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Browsing ECE Publications by Author "Aazhang, Behnaam"
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Item Achievable Rates for Arbitrary Network Topologies with â Cheapâ Nodes(2003-05-01) Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)In this paper, we derive achievable rates for arbitrary network topologies consisting of â cheapâ nodes. A node is labeled â cheapâ if its radio can only operate in TDD mode when transmitting and receiving in the same frequency band. Two main results are shown. The first result provides an achievable rate for the channel with either continuous alphabet or discrete channel. The second result provides the achievable rate for the Gaussian channel with average power constraint. The two results are applied to the case of Gaussian relay channel and concatenated channel with cheap nodesItem Addressing indirect frequency coupling via partial generalized coherence(Springer Nature, 2021) Young, Joseph; Homma, Ryota; Aazhang, BehnaamDistinguishing between direct and indirect frequency coupling is an important aspect of functional connectivity analyses because this distinction can determine if two brain regions are directly connected. Although partial coherence quantifies partial frequency coupling in the linear Gaussian case, we introduce a general framework that can address even the nonlinear and non-Gaussian case. Our technique, partial generalized coherence (PGC), expands prior work by allowing pairwise frequency coupling analyses to be conditioned on other processes, enabling model-free partial frequency coupling results. By taking advantage of recent advances in conditional mutual information estimation, we are able to implement our technique in a way that scales well with dimensionality, making it possible to condition on many processes and produce a partial frequency coupling graph. We analyzed both linear Gaussian and nonlinear simulated networks. We then performed PGC analysis of calcium recordings from mouse olfactory bulb glomeruli under anesthesia and quantified the dominant influence of breathing-related activity on the pairwise relationships between glomeruli for breathing-related frequencies. Overall, we introduce a technique capable of eliminating indirect frequency coupling in a model-free way, empowering future research to correct for potentially misleading frequency interactions in functional connectivity analyses.Item All-optical CDMA with bipolar codes(1995-03-20) Nguyen, Lim; Aazhang, Behnaam; Young, James F.; Center for Multimedia Communications (http://cmc.rice.edu/)A method for the transmission and detection of bipolar sequences in a unipolar system is presented. It allows all-optical implementation, in noncoherent optical CDMA systems, of the bipolar codes that have been developed for the radio domain. A practical design is described that encodes the spectrum of a broadband optical source to support a large number of subscribers.Item Analysis of Decision-Feedback Based Broadband OFDM Systems(2005-11-01) de Baynast, Alexandre; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)In wireless communications, about 25% of the bandwidth is dedicated to training symbols for channel estimation. By using a semi-blind approach, the training sequence length can be reduced while improving performance. The principle is as follows: the detected symbols (hard decision) are fed back to the channel estimator in order to re-estimate the channel more accurately. However, semi-blind approach can significantly deteriorate the performance if the bit error rate is high. In this paper, we propose to determine analytically the minimum Signal to Noise Ratio (SNR) from which a semi-blind method starts to outperform a training sequence based only system.Item Antenna Packing in Low Power Systems: Communication Limits and Array Design(2008) Muharemovic, Tarik; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)In this paper, we study design of transceiver antenna arrays and its impact on spectral efficiency of low-power systems. Our primary motivation is construction of practical and portable multi-antenna configurations with a very small and a-priori fixed volume for placing antennas. Using spectral efficiency as a target metric for array optimization, we show that any array configuration, transmit or receive, can be characterized via a parameter that we interpret as "effective degrees of freedom." For any array configuration, effective degrees of freedom describes an equivalent uncorrelated array, which results in the same low-power behavior of spectral efficiency. Joint optimization of transmit and receive antenna configurations decouples into maximizing effective degrees of freedom for transmitter and receiver separately. To achieve this goal, we introduce and study a theoretical benchmark of "limiting degrees of freedom," which is the least upper bound on effective degrees of freedom, evaluated over all configurations with finite number of