Wireless communication system

Abstract

In order to transmit a large amount of data in series at a time with a mimo communication scheme while avoiding deterioration of decoding characteristics due to change over time by a channel matrix, a wireless communication system uses an open-loop type and a closed-loop type of mimo communication modes in combination and switches to the open-loop mimo communication mode in response to the information that the amount of data to be transmitted at a time has exceeded a predetermined amount of bits or a predetermined transmission time during data transmission under the closed-loop mimo communication mode. By discontinuing useless closed-loop mimo communication and switching to the open-loop mimo communication mode that is better than Zero-forcing, the decoding characteristics are prevented from simply becoming deteriorated.

Description

where x is a transmission signal, H is propagation channel information, and n is a noise term.

By performing singular value decomposition (SVD) of the channel matrix H, the channel matrix H is expressed as follows:
H=UDV^H   (2)

where a matrix V^H denotes a complex conjugate transpose matrix (Hamiltonian) of a matrix V. In general, the product of an original matrix multiplied by its complex conjugate transpose matrix is an identity matrix (V·V^H=I) Therefore, in the closed-loop type of MIMO communication as shown in FIG. 8 for example, assuming that the receiver acquires the channel matrix H from TxPreamble and the transmitter acquires a transmission antenna weight V based on the feedback RxPreamble from the receiver to perform weighted transmission using the transmission antenna weight V, a reception signal y′ at the receiver is expressed as follows:
y′=H(Vx)  (3)
=UDx

Further, assuming that the receiver performs singular value decomposition of the channel matrix H acquired from TxPreamble and performs weighted reception using the obtained reception weight U^H, a reception signal y″ is expressed as follows:
y″=U^Hy′  (4)
=U^HUDx
=Dx

where D denotes a diagonal matrix and is expressed as follows:

$\begin{array}{cc}D=\left[\begin{array}{ccc}\sqrt{{\lambda }_1}& & \\ & \sqrt{{\lambda }_2}& \\ ⋮& & \ddots \\ & & \\ 0& & \end{array}|\begin{array}{cc}& 0\\ & \\ & \\ \sqrt{{\lambda }_{n-1}}& \\ & \sqrt{{\lambda }_n}\end{array}\right]& \left(5\right)\end{array}$

That is, in the SVD-MIMO transmission, UDV^H is obtained by performing the singular value decomposition of a numerical matrix whose elements denote channel information corresponding to each antenna pair, namely the channel information matrix H, and the transmission antenna weighting factor matrix V and the reception antenna weighting factor matrix U^H are obtained. Thus, each MIMO channel is expressed as the diagonal matrix D having the diagonal elements that are the square root of each eigenvalue λ_i, and signals can be multiplexed to be transmitted without any crosstalk.

Equation (5) shows that a transmission signal is given by power gain. Therefore, reception architecture in the closed-loop type of MIMO communication is basically the same as Zero Forcing (or Nulling) except for the difference that H- or U^H is employed as the reception weight.

In such a case of transmitting a large amount of data in series at a time, there is a problem of change over time by the channel matrix. That is, in practice, the accuracy of the channel matrix H obtained from the preamble information located at the beginning of a data division from the transmitter becomes deteriorated in the case of a large channel fluctuation or sending long data at a time. Assume that the reception signal y′ is originally obtained by the following equation:
y′=H(Vx)  (6)
=UDV^H(Vx)

However, assuming that the channel matrix changes to H_new due to a propagation path fluctuation, a practical reception signal y_new is expressed as follows:
y_new=H_new(Vx)  (7)
=UnewDnewV^Hnew(Vx)

As a matter of course, even though the receiver receives signals using a reception antenna weighting factor U_new based on H_new obtained from the preamble information, orthogonal channels are not formed, crosstalk occurs between MIMO channels which should be logically independent, and it results in a reception series in which interference waves remain.

This problem occurs because the accuracy of the transmission antenna weight V becomes deteriorated due to a fading channel fluctuation. In this embodiment, by switching from a closed-loop type of MIMO communication to an open-loop type of MIMO communication seamlessly, the accuracy deterioration of the transmission weight V is coped with by operation of the receiver alone to improve decoding characteristics.

