Code tracking for direct-sequence spread-spectrum systems under multiuser environment and multipath fading channels

Abstract

Direct-sequence spread-spectrum (DS/SS) techniques play an important role in wireless communication systems due to their well-known advantages such as anti-jam capability and high bandwidth efficiency. The synchronization between the incoming and the local despreading pesudonoise (PN) codes at the receiver is crucial for the successful operation of DS/SS systems. This thesis focuses on the code tracking, whose function is to achieve and continuously maintain the fine synchronization. Traditional delay lock loop (T-DLL), although well suited for code tracking under additive white Gaussian noise (AWGN) channels, suffers severe performance degradation by multiple access interference (MAI) and multipath fading. The discriminator characteristic or S-curve of T-DLL is distorted and randomly biased by the time-varying multipath interference (MPI) and MAI. The resultant tracking bias not only harms the tracking capability of the loop, but also degrades the performance of the receiver, since it causes a loss in signal power by the square of the PN code’s autocorrelation function. The main contribution of this thesis can be divided into three parts: First, performance analysis of T-DLL is presented, showing that T-DLL has noticeable tracking bias under multiuser environments. The maximum tracking biases and the instability probabilities of T-DLL are calculated for various multiuser scenarios. Similarly, MPI causes the S-curve to be biased by the residual cross correlation from adjacent paths, hence the shift of lock point. In the second contribution, several schemes are proposed to combat MAI effects. a) A modified de-correlated delay lock loop (MD-DLL) is first proposed for synchronous DS/SS systems under multiuser environments. The basic principles of MD-DLL are: i) to employ the de-correlating technique of multiuser detector to remove the MAI at the on-time code position; and ii) to use a suboptimum search algorithm to find proper timing offsets for the reference codes to minimize the tracking bias and suppress noise enhancement. b) De-correlated modified code tracking loop (D-MCTL) is derived by applying the de-correlating idea to modified code tracking loop (MCTL). Detailed analysis of D-MCTL and T-MCTL is performed under AWGN and multiuser environment. Performance measure includes S-curve, stability, tracking bias, mean square tracking error and mean-time-to-lose-lock (MTLL). Unlike MD-DLL which requires a search algorithm to compensate the noise imbalance that may introduce a small tracking bias under low signal-to-noise ratio (SNR) condition, D-MCTL has much lower computational complexity and exhibits zero tracking bias for the whole range of SNR, regardless of the number of interfering users. c) The above two proposed schemes are analogous to zero-forcing equalizers for inter-symbol interference (ISI) channels. The advantages are near-far resistance and that no knowledge of users’ powers and channel noise power is required. However, the noise enhancement coming with the de-correlating structure may degrade the error performance. Therefore, taking the important properties of S-curve including tracking range and lock point, and error performance together into consideration, a constrained minimum mean square error code tracking loop (CMMSE-CTL) is proposed. By introducing weighted assistant despreading signals into the local reference signals, the resultant extra degrees of freedom is exploited to improve the loop’s tracking performance. The derived mean square tracking error is taken as the cost function and minimized with respect to the weight parameters, under the constraints which ensure an odd-symmetric bias-free S-curve with enhanced tracking range. Simulations show that CMMSE-CTL provides unbiased stable code tracking with reduced mean square tracking error, enhanced tracking range and near-far resistance. The final contribution is a proposed low-complexity noncoherent code tracking loop (LC-CTL) with multipath diversity and enhanced S-curve for DS/SS systems over multipath fading channels. Instead of using channel estimators or multipath interference canceller which require additional hardware complexity as the common approaches in the literature, the proposed scheme introduces the early and late signals of the neighboring paths into the local reference signals for each path, and combines the residual cross-correlation from adjacent paths constructively to form an enhanced S-curve. The S-curve of the new loop is shown to be bias-free and odd-symmetric. The new loop outperforms the traditional scheme greatly in terms of mean square tracking error and MTLL. Keywords— Direct-sequence spread-spectrum (DS/SS), multipath fading, multiple access interference (MAI), code tracking, delay lock loop, synchronization.