Patent Publication Number: US-2017374660-A1

Title: TDD and FDD Joint Carrier Aggregation Enhancements in LTE-Advanced

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This patent is a continuation of and claims the benefit of U.S. Non-Provisional application Ser. No. 14/187,157, filed Feb. 21, 2014, the entire contents of which is hereby expressly incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to data transmission in communication systems and, more specifically, to methods and systems for supporting time division duplex (TDD) and frequency division duplex (FDD) joint carrier aggregation. 
     BACKGROUND 
     In wireless communication systems, such as third generation partnership project (3 GPP) long term evolution (LTE) systems, downlink and uplink transmissions may be organized into two duplex modes: frequency division duplex (FDD) mode and time division duplex (TDD) mode. The FDD mode uses a paired spectrum where the frequency domain is used to separate the uplink (UL) and the downlink (DL) transmissions. The TDD mode, on the other hand, uses a common spectrum and relies on the time multiplexing to separate the UL and the DL transmissions. 
     In wireless communication systems, such as LTE systems, a User Equipment (UE) may transmit Hybrid Automatic Repeat reQuest (HARQ) bits in response to Physical Downlink Shared Channel (PDSCH) transmissions received on the DL. A HARQ bit may be an acknowledge (ACK), which signals successful receipt of data. A HARQ bit may also be a negative acknowledgement (NACK), which signals that data was not received successfully. A UE may transmit HARQ bits on a Physical Uplink Control Channel (PUCCH). 
     In LTE-advanced (LTE-A) systems, carrier aggregation (CA) enables multiple component carriers (CCs), which use up to 20 MHz bandwidth each, to be aggregated to form a wider total bandwidth. To support CA, a LTE-A UE may simultaneously receive or transmit on one or multiple CCs. These multiple CCs may be from the same evolved NodeB (eNB) or from difference eNBs. In a FDD system, the number of CCs aggregated in the DL may be different from the CCs in the UL and the bandwidths configured for the UL may also be different from the bandwidths for the DL. 
     For CA, a UE may have one independent HARQ entity per CC and in each of uplink or downlink. Each UE may have one Radio Resource Control (RRC) connection with the network. The serving cell handling RRC connection establishment/re-establishment/handover is referred to as the Primary Cell (PCell). The carrier corresponding to the PCell is termed the primary component carrier (PCC). Other serving cells are referred to as secondary cells (SCells) and their corresponding carriers are referred to as secondary component carriers (SCC). The carriers could be aggregated intra-band and/or inter-band. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is an example mobile communication system supporting TDD-FDD joint CA. 
         FIG. 2  is an example HARQ timing relationship in TDD-FDD joint CA. 
         FIG. 3  is an example bitmap for a PUCCH resource allocation. 
         FIGS. 4A-4B  are an example information element definition for a PUCCH resource allocation. 
         FIG. 5  is an example configuration for PUCCH transmissions. 
         FIG. 6  is an example configuration for PUCCH transmissions. 
         FIG. 7  is an example information element definition for an additional N RB   (2)  parameter. 
         FIG. 8  is a flowchart illustrating a method for supporting TDD-FDD joint CA, according to one example of principles described herein. 
         FIG. 9  is a flowchart illustrating a method for supporting TDD-FDD joint CA, according to one example of principles described herein. 
         FIG. 10  is an example HARQ timing relationship on a TDD carrier with self-scheduling. 
         FIG. 11  is an example HARQ timing relationship on a TDD carrier with cross-carrier scheduling. 
         FIG. 12  is a flowchart illustrating a method for supporting TDD-FDD joint CA, according to one example of principles described herein. 
         FIG. 13  is a flowchart illustrating a method for supporting TDD-FDD joint CA, according to one example of principles described herein. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an example mobile communication system  100  that supports TDD-FDD joint CA. For example, in a mobile communication system, a UE may receive a PUCCH resource allocation from an eNB. The eNB may transmit the PUCCH resource allocation in a RRC message. The PUCCH resource allocation may include a first PUCCH configuration for a first subframe and a second PUCCH configuration for a second subframe, and the second PUCCH configuration is different than the first PUCCH configuration. The UE may transmit a first group of HARQ bits based on the first PUCCH configuration in the first subframe and transmit a second group of HARQ bits based on the second PUCCH configuration. 
     In some implementations, the first group of HARQ bits include one or more HARQ bits corresponding to PDSCH transmissions on a TDD carrier and one or more HARQ bits corresponding to PDSCH transmissions on a FDD carrier. In some implementations, the first PUCCH configuration indicates a first PUCCH format, and the second PUCCH configuration indicates a second PUCCH format. In some implementations, the first PUCCH format may be PUCCH format 3. In some implementations, the PUCCH resource allocation is a bitmap. The bitmap may indicate a PUCCH format for each subframe in a radio frame. In some implementations, the UE may transmit the first group of HARQ bits in a resource block that is between resource blocks used to transmit PUCCH format 1 and resource blocks used to transmit Physical Uplink Shared Channel (PUSCH). In some implementations, the UE may receive a resource block allocation from the eNB. The resource block allocation may include a first parameter and a second parameter that is different than the first parameter. The UE may transmit the first group of HARQ bits based on the first parameter, and transmit the second group of HARQ bits based on the second parameter. 
     In some implementations, a UE may identify a plurality of subgroups of HARQ bits in the first group of HARQ bits. The UE may identify a plurality of PUCCH resources for the plurality of subgroups of HARQ bits. In some implementations, the plurality of PUCCH resources is consecutive. The UE may transmit the first group of HARQ bits based on the plurality of PUCCH resources. 
     In some implementations, a UE may receive a TDD-FDD joint CA configuration from an eNB. The TDD-FDD joint CA configuration may include a FDD carrier as a PCell and a TDD carrier as a SCell. The UE may receive a PDSCH on the TDD carrier. The UE may transmit a HARQ bit corresponding to the received PDSCH based on a FDD HARQ timing. In some implementations, the UE receives a PDSCH assignment for the TDD carrier on the TDD carrier. In some implementations, the UE receives a PDSCH assignment for the TDD carrier on the FDD carrier 
     In some implementations, an eNB configures a TDD-FDD joint CA, which includes a TDD carrier and a FDD carrier. The eNB may use only the FDD carrier to configure a PCell. In some implementations, the eNB may use a TDD carrier to configure a PCell, and transmits scheduling assignments for the TDD carrier only on the TDD carrier. 
