WO2014007030A1 - Transmitting device, receiving device, communication system and interpolation method - Google Patents

Description

DESCRIPTION

Title of Invention

TRANSMITTING DEVICE, RECEIVING DEVICE, COMMUNICATION SYSTEM AND INTERPOLATION METHOD

Technical Field

[0001]

The present invention relates to a transmitting device, receiving device, wireless communication system and interpolation method used in wireless

communications.

Background Art

[0002]

MIMO (Multiple input - multiple output) technology in wireless

communications uses multiple transmit and receive antennas to improve communication performance. MIMO provides a variety of SU-MIMO (Single-user MIMO) and MU-MIMO (Multi-user MIMO). MISO (Multiple input - single output), SIMO (Single input - multiple output) and SISO (Single input - single output) are degenerate cases of MIMO. MIMO may be combined with OFDM (orthogonal

frequency-division multiplexing) or OFDMA (orthogonal frequency-division multiple access) to handle efficiently the problems created by multi-paths. In OFDM, a large number of closely spaced orthogonal sub-carriers are used to carry data.

[0003]

MIMO uses precoding (Non-Patent Document 1). It means that multiple data streams are emitted simultaneously on the same subcarrier from transmit antennas with independent and appropriate weightings such that throughput (average rate of successful message delivery over a communication channel) is maximized at receive antennas. In a closed loop MIMO, the transmitting device must be informed about the channel.

Therefore, the transmitting device sends sounding reference signals carried by subcarriers to a receiving device. The receiving device estimates the channel states using them and sends back CSI (Channel state information) to the transmitting device, which then precodes MIMO data to make beamformed data signals.

In Non-Patent Document 2, it is described that a receiving device sends channel matrix on every second or fourth subcarrier when Grouping Ng is 2 or 4 (Table Citation List

Non Patent Literature

[0004]

[NPL 1] M. Joham, J. Brehmer and W. Utschick, "MMSE Approaches to Multiuser Spatio-Temporal Tomlinson-Harashima Precoding" Proc. 5th Int. ITG Conf. on Source and Channel Coding, pp.387-394, January, 2004.

[NPL 2] "IEEE P802. l l Wireless LAN Specification Framework for TGac "IEEE 802.11-09/0992r21, January, 2011.

Summary of Invention

Technical Problem

[0005]

According to the technique disclosed in Non-Patent Document 2, it would be expected to improve throughput because the channel state information conveyed by a feedback frame is reduced. However, the transmitter needs to interpolate the channel values in missing subcarriers to precode MIMO data, and furthermore channel characteristics changes moment by moment. It could not be possible to improve the throughput as expected by the technique disclosed in Non-Patent Document 2, since the channel characteristics could not be adequately recovered and the interference between streams remains.

[0006]

One of the technical problems to be solved by the present invention is to optimize the recovery of channel characteristics using reduced channel state information conveyed by a feedback frame. Other technical problems to be solved will become apparent by the following descriptions of the embodiments according to the present invention.

Solution to Problem

[0007]

(1) The present invention has been made to solve the above problem. The present invention is a transmitting device in a wireless communication which comprises a control module to indicate a method out of a plurality of interpolation methods each of which interpolates the channel values in the missing subcarriers based on the positions of the representative subcarriers, which are selected out of the subcarriers constituting the frequency band used in the wireless communication, and the values in the representative subcarriers, and a transmitting module to transmit the information about the selected interpolation method.

[0008]

(2) Additionally, the transmitting device according to the present invention is configured to transmit the information about the selected interpolation method according to the channel state between the transmitting and receiving devices.

[0009]

(3) Further, the present invention is a transmitting device in a wireless communication which comprises a control module to indicate several methods out of a plurality of interpolation methods each of which interpolates the channel values in the missing subcarriers based on the positions of the representative subcarriers, which are selected out of the subcarriers constituting the frequency band used in the wireless communication, and the values in the representative subcarriers, and a transmitting module to transmit the information about the several interpolation methods selected.

[0010]

(4) Further, the present invention is a receiving device in a wireless communication which comprises a reception module to receive information about several interpolation methods each of which interpolates the channel values in missing subcarriers based on the positions of representative subcarriers, which are selected out of the subcarriers constituting the frequency band used in the wireless communication, and the values in the representative subcarriers, and an interpolation choice module to select a method out of the several interpolation methods.

[0011]

(5) Further, the present invention is a transmitting device in a wireless communication between the transmitting device and a plurality of receiving devices which comprises a control module to indicate in every receiving device a method selected out of a plurality of interpolation methods each of which interpolates the channel values in the missing frequency points based on the positions of representative subcarriers , which are selected out of the subcarriers constituting the frequency band used in the wireless communication, and the values in the representative subcarriers, and a transmission module to transmit the selected interpolation methods.

[0012]

(6) Additionally, the transmitting device according to the present invention is configured to transmit the information about the maximum and/or minimum of the number of representative subcarriers.

[0013] (7) Additionally, the transmitting device according to the present invention is configured to transmit information about the quality of feedback from the receiving device to the transmitting device.

[0014]

(8) Further, the present invention is a wireless communication system consisting of a transmitting device and a plurality of receiving devices in which a receiving device which performs an interpolation optimized feedback of the

representative subcarriers, which are selected out of subcarriers constituting the frequency band used in the wireless communication, to optimize the recovery of channel characteristics using reduced channel state information conveyed by a feedback frame, and another receiving device which does not perform the interpolation optimized feedback are included in the plurality of receiving devices.

[0015]

(9) Further, the present invention is a receiving device in a wireless

communication which comprises an optimum selection module to select representative subcarriers out of subcarriers constituting the frequency band used in the wireless communication at irregular intervals.

[0016]

(10) Further, the present invention is an interpolation method used in a wireless communication, which comprises a step of identifying representative subcarriers, selected out of subcarriers constituting the frequency band used in the wireless communication, to the channel values, a step of finding the slopes of the channel characteristics in the representative subcarriers, and a step of interpolating channel values in the missing points by the channel values, slopes and positions in the representative subcarriers.

Advantageous Effects of Invention

[0017]

According to the present invention, the channel characteristics are optimized by the reduced channel state information conveyed by a feedback frame.

Brief Description of Drawings

[0018]

[Fig. 1] Fig. 1 is a block diagram of an exemplary wireless communication system according to the present invention.

[Fig. 2] Fig. 2 is a protocol of the communication in Fig. 1. [Fig. 3A] Fig. 3A shows an example of VHT Capabilities field in a beacon.

[Fig. 3B] Fig. 3B shows an example of VHT Capabilities field in a beacon.

[Fig. 3C] Fig. 3C shows a table of supported interpolation.

[Fig. 4A] Fig. 4A shows the structure of NDPA according to the first embodiment.

[Fig. 4B] Fig. 4B shows the structure of NDPA according to the first embodiment.

[Fig. 5] Fig. 5 is the structure of NDP according to the first embodiment.

[Fig. 6 A] Fig. 6 A shows the structure of SND FB.

[Fig. 6B] Fig. 6B shows the structure of SND FB.

[Fig. 6C] Fig. 6C shows an illustrative configuration for Quantization.

[Fig. 7A] Fig. 7A shows one configuration of Number of subcarriers.

[Fig. 7B] Fig. 7B shows another configuration.

[Fig. 7C] Fig. 7C shows another configuration.

[Fig. 7D] Fig. 7D shows another configuration.

[Fig. 7E] Fig. 7E shows another configuration.

[Fig. 8] Fig. 8 is the structure of FB Poll according to the first embodiment.

[Fig. 9] Fig. 9 is a schematic block diagram of an access point according to the first embodiment.

[Fig. 10] Fig. 10 is the example of a precoding module.

[Fig. 11] Fig. 11 is a flowchart showing the procedure of a central value interpolation. [Fig. 12] Fig. 12 is a flowchart showing the procedure of a linear interpolation.

[Fig. 13] Fig. 13 is a flowchart showing the procedure of a cubic interpolation.

[Fig. 14] Fig. 14 is a flowchart showing the procedure in which slopes of the channel characteristic are calculated.

[Fig. 15] Fig. 15 is a schematic block diagram of the station according to the first embodiment.

[Fig. 16] Fig. 16 is a flowchart showing the procedure in which the selection of representative frequency points is performed.

[Fig. 17] Fig. 17 is a graph explaining supplementarily the procedure of Fig. 16. [Fig. 18] Fig. 18 is a flowchart showing the procedure, in which the incremental mapping is calculated.

[Fig. 19] Fig. 19 is a table of incremental mapping.

[Fig. 20A] Fig. 20A shows tables of supported interpolations according to the second embodiment.

[Fig. 20B] Fig. 20B shows tables of supported interpolations according to the second embodiment.

[Fig. 21 A] Fig. 21 A shows the details of VHT MIMO Control according to the second embodiment.

[Fig. 2 IB] Fig. 2 IB shows tables of interpolation.

[Fig. 21C] Fig. 21C shows tables of interpolation.

[Fig. 22] Fig. 22 is a schematic block diagram of the access point according to the second embodiment.

[Fig. 23] Fig. 23 is a schematic block diagram of the station according to the second embodiment.

[Fig. 24] Fig. 24 is a flowchart showing the procedure to choose an interpolation method and then to select representative points at the station.

[Fig. 25] Fig. 25 is the structure of NDPA according to the third embodiment.

[Fig. 26A] Fig. 26A shows interpolation choice aid of NDPA shown in Fig. 25.

[Fig. 26B] Fig. 26B shows interpolation choice aid of NDPA shown in Fig. 25.

[Fig. 26C] Fig. 26C shows interpolation choice aid of NDPA shown in Fig. 25.

[Fig. 26D] Fig. 26D shows interpolation choice aid of NDPA shown in Fig. 25.

[Fig. 27] Fig. 27 is a flowchart explaining the actions of interpolation adaptation at a station.

[Fig. 28] Fig. 28 is the structure of NDPA according to the fourth embodiment.

[Fig. 29] Fig. 29 is a flowchart showing the procedure of a station according to the fifth embodiment.

Description of Embodiments

[0019]

Now, preferred embodiments of the present invention will be described in detail with reference to the annexed drawings. The embodiments relate to a WLAN (Wireless Local Area Network), but they are not restricted to the WLAN, but are also applicable to a mobile phone network.

THE FIRST EMBODIMENT

[0020]

Fig. 1 is a block diagram of an exemplary wireless communication system according to the present invention.

The system includes BSSs (Basic Service Sets) 1 to 3 which form wireless networks respectively. BSS 1 includes an access point 101 and stations 111 to 118. BSS 2 includes an access point 102 and stations 111, 112 and 121. BSS 3 includes an access point 103 and stations 116, 117 and 131.

Stations 111 to 118 receive a beacon from the access point 101. Stations 111, 112 and 121 receive a beacon from the access point 102. Stations 116, 117 and 131 receive a beacon from the access point 103. Stations 111 and 112 receive beacons from both of access points 101 and 102 respectively. Stations 116 and 117 receive beacons from both of access points 101 and 103 respectively. Either one or all of BSSs 1 to 3 may be connected to a WAN (Wide Area Network) which is not shown in Fig. 1.

In the following descriptions, an access point and a station may be abbreviated to AP and STA respectively. An access point and stations may also be referred to as transmitting and receiving devices respectively.

[0021]

Each of access points 101 to 103 has n transmit antennas. Each of stations 111 to 118, 121 and 131 has m receive antennas. In Fig. 1, four transmit antennas of each of access points 101 to 103 and one receive antenna of each of stations 111 to 118, 121 and 131 are depicted in Fig. 1 in order to make the drawing clear and simple.

For example, the access point 101 takes four stations 111 to 114 and communicates with them. In SU-MIMO, for example, the access point 101 has four transmit antennas and each of stations 111 to 1114 has four receive antennas. In that case, the four transmit antennas emit streams and the four receive antennas of any one of stations 111 to 114 receive four streams respectively. Access point 101

communicates with one of stations 111 to 114 at once, and then it communicates with another station successively.

In MU-MIMO, for example, access point 101 has four transmit antennas, and each of stations 111 to 1114 has one receive antenna. The four transmit antennas of access point 101 emit streams and one receive antenna of each of stations 111 to 1114 receives the stream that is directed to itself.

[0022]

The above mentioned numbers of antennas are only for example. The present embodiments are applicable to MISO, SIMO and SISO, and are even applicable to a wired network.

In MU-MIMO, a station may even have multiple receive antennas. In SU-MIMO, the number m of receive antennas of the station may be even larger or smaller than the number n of transmit antennas of the access point.

[0023]

Fig. 2 is a protocol of the communication in Fig. 1.

Access point 101 periodically transmits Beacon 201 to inform stations 111 to 118. Stations 111 to 118 detect and identify Beacon 201 and examine parameters to join the network respectively.

After sending Beacon 201 and before sending the next, access point 101 sends NDPA (No Data Packet Announce) 211. NDPA 211, like a beacon, is a control frame and designates one of the stations 111 to 118 which responds first. NDPA 211 may specify another station which responds next. Access point 101 sends NDP (No Data Packet) 212 after SIFS (Short Inter- Frame Space). NDP 212 carries out the sounding of channels between the access point and stations. The sounding process starts with NDPA 211.

The station which responds first feeds back SND FB (Sounding Feedback) 213-1 after SIFS. The result of the sounding is written in a data field of SND FB 213-1. After receiving SND FB 213-1, access point 101 sends FB Poll (Feedback Polling) 214-1 after SIFS. The station which responds second feeds back SND FB

213- 2 after SIFS. After receiving SND FB 213-2, the access point 101 sends FB Poll

214- 2 after SIFS. The station which responds third feeds back SND FB 213-3 after SIFS. The above process continues until station 118 responds.

After the sounding sequence, access point 101 sends Beamforemed Data 221. Beamforemed Data 221 includes any of the text data, audio data, still image data, moving image data, etc.

In the following, SND FB 213-1 to SND FB 213-3 are named SND FB and FB Poll 214-1 to FB Poll 214-2 are named FB Poll 214 generically.

[0024]

Fig. 3 A and 3B shows an example of VHT Capabilities field in a beacon. Any station that intends to connect to an access point reads the beacon in order to know the capabilities of the access point.

[0025]

In Fig.3A, Element ID 301 indicates that the elements to follow correspond to VHT Capabilities element. Length 302 gives the length of VHT Capabilities element. VHT Capabilities Info 303 specifies the capabilities of the access point 101. A-MPDU Parameters 304 indicates the maximum length of aggregated MPDU (MAC Protocol Data Unit) which a station can receive. Supported MCS Set 305 is used to convey the combinations of MCSs (Modulation and Coding Sets) which a station supports for both reception and transmission.

[0026]

Fig. 3B shows VHT Capabilities Info 303 in more detail.

