Radio communication method in radio communication system, terminal apparatus, base station apparatus, and radio communication system

A radio communication method in a radio communication system which performs radio communication between a terminal apparatus and a base station apparatus, the radio communication method including: selecting one of a first transmission method or a second transmission method on the basis of transmission power of transmission signal transmitted from the terminal apparatus, in the base station apparatus; and transmitting the transmission signal to the base station apparatus by the selected first or second transmission method, in the terminal apparatus.

TECHNICAL FIELD

The present invention relates to a radio communication method, a terminal apparatus, a base station apparatus, and a radio communication system.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project), LTE (Long Term Evolution, or Evaluated UTRA and UTRAN) is under investigation as a next-generation radio communication standard (Non-Patent Document 1 illustrated below, for example).

In LTE, OFDM (Orthogonal Frequency Division Multiplexing) is utilized for a downlink from a base station to a terminal, and SC-FDMA (Single-Carrier Frequency Division Multiple Access) is utilized for an uplink from the terminal to the base station.

The OFDM is a transmission method in which a frequency band is divided into a plurality of sub-carriers and data are transmitted while carried directly on each sub-carrier. On the other hand, the SC-FDMA is a transmission method in which data transformed by DFT (Discrete Fourier Transform) is carried on the sub-carrier and is transmitted.FIG. 18andFIG. 19illustrate configuration examples of signal processing circuits employed in SC-FDMA and OFDM, respectively. Referring toFIG. 18, a DFT (Discrete Fourier Transform) unit101is included in front of a sub-carrier mapping unit, and DFT-processed signal is input successively into the sub-carrier mapping unit102, an IDFT (Inverse Discrete Fourier Transform) unit103, and a CP (Cyclic Prefix) insertion unit104. Referring toFIG. 19, transmission data is input into a sub-carrier mapping unit111and then input successively into an IDFT unit112and a CP insertion unit113.

On the other hand, the base station or the terminal uses an amplifier to transmit data. The amplifier has a problem that linearity cannot be maintained and the data are distorted, when an input power is large. When the data are distorted, out-of-band emission power increases. An upper limit value of the out-of-band emission power (hereinafter, “ACLR”) is determined by an ACLR (Adjacent Carrier Leakage Ratio) standard, and when the data distortion is large, the ACLR can no longer be satisfied.

In consideration of the ACLR, the SC-FDMA is a favorable method due to its low PAPR (Peak to Average Power Ratio), and therefore SC-FDMA is applied to the uplink from the terminal in LTE.Non-Patent Document 1: 3GPP TS 36. 211V8. 0. 0 (2007-09)Non-Patent Document 2: Hikmet Sari, Geroges Karam and Isabell Jeanclaude, “Transmission Techniques for Digital Terrestrial TV Broadcasting”, IEEE Communication Magazine, pp. 100-109, February 1995

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

Although the SC-FDMA is advantageous in terms of the PAPR, because the sub-carrier which is continuous on a frequency axis is used, the sub-carrier cannot be selected non-continuously on the frequency axis, and the SC-FDMA has a constraint in terms of scheduling during resource allocation and the like. Also, as illustrated inFIG. 20(Non-Patent Document 2, for example), errors are likely to occur in relation to another method even under an identical reception E/N condition.

Accordingly, it is an object of the present invention to improve inconvenient which occurs when the SC-FDMA is applied.

More preferably, it is an object to improve in consideration of scheduling flexibility or quality when the SC-FDMA is applied.

Means for Solving the Problem

According to an aspect of the present invention, a radio communication method in a radio communication system which performs radio communication between a terminal apparatus and a base station apparatus, the radio communication method including: selecting one of a first transmission method or a second transmission method on the basis of transmission power of transmission signal transmitted from the terminal apparatus, in the base station apparatus; and transmitting the transmission signal to the base station apparatus by the selected first or second transmission method, in the terminal apparatus.

Also, according to an another aspect of the present invention, a radio communication method in a radio communication system which performs radio communication between a terminal apparatus and a base station apparatus, the radio communication method including: selecting a first transmission method if the terminal apparatus transmits transmission signal by MIMO or selecting a second transmission method if not, in the base station apparatus; and transmitting the transmission signal to the base station apparatus by the selected first or second transmission method, in the terminal apparatus.

