Adaptive bit loading scheme using COFDM at low elevation over LEOsatellite communication channel

Much interest has been shown in employing LEO satellite communication systems. A novel algorithm is proposed for adaptive bit loading (multilevel modulation assignment) between COFDM subcarriers in a frequency selective fading channel. Two-dimensional alignment has been carried out. The system performance is greatly improved at low elevation angles compared to that using fixed bit loading. Furthermore. The comparison shows that this adaptive bit loading COFDM system is suitable for frequency selective slow fading channels, which have time-varying deep nulls in the frequency response     

An analytical model to predict the probability density function ofelevation angles for LEO satellite systems

Based on Earth-satellite geometry, an analytical model is proposed to predict the probability density function (pdf) of elevation angles from an Earth station to a circular low-Earth-orbit (LEO) satellite constellation. Unlike currently available empirical models using a great deal of path data for model-fitting to obtain results applicable only to the specified systems (for example, the Iridium or Globalstar systems), the proposed model is fast, exact and provides greater flexibility for the user-specified satellite systems. The results may help to develop the LEO satellite communication channel models under the propagation environments characterized by various empirical elevation-angle-dependent channel models     

低轨卫星移动通信系统接入方案

在低轨卫星移动通信系统中,由于卫星和移动用户间的相对运动使得呼叫切换频繁发生.为了降低星间切换请求到达率,减小系统切换开销,本文在距离优先接入方案基础上进一步提出了两种接入策略:覆盖时间优先方案和仰角加权的覆盖时间优先方案.构造了非均匀分布全球话务密度模型.并参照某实际系统参数,对不同接入方案准则下的全球话务服务进行了系统仿真,得到了相应的系统性能参数.     

Performance comparison of space-time block and trellis codes in the MIMO land mobile satellite channels

Due to the crowded orbits and shortage of frequency resources, the use of MIMO technology to improve spectrum efficiency and an increase of the capacity have become a necessary trend of broadband satellite communication. Firstly, we analyze the main influenced factors and compare the bit error rate (BER) performance of space-time block code (STBC) scheme and space-time trellis code (STTC) scheme. Then we build up the model of land mobile satellite (LMS) channel under different environments by using 3-state Markov chain. This paper emphatically studies the BER performance of STTC and STBC in the MIMO satellite channel. The main emphasis is placed on the effects of different factors, such as terminal environment and elevation angles, on the BER performance of STBC and STTC schemes. Simulation results indicate that performance of STTC in Rayleigh channel is obviously improved with the increasing number of transmitting and receiving antennas, but the encoder state has little impact on the performance. In the Rayleigh channel, the performance of Alamouti code is better than that of STTC. In the LMS channel, performance of these two kinds of space-time coding in open area is optimal, and in the urban area it is the worst. Nevertheless, performance of STTC is slightly superior to the performance of STBC under different circumstances. Under the same environmental conditions, BER of STBC and STTC reduces with the increase of the satellite altitude angle, and therefore, the BER curves of STTC fall faster.     

Interference considerations in frequency sharing between low earth orbit satellites and the geostationary fixed satellite and terrestrial service

Recently many companies and consortia have considered launching LEO satellites for such projects as remote sensing, transportation and mining operations. When the multiple low Earth orbit (LEO) satellites are transmitting they can interfere with existing terrestrial microwave and satellite earth-stations in the fixed and mobile service. The interference problem is related to the number of proposed LEOs and the altitude and consequently the time to orbit the earth. These systems usually consist of many small satellites, and each satellite stays in the beam of a terrestrial station for up to 72 seconds in each 222 minute orbit. Earth coverage could be obtained by 48 (LEO) satellites 1500 km above the earth in polar orbit, and hence at least one LEO would always be interfering with terrestrial networks. A technical evaluation would then be required to determine the resultant BER (bit error rate) effecting existing terrestrial services. A determination can then be made to support such a LEO system or object via official channels such as the ITU.     