     Transmitting HARQ bits according to methods and systems described herein may provide one or more advantages. This approach may improve system performance by increasing throughput and reducing latency. For example, an eNB may allocate PUCCH format 3 flexibly to allow more HARQ ACK/NACK opportunities when needed without causing backward compatible problems for legacy UEs. Furthermore, a flexible allocation of PUCCH resources may allow efficient use of PUCCH resources. In addition, improvements on HARQ timing may reduce the round trip time for PDSCH transmissions on the TDD carrier. 
     At a high level, the example mobile communication system  100  includes a UE  102 , a LTE or LTE-A network  110 , which includes an eNB  104  that is communicably coupled with the UE  102 . The UE  102  receives a TDD-FDD joint CA configuration from the eNB  104 . The TDD-FDD joint CA configuration includes a FDD carrier  120  and a TDD carrier  130 . In some instances, the UE  102  receives a PUCCH resource allocation from the eNB  104 . The PUCCH resource allocation includes a first PUCCH configuration for a first subframe and a second PUCCH configuration for a second subframe that is different than the first PUCCH configuration. The UE transmits a first group of HARQ bits based on the first PUCCH configuration in the first subframe. The UE transmits a second group of HARQ bits based on the second PUCCH configuration in the second subframe. In some implementations, the first group of HARQ bits includes one or more HARQ bits corresponding to PDSCH transmission on the FDD carrier  120  and one or more HARQ bits corresponding to PDSCH transmissions on the TDD carrier  130 .  FIG. 2-9  and associated descriptions provide additional details of these implementations. 
     In some instances, the eNB  104  configures the FDD carrier  120  as a PCell and the TDD carrier  130  as a SCell. The UE  102  receives a PDSCH on the TDD carrier  130 . The UE  102  transmits a HARQ bit corresponding to the received PDSCH based on a FDD HARQ timing.  FIG. 10-12  and associated descriptions provide additional details of these implementations. In some instances, the eNB  104  configures a PCell using only the FDD carrier  120 . In some instances, the eNB  104  configures a PCell using the TDD carrier  130  and then transmits scheduling assignments for the TDD carrier  130  only on the TDD carrier  130 .  FIG. 13  and associated descriptions provide additional details of these implementations. 
     Turing to a general description of the elements, the UE  102  may be referred to as mobile electronic device, user device, mobile station, subscriber station, portable electronic device, mobile communications device, wireless modem, or wireless terminal. Examples of a UE (e.g., the UE  102 ) may include a cellular phone, personal data assistant (PDA), smart phone, laptop, tablet personal computer (PC), pager, portable computer, portable gaming device, wearable electronic device, or other mobile communications device having components for communicating voice or data via a mobile communication network. 
     Other examples of a UE include, but are not limited to, a television, a remote controller, a set-top box, a computer monitor, a computer (including a tablet, a desktop computer, a handheld or laptop computer, a netbook computer), a microwave, a refrigerator, a stereo system, a cassette recorder or player, a DVD player or recorder, a CD player or recorder, a VCR, an MP 3  player, a radio, a camcorder, a camera, a digital camera, a portable memory chip, a washer, a dryer, a washer/dryer, a copier, a facsimile machine, a scanner, a multi-functional peripheral device, a wristwatch, a clock, and a game device, etc. The UE  102  may include a device and a removable memory module, such as a Universal Integrated Circuit Card (UICC) that includes a Subscriber Identity Module (SIM) application, a Universal Subscriber Identity Module (USIM) application, or a Removable User Identity Module (R-UIM) application. The term “UE” can also refer to any hardware or software component that can terminate a communication session for a user. In addition, the terms “user equipment,” “UE,” “user equipment device,” “user agent,” “UA,” “user device,” and “mobile device” can be used synonymously herein. 
     A radio access network (RAN) is part of a mobile telecommunication system which implements a radio access technology, such as UMTS, CDMA2000 and 3GPP LTE/LTE-A. In many applications, a RAN includes at least one eNB  104 . An eNB (e.g., the eNB  104 ) can be a radio base station that may control all or at least some radio related functions in a fixed part of the system. The at least one eNB  104  can provide radio interface within their coverage area or a cell for UEs  102  to communicate. eNBs  104  may be distributed throughout the cellular network to provide a wide area of coverage. The eNB  104  directly communicates to one or a plurality of UEs  102 , other eNBs, and one or more core network nodes. 
     The LTE network  110  may include one or a plurality of radio access networks, core networks (CNs), and external networks. In certain implementations, the radio access networks may be Evolved Universal Mobile Telecommunications System (UMTS) terrestrial radio access networks (EUTRANs). In addition, in certain instances, core networks may be evolved packet cores (EPCs). In some implementations, 2G/3G systems, e.g., Global System for Mobile communication (GSM), Interim Standard 95 (IS-95), Universal Mobile Telecommunications System (UMTS) and CDMA2000 (Code Division Multiple Access) may also be integrated into the LTE network  110 . 
     Though described in terms of  FIG. 1 , the present disclosure is not limited to such an environment. In general, mobile communication systems may be described as cellular networks made up of a number of radio cells, or cells that are each served by a base station or other fixed transceiver. The cells are used to cover different areas in order to provide radio coverage over an area. Example mobile communication systems include Global System for Mobile Communication (GSM) protocols, Universal Mobile Telecommunications System (UMTS), 3GPP Long Term Evolution (LTE), and others. In addition to mobile communication systems, wireless broadband communication systems may also be suitable for the various implementations described in the present disclosure. Example wireless broadband communication system includes IEEE 802.11 wireless local area network, IEEE 802.16 WiMAX network, etc. 
       FIG. 2  is a schematic  200  illustrating an example HARQ timing relationship in TDD-FDD joint CA. The example HARQ timing relationship includes an example HARQ timing relationship for PDSCH transmission on a FDD carrier  202 , and an example HARQ timing relationship for PDSCH transmission on a TDD carrier  204 . In a 3GPP LTE TDD system, a subframe of a radio frame can be a downlink, an uplink or a special subframe. The special subframe comprises downlink and uplink time regions separated by a guard period for downlink to uplink switching. The 3GPP specification standards define seven different UL/DL configuration schemes for LTE TDD operations. These schemes are listed in Table 1. D represents downlink subframes, U represents uplink subframes and S represents the special subframe. The special subframe includes three parts, (1) the downlink pilot time slot (DwPTS), (2) the uplink pilot time slot (UpPTS) and (3) the guard period (GP). Downlink transmissions on the PDSCH may be made in DL subframes or in the DwPTS portion of a special subframe. The table below illustrates LTE TDD uplink-downlink configurations. 

