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In mobile radio systems, especially in urban mobile service, one fixed transmitting station is located at the center part of the area, while several fixed receiving stations are installed at different location. In such systems the receiving station with the highest signal-to-noise ratio should be selected at all times. This paper presents two methods to select the best receiving station. One is an analog processed switching system. The other is a digital processed system. The design objectives, circuit diagrams and characteristics are described in detail. The performance of these two systems were compared in a field test. It became clear that there were no essential differences in the performance. The analog system was adopted in the actual mobile system taking into consideration the advantage in cost.

Radio equipment and systems designers face an alarming complexity of considerations, when they attempt to calculate probable interference rates. Only a computer is able to handle all of these variables and predict system performance to a meaningful degree of probability. Therefore, this paper describes a computer simulation approach which has been used to: (1) Explore the relation between interference rates and equipment characteristics; (2) Determine the manner in which channel loading affects the various types of interference; and (3) Analyze multi-system configurations of both the concentrated and the geographically spread types.

This paper describes the original VHF mobile radio system in the Indiana and Michigan Electric (I&ME) Company and how it operated with standard squelch. It points out some of the problems which were encountered at the relatively low operating frequency in the 37-MHz region and goes on to show the solution for most of these problems by converting the entire system of 19 base stations and 300 mobile units from standard squelch to a tone coded squelch. The entire changeover was done in an economical manner and without disrupting operation of the original system while the conversion was being made. The benefits ensuing from this conversion are covered in detail.

A signal-to-interference ratio for the transmission of scanned images and digital data in the dynamic multipath environment was derived in an earlier paper. The dynamic case depends upon a Doppler environment-signal parameter ratio, while the static result is independent of the signal structure. The dynamic case is asymptotic to, but always worse than, the static one. For a Doppler-signal ratio of less than 10 percent the dynamic signal-to-noise ratio is essentially equal to the static one. Most real systems today fall in this region. This indicates why certain simple multipath analyses in use today have been successful. The method of designing a signal to fall below the 10-percent value is indicated. Signal-to-noise ratio curves are provided for system design purposes. These results permit system analysis and tradeoff studies to be based on earth reflection coefficient distributions and aircraft or satellite operation envelopes.

This paper presents some of the economic and technical considerations for development of future noncooperative vehicle-detector systems. Vehicle-detector accuracy requirements, installation cost, location, and the roadway environment are used to develop a set of preferred vehicle detector characteristics. In addition, a description of vehicle detectors, considered for development by the Bureau of Public Roads, and some of the current vehicle-detector concepts under development, are presented.

Many agencies, both governmental and private, in the highway transportation field need a system which would automatically identify motor vehicles. The basic concept of such a system requires a vehicle-mounted coded device, a transponder, that uniquely identifies that vehicle. As this vehicle comes within the range of a system interrogator installed along or on the highway, the code is read by this interrogator and the identification of the vehicle is obtained. The ideal transponder is passive and requires no power; has no effect on, and is not affected by the vehicle; does not require physical contact with the interrogator, and requires no action by the vehicle operator. Sensing at a distance (noncontact) involves the detection of some type of energy; either radiated or reflected from the object being monitored. Many different sensing techniques, including magnetic, infrared, optical, sonic, radioactive, radio frequency (RF) transmission, microwaves, have been surveyed, investigated, and analyzed to determine their applicability for an automatic vehicle identification (AVI) system. This analysis has concluded that a low-frequency induction system is presently the most appropriate method for use on the highway environment, Several diverse systems differing in design details have been constructed and tested, utilizing this basic technique. This paper describes the design of these systems as well as the methods that have been proposed and/or developed using the other sensing technologies.

The IEEE Third Symposium on Vehicular Communications Systems was held on 13 May 1969, in Los Angeles, CA, USA, and treated the subject of electromagnetic spectrum utilization. This subject is very broad in scope and very timely in that the recent release of the JTAC report entitled "Spectrum engineering - the key to progress" should produce a long needed reevaluation of the approach to electromagnetic spectrum utilization. The content of the program is reviewed here either by the paper, an abstract, or comment by the Symposium Chairman.

