Forcing Criterion Research Articles

FrFT equalization for underwater sweep-spread communication system

In underwater acoustic channels, multipath propagation and Doppler frequency can distort the received signal and reduce system reliability. In order to effectively combat multipath interference, the Sweep-Spread Carrier (SSC) system using frequency swept signal is proposed, however, the SSC receiver performs well only when the maximum delay spread of path arrivals and the Doppler spread are moderate. In this paper, fractional Fourier transform (FrFT) is applied to channel equalization of the SSC system, which improves the ability to resist the Doppler effect and intersymbol interference (ISI). FrFT can simplify the channel characteristics by selecting an appropriate transform order. We analyze the FrFT equalizer in the SSC system under the zero forcing (ZF) criterion and minimum mean squared error (MMSE) criterion. Compared with typical nonlinear Turbo equalization, FrFT equalizer only requires lower complexity to achieve good performance. The numerical simulation results in bellhop and ray tracing channels and the results of the Wu-yuan Bay test channel show the high robustness of the proposed method in doubly dispersive channels.

Optimal excitation mechanism for combustion enhancement of supersonic shear layers with pulsed jets

Supersonic shear layers (convection Mach number = 0.6) with flow direction excitation are studied by a canonical Large Eddy Simulation (LES) method with quasi-laminar turbulent reaction model. By modulating velocity of central jet through ideal square wave under four excitation periods (τ = 20, 40, 60, and 80 μs), the present study aims to explore the optimal excitation mechanism for combustion enhancement. As the excitation period increases, combustion efficiency tends to increase and then decrease. The maximum combustion efficiency is achieved at τ=40μs, which is nearly four times that in the no excitation scenario. The analysis of mixing characteristics of the nonreacting cases reveals that mixing enhancement mainly explains combustion gain of different excitation modes. From the limiting formation of vortex, the size of the vortex core has fully developed to reach axial contact at τ=40μs. Any further increase in excitation intervals implies a larger vortex spacing, which leads to poor fluid mixing while lower excitation case forms continuous small vortices with less mixing region. The comparison of the circulation decomposition of the main vortex and trailing wakes indicates the pinch-off characteristic of the limiting formation of vortical structure. On the basis of these results, an optimal forcing criterion is proposed according to the limiting vortex formation with dimensionless excitation time scale τ∗≈1.64. This study provides a novel perspective for the optimal combustion enhancement of supersonic shear layers from the aspect of vorticity dynamics.

Cooperative In-Home Power Line Communication: Analyses Based on a Measurement Campaign

This work focuses on analyses of cooperative protocols to enhance the performance of power line communication systems. Based on a measurement campaign and considering a sum power constraint, achievable data rates for amplify-and-forward (AF) and decode-and-forward (DF) protocols are analyzed. Similar investigations are performed for the maximum data rates attained using Hermitian-symmetric orthogonal frequency division multiplexing (HS-OFDM) together with equal gain combining (EGC), selection combining (SC) and maximal ratio combining (MRC) techniques. The influences of optimally and uniformly allocated transmission power and frequency bandwidth are are also analyzed and the efficiency of combination before and after equalization is compared. Results show that the relative distances among source, relay and destination nodes significantly impact system performance. Also, they reveal a range of total transmission power and bandwidth in which benefits can be verified. Among combining techniques, MRC and SC present similar results, but MRC offers a slightly better performance. In relation to computational complexity, SC is the most favorable. Maximum data rate analyses of HS-OFDM with frequency domain equalization based on zero forcing and minimum mean square error criteria show that the former scheme offers almost the same performance as the latter. Furthermore, it is shown that equalization after combination is more advantageous.

Patrones históricos y escenarios térmicos futuros en mares mexicanos

Into different areas (e.g., academic, public) predictions of climate as part of the process of decision-making are required. Despite such information need, the inconsistency of global models to predict the state of the climate in small scales (regions) is widely recognized. Considering this, we tested predictions of sea surface temperature (SST) in 10 marine regions off the coast of Mexico. Using classification and regression trees, Mexican coastal states were grouped accordingly to their similarity in instrumental records of air temperature (AST). Such AST groups were considered explanatory variables together with regional climatic scale indices (e.g., Pacific Decadal Oscillation, PDO). Historical patterns of change (period, amplitude and phase) of AST and climate indices were characterized, and then its relationship with SST was analyzed using generalized additive models (GAM). The SST response to climatic scenarios was evaluated with 3 different forcing criteria. The GAM models showed significant fits and relatively high values of R² and deviance. Projections of regional climate variability showed substantial differences in comparison to the monotonic increase in SST global models outputs. The re-scaling strategy applied in this work for Mexican seas surface temperature, proved to be useful to integrate the historical variation with different forcing criteria.

Optimal joint power allocation and sub‐carrier pairing in amplify‐and‐forward SC‐FDMA relaying networks

Optimal power allocation (PA) and sub-carrier pairing (SP) strategies for a dual-hop amplify-and-forward single-carrier frequency division multiple access network are derived. Frequency domain equalisers based on minimum mean square error (MMSE) and zero forcing (ZF) criteria are considered. For each equaliser, the problem of joint PA and SP is solved to maximise the system capacity subject to the total network power constraint. The optimality of ordered SP (OSP) and reverse OSP scenarios for MMSE and ZF equalisers, respectively, is proved.

