Showing papers in "IEEE Transactions on Vehicular Technology in 1987"

Antenna diversity in mobile communications

Abstract: The conditions for antenna diversity action are investigated. In terms of the fields, a condition is shown to be that the incident field and the far field of the diversity antenna should obey (or nearly obey) an orthogonality relationship. The role of mutual coupling is central, and it is different from that in a conventional array antenna. In terms of antenna parameters, a sufficient condition for diversity action for a certain class of high gain antennas at the mobile, which approximates most practical mobile antennas, is shown to be zero (or low) mutual resistance between elements. This is not the case at the base station, where the condition is necessary only. The mutual resistance condition offers a powerful design tool, and examples of new mobile diversity antennas are discussed along with some existing designs.

Channel occupancy time distribution in a cellular radio system

Abstract: 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 near/far effect in local ALOHA radio communications

Abstract: The ALOHA protocol allows many uncoordinated terminals to transmit data packets through a common channel. The application of ALOHA to short range (indoor or mobile) radio systems containing mobile terminals that send packets to a central base station is studied. In such systems, terminal-to-base distances can be vastly different and there can be pronounced shadowing. This gives rise to a capture effect at the receiver, where the packet arriving with the highest energy has a good chance of being detected accurately, even when other packets are present. It is well known that this capture effect improves the throughput and delay characteristics of the system as a whole. One of the principal findings is that even the user with the weakest signal shares in these improvements. This result is derived for the realistic case in which each terminal has a buffer to store packets that arrive while the terminal is waiting to transmit packets that were destroyed in earlier collisions.

Mobile digital communications via tone calibration

Abstract: A new digital signaling technique that is particularly suited for channels impaired by multipath fading is presented. The proposed modulation scheme employs a continuous-wave (CW) tone to calibrate the mobile channel against the multipath-induced phase uncertainties. This technique is applicable to quaternary phase shift keying as well as to more complicated signal constellations such as M-ary phase shift keyed schemes. The advantages of tone calibration are: 1) robustness of the receiver and 2) elimination of the link dependent error floor. Furthermore, since the CW tone can also be used for coherent signal demodulation, carrier phase acquisition can be achieved within a bit time. This property is particularly attractive when a burst of data with a short burst length has to be detected. This radio technique is useful for both the terrestrial mobile and the newer satellite-aided mobile communication (SAMC) services.

Measurements and models of a land mobile satellite channel and their applications to MSK signals

Abstract: The fading and shadowing effects observed on land mobile satellite signals are characterized in statistical terms. Models are developed in terms of probability distribution of the signal's envelope and phase as well as its rate of change with time. The multipath fading and shadowing effects modelled are typical of those encountered in a mobile-satellite link in rural and suburban areas. A comparison of the models with experimental data at 870 MHz and 1542 MHz is given. Applications of the models to predict performance of a minimum shift keying (MSK) signal at 2400 bits/s show that large margins are required to compensate for the effects of fading and shadowing. In addition, the results show that conventional coherent demodulation of a MSK signal may not be feasible due to phase variation caused by fading and shadowing. On the other hand, the results show that random FM has negligible effect on the probability of error of the MSK signal at 2400 bits/s when frequency demodulation is used.

Measurements of the wide-band radio channel characteristics for rural, residential, and suburban areas

Abstract: A setup for impulse response measurement using short RF pulses and a wide-band receiver is described. The impulse response is measured for every 10 cm of displacement. Results are presented showing the delay spread and the average power delay profiles for rural, residential, and suburban areas. Also some tools are presented to estimate the impulse response, using the radar equation and a diffuse scattering model.

New cellular schemes for spectral efficiency

Abstract: Assuming the constraint of an allocated frequency spectrum, two new schemes are suggested to increase spectral efficiency. Furthermore, the two new schemes can be integrated to become a hybrid system and offer greater performance which takes advantage of the merits and performance of each individual scheme.