The following fundamental technical questions concerning the use of the spectrum allocated to the land mobile radio service are discussed. 1) Is it technically feasible to increase the number of assignable channels within the allocated spectrum space? 2) Are other modulation methods more or less suitable for providing more channels? The primary resource material used to answer these questions is the response of the Land Mobile Section, Industrial Electronic Division, Electronic Industries Association [49]. Only the frequency spacing question is treated in this paper.

Automobile headlamp illumination is of concern because uniformity and shape of the beam pattern are quite important for optimum road visibility and safety for oncoming drivers. The Society of Automotive Engineers' specification on headlamp illumination [1] designates the maximum and minimum intensities at several spots within the beam. Many states require inspection of vehicle-mounted headlamps for illumination and aiming, and this test is commonly performed using a device which clamps on the headlamp lens. This clamp-on device is limited to checking the maximum intensity at beam center, rather than checking the overall beam pattern to the SAE-J579a specifications. In addition, mechanical limitations of this device have often resulted in the mis-aiming of vehicle headlamps. Present instrumentation available for measuring light intensity per SAE-J579a is primarily of the photo-cell type device, and the point-to-point measurement of light intensities is quite tedious and time consuming. A technique is described in this paper which incorporates electronic signal processing into a closed-circuit television system for displaying contours of constant illumination intensity. The technique is capable of rapidly performing vehicle-mounted headlamp testing per SAE-J579a and could be adapted for use at state inspection stations.

At the present time there is much interest in vehicle location systems [1]-[3]. For certain applications [4]-[7] precise location of the vehicle is not needed and determination that the vehicle is within a zonal area of the city is sufficient. This paper considers a vehicle location system in which an array of transmitters forms an "electronic fence" along the zone boundaries. As it passes throagh the electronic fence a suitably equipped vehicle receives and stores the zonal location information. This information is sent by the vehicle to a central location via a separate radio channel after receiving an interrogation message. The analysis of the communication performance of the short-range communications link between the zonal transmitters and the vehicle is the concern of this paper. Design curves and formulas are presented that allow performance calculations to be made. The implications of this predicted communication performance to a vehicle locator system are then considered. Most of the results obtained are directly applicable to a general class of vehicle location systems, which utilize the proximity of a vehicle to some coded electronic signpost to determine the vehicle's location. The electronic fence is a special case of the proximity-type system.

The design of future mobile radio systems will be based upon an estimated user demand. Systems probably will be engineered to serve the "average business day" spatial distribution of call attempts, which for the purpose of this study was assumed to be uniform. There will be day-to-day fluctuations about this long-term average, the magnitudes of which are only to be conjectured at this time. This paper compares the performance of computer simulated mobile radio systems operating with several demand profiles. The profiles were selected to give an indication of the effects on system performance of either uncompensated changes in average traffic or normal fluctuations which occur in the randomly offered traffic even when the long-term average value does not change. The dynamic channel assignment systems increased the traffic capacity at low blocking levels even over a perfectly designed fixed channel assignment system. In addition, it was found that the dynamic channel assignment systems were relatively insensitive to the periodic spatial demand distributions studied by this computer simulation. Even though new call attempts were made to fluctuate markedly between adjacent base stations within a reuse interval, the blocking rate of each base station remained constant. It appears that the blocking rate within a reuse interval depends (in the dynamic channel assignment systems) mainly on the average demand within that interval and not very strongly upon the distribution of that demand. In the fixed channel assignment system, uncompensated fluctuations in the spatial demand distribution away from the design value always degrade system performance.

Electronic clocks are rapidly becoming practical for automotive applications. These digital clocks promise not only superior timekeeping in automotive clocks, but they also could be used for a myriad of timing functions throughout the car. These range from turn signal flashers to top-speed limiting and safety-monitoring checks. Integrated circuits utilizing complementary metal oxide semiconductor devices (CMOS and silicon gate CMOS) make several system approaches feasible. This paper outlines some possible system approaches and summarizes considerations for CMOS IC's, quartz crystals, and electronic readout devices.

