Browsing by Author "Bhashyam, Srikrishna"

Now showing 1 - 16 of 16
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    Channel Estimation for Multirate DS-CDMA Systems
    (2000-11-20) Sabharwal, Ashutosh; Mitra, Urbashi; Bhashyam, Srikrishna; Center for Multimedia Communications (http://cmc.rice.edu/)
    The problem of channel estimation for multiple data rate Direct-Sequence Code-Division Multiple-Acess (DS-CDMA) systems is addressed. The derived algorithms are suitable for both variable chipping rate and/or variable spreading multirate systems. Furthermore, methods for uplink and downlink channel estimation are investigated. It is observed that direct exploitation of the multirate nature of the signal yields improved performance. The algorithms are evaluated via simulation.
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    Edge Localized Image Sharpening via Reassignment with Application to Computed Tomography
    (2000-07-01) Dorney, Timothy D.; Bhashyam, Srikrishna; Doran, Andrew; Choi, Hyeokho; Flandrin, Patrick; Baraniuk, Richard G.; Center for Multimedia Communications (http://cmc.rice.edu/); Digital Signal Processing (http://dsp.rice.edu/)
    Traditional filtering methods operate on the entire signal or image. In some applications, however, errors are concentrated in specific regions or features. A prime example is images generated using computed tomography. Practical implementations limit the amount of high frequency content in the reconstructed image, and consequently, edges are blurred. We introduce a new post-reconstruction edge enhancement algorithm, based on the reassignment principle and wavelets, that localizes its sharpening exclusively to edge features. Our method enhances edges without disturbing the low frequency textural details.
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    Efficient VLSI Architectures for Baseband Signal Processing for Wireless Base-Station Receivers
    (2000-07-20) Rajagopal, Sridhar; Bhashyam, Srikrishna; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    A real-time VLSI architecture is designed for multiuser channel estimation, one of the core base-band processing operations in wireless base-station receivers. Future wireless basestation receivers will need to use sophisticated algorithms to support extremely high data rates and multimedia. Current DSP architectures are unable to fully exploit the parallelism and bit level arithmetic present in these algorithms. These features can be revealed and efficiently implemented by task partitioning the algorithms for a VLSI solution. We modify the channel estimation algorithm for a reduced complexity fixed-point hardware implementation. We show the complexity and hardware required for three different area-time tradeoffs: an area-constrained, a time-constrained and an area-time efficient architecture. The area-constrained architecture achieves low data rates with minimum hardware, which may be used in picocell base-stations. The time-constrained solution exploits the entire available parallelism and determines the maximum theoretical data rates. The area-time efficient architecture meets real-time requirements with minimum area overhead. The orders-of-magnitude difference between area and time constrained solutions reveals significant inherent parallelism in the algorithm. All proposed VLSI solutions exhibit better time performance than a previous DSP implementation.
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    Efficient VLSI architectures for multiuser channel estimation in wireless base-station receivers
    (Kluwer Academic Pubishers, 2002-06-20) Rajagopal, Sridhar; Bhashyam, Srikrishna; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    This paper presents a reduced-complexity, fixed-point algorithm and efficient real-time VLSI architectures for multiuser channel estimation, one of the core baseband processing operations in wireless base-station receivers for CDMA. Future wireless base-station receivers will need to use sophisticated algorithms to support extremely high data rates and multimedia. Current DSP implementations of these algorithms are unable to meet real-time requirements. However, there exists massive parallelism and bit level arithmetic present in these algorithms than can be revealed and efficiently implemented in a VLSI architecture. We it re-design an existing channel estimation algorithm from an implementation perspective for a reduced complexity, fixed-point hardware implementation. Fixed point simulations are presented to evaluate the precision requirements of the algorithm. A dependence graph of the algorithm is presented and area-time trade-offs are developed. An area-constrained architecture achieves low data rates with minimum hardware, which may be used in pico-cell base-stations. A time-constrained solution exploits the entire available parallelism and determines the maximum theoretical data processing rates. An area-time efficient architecture meets real-time requirements with minimum area overhead.