antennas. Limiting degrees of freedom therefore describes the best possible performance for any transceiver array which confines its elements inside a given space. We compute a closed-form expression for limiting degrees of freedom of a circular geometry. Finally, we present numerical procedure and examples for designing linear and square arrays with non-uniform spacing, which typically exhibit significant spectral efficiency gains over uniform arrays.Item An Approach to Capacity Analysis of Coarsely Coordinated Low Power Multiple Access Systems(2004-07-01) Muharemovic, Tarik; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)We consider multiaccess problem in low power systems, where we allow each user to select its own data rate and transmit power locally and independently from other users. Here, every user has a set of low power codebooks, labeled a policy, which accomodates a range of small spectral efficiencies, while treating instantaneous data rates of other users as an unknown compound parameter. Even with such coarse user coordination, multiuser detection enables a system which is superior to any classic orthogonal division system. First we fully characterise the set of achievable policies, after which we demonstrate that in multiantenna systems, policies are be viewed as awarding protected receiver spatial dimensions to each user.Item Bayesian Blind PARAFAC Recievers forDS-CDMA Systems(2003-10-01) de Baynast, Alexandre; Declercq, David; De Lathauwer, Lieven; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)In this paper an original Bayesian approach for blind detec-tion for Code Division Multiple Access (CDMA) Systems in presence of spatial diversity at the receiver is developed. In the noiseless context, the blind detection/identification problem relies on the canonical decomposition (also re-ferred as Parallel Factor analysis [Sidiropoulos, IEEE SP 00], PARAFAC. The author in [Bro,INCINC 96] pro-poses a suboptimal solution in least-squares sense. How-ever, poor performance are obtained in presence of high noise level. The recently emerged Markov chain Monte Carlo (MCMC) signal processing method provide a novel paradigm for tackling this problem. Simulation results are presented to demonstrate the effectiveness of this method.Item Beamformer Design with Feedback Rate Constraints : Criteria and Constructions(2003-07-20) Mukkavilli, Krishna Kiran; Sabharwal, Ashutosh; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)In this work, we provide a geometrical framework for the analysis and design of beamformer codebooks with finite number of beamformer vectors. We present a design criterion for good beamfomer codebooks and show the equivalence of the beamformer design problem to two other known problems. First, the beamformer design problem can be directly posed as a problem of packing 2 dimensional subspaces in a 2t dimensional Grassmannian manifold, t being the number of transmit antennas. And second, under certain conditions, the beamformer design problem is equivalent to the construction of unitary space time constellations.Item BER of optical communication system using fiber source(1995-11-20) Nguyen, Lim; Aazhang, Behnaam; Young, James F.; Center for Multimedia Communications (http://cmc.rice.edu/)We analyze the bit-error rate (BER) of an optical communication system using the superfluorescent fiber source (SFS). The counting statistics of thermal light give improved performance relative to the Gaussian statistics that predict a BER floor. We consider a spectrum-sliced wavelength-division multiple access (WDMA) system that employs the SFS.Item Blind PARAFAC Receivers for Multiple Access-Multiple Antenna Systems(2003-10-01) de Baynast, Alexandre; De Lathauwer, Lieven; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)In this paper, we present a new blind receiver for multiple access channel with multiple transmit antennas per user and multiple receive antennas (MIMO channel). After being multiplied by a spreading sequence, each user s data is split into Nt streams that are simultaneously transmitted using Nt transmit antennas. The received signal at each receive antenna is a linear superposition of the Nt transmitted signals of the Nu users perturbed by noise. We propose a new blind detection/identification algorithm under the assumption that the fading is slow and frequency non-selective. This algorithm relies on a generalization of parallel factor analysis (PARAFAC analysis, [Kruskal, Lin. Alg. Appl. 77, Sidiropoulos, Tr. on Sig. Proc. 00]): we show that a generalized canonical decomposition (CANDECOMP) of the 3D data tensor is unique under mild assumptions without noise. Neither algebraic orthogonality nor independence between sources is needed for uniqueness of the decomposition. By performing this decomposi-tion, in rank-(Nt,Nt,1) terms, we are able to retrieve the three sets of parameters: the symbols, the channel fading coefficients (including the antenna gains) and the spreading sequences. In a noisy context, we