Performance of multi-beam CDMA-based LEO satellite systems in aRice-lognormal channel

The performance of multi-beam code-division multiple-access (CDMA)-based low Earth orbit (LEO) systems with convolutional coding and soft decision Viterbi decoding is analyzed. The multiple access interference due to all visible interferers of the given satellite is taken into account. The results show that the interference coming from the users located outside the footprint but still seen by the given satellite has a nontrivial effect on the system capacity. The multiple access interference, though, can be reduced by increasing the elevation restraint of each satellite footprint. Estimates on this reduction are given     

基于遍历容量的低轨卫星协作通信中继选择算法

在低轨卫星通信系统下行链路中引入协作技术,可获得协作分集增益,从而减小信道衰落的影响。目前在目标通信区域内多星的覆盖率非常高,合适的中继卫星选择策略对于提升卫星通信网络的整体性能至关重要。针对这一问题,本文提出一种基于遍历容量的中继选择算法。该算法借助卫星轨道信息以及统计信道信息,综合考虑卫星本身载荷受限和高动态的特点,通过公平性因子进行遍历容量和能耗之间的折中,选择具有最佳权重遍历容量的节点作为协作卫星。并在此过程中,得到了卫星间协作链路信噪比概率密度闭式表达式,该表达式用来求解协作链路遍历容量。仿真分析表明,该方法可以动态的调整协作卫星的优先级,在误比特率性能和公平性之间可以达到很好的平衡,并且可以在低EIRP情况下提高卫星的服务时间。      

A Measurement of the MARISAT L-Band Signals at Low Elevation Angles Onboard Mobil Aero

The MARISAT-to-shipL-band signals used in maritime mobile satellite communications service are degraded by fading and scintillation at low elevation angles. These degradations are attributed to multipath effects arising from ionospheric, tropospheric, and/or sea surface diffractions. To characterize such degradations, measurements were made at the Southbury earth station and onboard the S.S. Mobil Aero while the ship was en route from Norfolk, VA, to Texas City, TX. Measurements include monitoring the carrier-to-noise ratio (C/N) of time-division multiplex/time-division multiple access (TDM/TDMA) voice carriers and measuring the bit error rate (BER) of 2400/1200 bit/s digital data transmissions through a voice channel. Results indicated that bothC/Nand BER are severely degraded at elevation angles below 5° due to propagation anomalies. Information useful for design considerations of future maritime communications systems is summarized.     

一种低轨卫星通信终端的数字波控方法

针对低轨卫星通信场景下地面终端需自动追星的问题,提出了基于相控阵的波束控制方法.首先利用卫星的位置以及终端的位姿信息计算终端对星的初始指向角,并用该指向角计算相控阵各阵元的移向量完成初始对星,然后利用相控阵在数字域进行圆锥扫描实现对星的精跟踪.仿真结果表明,所提方法能较好完成终端的对星工作.     

On the performance of UNB for machine‐to‐machine low earth orbit (LEO) satellite communications

Ultra narrow band (UNB) is a widely used technology for machine‐to‐machine and low‐power wide‐area communications. Its properties, long range with small RF power, make it naturally attractive for satellite communications but also draw new challenges compared with terrestrial systems where this technology is already deployed. Indeed, the main advantage of UNB signals, their small bandwidth, makes them more sensitive to frequency drifts that are particularly present in the case of LEO satellite systems. It also implies the use of a random access method in which the carrier frequency is a parameter unknown by the receiver. In this paper, we propose a general semianalytical model to evaluate the performance of a terrestrial or satellite system using UNB technology, taking into account the multiuser interference and the frequency drift. This model is then used to assess the performance (packet loss ratio and throughput) on the return link medium access control (MAC) of a representative LEO satellite system. With our model, we analyze the effect of frequency drift on the system performance. This paper also proposes to investigate more deeply the multiuser interference modeling in order to estimate accurately the performances of a UNB system in terms of bit error rate (BER). We propose a semianalytical approach to study the interference in presence of arbitrary power imbalance that includes the effect of frequency offset and frequency drift and applicable for any linear modulation and any pulse‐shaping filter. The expression of the multiuser interference is established in the general case. We then propose a methodology to compare this exact model to the Gaussian interference approximation (mainly used through the central limit theorem) in order to assess its accuracy.     

