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This paper considers the suitability of a range of multi-input-multi-output (MIMO) orthogonal frequency-division multiplexing architectures for use in urban hotspots. A ray-tracing propagation model is used to produce realistic MIMO channel data. This information is used to determine the expected throughput and area coverage for various physical (PHY) layer schemes. Site-specific throughput predictions are generated in a city-center environment. Link adaptation (LA) is shown to play a key role in the choice of space-time algorithm, the use of adaptive modulation and coding, and the number of antennas employed at both ends of the radio link. No single PHY layer scheme is suitable to cover the entire coverage area. Results demonstrate the need for MIMO LA under a wide range of channel conditions. For the area under test, 2% of covered locations selected a spatial multiplexing (SM) scheme, 50% selected a space-time block coding (STBC) scheme, and 48 % selected a hybrid SM/STBC scheme. With suitable power control and LA, for the scenario under consideration, high peak capacities and good geographic coverage were achieved. Index Terms--Link adaptation (LA), orthogonal frequency-division multiplexing (OFDM), power control, propagation, space-time block codes (STBCs), spatial multiplexing (SM).

In this paper, orthogonal frequency-division multiple-access (OFDMA) systems with dynamic deterministic (as opposed to pseudorandom) allocation of subcarriers to users to exploit multiuser diversity are investigated. Previously published work on dynamic multiuser subcarrrier allocation for OFDMA systems with single-input--single-output (SISO) channels are surveyed. A near-optimal low-complexity algorithm tier SISO systems, which is structurally similar to the algorithm by Rhee and Cioffi, is extended to the case of multiple-input--multiple-output (MIMO) systems in this paper. The optimality and adaptability of this algorithm are analyzed by formulating an assignment problem and comparing with one optimal and two extended suboptimal strategies proposed based on previous work. Consideration of a MIMO channel creates further issues for the subcarrier-allocation process. In particular, methods whereby an appropriate subcarrier allocation may be exploited to minimize the effects of correlation in MIMO channels are of considerable interest. Several novel variants of the algorithm (referred to as 'schemes') are proposed and evaluated for MIMO systems employing both space-time block coding (STBC) and spatial multiplexing (SM) in both uncorrelated and correlated fading channels. Simulation results identify the most suitable schemes for both STBC and SM and, in particular, show that substantial improvements in performance tin terms of bit-error rate) in correlated channels can be achieved by means of suitable subcarrier allocation. In uncorrelated channels, the best scheme can tilter approximately 7-dB gain over the conventional MIMO channel; in highly correlated channels, even more substantial improvements (> 11-dB gain for STBC, > 20-dB gain for SM) in performance can also he achieved, demonstrating the ability of a well-designed subcarrier-allocation scheme to mitigate the debilitating effects of correlation on MIMO systems. Index Terms--Multiple input multiple output (MIMO), orthogonal frequency-division multiple access (OFDMA), single input single output (SISO), space-time block coding (STBC), spatial multiplexing (SM), subcarrier allocation.

A frequency-hopped version of the Cartesian feedback linear transmitter for use in future mobile communication systems is introduced. The essential requirements for the successful operation of such a transmitter are discussed. Using the results obtained from a hardware prototype, the problems generated by a hopping carrier are outlined and solutions introduced.

The availability of effective numerical absorbing boundary conditions (ABCs) for use with the Finite Difference Time Domain (FDTD) method is essential for efficient application of the technique to microwave circuit analysis. Although many published results exist which have made use of various ABCs, little information concerning the optimum choice has been given. In this contribution, the suitability of available ABCs for planar circuit analysis is investigated and a new technique is presented whereby second order ABCs can be optimized for this type of problem.

Asserts that school systems will have to convert from their traditional methods of organizing and operating if competencies beyond the basic skills are to be taught. (Author/IRT)

This paper considers low-frequency radar cohabitation with digital video broadcasting terrestrial (DVB-T). The spatial and temporal radio frequency propagation characteristics of the terrestrial and air-to-ground channels are modeled using state-of-the-art 3-D ray tracing for real-world broadcast television scenarios. Rooftop DVB-T receivers are assumed at user locations in two different urban environments, namely, the cities of London and Bristol, U.K.. The propagation characteristics are combined with appropriately orientated 3-D antenna radiation patterns, which include full polarization information for the DVB-T rooftop antennas, the base-station antennas, and the airborne radar antennas. A radar waveform generator is used to synthesize a representative range of radar signals. A DVB-T physical layer simulator has been developed to predict the performance of the received broadcast stream at each rooftop location. Factors, such as additive noise, the power and structure of the wanted DVB-T and the interfering radar signals, and the nature of the radio channels, are shown to influence the DVB-T bit error rate results. Statistics for the probability of high-quality DVB-T reception are reported as a function of the modulation scheme, propagation environment, aircraft orientation, radar antenna array configuration, and radar waveform type. [ABSTRACT FROM PUBLISHER]

In this paper, orthogonal frequency-division multiple-access (OFDMA) systems with dynamic deterministic (as opposed to pseudorandom) allocation of subcarriers to users to exploit multiuser diversity are investigated. Previously published work on dynamic multiuser subcarrrier allocation for OFDMA systems with single-input-single-output (SISO) channels are surveyed. A near-optimal low-complexity algorithm for SISO systems, which is structurally similar to the algorithm by Rhee and Cioffi, is extended to the case of multiple-input-multiple-output (MIMO) systems in this paper. The optimality and adaptability of this algorithm are analyzed by formulating an assignment problem and comparing with one optimal and two extended suboptimal strategies proposed based on previous work. Consideration of a MIMO channel creates further issues for the subcarrier-allocation process. In particular, methods whereby an appropriate subcarrier allocation may be exploited to minimize the effects of correlation in MIMO channels are of considerable interest. Several novel variants of the algorithm (referred to as "schemes") are proposed and evaluated for MIMO systems employing both space-time block coding (STBC) and spatial multiplexing (SM) in both uncorrelated and correlated fading channels. Simulation results identify the most suitable schemes for both STBC and SM and, in particular, show that substantial improvements in performance (in terms of bit-error rate) in correlated channels can be achieved by means of suitable subcarrier allocation. In uncorrelated channels, the best scheme can offer approximately 7-dB gain over the conventional MIMO channel; in highly correlated channels, even more substantial improvements (> 11-dB gain for STBC, > 20-dB gain for SM) in performance can also be achieved, demonstrating the ability of a well-designed subcarrier-allocation scheme to mitigate the debilitating effects of correlation on MIMO systems. [ABSTRACT FROM AUTHOR]

Presents insights into the behavior of combined analog locked-loop universal modulator (CALLUM) systems. Production of a linear output through the use of nonlinear, but highly efficient, radio frequency power amplifiers; Up-conversion and amplification of a baseband signal within a closed-loop feedback scheme; Results from an experimental CALLUM2 system.