移动通信系统中宽带功放线性化设计与仿真

自适应数字预失真技术能够解决宽带高峰均比信号经过功放产生的非线性失真问题.主要介绍了自适应数字预失真的结构、预失真器的数学模型及相关的自适应算法.仿真结果表明采用自适应数字预失真技术能够显著地改善功放输出信号的带外频谱失真,是一种有效、灵活的技术.     

Coexistence of broadband power line communication systems

es and the radiation field are finally determined.     

Equalization methods in OFDM and FMT systems for broadband wireless communications

Multicarrier systems are adopted in several standards for their ability to achieve optimal performance in very dispersive channels. In particular, orthogonal-frequency division multiplexing (OFDM) and filtered multitone (FMT) systems are two examples where the modulation filter has an ideal rectangular amplitude characteristic in time and frequency domains, respectively. In this letter, we propose new equalization schemes for FMT and compare their performances with OFDM. In general, FMT has a greater spectral efficiency than OFDM, due to the absence of the cyclic prefix and a reduced number of virtual carriers. However, it exhibits a higher distortion per subchannel, due to the imperfect equalization of the transmit filters. As a performance comparison, we considered both the achievable bit rate (ABR) and the bit error rate (BER) in a multipath Rayleigh fading channel. We note that while ABR gives a theoretical bound on the system bit rate, assuming the knowledge of the channel at the transmit side, the BER refers to an uncoiled system with a fixed modulation. Although FMT requires a fixed structure with a higher computational complexity than OFDM, it turns out that FMT, even with the simplest one tap per subchannel adaptive equalizer, yields a better performance than OFDM, both in terms of ABR and BER. Hence, FMT can be a valid alternative to OFDM for broadband wireless applications, also.     

Analysis of the methods to increase the output power and frequency of the broadband TWTs

The limitations on the output power and operating frequency of the broadband TWTs resulting from a relatively low thermal stability of helix and the self-excitation at the backward wave at a operating voltage of greater than 10 kV are considered. The analysis yields the methods to overcome limitations: the application of a bifilar helix with the azimuthally conducting rings and the application of combined metal-dielectric supports that allow a simultaneous improvement of the heat removal from the helix and an increase in the TWT band. It is demonstrated that a decrease in the slowing caused by the rings and a smaller period of the bifilar helix make it possible to shift the threshold of the self-excitation at the backward wave to 30–40 kV.     

Analysis and design of broadband grating couplers

The authors have assessed the ability of thin-film dielectric gratings to convert an incident guided wave into outgoing beams whose widths and intensities are maintained nearly constant over a large wavelength band. For this purpose, they have improved and extended a previously developed analytical approach and used it to investigate the pertinent coupling characteristics of typical planar gratings having symmetric rectangular or blazed triangular profiles. The results reveal that, by judiciously designing the grating configuration, it is possible to achieve broadband coupling operation over a wavelength ratio of 2:1 or larger. It is also found that rectangular gratings can offer wider bandwidths, but they couple energy almost equally into the cover and the substrate beams. By contrast, blazed gratings allow somewhat narrower bandwidths, but they can transfer energy almost entirely into either one of those two beams     

On the design of broadband quasioptical systems for submillimeter-wave radio-astronomy receivers

In this paper, we summarize the issues that should be considered when designing broadband quasioptical systems for submillimetre-wave radioastronomy receivers. We cover topics such as bandwidth, cross-talk, truncation, and aberrations, and we argue that it should be possible to manufacture high-efficiency systems that have several octaves of bandwidth. A key feature of the paper is that we use the language of multimode Gaussian optics throughout, and in this way, we emphasize that a receiver is a diffraction-limited imaging system rather than just a collection of components for guiding Gaussian beams. The whole discussion is conducted in terms of a particular system we are constructing for the James Clerk Maxwell Telescope in Hawaii.     

On the code and interleaver design of broadband OFDM systems

In this letter, we study the performance of space-frequency-coded orthogonal frequency-division modulation systems over multiple-input multiple-output frequency-selective channels. The diversity and coding advantages are derived in terms of the minimum Hamming distance and the minimum squared product distance of the code as well as the relative frequency locations (tones) where a pair of codewords with the minimum Hamming distance differ. These relationships between performance and well-defined code parameters provide new insight to code construction and interleaving design. In addition, we propose a block interleaver that yields nearly optimal coding advantage for space-frequency trellis codes.     

Input filter design for multiple-module DC power systems

When multiple DC/DC power converters are operated from a DC bus with a nonzero source impedance, undesirable interactions can occur between an individual regulator and the input impedance of the other regulators on the bus. Consequently, criteria for input filter design in the presence of a significant source impedance are developed, which, when used in conjunction with already-known input filter criteria, permit the input filter to be designed so that each regulator operates reliably. Proper filter design tends to decouple the negative regulator impedances from the bus, leaving only the passive input filter impedances to affect the other converters. These filter impedances appear in parallel with the source impedance and reduce the overall source impedance. Hence, the use of multiple modules on the same bus actually improves the performance of the individual regulators. An example, the buck current mode controlled power converter, is examined in detail. Extensive experimental evidence is presented to verify the analytical results     

AMCs for ultra-thin and broadband RAM design

A novel methodology for ultra-thin and broadband radar absorbing material (RAM) design is proposed. The method is based on periodically arranging two different types of thin planar slabs that are both artificial magnetic conductors (AMCs) with different phase characteristics that are arranged in a checkerboard configuration. By controlling the phase difference between the two types of AMC slabs, the energy which is normally incident on the screen is redirected, and the RCS along the boresight is reduced over a wide frequency range.     

