In this manner, if a large number of errors are occurring (high BLER), the power of the control channel is increased. Likewise, if a small number of errors are occurring (low BLER), the power of the control channel is decreased. 
   Referring again to  FIG. 7 , a downlink control channel  715  is processed using an error detection scheme, such as CRC, in CRC detection entity  720 . A BLER calculation  725  is performed on the output of the CRC detection entity  720  and a BLER report  730  is sent from the WTRU  705  to the UTRAN  710 . Furthermore, an uplink control channel  735  is processed using an error detection scheme, such as CRC, in CRC detection entity  740 . A BLER calculation  745  is performed on the output of CRC detection entity  740 . The results of the BLER calculation  745  and the BLER report  730  are used to adjust uplink and downlink control channel transmit power and coding entity  750 . 
   Not all transport channel attributes as currently defined are necessary to provide the benefits described hereinbefore. As understood by those of skill in the art, transport channels comprise both dynamic and semi-static attributes. Semi-static attributes are attributes that can be changed by Layer 3 signal (Radio Resource Control (RRC)) procedures. Dynamic attributes can be changed on a TTI basis by generating a different Transport Format Combination Indicator (TFCI). Semi-static attributes may include coding rate, CRC and TTI size. A dynamic attribute may be the transport block (payload) size. For this, a new type of transport channel is defined that is unique to AM&amp;C and H-ARQ signaling requirements. Individual transmissions on transport channels exist within specific TTIs. 
   When AM&amp;C and H-ARQ signaling is applied to transport channels, control channel TTIs can either be aligned with the current AM&amp;C and H-ARQ physical channel signaling and physical channel allocations, or existing dedicated channels as shown in  FIG. 8 . Dedicated channel TTIs exist over longer periods than AM&amp;C and H-ARQ channel allocation and signaling periods. 
   Referring to  FIG. 9 , if AM&amp;C and H-ARQ signaling is applied to transport channels with TTIs that exist over periods longer than AM&amp;C and H-ARQ signaling and channel allocation periods, then AM&amp;C and H-ARQ transport channel signaling messages will encompass information related to several AM&amp;C and H-ARQ channel allocations. 
   Each of the aforementioned transport channels has variable information fields from TTI to TTI. The fields incorporate a dynamic payload parameter, whereby a number of parameters of a variable payload may be increased (added) or decreased (removed) as needed. For example, using the TTIs, the amount of information that is signaled may be increased or decreased. This capability is useful in compensating for differences in different system program releases (e.g., RAN V, VI, VII) whereby one can change what is signaled by incorporating new features. Additionally, one may want to change the signaling used in particular cells of the system or for particular users. 
   In another example, for new allocations, it may be desired to use new fields that do not exist all of the time. Transport channels allow TTI fields to be varied to dynamically allocate new fields or vary the size of existing fields, which change what is signaled at any one time. One unique aspect of the transport channel is the TTI alignment. Present transport channels use a coded composite transport channel and each transport channel has a certain TTI. The present invention provides a transport channel that conveys this control information whereby TTIs are aligned with the AM&amp;C and H-ARQ transmissions. 
     FIG. 10  shows a UMTS  1000  operating in accordance with an alternate embodiment of the present invention. The UMTS  1000  includes a WTRU  1005 , a Node B  1010  and an RNC  1015 , where the AM&amp;C and H-ARQ signaling utilizes dedicated coded composite transport channels. Utilizing these dedicated channels for AM&amp;C and H-ARQ signaling, minimizes the number of simultaneous channels that need to be supported. 
   The WTRU  1005  includes a MAC-hs entity  1040  and a physical entity  1053 . The Node B  1010  also includes a MAC-hs entity  1045  and a physical entity  1043 . Although prior art dedicated transport channels are terminated at the RNC, the transport channel in accordance with the present invention is terminated at the Node B  1010 . At least one dedicated transport channel  1020  is currently terminated between MAC-d entities  1025 ,  1030  in the WTRU  1005  and the RNC  1015 , respectively. Since AM&amp;C and H-ARQ signaling over the transport channel  1035  exists between the WTRU  1005  and Node B  1010 , where the AM&amp;C and H-ARQ signaling transport channel  1035  is terminated between MAC-hs entities  1040 ,  1045 , the present invention separates processing of the new AM&amp;C and H-ARQ signaling transport channel  1035  in Node B  1010 . A CCTrCH  1050  is formed by the combination of the at least one dedicated transport channel  1020  and the AM&amp;C and H-ARQ signaling transport channel  1035 . 
