Cost-effective networking via digital satellite communications

This paper presents the major issues in forming a cost-effective and efficient network from a set of earth stations sharing the satellite resource. The discussion is in the context of satisfying the performance requirements for transmission of data, voice, and image presented to earth stations in digital form. Multiple access techniques have significant impacts on the network economy. Although wide-band single-carrier time-division multiple access (TDMA) uses the satellite capacity efficiently and is cost-effective for networks with large traffic volumes per node, it requires expensive earth stations and is not cost-effective for networks with small traffic volumes per node. The concept of multicarrier-TDMA (MC-TDMA) is described and its economic advantage over TDMA is demonstrated for networks with dispersed points of traffic origination. Application of demand assignment to MC-TDMA is also presented. Some implementation issues are also discussed.     

数字通信调制参数自动校准的研究

调制质量作为无线通信校准中的重要参数,对无线通信测试的性能起着至关重要的作用.针对调制参数变化大,手动测试出错率高等问题,给出一种数字移动通信综合测试仪数字调制参数自动校准的方法,并对测量结果进行了不确定度分析.实验表明,该自动测试方法精确度高,稳定性高,实用性强.     

Mobile communications

The principal problem faced by networks with mobile users is how to ensure that the service provider functions keep pace with the continually changing network state resulting from user mobility, without consuming large amounts of network resources in the process. Regardless of whether the users are mobile or stationary, all telecommunications networks must perform basic service provider functions such as controlling access to services, locating users, routing traffic, and accounting and billing for service use. However, in the presence of mobile users, all of these functions must be performed much more often and much more rapidly. Developing low-overhead high-performance service provider solutions has been and will continue to be a key area of research in mobile communications. This special issue on mobile communications comprises five articles, each of which covers a different aspect of mobile communications     

Mobile communications

An important issue facing the 1992 World Administrative Radio Conference, namely, determining how to provide additional spectrum for both existing and entirely new mobile services, is examined. At least six types of mobile systems will be seeking new or expanded allocations; three involve satellites and three rely on primarily terrestrial techniques. All but one will be forced to compete for allocations in the same crowded 1-3 GHz frequency range. The various allocation proposals are discussed     

Dual-pilot tone calibration technique

Pilot-based calibration techniques are used to reduce the effects of multipath fading in mobile satellite receivers. One of the more recent of these techniques, namely the tone calibration technique (TCT), suggests transmitting double sideband modulation with the pilot tone located at the center of its spectrum where the amplitude and phase characteristics of the channel are most stable. To "make room" for the pilot in the presence of the Doppler shift, the equivalent low-pass data sidebands must be shaped so as to have zero response in the neighborhood of dc. Other techniques such as transparent tone-in-band (TTIB) similarly "notch out" a hole in the center of the data spectrum for location of the pilot. An alternate possibility which is at the same time much more bandwidth efficient than TCT is a dual-pilot tone calibration technique (DPTCT) that symmetrically locates a pair of pilots outside the data spectrum near the band edges of the channel. The operation and performance of DPTCT are analyzed, and its effectiveness is compared to that of the single tone TCT technique.     

Mobile satellite communications

The paper presents an up-to-date overview as well as new results on mobile satellite communications. Specifically, the following subjects are addressed: worldwide activities; satellite orbits; multiple access techniques; Novstar global positioning system; economic and regulatory issues; future trends. Some of these technical and nontechnical subjects appeal disjoint, but they are unique and significant among the many facets, which effect the development and operation of mobile satellite communication systems     

Integrated digital communications networking

Described herein is an integrated communications system engineeredand installed to support the overseas design and manufacture of major body sections of a new commercial transport aircraft. The communications requirements include the transmission of voice, teletype, facsimile, and computer data. The system design approach was to digitize all input data including voice, utilizing a narrowband voice digitizer, and then combining all data onto a single dedicated channel 'for transmission. Also described is ttie use of statistical multiplexing for data compression and the demultiplexing and routing of computer data to several manufacturing plants in foreign countries.     

Alternatives in digital communications

The following major interrelated aspects in digital communications are described: source encoding, channel encoding and transmission, and receiver methods. Methods for digitizing speech and video are summarized. Digitized voice can be transmitted at 10 kb/s, while phone video requires 1.544 Mb/s. The major aspects of both forward-error control (FEC) and automatic request for repeats (ARQ) are considered. Power-limited forward channels are distinguished from bandwidth-limited ones. Signal distance concepts and maximum-likelihood detection principles provide the framework for describing both block-coded and convolutional-coded signaling. Convolutional coding with Viterbi decoding appears to achieve the most cost-effective forward-error control. The improved FEC techniques benefit the transmission of real-time digitized voice and video and any communication over severely power-llmited links. ARQ methods continue to be effective and practical for most cases where the delay associated with retransmission can be tolerated.     

Filter banks in digital communications

Digital signal processing has played a key role in the development of telecommunication systems over the last two decades. In recent years digital filter banks have been occupying an increasingly important role in both wireless and wireline communication systems. In this paper we review some of these applications of filter banks with special emphasis on discrete multitone modulation which has had an impact on high speed data communication over the twisted pair telephone line. We also review filter bank precoders which have been shown to be important for channel equalization applications.     