 
     As Table 1 shows, there are two switching point periodicities specified in the LTE standard: 5 ms and 10 ms. The 5 ms switching point periodicity is introduced to support the co-existence between LTE and low chip rate UTRA TDD systems and the 10 ms switching point periodicity is introduced to support the co-existence between LTE and high chip rate UTRA TDD systems. The supported configurations cover a wide range of UL/DL allocations from a DL heavy 1:9 ratio to a UL heavy 3:2 ratio. The DL allocations in these ratios include both DL subframes and special subframes, which can also carry downlink transmissions in DwPTS. Therefore, compared to FDD systems, TDD systems have more flexibility in terms of the proportion of resources assignable to uplink and downlink communications within a given assignment of spectrum. Specifically, it is possible to distribute the radio resources unevenly between uplink and downlink. This will provide a way to utilize the radio resources more efficiently by selecting an appropriate UL/DL configuration based on interference situation and different traffic characteristics in DL and UL. 
     Because the UL and DL transmissions are not continuous (i.e. UL or DL transmissions may not occur in every subframe) in a LTE TDD system, the scheduling and HARQ timing relationships are separately defined in the specifications. An example HARQ timing relationship for the downlink is shown below in Table 2. The example HARQ timing relationship associates an UL subframe n, which conveys ACK/NACK, with DL subframes n-ki, i=0 to M−1. The set of DL subframes for which ACK/NACK is provided is referred to herein as the bundling window, and the number of subframes for which ACK/NACK is provided, M, is referred to as the bundling window size. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Downlink association set index K: {k 0 , k 1 , . . . k M-1 } 
               
            
           
           
               
               
            
               
                 UL-DL 
                   
               
               
                 Config- 
                 Subframe n 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 uration 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 0 
                 — 
                 — 
                 6 
                 — 
                 4 
                 — 
                 — 
                 6 
                 — 
                 4 
               
