Wireless communication system and wireless communication method

There is provided a wireless communication system in which a feedback volume is reduced as exceeding a communication speed demanded by a mobile station. The wireless communication system includes: a plurality of base stations 102a and 102b having a plurality of antennas; a plurality of mobile stations 103a to 103d having a plurality of antennas; and a base station controller 101a collectively controlling a plurality of the base stations. During a certain period of time (time slot), there are provided an (MIMO+TDMA) communication mode by which only one of the base stations and one of the mobile stations are communicated with each other, an (SDMA+TDMA) communication mode by which one of the base stations and the plurality of the mobile stations are simultaneously communicated with each other, and a (multipoint SDMA) communication mode by which the plurality of the base stations and the plurality of the mobile stations are simultaneously communicated with each other by coordinating the plurality of the base stations with each other by the base station controller. And, it is judged by the base station or the base station controller whether or not the communication speed requested from the mobile station can be exceeded, so that an appropriate communication mode is automatically decided by switching the communication modes.

TECHNICAL FIELD

The present invention relates to a wireless communication system, and more particular, the present invention relates to a technique effectively applied to a method of transmitting/receiving data between a plurality of base stations having a plurality of transmitting/receiving antennas and a plurality of mobile stations associated with each base station.

BACKGROUND ART

As a technique relating to a wireless communication system, there are, for example, techniques described in Non-Patent Documents 1 to 5 and techniques described in Patent Documents 1 to 3.

In Non-Patent Document 1, for the communication capacity of a one-to-N point wireless communication (BC: Broadcast Channel) in which one point of a transmitting station is associated with N points of receiving stations, Dirty Paper Coding is disclosed as a system of setting an upper limit of the system capacity. Also, in Non-Patent Document 2, existence of a system for maximally providing the capacity is disclosed.

Non-Patent Document 3 discloses a concept of a system for improving a throughput of the entire system by apparently increasing a total number of antennas on the transmitting station end to execute the above-described Dirty Paper Coding with a plurality of transmitting stations coordinately associated with each other.

Non-Patent Document 4 discloses an eigenmode transmission system as a system of maximally providing the communication capacity obtained by wireless propagation channel in one-to-one point wireless communication (Point to Point) in which one point of a transmitting station is associated with one point of a receiving station.

Non-Patent Document 5 discloses a technique of estimating channel station information in a wireless area in a multiuser system having one point of a transmitting station and a plurality of points of receiving stations.

Patent Document 1 discloses a technique of simultaneous transmission by performing space division (SDMA: Space Division Multiple Access) for a plurality of mobile stations in a time slot of time division (TDMA: Time Division Multiple Access).

Patent Document 2 discloses a method of calculating a combination of antennas for developing an optimum performance among a plurality of transmitting/receiving antennas.

Patent Document 3 discloses a technique of increasing the communication capacity by adjusting a modulation system and a transmission bit rate such as a coding rate responding to the variation of SDMA environmental parameters.

PRIOR ART DOCUMENTS

Patent Documents

Non-Patent Document 1: “Writing on dirty paper”, IEEE Trans. Inform. Theory, vol. 29, issue 3, May 1983, Written by M. Costa, published by IEEE, p. 440, FIG. 1: Variation of Gaussian-Shannon channelNon-Patent Document 2: W. Yu and J. M. Cioffi, “Sum capacity of Gaussian vector broadcast channels”, IEEE Trans. Inform. Theory, Vol. 50, No. 9, pp. 1875-1892, September, 2004Non-Patent Document 3: S. Shamai and B. Zaidel, “Enhancing the cellular downlink capacity via co-processing at the transmitting end”, in Proceedings of IEEE Vehicular Tech. Conf., May 2001-Spring, pp. 1745-1749Non-Patent Document 4: “Applications of Space Division Multiplexing and Those Performance in a MIMO Channel”, the transactions of the Institute of Electronics, Information and Communication Engineers. B Vol. J-87_B, No. 9, September, 2004 published by the Institute of Electronics, Information and Communication Engineers, written by Takeo Ohgane, Toshihiko Nishimura, and Yasutaka OgawaNon-Patent Document 5: “An Improved User Selection Algorithm in Multiuser MIMO-BC (Broadcast) with Channel Prediction”, IEICE Technical Report RCS2008-11 (2008-05), published by the Institute of Electronics, Information and Communication Engineers, written by Shi Min and Tomoaki Ohtsuki.

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

Incidentally, in a wireless communication system, as accompanied with a high-speed system in recent years, a MIMO (Multiple Input Multiple Output) technique of transmitting data from a plurality of antennas and receiving the same by a plurality of antennas has been adopted for many wireless standard such as wireless LAN and mobile communication from a viewpoint of improving frequency utilization efficiency.

For example, in one-to-one wireless communication (point to point) in which one point of a transmitting station is associated with one point of a receiving station, there is known an eigenmode transmission system disclosed in Non-Patent Document 4 as a system of maximally providing communication capacity obtained by a wireless propagation channel. In this eigenmode transmission system, the capacity obtained by the transmission channel is maximally provided as characteristics of the wireless propagation channel between the transmitting and receiving antennas by performing eigen decomposition (singular Value Decomposition) to a channel matrix “H” and calculating the matrix obtained by the eigen decomposition to be weighted as a transmitting vector signal and a receiving vector signal. However, since the channel state information measured at the receiving end is fed back to the transmitting end, an actual channel state in the communication is different from the fed-back channel state when the wireless propagation channel significantly varies, and therefore, there arises a problem that a communication performance is deteriorated. In a system presuming a semi-static environment that a user does not move so much such as a wireless LAN environment, the system is effective because of low variation in the wireless propagation channel.

Further, many studies have been made on communication capacities of a one-to-N wireless communication (BC) in which one point of a transmitting station is communicated with N points of receiving stations and an M-to-one wireless communication system (MAC: Multiple Access Channel) in which M points of transmitting stations are communicated with one point of a receiving station, from a viewpoint of information theory. In the communication capacity of the BC, the Dirty Paper Coding disclosed in Non-Patent Document 1 has been introduced as a system of providing an upper limit of the system capacity, and Non-Patent Document 2 establishes existence of a system for maximally providing the capacity.

In the one-to-N BC as compared with the above-described one-to-one eigenmode transmission system, a total number of antennas on the receiving end is increased as increase in the number of users, and therefore, the communication capacity which can be provided in the entire system is increased.

The techniques disclosed in these Non-Patent Documents are not established unless the channel state information formed in the space between the transmitting/receiving stations is instantaneously acquired on the transmitting end. In an actual system, the channel state information is measured on the receiving end, and is notified to the transmitting end with using a feedback link connected from the receiving end to the transmitting end. The channel varies during a delay time required for this feedback, and the deterioration of the performance becomes large, and therefore, it is difficult to achieve the techniques.

Further, in the one-to-N communication capacity of BC, the communication capacity which can be provided is limited due to the limitation of the number of antennas of the transmitting station. Accordingly, the above-described Non-Patent Document 3 discloses a concept of a system of improving a throughput of the entire system by apparently increasing a total number of antennas on a transmitting station end by coordinately associating a plurality of transmitting stations with each other to execute the above-described Dirty Paper Cording.

Still further, in a conventional wireless communication system, various methods are known such as a method of FDMA (Frequency Division Multiple Access) in which a frequency is divided from that of an adjacent base station when signals are simultaneously transmitted from the base stations to the mobile stations and reusing the frequency at separated locations away from each other by design of a cell structure, and a method of CDMA (Code Division Multiple Access) in which, even when a plurality of base stations transmit signals with the same frequency, the frequency is multiplexed by coding and the signals are fetched by the same coding on the receiving end. Still further, a method is also known such that time division (TDMA) is performed to a plurality of base stations. Still further, in recent years, as disclosed in the above-described Non-Patent Document 3, a system in which a plurality of base stations are associated with each other to communicate with a plurality of mobile stations by space division (SDMA) has been also proposed. These multiple access techniques are decided and operated based on a system standard.

For example, in the above-described Patent Document 1 in which simultaneous communications by SDMA are performed to a plurality of mobile stations in a time slot of TDMA, although a SDMA communication for a mobile station contained by one base station is disclosed, a system of improving a throughput in the entire system by performing the SDMA communication in which the plurality of base stations are coordinated with each other is not disclosed.

