MIMO antenna apparatus provided with variable impedance load element connected to parasitic element

1. A MIMO antenna apparatus comprising:

a plurality of feeding antenna elements for respectively receiving a plurality of wireless signals;

a demodulator for demodulating the wireless signals received by the plurality of feeding antenna elements, by a MIMO (Multi-Input Multi-Output) method;

at least one parasitic element provided to be electromagnetically coupled to each of the feeding antenna elements;

at least one variable impedance load element connected to the parasitic element;

a comparator for detecting a received signal level of each of the wireless signals received by the feeding antenna elements and comparing the received signal levels with each other, thereby detecting the minimum received signal level; and

a controller for controlling an impedance value of the variable impedance load element based on the received signal levels detected by the comparator, such that the received signal level of the wireless signal having the minimum received signal level is substantially maximized.

2. The MIMO antenna apparatus as claimed in claim 1 ,

wherein the comparator further detects the received signal level smaller than a predetermined first threshold value, and

wherein the controller further controls the impedance value of the variable impedance load element based on the received signal levels detected by the comparator, such that the received signal level of the wireless signal having the minimum received signal level among wireless signals having the detected received signal level smaller than the first threshold value is substantially maximized.

3. The MIMO antenna apparatus as claimed in claim 1 ,

wherein the comparator further compares the received signal level of each of the wireless signals with a predetermined first threshold value, and when the received signal levels of all of the wireless signals are larger than or equal to the first threshold value, the comparator compares a signal level difference between the maximum received signal level and the minimum received signal level with a predetermined second threshold value, and

wherein, when the signal level difference is larger than or equal to the second threshold value, the controller further controls the impedance value of the variable impedance load element based on the received signal levels detected by the comparator, such that the received signal level of the wireless signal having the minimum received signal level is substantially maximized.

4. The MIMO antenna apparatus as claimed in claim 1 , further comprising a wireless transmitter for wirelessly transmitting a control signal to a sender-side wireless station apparatus which transmits the plurality of wireless signals, the control signal controlling a communication method used by the sender-side wireless station apparatus,

wherein the comparator further compares the received signal level of each of the wireless signals with a predetermined first threshold value, and when the received signal levels of all of the wireless signals are smaller than the first threshold value, the comparator detects the maximum received signal level, and

wherein, when the received signal levels of all of the wireless signals are smaller than the first threshold value, the controller further:

(i) controls the sender-side wireless station apparatus by making the wireless transmitter transmit the control signal and controls the MIMO demodulator, so as to change communication method used by each of the sender-side wireless station apparatus and the MIMO demodulator from a MIMO method to a SISO (Single-Input Single-Output) method; and

(ii) controls the impedance value of the variable impedance load element based on the received signal level detected by the comparator, such that the received signal level of the wireless signal having the maximum received signal level is substantially maximized.

5. The MIMO antenna apparatus as claimed in claim 1 , further comprising a wireless transmitter for wirelessly transmitting a control signal to a sender-side wireless station apparatus which transmits the plurality of wireless signals, the control signal controlling a communication method used by the sender-side wireless station apparatus,

wherein the comparator further compares the received signal level of each of the wireless signals with a predetermined first threshold value, and when the received signal levels of all of the wireless signals are larger than or equal to the first threshold value, the comparator compares a signal level difference between the maximum received signal level and the minimum received signal level with a predetermined second threshold value, and when the received signal levels of all of the wireless signals are smaller than the first threshold value, the comparator detects the maximum received signal level, and

wherein,

(a) when the received signal level of at least one wireless signal is smaller than the first threshold value and the received signal level of at least one wireless signal is larger than or equal to the first threshold value,

the controller further controls the impedance value of the variable impedance load element based on the received signal levels detected by the comparator, such that the received signal level of the wireless signal having the minimum received signal level among wireless signals having the detected received signal level smaller than the first threshold value is substantially maximized,

(b) when the received signal levels of all of the wireless signals are larger than or equal to the first threshold value and the signal level difference between the maximum received signal level and the minimum received signal level is larger than or equal to the second threshold value,

the controller further controls the impedance value of the variable impedance load element based on the received signal levels detected by the comparator, such that the received signal level of the wireless signal having the minimum received signal level is substantially maximized, and

(c) when the received signal levels of all of the wireless signals are smaller than the first threshold value, the controller further:

(i) controls the sender-side wireless station apparatus by making the wireless transmitter transmit the control signal and controls the MIMO demodulator, so as to change communication method used by each of the sender-side wireless station apparatus and the MIMO demodulator from a MIMO method to a SISO method; and

(ii) controls the impedance value of the variable impedance load element based on the received signal level detected by the comparator, such that the received signal level of the wireless signal having the maximum received signal level is substantially maximized.

6. The MIMO antenna apparatus as claimed in claim 4 , wherein in the case that the communication method used by each of the sender-side wireless station apparatus and the MIMO demodulator are changed from the MIMO method to the SISO method,

(a) when the received signal levels of all of the wireless signals have become larger than or equal to the first threshold value by controlling the impedance value of the variable impedance load element, or

(b) when a predetermined fixed control time has elapsed,

the controller further controls the sender-side wireless station apparatus by making the wireless transmitter transmit the control signal and controls the MIMO demodulator, so as to change the communication method used by each of the sender-side wireless station apparatus and the MIMO demodulator from the SISO method to the MIMO method.

7. The MIMO antenna apparatus as claimed in claim 5 , wherein in the case that the communication method used by each of the sender-side wireless station apparatus and the MIMO demodulator are changed from the MIMO method to the SISO method,

(a) when the received signal levels of all of the wireless signals have become larger than or equal to the first threshold value by controlling the impedance value of the variable impedance load element, or

(b) when a predetermined fixed control time has elapsed,

the controller further controls the sender-side wireless station apparatus by making the wireless transmitter transmit the control signal and controls the MIMO demodulator, so as to change the communication method used by each of the sender-side wireless station apparatus and the MIMO demodulator from the SISO method to the MIMO method.

8. The MIMO antenna apparatus as claimed in claim 1 , wherein the variable impedance load element has an impedance value which continuously changes according to control of the controller.

9. The MIMO antenna apparatus as claimed in claim 1 , wherein the variable impedance load element has a plurality of impedance values which are selectively changed according to control of the controller.

10. The MIMO antenna apparatus as claimed in claim 1 , further comprising:

a wireless communication circuit for receiving or transmitting a certain wireless signal; and

a switch for connecting either one of the variable impedance load element and the wireless communication circuit to the parasitic element.

11. A wireless communication apparatus comprising a MIMO antenna apparatus, the MIMO antenna apparatus including:

a plurality of feeding antenna elements for respectively receiving a plurality of wireless signals;

a demodulator for demodulating the wireless signals received by the plurality of feeding antenna elements, by a MIMO method;

at least one parasitic element provided to be electromagnetically coupled to each of the feeding antenna elements;

at least one variable impedance load element connected to the parasitic element;

a comparator for detecting the received signal level of each of the wireless signals received by the feeding antenna elements and comparing the received signal levels with each other, thereby detecting the minimum received signal level; and

a controller for controlling an impedance value of the variable impedance load element based on the received signal levels detected by the comparator, such that the received signal level of the wireless signal having the minimum received signal level is substantially maximized.