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    Feedback gain in Multiple Antenna Systems
    (2002-05-20) Bhashyam, Srikrishna; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Multiple antenna transmission and reception have been shown to significantly increase the achievable data rates of wireless systems. However, most of the existing analysis assumes perfect or no channel information at the receiver and transmitter. The performance gap between these extreme channel assumptions is large and most practical systems lie in between. Therefore, it is important to analyze multiple antenna systems in the presence of partial channel information. In this paper, we upper bound the outage probability performance of multiple antenna systems with preamble-based channel estimation and quantized feedback. We design causal feedback and power control schemes to minimize this upper bound on outage probability. We consider the following practical issues in our analysis and design: (i) the channel information is imperfect both at the receiver and at the transmitter, and (ii) part of the total available resources for the system need to be used for estimation and feedback. Our results demonstrate that for block fading channels, sending a periodic preamble and causally receiving channel state information via a feedback channel can lead to substantial gains in the outage performance over any non-feedback scheme. Most of the gains achieved by perfect feedback can be achieved by very few bits of feedback. Furthermore, it is demonstrated that these outage probability gains can be translated into improvements in frame error rate performance of systems using space-time codes. Thus, implementing a power control, even at the cost of reduced spectral resources for the forward channel is beneficial for block fading channels.
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    Multiuser Channel Estimation and Tracking for Long Code CDMA Systems
    (2002-01-15) Bhashyam, Srikrishna; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next generation wireless systems use long spreading codes whose period is much larger than the symbol duration. In this paper, we derive the maximum likelihood channel estimate for long code CDMA systems over multipath channels using training sequences and approximate it using an iterative alogrithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be neglible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI resistant multiuser channel estimation and tracking scheme with reasonable computational complexity is derived for long code CDMA systems over multipath fading channels.
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    Multiuser Channel Estimation for Long Code CDMA Systems
    (2000-09-20) Bhashyam, Srikrishna; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next generation wireless systems use long spreading codes whose period is much larger than the symbol duration. In this paper, we derive the maximum likelihood channel estimate for long code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each processing window. The asymptotic convergence of the mean of the iterative estimate to the actual channel is also shown. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. Finally, the proposed iterative channel estimation technique is extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI resistant multiuser channel estimate with reasonable computational complexity is derived for long code CDMA systems over multipath fading channels.
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    Partially Blind Multiuser Detection
    (1999-03-20) Das, Suman; Bhashyam, Srikrishna; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Multiple access interference (MAI) is a significant limiting factor in the performance of direct sequence code division multiple access (DS-CDMA) systems. Various multiuser detection techniques have been developed to combat the effects of MAI. These detection techniques either assume the knowledge of all the users in the system (conventional) or assume the knowledge of the user of interest only (blind). Due to the limitations of the blind algorithms in the presence of a large number of interferers, there is a significant performance gap between these two classes of detectors. Additionally, in practice, the receiver could have only partial knowledge of the interference. In this paper, we develop a new class of detectors, partially blind multiuser detectors, that use information about a subset of interferers and bridge the performance gap between the blind and the conventional multiuser detectors.
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    Partially Blind Multiuser Receivers for CDMA Systems
    (1999-12-20) Das, Suman; Bhashyam, Srikrishna; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    Multiple access interference (MAI) significantly limits the performance of code-division multiple access (CDMA) systems. Existing multiuser receivers combat MAI using the knowledge of either all (conventional) or none (blind) of the interfering users. However, in practice, partial knowledge of the interference is often available at the receiver. For example, the base station of a cellular communication system typically has information about the interfering users within its cell only (about 60\\% of the total interference). Therefore, conventional multiuser receivers cannot be used. We show the degradation in performance resulting from treating the unknown (out-of-cell) interference as noise or using the blind receiver. We then develop a new "partially blind receiver" for joint synchronization and detection that combines a nonlinear interference cancellation technique for the in-cell interferers with the linear blind minimum mean squares error (MMSE) technique for the out-of-cell users. The new receiver significantly outperforms the blind receiver and the receiver that ignores the out-of-cell interference. Additionally, due to the nonlinear interference cancellation technique, the proposed receiver can even outperform conventional linear multiuser receivers with the knowledge of all the users.
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    Performance of Iterative Multiuser Decoding and Channel Estimation in WCDMA Systems
    (2000-05-20) Bhashyam, Srikrishna; Hottinen, Ari; Center for Multimedia Communications (http://cmc.rice.edu/)
    This paper studies the performance of iterative multiuser decoding, interference cancellation, and channel estimation techniques applicable to third generation WCDMA systems. The concept uses a posteriori probabilities of code symbols to enhance detection, decoding and channel estimation in an iterative fashion. Performance is analyzed in a multi-path channel with simulations. It is seen that the proposed concept combats multiuser interference effectively and performs almost as well as single user systems.