Further, in the method disclosed in the above-described Patent Document 2, a common code book is used between transmitting and receiving stations as the feedback information. However, when a difference between the channel state information represented by the code book and the measured channel state information is large, there arises a problem such as the deterioration of the communication performance. More particularly, in the case of performing the SDMA communication to the plurality of mobile stations by coordinating the plurality of base stations with each other, for the channel state information between the plurality of mobile stations and the plurality of base stations, geographical distribution occurs in the variation of the channel state information which is significantly varied in some mobile stations while slightly varied in some mobile stations as increasing the containing area. If the channel state information is represented by the code book so as to also include this geographical distribution, a type of the channel state information handled by the code book has to be consequently increased. It is necessary that there arises a difference in a structure between the code book for the channel state information required for the SDMA communication for only one base station and the code book required for the case of the coordination among the plurality of base stations. In such a system as switching to be the scalable communication mode in a scalable manner, a system which can reduce the feedback volume and correctly acquire the channel state information is required.

Further, in the technique disclosed in Patent Document 3, a degree of reliability is judged based on correlation between the original channel state information and the channel state information estimated, followed, and calculated on the receiver end. This technique is different from that of the present invention in which the degree of reliability is judged based on correlation between the received channel state information and the channel state information which has been reported. In addition, in Patent Document 3, with using this reliability, a modulation system and an error-correcting code are switched. However, the present invention is also different in a point that this reliability is used for switching a communication mode such that the SDMA itself is not used and is replaced with a MIMO-TDMA or others.

In the wireless communication system as described above, the system in which a plurality of base stations are coordinated with each other to communicate with a plurality of mobile stations by the SDMA has a possibility of increasing the capacity provided by the system. On the other hand, due to the increase of the number of mobile stations and the number of transmitting/receiving antennas simultaneously handled by the system, there arises a problem that a volume of the channel state information communicated between the transmitting/receiving stations is increased. In the increase of the volume of the channel state information, when the channel state information is precisely fed back from the receiving end to the transmitting end, a state of the channel is changed due to delay of the feedback to cause a difference between the channel at the time of communication and the channel acquired by the feedback, and therefore, the transmission rate provided by the system is deteriorated. Also, even in a case of no change of the state of the channel, when a volume of the feedback is large, there arises a problem that a substantial throughput is deteriorated due to communication protocol overhead.

Further, since the mobile station has various pieces of the channel state information such as stationary and moving one, the feedback needs to be performed at an appropriate timing in order to always keep the latest channel state information which is changed every moment sometimes. Further, when a traffic volume of the communication demanded by the mobile station is not large, the demand may be satisfied even without the coordination among the base stations by a conventional communication method of performing communication of a single base station with one or a plurality of mobile stations, and therefore, it is not necessary to always perform the SDMA communication by a plurality of base stations.

In consideration of these circumstances, a wireless communication system is required, in which a communication mode capable of providing resources necessary for exceeding a demand speed of the mobile station can be automatically decided and switched. Further, for the problem of the increase of the feedback volume when the communication mode is switched, a communication mode is required to provide a mode for reducing the feedback volume to prevent efficiency decrease of the frequency utilization efficiency. Accordingly, a preferred aim of the present invention is to provide a wireless communication system which exceeds a transmission rate demanded by a mobile station and reduces a feedback volume.

The above and other preferred aims and novel characteristics of the present invention will be apparent from the description of the present specification and the accompanying drawings.

Means for Solving the Problems

The summary of the typical ones of the inventions disclosed in the present application will be briefly described as follows.

That is, as the summary of the typical ones, in a wireless communication system including: a plurality of base stations having a plurality of antennas; a plurality of mobile stations having a plurality of antennas; and a base station controller of collectively controlling the plurality of base stations. The wireless communication system includes, during certain constant time (time slot): a communication mode (called “MIMO+TDMA communication mode”) by which only one base station and one mobile station are communicated with each other; a communication mode (called “SDMA+TDMA communication mode”) by which one base station and a plurality of mobile stations are simultaneously communicated with each other; and a communication mode (called “multipoint SDMA communication mode”) by which a plurality of base stations and a plurality of mobile stations are simultaneously communicated with each other by coordinating the plurality of base stations with each other by the base station controller, and an appropriate communication mode is automatically decided by switching the communication modes based on judgment whether or not a communication speed demanded by the mobile station is exceeded, by the base station and/or base station controller.

For the judgment whether or not the demand transmission speed is exceeded, channel state information between the antenna of the base station and the antenna of the mobile station is collected, a communication capacity which can be provided in each communication mode is calculated from the collected channel state information, and it is judged whether or not the demand transmission speed is exceeded by a degree of the communication capacity.

In a method of collecting the channel state information between the antenna of the base station and the antenna of the mobile station by the base station or the base station controller, the base station includes: means for transmitting a preamble signal with an already-known pattern necessary for measuring the channel state information at the mobile station; and means for notifying information relating to a feedback method of deciding how the mobile station feeds back the channel state information. In the method, the mobile station measures the channel state information by the preamble signal, it is judged whether or not the measured channel state information is fed back or others in accordance with the information relating to the feedback method, and the channel state information is fed back based on a judgment result, so that a feedback volume of the channel state information is reduced.

More specifically, since the channel state information which the base station uses is based on the channel state information having already been fed back and notified to the base station in past by the mobile station, correlation between the channel state information used in present by the base station and the latest channel state information measured from the preamble signal by the mobile station is acquired. When the correlation is high, the feedback volume is reduced by feeding back only the information that the past channel state information can be used as it is. Alternatively, an actual error rate of the communication data or others is measured, and if the error rate is within an allowable range, it is judged that the channel state information used by the base station which is notified in the past can be continuously used, and only the information that the channel state information can be used is fed back.

Effects of the Invention

The effect obtained by typical aspects of the present invention will be briefly described below.

That is, as the effect obtained by typical aspects, a wireless communication system capable of exceeding a communication speed demanded by a mobile station and reducing a feedback volume can be provided.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment, and the repetitive description thereof will be omitted.

FIG. 1illustrates an example of the entire structure of a wireless communication system according to an embodiment of the present invention. The wireless communication system includes: a base station controller101a; a plurality of base stations102a,102b. . . connected to the base station controller101a; a plurality of mobile stations103a,103b,103c,103d. . . transmitting/receiving data to/from these base stations102a,102b. . . ; and others. Each of the base stations102a,102b. . . has a plurality (for example here, four) of antennas. Further, each of the mobile stations103a,103b,103c,103d. . . has a plurality (for example here, two) of antennas.

Note that, in this wireless communication system, the base station controller101ais connected to a gateway104, and further, this gateway104is connected to the base station controller101b. Signals among a plurality of base station controllers are transferred via the gateway104. Further, the gateway provides means for access to a core network105. The core network105is connected to Internet.

The base station controller101acontrols the base stations102a,102b. . . under its control to achieve to control the collaborative-coordination among the base stations. The example ofFIG. 1illustrates a state in which the base stations102aand102bcontrol the collaborative-coordination for SDMA communication at certain moment to transmit data to the mobile stations103aand103c. The mobile stations103aand103cperform the received signal processing by separating a desired signal. At this time, the base station controller101aforms a transmitting signal for the SDMA, acquires the channel state information for total four receiving antennas of the mobile stations103aand103cwith using total eight transmitting antennas of the base stations102aand102b, and performs the received signal processing for the broadcast channels of 8×4.

Further, the base station controller101acan be connected to the other base station controller101bvia the gateway104, and they mutually exchange information of the controlled base stations, so that interference with each other can be prevented by allocating a different frequency thereto, and the base stations under control of each base station controller can be coordinately controlled by collaborative-coordination between the base station controllers depending on cases. By coordinately controlling among the base stations, the number of transmitting antennas is increased, so that the throughput of the entire system can be improved.

FIG. 2illustrates an example of a control sequence in a MIMO+TDMA (referred to also as MIMO-TDMA) communication mode in which communication between the base station and the mobile station is time-divided so as to be in one-to-one correspondence with each other at a moment.

First, the base station102atransmits a preamble signal for acquiring the channel state information in a TDMA time slot (Slot) #1. It is assumed that the mobile station103apreviously acquires a right to communicate in the time slot #1 based on a communication protocol. The mobile station103areceives the preamble signal and performs channel estimation. The mobile station103afeeds back the acquired channel state information “Haa” to the base station102a. Here, the channel state information notified from the mobile station to the base station may be the channel estimation information acquired by receiving the preamble signal at the moment or the averaged channel state information acquired by receiving the preamble signal several times. Also, when the preamble signal has not been received in the time slot, the channel state information acquired when received in the previous time slot may be used.