Design study for a CDMA-based LEO satellite network: downlinksystem level parameters

The performance analysis of a new concept of a code-division multiple-access (CDMA) based low Earth orbit (LEO) satellite network for mobile satellite communications is presented and discussed. The starting point was to analyze the feasibility of implementing multisatellite and multipath diversity reception in a CDMA network for LEO satellites. The results are used to specify the design parameters for a system experimental test bed. Due to the extremely high Doppler, which is characteristic of LEO satellites, code acquisition is significantly simplified by using a continuous wave (CW) pilot carrier for Doppler estimation and compensation. The basic elements for the analysis presented are: the channel model, the pilot carrier frequency estimation for Doppler compensation, and multipath and multisatellite diversity combining     

基于STK的双层卫星星座设计及仿真

轨道与星座的设计是整个卫星通信系统设计的基础,合理的轨道设计与星座配置方案可以显著提高系统的整体性能。结合GEO卫星处理能力强,LEO卫星星地时延小的特点,以提供全球实时接入为目标,提出了一种由LEO卫星提供全球覆盖的GEO/LEO双层卫星网络星座设计方案。运用卫星覆盖带分析方法,确定由48颗LEO卫星完成全球覆盖。通过在STK仿真环境下进行计算机仿真验证,得到所设计的卫星星座可完成全天时全球覆盖。     

Adaptive Compensation Method Using the Prediction Algorithm for the Doppler Frequency Shift in the LEO Mobile Satellite Communication System

In low earth orbit (LEO) satellite communication systems, more severe phase distortion due to Doppler shift is frequently detected in the received signal than in cases of geostationary earth orbit (GEO) satellite systems or terrestrial mobile systems. Therefore, an estimation of Doppler shift would be one of the most important factors to enhance performance of LEO satellite communication system. In this paper, a new adaptive Doppler compensation scheme using location information of a user terminal and satellite, as well as a weighting factor for the reduction of prediction error is proposed. The prediction performance of the proposed scheme is simulated in terms of the prediction accuracy and the cumulative density function of the prediction error, with considering the offset variation range of the initial input parameters in LEO satellite system. The simulation results showed that the proposed adaptive compensation algorithm has the better performance accuracy than Ali's method. From the simulation results, it is concluded the adaptive compensation algorithm is the most applicable method that can be applied to LEO satellite systems of a range of altitude between 1,000 km and 2,000 km for the general error tolerance level, M = 250 Hz.     

Personal communications by satellite

Personal communications (PC) refers to two-way voice (and possibly data) communications to a small hand-held unit, capable of being carried by a person and used in various locations. PC via satellite refers to the case where this hand-held unit communicates directly with a satellite to provide the duplex voice (or data) service. Both geosynchronous earth orbit (GEO) and low earth orbit (LEO) satellites have been considered to provide this service. GEO and LEO satellite systems must compete with existing mobile cellular radio systems both in meeting performance requirements and service costs, if they are to be a significant supplier of PC services. GEO and LEO systems each have unique advantages and disadvantages when used to provide a PC service. While these general characteristics are identified in Section 1 of the paper, a more quantitative comparison is needed. This quantitative comparison is made by comparing a GEO PC satellite system, operating at EHF (K_a-band) frequencies with a LEO system operating at UHF (L-band) frequencies, including service costs for both systems. The two systems used in the comparison are examples of realistic GEO and LEO system designs for PC service, and although it is not exhaustive, the comparison points out some of the key differences between GEO and LEO systems that affect service performance and cost.     

Evaluation of handover mechanisms in shadowed low earth orbit land mobile satellite systems

This paper presents a quantitative analysis by computer simulation of the active set update (ASU) handover algorithm for a shadowed low earth orbit (LEO) land mobile satellite (LMS) environment. As a precursor to the handover analysis, the mutual visibility statistics for a 66 satellite polar and 48 satellite rosette-type constellation are presented. These results show the statistical nature of the levels of satellite diversity and mobile-to-satellite elevation angles (to the highest satellite) within each network and also indicate the influence of the channel characteristics on the handover strategy. A two-state Markov modulated channel model is assumed in the handover analysis, and this enables the assessment of increased levels of power and time hysteresis on the quality of service and network signalling load in a shadowed land mobile satellite environment. In particular, attention is given to the different modes of ASU operation for hard handover, switch diversity and soft handover.     

