Empirical models for rain fade time on Ku- and Ka-band satellitelinks

Rain attenuation data from the OLYMPUS satellite beacon measurements are used to investigate fade time in the Ku and Ka bands. Using statistical procedures, an empirical model is developed which predicts fade time as a function of attenuation level, frequency of operation, and fade duration interval. Total annual fade times are predicted over a frequency span of 12-30 GHz for attenuation levels in the 3-18 dB range and for fade duration intervals of 30-60 s, 60-120 s, 2-5 min, and 5-20 min. The predicted fade times are in good agreement with the measured values. An alternate model, described by two simple relationships in two different ranges of attenuation level, is also presented by simplifying the original single-equation model. The simplified model accounts for fades associated with stratiform rain and thunderstorms separately     

Efficient adaptive forward error control schemes for Ka-band satellite systems

Adaptive fade margin is required to counter the severe but varying rain attenuation in K_a-band satellite communications. In a search for a suitable rain countermeasure, the effectiveness of adaptive forward error control (AFEC) schemes is studied. Two AFEC schemes using convolution codes and concatenated codes are proposed and their performances analysed. The schemes can provide progressively adaptive fade margins of 10-1 and 10-4 dB, respectively, in excess of the system's fixed fade margin. To improve the efficient use of shared resources of the system, an AFEC scheme using double coding is introduced and its performance analysed. In this scheme a single codec is used repeatedly, resulting in reduced hardware cost and increased use. By using convolutional code and Golay code the above scheme can provide adaptive fade margins of 10°8 dB and 8°1 dB, respectively. Although concatenated codes have slightly better performance than double codes, the hardware implementation and decoding complexity of the latter are significantly less. Implementation complexity of AFEC resource sharing in the light of code complexity, time frame expansion, link condition monitoring and signalling is discussed in detail.     

Crossing rates and fade durations for diversity-combining schemes over ?-? fading channels

This paper derives exact expressions for the level crossing rate and average fade duration of multibranch pure-selection, equal-gain, and maximal-ratio combiners operating over independent non-identical alpha-mu (also called generalized Gamma or Stacy) fading channels. The derived expressions are in closed form for pure-selection combining and in integral form for equal-gain and maximal-ratio combining. For the two latter schemes, accurate closed-form approximations are then provided. The analytical results are validated by reducing the general expressions to known particular cases and, more generally, by means of simulation. Numerical examples are also given to illustrate the high accuracy of the proposed closed-form approximations.     

Throughput analysis for two ARQ schemes using combined transitionmatrix

The authors investigate throughput efficiencies for two basic automatic-repeat-request (ARQ) strategies, go-back-N and selective repeat disciplines, assuming that forward and backward channels have Markov error characteristics and they are mutually independent. For the two ARQ schemes, simple analytical formulas for the throughput efficiency are obtained. Particularly, in the case of the go-back-N scheme the authors show that the result of throughput analysis can be extended to a general form that includes results presented by C.H.C. Leung et al. (1988)     

Real-time prediction of attenuation for applications to fade countermeasures in satellite communications

The author has simulated the behaviour of a fade countermeasure control system that predicts the attenuation level on the satellite link on the basis of previous attenuation samples, using the experimental rain attenuation data measured at 11.6 GHz in three Italian stations with the Sirio satellite. Some dynamic characteristics of fading are investigated and the performances of prediction algorithms based on linear regression are evaluated.<>     

Security analysis and improvement of two attribute-based signature schemes

In order to overcome the drawbacks of current attribute-based signature (ABS) schemes in terms of security,efficiency and signing policy,Ma,et al.and Cao,et al.respectively proposed a threshold ABS with single attribute au-thority and a multi-authority ABS with signing policy supporting AND,OR,threshold gates,and presented the security proof of their schemes under computational Diffie-Hellman assumption.Both schemes were demonstrated have security pitfalls by presenting specified attacks against them.Specifically,their schemes are all vulnerable to forgery attack.Thus,they are not feasible for practical applications.In addition,the cause of the flaws in these ABS schemes are presented,as well as an improvement of Ma et al.'s scheme.     

