This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
Currently in 3GPP, one work item (WI) ‘Further enhancement of Carrier Aggregation (FeCA)’ is in progress. It is possible to aggregate up to 32 Component Carriers (CC), including both licensed carriers and unlicensed carriers. One important part of this work item is to design and improve the control channels including Downlink Control Information (DCI) and Uplink Control Information (UCI) and possibly some other control channels.
When UCI is to be transmitted in a subframe in which the UE has been allocated transmission resources for the PUSCH, the UCI is multiplexed together with the UL-SCH data prior to DFT spreading, in order to preserve the low CM single-carrier property. The PUCCH is never transmitted in the same subframe as the PUSCH in Releases 8 and 9. The multiplexing of CQI/PMI, HARQ ACK/NACK, and RI with the PUSCH data symbols onto uplink resource elements (REs) is shown in FIG. 1.
The number of REs used for each of CQI/PMI, ACK/NACK and RI is based on the MCS assigned for PUSCH and offset parameters, βoffsetCQI, βoffsetRI, βoffsetHARQ-ACK, which are semi-statically configured by higher-layer signaling.
As shown in FIG. 1, CQI/PMI resources are placed at the beginning of the UL-SCH data resources and mapped sequentially to all SC-FDMA symbols on one subcarrier before continuing on the next subcarrier. The UL-SCH data is rate-matched around the CQI/PMI data. The same modulation order as UL-SCH data on PUSCH is used for CQI/PMI. For small CQI and/or PMI report sizes up to 11 bits, a (32, k) block code, similar to the one used for PUCCH, is used, with optional circular repetition of encoded data (see 3GPP 36.212). No Cyclic Redundancy Check (CRC) is applied. For large CSI reports (>11 bits), an 8-bit CRC is attached and channel coding and rate matching is performed using the tail-biting convolutional code. The HARQ ACK/NACK resources are mapped to SC-FDMA symbols by puncturing the UL-SCH PUSCH data. Positions next to the RS are used, so as to benefit from the best possible channel estimation. The maximum amount of resource for HARQ ACK/NACK is 4SC-FDMA symbols. The coded RI symbols are placed next to the HARQ ACK/NACK symbol positions irrespective of whether ACK/NACK is actually present in a given subframe. The modulation of the 1- or 2-bit ACK/NACK or RI is such that the Euclidean distance of the modulation symbols carrying ACK/NACK and RI is maximized. The outermost constellation points of the higher-order 16/64-QAM PUSCH modulations are used, resulting in increased transmit power for ACK/NACK/RI relative to the average PUSCH data power. The coding of the RI and CQI/PMI are separate, with the UL-SCH data being rate-matched around the RI REs similarly to the case of CQI/PMI.
According to the current 3GPP standard which supports up to 5 DL CCs, the UCI can be transmitted over the UL SCH. In 3GPP 36.213 c40, feedback cell for UCI is specified. If the UE is configured with more than one serving cell and is not configured for simultaneous PUSCH and PUCCH transmission, then in Subframe n UCI shall be transmitted                on PUCCH using format 1/1a/1b/3 or 2/2a/2b if the UE is not transmitting PUSCH;        on PUSCH of the serving cell if the UCI consists of aperiodic CSI or aperiodic CSI and HARQ-ACK;        on primary cell PUSCH if the UCI consists of periodic CSI and/or HARQ-ACK and if the UE is transmitting on the primary cell PUSCH in subframe n unless the primary cell PUSCH transmission corresponds to a Random Access Response Grant or a retransmission of the same transport block as part of the contention based random access procedure, in which case UCI is not transmitted;        on PUSCH of the secondary cell with smallest cell index if the UCI consists of periodic CSI and/or HARQ-ACK and if the UE is not transmitting PUSCH on primary cell but is transmitting PUSCH on at least one secondary cell.        
In any specified case, the UCI transmission over more than one UL CC is not allowed for the aggregated cells.
For up to 5 DL CCs in the current specification, there are up to 10 HARQ ACKs bits for FDD at one time. For TDD, the number of HARQ feedback bits depends on the UL/DL configuration. Take UL/DL configuration 2 as one example, the total required bits HARQ ACK NACK bits are 40 bits for 5 DL CCs. In TDD case, HARQ ACK/NACK bundling may be applied between two code words. For FDD, assume 1/2 coding and QPSK modulation is applied, there are 10 symbols (i.e. RE) needed at least.
For up to 32 DL CCs, there are up to 64 HARQ ACKs at one time (Rank 2 transmission) depending on the number of configured DL CCs for FDD. For TDD, the number of HARQ-ACK bits to be feedback depends on the number of configured CCs and UL/DL subframe configuration of the DL CCs. Assume there are 32 DL CCs with UL/DL subframe configuration 2 and transmission mode 3, there are up to 256 (32*4*2) HARQ ACK/NACK bits. Assume 1/2 coding rate and QPSK modulation are applied, FDD needs 32 REs at least while TDD needs 256 symbols (32 symbols for FDD and 128 symbols for TDD respectively if the bundling is applied between two codewords) at least.
Considering the case that the UCI is only carried over the UL-SCH channel, when there are 32 DL CCs with UL/DL subframe configuration 2 for TDD systems, there can be up to 256/128 symbols (32 DL CC, UL/DL configuration 2, 1/2 coding and QPSK modulation) with/without bundling between two codewords for HARQ QCK/NACK feedback. However, one PRB can at most carry 48 (4 symbol×12 subcarrier) symbols. Considering the 1% error target for HARQ ACK/NACK detection, more symbols may be required for HARQ ACK/NACK carrying by configuring the mentioned parameters (e.g. βoffsetHARQ-ACK). There may be no enough room for HARQ-ACK/NACK feedback when the number of scheduled PRBs over the UL CC for HARQ ACK/NACK bits is too small while the number of the scheduled DL CCs is large. For instance, the available REs are not enough to carry the HARQ ACK/NACK bits if one UL CC is scheduled with one or two PRBs while 32 DL CCs are scheduled. Similarly RI and CSI feedback also need more resources.