In a conventional mobile communication system, there is a problem of an increase of reception errors that results from large deterioration in the quality of uplink signals received by a radio base station and the quality of downlink signals received by a mobile station, because a received signal level in a data receiving-side apparatus instantaneously varies due to multi-path fading and the like.
As a technique for overcoming the aforementioned problem, there is known hybrid ARQ (Auto Repeat reQuest and hereinafter referred to as HARQ).
As illustrated in FIG. 1, in the HARQ, a data receiving-side apparatus (a radio base station Node B or a mobile station UE) transmits a transmission acknowledgement signal (Ack or Nack) to a data transmitting-side apparatus (a mobile station UE or a radio base station Node B), in response to a received transmission data block.
In general, only when receiving a transmission acknowledgement signal (Ack) indicating that a transmission data block (for example, a transmission data block #1) has been correctly received, the data transmitting-side apparatus is configured to transmit a next transmission data block (for example, a transmission data block #2).
On the other hand, when receiving a transmission acknowledgement signal (Nack) indicating that a transmission data block has not been correctly received, the data transmitting-side apparatus is configured to transmit the transmission data block again.    [Non-Patent Document 1] “W-CDMA Mobile Communication System”, edited by Keiji Tachikawa, Maruzen Co., Ltd.    [Non-Patent Document 2] 3GPP TR25.896 v6.0.0
The HARQ is excellent in the point that a transmission data block can be transmitted to the data receiving-side apparatus without fail. However, it has a disadvantage that a transmission acknowledgement signal (Ack or Nack) is transmitted over a radio link in the opposite direction (a downlink is used when transmitting the transmission data block over an uplink, and the uplink is used when transmitting the transmission data block over the downlink), which results in an increase in load on the radio link in the opposite direction.
Moreover, in the mobile communication system, there is known a technique for controlling a transmission rate of a signal according to an availability of a radio link, a radio quality, and the like (technique for determining a transmission data block size).
For example, as a system to which the technique is to be applied, there are known “HSDPA (High Speed Downlink Packet Access) and “EUL (Enhanced Uplink)” that are being standardized by the 3GPP. Furthermore, in the system, the HARQ is to be applied.
In the system, the data receiving-side apparatus is configured to send a one-bit transmission acknowledgement signal indicating Ack or Nack, regardless of a transmission block data size.
In other words, when a transmission data block size is large, the load of a one-bit transmission acknowledgement signal indicating Ack or Nack is appropriate for its purpose, but when a transmission data block size is small, the load of the one-bit transmission acknowledgement signal indicating Ack or Nack is not an allowable condition.
Moreover, consider that the sums of transmission data block sizes in certain radio links are equal to each other. When each transmission data block size is large, only a small number of transmission data blocks are transmitted, so that only a small number of transmission acknowledgement signals are generated.
On the other hand, when each transmission data block size is small, a large number of transmission data blocks are transmitted, so that a large number of transmission acknowledgement signals are transmitted. This causes a problem that the radio load on the radio link in the opposite direction is increased.
Furthermore, as mentioned above, an influence on the mobile communication system caused by the load on the radio link in the opposite direction depends on the congestion degree of the radio link in the opposite direction.
Specifically, when the congestion degree of the radio link in the opposite direction is small, the influence on the mobile communication system caused by the load on the radio link in the opposite direction is small. However, when the congestion degree of the radio link in the opposite direction is large, there is a problem that the influence on the mobile communication system caused by the load on the radio link in the opposite direction becomes large.