               
                 1 
                 — 
                 — 
                 7, 6 
                 4 
                 — 
                 — 
                 — 
                 7, 6 
                 4 
                 — 
               
               
                 2 
                 — 
                 — 
                 8, 7, 4, 6 
                 — 
                 — 
                 — 
                 — 
                 8, 7,  
                 — 
                 — 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 4, 6 
                   
                   
               
               
                 3 
                 — 
                 — 
                 7, 6, 11 
                 6, 5 
                 5, 4 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 4 
                 — 
                 — 
                 12, 8, 7,  
                 6, 5,  
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                   
                   
                 11 
                 4, 7 
                   
                   
                   
                   
                   
                   
               
               
                 5 
                 — 
                 — 
                 13, 12, 9,  
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                   
                   
                 8, 7, 5, 4,  
                   
                   
                   
                   
                   
                   
                   
               
               
                 6 
                 — 
                 — 
                 7 
                 7 
                 5 
                 — 
                 — 
                 7 
                 7 
                 — 
               
               
                   
               
            
           
         
       
     
     The uplink HARQ timing linkage is shown in Table 3 below. The table indicates that the Physical Hybrid Indicator Channel (PHICH) ACK/NACK received in the DL subframe i is linked with the UL data transmission in the UL subframe i-k, k being given in Table 2. In addition, for UL/DL configuration 0, in subframes 0 and 5, I PHICH =1 and k=6. This is because there may be two ACK/NACKs for a UE transmitted on the PHICH in subframes 0 and 5, one is represented by I PHICH =1, the other is I PHICH =0. I PHICH  just serves as an index. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 k for HARQ ACK/NACK 
               
            
           
           
               
               
            
               
                 TDD UL/DL 
                 subframe number i 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Configuration 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 0 
                 7 
                 4 
                   
                   
                   
                 7 
                 4 
                   
                   
                   
               
               
                 1 
                   
                 4 
                   
                   
                 6 
                   
                 4 
                   
                   
                 6 
               
               
                 2 
                   
                   
                   
                 6 
                   
                   
                   
                   
                 6 
                   
               
               
                 3 
                 6 
                   
                   
                   
                   
                   
                   
                   
                 6 
                 6 
               
               
                 4 
                   
                   
                   
                   
                   
                   
                   
                   
                 6 
                 6 
               
               
                 5 
                   
                   
                   
                   
                   
                   
                   
                   
                 6 
                   
               
               
                 6 
                 6 
                 4 
                   
                   
                   
                 7 
                 4 
                   
                   
                 6 
               
               
                   
               
            
           
         
       
     
     The UL grant, HARQ and transmission/retransmission relationship is shown below in Table 4. Upon detection of a PDCCH with DCI format 0 and/or a PHICH transmission in subframe n intended for the UE, the UE may adjust the corresponding PUSCH transmission in subframe n+k, with k given in Table 4. For TDD UL/DL configuration 0, if the least significant bit (LSB) of the UL index in the DCI format 0 is set to 1 in subframe n or a PHICH is received in subframe n=0 or 5 in the resource corresponding to I PHICH =1, or PHICH is received in subframe n=1 or 6, the UE may adjust the corresponding PUSCH transmission in subframe n+7. If, for TDD UL/DL configuration 0, both the most significant bit (MSB) and LSB of the UL index in the DCI format 0 are set in subframe n, the UE may adjust the corresponding PUSCH transmission in both subframes n+k and n+7, with k given in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 k for PUSCH transmission 
               
            
           
           
               
               
            
               
                 TDD UL/DL 
                 subframe number n 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Configuration 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 0 
                 4 
                 6 
                   
                   
                   
                 4 
                 6 
                   
                   
                   
               
               
                 1 
                   
                 6 
                   
                   
                 4 
                   
                 6 
                   
                   
                 4 
               
               
                 2 
                   
                   
                   
                 4 
                   
                   
                   
                   
                 4 
                   
               
               
                 3 
                 4 
                   
                   
                   
                   
                   
                   
                   
                 4 
                 4 
               
               
                 4 
                   
                   
                   
                   
                   
                   
                   
                   
                 4 
                 4 
               
               
                 5 
                   
                   
                   
                   
                   
                   
                   
                   
                 4 
                   
               
               
                 6 
                 7 
                 7 
                   
                   
                   
                 7 
                 7 
                   
                   
                 5 
               
               
                   
               
            
           
         
       
     