Rigorous analysis on performance of LEO satellite ground station in urban environment

Microsatellites in low Earth orbits (LEOs) have been in use for the past two decades. LEO satellites are used for public communication and also for scientific purposes, and the orbits vary with the type of satellites and the primary purposes. LEO scientific satellites have a variety of applications, including Earth surveillance and astronomy applications. These satellites provide opportunities for investigations for which alternative techniques are either difficult or impossible to apply. Thus, it may be expected that such missions will be further developed in the near future especially in fields where similar experiments by purely Earth‐based means are impracticable. Ground stations have to be established in order to communicate with such satellites, and the quality of communication depends on the performance of the satellite ground station, in addition to that of the satellite. Before the implementation of the ground station, analyses related to environmental factors have to be considered, especially in urban areas. Rain effects, the impact of intermodulation products, and contact time duration at low elevation angles are some of the aspects that are considered in this work and which influence the final decisions on the design of the ground station. Measurements on the ground station based on Sun flux density are described, which provide an opportunity to check the performance of the ground station. Copyright © 2007 John Wiley & Sons, Ltd.     

Analysis of M-ary phase-shift keying with diversity reception forland-mobile satellite channels

An analytical technique well suited to numerical analysis is presented for computing the average bit-error rate (BER) and outage probability of M-ary phase-shift keying (PSK) in the land-mobile satellite channel (LMSC) with microdiversity reception. Closed-form expressions are found for L-branch microdiversity using both selection diversity combining (SDC) and maximal ratio combining (MRC). These expressions are extended to include both M-ary coherent PSK (M-PSK) and differential PSK [M-differential PSK (DPSK)]. Following previous empirical studies, the LMSC is modeled as a weighted sum of Rice and Suzuki distributions. Numerical results are provided illustrating the achievable performance of both M-PSK and M-DPSK with diversity reception. Using measured channel parameters, the performance in various mobile environments for various satellite elevation angles is also found     

卫星移动通信信道模型和误码率特性分析

在对卫星移动信道传输特性统计模型研究分类的基础上，给出了适合卫星移动信道的部分阴影扩展模型。用试验和修正的方法给出了模型参数，得到的模型适用于非静止轨道变仰角信道，分析了非静止轨道卫星通信系统窄带调制和宽带扩频调制的误码率性能，得到了有意义的结论。     

Theatre in the Sky: a ubiquitous broadband multimedia‐on‐demand service over a novel constellation composed of quasi‐geostationary satellites

To meet an ever‐growing demand for wideband multimedia services and electronic connectivity across the world, development of ubiquitous broadband multimedia systems is gaining a tremendous interest at both commercial and academic levels. Satellite networks will play an indispensable role in the deployment of such systems. A significant number of satellite communication constellations have been thus proposed using Geostationary (GEO), Medium Earth Orbit (MEO), or Low Earth Orbit (LEO) satellites. These constellations, however, either require a potential number of satellites or are unable to provide data transmission with high elevation angles. This paper proposes a new satellite constellation composed of Quasi‐GeoStationary Orbit (Quasi‐GSO) satellites. The main advantage of the constellation is in its ability to provide global coverage with a significantly small number of satellites while, at the same time, maintaining high elevation angles. Based on a combination of this Quasi‐GSO satellites constellation and terrestrial networks, the paper proposes also an architecture for building a global, large‐scale, and efficient Video‐on‐Demand (VoD) system. The entire architecture is referred to as a ‘Theatre in the Sky’. Copyright © 2006 John Wiley & Sons, Ltd.     

低轨卫星自跟踪技术分析

针对低轨(LEO)卫星无信标信号时,传统自跟踪方式无法对其进行有效跟踪的问题,提出了全数字跟踪体制。对采用频分时分复合多址(FDMA/TDMA)体制的低轨卫星,采用宽带检测结合窄带跟踪的方法,快速准确地提取方位俯仰误差,实现了对目标卫星的高精度跟踪。对采用码分多址(CDMA)体制的低轨卫星,采用频域跟踪的方法,实现了低信噪比条件下对目标卫星的高精度跟踪。