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Articles by Xuejun Sha
Total Records ( 8 ) for Xuejun Sha
  Xinyu Zhang , Xuejun Sha and Ali Nawaz Khan
  The extremely narrow pulses with the order of nanoseconds and multipath in Time Hopping Ultra Wide Band system result in long mean acquisition time as well as low acquisition precision. A novel Variable Step Acquisition scheme, which aims to improve both mean acquisition time and acquisition precision, is proposed. The proposed acquisition scheme employs nonconsecutive search and adjusts searching steps according to several thresholds, which makes the acquisition process a fast acquisition with variable steps, i.e., from large steps to small ones. The mean acquisition time performances of the proposed scheme and conventional acquisition schemes are evaluated and compared. It is evident that proposed acquisition scheme significantly outperforms the conventional ones.
  Xinyu Zhang , Xuejun Sha and Ali Nawaz Khan
  The influence of clock frequency offset between transmitter and receiver of IR UWB (Impulse Radio Ultra Wide Band) system is talked about in this study. Firstly, this study highlight the fact that clock frequency offset in UWB system will decrease the performance of the coherent receiver and then the necessity of frequency offset estimation before acquisition step is stated. To improve the acquisition failure situation caused by frequency offset, a new acquisition structure with frequency offset estimation, as well as a new method to estimate frequency offset, is proposed. The scheme employs the characteristics of the symmetry and linearity of correlation to approximate the frequency offset in a range of searching area and has good accuracy. As the simulation results reveal, the proposed acquisition structure with frequency offset estimation outperforms the traditional acquisition system and the acquisition failure caused by frequency offset is improved greatly.
  Wei Wu , Zhongzhao Zhang , Xuejun Sha and Chenguang He
  Some auto rate protocols at the MAC layer have been proposed to improve the throughput of Ad Hoc networks with multiple rates support at physical layer. However, all of them neglect the influence of network congestion. The network performance will deteriorate as a result of transmitting data to a congested node. In this study, an auto rate protocol based on congestion detection called auto rate based on congestion detection (ARCD) is proposed. In the ARCD protocol, congestion level is detected at the receiving nodes and fed back to the sending nodes along with the rate selection information and then the sending nodes transmit a limited number of back-to-back packets at appropriate rates. The simulation results show that the ARCD protocol can not only improve the throughput and packet delivery ratio of Ad Hoc networks by taking full advantage of channel condition, but also achieve hop-by-hop congestion control.
  Bo Ding , Lin Mei and Xuejun Sha
  By discussing the development of 4-WFRFT, a discrete sequence algorithm for 4-WFRFT based on DFT is introduced and this makes it possible to use 4-WFRFT in a discrete time system. On the basis of analyzing the rotating, squeezing effect to the digital signal constellation by 4-WFRFT process also the similarity of 4-WFRFT signal to Gaussian distribution and the anti-scanning competence of 4-WFRFT signal, an alterable-parameter secure communication system is introduced based on 4-WFRFT secure communication system. Moreover, in the light of frequency-hopping patterns, a dynamic alterable-parameter strategy for single parameter 4-WFRFT and three strategies for multi-parameters 4-WFRFT are designed to avoid a non-receiver’s intercepting and decoding. "Tracking coefficient" is defined to judge the anti-scanning capacity. The results of simulation show that the alterable-parameter 4-WFRFT secure communication system has strong practical significance.
  Yang Cui , Yubin Xu , Rongqing Xu and Xuejun Sha
  The integration of heterogeneous wireless network has become an overwhelming tendency. So, it is a huge challenge for the network designers that how to manage resource in these access networks with different Radio Access Technologies (RATs) in a cooperative mode. As a feasible scheme, Generic Link Layer (GLL) is introduced. In this study, a novel multi-radio packet scheduling algorithm, is proposed for real-time traffic. There are four outstanding contributions in QU. First, in order to guarantee Quality of Service (QoS) of real-time traffic and avoid wasting resource, a scheduling utility function is developed to represent the degree of satisfaction of performing a packet scheduling in term of avoiding wasting resource provided that guaranteeing QoS. Second, using this utility function, a packet scheduling model, which in fact is a NP problem, is designed and analyzed based on joint consideration of fairness. Third, for improving execution rate, Hopfield Neural Network (HNN) is used to fast find an optimal solution for the scheduling model. Final, simulation is set up and is compared with M-LWDF and PLR algorithms to evaluate performance of QU algorithm. Simulation results show that under high load QU algorithm has lower packet loss ratio and higher spectrum efficiency while offers allowable average packet delay.
  Jingning Wang , Xuejun Sha , Linan Sun and Zhongzhao Zhang
  In this study, a cooperative diversity scheme with decoded-and-forward plus amplify-and-forward is proposed by employing truncated stop-and-wait automatic repeat request for error control. All the transmission channels are assumed to exhibit Nakagami-m fading and the cross-layer performance is analyzed for the proposed scheme, such as the channel efficiency in physical layer, throughput and packet loss rate in link layer. The simulation results show that, the proposed scheme has the better cross-layer performance than other cooperative systems. By choosing a suitable partner, the proposed cooperative scheme can provide better performance than non-cooperative systems and spatial diversity gain can be obtained.
  Ali Nawaz Khan , Xuejun Sha and Xinyu Zhang
  In this study, we have presented and tested a method to calculate rate of handoff accurately by accounting for irregularities in cellular coverage area in next generation wireless and cellular communication networks. Traditionally, rate of handoff calculations are based on the assumption that cell boundaries can be represented by uniform geometrical shapes such as circle, square and hexagon. However, in real situations cell boundaries are irregular due to various signal-degradation factors and hence cannot be precisely represented by definite geometrical structures. The difference in calculated and measured values of rate of handoff introduces compounding error in probabilistic analysis of Next Generation Networks and hence, requires the multiplication of an appropriate correction factor with calculated rate of handoff to minimize the error. Extensive experimentation reveal that actual rate of handoff for a cellular environment is greater than the calculated rate by as much as 20% and is proportional to the degree of irregularity in the cell boundary.
  Yue Li , Xuejun Sha , Liang Ye and Xiaojie Fang
  Background: In present communication systems, the transmission signals are considered to be time-invariant and are processed by the time-invariant bandpass filter at the receiver. In this regard, all the signals in the pass band will pass through the filter, including the interferences. Materials and Methods: This study analyzes the time-frequency characteristic of the 4-QAM modulated pulse shaping signal and reveals that the energy concentration of the pulse shaping signal is time-varying, which makes some low-energy areas exist in the pass-band. Based on this characteristic, the masking threshold constrained time-varying filter is proposed and the optimal masking threshold is determined with MMSE criterion. Results: Simulation results show that based on the signal’s time-frequency distribution, the SINR and BER performances of the proposed filter outperform the conventional bandpass filter. Conclusion: According to the 4-QAM modulated pulse shaping signal’s energy concentration nature, the non-orthogonal or inter-cell interferences could be suppressed further by the masking threshold constrained time-varying filter.
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