[1, 2, 3, 4], and post-inverse estimation .
The figure 1 here shows a block diagram of the signal processing chain of the block LS approach. The feedback signal processes and updates the predistortion filter coefficients, similar to the principle of adaptive filtering.
The post-inverse calibration technique consists of finding the inverse path in the modulator, as illustrated in figure 3.10 here.
The predistortion filter approach requires significant processing, such as: generating a baseband modulated reference transmission signal in order to estimate I/Q phase and gain imbalance; matrix processing; adaptive filtering; and compensating for the transmission path in order to maximize the image rejection ratio (IRR). Furthermore, LO leakage needs to be mitigated prior to performing predistortion filter estimations. Synchronizing the transmitted data and the feedback data is mandatory for this approach. Bad synchronization introduces inter-symbol interference (ISI) to the feedback data, leading to incorrect estimation of the predistortion filter coefficients.
In the direct calibration approach, a simple sinusoid reference tone is transmitted with the signal; no synchronization between the transmission and feedback is required. This approach uses a simple gain and phase compensation with a few multipliers and adders in the transmission path, and an error measurement circuit in the feedback path.
LO leakage can also be estimated using the same error measurement circuit as in the feedback receiver in Figure 4, then the DC offset can be adjusted via the DAC inside the RF transmitter, or digitally compensated in baseband signal. The LMS6002D transceiver from Lime Microsystems has an integrated DAC per analog I/Q rail in order to compensate for the transmitted DC offset .
Nutaq’s Radio420X FPGA mezzanine card (FMC) product line includes an enhanced version of the Tx I/Q mismatch and LO leakage calibration techniques proposed by Lime Microsystems, which improve the IRR and reduce the LO leakage. An example of the transmitter I/Q imbalance and LO leakage before and after calibration in the Radio420X is shown in the following figures.
Transmitter I/Q imbalance and LO leakage before calibration
Transmitter I/Q imbalance and LO leakage after calibration
In these figures, the transmitter center frequency is at 1952.5 MHz, the 15 dBm (marker R) transmission tone is at 1953 MHz, and the sideband image is at 1952 MHz. The measured sideband IRR (marker 1) and LO leakage (marker 2) values after calibration are about 48 dBc and 46 dBc, respectively. Another transmitter I/Q imbalance and LO leakage calibration example can be found in our previous blog post.
It turns out that the low-cost wideband radio transceiver based on the LMS6002DFN is capable of achieving good transmitter RF performance without using and high cost analog components or calibration algorithms power processing hungry. Furthermore, the Radio420X package includes the transmitter I/Q imbalance and LO leakage calibrations, so no further calibration is required on the user’s end.
- Myllari, Olli, Lauri Anttila, and Mikko Valkama. 2010. “Digital Transmitter I/Q Imbalance Calibration: Real-Time Prototype Implementation and Performance Measurement.” European Signal Processing Conference. Aalborg, Denmark. http://www.eurasip.org/Proceedings/Eusipco/Eusipco2010/Contents/papers/1569290640.pdf
- Anttila, L., M. Vlkama, and M Renfors. 2008. “Frequency-Selective I/Q Mismatch Calibration of Wideband Direct-Conversion Transmitters.” IEEE Transactions on Circuits and Systems II: Express Briefs 55(4): 359-363, 2008. doi: 10.1109/TCSII.2008.919500
- Picinbono, B., and P. Chevalier. 1995. “Widely Linear Estimation with Complex Data.” IEEE Transactions on Signal Processing 43(8): 2030-2033. doi: 10.1109/78.403373
- Kiayani, Adnan Qamar. 2009. “DSP Based Transmitter I/Q Imbalance Calibration: Implementation and Performance Measurements.” Tampere University of Technology, Tampere. http://dspace.cc.tut.fi/dpub/bitstream/handle/123456789/6686/kiayani.pdf?sequence=3
- Ding, Lei, Zhengxiang. Ma, D. R. Morgan, M. Zierdt, and G. T. Zhou. 2008 “Compensation of Frequency-Dependent Gain/Phase Imbalance in Predistortion Linearization Systems.” IEEE Transactions on Circuits and Systems I: Regular Papers 55: 390-397. doi: 10.1109/TCSI.2007.910545
- Lime Microsystems. 2012. LMS6002DFN Multi-band Multi-standard Transceiver – Programming and Calibration Guide.