Universal digital portable radio communications

Providing voice and data communications to people away from their wireline telephones has become a major communication frontier. This frontier is being penetrated by evolving approaches to portable communications, e.g., cordless telephones, mobile radiotelephone, and radio paging. However, these approaches have many limitations; none can provide universal portable communications services. This paper discusses limitations of the evolving approaches and considers objectives and approaches for providing more universal digital portable communications as an integrated part of telephone exchange networks. These more universal communications could be accomplished by using demand-assigned radio links for the last thousand feet or so of telephone loops and sharing the remainder of the fixed distribution facilities. Fixed radio ports as integrated parts of telephone distribution networks could be placed throughout service regions. Efficient use of the radio spectrum could be insured by the planned reuse of radio frequencies throughout the regions. The severe multipath radio propagation environment within and around buildings that strongly influences the design of portable communications systems is described in the paper. System configurations and radio link techniques, that can provide reliable communications in the multipath environment, are discussed. Radio system calculations are illustrated for radio ports with 30-ft-high antennas in residential areas. The calculations indicate that radio link availability would be greater than 99 percent for 2000-ft port separations and 5-mW portable transmitters. Reuse of frequencies would require dividing the allocated frequency band into segments for use at adjacent ports. Calculations suggest that link availability in the cochannel interference environment would be greater than 99 percent, if 25 to 35 segments were used in residential areas.     

Robust detection in digital communications

This paper deals with the problem of communicating through unspecified noise. Detectors, robust against variations in the probability density function of the noise, are developed and discussed. The paper covers three issues. First, the relation between a distance measuring receiver and a correlating receiver in a general case is shown. Second, a theoretical method for the computation of an upper limit for the probability of symbol error is presented. This computation fits into the ordinary framework for computation of the error probability by changing the inverted noise density 2/N₀ to efficacy, ϵ. Efficacy is defined in the paper. Third, detectors based on M-, i.e., maximum likelihood type, and R-, i.e., rank, statistics are tested and compared for GMSK and π/4-shifted DQPSK. From numerical comparisons of the upper bounds and their simulated estimates for robust detectors, it is concluded that the loss in Gaussian noise is very small compared to the optimum quadratic detector. The gain, compared to a nonrobust receiver optimized to Gaussian noise, is around 0.5 to 2 dB for large SNR and around 2 to 4 dB for low SNR in impulsive noise. This offers new methods of significantly improving communication when the noise is unknown     

Synchronization in digital communications networks

The general aspects of timing and synchronization in digital communications networks are reviewed. The properties of the links and nodes carrying and switching time-multiplexed digital signals determine the throughput and performance of a network. The continuity and integrity conditions for the digital information flow, where timing faults cause errors and loss of information, determine the requirements on accurate clock signal distribution. The dominant type of architecture for network synchronization is based on the master-slave hierarchy principle     

Performance parameters for digital communications

The specification of error performance and outage of a digital system using a percent-of-time-interval measure is described.     

Comments on tone calibration in time varying fading

An analysis of tone calibration techniques (TCTs) that examines the validity of the slow fading assumptions is presented. TCT systems previously proposed will produce an error floor, albeit a very low one. The requirements for the elimination of the error floor and the error floor produced in a typical TCT system are examined. Monte Carlo integration techniques are used to evaluate integrals containing the isotropic fading spectrum     

Mobile radio group communications by satellite

Mobile radio group communications enable a closed group of users to communicate among themselves using push-to-talk (PTT), half duplex transmissions, such that one's transmission is received by all. New protocol designs are presented which support group communications over mobile satellite systems. A new demand assignment multiple access (DAMA) protocol is necessitated by the inappropriateness of call-based channel assignments. Channels are assigned to individual user-groups engaged in active conversations. As members of a user-group contend for access to the assigned channel, access control by the usual manual methods is ineffective due to the very long round trip delay. A novel signaling protocol which automates access control is proposed. Carrier sensing, signal capture, and collision detection are employed to minimize access contentions. The throughput and holding time of the assigned channel are analyzed. Numerical results show that the access control protocol improves the capacity of the assigned channel by up to 65%     

Monobit digital receivers for ultrawideband communications

Ultrawideband systems employ short low-power pulses. Analog receiver designs can accommodate the required bandwidths, but they come at a cost of reduced flexibility. Digital approaches, on the other hand, provide flexibility in receiver signal processing but are limited by analog-to-digital converter (ADC) resolution and power consumption. In this paper, we consider reduced complexity digital receivers, in which the ADC is limited to a single bit per sample. We study three one-bit ADC schemes: 1) fixed reference; 2) stochastic reference; and 3) sigma-delta modulation (SDM). These are compared for two types of receivers based on: 1) matched filtering; and 2) transmitted reference. Bit-error rate (BER) expressions are developed for these systems and compared to full-resolution implementations with negligible quantization error. The analysis includes the impact of quantization noise, filtering, and oversampling. In particular, for an additive white Gaussian noise channel, we show that the SDM scheme with oversampling can achieve the BER performance of a full-resolution digital receiver.     

Multilevel signals in digital optical-fibre communications

The theory for multilevel signal applications in digital optical-fibre communications is derived for systems with avalanche and nonavalanche photodetectors. It is shown that a multilevel signal can significantly increase the information rate and/or the repeater spacing. At the same time equaliser design can be simplified.     

VLSI for analog/digital communications

Present trends and future prospects are discussed, emphasizing the prospects for fuller VLSI integration of low-power digital radio, for applications such as in-building wireless radio receivers. The main concern is with the front end of the receiver, including continuous-time analog and sampled analog VLSI filtering, and technologies that can mix analog and digital on the same chip. Prospects for the use of bipolar complementary metal-oxide semiconductor (BiCMOS) technology in communications are examined. Continuous-time monolithic filtering is discussed. As an example of a central receiver/transmitter component that one would like to integrate monolithically, the frequency synthesizer is considered