N. Amitay

Radio propagation at microwave frequencies for line-of-sight microcellular mobile and personal communications

A propagation experiment has been designed and conducted at 900 MHz and 11 GHz to characterize microcell channels using various antennas at two distinct frequencies. It is found that propagation in rural areas is dominated by interference between the direct, line-of-sight ray and a specular roadway-reflected ray. In urban areas, the addition of four specular wall-reflected rays adequately represents microcell propagation. The dependence of mean power falloff, measured mean power and calculated power on distance was determined. The lambda /2 scale microvariations of the received power are reduced compared to the variations in present cellular radio systems. For urban sites using omnidirectional base and mobile antennas, the RMS delay spread due to road- and wall-reflected rays was obtained from a six-ray model. Using a 20-dB horn for the mobile antenna can reduce this delay spread. >

Modeling and computer simulation of wave propagation in lineal line-of-sight microcells

The model and implementation principles governing the computer simulation of line-of-sight (LOS) microcell wave propagation are presented. Multiray propagation above a plane earth constitutes the basic model. For rural microcells, two rays are used while, for urban microcells, 10 rays are used propagating in a dielectric canyon. The model is applied to two relevant problems. In the first, control of electric field falloff with distance, r, is studied. ln the second, the advantages of switched transversal antenna diversity are demonstrated. Power falloff rates of 1/r/sup 4/ and 1/r/sup 6/ were obtained for simple two-element array structures. This may provide an additional interference control tool in the layout and design of microcells. The interference between the various rays propagating in lineal urban microcells results in deep nulls at various locations. The deleterious effects of these nulls can be eliminated by employing simple switched transverse antenna diversity. >

Outdoor BLAST measurement system at 2.44 GHz: calibration and initial results

There are ever increasing demands for additional capacity in wireless communications to handle voice, data, and wideband Internet applications. These demands are constrained by the bandwidth that was allocated to wireless communications. The spectral efficiencies in present day wireless systems hover around 1 bit/s/Hz. Bell-labs Layered Space-Time (BLAST) is a communication technique for achieving very high spectral efficiencies in highly scattering environments using multiple transmit and receive antennas. A measurement campaign was undertaken to assess the BLAST gains in spectral efficiency in the suburban outdoor environment for stationary subscribers. The measurements employed directive antennas to better control interference from adjacent cells. The measurements were performed over a narrow band at 2.44 GHz with five transmitting and seven receiving antennas, respectively. Extensive calibration methods, assisted by simulations, were developed to assure accurate results for the BLAST capacities of the measured remote subscriber sites. Initial results indicate that BLAST capacities of C/sub B//spl ges/38 bits/s/Hz at 20% of the measured locations and C/sub B//spl ges/24 bits/s/Hz at 50% of these locations are feasible, for reasonable link parameters and negligible interference.

Multipath Outage Performance of Digital Radio Receivers Using Finite-Tap Adaptive Equalizers

Recent analysis/simulation studies have quantified the multipath outage statistics of digital radio systems using ideal adaptive equalization. In this paper, we consider the use of finite-tap delay line equalizers, with the aim of determining how many taps are needed to approximate ideal performance. To this end, we assume an M -level QAM system using cosine rolloff spectral shaping and an adaptive equalizer with either fractionally spaced or synchronously spaced taps. We invoke a widely used statistical model for the fading channel and computer-simulate thousands of responses from its ensemble. For each trial, we compute a detection signal-to-distortion measure, suitably maximized with respect to the tap gains. We can thereby obtain probability distributions of this measure for specified combinations of system parameters. These distributions, in turn, can be interpreted as outage probabilities (or outage seconds) versus the number of modulation levels. A major finding of this study is that, for the assumed multipath fading model, very few taps (the order of five) are needed to approximate the performance of an ideal infinite-tap equalizer. We also find that a simple, suboptimal form of timing recovery is generally quite adequate, and that fractionally spaced equalizers are more advantageous than synchronously spaced equalizers with the same number of taps. This advantage is minor for rolloff factors of 0.5 and larger but increases dramatically as the rolloff factor approaches zero.