In Fig.3B, Maximum MPDU Length 311 indicates the maximum MPDU length. Supported Channel Width Set 312 indicates the bandwidth which a station supports. LDPC Coding Capabilities 313 is set to 0 if LDPC (Low Density Parity Check) is not supported, and to 1 if LDPC is supported. Short GI for 20/40/80/160 314 indicates support for receiving packets using the short guard interval in various bandwidths. Tx STBC 315 indicates support for the transmission of at least 2x1 STBC (Space-Time Block Coding). Rx STBC 316 indicates support for the reception of PPDU (PLCP Protocol Data Unit) using STBC. SU Beamformer Capable 317 indicates support for operation as single user beamformer. SU Beamformee Capable 318 indicates support for operation as single user beamformee.

[0027]

Grouping Set 319 indicates acceptable values for the VHT MIMO Control Grouping parameter with sounding feedback. Compressed Steering Number of Beamformer Antennas Supported 320 indicates the maximum number of beamformer antennas which the beamformee can support when sending compressed beamforming feedback. Number of Sounding Dimensions 321 indicates the number of antennas used by the beamformer when sending beamformed transmissions. MU Tx Capable 322 indicates whether or not the station supports operation as an MU beamformer.

MU Rx Capable 323 indicates whether or not the station supports operation as an MU beamformee. VHT TXOP PS 324 indicates whether or not the access point supports VHT TXOP power save mode for stations already in the cell, while it indicates whether or not the station is in VHT TXOP power save mode when trying to associate or re-associate to the access point.

[0028]

Supported Interpolation 331 shows the interpolation method which access point 101 can support.

Fig. 3C shows a possible example of Supported Interpolation 331.

Access point 101 sets the field to '00' if the interpolation method which access point 101 supports is a linear Interpolation. Access point 101 sets the field to '01 ' if the interpolation method which access point 101 supports is a pchip Interpolation. The wording "pchip" is an abbreviation for piecewise cubic Hermit interpolation polynomial. Access point 101 sets the field to ' 10' if the interpolation method which access point 101 supports is a cubic spline Interpolation. The value ' 11 ' is reserved for future use in this example, although it could be assigned to a different interpolation method.

[0029]

Supported Interpolation 331 reflects the interpolation method access point 101 prefers to process at that given time. Access point 101 can decide to downgrade the accuracy of the interpolation method if its resources are being used to a point in which the risk exists of not being able to perform all the required computations in time.

Downgrading the performed interpolation method could grant some extra computational power to meet the time requirements when the number of users becomes large.

Turning back to Fig. 3B, Reserved 325 is reserved for future use.

[0030]

Fig. 4A and 4B show the structure of NDPA 211.

In Fig. 4A, Frame Control 401 identifies the frame as NDPA 211. Duration 402 indicates the duration of NDPA 211. RA 403 is set to the address of the destination in case of SU-MIMO, and to the broadcast address in case of MU-MIMO. TA 404 is set to the address of the access point. Sounding Sequence 405 indicates a sequence number associated to the current sounding sequence. STA Info 1 406-1, ..., STA Info n 406-n contain the AID (Association Identifier) of the sounded stations respectively. AID is an indication of whether it is for SU MIMO or for MU MIMO, and in the latter case an indication of how many dimensions are requested. FCS 407 is a CRC of the previous fields in order to be able to detect errors.

[0031]

NDPA 211 contains a STA Info field for each of the station that must return its feedback right after the NDP 212, starting from the STA in the first STA Info field and continuing in order of appearance. In any case, after the access point has finished receiving the feedback from one station, the access point emits FB Poll 213 to state the identity of the next station to return its feedback.

[0032]

Fig. 4B shows the structure of STA Info of Fig. 4A.

In Fig. 4B, AID 411 contains an association identifier by which the addressed station is associated to the BSS. Feedback Type 412 is set to '0' if the requested feedback is intended for SU-MIMO and is set to Ί ' if the requested feedback is intended for MU-MIMO. Nc Index 413 is reserved in SU-MIMO, and it indicates the requested feedback dimension in MU-MIMO.

[0033]

Fig. 5 shows the structure of NDP 212.

In Fig. 5, L-STF501 corresponds to the legacy short training field. L-LTF 502 corresponds to the legacy long training field. L-SIG (Legacy-Signal) 503 gives information about the length of the packet. VHT-SIG-A 504a contains information about the packet and indicates whether it is an SU-MIMO or MU-MIMO transmission. VHT-SIG-A 504b gives additional information about the packet. VHT-STF 505 is an extension of the short training field for the VHT case. VHT-LTFl 506-1 serves for the station to estimate the channel from the first transmit antenna at the access point.

Consequent VHT-LTF2 506-2 to VHT-LTFn 506-n allow the station to estimate the channel from the second transmit antenna to the n^th transmit antenna at the access point. VHT-SIG-B 507 is set to a fixed bit pattern known.

[0034]

Fig. 6A and 6B show the structure of SND FB.

In Fig. 6 A, the whole frame format is specified for clarity, including Legacy Preamble 601, VHT Preamble 602, Service Field 603, VHT-DATA 604 and Tail & Padding 605.

VHT-DATA 604 is also known as MPDU (MAC Protocol Data Unit).

VHT-DATA 604 contains MAC Header 611, Frame Body 612 and FCS (Frame Check Sequence) 613.

[0035]

Next, MAC Header 611 will be explained.

Frame Control 621 contains some fields that identify the purpose of the frame, in this case as an action frame. Duration 622 gives the duration of the frame.

Address 1 (DA (Destination Address)) 623 contains the destination address. SA (Source Address) 624 contains the address of the transmitting station. BSSID (Basic Service Set Identification) 625 identifies the BSS to which the frame belongs.

Sequence Control 626 contains the identifier of the current sequence. VHT Control 627 contains information about the VHT MIMO configuration.

[0036]

Next, Frame Body 612 will be explained.

Category 631 states that this action frame corresponds to VHT.

Action 632 indicates that the action is "Interpolation optimized feedback". VHT MIMO Control 633 will be explained later. Number of subcarriers 634 states the number of representative subcarriers chosen for each stream.

Mapping 635 contains information about which representative frequency points the feedback is sent for. The mapping is created for the real and imaginary parts of the channel values. In this particular embodiment, the mapping is an incremental mapping, for example.

VHT Beamforming Report 636 contains the quantized channel values in representative frequency points chosen for feedback. MU-Exclusive Beamforming Report 637 is present in the case of MU-MIMO, and gives additional information about the SNR affecting the different frequency points for each stream.

[0037] Fig. 6B details VHT MIMO Control 633.

In Fig. 6B, Nc Index 641 indicates the number of maximum space-time streams that the access point can use for beamforming. Nr Index 642 indicates the number of transmit antennas which the access point uses for beamforming. Channel Width 643 states the bandwidth for which the feedback is given. Reserved 644 is reserved for future use. Remaining Segments 645 indicates how many segments are to be sent after the current one. First Segment 646 is set to Ί ' if the current segment is the first one of the sequence, and is set to '0' otherwise.

[0038]

Fig. 6C shows an illustrative configuration for Quantization 647.

If the field is set to ΌΟ', the quantization is set to 4 bits for the real part and 4 bits for the imaginary part. If the field is set to Ό , the quantization is set to 5 bits for the real part and 5 bits for the imaginary part. If the field is set to ΊΟ', the

quantization is set to 6 bits for the real part and 6 bits for the imaginary part. If the field is set to ' 11 ', the quantization is set to 7 bits for the real part and 7 bits for the imaginary part.

[0039]

Turning back to Fig. 6B, Sounding Sequence Number 648 contains the identifier of the current sounding sequence.

