In the field of wireless communications, high speed downlink packet transport schemes are being developed, whereby a plurality of communication terminal apparatuses share a high speed and high capacity downlink channel and a base station apparatus transmits packets to the communication terminal apparatuses. High speed downlink packet transport schemes employ scheduling and adaptive modulation techniques for improved transmission rates.
A scheduling technique refers to a technique of having a base station apparatus set the communication terminal apparatus to be the transmission destination (hereinafter referred to as “transmission destination apparatus”) of high speed downlink packets on a per time slot basis and assign the packets to transmit to the transmission destination apparatus. Moreover, an adaptive modulation technique refers to a technique of determining modulation schemes and error correction coding schemes (MCS: Modulation and Coding Scheme) adaptively according to the propagation path conditions of the communication terminal apparatus to transmit the packets to.
Moreover, a wireless communication system that performs high speed packet transport employs ARQ (Automatic Repeat Request), especially H-ARQ (Hybrid-Automatic Repeat Request), for improved data reception performance. ARQ refers to a technique of having the transmitter apparatus automatically perform the process of retransmitting a data unit (e.g., packet) in which an error is detected in the receiver apparatus, and H-ARQ refers to a technique of having the transmitter apparatus select specific bits and transmit them to the receiver apparatus upon retransmission so that the receiver apparatus is able to combine the retransmission signal and an earlier received signal. H-ARQ has the CC (Chase Combining) scheme that applies to packets the same coding as applied in the initial transmission and retransmits these packets, and the IR (Incremental Redundancy) scheme that applies to packets different coding than in the initial transmission and retransmits these packets.
An overview of the operation of a base station apparatus and a communication terminal apparatus that perform high speed packet transport using the scheduling and adaptive modulation techniques will be explained below.
The base station apparatus predicts channel quality based on downlink channel condition report values (e.g., CQI: Channel Quality Indicator) transmitted from each communication terminal apparatus, determines the communication terminal apparatus of the highest channel quality as the transmission destination apparatus, and assigns packets for this transmission destination apparatus in each time slot. In addition, based on the channel quality predicted, the base station apparatus also determines the TBS (Transport Block Size), the number of spreading codes (hereinafter referred to as “the number of codes”), modulation scheme, code rate, and transmission power. Then, the base station apparatus performs the error correction coding and modulation of the packets according to the above-determined specifics and transmits the result to the transmission destination apparatus. The base station apparatus also transmits scheduling information representing the scheduling result to the transmission destination apparatus.
Based on the scheduling information received, each communication terminal apparatus performs demodulation, decoding, and CRC detection in the time slot in which the packets for the apparatus is assigned, according to the modulation scheme and code rate determined in the base station apparatus. When the packet data is accurately decoded, the communication terminal apparatus transmits an ACK signal indicating that to the base station apparatus, and, when the packet data is not decoded accurately, the communication terminal apparatus transmits a NACK signal indicating that to the base station apparatus.
The base station apparatus transmits new data upon receiving the ACK signal and retransmits the same data upon receiving the NACK signal.
Thus, according to the high speed downlink packet transport scheme, all the communication terminal apparatuses in a sector share one channel and transmit packets efficiently, thereby enabling effective use of code resources.
Conventionally, as described above, the base station apparatus determines the TBS, the number of codes, modulation scheme (combination of these will be referred to as “TFRC”: Transport Format and Resource Combination) and transmission power based on CQI's from each communication terminal apparatus. More specifically, these are determined with reference to a table, disclosed, for example, in 3GPP, R1-02-0675, “Revised CQI Proposal” by Motorola, Ericsson. FIG. 1 shows the CQI table disclosed in the above-identified document, in which TFRC's and CQI's are associated. Here, for each CQI, a TFRC to satisfy the FER (Frame Error Rate) of 0.1 in one transmission is determined by simulation.
The above document shows that in simulation the SIR (Signal to Interference Ratio) of an HS-PDSCH (High Speed-Physical Downlink Shared Channel) that will result in the FER of 0.1, is determined by following formula (1):SIR[dB]=−4.5+CQI  (1)
Here, in the case of FIG. 1, a CQI assumes a value between 1 and 30, and −4.5 is a constant, provided that the system adopts HSDPA, a communication terminal apparatus uses a RAKE receiver, and the propagation environment shows the static characteristics of one path, FER=0.1.
To achieve the relationship of (1), the relationship between the transmission power of the HS-PDSCH (PHS-PDSCH) the transmission power of a pilot channel (CPICH: Common Pilot Channel) (PCPICH), and offset Γ satisfy following formula (2).PHS-PDSCH=PCPICH+Γ  (2)
However, referring to FIG. 1, simulation is not performed for all possible TFRC combinations, and so FIG. 1 presents the simulation results only partially. FIG. 2 is FIG. 1 shown in a different chart, in which the CQI's and the numbers of codes are associated on a per modulation scheme basis. In the hatched portion of FIG. 2, it may be possible to set a greater TBS than the TBS indicated in the CQI table of FIG. 2, which corresponds to the problem that the throughput is not yet maximized.
In addition, the transmission power of the HS-PDSCH is determined depending on how the base station apparatus distributes transmission power resources to communication terminal apparatuses. When there is a shortage of transmission power resources, the relationship represented in formula (2) may not always be achieved. Consequently, when formula (2) is not possible, formula (1) is not possible either, and it is not adequate to determine the TFRC from the CQI table based on the CQI's reported from the communication terminal apparatuses without taking into consideration the transmission power of the HS-PDSCH.