Subsequently, the base station102agenerates a MIMO data signal for MIMO communication by transmission signal processing with using the fed-back channel state information Haa, and transmits this MIMO data signal. The mobile station103areceives the MIMO data signal, and performs the received signal processing of separating the signal multiplexed for a plurality of antennas. As this MIMO communication, for example, an eigenmode transmission system can be cited.

The communication is performed by a procedure that the base station102acommunicates (with using channel state information “Hab”) with the mobile station103bin a next time slot #2 by the same procedure as described above, the base station102bcommunicates (with using channel state information “Hbc”) with the mobile station103cin a next time slot #3, and further, the base station102bcommunicates (with using channel state information “Hbd”) with the mobile station103din the a time slot #4. In this communication mode, it is assumed that a schedule about which base station communicates with which mobile station at what time is previously made based on a communication protocol.

FIG. 3illustrates an example of a control sequence in a SDMA+TDMA (referred to also as SDMA-TDMA) communication mode in which a single base station performs SDMA communication with a plurality of mobile stations, and the transmission time between the base stations is time-divided. It is assumed that the time slot #1 is previously allotted for the base station102abased on a communication protocol. In addition, it is assumed that the mobile stations103aand103bare dependent on (associated with) the base station102abased on a communication protocol.

In the TDMA time slot #1, the base station102atransmits a preamble signal, and the mobile stations103aand103breceiving this preamble signal estimate the channel state information “Haa” and “Hab” and feed back the result to the base station102aas channel state information. As described above with reference toFIG. 2, the channel state information may be the averaged channel state information or the channel state information estimated by receiving the previous preamble signal.

The base station102agenerates a SDMA data signal by SDMA transmission signal processing as disclosed in, for example, the above-described Non-Patent Document 2 with using the acquired channel state information Haa and Hab, and transmits this SDMA data signal. The mobile stations103aand103bfetches a desired signal by performing the SDMA received signal processing as disclosed in, for example, the above-described Non-Patent Document 2.

In a next time slot #2, the base station102bcommunicates (with using channel state information “Hbc” and “Hbd”) with the mobile stations103cand103dby the same procedure as described above.

FIG. 4illustrates an example of a control sequence in a multipoint SDMA (referred to also as MP-SDMA) communication mode in which a plurality of base stations are coordinated with each other to perform SDMA communication with a plurality of mobile stations.

In a TDMA time slot #1, the base station controller101agenerates a preamble signal, and transmits the preamble signal with using antennas of the base stations102aand102b. Alternatively, each of the base stations102aand102bmay generate a preamble signal, and transmission time by the base station102aand transmission time by the base station102bmay be previously decided based on a communication protocol. The mobile stations103a,103b,103c, and103dreceive this preamble signal, and estimate the channel state information Haa, Hab, Hac, Had, Hba, Hbb, Hbc, and Hbd. Each mobile station notifies the acquired channel state information to the base station controller101a. As described above with reference toFIG. 2, the channel state information may be the averaged channel state information or the channel state information estimated by receiving the previous preamble signal. For example, since the mobile station103ais associated with the base station102a, the mobile station103acannot receive the preamble signal transmitted from the base station102b, and therefore, may not acquire the channel state information. In this case, no channel state information may exist, the previous channel state information may be used, or the averaged channel state information may be transmitted.

The base station controller101agenerates the SDMA data signal by the SDMA transmission signal processing (as disclosed in, for example, the above-described Non-Patent Document 2) based on the acquired channel state information Haa+Hba, Hab+Hbb, Hac+Hbc, and Had+Hbd, and simultaneously transmits the data to the mobile stations103a,103b,103c, and103dvia the base stations102aand102b. Each mobile station fetches a desired signal by performing the SDMA received signal processing (as disclosed in, for example, the above-described Non-Patent Document 2).

FIG. 5illustrates an example of the processing in a case of mixing a plurality of communication modes such as the MIMO-TDMA, the SDMA-TDMA, and the multipoint SDMA descried above. In addition,FIG. 5also illustrates a relationship with a frame.

Although each communication mode is mixed within one frame, it is judged whether or not the communication should be performed as follows (0: does not communicate and 1: communicates) in each communication mode (MIMO-TDMA (or SIMO: Single Input Multiple Output), SDMA-TDMA, and MP-SDMA). Note that the frame time is previously fixedly decided so as to exceed a demand delay time of an application for the mobile station. Alternatively, the frame time may be dynamically decided so as to exceed a minimum value of a delay time demanded by the application for the mobile station.

With using the channel state information utilized in the previous frame and/or a statistical value of the channel state information, a structure of a communication mode in a next frame is judged on an assumption that such same channel state information can be acquired in the next frame.

For example, a transmission rate acquired when a mobile station “k” performs the MIMO-TDMA communication with using the entire frame is expresses by “Cmimo_k”, and a demand speed of the mobile station is expressed by “R_k”. When a following formula (1) is satisfied for all of the mobile stations k, only the MIMO-TDMA communication is performed over the entire frame.
Cmimo—k≧R—kFormula (1)

Here, when a transmission rate acquired when the SIMO communication with using one transmitting antenna and N receiving antennas is performed by fixing one antenna on the transmitting end, selecting one antenna at random, or selecting one antenna having the largest stream of an eigen value is expressed by “Csimo_k”, even if the MIMO signal processing is not performed for the mobile station k satisfying a following formula (2), the SIMO communication may be performed in a provided time slot.
Csimo—k≧R—kFormula (2)

If a mobile station not satisfying the above-described formula (1) exists, when a transmission rate acquired when the SDMA-TDMA communication is performed with using the entire frame is expressed by “Csdma_k” and when a following formula (3) is satisfied, only the SDMA-TDMA communication is performed over that entire frame.
Csdma—k≧R—kFormula (3)

If the formula (3) is not satisfied, when a ratio of time for the SDMA-TDMA communication with respect to 1 representing time for the entire frame is expressed by “α” and a following formula (4) is satisfied for all of the mobile stations k, the MIMO-TDMA communication and the SDMA-TDMA communication are time-divided by the time ratio α to be operated within the frame.
(1−α)×Cmimo—k+α×Csdma—k≧R—kFormula (4)

If the formula (4) is not satisfied, when a transmission rate acquired when the MP-SDMA communication is performed with using the entire frame is expressed by “Cmpsdma_k” and when a following formula (5) is satisfied, only the MP-SDMA communication is performed over that entire frame.
Cmpsdma—k≧R—kFormula (5)

If the formula (5) is not satisfied, when ratios of time for the SDMA-TDMA communication and for the MP-SDMA communication with respect to 1 representing time for the entire frame are expressed by “α” and “β”, respectively, α and β satisfying a following formula (6) are searched by linear programming or others. If a satisfying solution exists, the communications are operated by respective time ratios.
(1−α−β)×Cmimo—k+α×Csdma—k+β×Cmpsdma—k≧R—kFormula (6)

In this manner, by deciding the SDMA-TDMA communication and MP-SDMA communication based on the MIMO-TDMA communication, the MIMO-TDMA communication or the SIMO-TDMA communication is performed when the demand transmission rate is small. This is because a size of the channel state information required for each communication mode becomes larger in order of the SIMO-TDMA communication, the MIMO-TDMA communication, the SDMA-TDMA communication, and the MP-SDMA communication, and therefore, the operation so as to reduce the volume of feedback information as less as possible if not required can be achieved.

However, when the demand transmission rate satisfies a case such as a best-effort system, a demand transmission rate “Rk” is set to be a theoretical limitation in using all of the transmitting/receiving antennas, and time ratio may be acquired so as to minimize the “(left-hand side)-(right-hand side)” of the formula (5).

FIG. 6illustrates, in a case of no overhead of the feedback of the channel state information, an example of a relationship between transmission power (Power) and frequency utilization efficiency per user of each communication mode. As the transmission power, total transmission power of all the transmitting antennas with respect to the power of thermal noise on each antenna is illustrated in decibel. As a channel model, i.i.d Rayleigh channel model is assumed, which is an example calculated so as to have two base stations having four antennas and four mobile stations having two antennas as illustrated inFIG. 1. Note that the transmitting power in the MP-SDMA is a total transmitting power in two base stations. In this manner, the frequency utilization efficiency becomes higher in order of the MIMO-TDMA, the SDMA-TDMA, and the MP-SDMA, and the processing is satisfied by the MIMO-TDMA if the demand speed is slow as described above. However, it is found that processing such as the SDMA-TDMA or the MP-SDMA is required as the demand speed is higher.