Analysis for spectrum efficiency in single cell trunked and cellular mobile radio

Abstract: Spectrum efficiencies for single-cell trunked and cellular mobile radio systems are analyzed, taking into consideration fading circumstances and call blocking probability. Two kinds of transmit power control methods are considered. One retains area mean power at the receiver constant and the other retains local mean power constant. For single-cell trunked systems, it has been shown how spectrum efficiency is limited by the required traffic quality or permitable adjacent channel interference. A narrower channel spaced system shows superiority to wide-band systems, if an adjacent channel interference protection ratio is lower than about -40 dB and a few percent interference probability is permitted. On comparing cellular systems, superiority in spectrum efficiency is determined by the product of protection ratio and channel spacing. Call blocking probability and interference have little effect on spectrum efficiency comparison. For both systems, the two kinds of transmit power control and the two interchannel interference criteria show no significant difference between their effects on spectrum efficiency.

Multipath delay spread in a hilly region at 210 MHz

Abstract: Investigations of multipath propagation in hilly terrain show that the delay spread between the first and the last significant path may be greater than 30μs. The method of measurement is described, and the propagation conditions are illustrated by representative examples.

The effects of multipath delay spread on timing recovery

Abstract: Equations for the recovered timing for a squaring timing recovery circuit under multipath radio propagation are derived. Both coherent and differential detections are studied. If delay spread is much smaller than the symbol duration, the recovered timing can be approximated by the centroid of the power delay profile, p(t). Two cases of timing loop bandwidth are considered. If the fading frequency is much lower than the bandwidth of the timing loop, the instantaneous sample of p(t) is used to generate the timing clock. If the fading frequency is much higher than the loop bandwidth, the ensemble average of p(t) over fading samples is used to recover the timing. A computer simulation is performed for a system operating in a frequency-selective, slowly fading environment. It is found that for root mean square (rms) delay spread less than or equal to 0.1 of the symbol duration, a squaring timing loop with either narrow or wide bandwidth can properly determine the timing detection. The main mechanism of the "irreducible bit error rate" in this case is the closure of the eye-pattern instead of timing error.

A comparison of data modulation techniques for land mobile satellite channels

Abstract: Several modulation schemes for transmitting data over land mobile satellite channels are compared using a Monte Carlo simulation. Schemes under consideration include differentially detected minimum shift keying (DMSK), differentially detected filtered offset quadrature phase shift keying (DOQPSK), and coherently detected binary phase shift keying with transparent tone-in-band processing (BPSK-TTIB). The transmission of data to and from a mobile radio, which is also capable of operating as an amplitude companded single sideband radio, is the application considered here. The nominal bit rate is 2400 bit/s, while the nominal channel spacing is 5 kHz. DOQPSK with nonredundant single-error correction (SEC) is shown to be a promising candidate. It is capable of outperforming DMSK with SEC by more than 1 dB. Techniques that send a reference signal along with a PSK signal and then perform coherent detection, such as BPSK-TTIB, are also shown to be inferior to DOQPSK with SEC for the class of channels considered here.

Idle-signal casting multiple access with collision detection (ICMA-CD) for land mobile radio

Abstract: A population of mobile terminals communicating with a central station over a packet-switched multiple-access radio channel is investigated. In land mobile radio communication, in which terminals cannot sense the carrier transmitted from other terminals (either because they are out-of-range or obstructed), a central controlled multiple access system in which a central station broadcasts idle/busy information of the access channel is used for multiplexing the packets from the terminals. Idle-signal casting multiple access (ICMA), which is a practical application of this central controlled system, has been used in the Nippon Telegraph and Telephone Public Corp. (NTT) mobile phone system for some years. ICMA-CD, which is an advanced ICMA scheme characterized by collision detection and evaluates its improved throughput, channel capacity and loss probability in a mobile radio fading environment with restrictions on retransmission, is proposed. It is clarified that ICMA-CD is suitable for the mobile multiple access scheme, especially in the case where packet detection delay and collision recovery time are short and offered traffic is heavy.

