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|Title: ||Code tracking for direct-sequence spread-spectrum systems under multiuser environment and multipath fading channels|
|Other Titles: ||Duo yong hu huan jing he duo jing shuai luo xia zhi jie xu lie kuo pin xi tong de ma gen zong ji shu|
|Authors: ||Wu, Yating (吳雅婷)|
|Department: ||Department of Electronic Engineering|
|Degree: ||Doctor of Philosophy|
|Issue Date: ||2009|
|Publisher: ||City University of Hong Kong|
|Subjects: ||Spread spectrum communications.|
|Notes: ||CityU Call Number: TK5103.45 .W8 2009|
xi, 112 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2009.
Includes bibliographical references (leaves 105-111)
|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
Keywords— Direct-sequence spread-spectrum (DS/SS), multipath fading, multiple
access interference (MAI), code tracking, delay lock loop, synchronization.|
|Online Catalog Link: ||http://lib.cityu.edu.hk/record=b3008260|
|Appears in Collections:||EE - Doctor of Philosophy |
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