Wireless communications system, wireless communications apparatus, wireless communications method and computer program for wireless communication

Abstract

In performing SVD-MIMO transmission, a set-up procedure is simplified while assuring a satisfactory decoding capability with a reduced number of antennas. A transmitter estimates channel information based on reference signals sent from a receiver, determines a transmit antenna weighting coefficient matrix based on the channel information, calculates a weight to be assigned to each of components of a multiplexed signal, and sends, to the receiver, training signals for respective signal components, the training signals being weighted by the calculated weights. On the other hand, the receiver determines a receive antenna weighting coefficient matrix based on the received training signals.

Claims

1. An electronic device operating as an access point, the electronic device comprising: a weight information calculating circuit configured to determine weight information based on pre-training information which has been received from a mobile device; and a transmitter configured to apply the weight information to estimate channel conditions, and to transmit signals including the weight information applied to the mobile device so that the mobile device can determine operating parameters using the signals including the weight information applied.

2. The electronic device of claim 1, wherein the transmitter is further configured to transmit data signals to the mobile device, and the transmitted data signals and the signals to estimate the channel conditions are transmitted by Orthogonal Frequency Division Multiplexing (OFDM).

3. The electronic device of claim 2, further comprising: a plurality of antennas via which the signals to estimate the channel conditions and the data signals are transmitted.

4. The electronic device of claim 3, wherein the weight information includes at least a first weight and a second weight, and the transmitter is configured to weight signals transmitted via a first antenna of the plurality of antennas with the first weight and weight signals transmitted via a second antenna of the plurality of antennas with the second weight.

5. The electronic device of claim 4, wherein the signals to estimate the channel conditions include at least a first training signal and a second training signal, and the transmitter is configured to weight the first training signal with the first weight and the second training signal with the second weight.

6. The electronic device of claim 1, wherein the transmitter is further configured to transmit preamble signals to the mobile device that are not weighted by the weight information prior to transmitting the signals to estimate the channel conditions in a frame.

7. The electronic device of claim 1, wherein the weight information calculating circuit is configured to generate the weight information based on channel state information between the electronic device and the mobile device from which the pre-training information is received.

8. The electronic device of claim 7, wherein the weight information calculating circuit is further configured to estimate the channel state information based on reference signals received as the pre-training information from the mobile device, wherein a number of the reference signals is determined according to a number of antennas at the mobile device.

9. A method performed by an electronic device operating as an access point, the method comprising: obtaining, from a mobile device, pre-training information to be referenced for determining weight information; determining, with circuitry, the weight information based on the obtained pre-training information; applying, with the circuitry, the weight information to signals to estimate channel conditions for the mobile device; and transmitting, with the circuitry, to the mobile device, the signals to estimate the channel conditions for the mobile device after application of the weight information.

10. The method of claim 9, wherein data signals to be transmitted to the mobile device, and the signals to estimate the channel conditions are transmitted by Orthogonal Frequency Division Multiplexing (OFDM).

11. The method of claim 10, wherein the signals to estimate the channel conditions are transmitted prior to the transmitting of the signals.

12. The method of claim 11, further comprising: transmitting preamble signals that are not weighted by weights included in the weight information prior to transmitting of the signals to estimate the channel conditions in a frame.

13. The method of claim 12, wherein the preamble signals are for at least one of signal detection synchronization or adjustment of gain at the mobile device.

14. The method of claim 10, wherein the electronic device includes a plurality of antennas via which the signals to estimate the channel conditions and the data signals are transmitted.

15. The method of claim 14, wherein the weight information includes at least a first weight and a second weight, and the method further comprises weighting signals transmitted via a first antenna of the plurality of antennas with the first weight and weighting signals transmitted via a second antenna of the plurality of antennas with the second weight.

16. The method of claim 15, wherein the signals to estimate the channel conditions include at least a first training signal and a second training signal, and the method further comprises weighting the first training signal with the first weight and weighting the second training signal with the second weight.

17. The method of claim 9, further comprising: generating the weight information based on channel state information between the electronic device and the mobile device.

18. The method of claim 17, further comprising: estimating the channel state information based on reference signals received as the pre-training information from the mobile device, wherein a number of the reference signals is determined according to a number of antennas at the mobile device.

19. The method of claim 18, further comprising: calibrating the channel state information to obtain channel information of a channel from the electronic device to the mobile device.