FIG. 7illustrates an example of the channel state information. The example shows the uncompressed channel state information in IEEE 802.11n standard of the wireless LAN.

When the number of receiving antennas is two, an SNR (signal-to-noise ratio) of each antenna is expressed by 8 bits first. Subsequently, the channel state information (H11) for each subcarrier of OFDM (orthogonal frequency division multiplexing) signals is shown. The first information “3 bits” represents an amplitude gain applied over the whole channel. In the receiving, 8 bits are allocated to each of a real part and an imaginary part of a complex number in a matrix composed of a total number (Ntx) of the transmitting antennas×the number (2) of the receiving antenna in the received preamble signal. This channel matrix is provided by the number of subcarriers (f1 to f_L). In such channel state information, as the number of transmitting antennas increases, the size of the channel state information increases.

When the example illustrated inFIG. 1is used, while the number of transmitting antennas is four in the MIMO-TDMA or SDMA-TDMA communication modes, the number of transmitting antennas is eight in the MP-SDMA communication mode to increase twice the size of the channel state information. Although there is a method for compressing this channel state information even in IEEE 802.11n standard, an error occurs on the channel state information to be fed back by the compression. Further, in a standard of the cellular communication system such as WiMAX (worldwide interoperability for microwave access) and LTE (long term evolution), there is a method for returning the channel state information to the transmitting end by previously sharing the channel state information at the transmitting end and the receiving end with using a code book and judging which one of the channel state information acquired at the receiving end is closest. Also in this case, the deterioration of the channel state information occurs.

FIG. 8illustrates an example of characteristics between the feedback volume of the channel state information and the frequency utilization efficiency per user in each communication mode. Even when the same volume of the channel state information is fed back from each mobile station, the channel state information only for one mobile station is collected in the MIMO-TDMA, one base station collects the channel state information for two mobile stations in the SDMA-TDMA, and the channel state information for four mobile stations is collected in the MP-SDMA, and therefore, the overhead for collecting the channel state information for one communication is increased. In addition, in the MP-SDMA, the channel state information is also transmitted from the adjacent base station, and beside, the channel state information is large. Therefore, time in which communication can be effectively performed becomes short, and thus, a degree of deterioration of the frequency utilization efficiency increases as the volume of the channel state information increases.

FIG. 9illustrates an example of an enhanced preamble signal according to the embodiment of the present invention. InFIGS. 2 to 4, the preamble signal for acquiring the channel state information by the mobile station is illustrated. However, by replacing this signal with the enhanced preamble signal illustrated inFIG. 9, a case of not requiring returning the measurement result of the channel state information itself can be provided. Similarly to the example of the conventional preamble signal, by a scattered-type method, a preamble signal with a fixed pattern is transmitted by changing an antenna for every symbol. The signal may be transmitted by a space-time coding type method of simultaneously transmitting the signal from each antenna.

Subsequently, feedback judgment information, which is necessary for deciding whether or not the mobile station feeds back the channel state information, is transmitted. First, the feedback judgment information is transmitted from an antenna #1, and the received signal processing is performed on the receiving end by a conventional method without performing the signal processing for the MIMO or the SDMA. In a next frame, a state in which the feedback judgment information is transmitted with using an antenna #2 is illustrated. In this example, the signal processing for the MIMO or the SDMA is not performed on the receiving end. However, when the signal processing for the MIMO or the SDMA is performed in the previous frame, the feedback judgment information may be transmitted with using all the antennas by using the signal processing in the previous frame.

FIG. 10illustrates an example of a format of the feedback judgment information. In addition,FIG. 10illustrates an example of information contents. The format of the feedback judgment information includes fields such as a feedback mode, a base station ID/base station controller ID, channel state information which a base station uses, a feedback judgment type, a feedback judgment threshold, a mobile station ID, and a used channel ID.

The feedback mode is a field which specifies how the mobile station having received the preamble signal feeds back the channel state information. For example, if 1 is specified in the field, only the channel state information transmitted from the transmitting antenna of the base station on which the mobile station is associated is fed back. This specification is used in the MIMO-TDMA or SDMA-TDMA communication mode. If 2 is specified in the field, preamble signals transmitted from all the base stations from which the mobile station can receive are received, and their channel state information are fed back. In this manner, the multipoint MIMO can be processed.

If 3 is specified in the field, the channel state information is judged as OK or NG depending on whether or not conditions described in the fields of the feedback judgment type and the feedback judgment threshold are satisfied, and 1 bit information for OK or NG is fed back. If 4 is specified in the field, although the same as the specification of 3, 1 bit information for NG of the channel state information is fed back only when NG of the channel state information exists. If there is no problem for the channel state information due to specifying 3 or 4, the volume of feedback information can be significantly reduced.

If 1 is set in the feedback judgment type, with using a channel correlation coefficient as a judgment reference, the channel information is judged by a correlation coefficient between the channel state information which the base station uses and a channel estimation result obtained by estimating a channel which the base station uses based on the channel state information acquired in receiving of the preamble signal. A specific value of the judgment is described in the feedback judgment threshold. For example, the channel state information is judged as OK if the correlation coefficient is larger than “A=0.9”, and the channel state information is judged as NG if the correlation coefficient is equal to or smaller than “A=0.9”. In the feedback judgment type, not only the channel correction coefficient but also, for example, an error rate (2 is set) of a decoding result is used, and the channel state information may be judged as NG if the decoding error rate is expressed by “BER>10^−3” when the feedback judgment threshold is expressed by “B=3”, and the channel state information may be judged as OK in other cases.

In addition, for the feedback judgment type, a received signal strength indication (RSSI) of the preamble signal or the data signal, the Doppler frequency, a mobile speed of the mobile station, or others may be used. That is, if under an environment such that the received power is low or the channel state information widely varies due to high speed movement, it is judged that usage of the content is not appropriate for the MIMO or the SDMA, and in this case, the feedback volume of the channel state information can be automatically reduced.

When the channel correlation coefficient is specified for the feedback judgment type, how to acquire the channel state information which the base station uses is to read from the field of the channel state information which the base station uses. If 1 is specified in the field, it is simply considered that the base station uses the channel state information that the mobile station has reported to the base station at the end. If 2 is specified in the field, as assuming that estimation tracking processing is performed on the base station end for a receiving channel of the mobile station with using a method disclosed in Non-Patent Document 5 or others, it is considered that the base station uses a result obtained by the channel estimation tracking processing with using the same algorism on the mobile station end.

If 3 is specified in the field, as assuming that, for example, both of the base station and the mobile station have the same channel database, the channel state information which the base station uses can be known by notifying an index of the channel which the base station uses as an ID. In this case, since the ID is different depending on which channel is used in each mobile station, a set of the mobile station ID and the used channel ID is provided as a field. Further, the channel database which is the same in the base station and the mobile station may be previously calculated such as the above-described code book, or the database of the channel state information may be prepared in both the base station and the mobile station by adding a protocol by which the base station provides an ID to the channel state information has been reported by the mobile station.

FIG. 11illustrates an example (mix of the multipoint SDMA and the MIMO-TDMA) of a control sequence according to the embodiment of the present invention. The example shows a state that acquirement of the channel state information of all the mobile stations in the previous frame has been already started and the MP-SDMA communication is performed.

In the MP-SDMA time slot #1, the base station controller101afirst transmits an enhanced preamble signal with a content for judging whether or not the channel state information is fed back via the base station102a, and each of the mobile stations103a,103b,103c, and103dstores the channel state information Haa, Hab, Hac, and Had by the channel estimation processing. Next, similarly, the enhanced preamble signal is transmitted via the base station102b, and the channel state information Hba, Hbb, Hbc, and Hbd are measured and stored by each of the mobile stations103a,103b,103c, and103d. Based on these pieces of the channel state information, each of the mobile stations103a,103b,103c, and103dnotifies the base station controller101awhether or not the channel state information which the base station uses is OK or NG to be used as described with reference toFIG. 10. For example, it is assumed that the channel state information is NG in only the mobile station103d. The base station controller101aacknowledges that the channel state information cannot be used for the mobile station103d, and generates the SDMA data signal and transmits it to the rest of the mobile stations103a,103b, and103c.

