1. Field of the Invention
The present invention relates to radio communication systems, and more particularly, to a method for feeding back ACK/NACK for downlink data transmission in a radio communication system, so as to support data transmission based on HARQ in the radio communication system.
2. Background of the Invention
The Long Term Evolution system of the 3GPP standard organization supports two duplex modes including Frequency Division Duplex (FDD) and Time Division Duplex (TDD). FIG. 1 is a schematic diagram illustrating a frame structure of an LTE TDD system. The length of each radio frame is 10 ms which is equally divided into two 5 ms half-frames. Each half-frame includes 8 slots with length of 0.5 ms and 3 special slots, i.e. a Downlink Pilot Slot (DwPTS), a Guard Period (GP) and an Uplink Pilot Slot (UpPTS). The total length of the 3 special slots is 1 ms. Each half-frame consists of 2 consecutive slots, i.e. the kth sub-frame includes slot 2k and slot 2k+1. The LTE TDD supports 7 uplink-downlink configurations, as shown in table 1. Herein, D denotes a downlink sub-frame, U denotes an uplink sub-frame and S denotes the 3 special slots in the special field.
TABLE 1Uplink-downlink configurations of the LTE TDDIndex ofSwitch-pointSub-frame numberconfigurationperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
The LTE system transmits data based on Hybrid ARQ (HARQ). A data receiver transmits ACK or NACK information based on whether the data is corrected received. Scheduling of dynamic downlink data transmission is implemented via a Physical Downlink Control Channel (PDCCH). Each PDCCH may include 1, 2, 4 or 8 Control Channel Elements (CCEs) and each CCE is mapped to one ACK/NACK channel. Thus, a PDCCH which includes multiple CCEs is actually mapped to multiple ACK/NACK channels. In the LTE system, the ACK/NACK channel used for transmitting ACK/NACK information is implicitly determined by a CCE which has a smallest index among all CCEs constituting the PDCCH. Except for Semi-Permanent Scheduling (SPS), no PDCCH is required to be transmitted during initial transmission of downlink data, and the ACK/NACK channel used for feeding back the ACK/NACK information is configured semi-statically. The re-transmission of the downlink data is implemented via PDCCH scheduling. Similarly as the dynamic scheduling, the ACK/NACK channel used for feeding back the ACK/NACK information is determined by the PDCCH. The ACK/NACK feedback signal of the HARQ transmission of the downlink data is transmitted on the ACK/NACK channel of the Physical Uplink Control Channel (PUCCH).
In the LTE system, there are multiple methods for feeding back ACK/NACK information. The first method transmits ACK/NACK feedback information of only one downlink sub-frame on one uplink sub-frame. This method is applicable for FDD and some TDD uplink-downlink configurations. When MIMO is not adopted for data transmission, 1 bit of ACK/NACK information is generated, while when MIMO is adopted, 2 bits of ACK/NACK information are generated. The second method is to bundle ACK/NACK feedback information of multiple downlink sub-frames as 1 or 2 bits, so as to transmit the ACK/NACK information using the same method as the first method. This method is only applicable for TDD system. The bundling operation herein is working on codeword, i.e. bundle ACK/NACK information of codewords having the same index of the downlink sub-frames having data transmitted as one piece of ACK/NACK information. The third method is to transmit ACK/NACK feedback information of multiple downlink sub-frames in one uplink sub-frame. The data transmission of each downlink sub-frame generates one piece of ACK/NACK information. Then, the multiple bits of ACK/NACK information are transmitted on an ACK/NACK channel among multiple ACK/NACK channels based on QPSK modulation. In the LTE system, the number of bits may be 2, 3 and 4. This method is applicable for only the TDD system.
In the LTE system, the downlink data transmission is dynamically scheduled by PDCCH. But a UE may be unable to correctly receive the PDCCH transmitted by a base station. Thus, in the LTE TDD, when it is required to transmit ACK/NACK information of multiple downlink data sub-frames in one uplink sub-frame, a scheme is necessitated to enable the UE to detect whether the PDCCH of one or more downlink sub-frames is lost. In order to realize this function, in the LTE TDD, the PDCCH includes a field of 2 bits, referred to as Downlink Assignment Index (DAI). The DAI is used to indicate that how many downlink sub-frames in which the base station has transmitted PDCCH up to a current downlink sub-frame within N (N is larger than 1) downlink sub-frames associated with one uplink sub-frame. The changing order is 1, 2, 3 and 4. For example, if the UE receives two PDCCHs and their DAI values are respectively 1 and 3, the UE may determine that a PDCCH with DAI value of 2 is lost. In addition, the DAI cannot detect the situation where the last several PDCCHs are lost. For example, in the above example, the UE is unable to determine whether the base station has transmitted a PDCCH with DAI value of 4. As to the second method for feeding back the ACK/NACK information, it is defined that the UE feeds back the ACK/NACK information on an ACK/NACK channel determined by a last sub-frame of the PDCCH it receives. In this method, the base station may know whether the UE has lost the PDCCH of the last one or last multiple sub-frames based on the ACK/NACK channel actually occupied.
Since the DAI is unable to detect the situation where the last several PDCCHs are lost, in the LTE TDD, another method is adopted to indicate the ACK/NACK information when it is required to transmit the ACK/NACK and a Scheduling Request (SR) at the same time or when it is required to transmit the ACK/NACK and a Channel Quality Indicator (CQI) at the same time. What is fed back is the number of ACKs in the ACK/NACK information of each downlink data the UE received. Suppose the number of downlink sub-frames is M. Then, the number of ACKs indicated by this method is 0, 1, . . . , M. The number 0 is also used to indicate that the UE detect a situation where at least one downlink data is lost. In this method, the base station can determine whether the UE has correctly received all the downlink data based on the number of downlink data the base station actually transmitted and the number of ACKs the UE reported, and then can transmit new data or perform re-transmission.
In order to support higher data rate, in LTE-Advance (LTE-A) system, multiple Component Carriers (CCs) are aggregated to obtain larger working bandwidth for downlink and uplink of the communication system, i.e. Carrier Aggregation (CA). For example, five 20 MHz CCs may be aggregated to support a 100 bandwidth.
As to the LTE-A, there is a method for feeding back ACK/NACK based on multi-code transmission. Suppose the UE receives downlink data on N CCs at the same time, and each downlink data corresponds to one ACK/NACK channel. Then, the UE transmits ACK/NACK information on the N ACK/NACK channels at the same time, and the ACK/NACK feedback information of each downlink data is transmitted on the ACK/NACK channel corresponding to this downlink data. This method is a simple extension to the LTE processing method. But it has a deficiency that multiple ACK/NACK channels are occupied in parallel, which destroys the single-carrier characteristic of the uplink signal and thus brings out a higher Cubic Measurement (CM). And the larger the number of the ACK/NACK channels transmitted in parallel, the larger the value of the CM. At the same time, the transmission power of the UE is divided onto multiple ACK/NACK channels, which restricts the coverage are of the uplink signal.
The present invention provides a corresponding processing method to implement a tradeoff between ACK/NACK information amount fed back and signal coverage are or CM value.