Showing papers on "Telecommunications link published in 1991"

Integrated cellular communications system

Abstract: A code division multiple access (CDMA) spread spectrum cellular communications system with both surface and satellite nodes and using forward error correction coding, to enhance effective gain and selectivity, is provided. The system network control center (12) directs the top level allocation of calls to regional resources in the system. The regional node control centers (14) are connected to the system network control center (12) and direct the allocation of calls to ground nodes. The ground nodes (16) under the control of one of the regional node control centers (14) receive calls over land lines, spread them, modulate them, and transmit them. Satellite node control centers (18) are also connected to the network control center (12), and uplink the spread, multiplexed calls to the designated satellite (20). User units (22) respond to signals of either satellite or ground node origin. A digital data interleaving feature reduces fading. User position is determined. An adaptative transmitter power control (188) compensates for signal strength variations. An intercell bus (90) in a satellitte node (20) connects together multiple cells.

Processor-to-processor communications protocol for a public service trunking system

Abstract: A downlink between a communications dispatch console and a digitally trunked radio repeater system site controller efficiently transfers message data between the site controller and the console over a standard 9.2 kilobit per second landline. A downlink trunking card identical in structure to trunking cards used in the preferred embodiment system for RF channel signal processing interfaces the site controller with the landline link. A similar trunking card on the other side of the landline link interfaces the dispatch console processor with the landline link. Message protocol and format is translated between the site controller and the landline link and between the landline link and the dispatch console. Data buffering at each end of the landline link maximizes data transfer rate over the landline, and in addition, a priority scheme insures that more important messages are transmitted over the downlink before less important messages. Retransmit-until-acknowledged protocol is used to insure reliable message transfer--and also to permit receiving units along the downlink to slow down transmitting units in order to avoid message traffic blockage.

High-speed nondirective optical communication for wireless networks

Abstract: The primary obstacles to high-speed communication, namely, shot noise from ambient light, high-capacitance photodiodes and multipath dispersion, are reviewed, and ways to counter them are demonstrated for a prototypical infrared system. The discussion is limited to the physical-layer issues involved in the design of a single high-speed optical link. The focus is on the downlink (from base station to portable), which is a more challenging problem technically than the uplink for two reasons. First, the complexity of the portable receiver is much more constrained than that of the base station, due to power consumption and cost restrictions. Second, the downlink data rates are likely to be much higher than the uplink rates, because downlink communication will include downloading large executable files and possibly video services, whereas uplink communication will be used for transferring working files and keyboard commands which tend to be much smaller. >

Method of channel assignment by matching channel interference with channel link loss

Abstract: A method is offered of achieving a desired C/I ratio within a cellular system, at reduced transmitter power levels, by measuring a signal loss on a downlink between a base site transmitter (31) and mobile receiver (20) and calculating an uplink signal magnitude at a base site receiver. A communication channel is then selected from a number of communication channels based upon comparison of the uplink signal magnitude with pre-measured interference levels for each channel of the number of channels to produce a desired C/I ratio.

Communication link identifier

Abstract: In a communications system having components which are interconnected via communications links, the system is provided with the capability of responding to the application of a predetermined test signal to a link with information which identifies that link. Such information can take a variety of forms including the name of one or more persons assigned to use the link, and/or the telephone number or numbers associated with the link. This capability greatly reduces the time and expense associated with the maintenance and testing of a variety of communications systems, such as PBXs and central office switching machines.

Downlink outage predictions for cellular radio systems

Abstract: A general model for predicting downlink outage due to co-channel interference in cellular radio systems is presented. The model accounts for path loss, log-normal shadowing, Rayleigh fading, and vehicle velocity. The outage predictions are obtained by computer simulation. Various outage control techniques are evaluated, including cell sectoring, transmitter control, and hands-off. It is shown that down link power control is quite useful when hand-offs are used, and that shadowing can have a large effect on the outage predictions. The outage predictions are applied to a digital cellular system that uses quadrature differential phase shift keying (QDPSK) modulation and Reed-Solomon (RS) coding. It is shown that the performance of a coded digital cellular system is largely limited by rapidly moving vehicles. >

