Kyunbyoung Ko

Performance analysis of OFDMA uplink systems with symbol timing misalignment

This letter presents our investigation for the effect of symbol timing errors in orthogonal frequency-division multiple access (OFDMA) uplink systems. We express the symbol timing errors between users as the symbol timing misalignments with respect to the desired user. Then, we derive an explicit expression of the signal-to-noise ratio (SNR) as a function of the maximum value of the symbol timing misalignments. Analyses and simulation results show that, to achieve an SNR of 20 dB, the maximum value of the symbol timing misalignments must be less than the cyclic prefix duration plus 6.25% of the useful symbol duration. Based on the resulting SNR degradation, we evaluate the SNR gain with guard subcarriers in order to mitigate the effect of the symbol timing misalignments.

Performance of an MC-CDMA system with frequency offsets in correlated fading

The performance of an MC-CDMA system with carrier frequency offsets in correlated multipath fading is mathematically analyzed. The bit error rate is obtained from such analysis for the uplink Rayleigh fading channel. The derived results show that the performance of the MC-CDMA system is sensitive to the frequency offset, and the correlation of the subcarriers. That is, the performance degradation of the MC-CDMA system is caused by both carrier frequency offset and correlation among subcarriers. From the derived results, however, the performance degradation due to correlation among subcarriers is more severe than effect of frequency offset.

Outage probability and channel capacity for the N th best relay selection AF relaying over INID Rayleigh fading channels

Cooperative diversity systems have recently been proposed as a way to form virtual antenna schemes without utilizing collocated multiple antennas. In this paper, we consider the Nth best opportunistic amplify-and-forward (AF) cooperative diversity systems. The AF type can be regarded as one on the basis of modified channel state information. Wireless channels between any pair of nodes (i.e., direct and dual hop links) are assumed quasi-static independent and nonidentically distributed (INID) Rayleigh fading. The best opportunistic AF (OAF) scheme requires two phases of transmission. During the first phase, the source node transmits a signal to all relays and the destination. In the second phase, the best relay is only selected on the basis of highest signal-to-noise ratio (SNR) scheme to forward the source signal to the destination. Therefore, the indirect link (i.e., source-selected relay destination) can give the highest received SNR. However, the best relay selection cannot be available so that we might choose the second, third, or generally the Nth best relay. In this paper, we derive the approximated outage probability and channel capacity for the Nth best OAF relay systems over INID Rayleigh fading channels. At first, the indirect links received SNR is approximated as harmonic mean upper bound. With this information, we obtain the given relays Nth best selection probability as the closed form. Finally, both outage probability and channel capacity are derived as the closed forms. Simulation results are finally presented to validate the analysis. Copyright

Performance analysis of channel estimation for OFDM systems with residual timing offset

The authors present an analysis of the effect of timing offset on channel estimation for comb-type pilot-aided orthogonal frequency division multiplexing (OFDM) systems. Residual timing offset does not negatively affect the channel estimation of the pilot subcarrier, but does corrupt the channel information obtained via interpolation. This paper provides the mean square error (MSE) channel estimation performance when a linear interpolation technique is used in a comb-type pilot-aided OFDM system. Analysis shows that the performance degradation of the channel estimator due to imperfect frame synchronization is dependent on the frequency correlation of the channels and the amount of timing offset

RBF multiuser detector with channel estimation capability in a synchronous MC-CDMA system

The authors propose a multiuser detector with channel estimation capability using a radial basis function (RBF) network in a synchronous multicarrier-code division multiple access (MC-CDMA) system. The authors propose to connect an RBF network to the frequency domain to effectively utilize the frequency diversity. Simulations were performed over frequency-selective and multi-path fading channels. These simulations confirmed that the proposed receiver can be used both for the channel estimation and as a multi-user receiver, thus permitting an increase in the number of active users.

Effects of asynchronous MAI on average SEP performance of OFDMA uplink systems over frequency-selective Rayleigh fading channels

This paper investigates the effects of asynchronous multiple access interference (MAI) on the performance of orthogonal frequency-division multiple access (OFDMA) uplink systems in frequency-selective Rayleigh fading environments. We first analyze the MAI and its statistical characteristics using the Gaussian approximation. Then the analytical expression for the uncoded average symbol error probability (SEP) is obtained with various modulation schemes. Comparison with simulation results shows a tight agreement of the proposed analysis over a wide range of asynchronous timing errors and for various numbers of asynchronous users. Numerical examples show that when asynchronous timing errors reside within the guard interval, a performance of uncoded SEP- 10-3 can be feasible even in high dispersive environments. Furthermore, the effect of the Gaussian approximation is examined, and the performance comparison between interleaved- and block-type subcarrier allocation schemes is provided.

Performance analysis for multistage interference cancellers in asynchronous DS-CDMA systems

The authors propose an accurate analytical method for an asynchronous direct-sequence code-division multiple-access system using a multistage interference cancellation technique. In this analysis, the RAKE receiver is considered over uplink fading channels. Monte Carlo simulations show that the proposed analysis agrees more closely with simulation results than previous analyzes.

Performance analysis of asynchronous MC-CDMA systems with a guard period in the form of a cyclic prefix

This letter presents an analysis of asynchronous multicarrier code-division multiple-access (MC-CDMA) systems with a guard period (GP) in the form of a cyclic prefix over frequency-selective multipath fading channels, which results in closed-form bit-error rate performance. Furthermore, it confirms that the frequency diversity of MC-CDMA systems is identical to the path diversity. The analytical approach proposed here is extended to the case without a GP. The derived analytical results show that a GP is required for MC-CDMA systems in order to mitigate not only the effect of intersymbol interference and intercarrier interference, but also the desired signal power degradation.

More tractable average error rate expression for the Nth best relay selection scheme of DF relaying over Rayleigh fading channels

SUMMARY In this paper, we derive another exact average error rate expression for the Nth best decode-and-forward relay systems over Rayleigh fading channels. On the basis of error-events at relay nodes, the Nth best relays selection probability is expressed as a more tractable form in which the number of summations and the length of each summation are specified. Then, exact average error performance metrics are obtained as two well-known forms. The first is expressed with a moment generating function, and the other is presented as a closed form. Simulation results are finally presented to validate that the derived analytical expression can be a general frame work covering all Rayleigh fading channel conditions. Copyright

EFFECTS OF NITRATE ON THE UV PHOTOLYSIS OF H2O2 FOR VOCs DEGRADATION IN AN AQUEOUS SOLUTION

ABSTRACT The major objective of this study was to delineate the effect of nitrate on the UV oxidation of benzene and toluene, dissolved in less than 100 μg l−1, by conducting a bench‐scale operation at various reaction times and with various initial concentrations of H2O2 and NO3 −. The oxidation of benzene and toluene can be expected to be only about 10% and 18%, respectively, through the photolysis of H2O2 (initial conc. of 50 mg l−1), where the reactor was operated at a reaction time of 2 min, with an initial NO3 −‐N concentration of 5 mg l−1. Nitrate clearly hindered UV oxidation when the initial H2O2 concentration in the reactor was less than 50 mg l−1. Even if approximately 40% removal could be achieved under the conditions mentioned above (an initial H2O2 concentration of 200 mg l−1 at a reaction time of 9 min, with a high UV dose), the operating conditions for the 40% removal might be beyond the practical limits applied for effluents discharged from wastewater treatment plants. The results of the experiment also indicate that benzene and toluene can be oxidized in very limited amounts through direct photolysis, without additional oxidation by hydroxyl radicals.