The demand speed cannot be exceeded in the terminal station103dwithout any action, and therefore, the enhanced preamble signal is transmitted in a next time slot #2, and the channel state information Hbd which is a result measured by the mobile station103dis notified to the base station controller101a. At this time, the MIMO data signal is transmitted by the signal processing based on the MIMO-TDMA, and is received by the mobile station103d.

In a next frame, it is heard with using the channel state information updated from the mobile station103dwhether or not the MP-SDMA can be used, and similarly, that the channel state information is OK or NG. In this manner, as repeating the hearing several times, the mobile station repeatedly notifying the channel state information of NG is considered as a candidate for the MIMO-TDMA or the SDMA-TDMA because of having a tendency of the channel state information inappropriate for the MP-SDMA processing.

In the channel state information, the MP-SDMA processing is inappropriate for the mobile station when variation of the channel state information of the mobile station is large to the adjacent base station, and the SDMA-TDMA processing is also inappropriate when variation of correlation of the channel state information with the other mobile station included in the SDMA group is large even if the SDMA-TDMA processing is performed, and therefore, the MIMO-TDMA processing is preferable.

FIG. 12illustrates an example of a control sequence relating to a dynamic operation such as shifting to or termination from the multipoint SDMA communication according to the embodiment of the present invention.

When the demand speed from the mobile stations103a,103b,103c, and103dis small, the demand speed is exceeded by performing the SDMA-TDMA communication, the MIMO-TDMA communication, or the SIMO-TDMA communication with the associated mobile stations by the base stations102aand102b. In this case, when the same frequency resource is used by the base stations102aand102b, the base stations102aand102bcommunicates with each other in time division.

When the demand speed of the mobile station is exceeded, the above-described communication is continued. However, for example, when it is detected here that the demand speed is not exceeded in the mobile station103bassociated with the base station102b, a control message for a MP-SDMA request is transmitted from the base station102bto the base station controller101a, and the base station controller101atransmits a control message for an MP-SDMA start notification to the base stations102aand102bbased on the information of the base station or others whose periphery can be caught by the mobile station not exceeding the demand speed, so that the MP-SDMA communication, the SDMA-TDMA communication, the MIMO-TDMA communication, or the SIMO-TDMA communication, which are mainly controlled by the base station controller101a, is performed.

When the demand speed of the mobile station cannot be exceeded even if the MP-SDMA communication is performed, the base station controller101ajudges resource shortage, and decided the mobile station which exceeds the demand speed in the entire system based on the priority information of the connection maintenance which the mobile station has, by disconnecting the mobile station having a low priority. To the mobile station to be disconnected, a control message for a connection refusal notification is notified, and the connection link is cut off. The disconnected mobile station stands by for a certain period of time, and then, tries the connection request again. However, the base station controller101arefuses the connection for the new connection request when a margin for providing the system capacity is not sufficient, or accepts the connection when the margin is sufficient.

Further, when a state in which the demand speed of the mobile station is exceeded continues for N slots even if the MP-SDMA communication is not performed, the base station controller101ajudges that the MP-SDMA processing is unnecessary, and notifies a MP-SDMA termination notification message to the base stations102aand102b, so that the base stations102aand102bperform the SDMA-TDMA communication, the MIMO-TDMA communication, or the SIMO-TDMA communication to their associated mobile stations.

FIG. 13illustrates an example of a control algorism of the base station according to the embodiment of the present invention.

First, the channel state information is collected from the mobile station associated with the base station (Step1). The SIMO-TDMA communication, the MIMO-TDMA communication, or the SDMA-TDMA communication is decided by using the formulas (1) to (4) from the collected channel state information and the demand speed of each mobile station (Step2). It is judged whether or not the estimated speed of the associated mobile station exceeds the demand speed (Step3). If it is not exceeded (No), the control massage for the MP-SDMA processing request is transmitted to the base station controller (Step4), and a waiting state for the MP-SDMA termination is transmitted from the base station controller. When the control message for the termination notification of the MP-SDMA communication is received from the base station controller, the first step (Step1) of the collection of the channel state information from the mobile station is started again.

If, as the result of the judgment in Step3, the estimated speed exceeds the demand speed (Yes) in all the associated mobile stations so as to satisfy the formulas (1) to (4), the base station communicates with the associated mobile stations by the selected communication mode (Step5) in performing the SIMO-TDMA communication, the MIMO-TDMA communication, or the SDMA-TDMA communication are performed. The processing from Step1is repeated until the actual speed exceeds the demand speed (Step6). Alternatively, even when a ratio of the mobile stations whose actual speed exceeds the demand speed is not 100%, if the ratio exceeds a predetermined ratio, the processing may proceed to Step7. In a next time slot in which the selected communication has been performed, first, it is judged that the channel state information which the base station currently uses is OK or NG by the mobile station with using the enhanced preamble signal, and the information on the judgment result is collected (Step7).

Then, one mobile station is selected (Step8), and it is judged whether or not the channel state information of the mobile station is OK (Step9). If the channel state information is NG (No), it is judged that the MIMO and SDMA communications are not effective for the mobile station, and the mobile station is memorized as a candidate for the TDMA communication in which the channel state information is unnecessary on the transmitting end (Step11). For example, on the mobile station end, the SIMO-TDMA communication or others which performs only a diversity technique for maximal-ratio combining is performed.

If the channel state information is OK (Yes), the mobile station is memorized as a candidate for the SDMA-TDMA communication or the MIMO-TDMA communication (Step10). The processing from Step 8 is repeated until all the mobile stations are memorized as a candidate for any communication mode (Step12). Based on the memorized communication candidate, in the mobile station whose channel state information is OK, any one of the SIMO-TDMA communication, the MIMO-TDMA communication, and the SDMA-TDMA communication is decided based on the formulas (1) to (4) (Step13). Another embodiment for reducing an amount of this calculation will be described.

With reference to the communication mode in the previous time slot, for example, when the SDMA-TDMA communication has been performed at the previous time, it may be judged that the SDMA-TDMA communication is to be continued if the channel state information of all the mobile stations belonging to a group for the SDMA is OK. Similarly, when the MIMO-TDMA communication has been performed at the previous time, it is judged that the MIMO-TDMA communication is to be continued if the channel state information is OK as long as the channel state information is OK, so that the communication mode performed in the previous time slot is continued.

For the communication mode decided as described above, it is judged whether or not the estimated speed of the mobile station whose channel state information is OK exceeds the demand speed (Step14). More specifically, it is judged whether or not the formulas (1) to (4) are satisfied. If the estimated speed of the mobile station whose channel state information is OK does not exceed the demand speed (No), the processing returns to the first Step1. If the estimated speed of the mobile station whose channel state information is OK exceeds the demand speed (Yes), the processing returns to Step5, and communication is performed to all the mobile stations whose channel state information is OK or NG with using the selected communication mode. Even here, even if the rate in which the estimated speed of the mobile station whose channel state information is OK exceeds the demand speed is not 100%, the processing may return to Step8if the rate exceeds a predetermined rate.

FIG. 14illustrates an example of a control algorism of the base station controller according to the embodiment of the present invention. The base station controller waits for an MP-SDMA request message from the base station.

When the MP-SDMA request message is transmitted from the base station, the MP-SDMA start notification message is issued to the related base station (Step1) based on the base station ID described in the preamble signal received by the mobile station associated with the base station to which the request is issued. When each base station receives the MP-SDMA start notification message, each base station switches so that the signal from the base station controller is directly transmitted to the antenna.

Subsequently, the related base station transmits the preamble signal to collect the channel state information from each mobile station (Step2). Based on the collected channel state information, the communication mode which is the SIMO-TDMA communication, the MIMO-TDMA communication, the SDMA-TDMA communication, or the MP-SDMA communication is decided responding to the demand speed of each mobile station with using the formulas (1) to (6) (Step3).

It is judged whether or not the estimated speed of all the mobile stations exceeds the demand speed (Step4). If a mobile station not exceeding exists (No), a connection refusal notification message is transmitted to the mobile station whose estimate speed does not exceed the demand speed (Step5). In a method of selecting this mobile station, the connection is simply refused to all the mobile stations whose estimated speed does not exceed the demand speed, and the processing may proceed to a next Step6.

Alternatively, a priority has been previously decided for each mobile station by authentication protocol or others, the connection is refused to a mobile station in order of a low priority, the communication mode of Step3is re-calculated without connecting the mobile station, and the processing loop from Step3to Step5may be repeated at Step4until the mobile station whose estimated speed does not exceed the demand speed does not exist. Alternatively, as the priority, magnitude of a difference between the estimated speed and the demand speed may be used. The connection may be refused to a mobile station in order of a large difference or a small difference.