Efficient time-slot assignment algorithms for SS/TDMA systems with variable-bandwidth beams

Abstract: The authors present efficient sequential and parallel algorithms for computation of time-slot assignments in SS/TDMA (satellite-switched/time-division multiple-access) systems with variable-bandwidth beams. These algorithms are based on modeling the time-slot assignment (TSA) problem as a network-flow problem. If M(N) is the number of uplink (downlink) beams, L is the length of any optimal TSA, and alpha is the maximum bandwidth of an uplink or downlink beam, the sequential algorithm takes O((M+N)/sup 3/ min (M N alpha , L)) time to compute an optimal TSA, when the traffic-handling capacity of the satellite is of the same order as the total bandwidth of the links. The parallel algorithm uses L/2 processors and has a time-complexity of O((M+N)/sup 3/ log L) on a probabilistic random access machine (PRAM) model of parallel computation. The authors then generalize this algorithm to P >

Call performance in a frequency reuse digital portable radio system

Abstract: A discrete-event stochastic simulation of many concurrent calls was used to generate signal-to-interference ratio (S/I) time histories and statistics on a time-division multiple-access (TDMA) uplink. The simulation integrates portable radio multiple access, uplink/downlink signal modeling, and the traffic process. Calls were held to completion, enabling observation of the effects of cochannel interference on time-dependent S/I statistics. It was found that the improvement obtained from measurement-based time-slot assignment in uplink S/I at access partially erodes over the life of a call, if it retains the time-slot assigned at the start. The amount and speed of erosion of S/I at the first percentile suggests that good transmission quality can be maintained for stationary users, with low overhead in the fixed radio network. >

Switchable on-board communication payload for multi-band and multi-beam applications

Abstract: An on-board satellite signal switching assembly receives uplink beams of any ITU-allocated satellite signal transmission band (i.e., C-band, Ku-band, Ka-band) and converts these received beams to electrical signals. The electrical signals are converted (preferably down-converted) to a common bandwidth and input to a switch matrix which defines transmission paths between uplink and downlink beams. Outputs of the switch matrix are re-converted to downlink beams for transmission to desired earth stations. In this way, complete connectivity among between uplink beams of any band and downlink beams of any band is accomplished. Ground-to-ground communications systems may also benefit from these techniques.

Performance analysis of a single cell direct-sequence mobile radio system

Abstract: The uplink performance of a digital cellular radio system that uses DS (direct-sequence) CDMA (code division multiple access) is evaluated. Expressions for bit error probability that account for path loss, multipath-fading, multiple-access interference, and background noise are obtained. Three differentially coherent receivers are considered: a multipath rejection receiver, a RAKE receiver with predetection selective diversity combining, and a RAKE receiver with postdetection equal gain combing. Error correction coding is also considered. >

Direct-sequence spread-spectrum code-division-multiple-access cellular systems in Rayleigh fading and log-normal shadowing channel

Abstract: Direct-sequence spread-spectrum code-division-multiple-access (DS-SS-CDMA) techniques for personal and mobile communications are considered. A service area is divided into cells. The radio link is modeled as an additive white Gaussian noise channel with Rayleigh fading and log-normal shadowing, and is also corrupted by mutual interferences due to other users. L-branch microscopic diversity combining, K-site macroscopic diversity selection, and uplink power control are assumed. Under these assumptions, for uplink and downlink transmissions between users and cell-sites, analytical expressions of the probability distribution of the ratio of bit-energy to noise-plus-interference-density are derived. By numerically evaluating the derived expressions, the number of users per cell as a function of outage probability are obtained. Numerical results indicate that DS-SS-CDMA provides high capacity. >

Time-Division Multiple Access

Abstract: In multiple accessing through a satellite, uplink carriers can be separated in time rather than in frequency Instead of assigning a frequency band to each uplink, we assign a specific time interval, and a given station transmits only during its allotted interval This type of operation is referred to as time-division multiple access (TDMA) As we shall see, TDMA theoretically avoids the problem of many carriers trying to pass through the satellite at the same time, thereby avoiding the intermodulation problem of FDM A However, while FDM A involves relatively simple frequency tuning for accessing, and providing essentially independent channel on-off operation, TDMA requires communication concepts that are relatively new To accommodate many users, TDMA time intervals must necessarily be short, requiring burst-type transmissions, and the time intervals of all users must be properly and accurately synchronized, requiring several levels of timing control The required high-speed hardware for these operations is relatively new and, in many cases, is still under development