The receiver stores an old V before measuring the accuracy of the transmission weight matrix V. The receiver calculates a correlation value with a certain period, based on a new transmission weight matrix V_new estimated using a new transmission preamble (located at the beginning of a data division), and compares the correlation value with a reference threshold value. For example, a correlation value p can be calculated according to the following equation:

$\begin{array}{cc}\rho =\frac{E\left[V_{\mathrm{ij}}^{*}{V}_{\mathrm{ij}}^{\mathrm{new}}\right]}{\sqrt{E\left[{V_{\mathrm{ij}}}^2\right]}\sqrt{E\left[{V_{\mathrm{ij}}^{\mathrm{new}}}^2\right]}}& \left(8\right)\end{array}$

where E[•] denotes an ensemble average.

In this case, if a transmission preamble is multiplied by an old transmission weight matrix V, a new transmission weight matrix V_new can be obtained by multiplying H_newV (estimated from the preamble information) by V^H and calculating the following equation:
H_newVV^H=H_new   (9)
=UnewDnewV^Hnew

The accuracy deterioration of the transmission weight V is due to a channel matrix fluctuation. The accuracy deterioration of V may be detected based on the deterioration of a reception error rate instead of the correlation between the old V and the new V_new by the accuracy deterioration of V.

When the receiver confirms that the accuracy of V has deteriorated beyond the reference threshold value, the receiver stops performing the closed-loop type of MIMO communication and determines an optimum reception scheme by the open-loop type of MIMO communication.

In this context, the optimum reception scheme refers to schemes such as MMSE (Minimum Mean Square Error), BLAST (Bell Layered Architecture Space Time coding), MMSE+BLAST, Maximum Likelihood Estimation, and the like. However, in the case where hardware design does not allow a plurality of reception schemes, a reception scheme other than ZF is set to be ready for use. FIG. 1 shows, for reference, Eb/No vs. bit error rate characteristics on the assumption of MIMO having three respective antennas for transmission and reception. As shown in FIG. 1, in general, better decoding characteristics of the open-loop type are obtained in the following order. (It is known that the characteristic of MMSE alone is nearly similar to that of BLAST.)

ZF<MMSE<BLAST<MMSE+BLAST<Maximum Likelihood Estimation

According to an algorithm selected above, the following equation is obtained based on the reception signal y_new=H_new(Vx) and using (H_new·V)⁻:
(H_newV)⁻y_new=(H_newV)⁻·H_new(Vx)  (10)
=x

In the case of also transmitting the preamble that is not multiplied by the transmission weight matrix V, the preamble is decoded using the stored old V to make (H_new·V)⁻.

If receiving operation is performed using equation (10), at the time of significant accuracy deterioration of V, it is anticipated that the decoding characteristics are approaching those of ZF stochastically under a complex iid. channel environment if the channel matrix H is updated using the periodical preamble information inserted into data divisions as shown in FIG. 9. That is, as a result of having broken off learning of the channel matrix during long data communication, the decoding characteristics of the receiver in the closed-loop type of MIMO communication become deteriorated to the same degree as those of ZF which has especially low decoding characteristics among MIMO schemes of the open-loop type.

In this embodiment, by switching reception schemes based on the fluctuation of the transmission weight V as described above, reception characteristics of extremely higher quality can be maintained compared to the case where only the closed-loop type continues to be used. Further, since signals are received using the closed-loop type at the beginning, better reception characteristics are obtained compared to the case where only the open-loop type is used. Furthermore, as a result, it is also possible to use only the open-loop type of reception scheme. The open-loop type of terminal can merely receive signals using (H_new·V)⁻ calculated from the preamble, and the closed-loop type and the open-loop type of MIMO communication schemes can coexist.

Moreover, there may be methods for stopping the transmission in which the transmission antenna weight V is employed and switching to wholly the closed-loop type of MIMO scheme with the following methods. A first method is as follows. The receiver detects the accuracy deterioration of V and notifies the transmitter to perform the open-loop type of communication with another communication device such as a PHS or a sensor network device, so that multiplication of the transmission antenna weight V is stopped. At this time, if different encoding or modulation depth is applied to each transmission branch, it is necessary to switch the encoding or the modulation depth as necessary before transmission.

A second method is as follows. After a series of procedures for the closed-loop type is executed, the number of transmissions of continuous bits, a period, etc. by which the accuracy of the transmission antenna weight V can be maintained within a permissible limit are determined beforehand, and the transmitter and the receiver are switched to the open-loop type of MIMO scheme based on the items predetermined above. At this time, if different encoding or modulation depth is applied to each transmission branch, it is necessary to switch the encoding or the modulation depth as necessary before transmission.