Furthermore, according to an another aspect of the present invention, a terminal apparatus for performing radio communication with a base station apparatus, the terminal apparatus including: a reception unit which receives from the base station apparatus transmission method selection information indicating selected transmission method, which is selected one of a first transmission method or a second transmission method on the basis of transmission power of transmission signal transmitted from the terminal apparatus; and a transmission unit which transmits the transmission signal to the base station apparatus by the first or second transmission method on the basis of the transmission method selection information.

Furthermore, according to an another aspect of the present invention, a terminal apparatus for performing radio communication with a base station apparatus, the terminal apparatus including: a reception unit which receives from the base station apparatus transmission method selection information indicating that a first transmission method is selected if the terminal apparatus transmits by MIMO or a second transmission method is selected if not; and a transmission unit which transmits the transmission signal to the base station apparatus by the first or second transmission method on the basis of the transmission method selection information.

Furthermore, according to an another aspect of the present invention, a base station apparatus for performing radio communication with a terminal apparatus, the base station apparatus including: a selection unit which selects one of a first transmission method or a second transmission method on the basis of transmission power of transmission signal transmitted from the terminal apparatus; and a transmission unit which transmits transmission method selection information indicating the selected first or second transmission method to the terminal apparatus, wherein the terminal apparatus transmits the transmission signal by the selected first or second transmission method.

Furthermore, according to an another aspect of the present invention, a base station apparatus for performing radio communication with a terminal apparatus, the base station apparatus including: a selection unit which selects a first transmission method if the terminal apparatus transmits transmission signal by MIMO or selects a second transmission method if not; and a transmission unit which transmits transmission method selection information indicating the selected first or second transmission method to the terminal apparatus, wherein the terminal apparatus transmits the transmission signal by the selected first or second transmission method.

Furthermore, according to an another aspect of the present invention, a radio communication system, including: a terminal apparatus; and a base station apparatus, wherein radio communication is performed between the terminal apparatus and the base station apparatus, the base station apparatus includes: a selection unit which selects one of a first transmission method or a second transmission method on the basis of transmission power of transmission signal transmitted from the terminal apparatus; and a transmission unit which transmits transmission method selection information indicating the selected first or second transmission method, and the terminal apparatus includes: a reception unit which receives the transmission method selection information; and a transmission unit which transmits the transmission signal to the base station apparatus by the first or said second transmission method on the basis of the transmission method selection information.

Furthermore, according to an another aspect of the present invention, a radio communication system, including: a terminal apparatus; and a base station apparatus, wherein radio communication is performed between the terminal apparatus and the base station apparatus, the base station apparatus includes: a selection unit which selects a first transmission method if the terminal apparatus transmits transmission signal by MIMO, or selects a second transmission method if not; and a transmission unit which transmits transmission method selection information indicating the selected first or second transmission method, and the terminal apparatus includes: a reception unit which receives the transmission method selection information; and a transmission unit which transmits the transmission signal to the base station apparatus by the first or second transmission method on the basis of the transmission method selection information.

Furthermore, according to an another aspect of the present invention, a radio communication system, including: a terminal apparatus; and a base station apparatus, wherein radio communication is performed between the terminal apparatus and the base station apparatus, the base station apparatus or the terminal apparatus includes: a modulation unit which is adaptable to a plurality of transmission method selected in accordance with magnitude of transmission power; and a transmission unit which transmits signal modulated by the modulation unit, and the plurality of transmission method include SC-FDMA method and OFDM method, and a selection is performed to switch from the OFDM method to the SC-FDMA method in response to increase in the transmission power.

Advantageous Effect of the Invention

According to the present invention, the inconvenient which occurs when the SC-FDMA is applied, can be improved.

EXPLANATION OF REFERENCE NUMERALS

BEST MODE FOR CARRYING OUT THE INVENTION

Best mode for carrying out the present invention will be described below.

First, a first embodiment will be described.FIG. 1illustrates a configuration example of a radio communication system1. The radio communication system1includes terminal apparatuses (“terminals” hereinafter)10-1to10-3and base station apparatuses (“base stations” hereinafter)50-1to50-4. Dotted lines indicate cell ranges of the respective base stations50-1to50-4. When the terminals10-1to10-3are positioned in a cell, the terminals10-1to10-3can perform radio communication with the corresponding base stations50-1to50-4.