Communication system for bidirectional message transmission between ground stations, using a communication satellite

Abstract: The invention relates to a communication system for mobile radio applications for bidirectional message transmission between earth stations (3) using a communication satellite, where upward signals are transmitted in frequency division multiplex or on different frequencies for each communication channel for the uplink from the ground stations to the satellite, and downward signals are transmitted in time division multiplex for the downlink from the satellite to the ground stations. To endow the system with a high degree of efficiency, it is proposed that a transmission device (10 to 15) be provided for the downward signals on the satellite side with a transmitting antenna (15) emitting a single spot beam (FS) with a narrow lobe width and a deflection device (14) to direct the spot beam successively to different sub-areas, so-called virtual spots (Sj).

On-Board Processing

Abstract: The previous chapters concentrated on relatively straightforward satellite carrier processing, in which uplink carriers were either directly turned around on the downlink or switched between satellite antennas. The current trend, however, is toward more sophisticated processing at the satellite prior to retransmission, an area referred to as on-boardprocessing. By designing satellite payloads to accomplish more internal processing, a more efficient overall satellite link can be realized, which invariably leads to a simplification of the required earth stations. In this chapter we examine some of the present directions in on-board processing, and the expected emphasis in the next generation of communication satellites.

A system optimization approach to FDMA loadings

Abstract: The author investigates an approach to maximize frequency division multiple access (FDMA) transponder performance by simultaneously optimizing transponder gain and operating point and uplink signal powers under the assumption that the system is not bandwidth limited. The methodology used, including the assumptions and approximations made, is described. The detailed analysis with a derivation of the equations for required uplink effective isotropic radiated power are presented. An example is included to illustrate the concepts presented. >

Frequency-Division Multiple Access

Abstract: In Chapter 4 we analyzed a single-channel transponder with a single uplink carrier. We now extend the discussion to transponders designed for multiple carriers. Recall that when multiple carriers are used in satellite communications, it is necessary that a multiple-accessing format be established over the system. This format allows distinct separation of the uplink transmissions in passing through the satellite processor. In Section 1.6 three of the most common multiple-access formats were described. Each format has its own specific characteristics, advantages, and disadvantages. Satellite anomalies, such as nonlinear amplification and power division, will therefore have widely different effects on system performance for each method. Hence, it is necessary to carry out separate analysis procedures to assess analytically (and therefore design) satellite systems of each type. In this and the next three chapters we present comparisons of the accessing formats described in that section. Our basic objective is to describe the relationship, analytically and graphically, between key system parameters of the link and the established performance criterion, such as SNR and bit-error probability, of the system receivers. In this chapter we concentrate on frequency-division multiple accessing (FDMA) systems.

Architecture of ASCAMP digital hardware

Abstract: The advanced single-channel antijam man-portable (ASCAMP) is a low-power, lightweight, portable EHF satellite communications terminal supporting the Satellite Data Link Standard (1582C). The author discusses the design philosophy and architecture of the ASCAMP downlink processor (DP), a subsystem which performs baseband processing and extraction of intelligence from the received signal. To ensure absolute minimum size and power consumption, the whole DP is a custom-designed hardware structure tailored to the specific needs. The architecture chosen consists of a number of hardware modules, each performing part of the needed data transformations. All share a common memory to which they have independent, time-multiplexed access. Besides performing assigned tasks, each also posts status and addressing information concerning its most recent data read-write operations. This self-documenting action provides control for rapid computational flow through the DP. >

