Hiroji Kusaka

Blind phase recovery in QAM communication systems using higher order statistics

We present a method for phase recovery in QAM communication systems based on higher order statistics. Under the assumption that the QAM signals have independent, identically distributed (i.i.d.) in-phase and quadrature components, we derive the relation between the phase error and the fourth-order cumulant of the output. Numerical performance of the approach is demonstrated by computer simulation.

Blind identification of noncausal AR models based on higher-order statistics

Abstract In this paper we develop a new method to identify noncausal AR models that are driven by non-Gaussian i.i.d. input. Under a few moderate assumptions, the necessary and sufficient conditions for rebuilding the AR models from second and third order statistics are derived. It is shown that the AR model parameters are directly related to the solution of an eigenproblem. Based on this approach we present a method of AR model identification, applying eigendecomposition. Unique identification of AR models is guaranteed up to sign and linear phase ambiguities. Model order determination is not crucial in the method. If the order is overestimated, several equivalent AR models with different linear phases will be obtained.

An analysis of nonlinear distortion due to TWT in asynchronous DS-SSMB communication systems

We analyze the effects of the amplitude nonlinearity and AM/PM conversion in a traveling wave tube (TWT) used in an asynchronous direct sequence spread spectrum multiple access (DS-SSMA) communication system, assuming that the nonlinearity is represented by the power series of the input signal and the conversion is relatively small. We propose an analytical method of the signal-to-noise ratio at the output of the correlation detector in the receiver. We also justify our method by comparing the theoretic results with the computer simulations in when the amplitude nonlinear circuit is modeled by an error function as a typical example and show that the third order intermodulation product can be almost neglected in the sense of the signal-to-noise ratio.<<ETX>>

An improvement of nonlinear characteristics in DS-SSMA communication systems using equalizers

This paper proposes an improvement method of the deteriorated characteristics on account of the nonlinearity in direct sequence spread spectrum multiple access (DS-SSMA) communication systems. Where, a traveling wave tube (TWT) is adopted as nonlinear circuit in which the amplitude nonlinearity and AM/PM conversion are assumed to be relatively small. The transmission signal is expressed in two quadrature components and an equalizer of feed-forward transversal type is used to improve the nonlinear characteristics. The calculation results show that the significant improvement can be obtained from a simple equalizer using the least mean square algorithm.