FIG. 2 schematically shows the function configuration of a transmitter 10 which operates in a MIMO communication system in which the closed-loop type and the open-loop type coexist, according to this embodiment.

As shown in FIG. 2, the transmitter 10 has a plurality of transmission antennas 11-1, 11-2, . . . , 11-m and performs the MIMO communication using a plurality of streams formed between the transmitter and a receiver (described later) having a plurality of reception antennas.

Data generation units 13-1, 13-2, . . . , 13-m encode and modulate transmission data of each stream. A transmission antenna weighting unit 12 applies an antenna weight to each stream in accordance with an operational mode of either the closed-loop or the open-loop.

A control unit 14 controls transmission weighting in accordance with an operational mode of either the closed-loop or the open-loop.

The transmitter 10 operates basically in the closed-loop type of MIMO communication mode, acquires an appropriate transmission antenna weight V based on feedback information from the receiver, and transmits information more appropriately by adding the transmission weight.

In response to the information that the amount of data to be transmitted at a time has exceeded a predetermined amount of bits or a predetermined transmission time, the control unit 14 estimates that the channel matrix has changed over time and switches to the open-loop type of MIMO communication mode. FIG. 3 shows an operation process of the transmitter 10. This kind of operation prevents the decoding characteristics of the receiver in the closed-loop type of MIMO communication from becoming deteriorated to the same degree as those of ZF which has especially low decoding characteristics among MIMO schemes of the open-loop type.

The control unit 14 informs the receiver that the transmitter 10 side has switched to the open-loop type of MIMO communication mode. Alternatively, in response to the information from the receiver that the receiver has switched to the open-loop type of MIMO communication mode, the control unit 14 may switch to the open-loop type of MIMO communication mode.

FIG. 4 shows an internal configuration of the transmission antenna weighting unit 12. As shown in FIG. 4, each stream transmission data is multiplied by an antenna weight under the closed-loop type of MIMO communication mode. However, under the open-loop type of MIMO communication mode, the transmission antenna weighting unit 12 is switched in such a way that data passes by, and the data is transmitted without a transmission weight.

FIG. 5 schematically shows the function configuration of a receiver 20 which operates in a MIMO communication system in which the closed-loop type and the open-loop type coexist, according to this embodiment.

As shown in FIG. 5, the receiver 20 has a plurality of reception antennas 21-1, 21-2, . . . , 21-n and performs the MIMO communication using a plurality of streams formed between the receiver and the transmitter having a plurality of transmission antennas.

Data reproduction units 23-1, 23-2, . . . , 23-n demodulate and decode the transmission data of each stream. A reception antenna weighting unit 22 applies an antenna weight to each reception stream in accordance with an operational mode of either the closed-loop or the open-loop.

The receiver 20 operates basically in the closed-loop type of MIMO communication mode, acquires the channel matrix H based on a reference signal from the transmitter, and transmits feedback information to the transmitter. The receiver 20 transmits information more appropriately by adding the reception antenna weight.

According to a change in the transmission weight V obtained successively by singular value decomposition of the channel matrix and deterioration of reception characteristics such as an increase in a reception error rate, etc., a control unit 24 estimates that the channel matrix has changed over time and switches to the open-loop type of MIMO communication mode. FIG. 6 shows an operation process of the receiver 20. This kind of operation prevents the decoding characteristics in the closed-loop type of MIMO communication from becoming deteriorated to the same degree as those of ZF which has especially low decoding characteristics among MIMO schemes of the open-loop type.

The control unit 24 informs the transmitter 10 that the receiver 20 side has switched to the open-loop type of MIMO communication mode. Alternatively, in response to the information from the transmitter 10 that the transmitter has switched to the open-loop type of MIMO communication mode, the control unit 24 may switch to the open-loop type of MIMO communication mode. The information that the receiver side has switched to the open-loop type of spatial multiplexing communication mode may be sent to the transmitter by the MIMO communication which is the same as the data communication channel; however, the information may be sent using a wireless communication scheme other than the MIMO communication channel, such as a wireless LAN transmission channel, a sensor network, or the like.