Code-Division Multiple Access

Abstract: In code-division multiple access (CDMA) satellite systems, uplink stations are identified by uniquely separable address codes embedded within the carrier waveform Each uplink station uses the entire satellite bandwidth and transmits through the satellite whenever desired, with all active stations superimposing their waveforms on the downlink Thus, no frequency or time separation is required Carrier separation is achieved at an earth station by identifying the carrier with the proper address These addresses are usually in the form of periodic binary sequences that either modulate the carrier directly or change the frequency state of the carrier Address identification is accomplished by carrier correlation operations CDMA carrier crosstalk occurs only in the inability to correlate out the undesired addresses while properly synchronizing to the correct address for decoding As in TDMA, CDMA carriers have the use of the entire satellite bandwidth for their total activity period, and CDMA has the advantage that no controlled uplink transmission time is required, and no uniformity over station bit rates is imposed However, system performance depends quite heavily on the ability to recognize addresses, which often becomes difficult if the number of stations in the system is large

An integrated Ka/Ku-band payload for personal, mobile and private business communications

Abstract: The Canadian Department of Communications has been studying options for a government-sponsored demonstration payload to be launched before the end of the century A summary of the proposed system concepts and network architectures for providing an advanced private business network service at Ku-band and personal and mobile communications at Ka-band is presented The system aspects addressed include coverage patterns, traffic capacity, and grade of service, multiple access options as well as special problems, such as Doppler in mobile applications Earth terminal types and the advanced payload concept proposed in a feasibility study for the demonstration mission are described This concept is a combined Ka-band/Ku-band payload which incorporates a number of advanced satellite technologies including a group demodulator to convert single-channel-per-carrier frequency division multiple access uplink signals to a time division multiplex downlink, on-board signal regeneration, and baseband switching to support packet switched data operation The on-board processing capability of the payload provides a hubless VSAT architecture which permits single-hop full mesh interconnectivity The Ka-band and Ku-band portions of the payload are fully integrated through an on-board switch, thereby providing the capability for fully integrated services, such as using the Ku-band VSAT terminals as gateway stations for the Ka-band personal and mobile communications services

A fast packet switching satellite communication network

Abstract: A multibeam fast packet switching satellite communication network serving multiple zones is studied. The synchronous time division multiple access protocol (TDMA) is used in transmitting messages in the uplink channels while the asynchronous time division multiple access protocol (ATDMA) is used in the downlink channels. Moreover, fast packet switching capabilities are assumed on-board the satellite. Alternatives for the architecture of the on-board fast packet switching fabric are considered. The performance of the considered approaches has been derived by theoretical analysis and computer simulations. A novel input queuing technique is also proposed and analyzed to show that it achieves better performance with respect to the classical input queuing approach. Therefore, by means of the novel input queuing approach it is possible to lower the end-to-end delay without increasing the complexity of the on-board equipment. >

Satellite control link for paging system

Abstract: A paging system includes a satellite uplink (40) for assembling paging information and transmitting it to a satellite (26). The satellite (26) is then operable to transmit in a simulcast manner all the information to various regional paging systems (42). Each of the paging systems (42) includes a plurality of towers (10) each containing a satellite receiver (50), a satellite dish (48) associated therewith and a paging transmitter (52). Each of the satellite receivers (50) are operable to receive the information transmitted from the satellite (26) at substantially the same time such that there is no phase difference between any of the towers, regardless of location. Therefore, the transmitters (50) can transmit through antennas (12) information at the paging frequency with substantially no relative delays relative to each of the antennas (12). A backup link receiver (28) is provided for interfacing with a backup link transmitter (70). In the event of a failure of the receivers (50) or the satellite (26), the link transmitter (70) will transmit the paging information directly to select ones of the towers (10) over a separate frequency.

