Showing papers on "Telecommunications link published in 1993"

Mobile communication satellite payload

Abstract: A communications satellite payload provides for efficient communications between user devices, such as hand held cellular telephones, and terrestrial gateways that may be connected to world-wide public telephone and telegraph (PTTs) networks. The satellite payload simultaneously supports a plurality of independent antenna beams, for example 12 or 16 beams, on both the uplink and downlink for user devices employing full duplex communications. The satellite payload furthermore supports an identical number of full duplex channels between a terrestrial gateway and the satellite. The communications satellite payload is carried by one satellite of a plurality of such satellites in a low earth orbit (LEO) constellation to provide worldwide communications. Downlink beam forming occurs prior to final amplification of transmitted signals, thereby reducing losses within the beam forming equipment. A multi-element phased array of receiving antennas and a multi-element phased array of transmitting antennas is also disclosed, wherein each element of each phased array is utilized for each individual one of a plurality of received or transmitted beams.

Performance issues and algorithms for dynamic channel assignment

Abstract: Using dynamic channel assignment (DCA) algorithms to select communications channels as needed, time-division multiple access (TDMA) or frequency-division multiple access (FDMA) systems can serve dynamic and nonuniform traffic demands without frequency planning as long as quality is sufficient and equipment is available. Here, performance issues and algorithms for DCA in a TDMA portable radio system are considered. A fixed number of traffic servers (time slots) per radio port is assumed: therefore, the system capacity is hard-limited by the equipment availability, and the DCA efficiency is compared mainly through the signal-to-interference ratio in both the uplink and downlink directions. >

Closed-loop downlink telemetry and method for implantable medical device

Abstract: A closed loop system for attaining premium telemetry performance for vector field electromagnetically coupled telemetry systems. The disclosed system utilizes a linear ramped envelope for downlink RF bursts, resulting in a detected pulse width in the implanted device which is proportional to the downlink detection margin. The detected pulse width, when uplink telemetered to an external programming unit allows the programming unit to generate a telemetry status feedback signal for the user positioning the programming head with respect to the implant site. Feedback based upon both uplink and downlink signal strength in especially advantageous in dual-coil systems where there are distinct and unique coupling coefficients for uplink and downlink.

Communications protocol for digital telemetry system

Abstract: The present invention relates to a communications protocol for a digital telemetry system which enables more efficient digital data transmission between a plurality of digital communications nodes The protocol is implemented using uplink and downlink packets and superpackets Each node forms an uplink packet which contains node data and protocols Those packets are packaged into uplink superpackets and transmitted to a control node Downlink superpackets containing information signifying which of the uplink superpackets were received correctly are formed and transmitted to the individual nodes In response, any uplink superpacket received incorrectly is retransmitted Additionally, the communications protocol also time stamps messages received from the nodes to provide for digital communications time correlation

Analysis of a direct-sequence spread-spectrum cellular radio system

Abstract: The uplink and downlink performance of a digital cellular radio system that uses direct sequence code division multiple access is evaluated. Approximate expressions are derived for the area averaged bit error probability while accounting for the effects of path loss, log-normal shadowing, multipath-fading, multiple-access interference, and background noise. 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 combining. The RAKE receivers are shown to improve the performance significantly, except when the channel consists of a single faded path. Error correction coding is also shown to substantially improve the performance, except for slowly fading channels. >

On importance sampling in digital communications. I. Fundamentals

Abstract: A simulation algorithm design strategy based on the combination of event simulation, conditional importance sampling, and asymptotically optimal biasing of Gaussian noise inputs is discussed. The utility of this approach is illustrated by presenting numerical results for a satellite channel model that includes uplink and downlink noise sources, a travelling-wave tube amplifier (TWTA) nonlinearity, and intersymbol interference (ISI) from both uplink and downlink filtering. An overview and comparison of various simulation design strategies and some results on the optimization of general mean translation and variance scaling biasing schemes for nonlinear systems are presented. >

Voice and data integration in the air-interface of a microcellular mobile communication system

