JPH11205277A - Radio communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the power consumption while enhancing the utilizing efficiency of a transmission frequency band of an OFDM signal by using number of sub carriers for transmission of information through an outgoing channel more than number of sub carriers for transmission of information through an incoming channel in the radio communication system where information is sent by orthogonal frequency division multiplex(OFDM) signals. SOLUTION: In the radio communication system with a non-symmetry characteristic where OFDM signals are used for transmission, number of sub carriers for transmission of information through an outgoing channel 5 is more than number of sub carriers for transmission of information through an incoming channel 4. In the case of employing a same modulation system for the outgoing channel 5 and the incoming channel 4, a ratio of number of sub carriers of both the channels for transmission of information is selected to be a ratio of transmission rates. The low power consumption and miniaturization of radio terminals 1 and improvement of communication quality of the incoming channel or the like are expected by realizing the radio communication system where the transmission rate of the incoming channel 4 is lower than the transmission rate of the outgoing channel 5 in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radio communication system for transmitting radio signals using OFDM (Orthogonal Frequency Division Multiplexing) signals, and more particularly to a radio communication system having different transmission rates for downlink channels and uplink channels. Things.

[0002]

2. Description of the Related Art In recent years, a high-speed wireless data communication system indoors or outdoors has been required. In a wireless communication system that realizes high-speed data communication, it is necessary to reduce multipath interference in which a transmitted signal is received through various paths due to reflection from a building or the like. When multipath interference occurs, the reception characteristics deteriorate significantly. In general, an equalizer is used as a multipath-resistant reception method.
Very disadvantageous and impractical in terms of low power consumption.

Therefore, as a measure against multipath, orthogonal frequency division multiplexing (OFDM: Orthogonal Fres).
quency Division Multiplex
ing) system is being studied. The OFDM system is a multicarrier transmission system in which information is transmitted by setting subcarriers at a minimum interval that is orthogonal to each other. Since the influence of intersymbol interference due to multipath can be greatly reduced, multipath occurs. Even this is a transmission method in which the deterioration of the reception characteristics is small. Also OFD
By using the M method, the transmission rate of each subcarrier can be significantly reduced, and batch processing by digital signal processing (high-speed inverse Fourier transform and high-speed Fourier transform)
Thus, modulation and demodulation of a multicarrier signal can be performed. However, in the OFDM system, since the time waveform fluctuates more severely than the single carrier transmission system, high-efficiency power amplification cannot be performed, and the power efficiency deteriorates. The use of a linearizer is also considered for higher efficiency, but the size of the device is obviously increased and power consumption is not reduced.

[0004]

As described above, in the above-mentioned conventional wireless communication system using the OFDM signal, the OFDM signal is used.
There is a problem that the power consumption of the wireless terminal increases due to the poor power efficiency of the scheme. In a wireless communication system, portability of a wireless terminal is emphasized, and for that purpose, the size and weight of the terminal need to be reduced. However, since a wireless communication system using an OFDM signal consumes a large amount of power for transmitting information, a battery included in the wireless terminal has become large, and it has been extremely difficult to reduce the size and weight of the wireless terminal.

[0005] In a service realized by a radio communication system, an uplink channel from a radio terminal to a base station transmits a small amount of information such as control information and a request, and a downlink channel from the base station to the radio terminal has high quality. In many cases, a large amount of information such as unusual voice and image is transmitted, and in a normal wireless communication system in which both channels have the same transmission speed, there is a problem that the utilization efficiency of the transmission band is deteriorated.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem. In a wireless communication system using an OFDM signal, it is possible to reduce the power consumption and the size while improving the use efficiency of the transmission band of the OFDM signal. It is another object of the present invention to provide a wireless communication system using a wireless terminal capable of reducing the weight.

[0007]

In order to achieve the above object, according to the first aspect of the present invention, there is provided a wireless communication system in which information is transmitted by an OFDM signal. Is characterized in that the number of subcarriers is larger than the number of subcarriers transmitting information on the uplink channel.

