Chien-Hwa Hwang

Analysis of asynchronous long-code multicarrier CDMA systems with correlated fading

The effects of inter-carrier interference (ICI) and aperiodic random spreading sequences on the performance of asynchronous multicarrier code-division multiple-access (CDMA) systems with correlated fading between sub-carriers are investigated in this research. To conduct theoretical analysis for the maximal ratio combining (MRC) receiver, random parameters including asynchronous delays, correlated Rayleigh fading, and spreading sequences are averaged to find the unconditional covariance matrix of the interference-plus-noise vector. We demonstrate that the ICI in the system proposed by Kondo and Milstein (1996) can be mitigated by assigning a common random spreading sequence over all sub-carriers for each user, rather than using a set of distinct spreading sequences, where code sequences are assumed perfectly time-synchronized. Moreover, the analytic expression for the bit error probability (BEP) can be obtained with the Gaussian approximation. Simulation results are used to demonstrate the accuracy of our analysis. Various design tradeoffs including the number of sub-carriers, fading correlations, ICI and multipath effect are presented in simulation. We conclude that, when properly choosing a sufficiently large number of sub-carriers, the benefit of mitigating multipath interference and exploiting potential hidden frequency diversity shall prevail the impairment brought by increased ICI.

Analysis of multistage linear parallel interference cancellation in CDMA systems using graphical representation

The bit error probability of the multistage linear parallel interference canceller in a long-code code division multiple access (CDMA) system is analyzed with a graphic approach in this research. The decision statistic is modelled as a Gaussian random variable, whose mean and variance can be expressed as functions of moments of (R-I), where R is the correlation matrix of the signature sequences. Since the complexity of calculating these moments increases rapidly with the growth of the stage index, a graphical representation of moments is developed to facilitate the computation. Propositions are presented to relate the moment calculation problem to several well known problems in graph theory, i.e., the coloring, the graph decomposition, the biconnected component, and the Euler tour problems. It is shown that the analytic results of the bit error probability match well with simulation results.

Primary User Signal Detection in OFDM Based Cognitive Radios

In this paper, detection of the primary user (PU) signal in an orthogonal frequency division multiplexing (OFDM) based cognitive radio (CR) system is addressed. According to the prior knowledge of the PU signal, three detection algorithms based on the Neyman-Pearson philosophy are proposed. In the first case, a Gaussian PU signal with completely known probability density function (PDF) except for its received power is considered. The frequency band that the PU signal resides is also assumed known. Detection is performed individually at each OFDM sub-carrier possibly interfered by the PU signal, and the results are then combined to form a final decision. In the second case, the sub-carriers that the PU signal resides are known. Observations from all possibly interfered sub-carriers are considered jointly to exploit the fact that the presence of a PU signal interferers all of them simultaneously. In the last case, it is assumed no PU signal prior knowledge is available. The detection is involved with a search of the interfered band. The proposed detector is able to detect an abrupt power change when tracing along the frequency axis.

An iterative approach to frequency offset estimation for multicarrier communication systems

An iterative approach to estimate the carrier frequency offset in multicarrier communication systems is proposed in this work. Based on the previous estimated channel coefficients and provided a sufficiently large number of subcarriers, an approximate maximum likelihood (ML) estimate of the normalized frequency offset, which in turn can be used to refine the previous obtained estimation of channel coefficients. Simulation results show that the proposed algorithm requires a reliable estimation of the channel fading coefficients. Through a fast acquisition of the frequency offset at the beginning symbol block of each frame, the proposed iterative process can be better initiated, and the possible drift of the carrier frequency offset can therefore be tracked accurately.

Performance of asynchronous long-code multicarrier CDMA systems in the presence of correlated fading and inter-carrier interference

The effect of inter-carrier interference (ICI) and aperiodic random spreading sequences on the performance of asynchronous multicarrier code division multiple access (CDMA) systems with correlated fading is investigated in this research. Random parameters including asynchronous delays, correlated Rayleigh fading and spreading sequences are averaged to determine the unconditional covariance matrix of the interference-plus-noise vector. An analytic expression for the bit error probability is obtained based on the Gaussian approximation. Then, we extend the result by considering situations where eigenvalues are not necessarily all identical or distinct. Finally, design tradeoffs among the number of sub-carriers, fading correlations, ICI and multipath effect are presented in simulation results.

