Calibration algorithms for asynchronous CDMA-based antenna arrays

Abstract

It is well known that we can effectively improve system capacity, service quality, and coverage of service area in wireless communications by adopting an antenna array at the base station. The space-time signal processing for code division multiple access (CDMA) systems is one of the promising technology. Many space-time receivers for the uplink in CDMA systems have been proposed, which use knowledge of the received spatial and temporal signature waveform for the desired user. The channel information needed for symbol detection is normally acquired through the use of training sequences (or symbols) known both by the transmitter and receiver. However, not many space-time algorithms for the downlink in FDD (Frequency Division Duplexing) have been proposed. This is partially because the channel information obtained in uplink can not be directly used for downlink beamforming in FDD. Especially, when antenna array is not well calibrated, then the finding optimum downlink beaforming algorithm gets more difficult. In this dissertation, we propose several calibration algorithms for asynchronous CDMA-based antenna array in the presence of unknown gain and phase errors. The algorithms are not dependent on the geometry of antenna array and can be used when the number of signals could be greater than the number of antennas. The first on-line calibration algorithm does not require a prior knowledge of the DOAs (direction of arrivals) and multi-path delays of the signals of any user, but requires the code sequence of a reference user which is already known to the base station. The proposed algorithm provides us with estimates of the DOAs, the multi-path impulse response of the reference signal source, and the calibration of the antenna gain and phase, even under multiple access interference. We also present a new iterative calibration algorithm which works when the carrier offset often caused by either unstable oscillator or Doppler effect is present. The algorithm provides ...