Digital IQ imbalance compensation for dual-carrier double conversion receiver
Digital IQ imbalance compensation is utilized for a dual-carrier double conversion receiver. First, the effect of IQ imbalance on OFDM-based digital baseband is analyzed, showing that, in the presence of IQ imbalance, the baseband signal of each carrier is obtained from its own branch as well as the other branch. Second, IQ imbalance parameters of interest are estimated using pilot signals and compensated using only digital baseband processing.
1. A method of compensating for imbalances between the In-phase (I) and Quadrature (Q) components (IQ imbalance) of communication signals processed by a multi-carrier multiple conversion receiver operative in a radio frequency (RF) wireless communication network, the receiver having two or more receiver chains, each operative to downconvert signals modulated onto different RF carrier frequencies to baseband via at least one intermediate frequency (IF), the method comprising:
processing, in a first receiver chain, a first signal modulated onto a first RF carrier;
processing, in a second receiver chain that utilizes one or more RF or IF components of the first receiver chain, a second signal modulated onto a second RF carrier, different from the first carrier;
downconverting the first and second signals to baseband by conversion from the respective RF carrier frequencies to an IF, and by further conversion from the IF to baseband;
processing known reference signals in each receiver branch;
deriving IQ imbalance estimates associated with each receiver chain as the values that best match each received signal with one of the corresponding known reference signal or the corresponding known reference signal modified by the corresponding channel estimate; wherein deriving IQ imbalance estimates associated with each receiver chain as the values that best match each received signal with the corresponding known reference signal further comprises:
estimating an IQ-imbalance-plus-channel matrix comprising IQ imbalance values scaled by channel estimates; and
normalizing the IQ-imbalance-plus-channel matrix to remove the channel estimates; and
compensating the received signals for the IQ imbalance based on the IQ imbalance estimates.
2. The method of claim 1 , further comprising:
compensating the received signals for the IQ imbalance based on the IQ imbalance estimates; and
performing equalization on the compensated received signals to remove both residual IQ imbalance gain factors and channel frequency response.
3. The method of claim 1 wherein the first and second signals are Orthogonal Frequency Division Multiplexed (OFDM) signals and wherein estimating an IQ-imbalance-plus-channel matrix comprises
selecting a plurality of contiguous subcarriers;
assuming multipath fading is constant over the selected subcarriers; and
estimating the IQ-imbalance-plus-channel matrix jointly over the selected subcarriers.
4. The method of claim 1 further comprising:
estimating the channel frequency response associated with each carrier, prior to deriving the IQ imbalance estimates associated with each receiver chain as the values that best match each received signal with the corresponding known reference signal modified by the corresponding channel estimate.
5. The method of claim 4 wherein deriving the IQ imbalance estimates associated with each receiver chain comprises expressing the received signals as IQ imbalance scaling factors multiplied by known reference signal values as modified by the corresponding channel estimates, and solving for the IQ imbalance scaling factors.
6. The method of claim 1 wherein the intermediate frequency is half of the difference between the first and second carrier frequencies.
7. The method of claim 1 wherein at least one of the first and second signals comprises a test signal generated in the multi-carrier multiple conversion receiver and injected into a receiver chain.
8. The method of claim 7 wherein the test signal comprises a baseband signal.
9. A method of compensating for imbalances between the In-phase (I) and Quadrature (Q) components (IQ imbalance) of communication signals processed by a multi-carrier multiple conversion receiver operative in a radio frequency (RF) wireless communication network, the receiver having two or more receiver chains, each operative to downconvert signals modulated onto different RF carrier frequencies to baseband via at least one intermediate frequency (IF), the method comprising:
processing, in a first receiver chain, a first signal modulated onto a first RF carrier;
processing, in a second receiver chain that utilizes one or more RF or IF components of the first receiver chain, a second signal modulated onto a second RF carrier, different from the first carrier;
downconverting the first and second signals to baseband by conversion from the respective RF carrier frequencies to an IF, and by further conversion from the IF to baseband:
processing known reference signals in each receiver branch;
deriving IQ imbalance estimates associated with each receiver chain as the values that best match each received signal with one of the corresponding known reference signal or the corresponding known reference signal modified by the corresponding channel estimate; wherein the first and second signals are Orthogonal Frequency Division Multiplexed (OFDM) signals and wherein deriving the IQ imbalance associated with each receiver chain further comprises:
deriving IQ imbalance estimates over a plurality of subcarriers having different pilot patterns; and
combining the IQ imbalance estimates for different subcarriers based on the pilot patterns; and
compensating the received signals for the IQ imbalance based on the IQ imbalance estimates.
10. The method of claim 9 wherein combining the IQ imbalance estimates for different subcarriers based on the pilot patterns comprises combining the IQ imbalance estimates for different subcarriers such that the IQ imbalance estimates are emphasized for those subcarriers having pilot patterns conducive to accurate IQ imbalance estimation.