propose a simple algorithm of the alternating least squares (ALS) type, which yields a performance close to the linear minimum mean square error (LMMSE) receiver which requires knowledge of the channel and spreading sequences.Item Bounds on Achievable Rates for General Multi-terminal Networks with Practical Constraints(2003-04-20) Khojastepour, Mohammad; Aazhang, Behnaam; Sabharwal, Ashutosh; Center for Multimedia Communications (http://cmc.rice.edu/)We consider the problem of communication in a general multi-terminal network where each node of the network is a potential sender or receiver (or both) but it cannot do both functions together. The motivation for this assumption comes from the fact that current radios in sensor nodes operate in TDD mode when the transmitting and receiving frequencies are the same. We label such a radio as a cheap radio and the corresponding node of the network as a cheap node. We derive bounds on the achievable rates in a general multi-terminal network with finite number of states. The derived bounds coincide with the known cut-set bound of network information theory if the network has just one state. Also, the bounds trivially hold in the network with cheap nodes because such a network operates in a finite number of states when the number of nodes is finite. As an example, application of these bounds in the multi-hop network and the relay channel with cheap nodes is presented. In both of these cases, the bounds are tight enough to provide converses for the coding theorems, and thus their respective capacities are derived.Item Broadcast Space-Time Coding(2001-08-20) Memarzadeh, Mahsa; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)In this paper, we study transmission techniques for broadcast channels with a single transmitter and multiple receivers. The transmitter is assumed to be equipped with multiple antennas. Further, each receiver is interested only in part of the transmitted information. Using Gaussian code based information theoretic bounds, we analyze two transmit beamforming techniques. The first technique, zero-forcing beamformer, uses the channel information for all users to send spatially orthogonal signals to different users, i.e., no user receives interference from the other user signals. The second method uses the single-user optimal beamformer with no effort to reduce interference at the receivers. It is shown that the above transmit beamforming techniques are similar to multiuser receivers used in non-orthogonal CDMA systems. In particular, the zero-forcing beamformer is similar to a decorrelating detector and the single-user beamformer is identical to a matched filter. The comparison with CDMA multiuser receivers is strengthened by results on spectral efficiency of proposed beamformers, which follow the behavior of decorrelating and matched-filter receiver.Item The Capacity of Average and Peak Power Constrained Fading Channels with Channel Side Information(2004-03-01) Khojastepour, Mohammad; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)We derive the ergodic capacity of discrete-time fading channel with additive Gaussian noise subject to both peak and average power constraint. The average power can be interpreted as the cost that we incur to achieve a certain rate. On the other hand, the motivation of this analysis comes from the fact that there is also a peak power limitation in practical communication system. It is been shown that the optimal power adaption is no longer water-filling or constant power adaption which is the case where there is no limitation on the peak power. The numerical results show that the importance of peak power constraint become negligible for relatively low available average power, while it is limiting the capacity to be finite even as available average power goes to infinity.Item A comparative study of multiple accessing schemes(1997-11-20) Erkip, Elza; Aazhang, Behnaam; Fargues, M.P., Hippenstiel, R.D.; Center for Multimedia Communications (http://cmc.rice.edu/)We compare the performance of different accessing schemes (frequency division, time division and code division) for the uplink in a wireless communication system. We assume an additive white Gaussian noise channel with multipath fading. We consider the case when the receiver can track the channel parameters, but the transmitters cannot. We look at three different measures of performance: the Shannon capacity, the delay limited capacity and probability of outage. Shannon capacity is better suited for systems where the delay requirements are not as stringent, whereas the delay limited capacity and probability of outage give the performance when there are strict delay requirements or when the channel is slowly fading. We observe that all three schemes have the same achievable rates of transmission in Shannon sense, but the code division multiple access scheme performs better than frequency and time division in terms of delay limited capacity and probability of outage.Item Computationally