   In this embodiment, at least one TTI of a transport channel is aligned using a coded composite with the AM&amp;C and H-ARQ transmissions. The new AM&amp;C and H-ARQ control signaling transport channel  1035  is demultiplexed from the dedicated coded composite transport channel  1050  and routed to the MAC entity responsible for AM&amp;C and H-ARQ processing in the Node B. The MAC entities located in the WTRU  1005  and the Node B  1015  invoke the AM&amp;C and H-ARQ transmissions. When activation of the AM&amp;C and H-ARQ transmissions is required, an indication is sent to an entity that performs TFC selection for the dedicated coded composite Transport channel. The MAC entities  1040 ,  1045  inform a dedicated coded composite MAC TFC selection function within the MAC-d entities  1025 ,  1030  so that a TFC is chosen that allocates the AM&amp;C and H-ARQ control Transport channel within the TTI of the transport channel. Alternatively, for each TFC representing dedicated transport channel combinations, a second TFC is configured so that the MAC entity responsible for AM&amp;C and H-ARQ processing can dynamically adjust the TFC to add the new control signaling transport channel  1035  as needed. 
   Rather than terminating in the RNC  1015 , the present invention provides a transport channel existing between the WTRU  1005  and the Node B  1010 . The new AM&amp;C and H-ARQ control signaling transport channel  1035  may be part of the existing dedicated coded composite  1050 , or it may be a new transport channel existing only between Node B  1010  and the WTRU  1005 . The present invention enables the transport channel to be removed (demultiplexed) from the dedicated coded composite and terminated in Node B  1010  because it is known as a unique transport channel type or has a unique transport channel identity known to Node B  1010  and the WTRU  1005 . 
   For the case where the transport channel is part of the existing dedicated coded composite, then a mechanism is in place where the MAC entities  1040 ,  1045  invoke the AM&amp;C and H-ARQ transmissions. When the AM&amp;C and H-ARQ transmissions become active, an indication is sent to the entity that performs TFC selection for the dedicated coded composite transport channel, which informs the dedicated coded composite MAC entity that uses the TFC that its transport channel will be active so that the proper TFC can be chosen. Alternatively, for each dedicated TrCH TFC, an additional TFC is defined that can replace the TFC chosen by the dedicated MAC entity  1040 ,  1045  at any time. 
   A concern with applying a new transport channel to the existing dedicated transport channel is that AM&amp;C and H-ARQ control signaling is not continuous over time. Therefore, a mechanism that dynamically allocates the AM&amp;C and H-ARQ transport channel improves utilization of dedicated physical resources. 
   Thus, in one embodiment of the present invention, the need for uplink or downlink control signaling is realized using the AM&amp;C and H-ARQ MAC entities  1040 ,  1045 . The MAC-d entities  1025 ,  1030  are informed so that a proper TFC is chosen to allow transmission on the new AM&amp;C and H-ARQ transport channel. The AM&amp;C and H-ARQ MAC entities  1040 ,  1045  then add the new TrCH for physical processing in that TTI. 
   In an alternative embodiment of the present invention, the AM&amp;C and H-ARQ MAC entities  1040 ,  1045  adjust the TFC chosen by the MAC-d entities  1025 ,  1030  to include the new control signaling TrCH  1035  on an as needed basis. 
   In the uplink, the need for AM&amp;C and H-ARQ control signaling is known in advance to the WTRU. In order to dynamically allocate the AM&amp;C and H-ARQ signaling transport channel, when the need is realized, the transport format selection process is informed of the requirement. Subsequent transport format selection for the required TTIs assigns transport format combinations to establish the AM&amp;C and H-ARQ signaling transport channel. 
   While the present invention has been described in terms of the preferred embodiment, other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.