MIMO communication method, terminal, and base station apparatus for transmitting and receiving pilot signals to estimate a channel

1. A MIMO communication method of performing MIMO communication between a base station having a plurality of antennas and each of a plurality of terminals covered by the base station using uplink data slots and downlink data slots that are alternately placed on a time axis, comprising:

in each of the plurality of terminals,

setting pilot transmission headers to be used for transmission of a pilot signal at predetermined header intervals as headers of the uplink data slots;

spreading the pilot signal in the pilot transmission headers using corresponding one of orthogonal codes that are assigned to the plurality of terminals and are orthogonal to one another and transmitting the spread pilot signal;

spreading the pilot signal in the uplink data slots using a data orthogonal code that is different from the orthogonal code used in the pilot transmission headers, superimposing the spread pilot signal on transmission data, and transmitting the transmission data;

in the base station,

despreading a received signal that is transmitted from each of the plurality of terminals and is included in the pilot transmission headers using the orthogonal code assigned to a respective one of the terminals, estimating a channel between each of all antennas of the base station and the respective terminal, and storing a first value of the estimated channel;

demodulating the transmission data transmitted from the respective terminal on a basis of the first value of the estimated channel;

decoding a received signal included in the uplink data slots using the data orthogonal code assigned to the respective terminal and estimating a current channel between each of all antennas of the base station and the respective terminal; and

comparing the stored first value of the estimated channel with a second value of the estimated current channel and updating the stored first value of the estimated channel to the second value of the estimated current channel when a difference between the values is larger than a third value set in advance.

2. The MIMO communication method according to claim 1 , further comprising:

in each of the plurality of terminals,

adjusting an amplitude of the pilot signal;

additively superimposing the amplitude-adjusted pilot signal on the transmission data in a time direction or a frequency direction; and

transmitting a signal obtained from the superimposition;

in the base station,

receiving a signal transmitted from the respective terminal; and

subtracting the amplitude-adjusted pilot signal from the received signal in the time direction or the frequency direction and performing demodulation for MIMO communication using a remaining signal.

3. The MIMO communication method according to claim 1 , wherein the uplink data slot is separated into a plurality of data sections and the pilot signal is superimposed on at least one of the plurality of data sections.

4. The MIMO communication method according to claim 3 , wherein the data sections are orthogonal frequency division multiplexing (OFDM) symbols.

5. The MIMO communication method according to claim 4 , wherein a frequency component in the OFDM symbol is separated into a plurality of blocks and the pilot signal is superimposed on each of the plurality of blocks.

6. The MIMO communication method according to claim 1 ,

wherein OFDM is used for data communication,

wherein each of the plurality of terminals performs, in a frequency domain, the superimposition of the pilot signal on the transmission data in the uplink data slots before performing inverse fast Fourier transform (IFFT) of OFDM, and

wherein the base station performs fast Fourier transform (FFT) on the received signal so as to convert the received signal into a frequency-domain signal and performs the subtraction, the demodulation, and the channel estimation in the frequency domain.

7. A terminal for performing MIMO communication with a base station apparatus having a plurality of antennas, the terminal comprising:

a transmitter configured to transmit data to the base station apparatus using uplink data slots and transmit pilot transmission headers to the base station using time slots separate from the uplink data slots; and

processing circuitry configured to

set the pilot transmission headers to be used for transmission of a pilot signal at predetermined header intervals as headers of the uplink data slots,

spread the pilot signal in one of the pilot transmission headers using corresponding one of orthogonal codes that are assigned to a plurality of terminals and are orthogonal to one another and transmit the spread pilot signal, wherein the pilot signals for all of the plurality of terminals are spread to overlap in time within a same time slot of the pilot transmission header,

wherein the processing circuitry is configured to spread the pilot signal in one of the uplink data slots using a data orthogonal code that is different from the orthogonal code used in the pilot transmission headers, and multiplex the spread pilot signal on transmission data, and

the transmitter is configured to transmit the transmission data in the one of the uplink data slots, wherein the transmission data for all of the plurality of terminals overlap in time within the one of the uplink data slots.

8. A base station apparatus for performing MIMO communication with a plurality of terminals covered by the base station apparatus comprising:

a plurality of antennas; and

processing circuitry configured to

despread a signal that is received from each of the plurality of terminals via the plurality of antennas and is included in pilot transmission headers using an orthogonal code assigned to a respective one of the terminals,

estimate a channel between each of all of the plurality of antennas of the base station and the respective terminal,

store a first value of the estimated channel,

process the signal received from each of the plurality of terminals,

demodulate transmission data transmitted from the respective terminal on a basis of the first value of the estimated channel,

decode a received signal included in the uplink data slots using a data orthogonal code assigned to the respective terminal and estimate a current channel between each of all of the plurality of antennas of the base station and the respective terminal, and

compare the stored first value of the estimated channel with a second value of the estimated current channel and updates the stored first value of the estimated channel to the second value of the estimated current channel when a difference between the values is larger than a third value set in advance.