Several approaches to the operation of automated ground transport systems for urban use are presented and evaluated. Such systems have been suggested principally in response to the problems of urban traffic congestian and are often proposed as solutions to such problems. Thus this paper, the first of a series of three, discusses the problems of urban transportation and their origins, identifying as the principal elements the institutional structure, urban geography, and design of facilities for urban automotive transport. Importantly, the origins of a significant portion of "the urban transportation problem" lie outside the transport sector. Six potential applications for automated transport systems are suggested, ranging from specialized single-link "horizontal elevators" to general-purpose wide-area networks.

This paper is a digest of the electrical system section of a report [8] on the feasibility of using a hybrid heat engine/electrical propulsion system as a means of reducing exhaust emissions from street-operated vehicles. The electrical system is composed of an electric traction motor, a generator, control system, and batteries. Batteries are not covered in this paper, but the remainder of the items will be treated here. First, the electrical system parameters or characteristics that have the greatest impact on the total system are considered; next, details of the advantages and disadvantages of various approaches are summarized; and, finally, development efforts are recommended.

The Automatic Route Control System (ARCS) is a self-contained on-board system that directs the operation of a conventional motor vehicle over predetermined routes and controls activities (such as the delivery or pickup of items) performed along the route. In effect, the system continuously measures the vehicle's location coordinates, compares these with the route coordinates, and then issues audio, visual, and/or printed instructions appropriate to the location. The system also detects driver errors and prescribes corrective action. The routes to be followed and the actions to be taken are defined on interchangeable magnetic tape cartridges, which may be updated daily or as required from a computerized central file. ARCS enables a driver to operate efficiently over a complex route without reference to maps or lists and without prior knowledge of the route. It also eliminates tedious manual report preparation because computer-ready data for route accounting can be acquired on a magnetic tape cartridge. Although applicable to most types of operations carried out over routes that can be defined in advance, the prototype application is for the delivery of newspapers. Installed in a van with paper throwers stationed at special windows on each side, the system signals each thrower with a tone through headphones at the proper time to throw papers to individual subscribers while driving through residential areas at 15-30 mi/h.

This paper describes in detail the criteria for the design of a mobile laboratory for the characterization and measurement of man-made electromagnetic noise in the urban, suburban, and rural environment. A significant difference from previous similar measurements is the bandwidth of the receiving system, which is here of the order of 1 MHz. A simple model is proposed to describe the impulsive noise together with a series of experiments to validate the model and determine the significant necessary parameters. The detailed requirements for each of the subsystems of the mobile laboratory are discussed and some typical results obtained in trial runs are presented.

Analytically tractable statistical-physical models of man-made noise environments have been constructed [1]-[3]. These permit quantitative description of the various types of electromagnetic interference appearing in typical radio receivers and, in particular here, for the communication links employed in mobile transportation systems and urban environments generally. This paper presents a summary of some of the principal analytical results obtained to date [1], [4], and includes some suggested next steps for joint theoretical and experimental study of these increasingly important phenomena. First-order probability density functions (pdf's) and probability distributions (pd's) are obtained explicitly; (higher order pdf's and pd's may also be found by similar methods) [2]. These models are based on a Poisson distribution of sources in space. The approach is canonical, in that the results are, in form, independent of particular emitted waveforms, propagation conditions, source distributions, beam patterns, and specific system parameters, as long as the interference is narrow-band following (at least) the aperture and/or the RF stages of a typical receiver. Considered here only are the cases of communication interference, where source and receiver bandwidths are comparable. The paper concludes with a short discussion of some features of suggested future interaction between theory and experiment.

In those radio communications applications where area coverage is not desired, a series of repeaters, each operating at low power, can be used to extend the radio over long distances. Microwave relay systems operate on this principle with each node representing a point of potential local distribution. Where it is desirable to have the nodes closely spaced, such as along a highway, it becomes desirable to utilize a single frequency for all transmissions in one direction in the interest of conserving spectrum. This paper covers such a system operating in the 935-MHz band. The system described provides both the "backbone" communications system which employs single-frequency repeaters and the "roadside" communications from the repeater node points to users in their vehicles. The functioning of the repeaters is described, and the relationship between signal levels, isolation, external noise levels, and thresholds is presented. Both analog and digital modulations are employed, and it is shown how each repeater is remotely switched between "backbone" and "roadside" communications modes. Extensions from current single-channel to multiple-channel operation is described in terms of the application to a variety of highway-related uses. The results of propagation experiments both along the "backbone" and on the "roadside" are described and related to theoretical models. From these data, conclusions can be drawn about the relationship between repeater spacing, height, and power level, as well as their relationship to user antenna height, output power, and receiver sensitivity.