Efficient Iterative Multiuser Detection and Decoding(1998-11-20) Das, Suman; Erkip, Elza; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)We describe an iterative detection and decoding scheme for the uplink in a convolutionally coded direct sequence code division multiple access (DS-CDMA) system. We consider maximum-a-posteriori (MAP) decoding and reduce the complexity through an iterative interference cancelation scheme combined with a suboptimal channel decoding algorithm. The MAP priors are updated at every iteration step. We investigate the performance of this low complexity scheme and observe that it is close to optimal.Item Computationally Efficient Multiuser Detectors(1997-09-20) Das, Suman; Cavallaro, Joseph R.; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)CDMA is becoming an increasingly popular multiplexing scheme in wireless communications and this has necessitated the development of efficient detection techniques. The exponential complexity of the optimal detector on one end and inferior performance of conventional single-user detctor at the other have led to the development of suboptimal multiuser detectors with lower complexity. Most of these detection techniques involve solution of a linear system. In their naive implementation this requires O(n³) operations in the size of the matrix. This cost can be reduced if we move towards modern iterative techniques for solution of the system. However maximum benefit can be achieved if we fully exploit the structure of the system. In this paper we have proposed several methods of reducing the computational complexity utilizing the above ideas. We have also come up with algorighms which computationally can achieve the lower bound in complexity.Item Constellations for Imperfect Channel State Information at the Receiver(2002-10-20) 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. We assume that the transmitter doesn't know the channel coefficients, and that the receiver has only an estimate of them with some known estimation error. Inspired by the Stein's lemma, we propose to use the Kullback-Leibler distance between distributions assigned to the transmitted symbols as a performance criterion. Using this performance criterion, we derive a design criterion based on maximizing the minimum KL distance between constellation points. As an example, we design constellations for a single transmit antenna system using the above criterion, and through simulation, show that the new constellations can provide a substantial improvement in the performance over existing and widely-used constellations.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 Cooperative Communications in the Fading Channel(2005-03-01) Ahmed, Nasir; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)Cooperative coding is a communication paradigm that pools distributed resources of different nodes in a network, such that the nodes act like a collaborative system instead of greedy adversarial participants. Cooperation has shown promise in increasing throughput and providing better power efficiency in wireless networks. In this work, we consider a basic example of cooperative communication, relay coding, and consider methods to improve the power efficiency by employing feedback and using power control. We consider power control policies based on the degree of transmitter channel knowledge. First, when perfect feedback is available, we show results for the optimal power control policy for any network code. We show that by using the decode and forward relaying protocol, in some cases it is possible to approach the universal lower bound on the outage probability for the block fading relay channel. Second, when a finite rate of feedback is available, we see that only a few feedback bits are necessary to achieve most of the gains that the perfect feedback policy has over constant power transmission. Based on these results, it is evident that future network protocols should utilize feedback in order to fully exploit the potential gains of network coding.Item COOPERATIVE COMMUNICATIONS: Fundamental Limits and Practical Implementation(2006-01-01) Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam; Center for Multimedia Communications (http://cmc.rice.edu/)This chapter summarizes theoretically achievable gains and the construction of practical codes for user-cooperation. Most of these results relate to the relay channel, which is a three-terminal channel that captures the essence of usercooperation and serves as one of the primary building blocks for cooperation on a larger scale. In investigating the fundamental limits of relaying, we present information-theoretic results on the achievable throughput of relay channel in mutual-information terms. We also include results on Gaussian channels, and for the practically important case of half-duplex relaying. In the domain of relay coding, we specifically discuss pragmatic code constructions for half as well as full-duplex relaying, using LDPC codes as components.