Narrowband and broadband infrastructure design for wirelessnetworks

There has been an explosion in the number of wireless subscribers. A number of air interface technologies, such as GSM, TDMA and CDMA, are available to wireless service providers for offering wireless services. In addition, a variety of networking technologies, such as STM, ATM and frame relay, are available to the wireless services provider for designing their infrastructure networks. The abundant choice of technologies, and their associated capabilities and costs, creates a need for network design tools which can help vendors and wireless service providers to understand the economics of investing in different technologies. This article is concerned with the design of narrowband and broadband infrastructure networks for wireless access. The article first describes the different technology alternatives and tariff structures and their impact on wireless infrastructure network design. The general infrastructure design problem is then stated and a solution methodology outlined. Examples of the economic trade-offs involved in narrowband and broadband networking technologies are also presented     

Topological design,routing and capacity dimensioning for ISL networks in broadband LEO satellite systems

This paper considers the network design of intersatellite link (ISL) networks in broadband LEO satellite systems, where the major challenge is the topology dynamics. First, a general method to design convenient ISL topologies for connection‐oriented operation is presented, and a reference topology for numerical studies is derived. A permanent virtual topology is then defined on top of the orbiting physical one, thus forming a framework for discrete‐time dynamic traffic routing. On this basis, heuristic and optimization approaches for the combined routing and dimensioning task, operating on discrete time steps, are presented and their performance is numerically compared. It is shown that minimizing the worst‐case link capacity is an appropriate target function, which can be formulated as linear optimization problem with linear constraints. Using linear programming (LP) techniques, the dimensioning results are clearly better than with simple heuristic approaches. Copyright © 2001 John Wiley & Sons, Ltd.     

On architectures for broadband wireless systems

The growing popularity of portable and mobile computing and communication devices, along with the introduction of wireline broadband networks, is fueling demand for wireless broadband networks. Since ATM has been accepted as the standard for broadband integrated service networks, the authors consider the problem of providing “ATM-like services” to mobile devices. The fundamental issues that have to be tackled in order to provide broadband services which are currently available (or will be very shortly) to wireline hosts are discussed. The authors compare and contrast the architectures that have been proposed for extending the broadband wireline network infrastructure to the wireless environment. It is argued that transporting small ATM cells over the air leads to inefficient utilization of the already scarce transmission capacity. Subsequently, the authors propose an architecture for integrated wireless and wireline broadband networks which allows the wireline and wireless parts of the network to be designed independently, therefore allowing each to be optimized for the environment in which it operates. At the same time, the proposed architecture also permits simple and efficient interconnection between the wireline and wireless infrastructures     

Antenna systems for broadband wireless access

Broadband wireless access along with evolving mobile Internet and multimedia services are driving the surge of research and development activities for future wireless communication systems. We provide an overview of antenna systems for broadband wireless communications and introduce some of the important issues surrounding them. The approach we use is to first provide a general framework of how antenna systems may be utilized in wireless communication systems and then describe the antenna systems themselves. In particular, we consider antenna systems for the base station, mobile station, and then finally multiple-input multiple-output antenna systems where antenna systems are utilized at both the base and mobile stations     

MIMO systems for broadband wireless communications

The emergence of broadband wireless access (BWA) and mobility poses significant challenges to the future telecommunications landscape. Multiple input multiple output (MIMO) is a potential technology that can be used to improve system performances such as coverage, capacity and data rates for broadband wireless networks. Although this technology has proved to be beneficial for both current and future services by many research results, it is not clear how these benefits can be practically achieved with a MIMO system. Fundamental issues on capacity enhancement and performance limits need to be addressed before operators will be willing to deploy MIMO systems in their networks. This paper addresses some of the key issues that remain open regarding MIMO technology for BWA. The paper focuses on MIMO channel modelling and capacity analysis.     

宽带双极化四脊圆喇叭天线的分析与设计

基于航空电子系统射频综合实验室的需求,应用圆锥喇叭天线的传输原理和宽频带波导的匹配特性,研制了一种宽带双极化四脊圆喇叭天线。对其工作原理、性能及电气结构进行了分析与设计,实际制作了一批宽带双极化四脊圆喇叭天线并对其进行了调试,电性能参数优良,实测结果与设计基本一致:此天线在C,X,Ku波段内具有大于3倍频程的宽带匹配特性,相当高的交叉极化隔离度(B>22 dB)和高的增益(10 dB~20 dB)。结果表明该方法是正确和有效的。     

宽频带平板反射器阵列天线分析与设计

采用表面电流有间隙馈电的Hallén积分方程分析了大直径振子的驻波特性,对限制平板反射器阵列天线工作带宽的主要因素进行了研究,探讨了展宽天线带宽的基本方法和途径;以大直径振子为单元设计了一副八单元平板反射器阵列天线,在一个倍频程内,驻波比小于1.5,增益可达18 dBi,带内起伏小于2 dB,具有较好的方向性,在工程实践中具有广阔的应用前景.     

一种改进型全向宽带双锥天线的研究

研究了一种用于移动通信的改进型全向宽带天线,采用双锥结构,在馈电处加入短路枝节和寄生套筒,用于调节天线的输入阻抗特性,减小天线尺寸。该天线工作频率为696-2500MHz,高度仅为0．28λmax,直径为6cm,电压驻波比在整个频带内小于1．7,全向特性良好。仿真和实测结果吻合良好,表明该改进型双锥天线在较宽的频带内达到了满意的阻抗特性和方向图特性。