Single-Carrier Cyclic Prefix-Assisted PLC Systems With Frequency-Domain Equalization for High-Data-Rate Transmission

Achieving reliable high-speed communications over the power line requires robust modulation and equalization techniques to mitigate various hostile channel impairments. Multicarrier orthogonal frequency-division multiplexing (OFDM) has been proposed as the dominant modulation format for power-line communications (PLC). In this paper, we study a single-carrier cyclic prefix (SCCP)-assisted PLC system that employs block data transmission similar to OFDM and propose two frequency-domain equalizers based on the zero forcing and minimum mean-square error criteria. Performance comparisons with OFDM systems of identical system specifications are made via extensive computer simulations under different realistic channel parameters and data rates. The results show that the SCCP-assisted scheme outperforms the OFDM counterpart in most cases through higher frequency diversity exploitation while maintaining almost identical computational complexity. Other salient features of the SCCP technique for PLC applications are discussed. The proposed SCCP-assisted scheme is found to be an attractive alternative modulation technique for broadband PLC.

Ordered Successive Interference Cancellation (OSIC) in V-BLAST Systems with Asynchronous Transmission Mode

In this paper, we first present a successive interference cancellation (SIC) architecture for the V-BLAST system with asynchronous transmission mode, which is proposed in Shao and Tang (IEEE Trans Veh Technol 56(6):3827---3837, 2007) and characterized by achieving maximum possible diversity only by using a simple linear detection scheme under zero forcing (ZF) criterion. The SIC architecture that intuitively derived form the mathematical expression of the sampled output vector of a bank of matched filters at each receive antenna has a very low complexity, however, it can not employ the optimal detection order to minimize the propagation of errors from one step of detection to the next. Furthermore we find that it actually consumes extra energy of the previously detected signals to recover a signal. Therefore a ordered successive interference cancellation (OSIC) architecture is subsequently proposed to overcome the two shortcomings mentioned above. Simulations demonstrate that the OSIC in the asynchronous V-BLAST system can give much better performance at a little cost of complexity than it dose in conventional synchronous scheme.

Robust Linear Beamforming for MIMO Relay Broadcast Channel With Limited Feedback

This letter considers the relay-assisted multiuser downlink transmission with limited feedback in a wireless cellular network. Taking into account the channel quantization errors, we propose two robust linear beamforming schemes at the relay station based on zero forcing (ZF) criterion and minimum mean square error (MMSE) criterion, respectively. It is noted that the channel direction information (CDI) feedback is sufficient for determining the beamforming vectors in our proposed schemes, which reduces the feedback load. Simulation results show that compared with the conventional ZF beamforming and MMSE beamforming in limited feedback systems, the proposed precoding schemes achieve an improvement in the average bit error rate (BER) performance of the system.

Successive interference cancellation (SIC) in V-BLAST systems with asynchronous transmission mode

The V-BLAST system with asynchronous transmission mode first proposed by Shao can achieve full diversity only by using a simple linear detection scheme under zero forcing (ZF) criterion; therefore it gives a reasonable tradeoff between complexity and performance. In this paper, we propose two types of successive interference cancellation (SIC) detection schemes for the asynchronous V-BLAST system, one is characterized by applying M r successive interference cancellators before a maximal ratio combiner (where M t is the number of receive antennas), and the other has a maximal ratio combiner before a successive interference cancellator. Since Type I consumes more energy of the previously detected signals to recover a signal, Type II can offer a better performance and simulations demonstrate its validity.

On the Design of Optimal Training Sequence for Bi-Directional Relay Networks

In this letter, the problem of training sequence design in a bi-directional relay network is investigated. The two nodes, Tx1 and Tx2, communicate with each other via a relay (RL). The RL receives the signals that are transmitted simultaneously from both nodes and then broadcasts the superposition back to the two nodes. Single carrier cyclic prefix (SCCP) is deployed in the transmission from the two nodes and the RL as well for combating intersymbol interference (ISI). We are interested in the channel estimation problem in this scenario. We propose a design of training sequences from two nodes to minimize the mean-square error (MSE) of the channel estimation according to zero forcing (ZF) criterion. We do not separate the links between each node to RL but incorporate them into the composite channels. Simulation results show that our design performance suffers only 1-dB loss in terms of bit-error rate (BER) as compared to the case with perfect channel information.

Joint transceiver vector precoding based on GMD method for MIMO systems

We heuristically propose a joint transceiver vector precoding (VP) based on geometric mean decomposition (GMD) method for multiple input multiple output (MIMO) systems. In fact, it is the generalization and improvement of the GMD transceiver with Tomlinson-Harashima precoding (THP). We first propose the GMD-VP design with zero forcing (ZF) criterion, then exploit the extended channel matrix to obtain the improved regularized scheme. Simulation results show that the proposed schemes obtain performance improvement and complexity reduction.

Tomlinson–Harashima precoding with imperfect channel state information

Nonlinear Tomlinson–Harashima precoding (THP) is an attractive solution for a scenario where the transmission system employs multiple antennas at transmitter and multiple users with a single antenna at the receiver, so that the cooperation among the receive antennas are impossible (downlink scenario). THP solution based on zero forcing (ZF) and minimum mean square error (MMSE) criteria is one of the important techniques to achieve near multiple input multiple output channels capacity with reasonable complexity. In this paper, the effect of channel imperfection on THP is considered. At first, the achievable rate of THP with respect to ZF criterion in an imperfect channel state information (CSI) scenario is calculated. Moreover, based on MMSE criterion, a new robust solution is derived which provides a significant improvement with respect to the conventional optimisation method. Then, the effect of channel estimation error on THP is considered as an improved optimisation where THP filters are optimised together with a channel estimator. Spatial power loading is found to be important to the THP performance. This loading for robust/joint optimisation of MMSE THP is developed by minimum average symbol error rate sense. Simulation results show the capacity loss, the performance advantage attained by the robust/joint optimisation and the power loading in an imperfect CSI scenario.