Abstract: A multiple-access protocol and a call acceptance algorithm for voice and data integration in a microcellular mobile communication system are presented. The protocol supports circuit-mode voice, burst-mode voice, and data. A hybrid multiplexing scheme with no boundaries performs statistical multiplexing, the call-level (for circuit-mode voice) and the talkspurt/message-level (for burst-mode voice and data). This scheme achieves high utilization of the available bandwidth compared to pure circuit switching, but with a lower quality in the latter two classes, due to delay during channel access on each talkspurt/message. A two-party transaction model for each class is implemented, giving a realistic load on uplink and downlink. A unified access procedure is presented, and the structure of the required control bursts is described. Performance is analyzed using simulation, and the optimum data-segment size is obtained. The maximum acceptable load is determined for various traffic mixes. A call acceptance algorithm is implemented, and typical simulation results for delay and call blocking are given. >

Capacity improvement of base-station antenna arrays cellular CDMA

Abstract: In this paper, the use of antenna arrays at base-station for cellular CDMA is discussed. We present a performance analysis for a multiple cell CDMA cellular system with an antenna array at base-station for both base-station to mobile (downlink) and mobile to base station (uplink) links. Considering the effects of path loss, Rayleigh fading, log-normal shadowing, multiple access interference, and the thermal noise, we show that using an antenna array at base-station, both in receiving and transmitting, to resolve angular positions of the mobiles, leads to a many-fold increase in system capacity. We also present simulation results. >

Soft handoff in a cellular telecommunications system

Abstract: A soft handoff in a cellular telecommunications system in which a user mobile station (30) relays user communication radio signals via at least two base stations (BS) to and from a controlling mobile exchange (MSC) connected to another system user. The relay of user information signals is performed by using digital frames carried on digital transmission connections (70-76, 80-86) in the uplink and downlink direction. The differential delay of the two or more terrestrial routes between the base stations and the exchange is compensated by a novel time alignement (TA) procedure. Each of the base stations sends time aligment request to the mobile exchange which performs the frame time alignement of the downlink transmissions by taking into account TA requests from all base stations.

Low Cost Channel Estimation in the Uplink Receiver of CDMA Mobile Radio Systems

Abstract: Optimum signal processing in the receiver of future cellular CDMA mobile radio systems requires the estimation of the actual channel impulse responses on the basis of training signals which are embedded into the transmitted signal. Up to now, channel estimation in the case of the uplink, in which each user signal passes through a different radio channel and in which a different training signal is needed for each user turns out as so expensive that the application of CDMA in spite of its advantages appears questionable. In this paper, a channel estimation method is proposed and analyzed in which the different training signals of all users are time displaced versions of one and the same basic signal. Thus a low cost realization of the channel estimator in the form of a correlator becomes possible. Übersicht: Die optimale empfängerseitige Signalverarbeitung in künftigen zellularen CDMA-Mobilfunksystemen erfordert in den Empfängern das Schätzen der aktuellen Kanalimpulsantworten auf der Basis von Trainingssignalen, die in die Sendesignale eingebettet werden. Die Kanalschätzung erweist sich in der Aufwärtsstrecke, bei der jedes Teilnehmersignal eine andere Funkstrecke durchläuft, und bei der man für jeden Teilnehmer ein anderes Trainingssignal benötigt, bisher so aufwendig, daß das Verwenden von CDMA trotz der Vorteile dieses Vielfachzugriffsverfahrens in Frage zu stellen ist In dieser Arbeit wird ein für die Aufwärtsstrecke geeignetes Verfahren der Kanalschätzung vorgeschlagen und analysiert, bei dem die unterschiedlichen Trainingssignale aller Teilnehmer zeitverschobene Versionen ein und desselben Signals sind. Dadurch wird es möglich, den Kanalschätzer sehr aufwandsgünstig als Korrelator zu realisieren. Für die Dokumentation: CDMA / Mobilfunk / Kanalschätzung / Maximum-Likelihood-Schätzung / Korrelation / Inverses Filter

Coherent detection with reference-symbol based channel estimation for direct sequence CDMA uplink communications

Abstract: A new coding/modulation arrangement for up-link direct-sequence code division multiple access (DS-CDMA) communication, which employs coherent detection with reference-symbol based channel estimation, is presented. It is shown that the required E/sub b//N/sub O/ for this scheme is about 2 dB to 2.5 dB lower than non-coherent detection of Walsh coding with bit interleaving and about 1 dB to 1.3 dB lower than noncoherent detection with Walsh symbol interleaving over the entire range of practical vehicle speeds. A frequency domain analysis of this scheme is given.

