DESCRIPTION WIRELESS COMMUNICATION SYSTEM Technical Field The present invention relates to a wireless communication system. Background Art A mobile communication system for assigning transmission using a scheduler as an HSDPA system standardized in a 3GPP has been partially put to practical use. Described below is an example of an HSDPA system for performing a high-speed downlink transmission using an example of the configuration of a terminal and an example of a configuration of a base station. FIGS. 1 through 5 are explanatory views of a conventional HSDPA system. In the terminal illustrated in FIG. 1, for example, a wireless channel quality measurement/calculation unit 13 measures and calculates a wireless channel quality indicator (hereinafter referred to as a CQI (channel quality indicator)) according to the pilot signal of a downlink signal received through an antenna 10, a radio unit 11, and a demodulation/decoding unit 12. As a practical example, an SIR is calculated by measuring the reception power and interference power of the pilot signal. The CQI value is assembled into a transmitting signal by a wireless channel quality indicator transmission unit 14, encoded and modulated by a coding/modulation unit 15, and transmitted to a base station on an uplink wireless channel through the antenna 10. On the other hand, the base station illustrated in FIG. 2 receives a signal carrying the CQI value transmitted from a terminal through an antenna 20, a radio unit 21, and a demodulation/decoding unit 22, collects a wireless channel quality indicator (CQI), and notifies a scheduler 24 of the indicator. The scheduler 24 calculates the priority of the terminal for each available frequency band using the wireless channel quality indicator (hereinafter referred to as a CQI (channel quality indicator) reported from the terminal, and selects a transmission parameter on a higher priority basis. A control signal generation unit 25 generates a transmitting control signal, and transmits the signal to a terminal through a coding/modulation unit 27, a radio unit 28, and the antenna 20. The transmission data of a transmission data buffer 26 is transmitted to a terminal after the control signal is transmitted. FIG. 3 is a flowchart of a scheduling process. Assume that there are terminals UE1 through UEn in the cell of a base station. In step S10, the CQI values (CQI1 through CQIn) of the terminals UE1 through UEn are received. In step Sll, the CQI1 through CQIn are stored. In step S12, a TTI is initialized. A TTI is short for a transmission time interval, and refers to a transmission interval of the data to a terminal. In this example, it is used as a variable indicating a transmission frequency. In step S13, the TTI is increased by 1. In step S14, the priority Pk of the terminal UEk is calculated. In step S15, the system is initialized to i=0, ]=1. In step S16, the wireless resources R1 is calculated. With i=0, the wireless resources have not been assigned. Therefore, R1 refers to the entire wireless resources. In step S17, if is determined whether or not the wireless resources R1 is smaller than 0. If the determination in step S17 is YES, control is passed to step S21. If the determination in step S17 is NO, the terminal UEj having the priority Pk of the maximum value Pk_max is calculated from the n-1 terminals in step S18. In step S19, the method of transmitting data (data length, modulation system, etc.) to the terminal UEj is selected. In step S20, l is increased by 1, j is increased by 1, and control is returned to step S16. In step S21, the transmitting method selected in step S19 is modulated as a control signal, and the result is transmitted to the terminal. In step S22, the transmission data is modulated for the terminal to which the control signal has been transmitted, transmits the result to the terminal, and control is returned to step S13. As a method of calculating the priority, the MAX CIR method of selecting a larger CQI value, and the PF (proportional fairness) method of selecting a larger CQI and performing a selection for egual opportunity. In the above-mentioned 3GPP, the specification of the E3G (evolved 3G) system is inspected as a next generation mobile communication system. In this respect, the implementation of the OFDMA system and the SC-FDMA system are studied respectively for downstream and upstream as a multi-connection method. In addition, in the E3G system, a scheduling process is performed as with the HSDPA system using the frequency band broader than the conventional HSDPA (for example, four times). Furthermore, the terminal used in the E3G system has different bandwidths between upstream and downstream. Additionally, in the downstream, the available bands by terminals depend on each terminal, for example, 1.25 MHz, 2.5 MHz, 5 MHz,10 MHz, 20 MHz, etc. Therefore, it is necessary to perform a scheduling process at the system band of 20 MHz by considering the available bandwidth. That is, as illustrated in FIG. 4, it is necessary to perform the scheduling process on the entire system using one scheduler. Furthermore, assume that the downlink system bandwidth is 20 MHz, and the downlink bandwidth of a terminal is 5 MHz. At this time, the frequency used during operation is variable with the relationship with other terminals taken into account, and there are four options. Therefore, to allow the scheduler of a base station to select the optimum band from among a plurality of bands with the available bandwidths by other terminals taken into account, the CQI is measured and calculated