Fuqin Xiong

Distinct brain activation patterns for human maximal voluntary eccentric and concentric muscle actions.

Eccentric muscle contractions generate greater force at a lower level of activation and subject muscles to more severe damage than do concentric actions. A recent investigation has revealed that electroencephalogram (EEG)-derived movement-related cortical potential (MRCP) is greater and occurs earlier for controlling human eccentric than concentric submaximal muscle contractions. However, whether the central nervous system (CNS) control signals for high-intensity or maximal-effort eccentric movements differ from those for concentric actions is unknown. The purpose of this study was to determine whether the MRCP signals differ between the two types of maximal-effort contractions. Eight volunteers performed 40 maximal voluntary eccentric and 40 maximal voluntary concentric elbow flexor contractions on a Kin-Com isokinetic dynamometer. Scalp EEG signals (62 channels) were measured along with force, joint angle, and electromyographic (EMG) signals of the performing muscles. MRCP-based two-dimensional brain maps were created to illustrate spatial and temporal distributions of the MRCP signals. Although the level of elbow flexor muscle activity was lower during eccentric than concentric movements, MRCP-indicated cortical activation was greater both in amplitude and area dimension for the eccentric task. Detailed comparisons of individual electrode signals suggested that eccentric movements needed a significantly longer time for early preparation and a significantly greater magnitude of cortical activity for later movement execution. The extra preparation time and higher amplitude of activation may reflect CNS activities that account for the higher risk of injury, higher degree of movement difficulty, and unique motor unit activation pattern associated with maximal-level eccentric muscle actions.

M-ary amplitude shift keying OFDM system

Coherent M-ary amplitude-shift keying (MASK) is proposed for use in orthogonal frequency-division multiplexing (OFDM) systems. The frequency separation between subcarriers is only 1/2T instead of 1/T. With a slightly wider bandwidth, an /spl radic/M-ary ASK OFDM can achieve the same bit-error rate (BER) of M-ary quadrature amplitude modulation (QAM) OFDM and a better BER than that of M-ary phase-shift keying (MPSK) OFDM. The /spl radic/M-ary ASK OFDM has the same peak-to-average-power ratio as that of the M-ary QAM OFDM. The MASK OFDM can be implemented digitally and efficiently by fast cosine transform and demodulated by inverse fast cosine transform. Comparisons show that implementation complexity is reduced for additive white Gaussian noise channels with the use of the new scheme.

Modem techniques in satellite communications

Since the mid 1980s, research and development in satellite modem techniques has been very active and has yielded promising results. In the article the author presents a summary of the current status of work in this area, and discusses the directions this work is taking. >

M-ary energy detection of a Gaussian FSK UWB system

The energy detection M-ary Gaussian frequency-shift keying (FSK) system is proposed in this paper. The system performance is analyzed in additive white Gaussian noise channels, multipath channels, and in the presence of synchronization errors. The numerical results show that the M-ary modulation achieves the higher data rate than the binary modulation. However, it also results in performance degradation.

Performance of MHPM in Rician and Rayleigh fading mobile channels

This paper evaluates the error probability of the rectangular frequency pulse multi-h modulation (MHPM) scheme in slowly-fading, frequency-nonselective or frequency-selective Rician and Rayleigh channels. The evaluation is performed with a method combining analysis and computer simulation. Performance degradations are evaluated for various direct-to-reflected signal ratio, Doppler shifts, and relative time delays in Rician fading channels. Compared with minimum shift keying (MSK), MHPM schemes appear to have retained their coding gains.

UWB system based on energy detection of derivatives of the Gaussian pulse

A new method for energy detection ultra-wideband systems is proposed. The transmitter of this method uses two pulses that are different-order derivatives of the Gaussian pulse to transmit bit 0 or 1. These pulses are appropriately chosen to separate their spectra in the frequency domain. The receiver is composed of two energy-detection branches. Each branch has a filter which captures the signal energy of either bit 0 or 1. The outputs of the two branches are subtracted from each other to generate the decision statistic. The value of this decision statistic is compared to the threshold to determine the transmitted bit. This new method has the same bit error rate (BER) performance as energy detection-based pulse position modulation (PPM) in additive white Gaussian noise channels. In multipath channels, its performance surpasses PPM and it also exhibits better BER performance in the presence of synchronization errors.

The development of aeronautical mobile satellite services over the past thirty years

This paper presents an overview of the development of aeronautical mobile satellite services (AMSS) over the past 30 years. The inherent shortcomings of present air-ground HF communications have hindered the development of civil aviation, but according to the Future Air Navigation Systems (FANS) concept aeronautical satellite communication-including Automatic Dependent Surveillance (ADS)-will be the key to eliminating the shortcomings of HF communication systems. Satellite-based communication and surveillance will significantly improve air traffic control (ATC) over the oceanic and remote terrestrial airspace, and it will benefit civil aviation authorities, airlines as well as passengers. This paper discusses the availability of system elements, and world wide trials, demonstrations and preoperational use of aeronautical satellite communications over past years are described. Future satellite systems possible for aeronautical communications are also discussed. >

An all-digital clock-smoothing technique - counting-prognostication

This article presents a novel universal all-digital clock-smoothing technique - counting-prognostication. Operation principles, performance analysis, and comparisons are given. Analysis and measurement results show that this technique can efficiently smooth jitter and wander for a wide pull-in range and pull-out range, and jitter accumulation is small. A cycle-varying counting-prognostication method, which decreases pull-in time, is also suggested.

Coding, decoding, and recovery of clock synchronization in digital multiplexing system

High-speed broadband digital communication networks rely on digital multiplexing technology where clock synchronization, including processing, transmission, and recovery of the clock, is the critical technique. This paper interprets the process of clock synchronization in multiplexing systems as quantizing and coding the information of clock synchronization, interprets clock justification as timing sigma-delta modulation (T/spl Delta/-/spl Sigma/M), and interprets the jitter of justification as quantization error. As a result, decreasing the quantization error is equivalent to decreasing the jitter of justification. Using this theory, the paper studies the existing jitter-reducing techniques in transmitters and receivers, presents some techniques that can decrease the quantization error (justification jitter) in digital multiplexing systems, and presents a new method of clock recovery.

Sliding window symbol timing synchronizer

A novel non-data-aided symbol timing synchronizer for M-ary phase shift keying (MPSK) modulation schemes is studied. It calculates the post-demodulation baseband signal energy in a sliding window. If the symbol timing is correct, the average signal energy will be the maximum. Based on this principle, the synchronizer detects the phase errors and/or frequency errors. The synchronizer structure is designed; its operational principles are described. Simulation is performed to evaluate the performance and results are presented in this paper. Simulation results show that the synchronizer can work for QPSK at signal-to-noise ratio (E/sub b//N/sub 0/) as low as -3 dB (E/sub s//N/sub 0/ = 0 dB), with a near zero phase error.