Distributed switching and control with fast resource assignment/handoff for personal communications systems

An architecture for a combined personal, mobile, and (possibly) loop access microcellular communications system in which calls are digitized and packetized is presented. A network of microcells is utilized to communicate with the various subscribers via radio. A fast handoff/resource assignment process is introduced. It is subscriber-controlled and is jointly performed by the subscribers and base station employing the resource auction multiple access (RAMA) algorithm. In this deterministic algorithm, resource utilization is independent of traffic load. Using selected GSM parameters with RAMA, for the purpose of illustration, it is shown that, in microcells with delays of 1 mu s, 1080 resource assignments per second are feasible: and with delays as high as 45 mu s, 216 assignments per second are feasible. >

Numerical solutions for an infinite phased array of rectangular waveguides with thick walls

A numerical analysis of an infinite phased array of open rectangular waveguides has been made which includes the effects of wall thickness. Two planes of scan, the H and quasi- E planes, have been considered. In these cases, the general vector problem can be expressed in the form of a scalar one-dimensional Fredholm integral equation of the first kind. The approximate fields obtained numerically from the integral equation are used for the evaluation of the input complex reflection coefficient. A variational expression for the reflection coefficient is developed and used for improving the accuracy. Numerical results for the H and quasi- E plane scans are presented as a function of wall thickness and scan angle. Agreement with experimental results is very good.

Resource auction multiple access (RAMA) for statistical multiplexing of speech in wireless PCS

The excess capacity of resource auction multiple access (RAMA), originally proposed for fast handoffs and resource allocations in wireless personal communications systems (PCS), is evaluated for statistical multiplexing of speech. Using selected GSM parameters in conjunction with M-ary FSK for signaling, it is shown that, in cells with propagation delays of up to 45 /spl mu/s, 216 assignments/s are feasible. The aim is to exploit this large assignment capacity to increase channel utilization. The authors show that, for packet dropping probabilities of 1%, RAMA can have a multiplexing gain as high as 2.63 with fast speech detection and 2.28 with slow speech detection. RAMA permits graceful degradation during peak traffic demand by operating at higher packet dropping probabilities. The authors also observe that, at low values of packet dropping probability, delays experienced by transmitted packets are more evenly distributed for the case of fast speech detection while the bulk of the packets experience less delay with slow speech detection. Speech clipping statistics associated with various values of packet dropping probabilities are also presented. >

Resource auction multiple access (RAMA) in the cellular environment

Resource auction multiple access (RAMA) is a technique for rapidly assigning communication resources (e.g., frequency carriers and/or TDMA time slots) in a wireless environment. This scheme is deterministic in that, barring errors caused by fading and noise, a resource assignment is made in each and every assignment cycle. Previous papers reported on the ultimate call assignment capacity of this technique, limited only by propagation delays, and also demonstrated its capabilities for statistical speech multiplexing in a PCS environment. We extend the previous work in two directions: (1) we examine the effects of channel fading and noise; and (2) we consider both GSM and IS-54 cellular environments. Propagation is modeled, for purposes of this study, as flat Rayleigh fading, with mean propagation loss as a parameter. We focus on the subscriber-to-base link and consider several forms of diversity at the base. We also consider two RAMA modes (call assignment without and with statistical speech multiplexing). We show that a simple form of two-branch diversity can be very effective. In both GSM and IS-54, for example, using simple diversity and an average transmit power of 100 mw yields assignment error probabilities of only 1% for losses as high as 151 dB.<<ETX>>

On energy conservation and the method of moments in scattering problems

Electromagnetic scattering problems, including waveguide discontinuity, phased array, and scattering (exterior type) problems, are frequently described by integral equations that can be solved by the Ritz-Galerkin or generalized method of moments. Under appropriate conditions, it has been shown that reciprocity and variational properties are, in fact, preserved in the approximate solutions. It is shown here that in the Ritz-Galerkin method, energy is also conserved under certain conditions, even in those scattering problems where reciprocity does not exist. Hence energy conservation cannot serve as a check for accuracy of a numerical solution obtained by the Ritz method or other related methods.

Propagation measurements at microwave frequencies for microcellular mobile and personal communications

A series of measurements was conducted in both rural and semiurban areas to measure radio propagation characteristics at microwave and millimeter-wave frequencies in a microcellular mobile radio environment where transmission is essentially line-of-sight (LOS). Measurements were made at 11 GHz with a fixed-site antenna located at the side of the road at heights of up to 30 ft. Data have been taken with both omnidirectional and directional antennas on the mobile, to compare received signal characteristics. It was found that, in rural and semiurban environments, propagation is dominated by interference between the direct ray and a specular, roadway-reflected ray. The average power received does not decay any faster than 1/r/sup 2/ over the measured range of transmitter-to-receiver separation. A wideband communication channel looks feasible, since the dual-ray interference effects and the random components of the received signals can be overcome by simple methods such as higher antenna directivity or space diversity.<<ETX>>