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851.
852.
地面数字电视广播系统是我国广播电视综合覆盖体系中的重要组成部分,其与卫星数字电视广播系统和有线数字电视广播系统以及其他辅助系统一起相互协同提供全面的受众覆盖.据统计,在全国3.78亿户家庭中,有线电视用户为1.4亿户,其余2.38亿户家庭主要通过无线方式接收广播电视,如加上车载移动电视的收视用户,采用无线接收广播电视的用户规模将更为庞大. 相似文献
853.
854.
基于LDPC编码的IDMA编码和扩频均衡研究 总被引:1,自引:0,他引:1
将一种接近容量限的码型低密度奇偶校验码(LDPC)应用于交织多址接入系统,即基于LDPC编码的IDMA系统;深入研究了与此系统相关的编码和扩频的均衡问题。在高斯白噪声(AWGN)信道中,在以不同码率编码的该系统中,分别以误比特率10-6和10-3为标准,根据要达到的可靠数据和语音通信所需要的最小信噪比(SNR),阐明编码和扩频的均衡问题。仿真结果表明,在AWGN信道中,对于可靠的通信传输服务,基于LDPC编码的IDMA系统传输的最佳LD-PC编码码率在0.4和0.5之间;并且,以这种码率传输的系统相对于其他系统的优势是相当明显的。 相似文献
855.
Hirose T. Saito K. Kojima S. Yao B. Ohsono K. Sato S. Takada K. Ikushima A.J. 《Electronics letters》2007,43(8):443-445
Long-period fibre grating (LPFG) writing by a CO2 laser-annealing using a fibre-drawing process is demonstrated. The fibre in the drawing process was irradiated periodically by a CO2 laser to modify the refractive index. An LPFG with transmission loss of -10 dB and full width at half-maximum of 13 nm has been fabricated. Results show that the refractive index change was induced by stress at the moment of laser annealing 相似文献
856.
In this paper, we study the effect of imperfect channel estimation (ICE) on the performance of M-level quadrature amplitude modulation (M-QAM) with maximum ratio combining (MRC) and pilot-symbol assisted modulation (PSAM) in generalized Rician fading channels. By expressing the bit error rate (BER) of MRC diversity M-QAM in terms of the distribution of new decision variables, we derive novel, exact, and easy-to-evaluate BER expressions for diversity M-QAM with channel estimation errors. Our results include versatile system and fading channel parameters (e.g., arbitrary spatial and temporal correlation patterns among the diversity branches), and are valid for arbitrary linear channel estimators and square and rectangular M -QAM with different constellation sizes. In addition, we evaluate the performance of minimum mean-squared error (MMSE)- and sinc-interpolator-based channel estimators with PSAM, and provide some new insights into the performance of M-QAM with PSAM in generalized fading channels 相似文献
857.
Yang Yubin Lin Hui Zhang Yao 《IEEE transactions on systems, man and cybernetics. Part C, Applications and reviews》2007,37(6):1081-1098
As the number of available 3D models grows, there is an increasing need to index and retrieve them according to their contents. This paper provides a survey of the up-to-date methods for content-based 3D model retrieval. First, the new challenges encountered in 3D model retrieval are discussed. Then, the system framework and some key techniques of content-based 3D model retrieval are identified and explained, including canonical coordinate normalization and preprocessing, feature extraction, similarity match, query representation and user interface, and performance evaluation. In particular, similarity measures using semantic clues and machine learning methods, as well as retrieval approaches using nonshape features, are given adequate recognition as improvements and complements for traditional shape-matching techniques. Typical 3D model retrieval systems and search engines are also listed and compared. Finally, future research directions are indicated, and an extensive bibliography is provided. 相似文献
858.
On Lifetime-Based Node Failure and Stochastic Resilience of Decentralized Peer-to-Peer Networks 总被引:1,自引:0,他引:1
To model P2P networks that are commonly faced with high rates of churn and random departure decisions by end-users, this paper investigates the resilience of random graphs to lifetime-based node failure and derives the expected delay before a user is forcefully isolated from the graph and the probability that this occurs within his/her lifetime. Using these metrics, we show that systems with heavy-tailed lifetime distributions are more resilient than those with light-tailed (e.g., exponential) distributions and that for a given average degree, k-regular graphs exhibit the highest level of fault tolerance. As a practical illustration of our results, each user in a system with n = 100 billion peers, 30-minute average lifetime, and 1-minute node-replacement delay can stay connected to the graph with probability 1 - 1/n using only 9 neighbors. This is in contrast to 37 neighbors required under previous modeling efforts. We finish the paper by observing that many P2P networks are almost surely (i.e., with probability 1 - o(1)) connected if they have no isolated nodes and derive a simple model for the probability that a P2P system partitions under churn. 相似文献
859.
On Probing Signal Design For MIMO Radar 总被引:7,自引:0,他引:7
A multiple-input multiple-output (MIMO) radar system, unlike a standard phased-array radar, can choose freely the probing signals transmitted via its antennas to maximize the power around the locations of the targets of interest, or more generally to approximate a given transmit beampattern, and also to minimize the cross-correlation of the signals reflected back to the radar by the targets of interest. In this paper, we show how the above desirable features can be achieved by designing the covariance matrix of the probing signal vector transmitted by the radar. Moreover, in a numerical study, we show that the proper choice of the probing signals can significantly improve the performance of adaptive MIMO radar techniques. Additionally, we demonstrate the advantages of several MIMO transmit beampattern designs, including a beampattern matching design and a minimum sidelobe beampattern design, over their phased-array counterparts. 相似文献
860.
Yu Yan Jianping Yao 《Photonics Technology Letters, IEEE》2007,19(19):1472-1474
A novel tunable photonic microwave filter with complex coefficients is proposed and experimentally demonstrated. The complex coefficient is generated using a wideband tunable optical RF phase shifter that consists of two electrooptic intensity modulators. The phase of the RF signal is shifted by simply adjusting the bias voltages applied to the two electrooptic intensity modulators, and the phase shift remains constant over the microwave spectral region of interest. A two-tap photonic microwave filter with one tunable complex coefficient, with a wide and continuous tuning range, is experimentally demonstrated. 相似文献