ISSN:1124-318X    

DOI:10.1002/ett.4460120403

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractThis letter reports the results of a measurement campaign aimed at characterizing the traffic in Internet dial—up sessions. This is done at the data transfer level, i. e. considering the average data rate and the volume of data transferred during calls to Internet service providers. Major statistics (mean and two percentile values) are provided for both quantities with a hourly time resolution together with the empirical probability density functions relative to the peak hou

ISSN:1124-318X    

DOI:10.1002/ett.4460120404

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractThe capacity of a random—phase additive white Gaussian noise (AWGN) channel, referred to asnoncoherent channel, is investigated in the case of a transmission ofNinformation symbols. The non—Gaussianity of die capacity—achieving distribution is shown and a lower bound on the channel capacity is derived. For increasing values of the number of transmitted symbolsN, the capacity of a noncoherent channel is shown to asymptotically approach that of a coherent channel, i. e., a known—phase AWGN channel. The asymptotical Gaussianity of the capacity—achieving distribution is also shown. Based on the derived lower bound, the inherent capacity loss of a noncoherent channel, as compared to a coherent one, may be considered very limited for all but very small values ofN.This result may be viewed as the information theoretic counterpart of a similar conclusion derived by many authors with reference to the probability of detect

ISSN:1124-318X    

DOI:10.1002/ett.4460120405

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractButterworth and Square—Root—Raised—Cosine (SRRC) filtered OQPSK, and GMSK withBT= 0. 5 andBT= 1 are compared in the presence of narrow band and wide band interferers, over AWGN and nonlinear channels. The purpose of this work is to help finding out the most suitable modulation scheme for the increasingly crowded frequency bands available for space research mis

ISSN:1124-318X    

DOI:10.1002/ett.4460120406

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractMobile computing is a newly emerging computing paradigm. Whether using the term mobile database system or some other form, the database concept is essential to the mobile computing environment. This work employs a more thorough yet general means of defining the mobile database system, along with several application scenarios. Possible secure access schemes are also presented for databases in a mobile computing environment. The tradeoffs of these schemes are also analyzed. Moreover, the feasibility of applying some possible secure access methods is explored, along with the potential difficulties and unresolved issues. Finally, concluding remarks and recommendations for future works are made.

ISSN:1124-318X    

DOI:10.1002/ett.4460120407

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractThe commonly recommended approach to meet the Constant Bit Rate (CBR) service requirements in ATM networks is to use the playback buffer mechanism implemented at the AAL layer [1],[2],[3]. This solution was originally proposed for the fibre optics ATM networks, where cell loss ratio is negligible (e. g. 10—9). However, for the wireless ATM network concept the designers have to take into account the fact that an essential part of sending cells can be simply lost due to transmission errors and these losses can also occur in bursts (despite applying error protection mechanisms). Moreover, the variation of cell transfer delay is expected to be significantly larger and this is mainly due to radio access protocol behaviour. As a consequence, the above leads to a need to modify the playback buffer mechanism in order to improve its functionality in the wireless ATM network environment. The solution proposed in this paper is based on some enhancements of the dummy cell insertion (instead of lost cells) algorithm and on the K times extension of AAL1 PDU numbering range, by assigning the same number to K consecutive cells. The justification of the approach is provided in terms of theoretical considerations and simulation result

ISSN:1124-318X    

DOI:10.1002/ett.4460120408

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractA novel communication receiver which uses lapped transform (LT) incorporating modified median filter (MMF) algorithm is designed for narrow—band interference (NB1) excision. Comparing to traditional Fourier Transform, LT has longer basis vectors, less spectral leakage, thus better frequency resolution. The LT domain MMF algorithm takes full advantages of the direct sequence spread spectrum signal, as well as the characteristics of LT, performs the transform domain filtering twice. The first filtering locates the position of interference and mitigates most of them. The second filtering was performed in a small neighborhood of the located interference. So LT domain MMF algorithm can completely mitigate the interference without distorting the desired signal. Simulation results demonstrated the improved BER performance and increased robustness of our approac

ISSN:1124-318X    

DOI:10.1002/ett.4460120409

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractNovel non data—aided (NDA) algorithms are proposed for joint estimation of timing and carrier phase in MSK—type modulations. They are based on maximum likelihood methods and have a feedforward structure which is suitable to fully digital implementation. Performance with MSK and Gaussian MSK. (GMSK) is assessed by computer simulations and compared with that of other existing estimation sche

ISSN:1124-318X    

DOI:10.1002/ett.4460120410

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractWe investigate the interuser interference in cellular code—division multiple access (CDMA) and timedivision multiple access (TDMA) systems and compare the two multiple access techniques at different levels of user density per cell. The study includes the interference both on the downstream channel (from base station to users) and on the upstream channel (from users to base station). The purpose is to compare the two multiple access techniques when the total bandwidth occupancy and the maximum number of users per cell are the same. The results indicate that on a nondispersive radio channel with a path loss that is proportional to the fourth power of the distance, the interuser interference is approximately 10 dB higher in CDMA. The implication of this result, which holds for both the downstream and the upstream channels, is that for the same total bandwidth and number of users per cell, TDMA gives substantially superior bit error rate (BER) performance. This is confirmed by our numerical results evaluated for both additive white Gaussian noise (AWGN) channels and memoryless flat Rayleigh fading channels. Alternatively, CDMA needs approximately 10 times more bandwidth to achieve the same signal—to—interference (S/I) ratio, a result which may seem surprising, because the fact that CDMA employs a frequency reuse factor of 1 is often given as an evidence that this technique can support a higher number of users than TDMA in a given geographic area and frequency band

ISSN:1124-318X    

DOI:10.1002/ett.4460120411

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY

摘要:

AbstractWe address wide—area coverage for hand—held portables using an on—board signal processing satellite in geostationary (GEO) orbit at minimum payload cost. A GEO link, being power—limited, differs from terrestrial mobile communications systems that use direct radiating antennas by requiring a focal—fed parabolic reflector, which acts as a fixed analog beamformer. We propose to cascade a high—gain parabolic reflector and a digital maximum signal—to—interference—plus—noise ratio (SINR) multi—beamformer for spatial multi—user interference suppression to achieve coverage extension. This involves on—board antenna array signal processing of multiple feeds. It is shown that nearly uniform coverage of an area the size of Canada can be provided using only five parbolic reflector apertures. Compared to conventional focussed multibeam system, link margins are typically increased from a minimum requirement of 5. 25 dB to 7. 5 dB in border areas. The more uniform coverage reduces dynamic p

ISSN:1124-318X    

DOI:10.1002/ett.4460120412

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:2001

数据来源: WILEY