Next, the communication is performed with the mobile station whose estimated speed exceeds the demand speed by the selected communication mode (Step6). Subsequently, it is judged whether or not the MP-SDMA communication is available as the selected mode (Step7). For example, when it is found that the demand speed of the mobile station is exceeded by the SDMA-TDMA communication, the MIMO-TDMA communication, or the SIMO-TDMA communication (No) even without continuously performing the MP-SDMA communication for N times, the MP-SDMA is judged to be unnecessary, and the MP-SDMA termination notification message is transmitted to each base station (Step8), and the processing returns to the waiting state for the MP-SDMA.

When it is judged that the MP-SDMA communication is necessary (Yes), it is judged whether or not the actual transmission rate has exceeded the demand speed (Step9) with using the result obtained by the communication at Step6. If the actual transmission rate does not exceed the demand speed (No), the processing returns to Step2.

When the actual transmission rate exceeds the demand speed (Yes), the communication is performed in a next time slot by judging the channel state information stored in the base station controller as either OK or NG, so that the feedback volume can be reduced. More specifically, it is collected either OK or NG for the channel state information from the mobile station with using the enhanced preamble signal (Step10).

Here, even when the rate in which the actual transmission rate exceeds the demand speed is not 100%, the processing may proceed to Step10if the rate exceeds a predetermined rate. Otherwise, the processing may proceed to Step2.

One mobile station is selected (Step11), and it is judged whether or not the channel state information of that mobile station is OK (Step12). If OK (Yes), that mobile station is memorized as a candidate for the MP-SDMA communication, the SDMA-TDMA communication, or the MIMO-TDMA communication (Step13).

If the channel state information is NG (No), the mobile station is memorized as a candidate for the TDMA communication which is the communication mode that the transmitting end does not need to acquire the channel state information (Step14). For example, in the SIMO-TDMA communication, the receiving end is mentioned as a candidate or others for the communication of performing maximal-ratio combining or selective combining of the antenna diversity technique.

It is judged whether or not all the mobile stations are judged (Step15), and if any mobile station not judged yet still exists (No), the processing returns to Step11. If all the mobile stations are judged (Yes), the SIMO-TDMA communication, the MIMO-TDMA communication, the SDMA-TDMA communication, or the MP-SDMA communication is decided by calculating whether or not the formulas (1) to (6) are satisfied for the mobile station whose channel state information is OK (Step16).

In each communication mode, it is judged whether or not the estimated speed exceeds the demand speed for the mobile station whose channel state information is OK (Step17). If the estimated speed exceeds the demand speed (Yes), the processing returns to Step6, and the communication is performed with the mobile station by the selected mode. If the estimated speed does not exceed the demand speed (No), the processing returns to Step2, and the channel state information is collected from the mobile station.

Here, even if the rate in which the estimated speed exceeds the demand speed for the mobile station whose channel state information is OK is not 100%, the processing may return to Step6if the rate exceeds a predetermined rate. Otherwise, the processing may return to Step2.

FIG. 15illustrates an example of a structure of the base station according to the embodiment of the present invention. The base station102(corresponding to102aand102binFIG. 1) includes: antennas1501(1501ato1501d); a radio frequency block1502; a modem1506; a control block1522; and an external node interface1527.

The radio frequency block1502includes: duplexers1503(1503ato1503d) having transmission/reception changeover function which is connected to the antenna1501; an RF receiver1504(1504ato1504d) which is connected to the duplexer1503; and an RF transmitter1505(1505ato1505d) which is connected thereto. The RF receiver1504i(i=a to d) performs a filtering processing to a received signal transmitted from the antenna1501i, and converts the signal into an analog signal with a baseband band, and then, converts the signal into a digital signal (A/D conversion), and output the signal to the modem1506. On the other hand, the RF transmitter1501i(i=a to d) converts the digital signal outputted from the modem1506into an analog signal (D/A conversion), and performs conversion of a frequency band and power amplification, and then, outputs the signal to the duplexer1503i.

The modem1506includes: a received signal switch1507; a transmitting signal switch1508; a baseband signal receiver1509; a baseband signal transmitter1516; an enhanced preamble signal generator1515; a parallel/serial (P/S) convertor1514; and serial/parallel (S/P) convertor1521.

The received signal switch1507functions as a switch for switching either the received signal being the output from the radio frequency block1502is processed by the baseband signal receiver1509within the base station or the received signal is passed through without any change and the passing is notified to the base station controller via the external node interface1527.

When the received signal is passed through and the passing is notified to the base station controller, the received signal is converted into a serial signal by the parallel/serial convertor1514, and then, the signal is converted into a signal by a transmission signal processing block for external node1528in the external node interface1527so as to match the communication mode between the base station and the base station controller, and the passing is notified. The base station and the base station controller are connected with each other by an optical communication module, Ethernet, or an exclusive line.

The baseband signal receiver1509includes: a demodulator1510; a parallel/serial convertor1511; a decoder1512; and a data/control signal separator1513. When a signal processing is performed to the received signal by the baseband signal receiver1509, a demodulation processing for the SIMO, the MIMO, or the SDMA is performed by the demodulator1510. In the demodulation for the SIMO, for example, a signal obtained by the maximal-ratio combining is demodulated by the OFDM. In the example of the MIMO signal processing, a demodulation processing in the eigenmode transmission is performed, or a demodulation processing in Zero Forcing, MMSE (Minimum Mean Square Error), or MLD method is performed. Also in the SDMA, a well-known demodulation processing in Block Diagonalization, Tomlinson Harashima Precoding, or others is performed.

A stream signal outputted from the demodulator1510is returned to a serial signal by the parallel/serial convertor1511, and an error correction processing is performed to this serial signal by the decoder1512. For the error correction processing, a method such as Viterbi decoding, Turbo decoding, and LDPC decoding may be used. In the data/control signal separator1513, it is judged that the decoded received-signal is either a data signal for data communication or a control signal for signaling protocol by checking a header part, and the control signal is outputted to the control block1522. If the signal is the data signal, the signal is outputted to the external node interface1527, is converted by the transmission signal processing block for external node1528, and is data-transmitted to the base station controller or a device such as other router of receiving the data.

The transmitting signal switch1508switches between an output signal from the baseband signal transmitter1516and an outputted signal obtained by performing the serial/parallel conversion to a signal outputted from the base station controller by the serial/parallel convertor1521via the external node interface1527. When the multipoint SDMA communication is handled, the base station is switched over to the base station controller.

The enhanced preamble signal generator1515acquires the feedback judgment information from the control block1522, and adds this information to the preamble signal, so that the enhanced preamble signal described with reference toFIG. 9is generated. More particularly, if it is not necessary to add the feedback judgment information thereto, only the preamble signal is generated.

The baseband signal transmitter1516includes: a modulator1517; a serial/parallel convertor1518; an encoder1519; and a multiplexer1520. The control signal and the data signal are multiplexed by the multiplexer1520, and an encoding processing is performed in the encoder1519so that the error correction can be performed on the receiving end. For the encoding processing, Viterbi encoding, Turbo encoding, LDPC encoding, or others may be used. In this example, an outputted signal from the encoder1519is separated into four data streams by the serial/parallel convertor1518, and a modulation processing based on the OFDM is performed for the SIMO, the MIMO, or the SDMA by the modulator1517. Such a modulation processing as the demodulation processing described above is performed, and the modulation is performed by the same communication mode as the demodulation processing at the receiver of the mobile station. In the modulation processing, a signal processing is performed with using the channel state information between the base station and the mobile station. A database of channel state information1525included in the control block1522is stored on a physical memory, and the channel state information is fetched by accessing to this memory, and a transmission wait queue or others is calculated from this channel state information.

The control block1522includes: a control signal processing block1523; a generator block of information that judges whether to feed back1524; and a channel estimation tracking block1526. The data handled by this control block1522includes a database of the “database of channel state information”1525.

The control signal processing block1523executes the control algorism described above with reference toFIG. 13, and executes a control flow illustrated inFIGS. 2,3,4,11, and12. The “generator block of information that judges whether to feed back”1524generates the feedback judgment information described with reference toFIG. 10responding to a state of the control signal processing block1523. For example, a feedback mode in a case of collecting the channel state information by the base station from all the mobile stations is decided, or a feedback mode in a case of collecting only OK or NG from the channel state information is decided. Further, with reference to the database of the “database of channel state information”1525, the mobile station ID, the used channel ID, and others inFIG. 10are decided. Values of other contents are decided, when the base station is activated, by referring to the setting information stored in a non-volatile memory or others.

