1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and apparatus for transmitting an uplink control signal of a relay station in a wireless communication system.
2. Related Art
The institute of electrical and electronics engineers (IEEE) 802.16e standard was adopted in 2007 as a sixth standard for international mobile telecommunication (IMT)-2000 in the name of ‘WMAN-OFDMA TDD’ by the ITU-radio communication sector (ITU-R) which is one of sectors of the international telecommunication union (ITU). An IMT-advanced system has been prepared by the ITU-R as a next generation (i.e., 4th generation) mobile communication standard following the IMT-2000. It was determined by the IEEE 802.16 working group (WG) to conduct the 802.16m project for the purpose of creating an amendment standard of the existing IEEE 802.16e as a standard for the IMT-advanced system. As can be seen in the purpose above, the 802.16m standard has two aspects, that is, continuity from the past (i.e., the amendment of the existing 802.16e standard) and continuity to the future (i.e., the standard for the next generation IMT-advanced system). Therefore, the 802.16m standard needs to satisfy all requirements for the IMT-advanced system while maintaining compatibility with a mobile WiMAX system conforming to the 802.16e standard.
An orthogonal frequency division multiplexing (OFDM) system capable of reducing inter-symbol interference (ISI) with a low complexity is taken into consideration as one of next generation wireless communication systems. In the OFDM, a serially input data symbol is converted into N parallel data symbols, and is then transmitted by being carried on each of separated N subcarriers. The subcarriers maintain orthogonality in a frequency dimension. Each orthogonal channel experiences mutually independent frequency selective fading, and an interval of a transmitted symbol is increased, thereby minimizing inter-symbol interference. In a system using the OFDM as a modulation scheme, orthogonal frequency division multiple access (OFDMA) is a multiple access scheme in which multiple access is achieved by independently providing some of available subcarriers to a plurality of users. In the OFDMA, frequency resources (i.e., subcarriers) are provided to the respective users, and the respective frequency resources do not overlap with one another in general since they are independently provided to the plurality of users. Consequently, the frequency resources are allocated to the respective users in a mutually exclusive manner.
In an OFDMA system, frequency diversity for multiple users can be obtained by using frequency selective scheduling, and subcarriers can be allocated variously according to a permutation rule for the subcarriers. In addition, a spatial multiplexing scheme using multiple antennas can be used to increase efficiency of a spatial domain. In order to support the various schemes described above, a control signal must be transmitted between a mobile station (MS) and a base station (BS). Examples of the control signal include a channel quality indicator (CQI) for reporting a channel state from the MS to the BS, an acknowledgement/not-acknowledgement (ACK/NACK) signal in response to data transmission, a bandwidth request signal for requesting allocation of a radio resource, precoding information in a multiple antenna system, antenna information, etc. The control signal is transmitted through a control channel.
Meanwhile, a wireless communication system employing a relay station (RS) has recently been developed. The RS is employed for cell coverage extension and transmission capability improvement. A BS provides a service to an MS located in a coverage boundary of the BS via the RS, and thus can obtain an effect of extending the cell coverage. In addition, the RS improves signal transmission reliability between the BS and the MS, thereby improving transmission capacity. Even if the MS is located inside the coverage of the BS, the RS may be used when the MS is located in a shadow area.
The wireless communication system employing the RS requires a new frame structure different from the conventional frame structure. A frequency band used when the RS transmits a signal to the BS may be equal to a frequency band at which a signal is received from a relay MS. Alternatively, the frequency band used when the RS receives the signal from the BS may be equal to a frequency band at which a signal is transmitted to the relay MS. Due to self interference, it is difficult for the RS to simultaneously perform transmission and reception of a signal at the same frequency band. Therefore, a time for switching an operation mode between transmission and reception of the signal is required. In general, it is assumed that the RS cannot transmit or receive the signal during the operation mode switching time. Some subframes included in an RS frame may include a symbol used as a transition gap by considering the operation mode switching time. In this symbol, the RS cannot transmit an uplink control signal.
Therefore, when the conventional method of transmitting the uplink control signal between the BS and the MS is equally used between the BS and the RS, a problem may occur due to the transition gap. For example, there may be a problem in multiplexing of an uplink control signal transmitted by the RS and an uplink control signal transmitted by the MS or in maintaining orthogonality of multiplexed signals.
Accordingly, there is a need for a method of transmitting the uplink control signal by the RS to the BS in the RS frame including the transition gap.