This paper provides analyses of three types of diversity combining systems in practical use. These are: selection diversity, maximal-ratio diversity, and equal-gain diversity systems. Quantitative measures of the relative performance (under realistic conditions) of the three systems are provided. The effects of various departures from ideal conditions, such as non-Rayleigh fading and partially coherent signal or noise voltages, are considered. Some discussion is also included of the relative merits of predetection and postdetection combining and of the problems in determining and using long-term distributions. The principal results are given in graphs and tables, useful in system design. It is seen that the simplest possible combiner, the equal-gain system, will generally yield performance essentially equivalent to the maximum obtainable from any quasilinear system. The principal application of the results is to diversity communication systems and the discussion is set in that context, but many of the results are also applicable to certain radar and navigation systems.

Linear diversity combining techniques

This paper provides analyses of three types of diversity combining systems in practical use. These are: selection diversity, maximal-ratio diversity, and equal-gain diversity systems. Quantitative measures of the relative performance (under realistic conditions) of the three systems are provided. The effects of various departures from ideal conditions, such as non-Rayleigh fading and partially coherent signal or noise voltages, are considered. Some discussion is also included of the relative merits of predetection and postdetection combining and of the problems in determining and using long-term distributions. The principal results are given in graphs and tables, useful in system design. It is seen that the simplest possible combiner, the equal-gain system, will generally yield performance essentially equivalent to the maximum obtainable from any quasi-linear system. The principal application of the results is to diversity communication systems and the discussion is set in that context, but many of the results are also applicable to certain radar and navigation systems.

Linear Diversity Combining Techniques

1) The Myth: Sixty years of research following Shannon's pioneering paper has led to telecommunications solutions operating arbitrarily close to the channel capacity-“flawless telepresence” with zero error is available to anyone, anywhere, anytime across the globe. 2)The Reality: Once we leave home or the office, even top of the range iPhones and tablet computers fail to maintain "flawless telepresence" quality. They also fail to approach the theoretical performance predictions. The 1000-fold throughput increase of the best third- generation (3G) phones over second-generation (2G) GSM phones and the 1000-fold increased teletraffic predictions of the next decade require substantial further bandwidth expansion toward ever increasing carrier frequencies, expanding beyond the radiofrequency (RF) band to optical frequencies, where substantial bandwidths are available. 3) The Future: However, optical and quantum-domain wireless communications is less developed than RF wireless. It is also widely recognized that the pathloss of RF wireless systems monotonically increases with the carrier frequency and this additional challenge has to be tackled by appropriate countermeasures in future research. Hence, we set out to seek promising techniques of tackling the aforementioned challenges and for resolving the conflicting design constraints imposed on the flawless telepresence systems of the future. To disspell the myth, we evaluate both the operational 3G as well as the emerging fourth-generation (4G) wireless systems and demonstrate that there is a substantial difference between their theoretical and their practically attainable performance. The reality is that the teletraffic predictions indicate further thirst for bandwidth, which cannot be readily satisfied within the most popular 1-2-GHz carrier-frequency range, where the best propagation conditions prevail. We briefly consider the 10-300-GHz unlicensed band as a potential source of further spectrum, followed by a review of advances way beyond the upper edge of the RF range at 300 GHz, namely to the realms of optical wireless (OW) communications. As the carrier frequency is increased, the pathloss is also increased, which results in ever smaller cells. Furthermore, the high-frequency RF waves predominantly obey line-of-sight (LOS) propagation-like visible light. The future requires advances in both infrared and visible-light communications for circumventing the LOS nature of light. We hypothesize that light-emitting diode (LED) arrays acting as "massive" multiple-input-multiple-output (MIMO) components as well as transmitter/receiver cooperation might be conceived. The heterogeneous networks of the near future will rely on seamless, near-instantaneous handovers among OW hotspots, RF hotspots, and oversailing larger cells. These "massive" MIMOs might impose a high complexity, hence their reduced-complexity noncoherently detected counterparts might be favored. Finally, we conclude by touching upon the promising research area of quantum-domain communications, which might be expected to circumvent the aforementioned complexity problem of massive MIMOs with the aid of efficient quantum-domain search techniques-a truly exciting research era

Wireless Myths, Realities, and Futures: From 3G/4G to Optical and Quantum Wireless