According to the present invention, by changing the number of subcarriers for transmitting information between an uplink channel and a downlink channel, an asymmetric wireless communication system including a low-speed uplink channel and a high-speed downlink channel is realized. Effective use of the transmission band according to the service becomes possible.
Further, since the transmission power of the wireless terminal is reduced, power consumption can be reduced, and the size and weight of the wireless terminal can be reduced. Furthermore, by reducing the number of subcarriers in the uplink channel, the interval between subcarriers in the uplink channel is expanded, and as a result, the amount of interference by adjacent subcarriers is reduced, and important information such as control information and requests transmitted in the uplink channel. Communication quality can be improved.

According to a fourth aspect of the present invention, in a wireless communication system in which information is transmitted by an OFDM signal, the average of the frequency use efficiency of the modulation scheme used for each subcarrier of the downlink channel is the modulation used for each subcarrier of the uplink channel. It is characterized in that the efficiency is higher than the average of the frequency use efficiency of the system.

According to the present invention, an asymmetric wireless communication system composed of a low-speed uplink channel and a high-speed downlink channel is realized, and it is possible to effectively utilize a transmission band in accordance with a service of the wireless communication system. . Further, since the transmission power of the wireless terminal is reduced, power consumption can be reduced, and the size and weight of the wireless terminal can be reduced.
Furthermore, by using a modulation scheme with low frequency utilization efficiency in the uplink channel, it is possible to improve the reception characteristics at the base station even under the same transmission conditions, and it is also possible to improve the communication quality of the uplink channel.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram for explaining a wireless communication system according to the present invention.

As shown in FIG. 1, radio signals are transmitted between the radio terminal 1 and the base station 3, and between the radio terminal 2 and the base station 3. The transmission rate of the uplink channel 4 for transmitting information from the wireless terminal 1 to the base station 3 is determined by the base station 3
An asymmetric wireless communication system is constructed between the wireless terminal 1 and the base station 3, which is slower than the transmission speed of the downlink channel 5 for transmitting information to the downstream channel 5. In FIG. 1, a low-speed uplink channel 7 is also provided between the wireless terminal 2 and the base station 3.
Although an asymmetric wireless communication system composed of a high-speed downlink channel 6 is constructed, one of them may be a normal wireless communication system in which the transmission speeds of the uplink channel and the downlink channel are equal.

Thus, an asymmetric wireless communication system in which the transmission speed of the upstream channel 4 is lower than the transmission speed of the downstream channel 5 (the transmission speed of the downstream channel 5 is higher than the transmission speed of the downstream channel 4). By realizing
Low power consumption and miniaturization of wireless terminals, improvement in communication quality of uplink channels, and the like are expected.

An embodiment in which the present invention is applied to an asymmetric wireless communication system for transmitting an OFDM signal will be described below.

FIGS. 2, 3, 4, 5 and 6 illustrate an asymmetric wireless communication system in which the number of subcarriers for transmitting information on the downlink channel is larger than the number of subcarriers for transmitting information on the uplink channel. FIG.

When the same modulation scheme is used for each of the sub-carriers of the downlink channel and the uplink channel, an asymmetric wireless communication system is constructed in which the ratio of the number of sub-carriers for transmitting information is the transmission speed ratio. Since the transmission rate ratio varies depending on the service in the wireless communication system, it is preferable to set the transmission rate ratio to a value suitable for the service.

FIGS. 2, 3 and 4 show that the number of subcarriers for transmitting information on the downlink channel is larger than the number of subcarriers for transmitting information on the uplink channel, and the transmission bands occupied by the downlink channel and the uplink channel are different. FIG. 3 is a diagram illustrating a subcarrier arrangement in a wireless communication system of different types.

In general, a guard band is provided between the upstream channel and the downstream channel in order to prevent interference between the two channels. The guard band may be a wide band, a band as narrow as several subcarriers, or a configuration without a guard band.

FIG. 2 is an example of an arrangement diagram of subcarriers in an asymmetric wireless communication system when the subcarrier interval between the downlink channel and the uplink channel is equal. Since the subcarrier intervals of the uplink channel and the downlink channel are equal, the symbol lengths of the uplink channel and the downlink channel are equal, and the bandwidth occupied by the downlink channel is different from the bandwidth occupied by the uplink channel. , And an asymmetric wireless communication system using the wireless communication system can be realized, and the transmission band can be effectively utilized according to the service of the wireless communication system.