for every 5 MHz band at a terminal as illustrated in FIG. 5, and the result is to be reported to the base station. That is, four times as much as the measurement and calculation are required as compared with the HSDPA system. In addition, the frequency of reporting the CQIs to the base stations quadruples. As a result, the interference of the up-channel also quadruples. In the E3G system, when the entire system is scheduled by one scheduler, When simply compared with the scheduler of the conventional HSDPA system, the number of terminals to be scheduled is multiplied (for example, quadrupled). • As compared with the transmission interval of 2 msec of the conventional HSDPA system, the interval is 1/4, that is, 0.5 msec. For the two above-mentioned reasons, for example, 16 times scheduling speed as fast as the conventional system is demanded. That is, the priority calculation time is to be set to 1/16. On the other hand, the improvement of the performance of the process of the CPU and the DSP for performing the scheduling process approximately quadruples on the basis of the reference of year 2010 as the target of starting the service of the E3G, which is far from the above-mentioned 16 times with the Moore's Law (double process speed in 18 months) taken into account. Therefore, it is inevitable that the scheduling process is performed at a higher speed. The patent document 1 discloses the technology of grouping and scheduling terminals moving at a high speed. Furthermore, it specifies the bands to be scheduled. It is assumed that they are based on the HSUPA (high speed uplink packet access) of the 3GPP. However, in the descriptions, a terminal moving at a low speed or during halts is not scheduled. The patent document 2 discloses an example using an OFCDM (orthogonal frequency and code division multiplexing). That is, a spreading process is performed in the frequency and time directions, and then a multiplexing operation is performed. The patent document 3 groups the terminals using the amount of attenuation of transmission power. Since there are no descriptions about available frequency bands, it is considered that tne conventional OFDM is used. The patent document 4 discloses a base station detecting the moving speed of a mobile station using a Doppler frequency, and optimally selecting a coding rate and a modulation system. The patent document 5 discloses optimally determining the transmission rate of the communications of a mobile station and a base station according to the information about the Doppler frequency etc. of a mobile station. The patent document 6 discloses grouping a subcamer, acquiring channel quality information for each group, and transmitting and receiving the information. Patent Document 1: Japanese Laid-open Patent Publication No. 2006-060814 Patent Document 2: Japanese Laid-open Patent Publication No. 2005-318434 Patent Document 3: Japanese Laid-open Patent Publication No. 2001-036950 Patent Document 4: Japanese Laid-open Patent Publication No. 2003-259437 Patent Document 5: Japanese Laid-open Patent Publication No. 2005-260992 Patent Document 6: Japanese Laid-open Patent Publication No. 2005-160079 Disclosure of the Invention The present invention aims at providing a wireless communication system capable of speeding up a scheduling process at a base station. The wireless communication system according to the present invention having a base station communicating with a plurality of subordinate terminals using a plurality of frequency bands includes: a grouping device for assigning the plurality of terminals to a group of each of the frequency bands according to the wireless channel quality acquired for each frequency band used by a terminal in communicating with a base station; a scheduling device for scheduling the grouped terminal for each group; and a communication device for the base station communicating with a terminal according to a result of the scheduling. Brief Description of the Drawings FIG. 1 is an explanatory view (1) of a conventional HSDPA system; FIG. 2 is an explanatory view (2) of a conventional HSDPA system; FIG. 3 is an explanatory view (3) of a conventional HSDPA system; FIG. 4 is an explanatory view (4) of a conventional HSDPA system; FIG. 5 is an explanatory view (5) of a conventional HSDPA system; FIG. 6 is a sequence of the flow of the process according to an embodiment of the present invention; FIG. 7 is an explanatory view illustrating the case in which a grouping operation is performed in the easiest method on a basis of channel quality of each band during channel setting; FIG. 8 illustrates an image of measuring wireless channel quality for each available band; FIG. 9 is an explanatory view (1) of the method of grouping and scheduling a terminal; FIG. 10 is an explanatory view (2) of the method of grouping and scheduling a terminal; FIG. 11 is an explanatory view of an image of grouping and scheduling methods when the available bandwidth of a terminal is different from that illustrated in FIG. 10; FIG. 12 is an explanatory view (1) of a hierarchical grouping process; FIG. 13 is an explanatory view (2) of a hierarchical grouping process, FIG. 14 illustrates an example of a grouping table of a base station when a terminal is grouped; FIG. 15 is an explanatory view of other grouping methods; FIG. 16 illustrates an example of a grouping table of a base station for the grouping operation illustrated in FIG. 15; FIG. 17 is a view (1) of an example of the process when a terminal is grouped; FIG. 18 is a view (2) of an example of the process when