Base station, wireless terminal, and wireless communication system

1. A base station for wirelessly communicating with a wireless terminal having a plurality of transmit antennas using multiple-input and multiple-output, comprising:

a measuring unit configured to measure channel values of propagation channels based on a pilot signal transmitted from the wireless terminal;

an interpolation coefficient calculating unit configured to calculate interpolation coefficients used to estimate channel values for one of the transmit antennas of the wireless terminal by using channel values for rest of the transmit antennas, which channel values are measured by the measuring unit;

an estimate value calculating unit configured to estimate the channel values for the one of the transmit antennas based on the interpolation coefficients calculated by the interpolation coefficient calculating unit and the channel values for the rest of the transmit antennas measured by the measuring unit; and

an error calculating unit configured to calculate errors between the individual channel values for the one of the transmit antennas estimated by the estimate value calculating unit and corresponding channel values for the one of the transmit antennas measured by the measuring unit,

wherein the interpolation coefficient calculating unit calculates the interpolation coefficients based on the errors calculated by the error calculating unit and the channel values for the rest of the transmit antennas measured by the measuring unit.

2. The base station according to claim 1 , further comprising a determining unit configured to determine whether transmission of the pilot signal from the one of the transmit antennas is to be stopped, based on the errors calculated by the error calculating unit.

3. The base station according to claim 2 , wherein the measuring unit, the interpolation coefficient calculating unit, the estimate value calculating unit, and the error calculating unit perform the measurement of the channel values, the calculation of the interpolation coefficients, the estimation of the channel values, and the calculation of the errors with respect to each frequency block of the pilot signal.

4. The base station according to claim 2 , wherein when the determining unit has determined that transmission of the pilot signal from the one of the transmit antennas is to be stopped, the interpolation coefficient calculating unit, the estimate value calculating unit, and the error calculating unit repeat same processing on the rest of the transmit antennas.

5. A wireless terminal having a plurality of antennas and wirelessly communicating with a base station using multiple-input and multiple-output, the wireless terminal comprising:

a transmitting unit configured to transmit a pilot signal from each antenna and stop transmission of the pilot signal from the one of the antennas, indicated by an antenna information transmitted from the base station; and

a receiving unit configured to receive the antenna information indicating one of the antennas, whose transmission of the pilot signal is to be stopped;

wherein, when a data retransmission request is made by the base station, the transmitting unit transmits the pilot signal from all of the antennas.

6. The wireless terminal according to claim 5 , wherein the transmitting unit periodically transmits the pilot signal from all of the antennas.

7. The wireless terminal according to claim 6 , wherein the transmitting unit changes a periodic interval for transmitting the pilot signal from all of the antennas according to frequency of the data retransmission request made by the base station.

8. The wireless terminal according to claim 6 , wherein the transmitting unit changes a periodic interval for transmitting the pilot signal from all of the antennas according to moving speed of the wireless terminal.

9. A wireless communication system comprising a base station and a wireless terminal for wirelessly communicating with each other using multiple-input and multiple-output, the wireless terminal having a plurality of transmit antennas,

wherein the base station includes

a measuring unit which measures channel values of propagation channels based on a pilot signal transmitted from the wireless terminal; an interpolation coefficient calculating unit which calculates interpolation coefficients used to estimate channel values for one of a plurality of transmit antennas of the wireless terminal by using channel values for rest of the transmit antennas, which channel values are measured by the measuring unit;

an estimate value calculating unit which estimates the channel values for the one of the transmit antennas based on the interpolation coefficients calculated by the interpolation coefficient calculating unit and the channel values for the rest of the transmit antennas measured by the measuring unit, and

an error calculating unit configured to calculate errors between the individual channel values for the one of the transmit antennas estimated by the estimate value calculating unit and corresponding channel values for the one of the transmit antennas measured by the measuring unit, wherein the interpolation coefficient calculating unit calculates the interpolation coefficients based on the errors calculated by the error calculating unit and the channel values for the rest of the transmit antennas measured by the measuring unit, and

the wireless terminal includes

a receiving unit which receives, from the base station, antenna information indicating one of the transmit antennas, whose transmission of the pilot signal is to be stopped; and a transmitting unit which stops transmission of the pilot signal from the one of the transmit antennas, indicated by the antenna information received by the receiving unit.