PN code acquisition using smart antenna for DS-CDMA wireless communications

Recently, a smart antenna, i.e., a blind adaptive antenna array, has attracted much attention to improve the capacity of a future code division multiple access (CDMA) wireless communications system. Adachi et al. [1--2] employed a smart antenna of M elements for data demodulation but not for a pseudonoise (PN) code acquisition process. In [1--2] only one element is used for the PN code acquisition. The benefit of a smart antenna has not been used for the PN code acquisition in the literature. PN code acquisition is a coarse PN code synchronization process before data demodulation. If the PN code is not acquired at a receiver, data demodulation is impossible. In this dissertation, an efficient PN code acquisition scheme, which employs all antenna elements, is proposed. A goal of this dissertation is to exploit the benefit of a smart antenna for PN code acquisition in a DS-CDMA wireless communications system.;This dissertation has two main contributions. One is to propose an efficient PN code acquisition scheme using a smart antenna and fixed threshold. Its basic structure is the combination of an adaptive beamformer and a conventional PN correlation searcher. Theoretical upper and lower bounds of detection probability Pd and average acquisition time Tacq, respectively, are presented. Also the corresponding simulation results are presented under additive white Gaussian noise (AWGN) and Rayleigh fading environment. Both theoretical bounds and simulation results indicate that the proposed scheme can significantly improve the PN code acquisition performance. The other contribution is to propose an efficient PN code acquisition scheme using a smart antenna and an adaptive threshold. The basic structure is the combination of a conventional PN correlation searcher, an adaptive beamformer, and an adaptive threshold setting circuit. In addition to employment of all the antenna element outputs, this proposed scheme also uses an adaptive threshold for each PN code phase hypothesis testing by measuring the instantaneous power prior to the PN code despreading. Theoretical upper and lower bounds of performance of the proposed scheme are also presented as well as simulation results in terms of false alarm probability Pf, detection probability Pd and average acquisition time Tacq under Rayleigh mobile fading environment. Theoretical bounds and simulation results demonstrate that the proposed scheme is robust against the received signal and interference power level variations, and PN code acquisition time can be significantly shortened, e.g., at least twice for a given bit-energy-to-interference power spectral density ratio Eb/Io, by employing M = 5 elements, compared to the single element case. In addition, the transmitting power of a random access channel of a mobile user in a reverse link can be at least 4.5 dB lower, and interference can be reduced. Additional computational load and complexity to use the proposed PN code acquisition are insignificant.