a terminal is grouped; FIG. 19 is a view (3) of an example of the process when a terminal is grouped; FIG. 20 is a view (4) of an example of the process when a terminal is grouped; FIG. 21 is a view (5) of an example of the process when a terminal is grouped; FIG. 22 is a view of the configuration illustrating the principle of the terminal according to the present invention; FIG. 23 is a view illustrating the configuration of the principle of the base station according to the present invention; FIG. 24 illustrates an example of a configuration illustrated in FIG. 22 applied to a case when a CQI is measured as wireless channel quality; FIG. 25 illustrates an example of a configuration illustrated in FIG. 23 applied to a case when a CQI is measured on as wireless channel quality; FIG. 26 illustrates the second example of a configuration of a base station according to an embodiment of the present invention; FIG. 27 illustrates the third example of a configuration of a base station according to an embodiment of the present invention; FIG. 28 illustrates the second example of a configuration of a terminal according to an embodiment of the present invention corresponding to FIG. 27; FIG. 29 illustrates the fourth example of a configuration of a base station according to an embodiment of the present invention; FIG. 30 illustrates the fifth example of a configuration of a base station according to an embodiment of the present invention; Best: Mode for Carrying Out the Invention Described below is a downlink transmission as an example. FIG. 6 is a sequence of the flow of the process according to an embodiment of the present invention. In FIG. 6, a terminal measures the wireless channel quality for each frequency band (1) . That is, an SIR is calculated from received data, and a CQI value is obtained on the basis of the calculated SIR. The measured wireless channel quality is notified to a base station (2). The base station determines the available frequency band by the terminal from the information about the received wireless channel quality (3) , and classifies all terminals that have transmitted the wireless channel quality into groups (4) . When the grouping process is completed, the base station notifies each terminal of the terminal group to which the terminal belongs (5) . Upon receipt of the terminal group notification, the terminal sets an available frequency band and the terminal group (6). The terminal measures the wireless channel quality at the available frequency bands set for the terminal (7) , and notifies the base station of the measurement result (8). The base station performs a scheduling process for each available frequency band on the basis of the notified wireless channel quality. That is, the base station selects a technique for transmission on the basis of the priority of the terminal, and selects a transmitting method. Then, it generates control information to be received by the terminal (9), notifies the terminal of the transmission control information (10), and then transmits data (11). Thus, in the OFDMA system and the MC-CDMA system, terminals are grouped depending on the possible available bandwidths and the available frequencies. The grouping process can be performed when a wireless channel is set, or can be performed at predetermined intervals after setting the wireless channel. The information for the grouping process can be a possible available bandwidth of a terminal, the channel quality of each band, the use of a channel (load) of each band, etc. FIG. 7 is an explanatory view illustrating the case in which a grouping operation is performed in the easiest method on a basis of channel quality of each band during channel setting. Practically, assume a case in which a terminal has the maximum possible available bandwidth of 5 MHz, and the bandwidth of the system of 20 MHz. When a characteristic is set, the terminal measures the wireless channel quality for each band obtained by dividing the system band of 20 MHz by the maximum possible available bandwidth of 5 MHz, calculates a wireless channel quality indicator (1), and notifies the base station of the calculated indicator (2) . The base station (or a wireless channel control station) determines the available frequencies on the basis of the information and the possible available bandwidth about the terminal (3) , divides the terminal for each available bandwidth and available frequencies, and performs the grouping process (4) . It is also possible to determine the available frequencies by considering the channel load between the frequencies that can be accommodated. FIG. 7 is substantially the same as FIG. 6, but the available frequency bands and the terminal group are set when a channel is set, and the wireless channel quality of the available frequency bands of each terminal is measured by each terminal in a normal state, and the base station performs the scheduling process on the basis of the reported wireless channel quality, and starts communications. The operation in the normal state is the same as in FIG. 6, and the description is omitted here. FIG. 8 illustrates an image of measuring wireless channel quality for each available band. As described above, the terminal for which a terminal group is determined measures the channel quality only for the determined available frequencies, calculate the CQI, and reports the results to the base station. Thus, the number of CQI reports decreases, thereby reducing the uplink interference. The base station that has received the CQI classifies the CQI for each group of the