11. The method of claim 10 wherein combining the IQ imbalance estimates for different subcarriers based on the pilot patterns comprises forming a weighted linear average of IQ imbalance estimates for different subcarriers, wherein the weights depend on the pilot pattern for the respective subcarriers.
12. A wideband intermediate frequency (IF) dual-carrier double conversion receiver operative to process first and second communication signals, modulated onto first and second radio frequency (RF) carriers, respectively, comprising:
a first receiver chain comprising RF local oscillators (LO) operative to perform RF mixing and RF LOs operative to perform IF mixing, the first receiver chain operative to double convert a first signal from a first RF carrier frequency to an IF and then to baseband;
a second receiver chain sharing one or more RF LOs with the first receiver chain and operative to double convert a second signal from a second RF carrier frequency, different than the first RF carrier frequency, to the IF and then to baseband;
wherein the first and second receiver chains are operative to process known reference signals;
an IQ imbalance estimator operative to estimate IQ imbalance in each receiver chain as the values that best match each signal with one of a corresponding known reference signal or a corresponding known reference signal modified by a corresponding channel estimate; and
an equalizer operative to compensate the first and second baseband signals so as remove the estimated IQ imbalance, wherein the IQ imbalance estimator is operative to estimate IQ imbalance by:
estimating an IQ-imbalance-plus-channel matrix comprising IQ imbalance values scaled by channel estimates; and
normalizing the IQ-imbalance-plus-channel matrix to remove the channel estimates.
13. The receiver of claim 12 wherein
the IQ imbalance estimator is further operative to estimate IQ imbalance by compensating the received signals for the IQ imbalance using a least square estimation; and
the equalizer is operative to compensate the first and second baseband signals by performing equalization on the compensated received signals to remove residual IQ imbalance gain factors.
14. The receiver of claim 13 wherein the first and second signals are Orthogonal Frequency Division Multiplexed (OFDM) signals and wherein the IQ imbalance estimator is operative to estimate an IQ-imbalance-plus-channel matrix by:
selecting a plurality of contiguous subcarriers;
assuming multipath fading is constant over the selected subcarriers; and
estimating the IQ-imbalance-plus-channel matrix jointly over the selected subcarriers.
15. The receiver of claim 12 wherein the IQ imbalance estimator is operative to estimate IQ imbalance by estimating the channel frequency response associated with each carrier, prior to estimating the IQ imbalance in each receiver chain as the values that best match each received signal with a corresponding known reference signal.
16. The receiver of claim 15 wherein estimating the IQ imbalance in each receiver chain comprises expressing the received signals as IQ imbalance scaling factors multiplied by known reference signal values as modified by the corresponding channel estimates, and solving for the IQ imbalance scaling factors.
17. The receiver of claim 12 wherein the intermediate frequency is half of the difference between the first and second carrier frequencies.
18. The receiver of claim 12 further comprising a test signal generator operative to generate and inject a test signal into at least one of the first and second receiver chains.
19. The receiver of claim 18 wherein the test signal generator is operative to generate a baseband frequency test signal.
20. A wideband intermediate frequency (IF) dual-carrier double conversion receiver operative to process first and second communication signals, modulated onto first and second radio frequency (RF) carriers, respectively, comprising:
a first receiver chain comprising RF local oscillators (LO) operative to perform RF mixing and RF LOs operative to perform IF mixing, the first receiver chain operative to double convert a first signal from a first RF carrier frequency to an IF and then to baseband;
a second receiver chain sharing one or more RF LOs with the first receiver chain and operative to double convert a second signal from a second RF carrier frequency, different than the first RF carrier frequency, to the IF and then to baseband;
wherein the first and second receiver chains are operative to process known reference signals;
an IQ imbalance estimator operative to estimate IQ imbalance in each receiver chain as the values that best match each signal with one of a corresponding known reference signal or a corresponding known reference signal modified by a corresponding channel estimate; and
an equalizer operative to compensate the first and second baseband signals so as remove the estimated IQ imbalance, wherein the first and second signals are Orthogonal Frequency Division Multiplexed (OFDM) signals and wherein the IQ imbalance estimator is operative to:
derive IQ imbalance estimates over a plurality of subcarriers having different pilot patterns; and
combine the IQ imbalance estimates for different subcarriers based on the pilot patterns.
21. The receiver of claim 20 wherein combining the IQ imbalance estimates for different subcarriers based on the pilot patterns comprises combining the IQ imbalance estimates for different subcarriers such that the IQ imbalance estimates are emphasized for those subcarriers having pilot patterns conducive to accurate IQ imbalance estimation.
22. The receiver of claim 21 wherein combining the IQ imbalance estimates for different subcarriers based on the pilot patterns comprises forming a weighted linear average of IQ imbalance estimates for different subcarriers, wherein the weights depend on the pilot pattern for the respective subcarriers.