Method and apparatus for controlling signal quality in a cdma cellular telecommunications system.

Abstract: A system and a method for controlling the radio signal quality between a plurality of mobile stations (MS) and a plurality of base stations (BS) connected to a mobile switching center (MSC) of a code division multiple access (CDMA) cellular telecommunications system is described. According to the invention, each base station (BS) is adapted to measure the quality of each active CDMA communication link and to report for each active CDMA radio communication to the mobile switching center (MSC) the corresponding measured communication signal quality. The mobile switching center (MSC) is adapted to perform an averaging process on said reported quality measure values, in order to calculate a recommended quality measure threshold value. The base stations (BS) are adapted to receive said recommended threshold value for use in an uplink closed loop power control algorithm for the mobile stations (MS).

Asynchronous transfer mode link recovery mechanism

Abstract: An inter-node communications link failure recovery system for ATM nodes in which connections are quickly switched from a faulty link to one or more existing links. Within a node, alternate connection routes are predetermined for every connection originating at or terminating to a circuit connected to a link of interest (i.e., a link where recovery from a link fault is desired). The routing information for alternate connections is stored in the ATM node CPU memory. In response to an alarm indicating a break or other failure of link, the Network Management System sends a command to the node to establish hardware "loopback" path in the node interface circuit connected to the faulty link. The ATM node 10 then down-loads the alternate routing information for the failed link to the node routing tables used to route a cell onto a link in accordance with the cell header data. This causes all cells originally set up for transmission over the faulty link to be re-routed to a predetermined alternate link.

Fiber-optic microcell radio systems with a spectrum delivery scheme

Abstract: The authors examine microcell radio systems that use subcarrier transmission and spectrum delivery switch (SDS) techniques to meet traffic demands for a large number of portable phones. Radio signals are transmitted over fibre between base stations (BS) and a central station (CS). Relationships between blocking probability and offered traffic are described as a parameter of SDS ports. Optical link performances, such as link gain and carrier-to-noise ratio (CNR), are presented. Moreover, it is found that using FM techniques can improve the dynamic range by 20 dB compared to conventional methods for the uplink from BS to CS. The proposed technique can also increase the number of transmittable carriers compared to conventional methods for the downlink from CS to BS. This network strategy results in more flexible transmission. >

Beacon detection method and apparatus for sharing spectrum between wireless communications systems and fixed microwave systems

Abstract: A system for spectrum sharing between a point-to-point microwave system and a TDM/TDMA wireless communications system in a common geographical area is described. Each transmitter/receiver in the point-to-point microwave system transmits, in addition to its normal information signal, a beacon signal which is uniquely associated with the receiver receive frequency. In selecting uplink and downlink frequencies for port assignment, each port monitors the beacon frequencies and selects as uplink and downlink frequencies, frequencies which associated beacons fall below a predetermined threshold. Similarly, before accessing the wireless communications system through a selected port, a portable monitors the beacons associated with the selected port's uplink frequency. If the beacon associated with the uplink frequency exceeds a threshold, the portable selects an alternate port through which to access the network. Also, during a call an emergency link transfer to another port is made if the monitored beacon corresponding to the portable's uplink frequency rises above the threshold.

