Code Tracking Error in the Presence of Interference Signal

The performance of a code tracking loop for a spread spectrum signal can be severely deteriorated in the presence of an interference signal. The interference signal is modeled as a signal with the same code but with a different delay and carrier frequency. The variance of the tracking delay error is derived in terms of loop bandwidth, chip duration, the interference and signal power and bandpass filter bandwidth and the power spectral density of the additive Gaussian noise. INTRODUCTION A noncoherent early and late gate tracking loop can be used to track the delay in a spread spectrum (SS) signal [1]. The PN coded SS signal S(t) is given by (1) where Ps is the signal power, d(t) the binary data of 1 and !1, c(t) the direct sequence PN code and ) the delay, Tc the angular carrier frequency and 2c the carrier phase. The signal is subject to an interference signal given by (2) where PI is the interference signal power, )T is the frequency offset ) = $), $ is the sweeping rate and 2i is the interference signal phase. The received signal is given by y(t) = S(t) + I(t) + n(t) (3) where (4) nct and ns(t) are independent stationary Gaussian processes with power spectral density (PSD) equal to No/2. The purpose of this correspondence is to evaluate the variance of the tracking delay error , = )!) where ) is the estimate of the delay, using the noncoherent early and late gate delay lock loop [1]. Holmes [2] has investigated the performance of the tracking loop in the presence of single tone interference with no coding. The behavior of the phase lock loop (PLL) in the presence of continuous wave (CW) interference signal has been investigated by many authors, e.g., Bruno [3] , Blanchard [4] and Levitt [5]. TRACKING ERROR The noncoherent early and late gate delay lock loop is shown in Figure 1. The outputs of the early and late gate correlator are given as follows: