257 research outputs found
Fatigue Behavior of Electrodeposited Nanocrystalline Nickel Films
AbstractMaterial characterization of thin film for MEMS/nano structures is a building block for the reliability assessment. One of the most significant barriers for reliable MEMS/nano structure is the long term reliability which is different from those of bulk materials. Nano-indenter has been used widely to get the elastic modulus of materials in a very simple way but other testing machines are required for tensile and endurance properties. For the long term reliability assessment of micro/nano structures, a new micro fatigue testing machine was developed to obtain the high cycles fatigue behavior of thin films. Nanocrystalline nickel thin films exhibited significant rate dependency on tensile and fatigue behavior even in room temperature due to grain size related behavior in contrast to coarse grained wrought nickels
Understanding the Visitation Aspect of Black-headed Gull (Larus ridibundus) in the Long Period at Nakdong Estuary, Busan, Republic of Korea
AbstractThis study analyzes data in the first half of 1990 (May, 1989 - April, 1993) and mid-2000 (May, 2002 - April, 2006) to understand long-term status of black-headed gulls (Larus ridibundus) in the estuary of the Nakdong River under rapid changes. During the period (May, 1989 - April, 1993 and May, 2002 - April, 2003 for a total of 8 years), a total of 201,519 individuals of Laridae were discovered at the estuary and the number of the black-headed gulls (Larus ridibundus) was 32,538 (16.15%). Monthly average of the black-headed gulls showed that there were just a small number of individuals arrived in July and August, but the number increased from September, reached the highest in April for the first term (1989 - 1993). The second term showed that some individuals were discovered in summer (June - August), then the number increased from October and reached the peak in April. Monthly average in the latter period marked 5.7 times higher than that of the former. In the aspect of individuals, there is a significant difference between the two period (P<0.05) and it is discovered that the number of individuals in 2000s (Mean= 6923.50) was higher than that of 1990s (Mean=1211.00). Comparing individuals of gulls, herring gulls (Larus argentatus) and black-headed gulls for 8 years, it was found out that the herring gulls (Larus argentatus) showed the largest number (average=5313.13), followed by the black-headed gulls (average=4067.25) and gulls (average= 1207.63). The fluctuation on individuals of black-headed gulls in the mid-2000s (May, 2005 - April, 2006) recorded the average number of 472.75, then 113.50 in DMD, 244.00 in JJ and SJD, 635.25 in SJ and DY, 727.50 in LUD and 643.50 in USD, showing significant differences among regions (P<0.05)
Use of Multiple Antennas for DS/CDMA Code Acquisition
Manuscript received August 13, 2001; revised February 19, 2002; accepted
March 6, 2002. The editor coordinating the review of this paper and approving
it for publication is R. A. Valenzuela.A generalized acquisition scheme is proposed for direct
sequence code-division multiple-access systems with multiple
antennas. The proposed scheme employs grouping of multiple antennas
as a means of a tradeoff between two important factors determining
the mean acquisition time, combining gain and search
time. The performance of the proposed acquisition scheme is analyzed
in frequency-selective Rayleigh-fading channels with consideration
of spatial correlations. Numerical results show that the use
of the largest number of antenna groups is preferable to reducing
the mean acquisition time at low signal-to-interference ratio (SIR)
values. At high SIR values, on the contrary, the mean acquisition
time is found to increase in proportion to the number of antenna
groups. In a typical environment, the presence of spatial correlation
is shown to increase or decrease the mean acquisition time
within 50% compared with the uncorrelated fading case.This work was supported in part by the
Brain Korea 21 Project
Utilization of Multipaths for Spread-Spectrum Code Acquisition in Frequency-Selective Rayleigh Fading Channels
Paper approved by Z. Kostic, the Editor for Wireless Communication of the IEEE Communications Society. Manuscript received February 22, 2000; revised July 2, 2000.
This paper was presented in part at the 11th IEEE Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC 2000), London, U.K., September 2000.A novel acquisition scheme that utilizes multipaths
to improve acquisition performance is proposed for frequency-selective
fading channels. The proposed acquisition scheme employs
nonconsecutive search and joint triple-cell detection. The performance
is analyzed in frequency-selective Rayleigh fading channels.
Equations for the probabilities of detection and false alarm are derived,
and an expression for the mean acquisition time is developed.
The mean acquisition time performance of the proposed and
conventional acquisition schemes is evaluated and compared. It is
found that the proposed acquisition scheme significantly outperforms
the conventional one. The effects of various channel parameters
such as the number of resolvable paths, the shape of the multipath
intensity profile (MIP) and the signal-to-interference ratio
(SIR) on acquisition performance are also investigated.This work was supported by the Brain Korea 21 Project
Performance Comparison of FFH and MCFH Spread-Spectrum Systems with Optimum Diversity Combining in Frequency-Selective Rayleigh Fading Channels
Paper approved by Z. Kostic, the Editor for Wireless Communication of the IEEE Communications Society. Manuscript received June 5, 1999; revised May 12, 2000.