Satellite identification and antenna alignment

Abstract: A satellite identification system for identifying a communications satellite from which a broadcast communication signal is being received by an antenna, wherein the communication signal includes data identifying a programmer that broadcast the communication signal and/or an uplink location from which the communication signal is broadcast. The system includes a memory storing a look-up table correlating satellite identification data for a plurality of satellites with the programmer identification data and/or the uplink location data for the plurality of satellites; means for detecting the programmer identification data and/or the uplink location data from a communication signal received by the antenna from one of the plurality of satellites; and means for accessing the look-up table in response to the detected programmer identification data and/or the uplink location data to retrieve the satellite identification data for the satellite from which the communication signal is received. The satellite identification system is included in a satellite antenna alignment system for automatically identifying reference satellites. The satellite antenna alignment system also automatically aligns the antenna to a position at which optimum quality is achieved for a communication signal received from a reference satellite; and the alignment positions of the antenna are measured and stored for at least two reference satellites to which the antenna is automatically aligned. Current correlated data is provided in the look-up tables by detecting updated correlated data in a communication signal received by the antenna and loading the updated correlated data into the look-up tables.

BER of multi-carrier CDMA in an indoor Rician fading channel

Abstract: The paper analyzes a novel digital modulation technique called multi-carrier code division multiple access (MC-CDMA) in which data symbols are transmitted at multiple subcarriers where each subcarrier is modulated by a 1 or -1 based on a spreading code. Analytical results are presented on the performance of this modulation scheme in the downlink of an indoor wireless Rician fading channel. In addition, the performance of a controlled equalization technique that attempts to restore the orthogonality between users is evaluated. >

GSM base-station antenna diversity using soft decision combining on up-link and delayed-signal transmission on down-link

Abstract: Two branch base-station antenna diversity performance for the GSM (Groupe Speciale Mobile) system is simulated. For the uplink combining, soft decision signals from two SOVA data-detectors are used for a (nearly) optimum post-detection combining. For downlink, a delayed-signal (on the order of 2-b periods) is transmitted on a second antenna branch in order to introduce "artificial" time dispersion in the radio channel. This technique takes advantage of the equalizing properties of the GSM data-detector. The simulation results show that the multipath fading margin can be reduced by 6-13 dB on uplink and by 3-10 dB on downlink for the GSM specified test channels RA250 and TU50.

Beacon detection system for sharing spectrum between wireless communications systems and fixed microwave systems

Abstract: A system for spectrum sharing between a point-to-point microwave system and a TDM/TDMA wireless communications system in a common geographical area is described. Each transmitter/receiver in the point-to-point microwave system transmits, in addition to its normal information signal, a beacon signal which is uniquely associated with the receiver receive frequency. In selecting uplink and downlink frequencies for port assignment, each port monitors the beacon frequencies and selects as uplink and downlink frequencies, frequencies which associated beacons fall below a predetermined threshold. Similarly, before accessing the wireless communications system through a selected port, a portable monitors the beacons associated with the selected port's uplink frequency. If the beacon associated with the uplink frequency exceeds a threshold, the portable selects an alternate port through which to access the network. Also, during a call an emergency link transfer to another port is made if the monitored beacon corresponding to the portable's uplink frequency rises above the threshold.

Optimizing the RAKE receiver for the CDMA downlink

Abstract: The optimal filter solution for the code division multiple access (CDMA) downlink is derived. A practical suboptimal filter that recovers part of the loss introduced by the RAKE receiver is presented. A significant performance advantage is obtained when the proportion of total interference that comes from the same base station as the signal is large. There is a design tradeoff between the complexity of the practical filter and its performance. As the complexity increases, the performance approaches that of the optimal filter.

System for handling data errors on a cellular communications system PCM link

Abstract: A digital cellular communications system has a bidirectional pulse code modulation (PCM) link for conveying encoded speech and control data in the form of blocks, a switching unit coupled to a first end of the PCM link, and a base station coupled to a second end of the PCM link. The switching unit includes a speech encoder for encoding speech signals, an error detection code generator for generating an error detection code from the encoded speech signal, and a formatter, coupled to the error detection code generator, for producing a formatted block to be transmitted on the PCM link, the formatted block having a header, the encoded speech dam, and the error detection code. The base station has a PCM link error detector for signalling the existence of an error in a received block in response to the received block containing an error. The base station further includes a downlink error handler coupled to the PCM link error detector, for discarding a received block in response to the signal indicating the existence of an error therein, and for substituting therefor a default control message to be relayed to a mobile station, the default control message having no meaning defined by the digital cellular communications system, whereby the mobile station, in response to receiving the default control message, performs no operation other than a bad frame masking operation. Uplink error detection and handling is also provided.