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    Real-Time Algorithms and Architectures for Multiuser Channel Estimation and Detection in Wireless Base-Station Receivers
    (IEEE, 2002-07-20) Rajagopal, Sridhar; Bhashyam, Srikrishna; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    This paper presents alogrithms and architecture designs that can meet real-time requirements of multiuser channel estimation and detection in future wireless base-station receivers. Sophisticated algorithms proposed to implement multiuser channel estimation and detection make their real-time implementation difficult on current Digital Signal Processor (DSP)-based receivers. A maximum-likelihood based multiuser channel estimation scheme requiring matrix inversions is redesigned from an implementation perspective for a reduce complexity, iterative scheme with a simple fixed-point VLSI architecture. A reduced-complexity, bit-streaming multiuser detection algorithm that avoids the need for multishot detection is also developed for a simple, pipelined VLSI architecutre. Thus, we show that real-time solutions can be achieved for third generation wireless systems by (1) designing the alogrithms from a fixed-point implementation perspective, without significant loss in error rate performance, (2) task partitioning and (3) designing bit-streaming fixed-point VLSI architectures that explore pipelining, parallelism and bit-level computations to achieve real-time with minumum area overhead.
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    Signal and Information Processing for Wireless Communication Systems
    (2001-04-20) Bhashyam, Srikrishna; Center for Multimedia Communications (http://cmc.rice.edu/)
    Next generation wireless communication systems need to support access to multimedia data available on the internet. This universal wireless access to multimedia data requires data rates and quality of service that are orders of magnitude better than those available in current wireless systems. The two major problems posed by the wireless channel, multipath fading and multiple access interference, are addressed here. The time-varying and shared nature of the wireless channel lead to multipath fading and multiple access interference (MAI) respectively. In this work, signal and information processing algorithms are developed to combat these problems and, consequently, increase data rates and improve performance. First, multiuser signal processing algorithms that combat MAI are developed for code division multiple access (CDMA) systems. Although multiuser signal processing has been extensively studied in the past, very little attention has focused on the practically important case of CDMA with long spreading codes that forms the basis of next generation wireless cellular communications. This work proposes new multiuser channel estimation and tracking algorithms for long code CDMA and demonstrates the significant gains achievable in performance. We also propose new channel estimation algorithms for short code CDMA systems that support multiple data rates to achieve multimedia communication. The new problems posed by the multirate nature of this system, key to support multimedia communication, are studied. Then, in the rest of this work, the use of multiple antenna systems with channel feedback is studied to combat fading. Multiple antenna systems provide multiple independent channels for communication and can effectively combat fading. The following practical issues, ignored in most existing work, are considered in the analysis and design: (i) the channel information is imperfect both at the receiver and at the transmitter, and (ii) part of the total available resources for the system need to be used for channel estimation and feedback. Feedback and power control schemes are designed to achieve significant gains in performance (in terms of outage probability) and data rates. These gains, based on the information-theoretic analysis, are then translated into frame error rate improvements for practical systems that employ space-time codes.
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    Signal and information processing for wireless communication systems
    (2001) Bhashyam, Srikrishna; Aazhang, Behnaam
    Next generation wireless communication systems need to support access to multimedia data available on the internet. This universal wireless access to multimedia data requires data rates and quality of service that are orders of magnitude better than those available in current wireless systems. The two major problems posed by the wireless channel, multipath fading and multiple access interference, are addressed here. The time-varying and shared nature of the wireless channel lead to multipath fading and multiple access interference (MAI) respectively. In this work, signal and information processing algorithms are developed to combat these problems and, consequently, increase data rates and improve performance. First, multiuser signal processing algorithms that combat MAI are developed for code division multiple access (CDMA) systems. Although multiuser signal processing has been extensively studied in the past, very little attention has focused on the practically important case of CDMA with long spreading codes that forms the basis of next generation wireless cellular communications. This work proposes new multiuser channel estimation and tracking algorithms for long code CDMA and demonstrates the significant gains achievable in performance. We also propose new channel estimation algorithms for short code CDMA systems that support multiple data rates to achieve multimedia communication. The new problems posed by the multirate nature of this system, key to support multimedia communication, are studied. Then, in the rest of this work, the use of multiple antenna systems with channel feedback is studied to combat fading. Multiple antenna systems provide multiple independent channels for communication and can effectively combat fading. The following practical issues, ignored in most existing work, are considered in the analysis and design: (i) the channel information is imperfect both at the receiver and at the transmitter, and (ii) part of the total available resources for the system need to be used for channel estimation and feedback. Feedback and power control schemes are designed to achieve significant gains in performance (in terms of outage probability) and data rates. These gains, based on the information-theoretic analysis, are then translated into frame error rate improvements for practical systems that employ space-time codes.
