PLL PERFORMANCE OF DS-CDMA SYSTEMS IN THE PRESENCE OF PHASE NOISE, MULTIUSER INTERFERENCE, AND ADDITIVE GAUSSIAN-NOISE

The performance of decision-directed phase-locked loops (PLL) for use in direct-sequence code-division multiple-access (DS-CDMA) systems is investigated in the presence of phase noise, multiuser interference, a nd additive white Gaussian noise (AWGN), A unified and accurate analys is of the effect of these sources of noise on the PLL is carried out b ased on a nonlinear model (the Fokker-Planck method) since a linear an alysis yields a large deviation between analytical results and actual performance at low signal-to-noise ratios (SNR:). After describing the implementation of the PLL, the steady-state probability density funct ion (PDF) of the phase estimator error in a first-order loop is derive d. It is shown that a tradeoff exists between the effect due to the ph ase noise, the magnitude of the multiuser interference, and the AWGN a s a function of loop bandwidth, The optimal loop bandwidth minimizing the impact of these sources of noise on the PLL and the requirements o n the, frequency uncertainty of the carrier source for a required accu racy of the phase estimator are detailed, Numerical results of the var iance of the phase estimator error are discussed for Gold codes.