A method of bidirectional communication in a cellular mobile radio communication system wherein different base stations are used for the downlink and uplink connection

Abstract: The invention relates to methods for handling a bidirectional connection involving a mobile station in a cellular mobile radio communication system having adjacent cells of substantially different sizes, wherein the mobile station estimates the strength of radio signals received from base stations for cells of different size, the signal strength of radio signals received from the mobile station by base stations for cells of different size are estimated, decisions on which base station(s) should be responsible for an uplink channel of a bidirectional connection are based on uplink parameters including estimated strength for signals received at base stations, the signal strength at the base station responsible for the uplink channel is used for controlling the output power of the mobile station, and decisions on which base station(s) should be responsible for the downlink channel of a bidirectional connection are based on downlink parameters including estimated strength of radio signals from base stations received at the mobile station.

Integrated data link concept for air traffic control applications

Abstract: An integrated full duplex radio data link for aircraft capable of a sustained 4 to 8 Mb/sec raw data transfer rate communicates on newly proposed dedicated digital channels (DDC) for radio communications between the existing analog VHF amplitude modulated (AM) voice channels, and causes no perceptible interference to the remaining AM voice channels. While the present invention is intended primarily for terminal areas and airport surface applications, it may be employed with enroute applications. The present invention transmits AM encoded digital aircraft identification information appended to AM voice transmissions. An ground unit strips off the aircraft identification and assigns a DDC frequency to that aircraft unit over a control uplink channel (CUC) for further data communication. Downlink information, such as acknowledgement of the DDC assigned is transmitted over a digital control downlink channel (CDC). Data communication then proceeds on the DDC. The data is transmitted as a low power encoded signal having a constant envelope, and narrow bandwidth. Any interference from the DDC into the AM voice channel appears as background "white noise" to the AM receiver. The channel assignment scheme allows the present invention to function at an acceptable bit error rate.

Communication protocol for a mobile communication system

Abstract: A communication protocol for use in mobile network such as a TETRA or GSM network is proposed. The protocol more particularly is concerned with the signalling associated to half-duplex communications wherein a mobile terminal (A, B) communicates with a base station (BS) in one direction only by using a plurality of time slots per communication frame (USF1/17, DSF1/17) of either a downlink (DL) or uplink (UL) channel. The protocol associates to these communications a full-duplex signalling channel by allocating a single signalling time slot of both uplink and downlink signalling frames thereto. In this way an hitherto unnoticed drawback of known protocols is avoided, namely the wasting of bandwidth through allocation of multiple signalling time slots as well as the ambiguity of the network terminals as to which of these time slots is to be used by them or will be used by their communication partners.

Power control and diversity for the downlink of CDMA systems

Abstract: The authors study the capacity of a CDMA-based microcellular wireless system in the base to portable (downlink) direction. Narrowband CDMA with system bandwidth of 1.25 MHz is considered. Coherent reception using a pilot signal, BPSK modulation with 1/2 convolution codes, and power control are considered. Capacity of the downlink is evaluated as a function of multipath diversity, path loss exponent, and power control strategies. Both slow and fast power control on the downlink are considered. With slow power control, the capacity is severely limited. It is observed that fast power control significantly improves the capacity of the downlink. Finally, the effect of path loss exponent on the downlink capacity is illustrated.

