An Embedded System to Correct In-Phase and Quadrature-Phase Imbalance for WCDMA Direct Conversion Receiver

1. An Embedded System to Correct In-Phase and Quadrature-Phase Imbalance for WCDMA Direct Conversion Receiver Ye Shi Helsinkin University of Technology Supervisor:Professor Sven-Gustav Häggman 11,November 2003

2. Contents • Motivations • Thesis Objectives • Direct Conversion Receivers & I/Q imbalances • Estimating & Compensating I/Q Imbalances • Tests • Conclusions

3. Motivations • Direct Conversion Receivers (DCR) are used in WCDMA base stations • DCR is sensitive to I/Q imbalances • Develop an embedded system to estimate and compensate the I/Q imbalances

4. Thesis Objectives • Study WCDMA base station • Study software and hardware of transceiver ( transmitter and receiver) • Develop an embedded system to estimate and compensate I/Q imbalances

5. BB Filter I LNA RX Filter 900 F1 BB Filter Q DCR • Advantages: • Not many components, easier to integrate • Low power consumption • Disadvantages: • DC-offset • High sensitivity not easily available • Accurate I/Q phase and amplitude balance required

6. Most Common I/Q Imbalances-1 • Amplitude imbalance • Since I and Q are two separate signals, each one is created and amplified independently. Inequality of this gain between the I and Q branches results in amplitude imbalance. • A perfect signal will have a uniform constellation that is perfectly symmetric about the origin. I/Q amplitude imbalance is indicated when the constellation is not "square," i.e., when the Q-axis height does not equal the I-axis width, as seen in the Figure.

7. Most Common I/Q Imbalances-2 • Quadrature error • If the phase shift between the RF LO signals that mix with the I and Q base band signal at the modulator is not 90 degrees, a quadrature error occurs. • The constellation of the signal is distorted, which may cause error in the interpretation of the recovered symbols. The quadrature errors result in a "tipped" or skewed constellation.

8. Q Q 2 I I 3 3 1,4 1,4 Most Common I/Q Imbalances-3 • Delays in the I or Q branches • Delays can be caused by the modulator or by the filters I/Q without dalay Q delayed relative to I 2

9. Q I Most Common I/Q Imbalances-4 • DC-offset • DC offsets may be introduced in the I and Q branches. They may be added in the amplifiers in the I and Q branches. • DC offset errors result in a constellation whose center is offset from the reference center, as seen in the figure.

10. Estimating & Compensating I/Q Imbalances (1) • Developing a mathematical model to estimate • Amplitude imbalance • Phase imbalance between the I/Q branches • Quadrature error • Delay • Implementing the algorithm in PowerPC processor • Get the inverse values of the estimated I/Q imbalances • Caculate the filter tap coefficients based on the estimation

11. Estimating & Compensating I/Q Imbalances (2) • Compensating the I/Q Imbalances • Construct an equalizer filter based on the tap coefficients • Equalize the signals in the Q branch

12. Tests • The estimator and equalizer synthesis are implemented and tested • The simulation system is tested

13. Conclusions • This embedded system is specially used in dual-carrier DCR to remove the effects of I/Q imbalances • Advantages of the embedded system • The compensation method is frequency dependent • The pilot signal is not needed • Further study is needed

14. Thank you !!! • Questions?