20. A non-transitory computer-readable medium including computer program instructions, which when executed by processing circuitry of an electronic device operating as an access point, cause the processing circuitry to perform a method comprising: obtaining, from a mobile device, pre-training information to be referenced for determining weight information; determining the weight information based on the obtained pre-training information; applying the weight information to signals to estimate channel conditions for the mobile device; and transmitting, with the circuitry, to the mobile device, the signals to estimate the channel conditions for the mobile device after application of the weight information.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically shows a construction of a MIMO communication system according to a first embodiment of the invention.

(2) FIG. 2 shows an example of an antenna configuration of a transmitter and a receiver of the system.

(3) FIG. 3 shows an operational sequence between the transmitter and receiver of the system where an RTS/CTS function is employed.

(4) FIG. 4 schematically shows a construction of a communication system according to a second embodiment of the invention.

(5) FIG. 5 illustrates a concept of MIMO communication system.

(6) FIG. 6 illustrates a concept of SVD-MIMO transmission system.

(7) FIG. 7 illustrates a concept of V-BLAST communication system.

(8) FIG. 8 shows an example of an antenna configuration of a transmitter and a receiver in a V-BLAST communication system.

(9) FIG. 9 illustrates multiple devices, each including a transmitter and a receiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) There will be described embodiments of the invention by reference to the drawings.

(11) FIG. 1 schematically shows a construction of a first embodiment of the invention.

(12) A transmitter space-time encodes each transmitted signal to multiplex the signal and distributes the multiplexed signal to three antennas to send the signal therefrom to a receiver over a channel. The receiver receives the multiplexed signal via the channel through two antennas and space-time decodes the signal to obtain received signal or data.

(13) The communication system shown resembles the V-BLAST system of FIG. 7 in general. However, the transmitter, not the receiver, provides an antenna weighting coefficient when transmitting the data, and the antenna configuration of the transmitter and receiver is such that the number of the transmit antennas is larger than that of the receive antennas. The number of the receive antennas corresponds to the number of signal sub-channels.

(14) In the system shown in FIG. 1, the part of the transmitter has a redundancy in the degree of freedom of the antennas. To take advantage of this redundancy for improving the S/N ratio of the received signal, the transmitter sends a signal weighted by MSN (Maximum Signal-to-Noise ratio) which is criteria for maximizing the S/N ratio of signal of self, by zero-forcing, or by a combination of the MSN and zero-forcing. As a result, even where a redundancy in the degree of freedom of the antennas on the part of the receiver is not available (that is, the number of receive antennas is relatively small), the degree of freedom on the part of the transmitter can compensate this, to assure a satisfactory decoding capability.

(15) The operational procedure in the present communication system will be described.

(16) As a preparatory step, a training signal Pre-training Signal with respect to each antenna is sent from the receiver 20 in a time division fashion. In the specific example of FIG. 1, the receiver has two receive antennas, and therefore two Pre-training Signals are sent. A preamble Preamble prefixed to the Pre-training Signal is an additive signal for serving a signal detection, a timing synchronization and an adjustment of receiver gain.

(17) The transmitter 10 receives the training signal from the receiver 20 as a reference signal, calculates the channel information matrix H by a channel estimator 11 of the transmitter 10, and determines a transmit antenna weighting coefficient matrix Z.sub.T by a transmit antenna weighting coefficient matrix calculator 13 by applying the MSN, zero-forcing, or combination of these, with respect to each antenna.

(18) At this point, there may be a difference between the channel characteristics of transmitting and receiving circuitry of the transmitter 10. This is because of the following fact: Although the reversibility can be established in the spatial transfer function, the channel information matrix H is a function of a transfer function related to the transmitter 10, a spatial construction (transfer function), and a transfer function related to the receiver 20; the transfer functions related to the transmitter 10 and receiver 20 show fluctuation derived from variation of the analog circuitry for RF transmission/reception, and can not assure the reversibility. This irreversibility does not matter when the transmit antenna weighting coefficient matrix V obtained by the transmitter 10 by the singular value decomposition of the channel information matrix as acquired by the receiver 20 fed back to the transmitter 10 itself. However, in the present embodiment where the antenna weighting coefficient matrix which is necessary for the transmitter to send the data is obtained by the transmitter 10 by sending the Pre-training Signals from the receiver 20 to the transmitter 10, the variation of analog circuitry of both the receiver and transmitter affects the calculation of the matrix Z.sub.T. In this case, a transmission/reception calibrator 12 performs a suitable calibration on the matrix H.