terminal, and performs the scheduling process for each terminal group (each available frequency band) . Thus, since the number of terminals to be scheduled decreases, the computational complexity in calculating a priority of the terminal in the scheduling process is reduced, thereby speeding up the entire process. Furthermore, since the scheduling process is performed on each terminal group, the entire process can be furthermore sped up by concurrently operating a plurality of schedulers. FIGS. 9 and 10 are explanatory views of the method of grouping and scheduling a terminal. In FIGS. 9 and 10, the band of the system is 20 MHz, the available bandwidth of the terminal is 5 MHz, and the terminals UE 100 through 139 are classified into four groups. Using a frequency band 1, a group 1 is scheduled by a scheduler 1 in the four schedulers. Similarly, a group 2 is assigned a band 2 and a scheduler 2, a group 3 is assigned a band 3 and a scheduler 3, and a group 4 is assigned a band 4 and a scheduler 4. They are illustrated by (a) in FIG. 10. Since the data transmission interval is 0.5 ms, the scheduling process of each group is performed every 0.5 ms. Thus, when a plurality of schedulers are provided, one scheduler is assigned to each terminal group. That is, the group 1 is scheduled by, for example, the scheduler 1, and the group 2 is scheduled by the scheduler 2. The scheduling processes can be concurrently performed as illustrated by (b) in FIG. 10. FIG. 11 is an explanatory view (1) of an image of grouping and scheduling methods when the available bandwidth of a terminal is different from that illustrated in FIG. 10. In FIG. 11, the available bandwidth of the terminal is 10 MHz, and there are a group 5 scheduled by a scheduler 5 using bands 1 and 2, and a group 6 scheduled by a scheduler 6 using the bands 3 and 4. FIGS. 12 and 13 are explanatory view of hierarchical grouping. As described above, the possible available bandwidth by a terminal depends on the performance of a terminal. Therefore, there can be a method of performing a grouping process on the basis of a possible available bandwidth. In the case illustrated in FIG. 12, the terminals UE 160 through 169 capable of using 20 MHz are classified into a group 7, and scheduled by a scheduler 7. On the other hand, the terminals UE 140 through 149, and the UE 150 through 159 having the available band of 10 MHz are respectively classified into the group 5 scheduled by the scheduler 5 using the bands 1 and 2 and the group 6 scheduled by the scheduler 6 using the bands 3 and 4. The terminals UE 100 through 109, UE 110 through 119, UE 120 through 129, and UE 130 through 139 having the available band of 5 MHz are respectively classified into the group 1 scheduled by the scheduler 1 using the band 1, the group 2 scheduled by the scheduler 2 using the band 2, the group 3 scheduled by the scheduler 3 using the band 3, and the group 4 scheduled by the scheduler 4 using the band 4. As illustrated by (a) and (b) in FIG. 13, it is assumed that all of the possible available bands are used, and that, for example, a group having a broad loop such as 10 MHz as a possible available band is defined as a higher order group, and a group having a narrow loop such as 5 MHz as a possible available band is defined as a lower order group. At this time, the scheduling process is performed from the higher order group to the lower order group. As illustrated by (a) in FIG. 13, the group 7 is first scheduled every 0.5 ms as the transmission time of each piece of data, and then the groups 5 and 6, and finally the groups 1 through 4 are scheduled. Part (b) in FIG. 13 illustrates the image of hierarchical scheduling. The scheduling process is performed sequentially and hierarchically from the scheduler 7. Since the two schedulers, that is, the schedulers 5 and 6, and the four schedulers, that is, the schedulers 1 through 4, are concurrently operated, the scheduling processes can be expected to be sped up. FIG. 14 illustrates an example of a grouping table of a base station when a terminal is grouped. Corresponding to each terminal group number, the central frequency of the available band of each group, the bandwidth, and the identification number of the terminal belonging to each group is entered. FIG. 15 is an explanatory view of other grouping methods. The necessary transmission speed depends on the data to be transmitted. Therefore, the necessary bandwidth depends on the data. That is, QoS may need an available broad bandwidth, or an available narrow bandwidth. Furthermore, if a transmission can be performed by narrowing the bandwidth for the relationship with other terminals although a necessary transmission speed cannot be satisfied, the transmission can be performed. Therefore, when the possible available bandwidth of a terminal is 20 MHz, it cannot only belong to the terminal group of 20 MHz, but also belong to the terminal group of a narrower bandwidth such as 10 MHz, 5 MHz, etc. Therefore, the terminal group is hierarchically defined in the descending order of size of the available bandwidth. In FIG. 15, the terminal having the available band of 20 MHz can perform communications at 10 MHz and 5 MHz. In addition, the terminal having the available band of 10 MHz can also perform communications at 5 MHz. The terminals UE 160 through 169 having the available band of 20 MHz not only belong to the group 7 scheduled by the scheduler 7, but also belong to all groups 1 through 6. Accordingly, when the terminals UE 160 through 169 cannot use the band of 20 MHz, they can be assigned