The design of electronic fuel injection (EFI) controls for the automobile must take into account limitations imposed by the automotive environment and by mass production. This paper outlines the fundamentals of EFI controls and provides insight into the unique problems encountered in the design of a viable system. The impact of the electrical and physical automotive environment is analyzed, and the constraints arising from volume production and cost considerations are discussed.

Proper selection of the propulsion system to meet the requirements of an electric passenger vehicle is of prime importance. Choice may be made among alternating current, direct current, or combined systems. Each approach can be implemented by utilizing various control techniques and power elements. For instance, an ac system may use an adjustable voltage inverter (AVI) or a pulse-width-modulated inverter (PWMI) for the power processor, and an induction motor or a synchronous motor for the rotating element. Similar alternatives exist for a dc system. The choice of a suitable manner of implementation must be. based on information characterizing the various elements and tradeoffs matching them to the vehicle requirements. This paper describes various ac and dc techniques, characterizes AVI, PWM, and representative dc chopper power processors and examines their interface with a variety of motors such as series, shunt, induction, etc.

Advances in the design, development, and fabrication of key microwave components along with those in electronics have resulted in growing optimism about the application of radar technology to solving the automotive highway collision problem. However, before an automotive radar system can be fully realized, a major effort will be required to define the environment in which the system must operate, and to obtain a fundamental understanding of the influence of these RF parameters. One such parameter which will have a considerable effect on the design of automobile radars is that due to the presence of natural and man-made surfaces within the beamwidth of the antenna. Generally, the presence of such a surface will result in incident RF energy being reflected off the surface in a forward direction and some portion being scattered in a backward direction. This paper presents the results of a program which was designed to measure this type of reflectivity characteristic for common road surfaces. Measured data are presented along with a theoretical study demonstrating the influence that this phenomenon can have on highway collision radar systems.

This paper reports some propagation and coverage prediction results using various models provided to the Federal Communications Commission, Spectrum Management Task Force.

This paper begins with a discussion of multipath interference. The spatial description of the field impinging upon a mobile radio antenna is derived and the power spectrum and other properties of the signal envelope are considered. Next, large scale variations of the average signal are discussed. Measurements of observed attenuation on mobile paths over both smooth and irregular terrain are summarized. The paper concludes with a discussion of methods of predicting the area of coverage from a base station.

High capacity personal rapid transit (HCPRT) is a system concept which utilizes small, 4 to 6 passenger, vehicles at very short headways on exclusive guideway networks. The automatic operation of small vehicles at headways of 1 s or less presents a major technical problem which is amenable to a combination of design approaches. This paper explores the effect of basic parameters such as vehicle length, reaction time, emergency and failed vehicle deceleration rates, and emergency jerk rate on potential minimum operating headway. The results of this analysis are then discussed in the context of five HCPRT programs.

The design of a mobile or portable radio communications system requires the consideration of a number of unique engineering factors which do not necessarily apply to the design of a fixed, or point-to-point system. The design procedure is a combination of theoretical and empirical methods, which when taken with measured data, produce a system design meeting the required specifications. This paper will treat that portion of the system design pertaining to radio frequency propagation and system coverage. It stresses a realistic measurement program which will provide coverage data in support of the system design.

It is known that mobile communication systems are susceptible to radio-frequency interference (RFI) generated by automotive ignition systems. In this paper the automotive ignition noise generated by individual engine cylinders is investigated. Using specially designed instrumentation, the pulse amplitude densities of individual cylinder ignition noise are measured and analyzed. It is found that the ignition noise generated by the individual cylinders is not statistically uniform and the modes of the pulse amplitude densities of different cylinders may differ by as much as 30 dB (as measured by an antenna positioned to one side of the vehicle). The measurement procedure and results discussed in this paper may prove to be valuable in the diagnosis of ignition system RFI.