Since its development in the 1960's, the technique of obtaining high-resolution radio images of astronomical objects using Fourier synthesis has advanced sufficiently so that today such images often provide better angular resolution than is obtainable with the largest optical telescopes. A synthesis array measures the Fourier transform of the observed brightness distribution by cross-correlating the signals from antennas separated by distances up to tens of kilometers. The antennas must be equipped with low-noise receiving systems and connected together by phase-stable transmission links. Wide-bandwidth digital correlators are used to perform the cross correlation. The data-reduction algorithms and computing system play a critical role in determining the quality of the images produced by the array. The Very Large Array (VLA) synthesis telescope, recently constructed in New Mexico, consists of twenty-seven 25-m-diameter antennas arranged in a Y-shaped array. Each arm of the Y is approximately 21 km long and the antetmas can be moved to various positions on the arms by a rail-mounted transporter. The antennas are equipped with cryogenically cooled receiving systems and are interconnected by low-loss, TE 01 -mode, large-diameter waveguide. The cross-correlation products for each of the 351 pair combinations of antennas are measured for 4 IF signals by a 50-MHz bandwidth digital correlator. In this paper we discuss the design of synthesis arrays in general, and describe the design and performance of the VLA in particular, under the seven headings: array geometry design, sensitivity considerations, phase stability requirements, signal transmission system, delay and correlator system, control system, and data-reduction requirments. In each section, we review the underlying instrumental requirements and provide details of how the VLA was designed to meet them. Recently developed data-reduction algorithms provide effective ways of correcting synthesis images for the effects of missing Fourier components and instrumental and atmospheric amplitude and phase errors. The power of these algorithms is demonstrated using actual VLA images.

The very large array: Design and performance of a modern synthesis radio telescope

In wireless systems there is always a trade-off between reducing the transmit power and mitigating the resultant signal-degradation imposed by the transmit-power reduction with the aid of sophisticated receiver algorithms, when considering the total energy consumption. Quantum-assisted wireless communications exploits the extra computing power offered by quantum mechanics based architectures. This paper summarizes some recent results in quantum computing and the corresponding application areas in wireless communications.

Quantum-assisted and Quantum-based Solutions in Wireless Systems

A system for the measurement of the variations in electrical lengths of radio propagation paths is described The observed path-length instabilities are considered to be caused bythe same atmospheric turbulence responsible for the existence of very high frequency and ultra high frequency fields far beyond the radio horizon Results obtained on 1728 mc and 1,046 mc along 3?-, 10-, and 60-mile paths are reported It is pointed out that measurements of this type provide a powerful tool for the study of the size and intensity of the refractivity variations of the atmosphere giving rise to the observed phenomenon

Measurements of the Phase of Radio Waves Received over Transmission Paths with Electrical Lengths Varying as a Result of Atmospheric Turbulence

Amplitude spectra in the vicinity of the 60-GHz molecular oxygen resonant line are interpreted as a superposition of an absorption mechanism plus a scattering mechanism. Also, a simple derivation for the amplitude and phase spectra in the limit of small fluctuation frequencies is given.

Atmospheric amplitude spectra in an absorption region

Signals at 9.6 and 34.52 GHz, propagated simultaneously over a slant line-of-sight overwater path, have been analyzed to compare the power spectra of phase-of-arrival variations and fading and to determine the coherence of these signals with regard to both phase variations and fading. The phase data at the two radio frequencies exhibited nearly identical power spectra from 0.01 to 5 Hz and very high coherence from 0.01 to 0.1 Hz. The coherence dropped rapidly above 0.1 Hz and was in most cases less than 0.4 above 0.5 Hz. The power spectra of fading were similar in shape at the two frequencies, but the fading spectral density was consistently higher at 34.52 GHz than at 9.6 GHz from 0.1 to 5 Hz. The shape of the coherence function for fading was similar to that of the corresponding phase coherence function, but the fading coherence was lower at the low spectral frequencies. The possible effect of the small spatial separation of the propagation path on the coherence analysis is discussed.

Comparison of simultaneous line-of-sight signals at 9.6 and 34.5 GHz

The National Bureau of Standards has been conducting a series of measurements for a study of the characteristics of the turbulent lower atmosphere and its effect on the accuracy of radio direction-finding, guidance, and geodetic measurement systems. The results of three experiments are presented, one consisting of recordings of refractive index and apparent path length variations at 9400 Mc/s over a 15.5-mile path on Maui, Hawaii and the other two consisting of similar measurements made over a 9.5-mile path near Boulder, Colorado. The correlation of refractive index and apparent path length fluctuations is discussed as well as the power (variance) density spectra of both variables.

An analysis of time variations in tropospheric refractive index and apparent radio path length

A practical system is described for making single‐path phase measurements at X‐band over ranges up to about 20 miles with instrumental noise of the order of a fraction of one degree. The techniques for obtaining the necessary transmitter frequency stability of 1:109 are described. The use of such techniques to form a simple microwave repeater system with power gain of the order of 50 db is discussed.

Single Path Phase Measuring System for Three‐Centimeter Radio Waves

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