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.

Antenna diversity in mobile communications

A new model is presented for the prediction of attenuation by rain on either terrestrial or slant earth-to-space propagation paths. The model was developed using geophysical observations of the statistics of point rain rate, of the horizontal structure of rainfall, and of the vertical temperature structure of the atmosphere. The model was tested by comparison with attenuation distribution observations. The results show excellent agreement; the observations differ from model predictions by less than the rms deviations predicted by the model.

Prediction of Attenuation by Rain

This paper reviews the theories and the results of numerical calculations of interactions between hydrometeors and microwaves or millimeter waves propagating in media containing these hydrometeors. At frequencies above about 5 GHz, rain, snow, or ice crystals in high altitudes cause various degrading effects on the performance of communication links. The attenuation of the transmitted signals is a well-known example of such effects. Because of the demand for more communication channels, operating frequencies of both terrestrial and earth--scace communication systems become increasingly higher, and the communication technology becomes more and more sophisticated with the use of digital techniques, or the use of both orthogonaly polarized channels. These have given rise to the need of acquiring more detailed knowledge of interactions between hydrometeors and waves than is necessary for the calculation of attenuation. To obtain such knowledge, we have to know the detailed microphysical properties of hydrometeors, such as shape, size distributions, or the motion of hydrometeors while they are falling. These properties, some obtained previously by meteorolgists and some obtaianed recently by radio engineers to meet the requirements of present radio problems, are reviewed first. Various methods of calculations, and the detailed knowledge obtained from calculations of the single scattering properties of hydrometeors, whose sizes are generally comparable to wavelength, are then reviewed. With the information of these scattering properties, various propagation phenomena are discussed from a theoretical point of view. Specific subjects treated include attenuation, cross polarization due to rain or ice crystals in high altitudes, the methods of depolarization cancellation, rain scattering other then forward and backward which is necessary in the estimation of rain scatter interference between two communication links operating at the same frequency, various multiple or incoherent scattering effects on copolar and cross-polar signals, pulse shape, channel transfer characteristics, etc. The aim of this review is to provide fundamental background informations on the interactions between hydrometeors and radio waves, and hence the more practical investigations, such as long-term statistics of attenuation, are beyond the scope of this review.

Electromagnetic wave propagation and scattering in rain and other hydrometeors

Fading channels may improve the capacity of slottedALOHA networks. Rayleigh fading and spatially distributed packet transmitters diminish mutual packet interference and thus reduce the average blocking probability of a shared receiver. A transmission model, combining Poisson statistics of the offered data traffic with Rayleigh statistics for the fading channel, is developed. Analytical and numerical results are presented, indicating significant improvements to signaling in cellular mobile networks and in packet radio systems with fading radio channels.

Capacity of Slotted ALOHA in Rayleigh-Fading Channels

The broadcast nature of a wireless link provides a natural eavesdropping and intervention capability to an adversary. Thus, securing a wireless link is essential to the security of a wireless network, and key generation algorithms are necessary for securing wireless links. However, traditional key agreement algorithms can be very costly in many settings, e.g. in wireless ad-hoc networks, since they consume scarce resources such as bandwidth and battery power.Traditional key agreement algorithms are not suitable for wireless ad-hoc networks since they consume scarce resources such as bandwidth and battery power.This paper presents a novel approach that couples the physical layer characteristics of wireless networks with key generation algorithms. It is based on the wireless communication phenomenon known as the principle of reciprocity which states that in the absence of interference both transmitter and receiver experience the same signal envelope. The key-observation here is that the signal envelope information can provide to the two transceivers two correlated random sources that provide sufficient amounts of entropy which can be used to extract a cryptographic key. In contrast, it is virtually impossible for a third party, which is not located at one of the transceiver's position, to obtain or predict the exact envelope; thus retrieve the key. Since in the presence of interference strict reciprocity property can not be maintained; our methodology is based on detecting deep fades to extract correlated bitstrings. In particular, we show how a pair of transceivers can reconcile such bitstrings and finally flatten their distribution to reach key agreement. In our constructions we use cryptographic tools related to randomness extraction and information reconciliation. We introduce "secure fuzzy information reconciliators" a tool that enables us to describe robust key generation systems in our setting. Finally we provide a computational study that presents a simulation of a wireless channel that demonstrates the feasibility of our approach and justifies the assumptions made in our analysis.