to the group 5 or 6 having the band of 10 MHz. When they cannot also use the band of 10 MHz, they can be assigned to any of the groups 1 through 4 of the band of 5 MHz. Thus, the possibility that the terminals UE 160 through 169 cannot perform communications can be reduced. Similarly, the terminals UE 140 through 149 and UE 150 through 159 having the available band of 10 MHz can also be assigned to the groups 1 through 4 so that communications can also be performed at 5 MHz when the communications cannot be performed at the band of 10 MHz. Since the terminals UE 100 through 109, UE 110 through 119, UE 120 through 129, and UE 130 through 139 belong only to the groups 1 through 4 because there is no available band lower than 5 MHz. The scheduling process is performed from a higher order group (for example, 20 MHz) to a lower order group (for example, 5 MHz) . Thus, the number of terminals to be scheduled in a group can be reduced, and the priority calculating process can also be reduced, thereby speeding up the entire scheduling process. FIG. 16 illustrates an example of a grouping table of a base station for the grouping operation illustrated in FIG. 15. The central frequency of the available band of each group, the bandwidth, and the identification numbers of the terminals belonging to each group are entered in the table corresponding to each of the terminal group numbers 1 through 7. As with the case illustrated in FIG. 14, when a plurality of schedulers are provided, the number of schedulers are to be equal to the number of groups. By providing a scheduler for each group and concurrently and hierarchically operating the plurality of schedulers, the scheduling process can be sped up. In addition, the plurality of schedulers can be replaced with one scheduler capable of performing concurrent operations. FIGS. 17 through 21 are views of examples of the process when a terminal is grouped. In the example illustrated in FIG. 17, in step S30, the maximum possible available bandwidth of a target terminal is confirmed. In step S31, the CQI of each band is received from the terminal. In step S32, the available band is selected from the maximum value of the CQI. In step S33, a terminal group corresponding to the selected band is selected. In the example illustrated in FIG. 18, the maximum possible available bandwidth of a target terminal is confirmed in step S35, and the CQI of each band is received in step S36. In step S37, the available band is selected from the CQI and the use state of each band, and in step S38, a terminal group is selected. The use state of each band refers to the number of terminals already assigned to each band, etc. When the number of terminals assigned to a certain band becomes too large, the frequency of the selection by the scheduler is reduced and the transmission speed becomes lower. In this case, performed is the process of selecting the band of the second largest CQI, not the band of the largest CQI, etc. In the example illustrated in FIG. 19, the maximum possible available bandwidth of a target terminal is confirmed in step S40. In step S41, the bandwidth and the CQI for each band are received from the terminal. InstepS42, the available bandwidth and the available band are selected from the maximum value of the CQI. In step S43, a terminal group is selected. In FIG. 19, the terminal can use a plurality of available bands. For example, when the system band is 20 MHz and the available band of the terminal is 10 MHz, the terminal can use 10 MHz and 5 MHz. Therefore, the terminal measures the CQIs of two bands having a 10 MHz width and four bands having a 5 MHz, and the base station selects the available band from the measurement results. In the example illustrated in FIG. 20, for example, assume the case in which the GBR (guaranteed bit rate) of the QoS is set. That is, assume the case in which a service of the regulated lowest transmission speed is set. For example, assume that the possible transmission speed is 3 Mbps with the band of 5 MHz, the modulation system of the QPSK, and at the coding rate of 1/3. At this time, when the GBR of a terminal is 5 Mbps, it is necessary to have the bandwidth of 10 MHz to satisfy the GBR. Therefore, the terminal is assigned to the group having the available bandwidth of 10 MHz. The modulation system can be a QPSK, and a multivalue modulation system of 16 QAM, 64 QAM, etc. , the coding rate can be variable, and the MIMO function can be used. In step S45, the maximum possible available bandwidth of a target terminal is confirmed. In step S46, the QoS of transmission data to the target terminal is confirmed. In step S47, the necessary bandwidth is calculated. In step S48, the CQI for each band of the necessary bandwidth is received from the terminal. In step S4 9, the available bandwidth is selected from the maximum value of the CQI, the possible available bandwidth, and the necessary bandwidth. InstepS50, a terminal group is selected. In the example illustrated in FIG. 21, the degradation of the transmission characteristic by the movement of a terminal is considered. That is, the Doppler frequency is determined by the moving speed of the terminal, and the level of the degradation of the transmission characteristic is determined by the Doppler frequency. Since the Doppler frequency is enhanced with increasing available frequencies, it is desired to use a lower frequency in the communications with the terminal to correspond to a high-speed movement. Then, for example, when the system bandwidth is 20 MHz, and the central frequency is fl