Design of Reduced-Rank MMSE Combiner in Asynchronous MC-DS-CDMA

In this paper, we consider the design of a reduced- rank minimum mean-squared error (MMSE) combiner in an asynchronous multicarrier-direct sequence-code division multiple access (MC-DS-CDMA) system. It is shown that the combiner weights are related to quantities whose expressions resembles the eigenvalue moments of the interference autocorrelation matrix. To lift the burden of recomputing the weights from time to time, the weights are evaluated in a large system regime and averaged over random parameters including asynchronous delays, correlated fading coefficients and spreading sequences. Explicit expressions for the asymptotic weights of the MMSE combiner are derived. The large system setting is with the spreading gain N and user number K approaching infinity, but K/N = beta and the number of sub-carriers both finite.

Performance evaluation of asynchronous MC-DS-CDMA under random spreading and correlated fading

The effects of inter-carrier interference (ICI) and aperiodic random spreading sequences on the performance of asynchronous multicarrier code division multiple access (CDMA) systems with correlated fading among sub-carriers are investigated in this research. To obtain the maximal ratio combining (MRC) filter, random parameters including asynchronous delays, correlated Rayleigh fading and spreading sequences are averaged to find the unconditional covariance matrix of the interference-plus-noise vector. We demonstrate that the ICI in the system proposed by Kondo and Milstein can be mitigated by assigning a common random spreading sequence over all sub-carriers for each user, rather than using a set of distinct spreading sequences. Moreover, the analytic expression for the bit error probability (BEP) can be obtained with the Gaussian approximation. Simulation results are used to demonstrate the accuracy of our analysis. Finally, various design tradeoffs including the number of sub-carriers, fading correlations, ICI and multipath effect are also presented in simulation.

A statistical approach to distributed edge sensor detection

Edge sensor detection is often used in identifying regions that are affected by various factors in wireless sensor networks. A statistical methodology based on distributed detection theory and the Neyman-Pearson criterion is developed for edge sensor detection in this research. The input sensor statistics are assumed to be identically independently distributed in our framework. Edge regions and sensors are determined using a hypothesis test, where the observation model for each hypothesis is derived. A sub-optimal distributed detection scheme, which is optimal among detectors having the same test at all local sensors, and the way of choosing the optimal operating point are described. The condition under which the proposed scheme outperforms the optimum detector based on a single sensor is presented. Furthermore, the noisy channel effect is considered, and a method to overcome this noisy effect is addressed. The performance of the proposed distributed edge sensor detection scheme is studied via computer simulation, where the ROC curves are used to demonstrate the tradeoff between the cost (in terms of the sensor density) and detection accuracy.

Graphical approach to performance analysis of linear multistage interference cancellation in asynchronous CDMA systems

The bit error probability (BEP) of a multistage linear parallel interference canceller (LPIC) is a long-code code division multiple access (CDMA) s ys tem with chip-synchronized asynchronous transmission is analyzed in this work. By assuming the decision statistic to be a Gaussian random variable, the BEP can be obtained by plugging the conditional mean and variance of the decision statistic into the Q function. We formulate the effective correlation matrix D(i) in the asynchronous environment, and show that the conditional mean and the variance of the decision statistic can be expressed as functions of moments of D(i). The computation of moments of D(i) can be divided into two parts. One is the expectation of cosine terms, which can be solved by the method introduced in our previous work of a synchronous system [1]. The ot her is the expectation of cross-correlation terms, which is much more complex than the case in [1]. Besides tools in [1], we develop an evolutionary forest to deal with manipulations of D(i) and show that the computation of the expectation of cross-correlation terms is equivalent to solving a list coloring problem.

Performance Evaluation of Data Transmission over Multicarrier CDMA Systems

The performances of direct sequence-code division multiple access (DS-CDMA), multicarrier-CDMA (MC-CDMA) and multicarrier-direct sequence-CDMA (MC-DS-CDMA) systems under different channel conditions are compared in this work. In a frequency-selective slowly fading channel, MC-CDMA and MC-DS-CDMA outperform DS-CDMA, since the former two systems partition the frequency band into sub-channels, each of which has a nearly constant frequency response. Thus, MC-CDMA and MC-DS-CDMA do not suffer much from the multipath effect. The performance of MC-CDMA and MC-DS-CDMA can be further differentiated in severe fading conditions. In a frequency-selective fast fading channel, the larger spreading ratio of MC-DS-CDMA in the time domain prevents the chip duration of a sub-carrier from being longer than the channel coherence time. Hence, the sub-carrier orthogonality is maintained in MC-DS-CDMA, leading to its better performance in this case.