In the next-generation wireless relay system which demands high-speed transmission of 100 M to 1 G bits per second, it is assumed that a high frequency band is allocated. However, in general, a signal with a high frequency band has a strong straight-line property, compared to a signal with a low frequency band, and it is known that many dead areas where radio waves do not arrive are formed. Therefore, when the transmission power of a base transceiver station is the same as that of a currently available wireless relay system, the coverage of a cell (service area) will decrease when a high frequency band is allocated. The decrease in the coverage of a cell is not preferable, not only because the cost increases with an increase in the number of base transceiver stations but also because handover frequently occurs.
Accordingly, a wireless relay system has been suggested which includes a relay station RN (Relay Node) serving as a wireless relay device that relays wireless communication between a base transceiver station and a mobile station. In general, the cost of a relay station RN is lower than that of a base transceiver station BTS (Base Transceiver Station). Therefore, by introducing a relay station RN, it is possible to reduce the cost of the entire system, while ensuring a sufficient coverage.
As the relay station RN, a so-called amplify-and-forward (AF) type relay station and a so-called decode-and-forward (DF) type relay station are known. For example, the AF type relay station RN amplifies a signal received from a base transceiver station BTS and transmits the amplified signal to a mobile station UE (User Equipment). For example, the DF type relay station RN demodulates and decodes a signal received from a base transceiver station BTS, codes and modulates the signal to reform its frames, amplifies the signal, and transmits the amplified signal to the mobile station UE. The DF type relay station RN can realize the property of a more satisfactory reception error ratio in a receiving device (a base transceiver station or a mobile station), compared to the AF type relay station RN.
To retransmit data from the relay station RN to a receiving device, the relay station RN stores transmitted data in a retransmission buffer, for example, until the relay station RN acquires a signal (ACK) indicating reception completion from the receiving device. When a signal (NACK/DTX) indicating non-completion of the reception from the receiving device is feedback, the data is retransmitted to the receiving device. Therefore, for example, in the relay of the wireless communication from the base transceiver station BTS to the mobile station UE, the retransmission buffer may store many transmission data when there are abundant mobile stations UE (receiving devices) each of which is a relay destination. Therefore, there is a concern that an overflow in the retransmission buffer may occur.
In regard to the control of a buffer memory corresponding to the retransmission buffer, there is a known technology for controlling data in the buffer memory and transmission of the data depending on the use state of the buffer memory. According to this technology, when the amount of data accumulated in the buffer memory is equal to or greater than a predetermined level, the data in the buffer memory is controlled such that the data is released (removed). Further, according to this technology, when the amount of data accumulated in the buffer memory is less than the predetermined level, the data in the buffer memory is controlled such that the data is rarely accumulated in the buffer memory by reducing the amount of data transmitted from a transmitting device. For the related technology, see Japanese Laid-open Patent Publication No. 2003-319458, for example.
In the future, however, the capacity of a retransmission buffer will be reduced in order to miniaturize the relay station RN with the popularization of the relay station RN. In the above-described conventional technology, however, the data is merely controlled depending on whether the amount of data accumulated in the buffer memory is equal to or greater than the predetermined level, and the reduced capacity of the buffer memory is not considered. Further, when high-priority data, such as data with high instantaneousness, is removed from the buffer memory, the data is retransmitted from the transmitting device. Therefore, there is a problem that a delay occurs until the data is received by the receiving device.