Further, since the transmission band of the uplink channel is narrowed, the transmission power of the wireless terminal can be reduced, and the power consumption of the wireless terminal can be reduced. By reducing the power consumption of the wireless terminal, the size of the battery can be reduced and a wireless communication system that can be reduced in size can be realized.

FIG. 3 is an example of an arrangement diagram of subcarriers in an asymmetric wireless communication system when the subcarrier interval between the downlink channel and the uplink channel is different. The occupied bandwidths of the downlink channel and the uplink channel may be the same or different. By making the uplink channel subcarrier interval wider than the downlink channel subcarrier interval, the number of uplink channel subcarriers becomes smaller than the number of downlink channel subcarriers, and the uplink transmission speed is lower than the downlink transmission speed. It is possible to construct a wireless communication system having asymmetrical characteristics, and it is possible to effectively utilize a transmission band according to a service of the wireless communication system.

Further, since the interval between the sub-carriers of the uplink channel is widened, it is possible to reduce the interference between adjacent carriers and to improve the communication quality of the uplink channel. From another point of view, if the amount of interference in the uplink channel can be tolerated to the same extent as the downlink channel,
The increase in the distortion of the received signal can be tolerated by the extension of the subcarrier interval, and the transmission amplifier can be used in a slightly saturated state, so that the transmission power efficiency increases. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and the size of the wireless terminal can be reduced.

Further, when the subcarrier interval is longer in the uplink channel than in the downlink channel, the symbol length of the OFDM signal is shorter in the uplink channel than in the downlink channel, the transmission time of the uplink channel is reduced, and the Transmission power can be reduced. Therefore, with the subcarrier arrangement of FIG. 3, it is possible to realize a wireless communication system that can reduce the size of wireless terminals using OFDM signals.

FIG. 4 shows an example of a subcarrier arrangement diagram of an asymmetric radio communication system in which the subcarrier intervals of the uplink channel and the downlink channel are equal, but some of the subcarriers of the uplink channel are null carriers. It is.
A null carrier is a subcarrier that does not transmit information. The null carrier can be arranged at an arbitrary position, and the insertion position may be changed for each symbol. Null carriers may be included in the subcarriers of the downlink channel, but if the OFDM symbol is configured such that the subcarriers of the uplink channel include more null carriers than the subcarriers of the downlink channel, the A slower asymmetric wireless communication system can be realized. By using this method, an asymmetric wireless communication system can be realized even if the symbol length and the transmission band of the uplink channel and the downlink channel are equal. This makes it possible to use the transmission band effectively. In addition, since the inter-carrier interference is reduced in the subcarriers adjacent to the null carrier, it is possible to improve the uplink channel communication quality. From another point of view, if the amount of interference in the uplink channel can be tolerated to the same extent as the downlink channel,
The increase in the distortion of the received signal can be tolerated by the extension of the subcarrier interval, and the transmission amplifier can be used in a slightly saturated state, so that the transmission power efficiency increases. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and a wireless communication system capable of reducing the size of the wireless terminal can be constructed.

FIGS. 5 and 6 show that the number of subcarriers for transmitting information on the downlink channel is greater than the number of subcarriers for transmitting information on the uplink channel, and that the time is switched between the downlink channel and the uplink channel to perform OFDM in the same band. FIG. 1 is a diagram illustrating a subcarrier arrangement and a channel configuration diagram of a wireless communication system that transmits signals.

FIG. 5 is a diagram showing a subcarrier arrangement diagram and a channel of an asymmetric line communication system in which the number of uplink subcarriers is made smaller than the number of downlink subcarriers and is made wider than the uplink channel subcarrier interval. It is an example of a block diagram. By making the uplink channel subcarrier interval wider than the downlink channel subcarrier interval, the number of uplink channel subcarriers becomes smaller than the number of downlink channel subcarriers, and the uplink transmission speed is lower than the downlink transmission speed. It is possible to construct a wireless communication system having asymmetrical characteristics, and it is possible to effectively utilize a transmission band according to a service of the wireless communication system.