FIG. 2andFIG. 3illustrates configuration examples of the terminal10and the base station50, respectively. The terminal10includes a known signal reception unit11, a path loss calculation unit12, a transmission power calculation unit13, a known signal transmission unit14, a Δ(maximum power−current power) transmission unit (Δ transmission unit hereinafter)15, a transmission data buffer16, a scheduling request transmission unit17, a transmission method reception unit18, a data signal modulation unit19, and a data signal transmission unit20.

The known signal reception unit11receives a known signal from the base station50and outputs the received known signal to the path loss calculation unit12. For example, the known signal is transmitted periodically from the base station50.

The path loss calculation unit12calculates a downlink direction propagation path loss (a path loss PL) relative to the base station50on the basis of the known signal, and outputs the calculated path loss PL to the transmission power calculation unit13.

The transmission power calculation unit13calculates a transmission power on the basis of the path loss PL and so on. A following equation is used in the calculation.

Here, Ptindicates a data transmission power of the terminal10envisaged in accordance with the position of the terminal10, Pmaxis a maximum transmission power determined from capacity of the terminal10, PL is the path loss, and PLx-ileand Rminare constants of power control. The maximum transmission power Pmaxand the two constants PLx-ile, Rminare stored in a memory, for example, the transmission power calculation unit13reads from the memory, and calculates together with the path loss PL from the path loss calculation unit12.

The known signal transmission unit14transmits the known signal periodically to the base station50, for example.

The Δ transmission unit15calculates a difference Δ between the maximum transmission power Pmaxand the transmission power Pt, and transmits the calculated difference Δ to the base station50. The difference Δ indicates a decrease width from the maximum transmission power Pmaxcorresponding to a current position of the terminal10. Note that the difference Δ may be calculated by the transmission power calculation unit13.

The transmission data buffer16stores transmission data from an application unit or the like.

The scheduling request transmission unit17transmits a scheduling request to the base station50when transmission data is transmitted. The scheduling request transmission unit17calculates data amount of the transmission data stored in the transmission data buffer16or the like, and transmits the scheduling request including the data amount and a data rate.

The transmission method reception unit18receives a transmission method transmitted from the base station50and outputs to the data signal modulation unit19.

The data signal modulation unit19reads the transmission data from the transmission data buffer16and modulates the transmission data on the basis of the transmission method from the transmission method reception unit18.

The data signal transmission unit20transmits the modulated transmission data to the base station50.

On the other hand, as illustrated inFIG. 3, the base station50includes a known signal transmission unit51, a Δ (maximum power−current power) reception unit (Δ reception unit hereinafter)52, a scheduling request reception unit53, an MPR (Maximum Power Reduction) table54, a transmission method determination unit55, a transmission method transmission unit56, and a data reception unit57.

The known signal transmission unit51transmits the known signal to the terminal10periodically, for example.

The Δ reception unit52receives the difference Δ from the terminal10and outputs to the transmission method determination unit55.

The scheduling request reception unit53receives the scheduling request from the terminal10and outputs to the transmission method determination unit55.

The MPR table54stores respective values of a transmission method (OFDM or SC-FDMA), a modulation scheme (QPSK, 16QAM, and so on), a number of resource blocks (a number of sub-carriers that can be allocated on a frequency axis), and a reduction amount (a transmission power reduction amount hereinafter) Prfrom the maximum transmission power of the terminal10.

The terminal10includes an amplifier to transmit the transmission data, and the transmission power reduction amount Pris a value indicating a decrease width by which the transmission power must be reduced from the maximum transmission power in order to satisfy so-called ACLR (the upper limit value of out-of-band emission power) due to constraint of the amplifier in the terminal10.

FIG. 4illustrates an example of the MPR table54. As illustrated inFIG. 4, the value of the transmission power reduction amount Prdiffer in accordance with the transmission method, the modulation scheme, and the number of resource blocks. The reason is that transmission waveform transmitted from the terminal10differs according to the transmission method and so on, and the transmission power reduction amount Prtakes different value according to the transmission waveform. If the transmission method is different even if the modulation scheme and the number of resource brocks are same, the transmission power reduction amount Pris different. The reason is that in OFDM, the PAPR of the transmission power is larger than in SC-FDMA, and therefore the transmission power must be reduced to satisfy the ACLR.

Returning toFIG. 3, the transmission method determination unit55determines the transmission method by selecting one of OFDM and SC-FDMA in accordance with the difference Δ and a maximum value of the transmission power reduction amount Prread from the MPR table54. Determination processing will be described below. The transmission method determination unit55performs the determination processing when the scheduling request reception unit53receives the scheduling request, for example.