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    Time-Selective Signaling and Reception for Communication over Multipath Fading Channels
    (2000-01-20) Bhashyam, Srikrishna; Sayeed, Akbar M.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    The mobile wireless channel affords inherent diversity to combat the effects of fading. Existing code division multiple access (CDMA) systems, by virtue of spread-spectrum signaling and RAKE reception, exploit only part of the channel diversity via multipath combination. Moreover, their performance degrades under fast fading commonly encountered in mobile scenarios. In this paper, we develop new signaling and reception techniques that maximally exploit channel diversity via joint multipath-Doppler processing. Our approach is based on a canonical representation of the wireless channel which leads to a time-frequency generalization of the RAKE receiver for diversity processing. Our signaling scheme facilitates joint multipath-Doppler diversity by spreading the symbol waveform beyond the inter-symbol duration to make the channel time-selective. A variety of detection schemes are developed to account for the inter-symbol interference (ISI) due to overlapping symbols. However, our results indicate that the effects of ISI are virtually negligible due to the excellent correlation properties of the pseudo-random codes. Performance analysis also shows that relatively small Doppler spreads can yield significant diversity gains. The inherently higher level of diversity achieved by time-selective signaling brings the fading channel closer to an additive white Gaussian noise channel, thereby facilitating the use of powerful existing coding techniques for Gaussian channels.
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    Time-Selective Signaling and Reception for Communication over Multipath Fading Channels
    (1998-04-20) Bhashyam, Srikrishna; Center for Multimedia Communications (http://cmc.rice.edu/)
    The mobile wireless channel offers inherent diversity by virtue of multipath and Doppler shifts. Multipath diversity is exploited via spread-spectrum signaling employed in code division multiple access (CDMA) systems. However, the RAKE receiver commonly used in CDMA systems exploits only multipath diversity, and consequently suffers from significant performance degradation under fast fading. We develop new signaling and reception schemes in the context of CDMA systems that fully exploit the channel via joint multipath-Doppler diversity. The signaling waveforms are spread in time and frequency. Receiver structures are developed to deal with the inter-symbol interference (ISI) introduced by overlapping successive symbols. Analytical and simulated performance results indicate that the effects of ISI are negligible due to the excellent correlation properties of the spreading codes. Moreover, even the small Doppler spreads encountered in practice can yield significant performance gains. Additionally, the time-selective signaling scheme allows for substantially higher level of diversity and thereby brings the fading channel closer to an additive white Gaussian noise channel. This facilitates the use of error control codes developed for the Gaussian channel.
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    Time-selective signaling and reception for communication over multipath fading channels
    (1998) Bhashyam, Srikrishna; Aazhang, Behnaam
    The mobile wireless channel offers inherent diversity by virtue of multipath and Doppler shifts. Multipath diversity is exploited via spread-spectrum signaling employed in code division multiple access (CDMA) systems. However, the RAKE receiver commonly used in CDMA systems exploits only multipath diversity, and consequently suffers from significant performance degradation under fast fading. We develop new signaling and reception schemes in the context of CDMA systems that fully exploit the channel via joint multipath-Doppler diversity. The signaling waveforms are spread in time and frequency. Receiver structures are developed to deal with the inter-symbol interference (ISI) introduced by overlapping successive symbols. Analytical and simulated performance results indicate that the effects of ISI are negligible due to the excellent correlation properties of the spreading codes. Moreover, even the small Doppler spreads encountered in practice can yield significant performance gains. Additionally, the time-selective signaling scheme allows for substantially higher level of diversity and thereby brings the fading channel closer to an additive white Gaussian noise channel. This facilitates the use of error control codes developed for the Gaussian channel.