The St. Louis Metropolitan Police Department is the first major urban police department to implement an automatic vehicle monitoring (AVM) system. The AVM technology incorporates computer-aided dead-reckoning, thus facilitating vehicle tracking on individual streets in a city. Implemented as a Phase I prototype system in one police district early in 1975, the test system is evaluated in this paper utilizing a three-pronged approach. Focusing on 1) technology, 2) police operations, and 3) attitudes and organizational impact, attention is given to operational performance in Phase I, to ameliorative action for Phase II, and to the affects of AVM on response time, officer safety, voice-band congestion, and command and control.

Today's electric vehicles are severely limited in multistop and go driving range and accelerating capability by the lead acid battery's inability to handle high power peaks while maintaining maximum energy storage capability. A hybrid flywheel/battery system can be used to isolate the battery from the accelerating power peaks, and should recover a substantial part of the braking energy. This paper describes the development of a small, high speed, lightweight flywheel/ ac synchronous motor alternator sealed energy storage package coupled into the battery and dc drive motor system through a simple rectifier/inverter power circuit. This system stores just enough energy in the rotor of the machine for one start-stop cycle. Provision is made to add a flywheel to store energy for several cycles, or enough energy for climbing or descending long grades. The fields of the two machines are electronically controlled to achieve optimum performance and effective energy utilization.

Sensors faithfully transduce automotive control parameters into electric signals suitable for transmission to the electronic controls. Two parameters key to engine control are crankshaft position and manifold absolute pressure, which are used in both fuel and ignition control systems. This paper describes a variety of sensor concepts for each of the two parameters, and provides guidelines for expediting screening tests. The case for including sensor selection as an integral part of system trade-off studies is also argued.

Lateral control is an essential function for all forms of individual automated ground transport. Here two aspects of such control--the magnetic field distributions associated with a guideway-based, wire-reference configuration, and the associated vehicle-based sensors--are reviewed. Two wire-reference schemes have thus far been suggested. In the first, the amplitude characteristics of the magnetic field are employed to obtain the lateral control signal. In this paper, a theoretical analysis and detailed field measurements are used to define both "ideal" characteristics and those which would be encountered in a realistic operating environment. The resulting problems, which involve amplitude distortions of the field due to the proximity of steel-reinforcing materials, are defined, and their effects on vehicle control--poor tracking and passenger discomfort--are discussed. In the second approach, the lateral control signal is primarily dependent on the phase characteristics of the magnetic field. A theoretical analysis and a corresponding experimental field study, which are discussed herein, indicated that the problems associated with the first approach were largely overcome, as evidenced by successful full-scale tests of an automatically steered vehicle.

The basic purpose of this paper is to identify the potential requirements for maritime satellite communication and position fixing as it relates to both present and future operational systems (e.g., MARISAT, INMARSAT, etc.). Particular emphasis is given to the need for position fixing and ship terminal equipment utilization in national waters, as well as other major aspects. Present MARISAT operation and industry evaluation activities are briefly discussed, highlighting a few major programs and some of the potential benefit area thus far generally identified by the users of MARISAT. A comparison is made between satellite communication and the existing MF/HF system, including direct-printing radio telegraph (marine radio telex). From various standpoints, it appears that maritime satellite systems are more desirable than conventional (nontelex) marine telegraph/telephone. In addition, the improved performance, capability, and potential of satellites could outweigh the slightly lower annual traffic and equipment cost for HF marine telex.

Mobile communication through electromagnetically obstructed areas can be established by using wayside leaky coaxial cables. Available experimental data had been limited to 450 MHz. This paper presents the results of a recent experiment of the RADIAX cable at 900 MHz in near-field. These data are used to supplement an earlier RADIAX experiment at frequencies below 450 MHz and to compare with a theoretical study at 1 GHz.