     In the illustrated example, the TDD carrier  204  uses UL/DL configuration 1. The TDD carrier  204  uses subframe 7 to transmit HARQ bits corresponding to the received PDSCHs on the TDD carrier  204  in subframe 0 and subframe 1. The TDD carrier  204  uses subframe 8 to transmit HARQ bits corresponding to the received PDSCHs on the TDD carrier  204  in subframe 4. 
     In the illustrated example, the TDD carrier  204  is configured as a PCell. A UE may therefore use the TDD carrier  204  to transmit HARQ bits corresponding to PDSCH transmissions on the FDD carrier  202 . In some instances, the FDD carrier  202  has a HARQ timing relationship of 4, which means that a UE may transmit a HARQ bit in subframe n corresponding to PDSCH transmissions on the FDD carrier  202  in subframe n-4. In the illustrated example, when the UE receives a PDSCH transmission on the FDD carrier  202  in subframe 0, the earliest time that the UE may transmit a corresponding HARQ bit is in subframe 4. This delay can give the receiver time to processing the received data block and generate the corresponding ACK/NACK. However in the illustrated example, because the TDD carrier  204  uses UL/DL configuration 1, subframe 4 is a DL subframe. For UL/DL configuration 1, subframe 5 is also a DL subframe, and subframe 6 is a special frame. Therefore, for a PDSCH received in subframe 0 on the FDD carrier  202 , the UE may transmit a corresponding HARQ bit in subframe 7, which is a UL subframe. The UE may also transmit HARQ bits corresponding to subframes 1, 2, and 3 on the FDD carrier  202  in subframe 7 on the TDD carrier  204 . In another alternative, the relationship on the TDD UL subframe to transmit the HARQ bit for a particular FDD DL subframe may be specified in the standards or pre-configured by the eNB. 
     In the illustrated example, the UE may therefore transmit up to six HARQ bits in subframe 7 on the TDD carrier  204 . Two of the six HARQ bits correspond to PDSCH transmissions on the TDD carrier  204  in subframe 0 and 1, and the remaining four HARQ bits correspond to PDSCH transmissions on the FDD carrier  202  in subframe 0, 1, 2, and 3, respectively. The UE may also transmit up to six HARQ bits in subframe 2 on the TDD carrier  204 . The UE may transmit more HARQ bits if it receives PDSCH transmissions in multiple layers. On the other hand, the UE may transmit up to two HARQ bits in subframe 8 on the TDD carrier  204 . One of the two HARQ bits corresponds to PDSCH transmissions on the TDD carrier  204  in subframe 4, and the other HARQ bit corresponds to PDSCH transmissions on the FDD carrier  202  in subframe 4. 
     In some implementations, an eNB may configure a flexible PUCCH resource allocation. For example, a UE may receive a PUCCH resource allocation from an eNB. The resource allocation may include two PUCCH configurations. The UE may use one PUCCH configuration to transmit HARQ bits on PUCCH in one subframe, and use the other PUCCH configuration to transmit HARQ bits on PUCCH in another subframe. This approach may enable PUCCH resource allocation on a subframe basis and improve the radio resource utilization efficiency. For example, one PUCCH configuration may indicate a PUCCH format and the other PUCCH configuration may indicate another PUCCH format. In a LTE system, a UE may use PUCCH format 1 or format 3 to transmit HARQ bits. PUCCH format 3 may transmit more HARQ bits than PUCCH format 1, but PUCCH format 3 may also occupy more resources. In the illustrated example, the eNB may configure the UE to use PUCCH format 3 in subframes 2 and 7 because the UE may need to transmit more HARQ bits in these subframes. On the other hand, the eNB may configure the UE to use PUCCH format 1 in subframes 3 and 8, and thereby save resources in these subframes. 
     The eNB may transmit the PUCCH resource allocation using higher layer signaling. For example, the eNB may transmit the PUCCH resource allocation in a RRC message. The UE may also indicate whether it supports flexible PUCCH resource allocation using a Feature Group Indicator (FGI). For example, the UE may set the FGI bit to ‘1’ to indicate that it supports a PUCCH resource allocation that includes more than one PUCCH configurations. Conversely, the UE may set the FGI bit to ‘0’ to indicate that it does not support a PUCCH resource allocation that includes more than one PUCCH configurations. In another example, the eNB may transmit the allocation in a MAC control elements. 
       FIG. 3  is a schematic illustrating an example bitmap  300  for a PUCCH resource allocation. The bitmap  300  includes bit P 5   302   a , P 4   302   b , P 3   302 C, P 2   302   d , P 1   302   e , and P 0   302   f . In some implementations, an eNB may use a bitmap which has the same number of bits as the maximum number of UL subframes in TDD UL/DL configurations. For example, as shown in Table 1, the maximum number of UL subframes in TDD UL/DL configurations 0-6 is six. An eNB therefore may use a bitmap with 6 bits. In the illustrated example, each bit in the bitmap  300  indicates a PUCCH configuration for one UL subframe. For example, each bit in the bitmap  300  may indicate whether the UE may use format 3 to transmit HARQ bits in the respective subframe. For example, if the UE uses TDD UL/DL configuration 0, then the UE may transmit HARQ bits in subframe 2, 3, 4, 7, 8, and 9. The eNB may set bit P 5   302   a  to “1,” which indicates that the UE may use PUCCH format 3 to transmit HARQ bits in subframe 2. The eNB may set bit P 4   302   b  to “0,” which indicates that the UE may use PUCCH format 1 in subframe 3. In some implementations, if the UE uses TDD UL/DL configuration that has m UL subframes, where m&lt;6, the eNB may use a m-bit bitmap instead of a 6-bit bitmap to indicate the PUCCH resource allocation. In one alternative, to be simple, the m could be set to the maximum number of UL subframes of all possible TDD configurations. In some implementations, the eNB may use a 6-bit bitmap and use either the left most m bits or the right most m bits in the bitmap to indicate the PUCCH resource allocation. 
