Rice Wireless

Permanent URI for this collection

https://hdl.handle.net/1911/21945
Formerly the Center for Multimedia Communications, Rice Wireless is part of the university's Electrical and Computer Engineering Department. More information about the group can be found at http://wireless.rice.edu/.

Browse

Browsing Rice Wireless by Author "Aazhang, Behnaam"

Now showing 1 - 20 of 26
  • Thumbnail Image
    Item
    Data Collection by a Mobile Observer in a Single-hop Sensor Network
    (2005-01-15) Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we demonstrate significant reduction in communication power by using a mobile observer to collect data from a wireless single-hop network of static sensors. The key challenge in using a mobile observer is that it remains within range of any sensor for a limited period, and inability to transfer data in this period causes data loss. This prompts us to choose the fraction of nodes successful in communicating data as our performance constraint as we try to minimize network power. First, we establish that the process of data collection by a mobile observer is analogous to a queue. The queuing-based data collection model is then used as a design tool to identify the combination of system parameters that satisfies the performance metric with minimum power. Second, we derive a sufficient condition on node placement for zero data loss in a mobile observer network. As our third contribution, we demonstrate that in a dense network, the data collection analysis is much simpler, which allows us to obtain closed expressions for the network power. Power comparisons with static observer networks show that power reduction by multiple orders of magnitude is typical. The scenarios chosen for power comparisons also provide guidelines on the choice of path, if such a choice is available. As an example, a disc shaped network is analyzed and among the set of concentric circular paths, the best path is identified.
  • Thumbnail Image
    Item
    Delay and Rate Constrained Transmission Policies over Wireless Channels
    (2001-11-20) Rajan, Dinesh; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we study delay and rate-constrained transmission of bursty traffic over wireless channels. We characterize the minimum power requirements via bounds for both single user and multiuser downlink problems, using a class of randomized first-come first-serve policies. We show that larger tolerable delay leads to power reduction, even for single-user Gaussian channels.
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    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.
  • Thumbnail Image
    Item
    Fast Multi-user Detector for a Time-varying CDMA System
    (SPIE, 1997-07-01) Das, Suman; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communication
    This paper investigates methods to reduce the amount of computation needed to detect information bits using a linear detector for a CDMA system. We show windowing technique coupled with pipelining can reduce the amount of computation without significantly sacrificing the performance of linear feedback detector. We also describe efficient techniques to adapt to a dynamic system where the system parameters vary due to the change in delays associated with individual users.
  • Thumbnail Image
    Item
    Gradient Estimation for Sensitivity Analysis and Adaptive Multiuse Interface Rejection in Code-division Multiple-access Systems
    (1997-07-20) Mandayam, Narayan B.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we consider a direct-sequence code-division multiple-access (DS-CDMA) system in the framework of a discrete-event dynamic system (DEDS) in order to optimize the system performance. Based on this formulation, we develop infinitesimal perturbation analysis (IPA) for estimating the sensitivity of the average probability of bit error to factors ranging from near-far effects to imperfections in power control. The above estimates are shown to be unbiased, and this technique is then further incorporated into a stochastic gradient algorithm for achieving adaptive multiuser interference rejection for such systems, which is also subject to frequency nonselective slow fading. We use an IPA-based stochastic training algorithm for developing an adaptive linear detector with the average probability of error being the minimization criterion. We also develop a practical implementation of such an adaptive detector where we use a joint estimation-detection algorithm for minimizing the average probability of bit error. A sequential implementation that does not require a stochastic training sequence or a preamble is also developed.
  • Thumbnail Image
    Item
    Gradient Estimation for Stochastic Optimization of Optical Code-Division Multiple-Access Systems: Part I -- Generalized Sensitivity Analysis
    (1997-05-20) Mandayam, Narayan B.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    For optimizing the performance of optical code-division multiple-access (CDMA) systems, there is a need for determining the sensitivity of the bit-error rate (BER) of the system to various system parameters. Asymptotic approximations and bounds, used for system bit-error probabilities, seldom capture the sensitivities of the system performance. We develop single-run gradient estimation methods for such optical CDMA systems using a discrete-event dynamic systems (DEDS) approach. Specifically, computer-aided techniques such as infinitesimal perturbation analysis (IPA) and likelihood ratio (LR) methods are used for analyzing the sensitivity of the average BER to a wide class of system parameters. It is shown that the above formulation is equally applicable to time-encoded and frequency-encoded systems. Further, the estimates derived are unbiased, and also optimality of the variance of these estimates is shown via the theory of common random variates and importance sampling techniques.
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Low Complexity Iterative Multiuser Detection and Decoding for Real-Time Applications
