Determining transmitter impairments using offset local oscillators
View Patent ↗Systems and methods for measuring transmitter and/or receiver I/Q impairments are disclosed, including iterative methods for measuring transmitter I/Q impairments using shared local oscillators, iterative methods for measuring transmitter I/Q impairments using intentionally-offset local oscillators, and methods for measuring receiver I/Q impairments. Also disclosed are methods for computing I/Q impairments from a sampled complex signal, methods for computing DC properties of a signal path between the transmitter and receiver, and methods for transforming I/Q impairments through a linear system.
1. A method for determining I/Q impairments of a transmitter, the method comprising:
(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 tone in order to obtain an adjusted signal, 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 signal;
(b) computing raw I/Q impairments based on a frequency-shifted signal, wherein the frequency-shifted signal is a result of frequency shifting a sampled signal by an amount of offset between a local oscillator (LO) of the transmitter and a local oscillator of a receiver, wherein the sampled signal has been captured by a receiver in response to said transmission of the transmit signal;
(c) removing a current estimate of a signal path from the raw I/Q impairments to obtain path-compensated I/Q impairments, wherein the signal path includes a path from an I/Q modulator of the transmitter to a demodulator of the receiver; and
(d) updating the current estimate of the I/Q impairments of the transmitter based on the path-compensated I/Q impairments.
2. The method of claim 1 , further comprising:
repeating a set of operations including (a) through (d) 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 of frequency of the tone.
4. The method of claim 1 , further comprising:
removing measured I/Q impairments of the receiver from the sampled signal prior to said frequency shifting.
5. The method of claim 1 , wherein the pre-compensation transformation has the form of a 2×2 matrix, wherein a first diagonal element of the matrix is computed based on the current estimate of the I/Q impairments of the transmitter at a given frequency and its negative, wherein a first non-diagonal element of the matrix is computed based on the current estimate of the I/Q impairments of the transmitter at the given frequency and its negative.
6. The method of claim 1 , wherein the current estimate of the signal path includes a measured amplitude of the frequency-shifted signal.
7. The method of claim 6 , wherein the current estimate of the signal path also includes a measured rotation of the frequency-shifted signal.
8. The method of claim 1 , further comprising:
repeating a set of operations including (a) through (d), 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.
9. The method of claim 8 , further comprising determining the DC scaling and the DC rotation by:
supplying a zero vector signal to the transmitter;
supplying a non-zero DC vector signal to the transmitter;
computing the DC scaling and the DC rotation based on a first DC vector response and a second DC vector response, wherein the first DC vector response has been measured at the receiver in response to the zero vector signal, wherein the second DC vector response has been measured at the receiver in response to the non-zero DC vector signal.
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:
(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 tone in order to obtain an adjusted signal, 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 signal;
(b) compute raw I/Q impairments based on a frequency-shifted signal, wherein the frequency-shifted signal is a result of frequency shifting a sampled signal by an amount of offset between a local oscillator (LO) of the transmitter and a local oscillator of a receiver, wherein the sampled signal has been captured by a receiver in response to said transmission of the transmit signal;
(c) remove a current estimate of a signal path from the raw I/Q impairments to obtain path-compensated I/Q impairments, wherein the signal path includes a path from an I/Q modulator of the transmitter to a demodulator of the receiver; and
(d) update the current estimate of the I/Q impairments of the transmitter based on the path-compensated I/Q impairments.
11. The computer system of claim 10 , wherein the program instructions, when executed by the processor, further cause the processor to:
repeat a set of operations includes (a) through (d) 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 10 , wherein the program instructions, when executed by the processor, further cause the processor to:
remove measured I/Q impairments of the receiver from the sampled signal prior to said frequency shifting.
13. The computer system of claim 10 , wherein the pre-compensation transformation has the form of a 2×2 matrix, wherein a first diagonal element of the matrix is computed based on the current estimate of the I/Q impairments of the transmitter at a given frequency and its negative, wherein a first non-diagonal element of the matrix is computed based on the current estimate of the I/Q impairments of the transmitter at the given frequency and its negative.
14. The computer system of claim 10 , wherein the current estimate of the signal path includes a measured amplitude of the frequency-shifted signal.
15. The computer system of claim 14 , wherein the current estimate of the signal path also includes a measured rotation of the frequency-shifted signal.
16. A non-transitory 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 processor, cause the processor 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 tone in order to obtain an adjusted signal, 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 signal;
(b) compute raw I/Q impairments based on a frequency-shifted signal, wherein the frequency-shifted signal is a result of frequency shifting a sampled signal by an amount of offset between a local oscillator (LO) of the transmitter and a local oscillator of a receiver, wherein the sampled signal has been captured by a receiver in response to said transmission of the transmit signal;
(c) remove a current estimate of a signal path from the raw I/Q impairments to obtain path-compensated I/Q impairments, wherein the signal path includes a path from an I/Q modulator of the transmitter to a demodulator of the receiver; and
(d) update the current estimate of the I/Q impairments of the transmitter based on the path-compensated I/Q impairments.
17. The non-transitory memory medium of claim 16 , wherein the program instructions, when executed by the processor, further cause the processor to:
repeat a set of operations including (a) through (d) 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.
18. The non-transitory memory medium of claim 17 , 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 of frequency of the tone.
19. The non-transitory memory medium of claim 16 , wherein the program instructions, when executed by the processor, further cause the processor to:
remove measured I/Q impairments of the receiver from the sampled signal prior to said frequency shifting.
20. The non-transitory memory of claim 16 , wherein the program instructions, when executed by the processor, further cause the processor to:
repeat a set of operations including (a) through (d), 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.