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Browsing DSP Publications by Author "Aazhang, Behnaam"
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Item Contraction, Smoothness, and Low-Pass Filtering(2004-05-01) Khojastepour, Mohammad; Aazhang, Behnaam; Baraniuk, Richard G.; Digital Signal Processing (http://dsp.rice.edu/)We introduce a generalized definition for "low-pass" filters that covers time-varying and nonlinear systems under the same umbrella. We show that the qualitative concept of signal smoothing can be made precise through the concept of contractions in probabilistic metric spaces. For illustration, we consider classical linear time-invariant low-pass filters, nonlinear median filters, and time-varying guaranteed maximum delay schedulers employed in communication systems.Item DSP architectural considerations for optimal baseband processing(2002-08-20) Rajagopal, Sridhar; Rixner, Scott; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)The data rate requirements for future wireless systems has increased by orders-of-magnitude (from Kbps to several Mbps), requiring more sophisticated algorithms for their implementation. This tutorial will explore different architectural issues to consider for optimal wireless baseband processing. It will look at research into real-time architectural design issues such as number of functional units, data access from memory and sequential traceback for Viterbi decoding using digital signal processorsItem 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.Item 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.Item Multiuser detection in fast-fading multipath environments(2001-10-20) Sayeed, Akbar M.; 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.Item 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.Item On Design Criteria and Construction of Non-coherent Space-Time Constellations(2002-07-20) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Johnson, Don; Center for Multimedia Communications (http://cmc.rice.edu/)We consider the problem of digital communication in a Rayleigh flat fading environment using a multiple antenna system, when both the transmitter and the receiver are unaware of the channel coefficients. Using Stein's lemma to bound the pairwise error probabilities, we propose to use the Kullback-Leibler distance between distributions assigned to the transmitted symbols to design space-time constellations for non-coherent communication. We show that optimal codes according to the proposed criterion can be obtained by partitioning the signal space into appropriate subsets and using unitary designs inside each subset. Actual constellation design is performed by solving an optimization problem to find the size and location of the subsets. Our simulation results show that the new constellations can provide a substantial improvement in the performance over known unitary space-time codes. Furthermore, the proposed constellations can be decoded using a simple low-complexity decoder.Item 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.Item 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.