Turning back to Fig. 6A, FCS 613 contains CRC to validate the integrity of the

MPDU.

[0040]

Fig. 7 A shows a configuration of Number of Subcarriers 634.

In Fig. 7A, "SCs for stream 1, real part" 701-1 a contains the number of representative frequency points (representative subcarriers) for the real part of the first stream. "SCs for stream 1, imaginary part" 701 -lb contains the number of frequency points for the imaginary part of the first stream. Segment 701 -2a contains the number of representative frequency points for the real part of the second stream. Segment 701 -2b contains the number of frequency points for the imaginary part of the second stream. This configuration continues until stream n.

[0041]

Fig. 7B is another configuration.

The optimum number of chosen representative frequency points is usually the same or very close for the real and imaginary parts of each stream. This symmetry can be used by sending the same number for representative frequency points for both of real and imaginary parts. In this case, "SCs for stream 1" 711-1 contains the number of representative frequency points sent for the first stream. "SCs for stream 1" 711-2 contains the number of representative frequency points sent for the second stream. This

configuration continues until stream n.

[0042]

Fig. 7C shows another configuration.

In this case, the characteristics of the channel are similar for all the streams. The number of representative frequency points sent for each stream is very close or the same. In this example, the station computes the average number of chosen points sent for each stream, and sends that information as "Average SCs for all streams" 721.

"Offset for stream 1, real" 722- la is the difference between "Average SCs for all streams" 721 and the number of chosen subcarriers sent for the real part of the first stream. "Offset for stream 1, imaginary" 722- lb is the difference between "Average SCs for all streams" 721 and the number of chosen subcarriers sent for the imaginary part of the first stream. This configuration continues until stream n.

[0043]

Fig. 7D shows another configuration

In Fig. 7D, the average number of representative frequency points is sent and the offset for each stream is sent, but similar to Fig. 7B, real and imaginary parts are forced to use the same number of subcarriers.

Fig. 7E shows another configuration in which "SCs for all streams" 741 contains the same number of representative frequency points sent for each of the streams.

[0044]

Fig. 8 shows the structure of FB Poll.

In Fig. 8, Frame Control 801 indicates that the current frame is FB Poll.

Duration 802 indicates the duration of FB Poll. RA 803 is set to the address of the station to send its feedback next. TA 804 is set to the address of the access point.

Segment Retransmission Bitmap 805 indicates which parts must be transmitted. FCS 806 is CRC of the previous fields in order to be able to detect errors.

[0045]

Fig. 9 is a schematic block diagram of the access point according to the first embodiment.

In Fig. 9, Transmission Buffer module 901 receives data bits from an upper layer. The data bits constitute the beacon, NDPA, NDP, FBPoll or Beamformed Data depicted in Fig. 2. Transmission Buffer module 901 stores the data bits and then conveys them to Coding modules 902-1 to 902-n as indicated by Selection module 914. Coding modules 902-1 to 902-n performs error correction coding to the data bits coming from Transmission Buffer module 901 as indicated by Selection module 915, respectively. Modulation modules 903-1 to 903-n perform to the output signals from Coding modules 902-1 to 902-n modulations indicated by Selection module 914, respectively.

Pilot multiplexing modules 904-1 to 904-n multiplex pilot signals (channel estimation signals) to the output signals from Modulation modules 903-1 to 903-n respectively when they constitute NDP.

[0046]

Precoding module 905, having as input the modulated signals from Pilot Multiplexing modules 904-1 to 904-n performs precoding to said input signals.

[0047]

Fig. 10 shows the details of Precoding module 905.

First, Filter Calculation module 1001 creates a filter W based on the channel matrices transferred from Feedback Storage module 915. The filter W may be a weighting matrix of Zero-Forcing, or the one obtained by MMSE criterion, for example.

Filter module 1002 multiplies input signals from Pilot Multiplexing modules 904- 1 to 904-n by filter W to make precoded signals which are output to IFFT (Inverse Fast Fourier Transform) modules 906-1 to 906-n respectively. The multiplication is performed subcarrier by subcarrier.

[0048]

If the signals input from Pilot Multiplexing modules 904-1 to 904-n to

Precoding module 905, are the ones of a control frame like beacon, a unit matrix is selected as the filter W. According to this way, the signals bypass Precoding module 905.

[0049]

Turning back to Fig. 9, IFFT modules 906-1 to 906-n change the precoded symbols to time domain signals respectively. GI Insertion modules 907- 1 to 907-n insert guard intervals in the time domain signals respectively. Wireless transmission modules 908-1 to 908-n carry out DA conversion of the signals to which GI has been added to analogue signals, convert them to high frequency band and make transmissions from antennas 909-1 to 909-n respectively.

[0050]

Wireless reception module 910 receives SND FB depicted in Fig. 2 from a station. Wireless reception module 910 down convert it to baseband signals, and carries out AD conversion to obtain digital signals. Then, Wireless Reception module 910 converts the digital signals into frequency domain signals through FFT (Fast Fourier Transform), and sends them to Feedback Analyzer module 911.

Feedback Analyzer module 911 extracts the channel values in representative frequency points from VHT Beamforming Report 636 depicted in Fig. 6 A and sends them to Feedback Interpolation module 913. Feedback Analyzer module 911 also extracts the positions of the representative frequency points from mapping 635 in Fig. 6A and sends them to Feedback Demapping 912. Feedback Analyzer module 911 extracts the number of subcarriers from Number of subcarriers 634 and sends it to Feedback Demapping module 912. Feedback Analyzer module 911 extracts the quantization level from Quantization 647 and sends it to Feedback Interpolation module 913.

[0051]

Feedback Demapping module 912 de-maps the positions of representative frequency points and gives this data to Feedback Interpolation module 913. The number of subcarriers sent from Feedback Analyzer 911 is used to check the operation of Feedback Demapping module 912. Feedback interpolation module 913 performs interpolation of the channel values in the missing points based on the information given by both of Feedback Demapping module 912 and Feedback Analyzer module 911. The number of quantization level sent from Quantization 647 is used to check the operation of Feedback interpolation module 913. Control module 916 indicates the interpolation method selected out of a plurality of interpolation methods. The channel values in the representative and missing points stream by stream are given to Selection module 914 and Feedback Storage module 916.

[0052]

Selection module 914 receives information about the kinds and destination of the data bits stored in Transmission Buffer module 901. Selection module 914 also receives information about the channels from Feedback Interpolation module 913. Selection module 914 decides which antenna paths the data bits stored in Transmission Buffer module 901 are to be directed to and sends the decision to Transmission Buffer module 901 and Feedback Storage module 915.

Feedback Storage module 915 reconstructs channel matrices based on the information received from Feedback Interpolation 913 and Selection module 915.

[0053]

(Interpolating method 1)

Fig. 11 is a flowchart showing the procedure of a central value interpolation performed in Feedback Interpolation module 913 depicted in Fig. 9.

An access point receives as feedback the channel values in representative frequency points selected among all of the frequency points in the bandwidth. The term "frequency point" means the central frequency of OFDM subcarrier. The term "frequency point" may be abbreviated as "point".

In the central value interpolation, a representative frequency point corresponds to a fixed set of points. The set is made by two, four or eight consecutive points, for example. The access point first identifies which representative points correspond to the received channel values respectively (1101), and assigns the values to all of the missing points of the identified sets respectively (1102).

[0054]

For the central value interpolation, a station may send the exact channel value in a representative point or the average channel value with respect to the points belonging to the set, for example. The latter operation can eliminate spikes of noise and results in better performance.

[0055]

(Interpolating method 2)

Fig. 12 is a flowchart showing the procedure of a linear interpolation.