The reception antenna weighting unit 22 determines an antenna weight according to a reception scheme determined by the control unit 24. Under the open-loop type of MIMO communication mode, there are applied communication schemes other than Zero-forcing, such as MMSE, BLAST, MMSE+BLAST, Maximum Likelihood Estimation, etc.

In each reception architecture of the open-loop and the closed-loop, circuit processing units including generalized inverse matrix arithmetic circuits such as Moore Penrose and LU decomposition, a complex conjugate multiplication circuit for antenna weighting, and arithmetic circuits regarding CDM and OFDM decoding can be shared among reception schemes.

The present invention has been described in detail with reference to a specific embodiment. However; it is obvious that those skilled in the art can alter or modify the embodiment without departing from the scope and sprit of the invention. That is, the present invention has been disclosed in the form of exemplification, and the contents of the specification should not be interpreted restrictively. To understand the subject matter of the present invention, the appended claims should be taken into consideration.

Claims

1. A wireless communication system in which a transmitter having a plurality of antennas and a receiver having a plurality of antennas form a pair and spatially multiplex and communicate a signal, the wireless communication system comprising:

a closed-loop spatial multiplexing communication mode in which the transmitter transmits a signal for acquiring channel information to the receiver and the receiver sends feedback of the channel information to the transmitter;

an open-loop spatial multiplexing communication mode in which only the transmitter transmits a signal for acquiring channel information to the receiver; and

a mode switcher configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode, and switch to the open-loop spatial multiplexing communication mode in response to information indicating that an amount of data to be transmitted has exceeded a predetermined data limit or a transmission time during data transmission under the closed-loop spatial multiplexing communication mode has exceeded a predetermined time limit.

2. The wireless communication system of claim 1, wherein

the mode switcher is further configured to switch to the open-loop spatial multiplexing communication mode in response to the information that an error rate has exceeded a predetermined value during data transmission under the closed-loop spatial multiplexing communication mode.

3. The wireless communication system according to claim 1, wherein the transmitter transmits data that the transmitter does not apply a transmission weight to under the open-loop spatial multiplexing communication mode.

4. The wireless communication system of claim 1, wherein

the receiver performs an open-loop spatial multiplexing communication operation other than a communication scheme for providing a reception weight based on an inverse matrix H⁻ of a channel matrix obtained from a propagation path condition under the open-loop spatial multiplexing communication mode.

5. The wireless communication system according to claim 1, wherein

said mode switcher is further configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode, and inform the receiver that a mode has been switched to the open-loop spatial multiplexing communication mode.

6. A wireless communication system in which a transmitter having a plurality of antennas and a receiver having a plurality of antennas form a pair and spatially multiplex and communicate a signal, the wireless communication system comprising:

a closed-loop spatial multiplexing communication mode in which the transmitter transmits a signal for acquiring channel information to the receiver, the receiver sends feedback of the channel information to the transmitter, the receiver finds a channel matrix H by receiving a reference signal sent from the transmitter, obtains a reception weight matrix U^H, a diagonal matrix D and a transmission weight matrix V by performing singular value decomposition of the channel matrix H into UDV^H, and sends feedback of the transmission weight matrix V to the transmitter, there is performed Singular Value Decomposition-Multiple Input Multiple Output (SVD-mimo) communication in which the transmitter performs weighted transmission with the transmission weight matrix V and the receiver performs weighted reception with the reception weight matrix U^H;

an open-loop spatial multiplexing communication mode in which only the transmitter transmits a signal for acquiring channel information to the receiver; and

a mode switcher configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode, and switch to the open-loop spatial multiplexing communication mode in response to the information that the transmission weight matrix V obtained by the singular value decomposition has changed beyond a predetermined value during data transmission under the closed-loop spatial multiplexing communication mode.

7. A wireless communication apparatus having a plurality of antennas that spatially multiplexes and transmits a signal to a receiver having a plurality of antennas, the wireless communication apparatus comprising:

a closed-loop spatial multiplexing communication mode in which the apparatus performs weighted transmission based on feedback information from the receiver;

an open-loop spatial multiplexing communication mode in which the apparatus performs non-weighted transmission; and

a mode switcher configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode, and switch to the open-loop spatial multiplexing communication mode in response to information indicating that an amount of data to be transmitted has exceeded a predetermined data limit or a transmission time during data transmission under the closed-loop spatial multiplexing communication mode has exceeded a predetermined time limit.

