Browsing by Author "Sendonaris, Andrew"

Now showing 1 - 9 of 9
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    Advanced Techniques for Next-Generation Wireless Systems
    (1999-08-20) Sendonaris, Andrew; Center for Multimedia Communications (http://cmc.rice.edu/)
    In order to meet the demands of next-generation wireless systems, which will be required to support multirate multimedia at high data rates, it is necessary to employ advanced algorithms and techniques that enable the system to guarantee the quality of service desired by the various media classes. In this work, we present a few novel methods for improving wireless system performance and achieving next-generation goals. Our proposed methods include finding signal sets that are designed for fading channels and support multirate, exploiting knowledge of the fading statistics during the data detection process, exploiting the existence of Doppler in the received signal, and allowing mobile users to cooperate in order to send their information to the base station. We evaluate the performance of our proposed ideas and show that they provide gains with respect to conventional systems. The benefits include multirate support, higher data rates, and more stable data rates. It should be mentioned that, while we focus mainly on a CDMA framework for analyzing our ideas, many of these ideas may also be applied to other wireless system environments.
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    Advanced techniques for next-generation wireless systems
    (1999) Sendonaris, Andrew; Aazhang, Behnaam
    In order to meet the demands of next-generation wireless systems, which will be required to support multirate multimedia at high data rates, it is necessary to employ advanced algorithms and techniques that enable the system to guarantee the quality of service desired by the various media classes. In this work, we present a few novel methods for improving wireless system performance and achieving next-generation goals. Our proposed methods include finding signal sets that are designed for fading channels and support multirate, exploiting knowledge of the fading statistics during the data detection process, exploiting the existence of Doppler in the received signal, and allowing mobile users to cooperate in order to send their information to the base station. We evaluate the performance of our proposed ideas and show that they provide gains with respect to conventional systems. The benefits include multirate support, higher data rates, and more stable data rates. It should be mentioned that, while we focus mainly on a CDMA framework for analyzing our ideas, many of these ideas may also be applied to other wireless system environments.
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    Increasing uplink capacity via user cooperation diversity
    (IEEE, 1998-08-20) Sendonaris, Andrew; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Mobile users' capacity is limited by the fact that within the duration of any given call, they experience severe variations in signal attenuation, thereby necessitating the use of some type of diversity. We are proposing a new form of diversity, whereby diversity gains are achieved via the cooperation of in-cell users. Results show that, even though the inter-user channel is noisy, cooperation leads not only to an increase in capacity but also to a more robust system, where users' achievable rates are less susceptible to channel variations.
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    Joint signaling strategies for approaching the capacity of twisted-pair channels
    (1998-10-20) Sendonaris, Andrew; Veeravalli, Venugopal V.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    A technique is presented for jointly optimizing the signaling in the two directions of transmission on a twisted-pair communications channel. It is then applied to twisted-pair channel models with monotonic channel response and crosstalk transfer functions. While the signaling strategy presented in this paper can achieve only a lower bound on the true channel capacity, it is a significant improvement over existing signaling schemes. In particular, in contrast with existing schemes, the maximum information rate for the joint signaling strategy increases without bound as the signal-to-noise ratio (SNR) approaches infinity. It is also shown through numerical results that the proposed signaling strategy generalizes naturally to more practical nonmonotonic twisted-pair channel models incorporating bridge taps and other nonidealities. Finally, the form of the optimal signaling strategy suggests a relatively straightforward implementation using multicarrier modulation.
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    Multiuser detection in fast-fading multipath environments
    (IEEE, 1998-12-20) Sayeed, Akbar M.; Sendonaris, Andrew; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    We propose a new framework for multiuser detection in fast-fading channels that are encountered in many mobile communication scenarios. Existing multiuser RAKE receivers, developed to combat multipath fading and multiuser interference in slow fading, suffer substantial degradation in performance under fast fading due to errors in channel state estimation. The detectors proposed in this paper employ a novel receiver structure based on time-frequency (TF) processing that is dictated by a canonical representation of the wide-sense stationary uncorrelated scatterer (WSSUS) channel model. The workhorse of the framework is a TF generalization of the RAKE receiver that exploits joint multipath-Doppler diversity. Analytical and simulated results based on realistic fast-fading assumptions demonstrate that the proposed multiuser detectors promise substantially improved performance compared to existing systems due to the inherently higher level of diversity afforded by multipath-Doppler processing.
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    Multiuser detectors for fast-fading multipath channels
    (1997-10-20) Sayeed, Akbar M.; Sendonaris, Andrew; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    We propose a new framework for multiuser detection in fast fading channels that are encountered in many mobile communication scenarios. The detectors proposed in this paper employ a novel multiuser receiver structure based on time-frequency processing that exploits joint multipath-Doppler diversity. Performance analysis shows that the proposed time-frequency multiuser receivers, due to their inherently higher level of diversity, can potentially deliver substantial performance gains relative to conventional multiuser RAKE receivers.
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    Studies on capacity and performance of digital transmission over copper loops
    (1995) Sendonaris, Andrew; Aazhang, Behnaam
    Digital transmission over copper loops is studied. Models are found of main sources of interference, including near-end crosstalk (NEXT), far-end crosstalk (FEXT), impulse noise (IN) and intersymbol interference (ISI). Importance Sampling theory is introduced, and an explanation is given of how it can be used to efficiently estimate the bit error rate (BER) of the above digital transmission system. Results demonstrate the efficiency of Importance Sampling simulations and the validity of the models for the channel interference. A suboptimal scheme for approaching capacity of the twisted pair is analyzed and its connection to multicarrier modulation is presented. The resulting lower bound on capacity is a significant improvement over what exists in the literature.
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    User Cooperation Diversity--Part I: System Description
    (2003-11-20) Sendonaris, Andrew; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Mobile users' data rate and quality of service are limited by the fact that, within the duration of any given call, they experience severe variations in signal attenuation, thereby necessitating the use of some type of diversity. In this two-part paper, we propose a new form of spatial diversity, in which diversity gains are achieved via the cooperation of mobile users. Part I describes the user cooperation strategy while Part II focuses on implementation issues and performance analysis. Results show that, even though the inter-user channel is noisy, cooperation leads not only to an increase in capacity for both users but also to a more robust system, where users' achievable rates are less susceptible to channel variations.
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    User Cooperation Diversity--Part II: Implementation Aspects and Performance Analysis
    (2003-11-20) Sendonaris, Andrew; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    This is the second in a two-part series of papers on a new form of spatial diversity, where diversity gains are achieved through the cooperation of mobile users. Part I described the user cooperation concept and proposed a cooperation strategy for a conventional CDMA system. Part II investigates the cooperation concept further and considers practical issues related to its implementation. In particular, we investigate the optimal and suboptimal receiver design, and present performance analysis for the conventional CDMA implementation proposed in Part I. We also consider a high rate CDMA implementation and a cooperation strategy when assumptions about the channel state information at the transmitters are relaxed. We illustrate that, under all scenarios studied, cooperation is beneficial in terms of increasing system throughput and cell coverage, as well as decreasing sensitivity to channel variations.