In the linear interpolation, a representative frequency point corresponds to a fixed set of points.

An access point first identifies which representative points correspond to the received channel values respectively (1201). The access point finds line segments joining each pair of the channel values in consecutive representative points (1202), and assigns the line segment values in missing points to the points respectively (1203).

The linear interpolation is a very simple interpolation method, with very low computational load as in the central value interpolation, and in addition, its application results in a clear improvement over the central value interpolation.

[0056]

(Interpolating method 3)

A sine interpolation will be explained next. The sine interpolation uses a sine cardinal or sine function like sine (x) = sin (x) /x. The sine interpolation

accommodates the representative frequency points to a series of sine functions as seen in the following equation.

[0057]

[Equation 1] k - n - T (k - n - T\

x(k) = x[n]- sine sine

T

(1)

[0058]

In Equation (1), x(k) means the interpolated value in the missing point k, [-L, L] means the range of the subcarriers in the given bandwidth, T means the sampling period which corresponds to the bandwidth of a subcarrier and x[n] means the channel value in the representative point n.

The procedure of sine interpolation method is easy because it is not needed to find slopes like a cubic interpolation, but the computational load becomes heavier compared to that of a central value or linear interpolation.

The sine interpolation method is affected by the Gibbs phenomenon, causing ringing that can be very severe. Therefore, an appropriate window, such as Hamming window, Kaiser window, Blackman window, etc. can be used together with sine interpolation method to eliminate such ringing.

[0059]

(Interpolating method 4)

Fig. 13 is a flowchart showing the procedure of a cubic interpolation.

An access point first identifies which representative points correspond to the received channel values respectively (1301). Then, the access point finds slopes of the channel characteristics in the representative points respectively (1302). With the received channel values and calculated slopes, the access point finds the channel values in missing points (1303).

[0060]

(Interpolating method 4-1)

Fig. 14 is a flowchart showing the step 1302 in greater detail. It is preferable especially for a pchip interpolation.

An access point determines whether a representative point is an interior or end point (1401). If it is an interior point, here let it be named "point B", the access point finds line segments AB and BC which join the values in the point B and neighboring representative points A and C at both sides respectively, and calculates slopes S_AB and SBC of the line segments (1402).

The access point compares signs of S_AB and SBC, and determines whether the signs are equal or neither is zero substantially (1403). If the judgment in step 1403 proved true, the access point finds the slope S_B in point B as shown in the following equation (1404). [0061]

Equation 2]

^WAB ~ 2 · d _AB + d_BC

^WBC ~ ^AB ^ ' ^dBC (2)

[0062]

If the judgment in step 1403 proved false, the access point finds that slope SB is zero (1405).

[0063]

Turning back to the first step 1401 , if the representative point is an end point, here let it be named "point A", the access point finds line segments AB and BC which join consecutively the values in point A and neighboring representative points B and C at one side respectively, and calculates the slopes SAB and SBC of the line segments. And, the access point calculates the tentative slope S_A,T, as shown in the following equation.

[0064]

[Equation 3] o _ ^WAB ^{" S}AB + ^WBC ' ^SBC

aAB ^{+ a}BC

WAB = ² ' ^dAB + ^dBC

^WBC ~ ^~d B ( )

[0065]

The access point compares the signs of S_A,T and S_Bc, and determines whether the signs are equal or neither is zero substantially (1413). If the judgment in step 1413 proved false, the access point finds the slope S_A is zero ( 1414). If the judgment proved true, the access point compares the signs of SAB and SBC , and determines whether the signs are equal (1415). If the judgment proved true, the access point finds the slope S_A is equal to S_A,T (1418).

If the judgment proved false, the access point determines whether the absolute value of SAB multiplied by number 3 is larger than the absolute value of S_A,T ( 1416). If the judgment proved true, the access point finds the slope S_A is equal to SAB multiplied by three (1417). If the judgment proved false, the access point finds the slope SA is equal to S_A)T ( 1418). The above procedure is repeated for all of interior and end points.

[0066]

(Interpolating method 4-2)

In the following, another way of carrying out the step 1302 in Fig. 13 will be explained. It is preferable for cubic interpolation, especially for cubic spline interpolation.

The slopes of representative points are calculated by solving the following simultaneous equations.

[0067]

[Equation 4]

(4)

[0068]

In the above equation, dk means the slope in a representative point Xk, hk means the distance between points Xk and Xk+i and the letter "delta" of Greek alphabet with suffix k means the slope of a line segment which joins the values in points Xk and Xk+i .

In Equation 4, it is assumed that the first and second derivatives of the channel function at point Xk are continuous. Beside that, a new point x₀ is created outside of end point, and the following equation is assumed. The letter "delta" of Greek alphabet with suffix 0 means the corresponding slope.

[0069]

[Equation 5]

<5₀ = <5,

(5)

[0070]

According to the interpolation method using Equation 4 and 5, the overshoot of a cubic spline can be avoided. Such a cubic spline is a natural spline.

Now, the details of the step 1303 shown in Fig.13 will be explained.

The values of missing points are acquired by the following equation.

[0071]

[Equation 6] 3 - h - s² - 2 - s³ h³ - 3 - h - s¹ + 2 - s

P(x) =

P'(x_k ) = d_k ;P'(x_k+l) h = x_k+]— x_k

(6)

[0072]

In Equation (6), P(x) means the interpolated value in points x (xk≤ x≤ Xk+ι Xk means a representative point, y_k means the channel value in point Xk, dk means the slope in point Xk, h means the distance of the k^tA subinterval (h = Xk+i - Xk), and s means the distance between the interpolated point x and the representative point Xk.

The cubic interpolation method achieves higher resemblance to the real channel, because it considers not only the values of representative points, but also the slopes respectively.

[0073]

Fig. 15 is a schematic block diagram of a station according to the first embodiment.

Wireless Reception module 1502 receives wireless signals through Antenna 1501, converts them to baseband signals, performs DA conversion and transfers the digital signals to GI Extraction module 1503. GI Extraction module 1503 extracts GI from the digital signals and transfers the remainder to FFT module 1504 to perform Fast Furrier Transform to get frequency domain signals. The result of FFT is sent to Pilot Demultiplexing module 1505.

Pilot Demultiplexing module 1505 extracts pilot signals (channel estimation signals) from the remainder. The remainder is transferred to Channel Compensation module 1506 and the pilot signals are sent to Channel Estimation module 1509.

Channel Estimation module 1509, based on the values of the extracted pilot symbols, estimates the channels between Antenna 1501 and transmit antennas of the access point.

[0074]

Channel Compensation module 1506 performs channel compensation to the received signals from Pilot Demultiplexing module 1505 based on the information from Channel Estimation module 1509. Demodulation module 1507 demodulates the signals output from Channel Compensation module 1506. Decoding module 1508 decodes the signals output from Demodulation module 1507 and retrieves data bits.

[0075]

Optimal Selection module 1510 performs the operation of finding

representative frequency points based on the channel information sent from Channel Estimation module 1509. Optimal Selection module 1510 sends the channel values in representative points and the quantization level to Feedback Creation module 1511 and sends the positions of representative points to Incremental Mapping module 1511.

Incremental Mapping module 1511 finds an incremental mapping and number of representative points, and sends them to Feedback Creation module 1512.

Feedback Creation module 1512 puts the channel values in representative points and the quantization level to VHT beamforming Report 636 in Fig. 6 A and Quantization 647 in Fig. 6B respectively, and puts the incremental mapping and number of representative points to Mapping 635 and Number of Subcarriers 634 in Fig. 6A respectively. Other fields of SND FB in Fig. 2 are sent from the upper layer to the Feedback Creation module 1512, but the detailed explanation of it will be omitted because of it belonging to well-known matters.

