Rayleigh Fading Channels

PN code tracking is one of the most important factors that affect the performance of spread spectrum receivers. It is desirable to maintain PN code tracking for a long period of time and with high accuracy. This paper presents a non-coherent digital delay-locked loop (DLL) for code tracking in direct-sequence spread spectrum (DS-SS) systems. The proposed technique utilizes three non-coherent correlators with time-diversity followed by a lock detector. Analytical results for the mean time to lose lock, also called mean hold-in time, are derived for both additive white Gaussian noise and Rayleigh fading channels using a Markov chain model. Computer simulation results are also presented to validate the analytical results. It is shown that the proposed loop can maintain lock for relatively long periods of time under practical range of signal-to-noise ratio. For instance, the loop maintains lock for more than10 6 frames for an energy per bit-to-noise power spectral density (Eb/No ) of 4 dB or better. It is also shown that the mean hold-in time performance is only slightly degraded due to fading because of the time diversity gain utilized by the loop in obtaining the timing information. Finally, the ability of the proposed technique to successfully track randomly and linearly varying delays is demonstrated.