44,550 research outputs found
The Nelson-Seiberg theorem revised
The well-accepted Nelson-Seiberg theorem relates R-symmetries to
supersymmetry (SUSY) breaking vacua, and provides a guideline for SUSY model
building which is the most promising physics beyond the Standard Model. In the
case of Wess-Zumino models with perturbative superpotentials, we revise the
theorem to a combined necessary and sufficient condition for SUSY breaking
which can be easily checked before solving the vacuum. The revised theorem
provides a powerful tool to construct either SUSY breaking or SUSY vacua, and
offers many practicable applications in low energy SUSY model building and
string phenomenology.Comment: 5 pages; v2: abstract and introduction revised; v3: condition of
perturbative superpotentials added, JHEP published versio
Optimal PMU Placement for Power System Dynamic State Estimation by Using Empirical Observability Gramian
In this paper the empirical observability Gramian calculated around the
operating region of a power system is used to quantify the degree of
observability of the system states under specific phasor measurement unit (PMU)
placement. An optimal PMU placement method for power system dynamic state
estimation is further formulated as an optimization problem which maximizes the
determinant of the empirical observability Gramian and is efficiently solved by
the NOMAD solver, which implements the Mesh Adaptive Direct Search (MADS)
algorithm. The implementation, validation, and also the robustness to load
fluctuations and contingencies of the proposed method are carefully discussed.
The proposed method is tested on WSCC 3-machine 9-bus system and NPCC
48-machine 140-bus system by performing dynamic state estimation with
square-root unscented Kalman filter. The simulation results show that the
determined optimal PMU placements by the proposed method can guarantee good
observability of the system states, which further leads to smaller estimation
errors and larger number of convergent states for dynamic state estimation
compared with random PMU placements. Under optimal PMU placements an obvious
observability transition can be observed. The proposed method is also validated
to be very robust to both load fluctuations and contingencies.Comment: Accepted by IEEE Transactions on Power System
"Credit Crunches and Household Welfare: The Case of the Korean Financial Crisis"
The financial crisis in 1997 caused serious deterioration of the Korean economy. We examined the credit crunch in Korea and how it affected household welfare. With household panel data from 1996-1998, we estimated a switching regression model of a consumption Euler equation, which is augmented by endogenous credit constraints. Several empirical findings emerged. First, households coped with the negative shocks by reducing consumption of luxury items while maintaining food, education, and health-related expenditures. Second, the estimated results suggest that the standard consumption Euler equation, i.e., the necessary condition of the life-cycle permanent income hypothesis, does not hold because of binding credit constraints. Especially between 1997 and 1998, the probability of facing credit constraints increased significantly for all households. The expected welfare loss from binding credit constraints increased by 45% during the crisis, suggesting the seriousness of the credit crunch at the household level.
Comparative Study of SVD and QRS in Closed-Loop Beamforming Systems
We compare two closed-loop beamforming algorithms, one based on singular
value decomposition (SVD) and the other based on equal diagonal QR
decomposition (QRS). SVD has the advantage of parallelizing the MIMO channel,
but each of the sub-channels has different gain. QRS has the advantage of
having equal diagonal value for the decomposed channel, but the subchannels are
not fully parallelized, hence requiring successive interference cancellation or
other techniques to perform decoding. We consider a closed-loop system where
the feedback information is a unitary beamforming matrix. Due to the discrete
and limited modulation set, SVD may have inferior performance to QRS when no
modulation set selection is performed. However, if the selection of modulation
set is performed optimally, we show that SVD can outperform QRS.Comment: Milcom 200
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