In preparing or revising an RF channel plan for a group of mobile radio nets operating in the same region, the order in which the nets are assigned channels can be crucial to success. One well-known principle for selecting such an assignment sequence is to rank the channel requirements of the various nets in descending order of "assignment difficulty." This paper describes an easily automated heuristic assignment technique in which the channel requirements that prove themselves to be "difficult," through repeated failures to be assigned channels, rise rapidly toward the top of the list of requirements. The heuristic technique is useful in solving complex frequency-assignment problems that involve cochannel, adjacent-channel, spurious and intermodulation interference; nonrepetitive zone structures; fixed pre-existing frequency assignments; and frequency-resource lists that contain gaps and vary from zone to zone.

Results are presented of UHF propagation measurements conducted at 820 MHz in urban Philadelphia. The Philadelphia tests departed from previous path-loss measurements in that emphasis was placed on transmission from relatively low land-site antennas at heights of 40-70 feet above local ground level. Composite path-loss statistics have been derived for transmissions from five separate locations covering local environments considered typical of an urban area. This paper describes the measurement configuration, summarizes the data processing techniques, and presents results of the analysis.

Mobile telephone control unit design guidelines for the Advanced Mobile Phone Service have been developed based on driving behavior, user preference, automobile environment, and calling procedure considerations. This paper explains these guidelines and illustrates their application by reviewing five control unit design approaches.

An important aspect in the design of automated guideway transit (AGT) systems is the communication requirements for the longitudinal control of vehicles, that is, the requirements to measure vehicle states, transmit this information between vehicles, and subsequently control the individual vehicles. This paper examines the trade-offs in the data rates, word lengths, and allowable transmission time lags needed to maintain an acceptable level of vehicle performance, and how these requirements relate to the allocation of control computation between vehicle- and wayside-based computers. A nonlinear control law designed for short-headway vehicle-follower systems provides a baseline to examine these trade-offs. The specification of the parmeters entering the problem is approached through analytically derived results, using simplified linear models with verification and expansion of these results through a detailed simulation.

The concept of a universal emergency number originated in Europe and in 1967 subsequently received federal government impetus for adoption in the United States. In 1968 the American Telephone and Telegraph Company agreed to clear the number 911 for any public safety agency and community groups that desired to develop a common public number for entry to local emergency services. Approximately 800 911 systems have been developed in the United States since 1968. In a large urban area a citizen requiring emergency assistance from area public safety agencies can be faced with choosing from over 200 emergency numbers. The choice depends variously on the location and time of day or week of the emergency. The 911 system concept was developed to transfer the agency selection process to the public safety agencies: taking it from a public generally lacking knowledge of emergency operations and telephone numbers. This paper discusses problems of public access and their resolution by 911 systems. Basic and advanced 911 systems are described. The finance, technology, operations, and management of the implementation of 911 are detailed. The description points out the necessary interactions between public support, various levels of government, and the telephone company. The difficulties encountered while resolving intra/interagency and jurisdiction disputes are stressed. Conclusions regarding the future of 911 are presented and center around the need for public support to smooth the way for cooperation among all public and private entities involved in 911.

In this paper, both simulations and measurements are used to explore the propagation properties of ultrahigh-frequency (UHF) waves traveling through building structures and, in particular, windows. The propagation influence of several features of a typical window structure is examined, including the electrical properties of the surrounding wall, the homogeneity and roughness of this wall, and the incidence angle of the UHF wave. In these studies, ray tracing is benchmarked against both narrow-band measurements and the results of a full-wave moment method technique. Some of the inherent limitations of ray tracing are assessed and corrective methods are proposed in order to improve its accuracy.

Multiple-antenna receiving-diversity was shown previously to be effective in mitigating the effects of random angular orientation and multipath radio propagation for portable radiotelephones. This paper shows that time-division adaptive retransmission used with appropriate antenna configurations can also mitigate these effects. The retransmission configurations require fewer antennas than the receiving diversity configurations for a given improvement in relative signal-to-noise ratio (S/N). Cumulative distributions of S/N were determined for adaptive retransmission and diversity using random orientation and multipath propagation models. Distributions of S/N for systems with two antennas at the portable set and two appropriately polarized antennas at the Portable Radiotelephone Terminal (PORT) are similar to distributions for two branch selection diversity in the fixed-oriented mobile radio environment. Systems with one portable antenna and two PORT antennas have distributions with slopes similar to two branch mobile radio distributions but the distributions for the portables range from 3 to 7 dB worse.