The “database of channel state information”1525stores the channel state information for each mobile station such that, when the number of antennas of the base station is “N” lines and the number of antennas of the mobile stations is “M” lines, one piece of the channel state information is an N×M matrix, and L pieces of this channel state information is stored. A component of each matrix represents a channel impulse response which is obtained by expressing an IQ signal with using a complex number.

A channel estimation tracking block1526estimates in what state at a present time the channel state information having been used by the base station in past is, adds the information on this estimation result to the database of channel state information1525, and returns it. As an estimation algorism, a method such as weighted calculation for a prediction filter of channel variation is known if it is assumed that a temporal-variability factor follows the Jake's model, and the predicted result is calculated in accordance with this method. Note that the channel estimation tracking block1526may be not included in the structure. A weighted filter may be restructured to be learned so that a difference between a result estimated with using a weighted filter for the prediction and the occasionally-collected channel state information is minimized.

The external node interface1527includes: a data/control signal separator1529; a transmission signal processing block for external node1528; and a received signal processing block for external node1530. The transmission signal processing block for external node1528multiplexes an output signal from the parallel/serial convertor1514in the modem1506, an output data signal from the baseband signal receiver1509, a control signal from the control block1522, and others, and transmits a signal to the base station controller or other communication device. The received signal processing block for external node1530performs a received signal processing to a signal transmitted from the base station controller and a received signal from other communication device, and then, separates the date signal and the control signal by the data/control signal separator1529, and transfers the data signal to the modem1506and the control signal to the control block1522.

FIG. 16illustrates an example of a structure of a base station controller according to the embodiment of the present invention. A base station controller101(corresponding to101ainFIG. 1) includes: an external node interface1601; a modem1609; and a control block1623.

The external node interface1601includes: a received signal processing block for external node1602; a transmission signal processing block for external node1603; a received signal switch1604; a transmitting signal switch1605; a serial/parallel convertor1606; a data/control signal separator1607; and a parallel/serial convertor1608.

A received signal from the base station is processed by the received signal processing block for external node1602, and is separated by the received signal switch1604into either a signal for which a baseband signal receiver1610in the modem1609is used or a signal for which it is not used. When a demodulation processing or a decoding signal processing for a radio section is performed by the base station controller101instead of the base station102, the baseband signal receiver1610is required.

The signal outputted to the data/control signal separator1607by the received signal switch1604in the external node interface1601is separated into the control signal or the data signal by analyzing the header information in the data/control signal separator1607, and the control signal is outputted to a control signal processing block1624in the control bock1623. When it is the data signal, the signal is transferred to a data signal switch1621in the modem1609. By the data signal switch1621, the transferred signal may be transferred as a return signal to the transmitting signal switch1605in the external node interface1601, or the signal may be transferred to the baseband signal transmitter1616in the modem1609in order to transmit a modulated signal in the radio section. The transmitting signal switch1605selects the signal returned by the data signal switch1621in the modem1609or a signal which is a serial signal converted by the parallel/serial converter1608from the signal modulated by the baseband signal transmitter1616in the modem1609, and transfers the selected signal to the transmission signal processing block for external node1603. The transmission signal processing block for external node1603transmits the received signal to the base station or other communication device as the data signal or the control signal.

The modem1609includes: the baseband signal receiver1610; the baseband signal transmitter1616; an enhanced preamble signal generator1615; the data signal switch1621; and a control signal switch1622. The baseband signal receiver1610and the baseband signal transmitter1616have basically the same function as the baseband signal receiver1509and the baseband signal transmitter1516in the base station102. That is, the baseband signal receiver1610includes: a demodulator1611; a parallel/serial convertor1612; a decoder1613; and a data/control signal separator1614. Further, the baseband signal transmitter1616includes: a modulator1617; a serial/parallel convertor1618; an encoder1619; and a multiplexer1620. This structure is different in that parallelism of the serial/parallel conversion and the parallel/serial conversion is increased so as to correspond to a total number of antennas of the base stations.

The control block1623includes: a control signal processing block1624; a generator block of information that judges whether to feed back1625; and a channel estimation tracking block1627. A data handled by this control block1623includes a “database of channel state information”1626.

The control signal processing block1624performs the operation of the control algorism described with reference toFIG. 14, and executes the control flow illustrated inFIGS. 2,3,4,11, and12. As a control signal handled by the control signal processing block1624, a signal acquired via the baseband signal receiver1610in the modem1609, a signal transmitted from the data/control signal separator1607in the external node interface1601, and others are received. When the multipoint SDMA processing is performed, the baseband signal receiver1610of the modem1609is required. However, the control signal transmitted from the control block1522of the base station102is transferred to the control block1623via the latter data/control signal separator1607. The control signal generated by the control signal processing block1624in the control block1623is transferred to be outputted the baseband signal transmitter1616via the control signal switch1622in the modem1609, or to be directly outputted to the transmitting signal switch1605in the external node interface1601.

The database of channel state information1626, the generator block of information that judges whether to feed back1625, and the channel estimation tracking block1627have the same function as described for the base station. An enhanced preamble signal is generated by the enhanced preamble signal generator1615based on the information generated by the generator block of information that judges whether to feed back1625, and is transmitted via each base station.

FIG. 17illustrates an example of a structure of a mobile station according to the embodiment of the present invention. A mobile station103(corresponding to103ato103dinFIG. 1) includes: antennas1701aand1701b; a radio frequency block1702; a modem1706; a control block1718; and an external node interface1724.

The radio frequency block1702is illustrated as a block structure having the same function as described for the base station102. That is, the radio frequency block1702includes: duplexers1703aand1073b; RF receivers1704aand1704b; and RF transmitters1705aand1705b. While the example ofFIG. 17shows a case that the number of antennas is two, the structure is the same as the base station in a case of four antennas.

The modem1706includes: a baseband signal receiver1707; and a baseband signal transmitter1713. The baseband signal receiver1707has basically the same structure as the baseband signal receiver1509in the base station. That is, the baseband signal receiver1707includes: a demodulator1709; a parallel/serial converter1710; a decoder1711; and a data/control signal separator1712. The demodulator1709performs channel estimation when it receives a preamble signal for a demodulation processing. A result of this channel estimation is stored as a database of channel state information1721in the control block1718.

The baseband signal transmitter1713also has the same structure as the baseband signal transmitter1516in the base station. That is, the baseband signal transmitter1713includes: a modulator1714; a serial/parallel converter1715; an encoder1716; and a multiplexer1717.

The control block1718includes: a control signal processing block1719; a judgment block of channel state information feedback mode1720; and a channel estimation tracking block1722. A data handled by the control block1718includes a database of channel state information1721and a database of decoding error rate1723. The database of channel state information1721stores the channel estimation result of the demodulator1709as a database. For this data, the channel estimation tracking block1722estimates temporal variability, and performs the same operation as the channel estimation tracking block1526in the base station. Also, it is necessary to provide a weighting filter for prediction in the base station in order to acquire the same result as the base station. This weighting filter is previously provided between the base station and the mobile station with using a protocol. In addition, the channel estimation tracking block1722may not be included in the structure.

The control signal processing block1719executes the control flow illustrated inFIGS. 2,3,4,11, and12. In accordance with the feedback judgment information specified by the base station, the channel state information may be notified to the base station or the judgment result either the channel state information is OK or NG may be notified.

A method of judging either the channel state information is OK or NG will be described below. First, the feedback judgment type and the feedback judgment threshold are extracted from a format of the feedback judgment information described with reference toFIG. 10. When a channel correlation coefficient is specified in the feedback judgment type, the correlation coefficient is acquired as follows, and is judged by comparison with the threshold. More specifically, by the database of channel state information1721, the channel state information which the base station uses and the channel estimation result outputted from the demodulator1709for a latest preamble signal are referred. For example, by representing a component with i row and j column of a response matrix of a channel estimation result in a subcarrier “k” of an OFDM signal at time “t” as Hijk(t) and representing the information at time “t−τ” of the channel state information which the base station uses as Hijk(t−τ) the correlation coefficient is calculated by a following formula (7). And, OK is notified when the correlation coefficient is equal to or larger than the threshold. Otherwise, NG is notified.

Here, “A [*]” represents a sample mean of N samples with using N samples of the data acquired by returning from time t to the past. Here, “H′” represents a complex conjugate transposed matrix for a channel “H”.