Further, since the interval between subcarriers in the uplink channel is widened, it is possible to reduce interference between adjacent carriers and to improve the communication quality of the uplink channel. From another point of view, if the amount of interference of the uplink channel can be tolerated to the same extent as that of the downlink channel, the increase of the received signal distortion can be tolerated as the subcarrier interval is widened, and the transmission amplifier can be used in a slightly saturated state. Therefore, transmission power efficiency increases. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and a wireless communication system capable of reducing the size of the wireless terminal can be constructed.

FIG. 6 is a diagram showing the arrangement of subcarriers in an asymmetric radio communication system in which the subcarrier intervals of the uplink channel and the downlink channel are made equal and some of the subchannels of the uplink channel are null carriers that do not transmit information. FIG. 3 is an example of a channel configuration diagram. As in the case of FIG. 4, the null carrier can be arranged at an arbitrary position, and the insertion position may be changed for each symbol. Null carriers may be included in the downlink channel subcarriers, but if the uplink channel subcarriers include more null carriers than the downlink channel subcarriers, the uplink channel has a lower speed. Asymmetric wireless channels can be configured. When this method is used, as in the case of FIG. 4, since the subcarrier intervals of the uplink channel and the downlink channel are equal, an asymmetric wireless communication system can be configured even if the symbol length and the transmission bandwidth are equal.
The transmission band can be effectively used in accordance with the service of the wireless communication system without complicating the configurations of the base station and the wireless terminal. In addition, in subcarriers adjacent to the null carrier of the uplink channel, the inter-carrier interference is reduced, so that the communication quality of the uplink channel can be improved. From another viewpoint, if the amount of interference of the uplink channel can be tolerated to the same extent as that of the downlink channel, the increase in the received signal distortion can be tolerated by the increase in the subcarrier interval, and the transmission amplifier is used in a slightly saturated state. As a result, transmission power efficiency increases. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and a wireless communication system capable of reducing the size of the wireless terminal can be constructed.

As described above, in the embodiment described above, the O
In a wireless communication system that transmits an FDM signal, the number of subcarriers that transmit information on the downlink channel is made larger than the number of subcarriers that transmit information on the uplink channel. A wireless communication system having an asymmetric configuration can be realized, and a system can be constructed according to the service of the wireless communication system. Further, since the transmission power of the wireless terminal is reduced, the size of the wireless terminal can be reduced. Further, interference between subcarriers for transmitting information can be reduced, so that communication quality of an uplink channel can be improved. From another viewpoint, if the amount of interference of the uplink channel can be tolerated to the same extent as that of the downlink channel, the increase in received signal distortion can be tolerated as the subcarrier interval is widened, and the transmission amplifier is used in a slightly saturated state. As a result, the transmission power effect increases. As a result, transmission power can be reduced, and a wireless communication system capable of reducing the size of a wireless terminal can be constructed.

Next, referring to FIGS. 7 and 8, the average of the frequency use efficiency of the modulation scheme used for each subcarrier of the downlink channel is larger than the average of the frequency use efficiency of the modulation scheme used for each subcarrier of the uplink channel. An embodiment of a highly asymmetric wireless communication system will be described.

FIG. 7 is an example of a subcarrier arrangement diagram of this asymmetric wireless communication system. Each channel may use a single modulation scheme for all subcarriers in each channel, or may use a plurality of modulation schemes in each channel. Here, the modulation scheme of each subcarrier of the downlink channel is QPSK, and the modulation scheme of each subcarrier of the uplink channel is BPSK.

QPSK is 2 bits / sec / Hz, BP
Since SK is 1 bit / sec / Hz, if the bandwidth of the downlink channel is equal to the bandwidth of the uplink channel, the transmission rate of the downlink channel is twice that of the uplink channel.

As described above, by making the average of the frequency use efficiency of the modulation scheme used for each subcarrier of the downlink channel higher than the average of the frequency use efficiency of the modulation scheme used for each subcarrier of the uplink channel, the asymmetric radio communication is achieved. A communication system is realized, and a system suitable for a service of a wireless communication system can be constructed. From another viewpoint, when the transmission power is equal between the downlink channel and the uplink channel, the communication quality of the uplink channel is improved as compared with the downlink channel because the uplink channel uses a modulation scheme with lower frequency use efficiency. If the communication quality of the uplink channel can be tolerated to about the same level as the downlink channel,
Since the uplink channel can tolerate an increase in received signal distortion, and the transmission amplifier can be used in a slightly saturated state, transmission power efficiency increases. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and a wireless communication system capable of reducing the size of the wireless terminal can be constructed.