[0076]

Wireless Transmission module 1513 performs DA conversion to the signals of FBPoll transferred from Feedback Creation module 1512, converts them to a wireless frequency band and transmits them to the access point through Antenna 1501.

Control module 1514 performs the necessary actions for the above mentioned modules.

[0077]

Next, the selection of representative frequency points at a station will be explained.

(Selecting Method 1)

The selection is performed by selecting every second point to obtain representative points.

(Selecting Method 2)

It is performed by selecting every fourth point to obtain representative points. (Selecting Method 3)

It is performed by selecting every eighth point to obtain representative points.

[0078]

(Selecting Method 4)

Selecting Method 4 will be explained referencing to the flowchart in Fig. 16. In Fig. 16, a station takes the real parts of each element of the channel matrices in all of the frequency points sequentially (1601). The station finds a differentiable channel function, based on the real parts, then finds the points that present a relative maximum or minimum, and incorporates them to a set K as its elements (1602). It incorporates also the first and last points to the set K (1603). Then, it separates one of the neighboring points in K except endpoints that are closer than a defined distance d_k, and incorporates the remainder to a set K_s as its elements (1604).

[0079]

If it is necessary, the station will perform the following steps 1605 and 1606. It finds points K₊, at the values of which the tangents are substantially parallel to the line segments joining the values of neighboring points in the set K_s respectively (1605). Then, it separates one of the neighboring points K₊ that are closer than a defined distance d_k+, and incorporates the remainder into a set K_extra as its elements (1606). The points of sets K_s and K_extra are chosen for feedback (1607). The steps 1602 to 1607 are repeated for the real parts of each element of the channel matrices at all of the frequency points.

[0080]

Then, the station decides whether the above steps have already been iterated for the imaginary parts (1608). If the above steps are not iterated for the imaginary parts, the station takes the imaginary parts sequentially (1609). The station processes the step 1602. If the imaginary parts have already been processed, the station creates information to let the access point know which points were selected (1610).

[0081]

Fig. 17 is a graph explaining the procedure of Fig. 16. In Fig. 16, the transverse axis shows frequency and the vertical axis shows channel value.

Curve 1701 shows the channel function. Black dots 1711 and 1712 denote the values of the points which are found in the step 1602 shown in Fig. 16. Line 1721 shows a line segment joining black dots 1711 and 1712. White dots 1731 to 1733 denote the values of the points which are found in the step 1605. Tangents of the curve 1701 at the white dots 1731 to 1733 are substantially parallel to the line segment 1721.

[0082]

(Selecting Method 5)

In Selecting Method 5, the procedure of steps 1602 and 1605 shown in Fig. 16 can be modified by using line segments joining the values of neighboring points instead of using the channel function, and by following the same way as shown in the flowchart in Fig. 14.

[0083] (Selecting Method 6)

In Selecting Method 6, steps 1604 to 1606 shown in Fig. 16 are omitted. It is substantially matched to the cubic spline interpolation rather than the pchip

interpolation.

In selection method 4 to 6, the representative frequency points (subcarriers) are selected irregularly from the subcarriers constituting the frequency band used in the wireless communication.

[0084]

Next, feeding back of the representative points will be explained.

The frequency positions of representative points can be fed back as they are. However, an incremental mapping is more efficient.

Fig. 18 is a flowchart showing the procedure, in which the incremental mapping is calculated.

The first and last frequency points that are fed back are the first and the last ones of the subcarriers, and thus no mapping is needed for them. The first value of the incremental mapping is the distance between the first and second representative points. The second value of the incremental mapping is the distance between the second and third representative points. This procedure continues until the penultimate

representative point.

[0085]

The station sets the minimum distance d_mi_n between consecutive representative points (1801). The distance d_mi_n is the minimum between the distance di as defined in the method 1604 and the distance 'dj +' as defined in the method 1606 of figure 16. Then, it sets the maximum distance d_raax (1802). For example, d_max may be the sum of two to the power of five and d_mi_n. It calculates the distance d of consecutive representative points respectively (1803), and finds the reduced number d+ that is the subtraction of d_mj_n from d ( 1804).

[0086]

Fig. 19 shows an exemplary coding using the incremental mapping.

In Fig. 19, '0000' means 7 points of distance, and ' 1111 ' means 22 points of distance. In this example, the distance d_mi_n is set to 7 points, therefore the distance between consecutive representative points conveyed by a predetermined number of bits will be increased.

Alternatively, the distance between mapped values could be only multiple of 2 (even), e.g. '000' means the distance between consecutive representative points is 2 points, Ό0 means the distance between consecutive representative points is 4 points, '010' means the distance between consecutive representative points is 6 points. In this way, the incremental mapping will be simplified.

[0087]

(Finding appropriate representative points and quantization level )

Lesser header of the feedback frame and optimum reconstruction of channel characteristics at the access point using the specified interpolation method can be achieved by the appropriate representative points and/or quantization level of channel values found by the station. To this end, the number of selected frequency points and/or quantization level must be small and the deviation of interpolated channel matrices from the real channel matrices must also be small.

The deviation of interpolated channel matrix from the channel matrix is shown in the following Equation 9, where H is the real channel matrix as perceived by the station, Hi_ntei oiated is the interpolated channel matrix resulting from the values to be sent as feedback, "i" is the row index of the matrix (from 1 to Nr); "j" the column index of the matrix (from 1 to NR) and "n " the subcarrier index of the matrix (from 1 to Nsc)- [0088]

[Equation 7]

[0089]

Generally, if the pchip or cubic spline interpolation method is designated by the access point, then the station may adopt one of the aforementioned Selecting Methods 4 to 6 and small number of quantization level. On the contrary, if central value, linear or sine interpolation method is designated, the station may adopt one of the

aforementioned Selecting Methods 1 to 3 and high number of quantization level.

[0090]

According to the first embodiment, it is possible that an access point designates the preferable interpolation method, and a station sends back an optimized channel state information.

THE SECOND EMBODIMENT

[0091]

The format of a beacon according to the second embodiment is the same to the first embodiment except for the Supported Interpolation 331 in Fig 3B.

Fig. 20A shows the details of Supported Interpolation field according to the second embodiment. The field is comprised of two bits, for example, and shows the highest level interpolation method. The highest level interpolation method means the one which consumes the largest power, and shows the highest precision when the number of representative points is small. If an access point can perform up to a linear interpolation, the field is set to '00'. If the access point can perform up to a pchip interpolation, the field is set to '01 '. If the access point can perform up to a cubic spline interpolation, the field is set to ' 10'. The value ' 11 ' is reserved for future use in this example, although it may be assigned to a different interpolation method.

[0092]

Generally, an increase in the accuracy of the results of an interpolation comes at the cost of a higher computational load. It is unlikely that an access point is able to perform one high computational load method such as the cubic spline interpolation and not a relatively simpler one such as the linear interpolation. Therefore it is not needed to reserve a bit to indicate the ability of performing each interpolation method. The access point can indicate the highest computational load interpolation method which it supports, and the station is able to implicitly understand that the access point can support lower computational load interpolation method.

[0093]

Figure 20B is another example. In this case, the field is comprised of four bits, for example. If the bit B0 of the field is set to Ί ', the access point supports a linear interpolation method. If B 1 is set to Ί ', the access point supports sine interpolation method. If B2 is set to ' 1 ', the access point supports a pchip interpolation method. If B3 is set to Ί ', the access point supports a cubic spline interpolation method. If any one of B0 to B3 is set to ' 1 ', the access point support all of the corresponding interpolation method.

