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

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

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

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

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 90 0 RX Filter BB Filter F1 LNA I Q

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.

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.

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 I Q 31,4 I Q 2 3 2

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. I Q

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

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

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

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

Thank you !!! Questions?