(19) Subsequently, the transmitter 10 sends a concatenation of training signals and a signal as a component of the signal indicative of the data of interest, which is obtained by multiplexing the signal by space division. The training signals are weighted for reflecting the characteristics of the respective corresponding antennas by using the matrix Z.sub.T obtained as described above. It is particularly noted that even in the period where the training signals are sent out, the weighting for reflecting the characteristics of the corresponding antennas (hereinafter referred to as antenna-weighting) is performed for each signal multiplexed. In the example of FIG. 1, training signals Training-1 and Training-2, which are respectively subjected to weighting by element vectors w.sub.1 and w.sub.2 of the antenna weighting coefficient matrix Z.sub.T (=[w.sub.1, w.sub.2]), are sent by time division.

(20) On the other hand, a channel estimator 21 of the receiver 20 calculates a channel information matrix H each element of which corresponds to a pair of one of the transmit weighting coefficient vectors and a corresponding receive antenna, based on the training signals Training-1 and -2 as weighted with respect to each of signal components sent in a multiplexed fashion.

(21) A first receive antenna weighting coefficient matrix calculator 22 performs zero-forcing for each component of the transmit weighting coefficient vector to cancel the unnecessary signals other than a signal related to the receive antenna itself, so as to obtain a receive antenna weighting coefficient matrix Z.sub.R. Among the signals retrieved after the matrix Z.sub.R is provided, the signal exhibiting the highest S/N ratio is first decoded by a decoder 23 into x.sub.1.

(22) Next, the encoder 24 encodes the signal as decoded once again to produce a replica (duplicate) of the transmitted signal, which is canceled from a signal just received by the antenna. A second receive antenna weighting coefficient matrix calculator 25 excludes the corresponding transmit weighting coefficient vector components from the transmitted signal subjected to the canceling, and performs again zero-forcing for the signal to calculate a receive antenna weighting coefficient matrix Z.sub.R. The signal x.sub.2 exhibiting the highest S/N ratio among the remaining received signals is retrieved to be decoded by the decoder 23. In the second decoding operation, since the transmitted signal as first decoded is eliminated, the degree of freedom of the receive antennas is increased, accordingly enhancing the effect of maximal ratio combining. By iterating the above-described processing, all the multiplexed transmitted signals are decoded in sequence.

(23) The first embodiment is such that the transmitter 10 performs transmission of signals by using the MSN, zero-forcing, or combination of these, in weighting the signals. Thus, the degree of freedom of the transmit antennas is fully exploited, enhancing the S/N ratio of the received signals. Hence, even where there is no redundancy in the degree of freedom of antennas on the part of the receiver 20, the redundancy of the degree of freedom on the part of the transmitter can compensate this.

(24) FIG. 2 is a diagram specifically showing an antenna configuration in a bidirectional MIMO transmission system where a communication system related to the invention and a conventional communication system such as the V-BLAST are combined and where communication in the uplink and downlink directions is possible.

(25) In the specific example of FIG. 2, the transmitter has three antennas, while the receiver has two antennas. Where a downlink communication, namely, a communication in the direction from the transmitter to the receiver, is performed, the first embodiment of the invention is applied. That is, the antenna configuration is such that the number of transmit antennas is larger than that of the receive antennas (which equals the number of signal sub-channels as multiplexed communication channel), there is no redundancy in the degree of freedom of the receive antennas (namely, the number of the receive antennas is relatively small), and the redundancy in the degree of freedom of the antennas on the transmitter part compensates for the low degree of freedom of the receive antennas, to assure a satisfactory decoding capability.

(26) On the other hand, where a communication in the uplink direction, namely, a communication in the direction from the receiver to the transmitter is performed, a MIMO transmission can be initiated without any set-up procedure as in the V-BLAST system shown in FIG. 7. Since the V-BLAST method is not applied when the downlink communication is performed, the number of antennas need not be increase, to achieve the satisfactory decoding capability.

(27) An efficient MIMO communication system can be designed, for instance, by arranging such that three antennas (or redundant degree of freedom of antennas) are provided to an access point (control station) which has relatively ample power source and implementation capacity, while two antennas (or non-redundant degree of freedom of the antennas) are provided to a station (mobile station) relatively small in size and having less power source and implementation capacity than an access point.

(28) Further, to assure the communication quality, the present communication system employs the RTS/CTS function where the transmitter sends a transmission requesting packet RTS (Request To Send), and the receiver sends a confirmation packet CTS (Clear To Send) in response to the RTS, so that the transmitter initiates data transfer upon receiving the CTS.