This paper studies optimum signal combining for space diversity reception in cellular mobile radio systems. With optimum combining, the signals received by the antennas are weighted and combined to maximize the output signal-to-interference-plus-noise ratio. Thus, with cochannel interference, space diversity is used not only to combat Rayleigh fading of the desired signal (as with maximal ratio combining) but also to reduce the power of interfering signals at the receiver. We use analytical and computer simulation techniques to determine the performance of optimum combining when the received desired and interfering signals are subject to Rayleigh fading. Results show that optimum combining is significantly better than maximal ratio combining even when the number of interferers is greater than the number of antennas. Results for typical cellular mobile radio systems show that optimum combining increases the output signal-to-interference ratio at the receiver by several decibels. Thus, systems can require fewer base station antennas and/or achieve increased channel capacity through greater frequency reuse. We also describe techniques for implementing optimum combining with least mean square (LMS) adaptive arrays.

This paper describes a feasibility study of the adaptive mobile access protocol (AMAP) for MSAT-X, a proposed experimental mobile satellite communication network. The mobiles are dispersed over a wide geographical area and the channel data rate is limited due to the size and cost limitations of mobile antennas. AMAP is a reservation based multiple-access scheme. The available bandwidth is divided into subchannels, which are divided into reservation and message channels. The ALOHA multiple-access scheme is employed in the reservation channels, while the message channels are demand assigned. AMAP adaptively reallocates the reservation and message channels to optimize system performance. It has been shown that if messages are generated at a rate of one message per hour, AMAP can support approximately 2000 active users per 2400 bit/s channel with an average delay of 1.4 s.

Aiming at the choice of a method for the field strength prediction over irregular terrain in land mobile radio service which could be adopted as the most suitable for engineering purposes, six out of the known methods have been analyzed. To verify their prediction accuracy, complex field tests have been carried out. With the moving vehicle, the field strength has been measured along well defined path profiles. Statistical processing of the recorded data has yielded the median field strength levels in a number of points of the path serving as reference for the accuracy assessment. Other features of the compared methods, such as their operativeness, complexity, suitability for the computer application, influence of subjective factors, and tolerances in determining the initial data, have been discussed. The "clearance angle" method, recently proposed and experimentally verified in detail during the field test described in this paper, has been chosen, after an argued analysis, as the most suitable field strength prediction method for engineering purposes.

Densensitization, or the blocking of the receiver portion of the communications system, is one of the principal causes limiting receiver operation, especially in mobile networks under cositing situations. A clear understanding of this nonlinear effect is thus essential when attempting to improve the reliability of mobile radio communication systems so as to prevent severe interferences. A thorough qualitative analysis of desensitization effects on receiver parameters is provided and semi-empirical and statistical relationships useful for performing quantitative computations are developed. The developed analysis initially treats the computation of desensitization effects on radio communication system scenarios for both a single interfering transmitter and several simultaneous interfering transmitters. The results of this qualitative analysis then allow us to compute quantitatively the desensitization effects of a typical mobile radio system which are presented in a second paper.

In this paper 16 kbit/s digital voice transmission with conventional channel spacing of 25 kHz, employing a 16 kbit/s adaptive delta modulation (ADM) coder-decoder (CODEC) is evaluated. The main characteristics of narrow-band digital FM modulation schemes, such as tamed FM, Gaussian filtered minimum shift keying (GMSK), four-level FM and phase locked loop-quaternary phase shift keying (PLL-QPSK), are compared by laboratory tests. Digitized voice quality in a digital channel incorporating a 16 kbit/s ADM CODEC and GMSK coherent detection was compared with voice quality of a conventional analog FM channel. Bit error ratio (BER) performance is shown to depend primarily on demodulation schemes. Digital voice quality is inferior to that of analog voice with an opinion score difference of about 0.5 in fading environments. This kind of digital voice transmission will be applicable for those systems that require high security at an expense of speech quality.

