Method and apparatus for estimating and correcting gain and phase imbalance in a code division multiple access system

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures.
This invention presents a method and apparatus for compensating for the IQ imbalance in a direct sequence CDMA systems by digital signal processing and thus enabling the direct conversion receiver to convert a spread spectrum passband signal to a clean digital baseband signal without incurring much extra noise.
This invention eases the IQ offset requirement on RF and analog chips and lowers the cost of a mobile receiver.
Most CDMA systems provide a pilot signal.
This invention estimates the gain and phase imbalance by monitoring the constellation of the despread pilot signal because despreading gives processing gain and boosts up the quality of the pilot signal.
The pilot constellation is distorted by the gain and phase imbalance.
The despread pilot signal is not however sufficient to determine both gain and phase imbalance because the receiver is not aware of the actual pilot strength or amplitude.
Given this, the present invention uses the IQ-swapped spreading sequence in addition to the regular spread pilot.
The IQ-swapped spreading sequence is the spreading sequence whose real and imaginary components are the real and imaginary components of the regular spreading sequence as shown in the attached figure.
As proved in the attached sheet, the gain imbalance can be estimated by a function of the real component of the pilot symbol despread by the normal spreading sequence and the imaginary component of the pilot symbol despread by the IQ-swapped spreading sequence.
In a similar fashion, the phase imbalance can be estimated by a function of the pilot symbol despread by the normal sequence and the pilot symbol despread by the IQ-swapped spreading sequence.
Since the IQ imbalance varies extremely slowly, in the preferred embodiment a communication receiver 100 as shown in FIG