Personal and thin-route communications via K-band satellite transponders

Abstract: The concept of a fully meshed network of a briefcase-sized terminals is presented for personal and thin-route communications over Ku-band satellite transponders. In this concept, undesirable double-hop delays are avoided for voice communications. The bandwidth and power resources of the transponder are efficiently shared by users in a simple demand-assigned manner via code-division multiple access (CDMA). Voice, data, and facsimile are statistically multiplexed at each terminal. In order to minimize terminal costs, frequency-precorrected and level-preadjusted continuous-wave tones are sent from the central network control station in each beam so that the terminals in each downlink beam can use these pilots as references for antenna acquisition and tracking, as reliable frequency sources, and as indicators of signal fade for uplink power control (ULPC). The potential CDMA near-far problem due to uplink fades is mitigated by using ULPC. Quasi-burst mode transmission is used to minimize the potential of clock and pseudorandom number code synchronization. >

Passive optical communications system for telecommunications network

Abstract: Passive optical communications system for telecommunications network has central fine delay device which adds to decentralised coarse delay

An optimal time slot assignment algorithm in SS/TDMA system with intersatellite link

Abstract: The time slot assignment problem in a cluster of two satellites interconnected via an intersatellite link is considered. It is shown how the optimal switching algorithm proposed by A.A. Bertossi et al. (1987) can be applied to a more general case where the number of uplink and downlink beams are not equal for each satellite in the cluster. Moreover, each uplink or downlink beam can have a different bandwidth. Details of the scheduling algorithm are given together with an illustrative example. >

A software control system for the ACTS high-burst-rate link evaluation terminal

Abstract: Control and performance monitoring of NASA's High Burst Rate Link Evaluation Terminal (HBR-LET) is accomplished by using several software control modules. Different software modules are responsible for controlling remote radio frequency (RF) instrumentation, supporting communication between a host and a remote computer, controlling the output power of the Link Evaluation Terminal and data display. Remote commanding of microwave RF instrumentation and the LET digital ground terminal allows computer control of various experiments, including bit error rate measurements. Computer communication allows system operators to transmit and receive from the Advanced Communications Technology Satellite (ACTS). Finally, the output power control software dynamically controls the uplink output power of the terminal to compensate for signal loss due to rain fade. Included is a discussion of each software module and its applications.

Hardware implementation of the advanced SCAMP downlink processor

Abstract: The advanced single-channel antijam man-portable (ASCAMP) downlink is a custom-designed digital signal processor implemented in hardware that runs a tailored and efficient microcode. Appropriate sections of microcode are executed in response to the downlink requirements. The different processing functions, hardware design, and hardware implementation are discussed. The author points out the unique feature of reprogramming field programmable gate array (FPGA) hardware in order to accomplish different processing tasks with minimal hardware. Advantages and disadvantages of using FPGAs in the advanced SCAMP implementation and ideas for future development are mentioned. The ASCAMP (EHF satellite communication terminal) has stringent size, power, and weight specifications, making for a challenging design project. The downlink processor implementation has been optimized both in concept and design to meet these specifications. >

Cellular communications system

Abstract: A code division multiple access (CDMA) spread spectrum cellular communications system with both surface and satellite nodes and using forward error correction coding, to enhance effective gain and selectivity, is provided. The system network control center (12) directs the top level allocation of calls to regional resources in the system. The regional node control centers (14) are connected to the system network control center (12) and direct the allocation of calls to ground nodes. The ground nodes (16) under the control of one of the regional node control centers (14) receive calls over land lines, spread them, modulate them, and transmit them. Satellite node control centers (18) are also connected to the network control center (12), and uplink the spread, multiplexed calls to the designated satellite (20). User units (22) respond to signals of either satellite or ground node origin. A digital data interleaving feature reduces fading. User position is determined. An adaptative transmitter power control (188) compensates for signal strength variations. An intercell bus (90) in a satellitte node (20) connects together multiple cells.

An algorithm to design a multiple-channel communications system

Abstract: The authors present a novel algorithm to design a phase-modulated residual carrier communications system for optimum performance. The algorithm has been developed as an aid in the design of two telemetry data channels operating simultaneously with a turn-around ranging signal. The algorithm provides the selection of an optimum subcarrier frequency and modulation indices for minimizing the mutual interferences among the channels to achieve desired link performance margins. The set of modulation indices provided by this algorithm will also achieve an optimum power division between the carrier, the ranging and the high data rate and low data rate channels. Finally, a software program using this algorithm has been developed for use with personal computers. This program provides the computation of the optimum subcarrier frequencies and modulation indices for a standard case that is recommended by the international Consultative Committee for Space Data Systems. >