Co-channel interference measurement method for mobile communication

Abstract: A new method of measuring co-channel interference for mobile communication systems is proposed. In this method, the carrier envelope is sampled, and signal-carrier-to-interference-carrier ratio (CIR) is calculated by signal processing. The circuits for measurement are composed of an envelope detector, an analog-to-digital converter, and a microcomputer. Theoretical and experimental evaluations of measurement errors are examined, and the method is proved to be promising. Using this method, the signal-to-interference ratio of up to 20 dB is measured, within 2 dB error for fading frequency of less than 80 Hz.

Novel receiver structures for systems using differential detection

Abstract: Novel receiver structures are introduced which improve the bit error rate performance of differentially detected MSK and QPSK systems. These new receiver structures are based on 1) combining with feedback, 2) a simple Viterbi decoder which uses one- and two-bit detector outputs for maximum-likelihood sequence estimation, and 3) using the correlation of noise for the partial removal of its effect prior to decision. The new receiver structures presented are useful in mobile radio and mobile satellite communications where power efficiency, synchronization, and implementation complexity are of primary concern.

Fractional-bit differential detection of MSK: A scheme to avoid outages due to frequency-selective fading

Abstract: A new scheme of fractional-bit differential detection of minimum shift keying (MSK) is proposed. This technique is aimed at reducing timing fluctuation of eye patterns, which is found to be a cause of abnormally long burst errors in a frequency-selective fading channel. We have confirmed in a detailed laboratory experiment that such long-burst errors can be eliminated by employing a differential delay shorter than one-bit interval, thereby yielding remarkable improvement in the measured average bit error rates. The result, in a way, highlights the influence of eye-pattern timing fluctuation on an actual digital mobile radio system.

UHF bent-slot antenna system for portable equipment—I

Abstract: A newly developed bent-slot antenna for UHF portable communication equipment is introduced. The bent-slot antenna, modified from a half-wave slot element, is designed to have both vertical and horizontal elements and is intended to achieve the improvement of the receiving performance in the urban area. The antenna system characteristics such as current distribution, radiation pattern, gain, and input impedance are investigated both theoretically and experimentally for such parameters as the slot location, the feed point and the width and height of the equipment box. In the calculation, the equipment box is assumed as a rectangular conducting box, and the surface of the box and the slot are modeled by wire grids. The method of moments is applied to find both the electric and magnetic current distributions on wires.

Microprocessor-based railway interlocking control with low accident probability

Abstract: A laboratory model incorporating a microprocessor-based system to control signals and points and to monitor track circuits is described. Approach locking and backlocking controlled through a programmable timer are included. The model consists of a control panel with extrance-exit type control, namely a switch and a pushbutton for each route. Light emitting diodes (LED's) take the place of signal lamps. Points are simulated by means of three position neutral polar relays. A method of reducing the probability of accidents associated with this type of control is presented. The method consists of duplicating the input channels and utilizing redundant input data.

Frequency offset receiver diversity for differential MSK

Abstract: A new receiver diversity scheme for differential detection of minimum shift keying (MSK) is proposed. The signal from each receiving branch is translated to a different IF frequency. The IF signals are summed and then detected by a common differential detector. The diversity scheme does not need phase adjusters, signal quality measurement circuits or a switching controller; moreover the error rate performance is equivalent to that of postdetection equal gain combining diversity, which needs plural complete receiving systems. Theoretical analysis and computer simulation results show that the diversity gain of the proposed scheme is about 13 dB at a 10-3error rate under Rayleigh fading and 5 dB under Rician fading when the signal-to-interference power ratio is 10 dB. The static error rate performance improvement was confirmed by experimental test results. Simulation results with data from mobile propagation studies are also shown. Even in heavily shadowed cases, the probability that fading attenuation is less than 17 dB is improved from 80 to 99 percent, and a 7 to 13 dB improvement in margin requirement for 99 percent of time operation is achieved.

