Device and method for calibrating MIMO systems

1. A calibration circuit comprising:

at least a first and a second input/output port, each arranged for being connected to a different transmitter/receiver pair of a multiple input multiple output (MIMO) system;

at least a third and a fourth input/output port, each arranged for being connected to a different antenna;

an attenuator having a first attenuator port and a second attenuator port;

a first and a second non-reciprocal switch, the first switch being arranged for establishing a connection between the first input/output port and either the third input/output port or the first attenuator port, and the second switch arranged for establishing a connection between the second input/output port and either the fourth input/output port or the second attenuator port.

2. The circuit of claim 1 , wherein a transfer function of the connection of the first switch to the first attenuator port and via the second attenuator port to the second switch is substantially equal to a transfer function of the connection of the second switch to the second attenuator port and via the first attenuator port to the first switch.

3. A system comprising a calibration circuit of claim 1 , further comprising at least two transmitter/receiver pairs of the MIMO system.

4. A method of determining a measure of a relative analogue front end mismatch of at least a first and second transmit/receive pairs of a multiple input multiple output (MIMO) system, the method comprising:

selecting a first transmitter/receiver pair of a MIMO system as a reference;

transmitting a known test signal from the reference transmitter to the receiver of the second transmitter/receiver pair and determining a corresponding first transfer function;

transmitting the known test signal from the transmitter of the second transmitter/receiver pair to the reference receiver and determining a corresponding second transfer function; and

determining from the first and second transfer function the measure of a relative analogue front end mismatch.

5. The method of claim 4 , further comprising multiplying a signal to be fed to the transmitter of the second transmitter/receiver pair with the determined measure of relative mismatch.

6. The method of claim 4 , wherein the method is applied to each individual subcarrier of a multicarrier system.

7. The method of claim 4 , wherein the known test signal is a BPSK IEEE 802.11a long training symbol.

8. The method of claim 4 , wherein the first transmitter/receiver pair is different from the second transmitter/receiver pair.

9. A device for determining a measure of a relative analogue front end mismatch of at least a first and second transmit/receive pairs of a multiple input multiple output (MIMO) system, the device comprising:

means for selecting a first transmitter/receiver pair of a MIMO system as a reference;

means for transmitting a known test signal from the reference transmitter to the receiver of the second transmitter/receiver pair and determining a corresponding first transfer function;

means for transmitting the known test signal from the transmitter of the second transmitter/receiver pair to the reference receiver and determining a corresponding second transfer function; and

means for determining from the first and second transfer function the measure of relative mismatch.

10. The device of claim 9 , wherein the first transmitter/receiver pair is different from the second transmitter/receiver pair.

11. The device of claim 9 , further comprising means for multiplying a signal to be fed to the transmitter of the second transmitter/receiver pair with the determined measure of relative mismatch.

12. The device of claim 9 , wherein the known test signal is a BPSK IEEE 802.11a long training symbol.