Comparative analysis on the performance of two concatenated codes for high-speed long-haul optical communication systems

After the development trend of high-speed long-haul optical communication systems and the theory of the concatenated code are analyzed,the comparative researches on the performances of the two concatenated codes of the inner-outer type and the improved interleaving type are performed in detail.The theoretical analyses and simulation results show that the inner-outer type concatenated code has the greater redundancy,and the improved interleaving type concatenated code is a superior concatenated code with the advantages of the better error correction performance,moderate redundancy and easy implementation.As a result,the improved interleaving type concatenated code can be better used in high-speed long-haul optical communication systems.     

Modeling and performance analysis for soft handoff schemes in CDMA cellular systems

This paper investigates the features of a cellular geometry in code-division multiple-access (CDMA) systems with soft handoff and distinguishes controlling area of a cell from coverage area of a cell. Some important characteristics of the cellular configuration in soft handoff systems are used to propose a new design of efficient call admission control (CAC) in CDMA systems. Then, the paper constructs a continuous-time Markov chain (CTMC) model for CAC in CDMA with a soft handoff queue, obtains closed-form solutions, and thus develops loss formulas as performance indices such as the new blocking probability and the handoff dropping probability. In order to determine handoff traffic arrival rate, a fixed-point strategy is developed. Algorithms are also provided to stably compute loss probabilities and to determine the optimal number of guard channels. A new soft handoff scheme-eliminating pseudo handoff calls (EPHC)-is proposed to improve channel utilization efficiency based on mobility information. As an application of the loss formulas, the proposed modeling techniques are used to evaluate and compare the performance of conventional and proposed EPHC soft handoff schemes. Numerical results show the effectiveness of the proposed Markov chain models and the benefits of the new soft handoff scheme.     

Statistical analysis of rain fade slope

An analysis is made of the measured distributions of the fade slope of rain attenuation, conditional for attenuation values, measured at Eindhoven University of Technology from the satellite Olympus. It is found that the distribution is similar for positive and negative fade slopes and independent of frequency in the range from 12 to 30 GHz. A distribution model for the conditional distribution is found. The only parameter of the distribution is the standard deviation, which is found to be proportional to attenuation level and dependent on rain type, on the low-pass filter bandwidth and on the time interval used in the slope calculation. The observed relation between the standard deviation and attenuation is compared with results from other measurement sites. From this comparison it is found that the fade slope standard deviation is likely to depend on elevation angle and on climate, through its dependence on rain type.     

Performance analysis of two serial-search acquisition schemes for FH spread spectrum signals

Initial synchronization (acquisition) schemes of frequency hopped (FH) spread spectrum signals often make use of serial-search acquisition techniques. Two FH serial-search acquisition schemes are introduced and analysed. These schemes are the fixed dwell-time with false alarm memory (FDT/FAM) and the double dwell-time with binary integration (DDT/BI) scheme. Statistical properties namely, probability density function, characteristic function and mean value of the random acquisiton time of each acquisition scheme are obtained. The performances of the FDT, FDT/FAM and DDT/BI acquisition schemes are established and compared in the presence of partial band noise jamming, partial band tone jamming and additive white gaussian noise (AWGN). The results indicate that the DDT/BI acquisition scheme outperforms the FDT and FDT/FAM acquisition schemes.     

Call admission control schemes and performance analysis in wirelessmobile networks

Call admission control (CAC) plays a significant role in providing the desired quality of service in wireless networks. Many CAC schemes have been proposed. Analytical results for some performance metrics such as call blocking probabilities are obtained under some specific assumptions. It is observed, however, that due to the mobility, some assumptions may not be valid, which is the case when the average values of channel holding times for new calls and handoff calls are not equal. We reexamine some of the analytical results for call blocking probabilities for some call admission control schemes under more general assumptions and provide some easier-to-compute approximate formulas     

Delay analysis for interlan traffic using two suitable TDMA satellite access schemes