    (IEEE, 2005-07-01) Das, Suman; Erkip, Elza; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communication
    This paper presents a low-complexity multiuser decoding technique that can be implemented in real time for a convolutionally coded direct sequence code division multiple access (DS-CDMA) system. The main contribution, denoted here as the iterative prior update (IPU), consists of iterative interference cancellation and prior updates on sequences of coded bits combined with M-algorithm and list decoding. We illustrate performance gains over other low-complexity sequence detection and decoding strategies and argue that the algorithm converges within a few iterations and requires only a small size buffer for keeping track of the priors along iterations. The fact that the we can use existing available architectures for Viterbi decoding with slight modifications and can meet the real-time processing constraints makes the IPU algorithm an attractive alternative for cellular systems.
  • Thumbnail Image
    Item
    Maximum Likelihood Multipath Channel Parameter Estimation in CDMA Systems
    (1998-03-01) Sengupta, Chaitali; Hottinen, Ari; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communication
    The problem addressed in this paper is the estimation of the channel parameters in a Code Division Multiple Access(CDMA) communication system, in the presence of multipath effects. Maximum likelihood estimation of these parameters has been investigated in the past with the main drawback being the complexity of the multi-dimensional algorithms. The algorithm presented in this paper elegantly decomposes the multiuser problem into a series of single user problems. The algorithm first estimates a composite channel impulse response of each user and then extracts the channel parameters of all the paths of each user from the channel impulse response. We evaluate the performance of the algorithm through simulation studies.
  • Thumbnail Image
    Item
    Maximum Weight Basis Decoding of Convolutional Codes
    (IEEE, 2002-11-01) Das, Suman; Erkip, Elza; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communication
    In this paper we describe a new suboptimal decoding technique for linear codes based on the calculation of maximum weight basis of the code. The idea is based on estimating the maximum number locations in a codeword which have least probability of estimation error without violating the codeword structure. In this paper we discuss the details of the algorithm for a convolutional code. The error correcting capability of the convolutional code increases with the constraint length of the code. Unfortunately the decoding complexity of Viterbi algorithm grows exponentially with the constraint length. We also augment the maximal weight basis algorithm by incorporating the ideas of list decoding technique. The complexity of the algorithm grows only quadratically with the constraint length and the performance of the algorithm is comparable to the optimal Viterbi decoding method.
  • Thumbnail Image
    Item
    MultiHop Communication is Order Optimal for Homogeneous Sensor Networks
    (2004-04-01) Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    The main goal of this paper is to show that multi-hop single-user communication achieves the per node transport capacity of O(lnN/N) in homogeneous sensor networks, making it order-optimal. Our contributions in this paper are threefold. First, we construct a route-discovery and scheduling scheme based on spatial TDMA for sensor networks. Second, we show that our schedule achieves a per node transport capacity of O(lnN/N), the same as that achievable by beamforming. Third, we compare multi-hop communication and beamforming based methods in terms of the network power consumption required to attain a fixed throughput. Based on our power calculations, we conclude that if the channel attenuation is above a certain threshold (which we calculate), then multi-hop communication performs better, whereas below the threshold, beamforming is preferable.
  • Thumbnail Image
    Item
    Multitier Wireless Communications
    (2001-06-20) Aazhang, Behnaam; Cavallaro, Joseph R.; Center for Multimedia Communications (http://cmc.rice.edu/)
    Next-generation computing systems will be highly integrated using wireless networking. The Rice Everywhere NEtwork (RENE) project is exploring the integration of WCDMA cellular systems, high speed wireless LANs, and home wireless networks to produce a seamless multitier network interface. We are currently developing a simulation acceleration testbed and a multitier network interface card (mNIC) consisting of DSP processors, custom VLSI ASICs, and FPGAs for baseband signal processing to interact with the various RF units and the host processor. This testbed will also allow us to explore high performance algorithm alternatives through computer aided design tools for rapid prototyping and hardware/software co-design of embedded systems.
  • Thumbnail Image
    Item
    Multitier Wireless Systems
    (Aalborg University, Denmark, 2000-05-01) Aazhang, Behnaam; Cavallaro, Joseph R.; Center for Multimedia Communication
    Next-generation computing systems will be highly integrated using wireless networking. The Rice Everywhere NEtwork (RENE) project is exploring the integration of WCDMA cellular systems, high speed wireless LANs, and home wireless networks to produce a seamless multitier network interface. We are currently developing a simulation acceleration testbed and multitier network interface card (mNIC) consisting of DSP processors, custom VLSI ASICs, and FPGAs for baseband signal processing to interact with the various RF units and the host processor. This testbed will also allow us to explore high performance algorithm alternatives through computer aided design tools for rapid prototyping and hardware/software co-design of embedded systems.
  • Thumbnail Image
    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.
  • Thumbnail Image
    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.
  • Thumbnail Image