The transmission method transmission unit56transmits the determined transmission method. The terminal10transmits the transmission data on the basis of the transmission method (seeFIG. 2).

The data reception unit57receives the transmission data from the terminal10and performs reception processing on the basis of the transmission method.

Next, the transmission method determination processing will be described in detail.FIG. 5illustrates an example of a sequence diagram indicating an overall operation, andFIG. 6illustrates a flowchart indicating an example of the transmission method determination processing.

First, the known signal transmission unit51of the base station50transmits the known signal to the terminal10(S10).

Next, the transmission power calculation unit13of the terminal10calculates the difference Δ between the maximum transmission power Pmaxand the transmission power Ptof the terminal10corresponding to its position (S11).

Next, the scheduling request transmission unit17of the terminal10transmits the scheduling request (S12). The Δ transmission unit15transmits the difference Δ at the transmission timing of the scheduling request. The Δ transmission unit15outputs the calculated difference Δ to the scheduling request transmission unit17, and the scheduling request transmission unit17may transmit the scheduling request including the difference Δ.

Next, the transmission method determination unit55of the base station50determines the transmission method (S13).

Next, the processing shifts to the transmission method determination processing (FIG. 6), in which the transmission method determination unit55compares the maximum value of the transmission power reduction amount Prwith the difference Δ (S20). If the maximum value of the transmission power reduction amount Pris larger than the difference Δ, the transmission method determination unit55selects SC-FDMA (S21). On the other hand, if the maximum value of the transmission power reduction amount Prand the difference Δ are identical or the difference Δ is larger than the maximum value of the transmission power reduction amount Pr, the transmission method determination unit55selects OFDM (S22).

Alternatively, the transmission method determination unit55may select SC-FDMA if the base station50detects that the transmission power of the mobile station exceeds a predetermined threshold, and may select OFDM if the base station50detects that the transmission power of the mobile station is smaller than the predetermined threshold.

The reason for comparing the two values in this manner will now be described with reference toFIG. 7AandFIG. 7B.FIG. 7Aillustrates an example in which the transmission power is set on the ordinate and the maximum value of the transmission power reduction amount Pris larger than the difference Δ.FIG. 7Billustrates an opposite example.

As described above, the difference Δ indicates the transmission power decrease width from the maximum transmission power corresponding to the position of the terminal10. On the other hand, (the maximum value of) the transmission power reduction amount Prindicates (a maximum value of) the decrease width by which the transmission power must be reduced from the maximum transmission power due to the constraints of the amplifier in order to satisfy the linearity of the amplifier in the terminal10and thereby satisfy the ACLR (the upper limit value of the out-of-band emission power). If (the maximum value of) the transmission power reduction amount Pris larger than the difference Δ (seeFIG. 7A), this indicates that the terminal10should be capable of transmission at the decrease width Δ in accordance with its position, but the terminal10may transmit by excess reduced power due to the constraints of the amplifier.

A case in which the transmission power is reduced further due to the constraints of the amplifier corresponds to a case in which the terminal10is far from the base station50. In other words, as described in the prior art, the PAPR is larger in OFDM than in SC-FDMA, and therefore, in OFDM, an average transmission power must be reduced below that of SC-FDMA in order to satisfy the linearity of the amplifier and thereby satisfy the ACLR standard. If the terminal10is positioned far from the base station50, data is transmitted at the maximum transmission power as much as possible in order to increase a reception characteristic of the base station50.

However, since the PAPR is large in OFDM, the average transmission power must be reduced to satisfy the linearity of the amplifier. If there is a case where the transmission power must be reduced in accordance with OFDM, SC-FDMA has better the reception characteristic of the base station50, in which the average transmission power is large, than OFDM.

Hence, if the transmission power is reduced further due to the constraints of the amplifier, or in other words if the maximum value of the transmission power reduction amount Pris larger than the difference Δ (FIG. 7A), the transmission method determination unit55selects SC-FDMA as the transmission method.