In this case, the station is able to explicitly understand which interpolation methods are supported at the access points.

[0094]

On a certain occasion, the workload required to calculate the optimum representative frequency points for the best performing interpolation method is beyond reach of the station's computing capabilities, either for lack of raw computational power or for some other simultaneous tasks requesting processor time. Sometimes, the station may prefer to use a lower computational calculation to save battery and so on. Therefore, in that case, the station adopts a low level interpolation method such as a central value interpolation method. Otherwise, it adopts a high level interpolation method which the access point permits. [0095]

The selection of representative frequency points and quantization levels may be performed as that of the first embodiment.

Also, the station may choose one interpolation method among the ones which the access point indicates, depending on the state of channel conditions. The station may evaluate (measure) the channel simply from the amount of relative maximums and minimums channel values in the transmission bandwidth. A high number of the relative maximums and minimums imply hard channel, therefore the interpolation method and representative points which level is high are selected. A small number of the relative maximums and minimums imply mild channel, therefore the interpolation method and representative points which level is low are selected. The station may also calculate slopes connecting the channel values in consecutive frequency points respectively, and find the average of the absolute values. A high average imply hard channel, while a low average means imply mild channel. The station may also consider the incidence of a slope steeper than a predetermined value. More slopes over this value reflect a harder channel.

[0096]

When the channel is hard, the station may send back a lot of representative points and quantization level, while the channel is mild, it may send back a small number of those things. Speaking to the selecting method, in the former case, either one of Selecting Methods 1 to 3 may be chosen, while in the latter case, either one of Selecting Methods 4 to 6 may be chosen.

[0097]

The format of SND FB frame according to the second embodiment is the same to the first embodiment except for VHT MIMO Control 633 in Fig. 6A.

Fig. 21 A shows the details of VHT MIMO Control 633a according to the second embodiment. Reserved 644 in Fig. 6B according to the first embodiment is changed to Interpolation 2101, Reserved 2102 and Feedback Type 2103, but other fields in Fig. 21 A are kept the same to those of Fig. 6B.

Fig. 21 B shows an exemplary configuration of Interpolation 2101. The station sets the field to '00' if the feedback is not optimized according to any

interpolation methods according to the embodiments of the present invention (legacy operation). It sets the field to ΌΓ, if the feedback is a linear interpolation. It sets the field to ' 10', if the feedback is optimized for a pchip interpolation. It sets the field to ' 11 ', if the feedback is a cubic spline interpolation.

Fig. 21C shows another possible configuration in which the legacy operation is not considered.

[0098]

Fig. 22 is a schematic block diagram of an access point according to the second embodiment. Compared to Fig. 9 according to the first embodiment, only

Interpolation Detection module 2201 is added to in Fig- 22 and other modules are kept the same as those of the first embodiment.

Interpolation Detection module 2201, receiving the interpolation field contents from Feedback Analyzer 910, detects the interpolation method for which the feedback is optimized for, and informs of this method to Feedback Interpolation module 913.

Control module 2202 indicates the several interpolation methods selected out of a plurality of interpolation methods.

[0099]

Fig. 23 is a schematic block diagram of a station according to the second embodiment. Compared to Fig. 15 according to the first embodiment, only

Interpolation Choice module 2301 is added and Control module 1514 (Fig. 15) is changed to Control module 2314 in Fig. 23, however other modules are kept the same.

Interpolation Choice module 2301, receiving the channel matrices from Channel Estimation module 1509 and the additional information from Control module 2314, decides which interpolation method is to be used to optimize the feedback for and informs it to Optimal Selection module 1510.

[0100]

Fig. 24 is a flowchart showing the procedure to choose an interpolation method and then to select representative points at the station.

The station assigns threshold values for its particular measurement methods (2401). These threshold values may be the predetermined values that are always the same regardless of the conditions of the station, or alternatively the station could vary them depending on its conditions. For instance, the station could raise the threshold value for computationally heavy interpolation methods when the station's battery charge level is low, or it could regulate the threshold values according to the idleness of the station's CPU.

The station evaluates channel values (2402).

The station then decides the interpolation method to use (2403).

The station then decides which selecting method to use to find representative frequency points just like the first embodiment (2404).

According to the second embodiment, a station is able to select the most appropriate interpolation method and then to select representative frequency points, considering its particular conditions.

THE THIRD EMBODIMENT

[0101]

In the third embodiment, an access point may send some more information aiding a station to choose the most suitable parameters.

Fig. 25 shows NDPA field of the third embodiment. Compared to the NDPA field of the first embodiment (Fig. 4a), it has Interpolation Choice Aid 2501 as an extra field. FB Poll according to the third embodiment could also furnish such Interpolation Choice Aid.

[0102]

Fig. 26 A shows the details of Interpolation Choice Aid 2501.

Fig. 26A shows an example in which an access point sets the minimum or maximum number of representative frequency points which a station can transmit. In this example, two fields are used. Maximum / Minimum 2621 is set to Ί ', when the constraint is referred to the maximum amount of subcarriers to be sent. It is set to Ό', when it is referred to the minimum amount of subcarriers to be sent. Maximum / Minimum value 2602 is a 7 bit representation of the value decided by the access point. For example, the access point can decide to limit the feedback to no more than a given number of frequency points (Maximum / Minimum 2611 set to 'maximum').

Alternatively, the access point can decide to have a given number of frequency points exceeding some minimum setting. For doing this, the access point sends the indication of a minimum number of frequency points to be used for feedback (Maximum / Minimum 3721 set to minimum).

[0103]

Alternatively, Fig. 26B shows an example in which the access point indicates both the minimum and the maximum amount of representative frequency points to be used for feedback. Minimum 2611 contain the minimum value of points to be used. Maximum 2612 contains the value of the maximum number of points to be used. In both cases, the value could be extended by transmitting only even values, in which case '00' = 2, '01 ' = 4, etc. or by using tables previously defined. Furthermore, it is unlikely that maximum 2612 is set to low values such as 2 or 4 subcarriers. A predefined offset can be applied to it to further extend the range.

[0104]

In another example, the access point tells each station which maximum MSE must be obtained. Interpolation Choice Aid 2501 gives the first station to transmit its feedback the maximum value of MSE. The station knows both the channel matrices and the interpolation method of the feedback it's sending to the access point. With both values, the station can calculate the MSE and adjust the feedback as needed.

[0105]

Next, the variation will be explained.

An access point can decide if the quality must be increased or reduced by observing the packet error rate which the link with a given station is incurring. If the packet error rate is too high, the access point could decide to increase the quality of the feedback to avoid retransmissions at the cost of a potentially higher overhead. If the packet error rate is very good, the access point could decide to reduce the quality of the feedback. The wording "quality of feedback" means the total number of representative points and quantization level.

[0106]

In a network with one or very few stations, an access point could decide to increase the quality of the feedback to acquire a better knowledge of the channel and minimize the interferences between stations. On the contrary, in a scenario in which many stations are present, reducing the quality of the feedback could have a beneficial effect in the overall throughput.

[0107]

Fig. 26C shows another example of Interpolation Choice Aid 2501.

Interpolation Choice Aid 2501 comprises Interpolation Adaptation 2601 of one bit and Reserved 2602 of seven bits for example.

Fig. 27 shows a flowchart explaining the actions of a station receiving such field of Interpolation Adaptation 2621 in Fig. 26C.