8. The wireless communication apparatus of claim 7, wherein

said mode switcher is further configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode, and switch to the open-loop spatial multiplexing communication mode in response to information that an error rate has exceeded a predetermined value during data transmission under the closed-loop spatial multiplexing communication mode.

9. A wireless communication apparatus having a plurality of antennas that spatially multiplexes and transmits a signal to a receiver having a plurality of antennas, the wireless communication apparatus comprising:

a closed-loop spatial multiplexing communication mode in which the apparatus performs weighted transmission based on feedback information from the receiver;

an open-loop spatial multiplexing communication mode in which the apparatus performs non-weighted transmission; and

a mode switcher configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode, and switch to the open-loop spatial multiplexing communication mode in response to the information from the receiver that a mode has been switched to the open-loop spatial multiplexing communication mode.

10. A wireless communication apparatus having a plurality of antennas that spatially multiplexes and transmits a signal to a receiver having a plurality of antennas, the wireless communication apparatus comprising:

a closed-loop spatial multiplexing communication mode in which the apparatus performs weighted transmission based on feedback information from the receiver;

an open-loop spatial multiplexing communication mode in which the apparatus performs non-weighted transmission;

a mode switcher configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode;

a feedback information receiving section configured to receive feedback information for acquiring a transmission weight matrix V from the receiver and perform Singular Value Decomposition-Multiple Input Multiple Output (SVD-mimo) transmission of weighted transmission by the transmission weight matrix V under the closed-loop spatial multiplexing communication mode; and

a transmitting section configured to transmit data to which the apparatus does not apply the transmission weight V under the open-loop spatial multiplexing communication mode.

11. A wireless communication apparatus having a plurality of antennas that receives a multiplexed signal from a transmitter having a plurality of antennas, the wireless communication apparatus comprising:

a closed-loop spatial multiplexing communication mode in which the apparatus sends feedback of channel information to the transmitter, finds a channel matrix H by receiving a reference signal sent from the transmitter, obtains a reception weight matrix U^H, a diagonal matrix D and a transmission weight matrix V by performing singular value decomposition of the channel matrix H into UDV^H, and performs Singular Value Decomposition-Multiple Input Multiple Output (SVD-mimo) communication using weighted reception with the reception weight matrix U^H and sends feedback of the transmission weight matrix V to the transmitter;

an open-loop spatial multiplexing communication mode in which the apparatus does not send feedback of channel information to the transmitter; and

a mode switcher configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode, and switch to the open-loop spatial multiplexing communication mode in response to the information that the transmission weight matrix V obtained by the singular value decomposition has changed beyond a predetermined value during data transmission under the closed-loop spatial multiplexing communication mode.

12. A wireless communication apparatus having a plurality of antennas that receives a multiplexed signal from a transmitter having a plurality of antennas, the wireless communication apparatus comprising:

a closed-loop spatial multiplexing communication mode in which the apparatus sends feedback of channel information to the transmitter;

an open-loop spatial multiplexing communication mode in which the apparatus does not send feedback of channel information to the transmitter; and

a mode switcher configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode, and inform the transmitter that a mode has been switched to the open-loop spatial multiplexing communication mode.

13. The wireless communication apparatus of claim 12, wherein the

mode switcher is further configured to switch to the open-loop spatial multiplexing communication mode in response to the information from the transmitter that a mode has been switched to the open-loop spatial multiplexing communication mode.

14. A wireless communication apparatus having a plurality of antennas that receives a multiplexed signal from a transmitter having a plurality of antennas, the wireless communication apparatus comprising:

a closed-loop spatial multiplexing communication mode in which the apparatus sends feedback of channel information to the transmitter;

an open-loop spatial multiplexing communication mode in which the apparatus does not send feedback of channel information to the transmitter;

a mode switcher configured to switch between the closed-loop spatial multiplexing communication mode and the open-loop spatial multiplexing communication mode; and

a communication section configured to perform an open-loop spatial multiplexing communication operation other than a communication scheme for providing a reception weight based on an inverse matrix I-V of a channel matrix obtained from a propagation path condition under the open-loop spatial multiplexing communication mode.