On-board closed-loop congestion control for satellite based packet switching networks

Abstract: NASA LeRC is currently investigating a satellite architecture that incorporates on-board packet switching capability. Because of the statistical nature of packet switching, arrival traffic may fluctuate and thus it is necessary to integrate congestion control mechanism as part of the on-board processing unit. This study focuses on the closed-loop reactive control. We investigate the impact of the long propagation delay on the performance and propose a scheme to overcome the problem. The scheme uses a global feedback signal to regulate the packet arrival rate of ground stations. In this scheme, the satellite continuously broadcasts the status of its output buffer and the ground stations respond by selectively discarding packets or by tagging the excessive packets as low-priority. The two schemes are evaluated by theoretical queuing analysis and simulation. The former is used to analyze the simplified model and to determine the basic trends and bounds, and the later is used to assess the performance of a more realistic system and to evaluate the effectiveness of more sophisticated control schemes. The results show that the long propagation delay makes the closed-loop congestion control less responsive. The broadcasted information can only be used to extract statistical information. The discarding scheme needs carefully-chosen status information and reduction function, and normally requires a significant amount of ground discarding to reduce the on-board packet loss probability. The tagging scheme is more effective since it tolerates more uncertainties and allows a larger margin of error in status information. It can protect the high-priority packets from excessive loss and fully utilize the downlink bandwidth at the same time.

A contention-based reservation protocol using a TDD channel for wireless local area networks: A performance analysis

Abstract: A typical configuration of a LAN having wireless terminals is a network of microcells. The authors report on a microcell which consists of a single base station and wireless terminals. A channel allocation scheme between uplink (terminal-to-base) and downlink (base-to-terminal) communications and a media access control (MAC) protocol used for the uplink communication are considered. In particular, a reservation protocol with a slotted ALOHA reservation channel utilizing the TDD (time division duplex) technique is studied. The performance of the protocol is analyzed in terms of throughput and average message delay by the equilibrium point analysis (EPA) method. The authors examine how the ratio of the uplink channel capacity to the downlink channel capacity affects the performance under various traffic conditions. >

Digital data transmission over nonlinear satellite channels

Abstract: The M-ary PSK system over nonlinear satellite channels, in the presence of an arbitrary number of interferers and additive Gaussian noise on the uplink and downlink paths, is investigated. The effect of intersymbol interference due to the transmitter filter is also included. The bit error probabilities are obtained using the Gauss quadrature formula and a two-dimensional moment technique. >

Method and apparatus for decentralizing uplink network control functions and minimizing terminal power level assessment overhead in radio frequency data communications networks

Abstract: Methods and apparatus are set forth which enable a radio frequency data communications network (1) to perform decentralized uplink network control functions (decentralized away from the centralized general communications controller (GCC) level of the network hierarchy); (2) to minimize, or in some cases eliminate, the overhead expended in making power lever (signal strength) determinations and assessments at both the GCC and base station levels of the network hierarchy (for the purpose of managing uplink communications); and (3) to allow individual subscriber radios (terminals) to dynamically determine the most desirable base station to communicate with when exchanging information a host computer via a shared RF communication channel (the link between a terminal and the base station). Additionally, an illustrative radio frequency data communication network, built in accordance with the teachings of the invention as a modified version of an existing network, is described where the modified network incorporates the aforementioned methods and apparatus.

On-board processing for Ka-band applications

Abstract: The work reported arose from a study carried out for the European Space Agency to examine the impact of 20/30 GHz operation on the on-board processing requirements for a satellite system intended to serve private user networks of small fixed terminals interconnected with 2 Mb/s ISDN-like links. Fading in Ka band leads to the requirement of expensive HPAs in the user terminals. It is therefore an advantage to use FEC on the uplink. Continuous mode transmission simplifies on-board decoding (and interleaving), and allows higher coding gains. A simple access scheme is FDMA with variable data-rate channels, therefore a nonuniform demultiplexer is required on-board. There are two methods that belong to the same general class. The fast convolution method is inherently more efficient than the analysis-synthesis method since clock synchronism can be exploited and the output sampling rate can be decimated down to the symbol rate. The frequency resolution is inherently larger and the placement of carrier frequencies are more flexible. CDMA is attractive as its complexity is similar to TDMA, since demultiplexing in the code domain requires only additions. >