This paper was presented in part at the 10th IEEE Symposium on Personal, Indoor and
Mobile Radio Communications (PIMRC99), Osaka, Japan, September 1999.In this letter, the performance of frequency-hopping
spread-spectrum systems employing noncoherent reception and
transmission diversity is analyzed for frequency-selective Rayleigh
fading channels. Two different types of transmission diversity systems,
a fast frequency-hopping (FFH) system and a multicarrier
frequency-hopping (MCFH) system, are investigated. In order to
combine received signals from transmit diversity channels, the optimum
diversity combining rule based on the maximum-likelihood
criterion is developed. Probability of error equations are derived,
and utilized to evaluate the performance of the two systems.
MCFH systems are found to outperform FFH systems when the
channel delay spread is severe, while FFH systems are superior
to MCFH systems when a channel varies rapidly. Furthermore,
it is found that performance enhancement due to an increase of
diversity order is more significant for MCFH systems than for
FFH systems in frequency-selective fading channels. The effect
of frequency-selective fading is also investigated in determining
optimum frequency deviations of binary frequency-shift keying
signals.This work was supported by the Brain Korea 21 Project
Differentially Coherent Combining for Double-Dwell Code Acquisition in DS-CDMA Systems
Paper approved by L. Wei, the Editor for Wireless CDMA Systems of the IEEE Communications Society. Manuscript received January 9, 2002; revised June 15, 2002 and December 3, 2002.
This paper was presented in part at the 12th IEEE Symposium on Personal, Indoor, and Mobile Radio Communications, San Diego, CA,
September-October, 2001.The use of differentially coherent combining is proposed
to improve the performance of a double-dwell acquisition
system by increasing the reliability of a decision in the verification
stage. The detection and mean acquisition time performance of the
acquisition scheme with the proposed combining scheme is analyzed
in frequency-selective Rayleigh fading channels, and compared
with that of two previously published double-dwell acquisition
schemes based on long correlation intervals and noncoherent
combining. It is shown that the proposed acquisition scheme outperforms
the previous ones, and that the performance improvement
increases as the frequency offset increases.This work was supported by the Brain
Korea 21 Project
Transmit Power Allocation for BER Performance Improvement in Multicarrier Systems
This paper was presented
in part at the IEEE International Symposium on Personal, Indoor, and Mobile
Radio Communications, Lisbon, Portugal, September 2002.In a multicarrier system, transmit power allocation
over different subchannels is an effective means of improving the
performance. We develop the optimal transmit power allocation
scheme to improve bit-error rate (BER) performance in a multicarrier
system with diversity reception. A simple suboptimal
scheme is also derived from the optimal one, and an asymptotic
case referred to as the equal-signal-to-noise ratio scheme
is discussed. Numerical results show that the optimal and suboptimal
power allocation schemes significantly outperform the
equal power allocation scheme. The effects of the modulation
level, the number of receiving antennas, and the number of
subchannels on the BER performance are also investigated
Transmit Power Allocation for Successive Interference Cancellation in Multicode MIMO Systems
Paper approved by X. Wang, the Editor for Multiuser Detection and
Equalization of the IEEE Communications Society. Manuscript received June
10, 2005; revised November 2, 2006 and July 23, 2007. This paper was
presented in part at the IEEE International Conference on Communications,
Seoul, Korea, May 2005.Multiple-input multiple-output (MIMO) system
with multicode transmission can provide high speed data services
by transmitting independent parallel substreams from multiple
antennas and through multicode channelization. In this paper,
we first introduce an iterative two-stage successive interference
cancellation (SIC) detection scheme for a multicode MIMO
system. The proposed technique cancels the interference signals
successively in the space domain followed by the code domain.
Next, we develop various transmit power allocation schemes over
different data substreams for the proposed detection process to
improve error rate performance. The joint transmit power allocation
is derived to make the post-detection signal-to-interferenceplus-
noise ratio (SINR) become the same for all substreams
in both the space and code domains. As a computationally
efficient scheme, we propose a two-stage power allocation scheme,
which allocates the total transmit power to the substreams in
the code domain at the first stage, and allocates this code
domain power to the substreams in the space domain at the
second stage. Furthermore, variable and constant power ratio
(PR) schemes are derived to reduce the feedback overhead. In
particular, the constant PR scheme utilizes the transmit power
ratio determined by the long-term statistical properties of the
fading channel amplitudes, and achieves significantly reduced
feedback rate. Numerical results show that the proposed transmit
power allocation schemes for the two-stage SIC significantly
outperform the equal power allocation scheme
Statistical Multimode Transmit Antenna Selection for Limited Feedback MIMO Systems
This paper was
presented in part at the Asia-Pacific Conference on Communications, Busan,
Korea, August 2006.In a wireless multiple-input multiple-output
(MIMO) system, transmit antenna selection is an effective
means of achieving good performance with low complexity. We
consider spatial multiplexing with linear receivers, and equal
power and equal rate allocation over different selected transmit
antennas in order to reduce feedback overhead. Under these
constraints, we address the problem of statistical multimode
transmit antenna subset selection to improve the capacity
of spatially correlated MIMO fading channels. In particular,
we first derive an analytical closed-form expression for the
expectation of the lower bound on the capacity using the
smallest eigenvalue distribution of a Wishart matrix. Then,
we propose a transmit antenna subset selection criterion of
maximizing this average lower-bound capacity
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