15. A wireless communication method for spatially multiplexing and transmitting a signal to a receiver having a plurality of antennas in a transmitter having a plurality of antennas, the wireless communication method comprising:

performing closed-loop spatial multiplexing communication based on feedback information from the receiver, the closed-loop spatial multiplexing communication allowing the transmitter to perform weighted transmission;

switching to open-loop spatial multiplexing communication in response to information indicating that an amount of data to be transmitted has exceeded a predetermined data limit or a transmission time during data transmission under the closed-loop spatial multiplexing communication mode has exceeded a predetermined time limit; and

performing the open-loop spatial multiplexing communication.

16. A wireless communication method for receiving a multiplexed signal from a transmitter having a plurality of antennas in a receiver having a plurality of antennas, the wireless communication method comprising the steps of:

performing closed-loop spatial multiplexing communication on the assumption that a transmission weight has been applied by the transmitter, the closed-loop spatial multiplexing communication allowing the receiver to perform weighted reception;

switching to open-loop spatial multiplexing communication in response to a change in a reception characteristic, the open-loop spatial multiplexing communication allowing the receiver to perform weighted reception;

performing the open-loop spatial multiplexing communication on the assumption that a transmission weight has been applied by the transmitter.

17. A computer-readable medium storing a computer program which when executed on a computer causes the computer to perform processing for spatially multiplexing and transmitting a signal to a receiver having a plurality of antennas from a transmitter having a plurality of antennas, comprising:

performing closed-loop spatial multiplexing communication based on feedback information from the receiver, the closed-loop spatial multiplexing communication allowing the transmitter to perform weighted transmission;

switching to open-loop spatial multiplexing communication in response to information indicating that an amount of data to be transmitted has exceeded a predetermined data limit or a transmission time during data transmission under the closed-loop spatial multiplexing communication mode has exceeded a predetermined time limit; and

performing the open-loop spatial multiplexing communication.

18. A computer-readable medium storing a computer program which when executed on a computer causes the computer to perform processing for spatially multiplexing and transmitting a signal to a receiver having a plurality of antennas, in a transmitter having a plurality of antennas, comprising:

performing closed-loop spatial multiplexing communication by performing weighted transmission based on feedback information from the receiver;

performing open-loop spatial multiplexing communication in which the transmitter performs non-weighted transmission;

switching between the closed-loop spatial multiplexing communication and the open-loop spatial multiplexing communication, and switching to the open-loop spatial multiplexing communication in response to information from the receiver that a mode has been switched to the open-loop spatial multiplexing communication mode.

19. An information processing apparatus used for a communication system including a transmitter having a plurality of antennas and a receiver having one or more antennas, comprising:

circuitry configured to:

perform wireless communication in a closed loop spatial multiplexing communication mode in which feedback information for weighting signals is received from the receiver or a different spatial coding communication mode in which the feedback information is not received; and

switch to the different spatial coding communication mode from the closed loop spatial multiplexing communication mode in response to an indication from the receiver according to time expiry in data transmission,

wherein the feedback information is transmitted via a mimo (Multiple-Input Multiple-Output) communication channel and the indication is notified via another communication channel that is different from the mimo communication channel.

20. A method, implemented by an information processing apparatus used for a communication system including a transmitter having a plurality of antennas and a receiver having one or more antennas, the method comprising:

performing wireless communication in a closed loop spatial multiplexing communication mode in which feedback information for weighting signals is received from the receiver or a different spatial coding communication mode in which the feedback information is not received; and

switching to the different spatial coding communication mode from the closed loop spatial multiplexing communication mode in response to an indication from the receiver according to time expiry in data transmission,

wherein the feedback information is transmitted via a mimo (Multiple-Input Multiple-Output) communication channel and the indication is notified via another communication channel that is different from the mimo communication channel.

21. A receiver for a communication system that includes a transmitter having a plurality of antennas and the receiver having one or more antennas, the receiver comprising:

circuitry configured to:

perform wireless communication in a closed loop spatial multiplexing communication mode in which feedback information for weighting signals is transmitted to the transmitter or a different spatial coding communication mode in which the feedback information is not transmitted; and

cause the transmitter to switch to the different spatial coding communication mode from the closed loop spatial multiplexing communication mode in response to an indication from the receiver according to time expiry in data transmission,

wherein the feedback information is transmitted via a mimo (Multiple-Input Multiple-Output) communication channel and the indication is notified via another communication channel that is different from the mimo communication channel.