FIG. 8 shows that the OFDM signal is transmitted in the same band by switching the time between the upstream channel and the downstream channel, and the average of the frequency use efficiency of the modulation scheme of the subcarrier transmitted on the downstream channel is transmitted on the upstream channel. It is an example of a channel configuration diagram and a subcarrier arrangement diagram of a wireless communication system that is higher than the average of the frequency use efficiency of the subcarrier modulation scheme.

In the radio communication system shown in FIG. 8, the bandwidth, symbol length, and number of subcarriers of the uplink and downlink channels are all the same, and only the subcarrier modulation method is different. The modulation method of each subcarrier of the uplink channel is BPS
K, modulation scheme of each subcarrier of downlink channel is QPS
If K, as in FIG. 7, it is possible to realize an asymmetric wireless communication system in which the downlink channel has a transmission rate twice that of the uplink channel. As described above, by changing the frequency use efficiency of the modulation scheme of the uplink channel and the downlink channel, an asymmetric wireless communication system is realized, and a system suitable for the service of the wireless communication system can be constructed. When the transmission power is equal between the downlink channel and the uplink channel, the communication quality of the uplink channel is improved as compared with the downlink channel because the uplink channel uses a modulation scheme with lower frequency use efficiency. From another perspective, if the communication quality of the uplink channel can be tolerated to the same level as the downlink channel, the uplink channel can tolerate an increase in received signal distortion,
Since the transmission amplifier can be used in a slightly saturated state, transmission power efficiency increases. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and a wireless communication system capable of reducing the size of the wireless terminal can be constructed.

As described above, according to the radio communication system for transmitting the OFDM signal of the embodiment shown in FIGS. 7 and 8, the average of the frequency use efficiency of the modulation scheme used for the subcarriers of the downlink channel is calculated based on the subchannel of the uplink channel. By making the frequency use efficiency of the modulation scheme used by the carrier higher than the average, a configuration of an asymmetric wireless communication system can be realized, and a system configuration suitable for a service of the wireless communication system can be realized. Further, since the frequency utilization efficiency of the modulation scheme of the uplink channel is low, it is possible to improve the communication quality of the uplink channel. In other words, if the communication quality of the uplink channel can be tolerated to the same level as that of the downlink channel, the uplink channel can tolerate an increase in received signal distortion, and the transmission amplifier can be used in a slightly saturated state. Goes up. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and a wireless communication system capable of reducing the size of the wireless terminal can be constructed.

[0038]

As described above in detail, according to the radio communication system of the present invention, the number of subcarriers for transmitting information on the downlink channel is made larger than the number of subcarriers for transmitting information on the uplink channel. Thereby, an asymmetric wireless communication system can be realized, and a system configuration suitable for the service of the wireless communication system can be realized. Also,
By increasing the interval between the subcarriers of the uplink channel, it is possible to reduce the interference between the subcarriers of the uplink channel, and it is possible to improve the communication quality of important information such as control information and requests transmitted on the uplink channel. In other words, if the amount of interference of the uplink channel can be tolerated to the same level as that of the downlink channel, an increase in received signal distortion can be tolerated as the subcarrier interval is widened, and the transmission amplifier can be used in a slightly saturated state. Therefore, transmission power efficiency increases. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and a wireless communication system capable of reducing the size of the wireless terminal can be constructed.

