1,720,977 research outputs found
Capacities of spectrally overlaid single-code and multicode CDMA systems
No full textThis paper is concerned with the capacities of spectrally overlaid narrow-band and wide-band systems using two code-division multiple-access (CDMA) techniques: a single-code and a multicode. The capacity is defined here as the maximally achievable total throughput in the narrow-band and wide-band subsystems. User signal power in the overlay systems affects the performance of the systems. The signal power is characterized as the relative power level of wide-band and narrow-band users according to the spreading bandwidth, spectral overlay ratio, and the number of spreading codes assigned to a user in single-and multiple-cell environments. The capacities of the overlay systems are then analyzed. The results of the analysis show that the single-code technique yields better performance than does the multicode scheme in terms of the capacities of the overlay systems. Furthermore, the use of a multicode technique for wide-band subsystems and a single-code scheme for narrow-band subsystems is desirable for enhancing the performance of CDMA overlay systems
The required signal power for multimedia traffic in multipath faded CDMA systems
No full textThe reverse link signal power required for multimedia traffic in multipath faded single-code (SC-) and multi-code CDMA (MC-CDMA) systems is investigated. The effect of orthogonality loss among multiple spreading code channels is characterized by introducing the orthogonality factor. The required signal power in both CDMA systems is analyzed with varying system parameters of spreading bandwidth, the orthogonality factor, and the number of spreading codes. Analytical results show that MC-CDMA users transmitting only a single traffic type require significantly more power than SC-CDMA users with only a single traffic type. On the other hand, MC-CDMA users transmitting multimedia traffic require power levels approximately identical to SC-CDMA users with multimedia traffic
Link capacity and signal power according to allocations of spreading codes and bandwidth in CDMA systems
No full textReverse link performance analysis in single-code and multi-code CDMA systems is presented. Results show that the single-code system yields better performance than does the multi-code system in terms of link capacity and signal power. This improvement increases as spreading bandwidth is reduced and the number of spreading codes assigned to a user is increased
Link capacity and signal power of CDMA systems according to spreading code and bandwidth allocations in multipath fading environments
No full textReverse link performance analyses of single-code (SC) and multi-code (NIC) CDMA systems in multipath fading environments are presented. The degree of orthogonality loss among multiple spreading code channels is characterized by introducing the orthogonality factor. This factor depends on the multipath delay power profiles of the propagation channel and the number of paths resolved at a Rake receiver. The link capacity and the signal power of both CDMA systems are then analyzed according to varying system parameters, including spreading bandwidth, traffic load, the orthogonality factor, and the number of spreading codes assigned to a user. Analytical results show that the SC-CDMA system provides a larger link capacity and NIC users require more power than SC users. The dominant parameters affecting the performance difference are the spreading bandwidth and multipath delay power profiles
Capacity analysis of spectrally overlaid macro/microcellular CDMA systems supporting multiple types of traffic
No full textThe reverse link capacity of a spectrally overlaid macrocell/microcell cellular code-division multiple-access system supporting various types of traffic is analyzed. Several narrow-band subsystems are overlaid with a wide-band subsystem in macrocells, while in a microcell a single narrow-band subsystem is operated with the same spectrum as one of the macrocell narrow-band subsystems. Using a typical propagation model, the reverse link signal power and interference are characterized as the relative user signal power and the cross-tier interference factors between the macrocell and the microcell, considering various system parameters. The reverse link capacity of the overlay system is then analyzed. Results show that the dominant parameters affecting the system performance are the spectral overlay ratio and the distance between the microcell and macrocell base stations. In particular, when the distance equals half of the macrocell radius, optimum performance can be achieved by minimizing the cross-tier interference factors. These results can be applied to network planning for future wireless communication services
Effect of Bandwidth and Spreading Codes on Spectrum Utilization of Spectrally Overlaid CDMA Systems
No full textPHASE-SHIFTING TWYMAN-GREEN INTERFEROMETER UTILIZING NEMATIC AND CHOLESTERIC LIQUID-CRYSTAL CELLS
No full textWe show a phase shifting Twyman-Green interferometer utilizing a nematic liquid crystal (NLC) cell and a cholesteric liquid crystal (CLC) cell. NLC cell is used as a phase shifter (PS) and CLC cell is used as an intensity stabilizer. The NLC PS is designed to induce a phase shift of 2 pi rad for He-Ne laser beam (lambda=632.8 nm) with the applied voltage of about 10 V, and a CLC cell which is designed to exhibit its selective reflection peak at 632.8 nm, is used to remove the fluctuation of the intensity of the interferogram. The interferometer system is evaluated by testing a concave mirror surface and the result is discussed
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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