On the other hand, if the difference Δ is equal to or greater than (the maximum value of) the transmission power reduction amount Pr(FIG. 7B), the power decrease width Δ corresponding to the position is equal to or greater than (the maximum value of) the transmission power reduction width Prfrom the constraints of the amplifier, and therefore the transmission power is reduced sufficiently to satisfy the constraints of the amplifier. If the transmission power can be reduced in this manner, data can be transmitted sufficiently even if the terminal10is close to the base station50, and even if data are transmitted using OFDM having a high PAPR, both the linearity of the amplifier and the ACLR are satisfied.

Hence, if the transmission power is low, or in other words if the transmission power reduction amount Pris equal to or smaller than the difference Δ, the transmission method determination unit55selects OFDM. By the selection of OFDM, a radio characteristic is improved in comparison with SC-FDMA and scheduling is flexible.

In the first embodiment, the transmission method determination unit55reads from the MPR table54the maximum value (4.5 dB in the example illustrated inFIG. 4) of the transmission power reduction amount Pr. Alternatively, the maximum value of the transmission power reduction amount Pris stored in the MPR table54alone as a threshold. The transmission method determination unit55may then compare the threshold with the difference Δ.

Returning toFIG. 5, the transmission method transmission unit56of the base station50notifies the determined transmission method to the terminal10(S14).

The data signal modulation unit19of the terminal10modulates the transmission data in accordance with the notified transmission method (S15).

Next, the data signal transmission unit20of the terminal10transmits the data signal to the base station50(S16).

Next, the data reception unit57of the base station50demodulates the data signal in accordance with the selected transmission method (S17). The series of processes is then terminated.

Hence, in this embodiment, data are not transmitted uniformly by SC-FDMA on the uplink, and the data may be transmitted after switching to OFDM, for example. OFDM has better radio characteristic than SC-FDMA, and therefore an improvement in the radio characteristic can be achieved in comparison with a case in which the data are transmitted uniformly by SC-FDMA.

Further, with OFDMA, resource allocation scheduling not using sub-carriers that are continuous on the frequency axis can be performed, and therefore scheduling flexibility can be secured in comparison with a case in which the data are transmitted uniformly by SC-FDMA.

As a result, an improvement in throughput can be achieved.

Note that in the example described above, the determinations as to whether or not the transmission power of the terminal10has exceeded the predetermined threshold and whether or not the maximum value of the transmission power reduction amount Pris larger than the difference Δ are made in the base station50, but the mobile station may include the transmission method determination unit55.

By inputting the difference Δ and the transmission power as is into the transmission method determination unit55of the mobile station from the transmission power calculation unit13, the transmission method can be determined by the mobile station.

More specifically, when the transmission method determination unit55of the mobile station detects that its own transmission power has exceeded the predetermined threshold or that the maximum value of the transmission power reduction amount Pris larger than the difference Δ, the transmission method determination unit55controls the data modulation unit19such that transmission is performed using the SC-FDMA method.

Also, when the transmission method determination unit55of the mobile station detects that its own transmission power is lower than the predetermined threshold or that the maximum value of the transmission power reduction amount Pris smaller than the difference Δ, the transmission method determination unit55controls the data modulation unit19such that transmission is performed using the OFDM method.

Preferably, before switching the method, the base station50can notify the switch destination method (the SC-FDMA method or the OFDM method) before the method is switched by making the switch destination method transmit to the base station50from the data signal transmission unit20. Even if the notification is not performed, the switch destination method can be detected by having the base station50perform reception processing in relation to both methods respectively.

Further, an embodiment in which the positions of the base station and the mobile station are interchanged may be applied.

Next, a second embodiment will be described. In the first embodiment, the transmission method determination unit55compares the maximum value of the transmission power reduction amount Prwith the difference Δ. In the second embodiment, the transmission method is determined by comparing the transmission power reduction amount Prwith the difference Δ, after selecting the modulation method and the number of resource blocks, and reading corresponding items from the MPR table54.

The overall configuration of the radio communication system1and the respective configurations of the terminal10and the base station50are identical to those of the first embodiment (seeFIG. 1toFIG. 3). Further, the processing up to the point at which the base station50receives the scheduling request from the terminal10(S12ofFIG. 5) is similar to that of the first embodiment.

The transmission method determination unit55inputs the scheduling request from the scheduling request reception unit53, and performs the transmission method determination processing (S13).

FIG. 8illustrates flowchart indicating operational example of the transmission method determination processing, andFIG. 9illustrates an example of the MPR table54.

When the transmission method determination unit55selects the transmission method (S30), the transmission method determination unit55selects on the basis of a determined format (the modulation scheme and the number of resource blocks) (S31).

