In this paper, effects of cyclic prefix (CP) are investigated for orthogonal frequency division multiplexing (OFDM) systems over time-varying channels. Although employing a CP as the guard interval is a simple way to combat the inter-symbol interference (ISI) and the inter-carrier interference (ICI), it reduces the transmission efficiency of the system, especially for channels with long channel-impulse response (CIR), such as hilly terrain channels. On the other hand, the time-varying channel also gives rise to ICI. We show that to achieve a signal-to-interference ratio of 20 dB, for slow fading channels, half the length of CIR is enough for CP. For fast fading channels, the Doppler frequency become domination factor to affect OFDM systems
Cyber-security research on networked multi-sensor systems is crucial due to the vulnerability to various types of cyberattacks. For the development of effective defense measures, attention is required to gain insight into the complex characteristics and behaviors of cyber attacks from the attacker’s perspective. This paper aims to tackle the problem of distributed consensus estimation for networked multi-sensor systems subject to hybrid attacks and missing measurements. To account for both random denial of service (DoS) attacks and false data injection (FDI) attacks, a hybrid attack model on the estimator-to-estimator communication channel is presented. The characteristics of missing measurements are defined by random variables that satisfy the Bernoulli distribution. Then a modified consensus-based distributed estimator, integrated with the characteristics of hybrid attacks and missing measurements, is presented. For reducing the computational complexity of the optimal distributed estimation method, a scalable suboptimal distributed consensus estimator is designed. Sufficient conditions are further provided for guaranteeing the stability of the proposed suboptimal distributed estimator. Finally, a simulation experiment on aircraft tracking is executed to validate the effectiveness and feasibility of the proposed algorithm.
In this paper we propose a novel PML (partial maximum likelihood) receiver with instantaneous SNR-based subspace search for multistream MIMO (multi-input multi-output) systems. The soft-decision outputs of the conventional ZF receiver are used as the initial estimates for PML. In contrast to the OSIC (ordered successive interference cancellation) making only one hard decision based on soft-decision estimates, the proposed PML receiver set up a subspace for ML search with multiple candidate constellation points. The number of candidates in the subspace search is based on the instantaneous SNR at each data stream. Finally, the hard-decision estimates can be obtained by minimizing the ML function over the subspace search. Numerical results proved that when the number of transmit antennas is less than 4, the proposed PML considerably outperforms the conventional OSIC with approximately the same or even less complexity.
In order to improve the precision of attitude estimation system, paper presents a signal extraction method of MEMS Gyroscope called VSCS-LMS algorithm based on FPGA. The design is established with the S-shaped function of sigmoid to reduce the sensitivity to noise of LMS algorithm. And its FPGA-based system structure is synthesized and simulated on Virtex II FPGA by reusing the single-order filter unit to complement the multi-order accumulation. The signal extraction simulation of MEMS gyroscope shows that it is reliable, high precision and high degree of real-time.
This paper investigates inter-cell interference mitigation of non-orthogonal multiple access (NOMA) in the cellular downlink. A novel adaptive power and frequency resource allocation algorithm in the NOMA system is proposed for performance enhancement. The objective of the algorithm is to improve both the average cell throughput and cell-edge throughput through adaptive resource allocation in the power and frequency domains. The proposed algorithm improves the cell-edge throughput by providing better protection against inter-cell interference in the power domain while increasing the average cell throughput by providing more resources in the frequency domain. Numerical results show that the proposed algorithm yields significant improvements in the cell average throughput and cell-edge throughput by reducing the interference for cell-edge users and expanding the bandwidth for cell interior users.
Vertical Bell laboratory layered space-time (V-BLAST) is one of the promising multiple input multiple output (MIMO) architectures. Layered space time processing in V-BLAST requires the number of receive antennas to be equal to or larger than the number of transmit antennas. However, it is impractical for small-sized mobile units to accommodate all the required antennas. Focusing on this problem, we have presented a novel concept of virtual receive antennas (VRA) for spatial multiplexing system in flat fading channels. In that work, time domain equalization is utilized to eliminate inter-symbol interference, which is generated in the VRA. However, the employed time domain equalizer has a high computational complexity. In this paper, the VRA system with low complexity frequency domain equalizer (FDE) is investigated. Moreover, as the noise in the VRA system is correlated due to oversampling, we also present the noise decorrelating filter design. Through complexity and performance evaluation, FDE is demonstrated to be more suitable for the VRA system.
Orthogonal frequency division multiplexing (OFDM) is the projected modulation of choice for fourth-generation broadband multimedia wireless systems. However, for mobile applications, channel variations during one OFDM symbol introduce intercarrier interference (ICI), which degrades the performance. This gets more severe as mobile speed, carrier frequency or OFDM symbol duration increases. We analyze ICI for mobile OFDM systems in terms of the complex weighting coefficients, which give the contribution of each transmitter subcarrier to each demodulated subcarrier. Then, we propose a new cancellation method to reduce the effects of Doppler frequency shift. The results show that by using the proposed cancellation scheme, the ICI can be reduced significantly.
Non-orthogonal multiple access (NOMA) is a promising radio access technique for further cellular enhancements toward the 5th generation (5G) mobile communication systems. Single-user multiple-input multiple-output (SU-MIMO) is one of the key technologies in long term evolution (LTE) /LTE-Advanced systems. It is proved that NOMA combined with SU-MIMO techniques can achieve further system performance improvement. In this paper, we focus on the impact of rank optimization on the performance of NOMA with SU-MIMO in downlink. Firstly, a geometry based rank adjustment method is studied. Secondly, an enhanced feedback method for rank adjustment is discussed. The simulation results show that the performance gain of NOMA improves with the proposed two rank adjustment methods. Compared to orthogonal access system, large performance gains can be achieved for NOMA, which are about 23% for cell average throughput and 33% for cell-edge user throughput.
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)