Design and performance of a single-chip minimum shift keying (MSK) coherent demodulator fabricated by complementary metal oxide semiconductor-integrated circuit (CMOS-IC) technology is described. The demodulator consists of a phase detection circuit, carrier recovery circuit, data recovery circuit, and timing-clock recovery circuit. For the carrier recovery circuit, three types of Costas-loop are reviewed from the viewpoint that MSK modulation format has a close relationship to binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). Among these loops, a loop of center-frequency locking scheme modified from a BPSK Costas-loop, termed MSK Costas-loop in this paper, is adopted for IC implementation. Digital IC design techniques are next described. Utilizing the sample-and-hold operation of the digital devices, a divided-frequency locking scheme of the quadrature coherent demodulation is proposed. Finally, IC demodulator performance is experimentally shown in the static and Rayleigh fading environments. The bit error rate performance and error-burst characteristic are measured. It is concluded that the single-chip coherent demodulator is suitable for digital mobile radio application.

Theoretical and experimental studies are made on radiation patterns of mobile antennas within the frequency range from 300 to 900 MHz. In order to calculate vertical plane patterns of quarter wavelength monopole and a half-wavelength dipole located on an automobile's roof, a theoretical model of an automobile body is developed, in conjunction with the geometrical theory of diffraction (GTD). The theoretical model consists of three-perfectly conducting plates. It is ascertained from a comparison between calculated and measured results for the feed point impedance and the radiation pattern that the effect of the body and the earth can be well examined with the theoretical model. The results of pattern calculation for the above two antennas show that the radiation patterns depend largely on the configuration of the automobile body and the earth condition. The technique described in this paper can be used as a design tool to estimate radiation performance of mobile antennas.

The distribution of the channel occupancy time in a cellular radio system is studied, namely, the distribution of the time spent by customers on one frequency channel within a given cell. Two approaches are used. The first one relies on a computer simulation allowing a general model for mobiles behavior. The second is analytical, and assume a simplified system where mobiles keep constant directions. The results of both methods are compared with a fitted exponential distribution, and excellent agreements are found for most practical situations. In addition, a simple expression giving the average number of handoffs for a random customer is obtained. The results derived in this paper provide useful and simple tools to analyze cellular systems.

The commenter calls the attention of the authors of the above paper (see ibid., vol.38, p.204-10, Nov. 1989) to very closely related earlier work that deals with the essential issues in a more fundamental way. Specifically, these consider traffic capacity comparisons for demand-assigned, multiple-access systems that use slotted or unslotted ALOHA-type request channels. The analysis and results are directly applicable to any arrangement of channels in an FDMA (frequency multiple access)/TDMA (time division multiple access) multiplex and also account for the possibility that requests cannot be served because specific assets are busy although message channels are available. The authors agree that using the blocked-calls-queued service discipline to increase traffic capacity has been analyzed in the previous work cited. They point out that this call-processing discipline is particularly useful in the satellite mobile radio dispatch application examined because of the dispatcher bottleneck. Another important issue in mobile satellite demand-assignment multiple-access for radio dispatch is the large fraction of signaling overhead relative to the very short holding time of individual mobile radio calls. The authors note that their paper offers the techniques of multiple-calls batch-processing and pipeline signaling as effective measures in dealing with this issue. >

A computer simulation tool for evaluating the speech traffic handling capability of digital cordless telecommunications systems has been developed. The focus of the paper lies on the teletraffic engineering problems posed by time division multiple access (TDMA) systems operating with time division duplex (TDD) on one radio transceiver in radio environments. The impacts of fixed station separation, fixed station capacity, spatial traffic variability, dynamic channel assignment, microdiversity, and propagation model on the system capacity are investigated. The simulator serves as an invaluable tool for conducting sensitivity studies and its outputs provide useful insights on traffic performance of digital cordless systems. >

Pilot symbol assisted modulation (PSAM) has been shown to give good performance in Rayleigh fading and additive noise. In cellular systems, though, it is cochannel interference, more than noise, that limits the system. The paper is the first analytical investigation of the performance of PSAM in fading, interference, and noise. The investigation yields both an exact solution and an accurate but much more computationally efficient approximation. >