Mechanism for measuring transmitter I/Q impairments using shared local oscillators

1. A method for determining I/Q impairments of a transmitter, the method comprising:

performing a set of operations, wherein the set of operations includes:

(a) supplying a pre-compensation transformation to a pre-compensation circuit of the transmitter, wherein the pre-compensation circuit is configured to apply the pre-compensation transformation to a complex exponential tone to obtain an adjusted complex signal, wherein the complex exponential tone is at frequency f, wherein the pre-compensation transformation is configured to pre-compensate for a current estimate of the I/Q impairments of the transmitter, wherein the transmitter is configured to transmit a transmit signal based on the adjusted complex signal, wherein a receiver is configured to receive the transmit signal and capture a sampled complex signal representing the received transmit signal;

(b) computing raw I/Q impairments based on the sampled complex signal;

(c) transforming the raw I/Q impairments to determine transformed I/Q impairments, wherein said transforming removes measured I/Q impairments of the receiver from the raw I/Q impairments;

(d) updating the current estimate of the I/Q impairments of the transmitter based on path-compensated I/Q impairments, wherein the path-compensated I/Q impairments are obtained by removing a current estimate of a signal path from the transformed I/Q impairments, wherein the signal path includes a path from an I/Q modulator of the transmitter to a demodulator of the receiver.

2. The method of claim 1 , further comprising:

repeating the set of operations to determine a converged estimate of the I/Q impairments of the transmitter, wherein the set of operations is repeated until a quality measure based on the path-compensated I/Q impairments is larger than a threshold.

3. The method of claim 2 , further comprising:

performing said repeating a plurality of times to determine said converged estimate at a plurality of different values for the frequency f.

4. The method of claim 3 , wherein the plurality of different values is symmetric about zero, wherein the plurality of different values is ordered so the values alternate in sign and are non-decreasing in magnitude.

5. The method of claim 1 , wherein a local oscillator of the transmitter and a local oscillator of the receiver are phase locked to the same frequency.

6. The method of claim 5 , wherein, in at least a first performance of said set of operations, the current estimate of the signal path is based on a DC scaling and a DC rotation of the signal path.

7. The method of claim 1 , wherein the pre-compensation transformation has the form of a 2×2 matrix, wherein at least a first diagonal element of the matrix is computed from the current estimate of the I/Q impairments of the transmitter at frequencies f and −f, wherein at least a first non-diagonal element of the matrix is computed from the current estimate of the I/Q impairments of the transmitter at frequencies f and −f.

8. The method of claim 1 , wherein the current estimate of the signal path includes:

a measured amplitude of the sampled complex signal at frequency f and/or

a measured rotation of the sampled complex signal at frequency f.

9. The method of claim 1 , wherein the transmitter conforms to a direct-conversion architecture, wherein the demodulator is an I/Q demodulator.

10. A computer system for determining I/Q impairments of a transmitter, the computer system comprising:

a processor; and

memory storing program instructions, wherein the program instructions, when executed by the processor, cause the processor to perform a set of operations, wherein the set of operations includes:

(a) supplying a pre-compensation transformation to a pre-compensation circuit of the transmitter, wherein the pre-compensation circuit is configured to apply the pre-compensation transformation to a complex exponential tone to obtain an adjusted complex signal, wherein the complex exponential tone is at frequency f, wherein the pre-compensation transformation is configured to pre-compensate for a current estimate of the I/Q impairments of the transmitter, wherein the transmitter is configured to transmit a transmit signal based on the adjusted complex signal, wherein a receiver is configured to receive the transmit signal and capture a sampled complex signal representing the received transmit signal;

(b) computing raw I/Q impairments based on the sampled complex signal;

(c) transforming the raw I/Q impairments to determine transformed I/Q impairments, wherein said transforming removes measured I/Q impairments of the receiver from the raw I/Q impairments; and

(d) updating the current estimate of the I/Q impairments of the transmitter based on path-compensated I/Q impairments, wherein the path-compensated I/Q impairments are obtained by removing a current estimate of a signal path from the transformed I/Q impairments, wherein the signal path includes a path from an I/Q modulator of the transmitter to a demodulator of the receiver.

11. The computer system of claim 10 , wherein the program instructions, when executed by the processor, further cause the processor to:

repeat the set of operations to determine a converged estimate of the I/Q impairments of the transmitter, wherein the set of operations is repeated until a quality measure based on the path-compensated I/Q impairments is larger than a threshold.

12. The computer system of claim 11 , wherein the program instructions, when executed by the processor, further cause the processor to:

perform said repeating a plurality of times to determine said converged estimate at a plurality of different values for the frequency f.

13. The computer system of claim 12 , wherein the plurality of different values is symmetric about zero, wherein the plurality of different values is arc ordered so the values alternate in sign and are non-decreasing in magnitude.

14. The computer system of claim 10 , wherein a local oscillator of the transmitter and a local oscillator of the receiver are phase locked to the same frequency.

15. The computer system of claim 14 , wherein, in at least a first performance of said set of operations, the current estimate of the signal path is based on a DC scaling and a DC rotation of the signal path.

16. The computer system of claim 10 , wherein the pre-compensation transformation has the form of a 2×2 matrix, wherein at least a first diagonal element of the matrix is computed from the current estimate of the I/Q impairments of the transmitter at frequencies f and −f, wherein at least a first non-diagonal element of the matrix is computed from the current estimate of the I/Q impairments of the transmitter at frequencies f and −f.

17. The computer system of claim 10 , wherein the current estimate of the signal path includes:

a measured amplitude of the sampled complex signal at frequency f; and/or

a measured rotation of the sampled complex signal at frequency f.

18. A non-volatile computer-readable memory medium for determining I/Q impairments of a transmitter, wherein the memory medium stores program instructions, wherein the program instructions, when executed by a computer system, cause the computer system to:

(a) supply a pre-compensation transformation to a pre-compensation circuit of the transmitter, wherein the pre-compensation circuit is configured to apply the pre-compensation transformation to a complex exponential tone to obtain an adjusted complex signal, wherein the complex exponential tone is at frequency f, wherein the pre-compensation transformation is configured to pre-compensate for a current estimate of the I/Q impairments of the transmitter, wherein the transmitter is configured to transmit a transmit signal based on the adjusted complex signal, wherein a receiver is configured to receive the transmit signal and capture a sampled complex signal representing the received transmit signal;

(b) compute raw I/Q impairments based on the sampled complex signal;

(c) transform the raw I/Q impairments to determine transformed I/Q impairments, wherein said transforming removes measured I/Q impairments of the receiver from the raw I/Q impairments; and

(d) update the current estimate of the I/Q impairments of the transmitter based on path-compensated I/Q impairments, wherein the path-compensated I/Q impairments are obtained by removing a current estimate of a signal path from the transformed I/Q impairments, wherein the signal path includes a path from an I/Q modulator of the transmitter to a demodulator of the receiver.

19. The memory medium of claim 18 , wherein the program instructions, when executed by the computer system, further cause the computer system to:

repeat the set of operations to determine a converged estimate of the I/Q impairments of the transmitter, wherein the set of operations is repeated until a quality measure based on the path-compensated I/Q impairments is larger than a threshold.

20. The memory medium of claim 19 , wherein the program instructions, when executed by the computer system, further cause the computer system to:

perform said repeating a plurality of times to determine said converged estimate at a plurality of different values for the frequency f.