For example, the transmission method determination unit55determines a format in which the modulation scheme is “16QAM” and the number of resource blocks is “1”. The transmission method determination unit55then reads corresponding items from the MPR table54.FIG. 9illustrates an example of the MPR table54indicating the corresponding items. The transmission method determination unit55then reads the transmission power reduction amount Prof the OFDM method from the corresponding items. In the example illustrated inFIG. 9, the transmission power reduction amount Pris “3”. The transmission method determination unit55compares the read transmission power reduction amount Pr(=“3”) of the OFDM method with the difference Δ, and then selects SC-FDMA if the transmission power reduction amount Pris greater than the difference Δ, and selects OFDM if the transmission power reduction amount Pris equal to or smaller than the difference Δ, similarly to the first embodiment (S32). Subsequent processing is similar to that of the first embodiment.

The reason why the transmission method determination unit55reads the transmission power reduction amount Prof OFDM, from among the two transmission power reduction amounts Prof OFDM and SC-FDMA, is that the power reduction amount Prof OFDM is larger than that of SC-FDMA and strict condition becomes standard.

Note that in the second embodiment, the format may be determined by the scheduling request reception unit53rather than the transmission method determination unit55. In this case, the scheduling request reception unit53outputs the determined format to the transmission method determination unit55, and the transmission method determination unit55performs the processing described above on the basis of the format.

Next, a third embodiment will be described. The transmission method is selected further taking the transmission bit count into consideration in the third embodiment in comparison with the second embodiment.

The terminal10transmits to the base station50the scheduling request including a data amount (the transmission bit count) (FIG. 2, S12ofFIG. 5). If the transmission bit count is small, the terminal10can reduce the transmission power further. The transmission method determination unit55sets a decrease width corresponding to the transmission bit count as Δ1and determines the transmission method by comparing (Δ+Δ1) (decrease width (Δ+Δ1) hereinafter) with the transmission power reduction amount Pr.

The configurations of the radio communication system1, the terminal10, and the base station50according to the third embodiment are similar to those of the first embodiment. However not that the transmission method determination unit55determines the power decrease width Δ1corresponding to the transmission bit count. For example, the determination is such that the transmission method determination unit55includes a table of decrease widths Δ1corresponding to transmission bit counts and reads the decrease width Δ1corresponding to the transmission bit count from the table of decrease width Δ1. Alternatively, the transmission method determination unit55stores inside a formula for calculating the decrease width from the transmission bit count, may calculates and determine the decrease width Δ1from the formula. Alternatively, the base station apparatus50further includes a transmission bit count table60as illustrated inFIG. 11, and the transmission method determination unit55may read the decrease width Δ1corresponding to the transmission bit count.

FIG. 10illustrates a flowchart indicating an example of transmission method determination processing according to the third embodiment. This processing is similar to the processing of the first embodiment up to the point at which the base station50receives the scheduling request.

If the transmission method determination unit55inputs the scheduling request from the scheduling request reception unit53, the transmission method determination unit55determines the power decrease width Δ1on the basis of the transmission bit count included in the scheduling request (S41) and selects the transmission method corresponding to the format in the similar manner to the second embodiment (S40, S42).

And, if the power reduction amount Prof OFDM is larger than the decrease width (Δ+Δ1), the transmission method determination unit55selects SC-FDMA, and if the power reduction amount Prof OFDM is not larger than the decrease width (Δ+Δ1), the transmission method determination unit55selects OFDM (S43). In other words, if the amount of transmission data is small enough to be transmitted at a low transmission power, OFDM is selected, and if not, SC-FDMA is selected. Subsequent processing is similar to that of the first embodiment.

Further, note that in the third embodiment, an encoding ratio may be used in addition to the transmission bit count. The transmission bit count table60stores decrease widths Δ1corresponding to encoding ratios. When the scheduling request reception unit53receives the scheduling request, the scheduling request reception unit53determines the encoding ratio and outputs to the transmission method determination unit55. The transmission method determination unit55then determines the transmission method by reading the decrease width Δ1corresponding to the encoding ratio from the table60.

Next, a fourth embodiment will be described. In the fourth embodiment, if the base station50receives an instruction to reduce the power of the terminal10from a network (another base station, for example, although the network may be the base station50itself), the transmission method is determined taking into consideration a power decrease width Δ2corresponding to the instruction.