    Item
    On Beamforming with Finite Rate Feedback in Multiple Antenna Systems
    (2003-10-20) Mukkavilli, Krishna Kiran; Sabharwal, Ashutosh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    In this paper, we study a multiple antenna system where the transmitter is equipped with quantized information about instantaneous channel realizations. Assuming that the transmitter uses the quantized information for beamforming, we derive a universal lower bound on the outage probability for any finite set of beamformers. The universal lower bound provides a concise characterization of the gain with each additional bit of feedback information regarding the channel. Using the bound, it is shown that finite information systems approach the perfect information case as (t-1)2-B/(t-1), where B is the number of feedback bits and t is the number of transmit antennas. The geometrical bounding technique, used in the proof of the lower bound, also leads to a design criterion for good beamformers, whose outage performance approaches the lower bound. The design criterion minimizes the maximum inner product between any two beamforming vectors in the beamformer codebook, and is equivalent to the problem of designing unitary space time codes under certain conditions. Finally, we show that good beamformers are good packings of 2-dimensional subspaces in a 2t-dimensional real Grassmannian manifold with chordal distance as the metric.
  • Thumbnail Image
    Item
    On Design Criteria and Construction of Non-coherent Space-Time Constellations
    (2003-10-20) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; 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 the channel state information is available neither at the transmitter nor at the receiver. It is known that at high SNR, or when the coherence interval is much larger than the number of transmit antennas, a constellation of unitary matrices can achieve the capacity of the non-coherent system. However, at low SNR, high spectral efficiencies, or for small values of coherence interval, the unitary constellations lose their optimality and fail to provide an acceptable performance. In this work, inspired by the Stein's lemma, we propose to use the Kullback-Leibler distance between conditional distributions to design space-time constellations for non-coherent communication. In fast fading, i.e., when the coherence interval is equal to one symbol period and the unitary construction provides only one signal point, the new design criterion results in PAM-type constellations with unequal spacing between constellation points. We also show that in this case, the new design criterion is equivalent to design criteria based on the exact pairwise error probability or the Chernoff information. When the coherence interval is larger than the number of transmit antennas, the resulting constellations overlap with the unitary constellations at high SNR, but at low SNR they have a multilevel structure and show significant performance improvement over unitary constellations of the same size. The performance improvement becomes especially more significant when a large number of receive antennas are used. This property, together with the facts that the proposed constellations eliminate the need for training sequences and are most suitable for low SNR, makes them a good candidate for uplink communication in wireless systems.
  • Thumbnail Image
    Item
    On Multipath Channel Estimation for CDMA Systems Using Multiple Sensors
    (2001-03-20) Sengupta, Chaitali; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)
    This paper focuses on the design of a multiuser receiver structure for the reverse link of a code-division multiple-access communication system, in the presence of multipath effects and using an antenna array at the base station receiver. The algorithm presented in this paper solves the complex multidimensional problem of channel estimation in this complex scenario using a maximum-likelihood approach. This channel estimation technique requires the transmission of a training sequence or feedback of detected data. Once a composite channel-impulse response of each user is estimated, it is directly used in the detection process instead of first extracting the individual channel parameters, such as path delays and attenuation factors. The paper presents a framework that facilitates a computationally efficient solution to the combined problem of channel estimation and detection in a scenario involving multiple users, multiple paths, and multiple sensors at the receiver.
  • Thumbnail Image
    Item
    Partially Coherent Constellations for Multiple-Antenna Systems
    (2003-11-20) Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam; Varshney, Prabodh; 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. We assume that the transmitter doesnâ t know the channel coefficients, and that the receiver has only an estimate of them. We further assume that the transmitter and receiver know the statistics of the estimation error. We will refer to this system as a partially coherent system. In an earlier work, we had derived a design criterion for the partially coherent constellations based on maximizing the minimum KL distance between conditional distributions. We had also designed single transmit antenna constellations using this criterion, which showed substantial improvement in the performance over existing and widely-used constellations. In this work, using the KL-based design criterion, we design partially coherent constellations for multiple antenna systems, and evaluate their performance through simulation. We show that, even with only a few percent channel estimation error, the new constellations achieve significant performance gains over the conventional constellations and existing multiple antenna techniques. The proposed constellations are multi-level, with multi-dimensional spherical constellations at each level. We also propose a recursive construction for the constituent spherical subsets of the multiple-antenna partially coherent constellations.