In 3GPP (3rd Generation Partnership Project), the HSDPA (High Speed Downlink Packet Access) standard for W-CDMA mobile communication has been standardized (see non-patent document 1).  In HSDPA, the MAC-hs protocol or MAC-ehs protocol is used for the MAC (Medium Access Control) layer.  HSDPA provides packet-based high-speed data communication on a downlink from an RNC (radio network controller) to a UE (user equipment) via a Node-B. In HSDPA data communication, flow control is performed between the RNC (radio network controller) and the Node-B (base station). 
In the flow control, the Node-B notifies the RNC of the data capacity, and the RNC transmits data within the data capacity to the Node-B. Here, the Node-B determines the data capacity, taking into consideration, e.g., the capacity of the radio channel, the product quality report provided by the UE, the priority allocated to the bearer, and the state of the transmission path between the RNC and the Node-B as parameters.  Notification of the data capacity is provided via a frame protocol control message called CAPACITY ALLOCATION. 
In the HSDPA data communication, there are three types of cases contemplated for communication modes.  Parameters conforming to each case are set for RNCs and Node-Bs. 
FIG.  1 is a chart illustrating an example of parameter settings for the respective cases of HSDPA.  Referring to FIG.  1, examples of parameter settings for respective cases 1 to 3 are illustrated.  Case 1 has already been defined in 3GPP Release 5 onwards, and cases 2 and 3 are expected to be defined in 3GPP Release 7 onwards. 
In case 1, the size of PDUs (protocol data units) in the RLC (radio link control) layer (hereinafter referred to as “RLC PDU size” has a fixed length, and for the MAC layer, the MAC-hs protocol is used.  A PDU is a unit of a transmit signal in a predetermined protocol.  For example, a PDU includes a header according to a predetermined protocol and a payload including data in the protocol. 
In the MAC-hs protocol, neither 64QAM (Quadrature Amplitude Modulation) nor MIMO (Multiple Input Multiple Output) are used. 
In case 2, the RLC PDU size has a fixed length as in case 1, but the MAC-ehs protocol is used for the MAC layer.  In the MAC-ehs protocol, 64QAM and MIMO can be used.  Also in the MAC-ehs, a transmission method called Improved Layer 2 in Downlink is used. 
64QAM, which is one of digital modulation methods, expresses 64 values through a combination of eight phase types and eight amplitude types.  MIMO is a radio communication technique for expanding a data communication band using a plurality of antennas simultaneously.  In Improved Layer 2, the MAC-ehs protocol provided in Node-B segments user data.  Improved Layer 2 enables more efficient data transfer compared to a transmission method in which user data is divided by a fixed length in an RLC. 
In case 3, the RLC PDU size has a variable length, and for the MAC layer, the MAC-ehs protocol is used.  In this case, a Node-B designates a maximum length of the RLC PDU size.  An RNC can select an RLC PDU size within a range equal to or less than the maximum length designated by the Node-B. In flow control, the Node-B can control the maximum value of the RLC PDU size. 
In flow control in 3GPP Release 7 into which the MAC-ehs protocol has been introduced, a format called CAPACITY ALLOCATION TYPE 2 is used instead of a format called CAPACITY ALLOCATION TYPE 1 that is used in 3GPP Release 5. 
With a frame in CAPACITY ALLOCATION TYPE 2, a Node B can control the following four elements. 
(1) Maximum MAC-d/c PDU Length (MAC-d PDU length)
(2) HS-DSCH Credit (the number of MAC-d PDUs that can be transmitted during an interval of transmission in an HS-DSCH)
(3) HS-DSCH Interval (duration in which the number of MAC-d PDUs indicated by the HS-DSCH credit are transmitted)
(4) HS-DSCH Repetition Period (repetition count indicating the number of repetitions of the above duration)
For example, where a radio channel is going into a congestion, the MAC-d/c PDU Length (Maximum MAC-d/c PDU length) may be reduced or the HS-DSCH credit may be reduced in order to suppress the downlink data amount.  An HS-DSCH (High-Speed Downlink Shared Channel) is a channel shared by a plurality of HSDPA data communications. 
As described above, in cases 2 and 3, which are to be defined in 3GPP Release 7 onwards, 64QAM and MIMO, which could not used in and before 3GPP Release 6, can be used. 
Between cases 2 and 3 to be defined in 3GPP Release 7 onwards, there is a difference in whether the RLC PDU size has a fixed length or a variable length. 
In case 3, since the RLC PDU size is variable, the maximum value of the RLC PDU size can be changed in a range equal to or less than 1504 octets in flow control.  As a result of such flow control, more efficient data communication can be provided according to the changing communication status. 
Meanwhile, case 2 enables use of 64QAM and MIMO while performing flow control using an existing and simple algorithm with the RLC PDU size fixed as in case 1.