Next, a method of judging either the channel state information is OK or NG when the feedback judgment type described with reference toFIG. 10specifies the decoding error rate will be described. In this case, the database of decoding error rate1723is referred. The decoding error rate is measured by a decoding error rate measurement block1725in the external node interface1724. In this measurement method, a frame error rate which judges whether or not the decoding error rate is correct in a frame unit by CRC judgment is calculated, and its result is notified. Other measurement method is to embed an already-known pattern into the data signal and calculate a number shifted between the pattern result and a decoded result. Alternatively, as the data of the error rate, the number of bits may be used, the number of bits being in error correction by coding a decoded data signal again and acquiring a difference between the coded data and an input signal of the decoder. By comparing the error rate of the data judged as described above with the feedback judgment threshold, the channel state information is judged as NG if the error rate is larger than the threshold, and the channel state information is judged as OK if smaller than the threshold.

The external node interface1724has an interface connected to, for example, a microphone or a speaker as, for example, an input/output device in the mobile station. When it functions as an audio terminal, it mounts a processing of a codec1726to communicate data with the modem1706.

FIG. 18illustrates an example (example that the base station measures the channel state information with a signal transmitted from the mobile station) of a control sequence according to another embodiment of the present invention. In the above-described embodiment, such a function is provided for the mobile station, in which the mobile station having received the preamble signal from the base station performs the channel estimation to acquire the channel state information, and notifies the channel state information to the base station or judges either the channel state information is OK or NG. However, in another embodiment illustrated inFIG. 18, an example that this function is provided for the base station is described. Since the base station also has the demodulator1510(FIG. 15), the same operation can be performed by using the channel estimation result.

First, in a channel state information collecting period, the base station102anotifies a control message for preamble signal request. At this time, a transmission judgment processing is performed, in which the mobile stations103a,103b,103c, and103djudge the same result with using the enhanced preamble signal as judged that the channel state information is OK or NG, and the mobile station transmits the preamble signal only when the channel state information is judged as OK. Without performing this transmission judgment processing for the preamble signal, the preamble signals may be sequentially transmitted from all the mobile stations. However, when the number of the contained mobile stations is large, by previously focusing on the mobile stations whose channel state information is OK and transmitting their preamble signals, time for the reception of the preamble signals by the base station can be relatively short.

For example, when the mobile station103adecides to transmit the preamble signal in the transmission judgment processing for the preamble signal, a transmission frequency is changed. If the system is a TDD (Time Division Duplex) system with using the same frequency in a down-stream signal from the base station to the mobile station and an up-stream signal from the mobile station to the base station, it is not necessary to change the frequency. In an FDD (Frequency Division Duplex) system with using a different frequency in the up-stream signal and the down-stream signal, in order to acquire the channel state information used in the down-stream signal, by changing the frequency into that of a signal used in the down-stream signal and then transmitting the preamble signal, the base station having received the preamble signal can acquire the same channel estimation result as the channel state information of the down-stream signal. Similarly, the mobile stations103b,103c, and103dalso transmit the preamble signals to collect the latest channel state information by the channel estimation.

Next, in the time slot #1 (MP-SDMA), by obtaining the correlation coefficient between the channel state information acquired in the channel state information collecting period and the channel state information which the base station uses from the formula (7), the channel state information is judged as OK if the correlation coefficient is equal to or larger than the threshold. Otherwise, the channel state information is judged as NG. Then, the result of the channel state information of OK/NG is exchanged between the base stations102aand102b. A transmission processing for the MP-SDMA is performed with using the channel state information of OK to transmit the SDMA data signal to each mobile station. At this time, since the base station102acollects the channel state information from not only the contained associate mobile stations but also the other mobile stations, the MP-SDMA signal processing can be performed by the base station102a, so that a signal processing can be performed without via the base station controller101a. Obviously, the MP-SDMA signal processing can be also performed by the base station controller101a.

FIG. 19illustrates an example of a control sequence in a case that a base station autonomously decides a multipoint SDMA communication, according to another embodiment of the present invention.

In this sequence, in a channel state information collecting period, the mobile station transmits the preamble signal with using a frequency of a down-stream signal from the base station102ato the mobile stations103a,103b,103c, and103d. In this manner, the base station can acquire the channel state information for all the mobile stations which can be communicated by the down-stream signal.

Next, in a communication mode deciding period, based on the channel state information collected in the channel state information collecting period, it is calculated which one of the SIMO-TDMA communication, the MIMO-TDMA communication, and the SDMA-TDMA communication exceeds the demand speed for the associated mobile station with using the formulas (1) to (4), and decides the communication mode.

The base station102adetecting that the communication speed demanded by the associated mobile station cannot be exceeded issues an MP-SDMA request message to a surrounding base station102bas a broadcast message. On this control signal, an ID list of mobile stations which are targets for the MP-SDMA is described. The surrounding base station102breceives the MP-SDMA request message, checks the ID list of mobile stations, and, if the participation for the MP-SDMA is possible, transmits a notification of being possible to participate in the MP-SDMA to the base station102awhich has requested to the surrounding base station102b. As this control signal, the mobile station ID and the channel state information for the mobile station ID are notified.

Since the channel state information relating to the mobile station is collected from the base station probably performing the MP-SDMA to the base station102a, the base station102ajudges whether or not the demand speed of the mobile station is exceeded in the MP-SDMA with using the channel state information. If the demand speed is exceeded, the base station102atransmits a notification of execution of MP-SDMA to the peripheral base station102band the base station controller101a. If the demand speed is not exceeded, the base station102atransmits a connection refusal message to the mobile station which does not exceed the demand speed.

When the notification of execution of MP-SDMA is issued, to the base station102bto be participated in the MP-SDMA communication, the base station102awhich has issued the request notifies the information (all pieces of the channel state information relating to the mobile station or the information for the pre-coding to be performed in the base station) for performing the SDMA signal processing or others, and provides the necessary information about what signal is to be transmitted from an antenna of the base station102b.

And, in the time slot #1 (MP-SDMA), the base station controller101acopies the same data signal and transmits it to a plurality of base stations which performs the MP-SDMA communication. Each base station performs an arithmetic processing for the MP-SDMA to generate the SDMA data signal, and transmits the SDMA data signal to the mobile station. In this manner, the base station can autonomously perform the MP-SDMA communication.

Note that, in the communication mode deciding period which is a phase for collecting the channel state information, a method for reducing the feedback information as described with reference toFIG. 18may be used.

Effects obtained by the embodiment and another embodiment of the present invention as described above can be summarized as follows.

(1) The communication mode can be appropriately switched for exceeding the demand communication speed of the mobile station, and resources in the radio section can be used responding to the demand speed as needed. More particularly, in the multipoint SDMA communication which performs simultaneous communication with a plurality of mobile stations by the coordination of the plurality of base stations with each other, there is an effect of increasing the system capacity which cannot be supported by one base station by apparently providing the large number of antennas on the transmitting end. By automatically switching between performing the multipoint SDMA communication and performing the SDMA communication or the MIMO communication within one base station, even the traffic not allowed to be contained so far can be contained. Further, it is not necessary at the beginning to fixedly structure the system as the multipoint SDMA communication, and therefore, the multipoint SDMA communication can be automatically and autonomously established between the necessary base stations when the traffic of the mobile station geographical-unevenly occurs, so that the necessary resource can be allocated depending on the distribution of the traffic.

(2) In each communication mode, since it is essentially not necessary to perform the feedback to a user having small variation amount of the channel state information between the transmitting antenna and the receiving antenna, a only data about whether or not the channel state information which the transmitting end uses can be used without any change is fed back, so that the information volume for the feedback can be reduced, and the frequency utilization efficiency per user can be improved by reducing the overhead. More particularly, such a problem as the multipoint SDMA communication is solved, the problem in which the feedback volume of the channel state information is dramatically increased because of increasing the number of a plurality of base stations and mobile stations under their control, and therefore, a merit of the multipoint SDMA communication is reduced due to overhead. Therefore, by solving this problem and obtaining the merit of the multipoint SDMA communication, so that the throughput of the entire system can be improved and the number of users contained within the area can be increased.

INDUSTRIAL APPLICABILITY

The present invention relates to a wireless communication system. More particularly, the present invention relates to a method for data transmission/reception between a plurality of base stations having a plurality of transmitting/receiving antennas and a plurality of mobile stations associated with each base station.