A station has a counter "Zero_COUnt" in the Controller. The station keeps

Zero_Count which is set to 0 when the station first establishes the connection with the access point. The station checks the value of Interpolation adaptation 2621 (2701). If its value is Ί ', the station increases the quality of feedback (2702) and Zero_COUnt remains zero (2703).

[0108]

Turning back to step 2701, the station increments the value of Zero_count by one (2712) if the value is Ό'.

The station determines whether the new "Zero_COUnt" equals a predefined number of occurrences 'n' (2713). If the new "Zero_COUnt" equals 'n', the station reduces the quality of feedback by one (2714) and sets "Zero_COUnt" to zero. If "Zero_count" is not equal to 'n', the station maintains the quality of feedback. Turning back to Fig. 26D, Fig. 26D shows a variation of Fig. 26C.

Interpolation Adaptation 1 2631-1 indicates the interpolation adaptation for the first station. Interpolation Adaptation 2 2631-2 indicates the interpolation adaptation for the second station. This configuration continues until Interpolation Adaptation 8 2631-8 for the last station.

According to the third embodiment, it is possible to improve the feedback quality much better.

THE FOURTH EMBODIMENT

[0109]

Fig. 28 shows NDPA of the fourth embodiment. Compared to NDPA of the first embodiment in Fig. 4A, it has a field of Interpolation Method 2801 as an extra field.

FB Poll according to the fourth embodiment could also furnish such field.

[0110]

Interpolation Method 2801 is an 8 bits field, while only 2 bits are needed to state the interpolation method, for example. In Interpolation Method 2801, '00' is set to a linear interpolation, Ό 1 ' is set to a pchip cubic interpolation, Ί 0' is set to a spline cubic interpolation and Ί 1 ' is reserved for future use. The other 6 bits can be used to indicate the interpolation method to be used for subsequent stations returning feedback, if they are also stated in the fields of STA Info 406-2 to 406-4. Further stations don't receive this information until FB Poll is addressed to them.

A control module incorporated in the access point according to the fourth embodiment, designates a particular interpolation method which is matched to the individual station.

A station follows the same procedure as that of the first embodiment.

According to the fourth embodiment, an access point designates a particular interpolation method which is matched to the situation in each individual station.

THE FIFTH EMBODIMENT

[0111]

In the fifth embodiment, the communication system includes a legacy station and legacy access point, and for example, the station 118 is replaced by a legacy station 118' and access point 102 is replaced by an access point 102' in Fig. 1. Namely, in the fifth embodiment, the stations 111 to 117, 121 and 131 are ones according to any of the first to fourth embodiments of the present invention, while the station 118' is a legacy station. And, the access points 101 and 103 are ones according to any of the first to fourth embodiments of the present invention, while the access point 102 is a legacy access point. The wording "legacy station" means the station with the feedback (herein after referred to "legacy feedback") described in Non-patent Document 1 which is not able to understand the beacon field of "Supported Interpolation" of Fig. 3B.

And, the wording "legacy access point" means the one with the beacon field of

"Supported Interpolation".

The legacy stations communicate with an access point using legacy action frames. The stations according to any of the first to fourth embodiments may communicate with the access point making use of the interpolation optimized feedback. Additionally, the stations supporting interpolation optimized feedback could opt for using legacy feedback.

[0112]

Fig. 29 shows a flow chart for stations to decide whether to use the

interpolation optimized feedback or the legacy feedback.

First, a station considers if an access point supporting only legacy feedback are also intended to receive the feedback information (2901). If the judgment in step 2901 proved true, then the station will use the legacy feedback (2905). Otherwise, the station will continue to consider if the available computing power is to low to create the interpolation optimized feedback (2902). If the judgment in step 2902 proved true, then the station will use the legacy feedback (2905). Otherwise, the station will continue to consider if the battery is to low to create the interpolation optimized feedback (2903). If the judgment in step 2901 proved true, then the station will use the legacy feedback (2905). Otherwise, the station will use the interpolation optimized feedback (2904).

[0113]

According to the fifth embodiment, even if a legacy station or legacy access point is included in the communication system, the communication can be established. And, the station according to the present embodiments could perform the legacy feedback to continue the communication with an access point.

[0114]

The access point or station according to the present invention could perform any of the embodiments 1 to 5 at any predetermined interval.

Reference Signs List

[0115] 101 to 103 : access points (APs)2 Input document 111 to 118, 121 and 131 : stations (STAs) 909-1 to 909-n: antennas

1501: antenna

Claims

[CLAIMS]

[Claim 1]

A transmitting device in a wireless communication characterized in that it comprises a control module to indicate a method out of a plurality of interpolation methods each of which interpolates the channel values in the missing subcarriers based on the positions of the representative subcarriers, which are selected out of the subcarriers constituting the frequency band used in the wireless communication, and the values in the representative subcarriers, and a transmitting module to transmit the information about the selected interpolation method.

[Claim 2]

A transmitting device according to Claim 1, characterized in that the transmitting device is to transmit the information about the selected interpolation method according to the channel state between the transmitting and receiving devices.

[Claim 3]

A transmitting device in a wireless communication characterized in that it comprises a control module to indicate several methods out of a plurality of

interpolation methods each of which interpolates the channel values in the missing subcarriers based on the positions of the representative subcarriers, which are selected out of the subcarriers constituting the frequency band used in the wireless

communication, and the values in the representative subcarriers, and a transmitting module to transmit the information about the several interpolation methods selected.

[Claim 4]

A receiving device in a wireless communication characterized in that it comprises a reception module to receive information about several interpolation methods each of which interpolates the channel values in missing subcarriers based on the positions of representative subcarriers, which are selected out of the subcarriers constituting the frequency band used in the wireless communication, and the values in the representative subcarriers, and an interpolation choice module to select a method out of the several interpolation methods.

[Claim 5]

A transmitting device in a wireless communication communicating between the transmitting device and a plurality of receiving devices characterized in that it comprises a control module to indicate in every receiving device a method selected out of a plurality of interpolation methods each of which interpolates the channel values in the missing frequency points based on the positions of representative subcarriers , which are selected out of the subcarriers constituting the frequency band used in the wireless communication, and the values in the representative subcamers, and a transmission module to transmit the selected interpolation methods.

[Claim 6]

A transmitting device according to any one of Claims 1 to 5, characterized in that the transmitting device is to transmit the information about the maximum and/or minimum of the number of representative subcarriers.

[Claim 7]

A transmitting device according to any one of Claims 1 to 5, characterized in that the transmitting device is to transmit information about the quality of feedback from the receiving device to the transmitting device.

[Claim 8]

A wireless communication system consisting of a transmitting device and a plurality of receiving devices, characterized in that a receiving device which performs an interpolation optimized feedback of the representative subcarriers, which are selected out of subcarriers constituting the frequency band used in the wireless communication, to optimize the recover of channel characteristics using reduced channel state information conveyed by a feedback frame, and another receiving device which does not perform the interpolation optimized feedback are included in the plurality of receiving devices.

[Claim 9]

A receiving device in a wireless communication characterized in that it comprises an optimum selection module to select representative subcarriers out of subcarriers constituting the frequency band used in the wireless communication at irregular intervals.

[Claim 10]

An interpolation method used in a wireless communication, characterized in that it comprises a step of identifying representative subcarriers, selected out of subcarriers constituting the frequency band used in the wireless communication, to the channel values, a step of finding the slopes of the channel characteristics in the representative subcarriers, and a step of interpolating channel values in the missing points by the channel values, slopes and positions in the representative subcarriers.