According to the radio communication system of the present invention, the average of the frequency use efficiency of the modulation scheme used for each subcarrier of the downlink channel is made larger than the average of the frequency use efficiency of the modulation scheme used for each subcarrier of the uplink channel. By increasing the value, an asymmetric wireless communication system can be realized, and a system configuration suitable for the service of the wireless communication system can be realized. Further, since the frequency use efficiency of the modulation scheme of the uplink channel is low, the communication quality of the uplink channel can be improved. In other words, if the communication quality of the uplink channel can be tolerated to the same level as that of the downlink channel, the uplink channel can tolerate an increase in received signal distortion, and the transmission amplifier can be used in a slightly saturated state. Go up. As a result, the transmission power is reduced, the power consumption of the wireless terminal can be reduced, and a wireless communication system capable of reducing the size of the wireless terminal can be constructed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a wireless communication system according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an example of a subcarrier arrangement in a wireless communication system according to an embodiment of the present invention, in which the number of subcarriers transmitting information on a downlink channel is greater than the number of subcarriers transmitting information on an uplink channel;

FIG. 3 is a diagram showing an example of a subcarrier arrangement in another wireless communication system in which the number of subcarriers for transmitting information on the downlink channel is larger than the number of subcarriers for transmitting information on the uplink channel according to the embodiment of the present invention;

FIG. 4 is a diagram showing an example of a subcarrier arrangement of still another wireless communication system in which the number of subcarriers for transmitting information on the downlink channel is larger than the number of subcarriers for transmitting information on the uplink channel according to the embodiment of the present invention;

FIG. 5 is a diagram illustrating an embodiment of the present invention, in which the number of subcarriers for transmitting an OFDM signal in the same band by switching the time between a downlink channel and an uplink channel and transmitting information in the downlink channel is the number of subcarriers for transmitting information in the uplink channel. The figure which shows the example of the subcarrier arrangement | positioning of the radio | wireless communication system more than the number

FIG. 6 is a diagram illustrating an embodiment of the present invention, in which the number of subcarriers for transmitting an OFDM signal in the same band by switching the time between the downlink channel and the uplink channel and transmitting information in the downlink channel is the number of subcarriers for transmitting information in the uplink channel. The figure which shows the example of the subcarrier arrangement | positioning of another wireless communication system more than the number of lines

FIG. 7 is a wireless communication system according to an embodiment of the present invention, in which the average of the frequency use efficiency of the modulation scheme used in each subcarrier of the downlink channel is higher than the average of the frequency use efficiency of the modulation scheme used in each subcarrier of the uplink channel. Diagram showing subcarrier arrangement

FIG. 8 shows another radio system according to the embodiment of the present invention, in which the average of the frequency use efficiency of the modulation scheme used in each subcarrier of the downlink channel is higher than the average of the frequency use efficiency of the modulation scheme used in each subcarrier of the uplink channel. Diagram showing subcarrier arrangement in communication system

[Explanation of symbols]

 1, 2 wireless terminal 3 base station 4, 7 uplink channel 5, 6 downlink channel

Claims (7)

[Claims] 1. A radio communication system for transmitting an orthogonal frequency division multiplexed signal between a base station and a radio terminal, wherein the number of subcarriers for transmitting information on a downlink channel from the base station to the radio terminal is equal to the radio frequency. A wireless communication system, wherein the number of subcarriers for transmitting information on an uplink channel from a terminal to the base station is larger than the number of subcarriers. 2. The wireless communication system according to claim 1, wherein a subcarrier interval of the uplink channel and a subcarrier interval of the downlink channel are equal, and a bandwidth occupied by the downlink channel and a bandwidth occupied by the uplink channel are different. A wireless communication system characterized by being different. 3. The wireless communication system according to claim 1, wherein a subcarrier interval of the downlink channel and a subcarrier interval of the uplink channel are different. 4. The radio communication system according to claim 1, wherein a subcarrier interval of an uplink channel is equal to a subcarrier interval of a downlink channel, and a part of a subcarrier group of the uplink channel is a null carrier. Wireless communication system. 5. The wireless communication system according to claim 1, wherein time is switched between a downlink channel and an uplink channel to transmit an OFDM signal in the same band. 6. A radio communication system for transmitting an orthogonal frequency division multiplexed signal between a base station and a radio terminal, wherein a modulation scheme used in each subcarrier for transmitting information on a downlink channel from the base station to the radio terminal. A wireless communication system wherein the average of the frequency use efficiency of the modulation scheme used in each subcarrier for transmitting information on the uplink channel from the wireless terminal to the base station is higher than that of the modulation scheme. 7. The wireless communication system according to claim 6, wherein time is switched between a downlink channel and an uplink channel to transmit an OFDM signal in the same band.