In the conventional multi-hop system, a transmission system may be implemented to have a plurality of relay stations as virtual multiple antennas. In this case, unlike a multi-antenna system, data can be effectively received since a decrease in correlation becomes significant due to a large spatial distance between relay stations. However, a time-synchronous error may occur when the relay stations are used as the virtual multiple antennas. Various techniques have conventionally been proposed in order to prevent deterioration caused by the time-synchronous error, and the techniques are divided into a data transmission technique and a data reception technique.
As a representative example of the data reception technique, there is a Minimum Mean Square Error-Decision Feedback Equalizer (MMSE-DFE) method. In the MMSE-DFE method, signals transmitted from a transmitter and relay stations are received by repeating signal processing without using information on a time-synchronous error or multi-fading channel information. With this method, a reception performance is improved since the signals transmitted from the transmitter and the relay stations are simultaneously received, and then a decoding index of each signal is regulated according to an MMSE. That is, a receiver using the MMSE-DFE method can achieve improvement in the reception performance while reducing consumption of resources for synchronization and channel estimation. Disadvantageously, however, cost of the receiver (i.e., mobile station) significantly increases due to a complex structure of the receiver, and real-time signal processing is impossible due to a long signal-processing time. As a result thereof, services cannot be rapidly provided. In addition, a load of signal processing exponentially increases in proportion to the number of relay stations in use, thereby decreasing effectiveness.
Meanwhile, there is a data transmission technique using a Space-Time Block Code (STBC) method. The STBC method uses a frame design by considering a maximum multi-path delay time and a maximum time-synchronous error in a condition where transmission paths between a transmitter and all relay stations have been determined. In the STBC method, signals are transmitted by the transmitter and one RS, and specific relay stations are determined to which the signals are transmitted in each hop. As a result, a diversity gain and a power gain are obtained in a receiving end irrespective of a multi-path channel and a time delay. That is, according to the data transmission technique using the STBC method, the spatial diversity gain and the power gain can be obtained through multi-hop transmission in a multi-path fading channel and a channel having a time-synchronous error. Since information on the multi-path fading channel and a range of the time-synchronous error have to be known for each hop transmission, the STBC method has a demerit in that a data transfer rate significantly decreases in proportion to the number of relay stations in use, thereby deteriorating the entire system efficiency.
Accordingly, there is a need for a data transmission technique in which a target mobile station (MS) for receiving data is robust to a synchronous error without having to estimate time synchronization of signals transmitted from each relay station (RS) in an Orthogonal Frequency Division Multiplexing (OFDM) communication system.