The instruction is also known as an Overload Indicator, and if the transmission power of the terminal10is large such that interference is applied to the terminal of another cell, the transmission power of the terminal10is reduced in accordance with the instruction.

The configuration of the radio communication system1and the terminal10is similarly to their counterparts in the first embodiment.FIG. 12illustrates a configuration example of the base station50. As illustrated in theFIG. 12, the base station50includes a network reception unit70so as to be capable of receiving the instruction (Overload Indicator) from another base station via the network.

FIG. 13illustrates a flowchart indicating an example of the transmission method selection processing. The processing up to the point at which the base station50receives the scheduling request (S12inFIG. 5) is similar to that of the first embodiment.

If the network reception unit70receives the instruction from another base station, the network reception unit70outputs the power decrease width Δ2corresponding to the instruction (S50). For example, the network reception unit70A includes inside a table, and reads the corresponding decrease width Δ2from the table and outputs.

The transmission method determination unit55determines the format in a similar manner to the second embodiment (S52), reads the corresponding items from the MPR table54, compares the decrease width (Δ+Δ2) with the transmission power reduction amount Prof OFDM, and determines the transmission method (S51, S53).

More specifically, if the power reduction amount Prof OFDM is larger than the decrease width (Δ+Δ2), the transmission method determination unit55selects SC-FDMA, and if the power reduction amount Prof OFDM is not larger than the decrease width (Δ+Δ2), the transmission method determination unit55selects OFDM (S53). In other words, if transmission can be performed at a sufficiently low transmission power in accordance with the instruction, OFDM is selected, and if not, SC-FDMA is selected. Subsequent processing is similar to that of the first embodiment and so on.

Next, a fifth embodiment will be described. The fifth embodiment is an example of a case in which the transmission method is determined according to whether or not the terminal10performs MIMO (Multiple-Input Multiple-Output) transmission.

MIMO is a method for obtaining a transmission signal by receiving transmission signal transmitted from a plurality of transmission antennae in a single reception antenna and synthesizing the reception signal such that the reception signal is canceled. MIMO is used to obtain further throughput in an environment having a favorable reception SIR (Signal to Interference Ratio).

However, if transmission is performed using SC-FDMA, the reception signal is processed on a reception side by using a frequency equalizer, and due to the frequency equalizer, inter-stream interference cannot be eliminated. As a result, weighting coefficients relating respectively to the MIMO inter-stream interference and multipath interference become contradictory, and problem to deteriorate the reception signal characteristic occurs.

On the other hand, in OFDM, the reception side may not use the frequency equalizer and the sub-carrier is orthogonal, and therefore multipath interference does not occur during reception signal processing even if any weighting coefficient is used. Accordingly, reception can be performed using the weighting coefficient for eliminating the inter-stream interference of MIMO.

Hence, the transmission method determination unit55according to the fifth embodiment selects OFDM if MIMO transmission is to be performed, and selects SC-FDMA if MIMO transmission is not to be performed (S60to S62inFIG. 14).

The scheduling request transmission unit17of the terminal10transmits the scheduling request including an information indicating whether or not MIMO transmission is to be performed. The transmission method determination unit55may read the information from the scheduling request, and determine the transmission method.

If MIMO transmission is performed, the data is transmitted from the terminal10by using OFDMA, and therefore the radio characteristic of the reception signal deteriorates to a smaller extent than if transmission is performed by using SC-FDMA.

Next, a sixth embodiment will be described. In the first to fifth embodiments, the calculation of the difference Δ is performed in the terminal10. The sixth embodiment is an example of a case in which the base station50calculates the difference Δ.

FIG. 15andFIG. 16illustrates a configuration example of the terminal10and the base station50respectively, andFIG. 17illustrates a sequence diagram of overall processing. In the sixth embodiment, the base station50calculates the difference Δ, and therefore the base station50includes the known signal reception unit11, the path loss calculation unit12, and the transmission power calculation unit13.

The known signal transmission unit14of the terminal10transmits the known signal to the base station50(S70).

Next, the known signal reception unit11of the base station50receives the known signal, and the transmission power calculation unit13calculates the difference Δ between the maximum transmission power Pmaxand the transmission power Ptcorresponding to the current position, by using (Numeral 1) and so on (S71). Subsequent processing is similar to that of the first embodiment. The sixth embodiment may also be applied to any of the second to fourth embodiments.