       FIG. 4  is an example information element definition  400  for a PUCCH resource allocation. The example information element definition  400  includes an example information element  402 . The example information element  402  defines PUCCH-ConfigDedicated-V13. In the illustrated example, the information element  402  specifies a 6-bit bitmap that may be used to indicate a PUCCH resource allocation. 
       FIG. 5  is a schematic  500  illustrating one example configuration for PUCCH transmissions. In a LTE system, a UE transmit PUCCH on both sides of PUSCH. In the illustrated example, the UE uses resource blocks  520  to transmit a PUSCH. The UE uses resource blocks  512   a - b ,  514   a - b ,  516   a - b , and  518   a - b  to transmit PUCCHs. In the illustrated example, the eNB uses a parameter N RB   (2)    502  to configure the starting point of the resource blocks for PUCCH format 1. In the illustrated example, the starting point of the resource blocks for PUCCH format 1 transmission is NRB number of resource blocks away from the transmission bandwidth edge. For example, the UE may use the resource blocks  512   a - b  to transmit PUCCH format 2. The UE may also use the resource blocks  514   a - b  to transmit mixed PUCCH format 1 &amp; 2, and the resource blocks  516   a - b  to transmit PUCCH format 1. The eNB may configure the parameter N RB   (2)    502  semi-statically. For example, the eNB may transmit the parameter N RB   (2)    502  in a broadcast message so that all the UEs in the cell may use the same resource blocks to transmit PUCCH format 1. In the illustrated example, the UE transmit PUCCH format 3 in the resource blocks  518   a - b . The resource blocks  518   a  is located between the resource block  520  and the resource block  516   a . Similarly, the resource blocks  518   b  is located between the resource block  520  and the resource block  516   b . Placing PUCCH format 3 inside the allocation for PUCCH format 1 may provide one or more advantages. For example, as described previously, a UE may use format 3 to transmit HARQ bits in some subframes, and use format 1 to transmit HARQ bits in some other subframes. An eNB may configure the parameter N RB   (2)    502  semi-statically and a legacy UE may continue to use the parameter NRB  502  to determine which resource blocks the legacy UE may use for PUCCH format 2 and PUCCH format 1 transmission, respectively. 
       FIG. 6  is a schematic  600  illustrating one example configuration for PUCCH transmissions. In the illustrated example, the UE uses resource blocks  620  to transmit a PUSCH. The UE uses resource blocks  612   a - b ,  614   a - b ,  616   a - b , and  618   a - b  to transmit PUCCH. In the illustrated example, the eNB uses a parameter N RB   (2)    602  to configure the starting point of the resource blocks for PUCCH format 1. In the illustrated example, the starting point of the resource blocks for PUCCH format 1 transmission is N RB   (2)  number of resource blocks away from the transmission bandwidth edge. For example, the UE may use the resource blocks  616   a - b  to transmit mixed PUCCH format 1 &amp; 2, and the resource blocks  618   a - b  to transmit PUCCH format 1. In the illustrated example, the resource blocks  612   a - b , which the UE may use to transmit PUCCH format 3, are located at the band edge. The resource blocks  612   a - b  are thus outside of the resource blocks  614   a - b , which the UE may use to transmit PUCCH format 2. This approach has one or more advantages. For example, the resource blocks at band edge may provide a better signal quality. PUCCH format 3 may be used to transmit a large number of HARQ bits and may be less resilient to noise and interference. Therefore, using the resource blocks at band edge to transmit PUCCH format 3 may improve the signal quality of PUCCH format 3 and improve system performance. 
     As described previously, in some implementations, a UE may use format 3 to transmit HARQ bits in some subframes. Therefore an eNB may transmit a resource block allocation that includes two N RB   (2)  parameters. The eNB may use one N RB   (2)  to indicate the starting point of resource allocation for PUCCH format 1 in the subframes where a UE may use PUCCH format 3 to transmit HARQ bits, and use the other N RB   (2)  to indicate the starting point of resource allocation for PUCCH format 1 in the subframes where a UE does not use PUCCH format 3 to transmit HARQ bits. These parameters needs to be delivered to the UE via the RRC messages, and may be updated from time to time. 
       FIG. 7  is an example information element definition  700  specifying an additional N RB   (2)  parameter. The example information element definition  700  includes an example information element  702 . The example information element  702  defines PUCCH-ConfigComm-v12. In the illustrated example, the information element  702  includes an nRB-CQI-r12 value to specify the additional NRB parameter. The information element  702  also includes an nRB-CQI-subframes value to specify the subframes that the additional N RB   (2)  parameter applies to. 