Two demand assignment time division multiple access (TDMA) satellite access schemes that are suitable for providing flexible interconnections of local area networks are compared by means of two simulations; Fifo Ordered Demand Assignment/Information Bit Energy Adaptive (FODA/IBEA) and Combined Fixed Reservation Assignment (CFRA). A unique simulation tool was used, a meshed network of very small aperture terminals (VSATs) under the control of a master station was considered and similar networking conditions were applied to both access schemes. The generated traffic had two components with different level of burstiness; bulk traffic and interactive traffic. The performance evaluated is the delay for each traffic component. We show that CFRA is best when connecting clusters of only a few stations and even individual stations exchanging light traffic. On the other hand, FODA/IBEA is better suited to interconnect networks, or within networks with many hosts, where heavy traffic is more likely. © 1997 John Wiley & Sons, Ltd.     

A performance analysis of trellis-coded modulation schemes overRician fading channels

This paper presents a saddle point approximation (SAP) method to compute the pairwise error probability (PEP) of trellis-coded modulation (TCM) schemes over Rician fading channels. The approximation is applicable under several conditions, such as finite and ideal interleaving, ideal coherent and pilot-tone aided detection, and differential detection. The accuracy of this approximation is demonstrated by comparison to the results of numerical integration. When ideal interleaving is assumed, an asymptotic approximation for the PEP of ideal coherent, pilot-tone aided or differentially detected TCM is derived. This asymptotic approximation of the PEP is in a product form and much tighter than the ordinary Chernoff bound on the PEP. Also, based on the SAP, the effect of finite interleaving depth on the error performance of TCM schemes over Rician and shadowed Rician channels is studied     

Comparative performance analysis of adaptive multispectraldetectors

The fully adaptive hypothesis testing algorithm developed by I.S. Reed and X. Yu (1990) for detecting low-contrast objects of unknown spectral features in a nonstationary background is extended to the case in which the relative spectral signatures of objects can be specified in advance. The resulting background-adaptive algorithm is analyzed and shown to achieve robust spectral feature discrimination with a constant false-alarm rate (CFAR) performance. A comparative performance analysis of the two algorithms establishes some important theoretical properties of adaptive spectral detectors and leads to practical guidelines for applying the algorithms to multispectral sensor data. The adaptive detection of man-made artifacts in a natural background is demonstrated by processing multiband infrared imagery collected by the Thermal Infrared Multispectral Scanner (TIMS) instrument     

过采样扫描探测性能对比分析

分析了单采样和过采样两种扫描探测体制的成像原理,并建立了扫描图像仿真模型.在不同云层杂波场景下从目标能量收集、背景杂波抑制和弱小目标检测三个方面对单采样和过采样扫描图像进行了对比分析.理论分析和仿真结果表明:过采样扫描探测体制更有利于收集目标能量,杂波抑制能力较优,对弱小目标的探测能力更强.     

过采样扫描探测性能对比分析

分析了单采样和过采样两种扫描探测体制的成像原理,并建立了扫描图像仿真模型。在不同云层杂波场景下从目标能量收集、背景杂波抑制和弱小目标检测三个方面对单采样和过采样扫描图像进行了对比分析。理论分析和仿真结果表明：过采样扫描探测体制更有利于收集目标能量,杂波抑制能力较优,对弱小目标的探测能力更强。     

On the analysis of the performance of coded modulation schemes for unequal error protection

A method of analysis for comparing the performance of time-division-coded modulation and superposition-coded modulation schemes with unequal error protection has been proposed. The performance of practical channel codes was modeled heuristically by using a signal-to-noise ratio loss factor /spl lambda/ and it was shown that the superiority of superposition-coded modulation depends on the degree of inequality in protection prescribed. A different approach, using the channel cutoff rate R/sub 0/ instead of /spl lambda/, is proposed here. Results of the analysis of the performance of both approaches are compared, and they are also compared to those results obtained from practical design examples.     

EPON接入保护架构及其性价比分析

高性价比的保护架构对EPON接入网安全稳定和经济有效的运行具有重要作用.文章首先概述了现有EPON接入保护架构类型,并且通过可靠性框图计算分析了各种保护架构的性能价格参数;最后结合电力系统应用环境,给出了满足电力系统通信业务需求的EPON接入最佳保护架构.