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Robust key generation from signal envelopes in wireless networks

Relationships between rain and ice attenuation and depolarization for several incident polarizations have been determined experimentally for the first time. The relationships are based on measurements made using 19- and 28-GHz beacon transmissions from a Comstar satellite. Vertical and horizontal incident polarizations experience much less depolarization on the average than circular polarization or linear polarization incident at 45\deg . The measurements confirm that the usual orientations of the major symmetry axes of nonspherical raindrops and ice crystals are nearly horizontal. Joint cumulative distributions of attenuation and depolarization for one year of continuous measurement at 19 and 28 GHz are also presented. These distributions are useful for determining joint attenuation and depolarization margins required to meet outage objectives of specific satellite communication systems. For example, the outage of a 19-GHz dual-polarized system received with a polarization angle 21\deg from horizontal and having an attenuation margin of 20 dB would be depolarization-dominated unless the system could also tolerate at least -15-dB depolarization.

Characteristics of rain and ice depolarization for a 19- and 28-GHz propagation path from a Comstar satellite

This paper presents measured fade duration and levelcrossing statistics of a hardware-simulated mobile radio channel. These measurements were taken over a wide range of fade depths. The measurements agree well with existing analytical results and extend them to show the distribution of fade durations for shallow fades. Fine detail in the fade-duration distributions for shallow fades is produced by peaks in the fading spectrum.

Fade-Duration Statistics of a Rayleigh-Distributed Wave

We present the analysis and design of an efficient hybrid digital/analog multichannel hardware simulator for frequencyselective Rayleigh-fading mobile radio paths. The simulator has eight RF outputs and is linear and reciprocal. All outputs exhibit mutually independent Rayleigh fading over a 45 dB range and have identical time delay spreads. Analog RF techniques are combined with a digital signal processor (DSP) to generate random path modulation. Frequency selectivity from time delay spread is created using several RF path delay elements. Simulated vehicle velocity is continuously variable from 6 to 60 mph. The simulator is thus a flexible tool for the study of mobile radio transmission techniques.

A Hybrid Multichannel Hardware Simulator for Frequency-Selective Mobile Radio Paths

Cumulative attenuation distributions for one year and relationships between attenuations at 19 GHz and 28 GHz are presented for an earth-space propagation path terminating at Crawford Hill, New Jersey. An example from the distributions indicates that 20 dB attenuation was exceeded for 0.012% of the year at 19.04 GHz and for 0.06% at 28.56 GHz. For both frequencies the polarization was linear and oriented 21° from vertical at the receiver. When the attenuation at 19 GHz was less than 20 dB, the attenuation in dB at 28 GHz averaged 2.1 times that at 19 GHz. This ratio was lower at higher attenuations.

Rain Attenuation Statistics from a 19 and 28 GHz COMSTAR Beacon Propagation Experiment: One Year Cumulative Distributions and Relationships Between the Two Frequencies

Three experimental programs at Bell Laboratories, Crawford Hill, NJ, have collected rain attenuation data at three frequencies in the 10-30 GHz range for over 2 years. As expected from a simple geometrical model, rain attenuation statistics scale approximately as the cosecant of the path elevation angle. Thus, greater attenuation is encountered on lower elevation angle paths. The increased rain attenuation at higher frequencies is illustrated by comparing cumulative rain attenuation distributions at three widely separated frequencies. Typical year-to-year variations in these distributions are presented. Periods of severe rain attenuation are shown to occur more frequently during the summer months and during the afternoon hours. Cumulative attenuation distributions are compared for these and other time periods.

H. Arnold

Papers

Characteristics of rain and ice depolarization for a 19- and 28-GHz propagation path from a Comstar satellite

Fade-Duration Statistics of a Rayleigh-Distributed Wave

A Hybrid Multichannel Hardware Simulator for Frequency-Selective Mobile Radio Paths

Rain Attenuation Statistics from a 19 and 28 GHz COMSTAR Beacon Propagation Experiment: One Year Cumulative Distributions and Relationships Between the Two Frequencies

Rain Attenuation at 10-30 GHz Along Earth-Space Paths: Elevation Angle, Frequency, Seasonal, and Diurnal Effects