       FIG. 8  is a flowchart  800  showing an illustrative method for supporting TDD-FDD joint CA. The flowchart begins at step  802  where a UE receives a PUCCH resource allocation from an eNB. The PUCCH resource allocation may include a first PUCCH configuration for a first subframe and a second PUCCH configuration for a second subframe. In some implementations, the first PUCCH configuration indicates a first PUCCH format, and the second PUCCH configuration indicates a second PUCCH format. In some implementations, the PUCCH resource allocation may include a bitmap. For example, the bitmap may indicate a PUCCH format 3 for a first subframe and a PUCCH format 1 for a second subframe. 
     In some implementations, at step  806 , the UE transmit a first group of HARQ bits using PUCCH format 3 in a first subframe as indicated by the bitmap. The first group of HARQ bits may include one or more HARQ bits corresponding to PDSCH transmissions on a TDD carrier and one or more HARQ bits corresponding to PDSCH transmissions on a FDD carrier. In some implementations, the UE may transmit the first group of HARQ bits in a resource block that is between resource blocks used to transmit PUCCH format 1 and resource blocks used to transmit PUSCH. At step  808 , the UE transmits a second group of HARQ bits using PUCCH format 1 in a second subframe. 
     In some implementations, at step  814 , the UE receives a resource block allocation. The resource block allocation may include a first parameter and a second parameter that is different than the first parameter. At step  816 , the UE may transmit a first group of HARQ bits using PUCCH format 3 in a first subframe as indicated by the bitmap. The first group of HARQ bits may include one or more HARQ bits corresponding to PDSCH transmissions on a TDD carrier and one or more HARQ bits corresponding to PDSCH transmissions on a FDD carrier. In some implementations, the UE may transmit the first group of HARQ bits in a resource block indicated by the first parameter. At step  818 , the UE transmits a second group of HARQ bits using PUCCH format 1 in a second subframe as indicated by the bitmap. The UE may transmit the second group of HARQ bits using a resource block indicated by the second parameter. 
     In some implementations, an eNB may divide a first group of HARQ bits into several subgroups, and allocate PUCCH resources to each of the subgroups.  FIG. 9  is a flowchart  900  showing an illustrative method for supporting TDD-FDD joint CA with multiple subgroups. The flowchart begins at step  902  where a UE receives a PUCCH resource allocation from an eNB. The PUCCH resource allocation may include a first PUCCH configuration for a first subframe and a second PUCCH configuration for a second subframe. In some implementations, the first PUCCH configuration may indicate the number of subgroups in the first group of HARQ bits. 
     At step  904 , the UE identifies a plurality of subgroups in the first group of HARQ bits. For example, as described previously, in some subframes, the UE may transmit a first group of HARQ bits that includes six HARQ bits, while in other subframes, the UE may transmit a second group of HARQ bits that includes two HARQ bits. For the first group of HARQ bits, the eNB may divide the six HARQ bits into three subgroups and indicate such an allocation using the first PUCCH configuration. Accordingly, the UE identifies the three subgroups of HARQ bits. For the second group of HARQ bits, the eNB may allocate one subgroup and the UE may identify one subgroup of HARQ bits based on eNB&#39;s allocation. 
     At step  906 , the UE identifies a plurality of PUCCH resources for the plurality of subgroups of HARQ bits. In some implementations, the eNB may allocate one PUCCH resource to each subgroup of HARQ bits. For example, the eNB may allocate three PUCCH resources for three subgroups of HARQ bits. Each PUCCH resource may be associated with one subgroup of HARQ bits. In some implementations, the plurality of PUCCH resources are consecutive. For example, if there are more than 20 ACK/NACK bits to be transmitted, the UE may use two PUCCH Format 3 resources. The eNB may configure the two PUCCH Format 3 resources such that the resource blocks used to transmit PUCCH format 3 are consecutive in frequency domain, e.g., PUCCH resources of index n PUCCH   (3,{tilde over (p)})  and n PUCCH   (3,{tilde over (p)}) +1. This approach has one or more advantages. For example, this approach may reduce peak to average power ratio (PAPR) at the UE because the multiple PUCCH resources are in the same resource block or in the contiguous resource block. 
     At step  908 , the UE transmits the first group of HARQ bits based on the plurality of PUCCH resources in a first subframe. For example, in a first subframe where the UE may transmit three subgroups of HARQ bits, the UE may use the three PUCCH resources associated with the three subgroups. At step  910 , the UE transmits the second group of HARQ bits in a second subframe. For example, in a second subframe where the UE may transmit one subgroup of HARQ bits, the UE may use the one PUCCH resources associated with the one subgroup. 