(29) FIG. 3 shows a transmission/reception operational sequence in the communication system where the RTS/CTS system is applied.

(30) As shown in FIG. 3, the receiver, once receiving the RTS, sends the CTS packet with the reference signals included therein in response to the RTS. The transmitter, once receiving the CTS packet, calibrates the channel information obtained based on the reference signals included in the CTS packet, so as to obtain the channel information in the direction from the transmitter to receiver to calculate the weight for each of signal component multiplexed, and sends data packet including the training signals for respective signal components sent in a multiplexed fashion. The data part of the data packet is sent as a signal multiplexed by space division multiplexing.

(31) FIG. 4 is a diagram illustrating a construction of a communication system according to a second embodiment of the invention.

(32) The system of FIG. 4 is identical with the system of FIG. 1 in that each transmitted signal multiplexed on the part of the transmitter is space-time decoded to be distributed to plural antennas through which the signal components are sent to the receiver over respective sub-channels of a channel in a multiplexed fashion, and the receiver space-time decodes the signal components received through plural antennas via the sub-channels to obtain a received signal or data.

(33) In the embodiment of FIG. 1, the transmit antenna weighting coefficient matrix calculator 13 determines, for each antenna, the transmit antenna weighting coefficient matrix Z.sub.T by the MSN, zero-forcing, or combination of these, based on the channel information matrix H obtained by a calculation using the training signals from the receiver 20. On the other hand, the second embodiment is such that a singular value decomposition unit 15 employs the SVD (Singular Value Decomposition) in calculating the transmit antenna weighting coefficient, and weights the signal by the weighting coefficient matrix V before transmission of the signal.

(34) When zero-forcing criteria is applied to the training signals sent with weighted by V, the weighting coefficient matrix on the part of the receiver 20 necessarily becomes U.sup.H. Therefore, it is obvious that if the SVD calculation on the part of the transmitter 10 is allowed, a SVD-MIMO transmission without communication of U.sup.H to the receiver 20 is enabled, omitting the necessity to perform the singular value decomposition on the part of the receiver 20. That is, according to the present embodiment, a MIMO system with 22 antennas can be relatively easily realized.

(35) It is noted that there may be some cases where the channel characteristic of the transmitting circuitry and that of the receiving circuitry of the transmitter 10 are different; in this case, the transmission/reception calibrator suitably calibrates the channel information matrix H, as described above.

(36) On the part of the receiver 20, the channel estimator 21 calculates a channel information matrix H each element of which corresponds to a pair of one of the transmit weighting coefficient vectors and a corresponding receive antenna. The first receive antenna weighting coefficient matrix calculator 22 performs zero-forcing for each component of a transmit weighting coefficient vector to cancel unnecessary signals other than the signal related to the receiver itself, to obtain a receive antenna weighting coefficient matrix U.sup.H. The signal exhibiting the highest S/N ratio among the received signals retrieved after U.sup.H is provided is decoded by a decoder 23 to obtain signal x^.sub.1.

(37) Thereafter, the decoded signal is again encoded by an encoder 24, to produce a replica (duplicate) of the transmitted signal which is canceled from a received signal just received by the antenna. A second receive antenna weighting coefficient matrix calculator 25 excludes components of transmit weighting coefficient vector corresponding to the transmitted signal subjected to the canceling, and again applies zero-forcing to the signal to calculate a receive antenna weighting coefficient matrix U.sup.H. The signal x.sub.2 exhibiting the highest S/N ratio among the remaining received signals is retrieved and decoded by the decoder 23. Or alternatively, the second multiplexed signal x^.sub.2 may be directly retrieved from each received signal retrieved after the first receive antenna weighting coefficient matrix calculator 22 has provided U.sup.H.

(38) FIG. 9 illustrates multiple electronic devices 102 and 104. Electronic device 102 includes a transmitter 10a and a receiver 20a, and the electronic device 104 includes a transmitter 10b and a receiver 20b. The electronic device 102 may be considered an electronic device, and the electronic device 104 may be considered another electronic device. The electronic devices of FIG. 9 may be constructed according to any of the teachings herein.

(39) Although the invention has been described in detail by reference to specific embodiments thereof, it is to be understood that modifications of the embodiments or substitution of some elements or features in the embodiments which may occur to those skilled in the art may be made, without departing from the gist of the invention. That is, the embodiments have been described for illustrative purposes only, and the invention is not limited to the details of the embodiments. In interpreting the gist of the invention, appended claims should be well taken into consideration.