UHF bent-slot antenna system for portable equipment—II: Receiving performance in urban areas

Abstract: The receiving performance of UHF portable equipment with a bent-slot antenna is studied on the basis of experimental results. The received power increase in the experiment is analyzed by applying the ray and random models to urban area structures. The analysis shows that the bent-slot antenna designed with both vertical and horizontal polarization components has contributed to an increase in received power for some limited angles of wave incidence, as a result of combined reception of cross-polarized waves in an actual environment.

On selection of system bit-rate in the mobile multipath channel

Abstract: A simple analytic model of the multipath fading channel is presented. For a given echo density and distance of communication, it is shown that as the system bit-rate is increased, the probability of unacceptable distortion will rapidly increase. As the bit-rate is reduced, flat fading will increase. In between these two cases there exists a bit-rate for which the mnltipath components can be exploited by an adaptive receiver to improve performance. The probability of this event is shown to have a maximum value for a particular bit-rate. It will also be shown how the maximum acceptable bit-rate depends on range, receiver complexity, and propagation characteristics.

Impact of frequency-selective fading on digital land mobile radio communication at transmission rates of several hundred kbit/s

Abstract: Based on measurements reported from New York City and the city of Berne, Switzerland, and its outskirts, computer simulations of the mobile radio channel transfer functions were performed. It is demonstrated that the impulse response may suffer considerably from large variations of the channel characteristics. It is concluded that powerful means of signal processing are necessary to safeguard digital land mobile radio communication at transmission rates of several hundred kbit/s.

The near field of annular antennas

Abstract: The circular loop antenna has been investigated by many authors beginning as early as 1897 with Pocklington's study of the thin wire loop excited by a plane wave. Later, both Hallen and Storer considered the case of driven antennas. All of these authors used a Fourier series expansion for the loop current, and the latter two found numerical difficulties with the approach. These difficulties, as pointed out by Wu, can be avoided by integration of the free-space Green's function over the toroidal surface of the wire. The theory developed here is an improvement on previous works because it specifically takes into account the finite dimension of the wire. Consequently, the range of validity for our solution extends to fatter wires than previously considered. Furthermore, some detail of the loop current around the cross section of the wire is revealed. The theory, if applied to thin wire loops, gives radiation admittances in excellent agreement with results obtained by previous investigators. The computations of the electromagnetic fields near resonant loops show that the electric field strength is larger by a factor of ten than previously published values. Experimental results validate the theory presented in the following sections.

An improvement effect by a hard-limiter on differential detection performance of PSK in frequency-selective fading

Abstract: Differential detection of PSK after an IF hard-limiter is described. This detection scheme is well known for the reason that the receiver structure is greatly simplified with saturated amplifiers and a digital-type differential detector. In addition, it can give the improved effect of reducing eye pattern timing fluctuation which is found to be a cause of excessive errors in a frequency-selective fading channel. Error rates were measured for binary and quadrature phase-shift keying. The results show great improvement over the detection scheme employing linear amplifiers and an analog differential detector.

Effects of channel narrowing in small zone FM mobile communication systems

Abstract: In small zone (cellular) FM mobile communication systems, greater geographical reuse is achieved when the modulation index is larger. A large index, however, imposes a small number of channels. The spectrum utilization factor in total thus is determined as a synthesis of available channel number and geographical frequency reuse. From this viewpoint, relationship between the modulation index and spectrum utilization factor is discussed under the Rayleigh fading condition, with and without diversity. To take into account the actual traffic condition not only uniform but also a concentrated traffic distribution condition is considered. It is shown that the optimum peak deviation that realizes the most efficient spectrum utilization is from 3 to 4 kHz for uniform traffic distribution, and 2 kHz for concentrated traffic distribution. These results show that narrow channel spacing, such as 12.5 kHz, is better than the conventional 25 kHz or wider channel spacing to achieve efficient spectrum utilization.