       FIG. 10  is a schematic  1000  illustrating an example HARQ timing relationship on a TDD carrier with self-scheduling. The schematic  1000  includes a TDD carrier  1010  and a FDD UL  1020 . In the illustrated example, an eNB configures a TDD-FDD joint CA for a UE. The eNB configures a FDD carrier having the FDD UL  1020  as a PCell and the TDD carrier  1010  as a SCell. The UE may therefore transmit HARQ bits corresponding to PDSCH transmissions on the TDD carrier  1010  on the FDD UL  1020 . In the illustrated example, the eNB configures the TDD carrier  1010  as self-scheduling. The UE may receive a PDSCH assignment in subframe 0 on the TDD carrier  1010 . The PDSCH assignment may indicate a PDSCH transmission for the TDD carrier  1010  in subframe 0. In the illustrated example, the UE transmits HARQ bits corresponding to PDSCH transmissions on the TDD carrier  1010  based on a FDD HARQ timing. For example, the UE receives a PDSCH transmission on the TDD carrier  1010  in subframe 0. Instead of using the TDD HARQ timing as described in Table 2, which means transmitting a corresponding HARQ bit in subframe 7, the UE transmits the corresponding HARQ bit in subframe. This approach reduces the round trip delay of a HARQ response, especially for PDSCH on subframe 0, 1, 5, and 6 in the illustrated example. 
       FIG. 11  is a schematic  1100  illustrating an example HARQ timing relationship on a TDD carrier with cross-carrier scheduling. The schematic  1100  includes a FDD DL  1102 , a TDD carrier  1110 , and a FDD UL  1120 . Similarly as  FIG. 10 , the eNB configures a FDD carrier having the FDD DL  1102  and the FDD UL  1120  as a PCell and the TDD carrier  1110  as a SCell. The UE may therefore use the FDD UL  1120  to transmit HARQ bits corresponding to PDSCH transmissions on the TDD carrier  1110 . In the illustrated example, the eNB configures the TDD carrier  1110  as cross-carrier scheduling. For example, the UE may receive a PDSCH assignment in subframe 0 on the FDD DL  1102 . The PDSCH assignment may indicate a PDSCH transmission for the TDD carrier  1110  in subframe 0. In the illustrated example, the UE may transmit HARQ bits corresponding to PDSCH transmissions on the TDD carrier  1110  based on a FDD HARQ timing. For example, the UE may receive a PDSCH transmission on the TDD carrier  1110  in subframe 0. Instead of using the TDD HARQ timing as described in Table 2, which means transmitting a corresponding HARQ bit in subframe 7, the UE may transmit the corresponding HARQ bit in subframe 4. 
       FIG. 12  is a flowchart  1200  showing an illustrative method for transmitting HARQ bits based on FDD HARQ timing. The flowchart begins at step  1202  where a UE receives a TDD-FDD joint CA configuration. In the illustrated example, the TDD-FDD joint CA configuration includes a FDD carrier as a PCell and a TDD carrier as a SCell. In some implementations, the TDD carrier is configured as self-scheduling and at step  1204 , the UE receives a PDSCH assignment for the TDD carrier on the TDD carrier. In some implementations, the TDD carrier is configured as cross-carrier scheduling, and at step  1206  the UE receives a PDSCH assignment for the TDD carrier on the FDD carrier. At step  1210 , the UE receives a PDSCH transmission on the TDD carrier. At step  1212 , the UE transmits a HARQ bit corresponding to the PDSCH transmission on the TDD carrier using a FDD HARQ timing. 
     In some implementations, an eNB may use a restrictive approach in configuring a TDD-FDD joint CA. In one example, an eNB may avoid configuring a TDD carrier as a PCell in a TDD-FDD joint CA. In another example, an eNB may configure a TDD carrier as a PCell in a TDD-FDD joint CA, and avoid using cross-carrier scheduling. This approach may enable all the subframes in a radio frame be utilized. 
       FIG. 13  is a flowchart  1300  showing an illustrative method for configuring a TDD-FDD joint CA using a restrictive approach. The flowchart begins at step  1302  where an eNB configures a TDD-FDD joint CA. The TDD-FDD joint CA may include a FDD carrier and a TDD carrier. In some implementations, the eNB may avoid configuring the TDD carrier as a PCell. Therefore at step  1304 , the eNB configures a PCell using only the FDD carrier. In some implementations, the eNB may configure a TDD carrier as a PCell at step  1310 . The eNB may avoid using cross-carrier scheduling. At step  1312 , the eNB transmits scheduling assignments for the TDD carrier only on the TDD carrier. 
     A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims. 
     Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementation descried above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a signal software product or packaged into multiple software products. 
     Also, techniques, systems, subsystems, and methods described and illustrated in the various implementations as discrete or separate may be combined or integrated with other systems, modules, techniques, or methods without departing from the scope of the present disclosure. Other items shown or discussed as coupled or directly coupled or communicating with each other may be indirectly coupled or communicating through some interface, device, or intermediate component, whether electrically, mechanically, or otherwise. Other examples of changes, substitutions, and alterations are ascertainable y one skilled in the art and could be made without departing form the spirit and scope disclosed herein. 
     While the above detailed description has shown, described, and pointed out the fundamental novel features of the disclosure as applied to various implementations, it will be understood that various omissions and substitutions and changes in the form and details of the system illustrated may be made by those skilled in the art, without departing from the intent of the disclosure. In addition, the order of method steps are not implied by the order they appear in the claims.