Now showing items 1-20 of 142

    • Achievable Rates for Arbitrary Network Topologies with â Cheapâ Nodes 

      Khojastepour, Mohammad; Sabharwal, Ashutosh; Aazhang, Behnaam (2003-05-01)
      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 ...
    • Addressing indirect frequency coupling via partial generalized coherence 

      Young, Joseph; Homma, Ryota; Aazhang, Behnaam (2021)
      Distinguishing 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 ...
    • All-optical CDMA with bipolar codes 

      Nguyen, Lim; Aazhang, Behnaam; Young, James F. (1995-03-20)
      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 ...
    • Analysis of Decision-Feedback Based Broadband OFDM Systems 

      de Baynast, Alexandre; Sabharwal, Ashutosh; Aazhang, Behnaam (2005-11-01)
      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 ...
    • Antenna Packing in Low Power Systems: Communication Limits and Array Design 

      Muharemovic, Tarik; Sabharwal, Ashutosh; Aazhang, Behnaam (2008)
      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, ...
    • An Approach to Capacity Analysis of Coarsely Coordinated Low Power Multiple Access Systems 

      Muharemovic, Tarik; Sabharwal, Ashutosh; Aazhang, Behnaam (2004-07-01)
      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 <i>policy</i>, which accomodates a range of small spectral efficiencies, while treating instantaneous data rates of other ...
    • Bayesian Blind PARAFAC Recievers forDS-CDMA Systems 

      de Baynast, Alexandre; Declercq, David; De Lathauwer, Lieven; Aazhang, Behnaam (2003-10-01)
      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 ...
    • Beamformer Design with Feedback Rate Constraints : Criteria and Constructions 

      Mukkavilli, Krishna Kiran; Sabharwal, Ashutosh; Erkip, Elza; Aazhang, Behnaam (2003-07-20)
      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 ...
    • BER of optical communication system using fiber source 

      Nguyen, Lim; Aazhang, Behnaam; Young, James F. (1995-11-20)
      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.
    • Blind PARAFAC Receivers for Multiple Access-Multiple Antenna Systems 

      de Baynast, Alexandre; De Lathauwer, Lieven; Aazhang, Behnaam (2003-10-01)
      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 <i>N<sub>t</sub></i> streams that are simultaneously transmitted using <i>N<sub>t</sub></i> transmit antennas. ...
    • Bounds on Achievable Rates for General Multi-terminal Networks with Practical Constraints 

      Khojastepour, Mohammad; Aazhang, Behnaam; Sabharwal, Ashutosh (2003-04-20)
      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 ...
    • Broadcast Space-Time Coding 

      Memarzadeh, Mahsa; Sabharwal, Ashutosh; Aazhang, Behnaam (2001-08-20)
      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 ...
    • The Capacity of Average and Peak Power Constrained Fading Channels with Channel Side Information 

      Khojastepour, Mohammad; Aazhang, Behnaam (2004-03-01)
      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 ...
    • A comparative study of multiple accessing schemes 

      Erkip, Elza; Aazhang, Behnaam (1997-11-20)
      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 ...
    • Computationally Efficient Iterative Multiuser Detection and Decoding 

      Das, Suman; Erkip, Elza; Aazhang, Behnaam (1998-11-20)
      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 ...
    • Computationally Efficient Multiuser Detectors 

      Das, Suman; Cavallaro, Joseph R.; Aazhang, Behnaam (1997-09-20)
      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 ...
    • Constellations for Imperfect Channel State Information at the Receiver 

      Borran, Mohammad Jaber; Sabharwal, Ashutosh; Aazhang, Behnaam (2002-10-20)
      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 ...
    • Contraction, Smoothness, and Low-Pass Filtering 

      Khojastepour, Mohammad; Aazhang, Behnaam; Baraniuk, Richard G. (2004-05-01)
      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 ...
    • Cooperative Communications in the Fading Channel 

      Ahmed, Nasir; Aazhang, Behnaam (2005-03-01)
      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 ...
    • COOPERATIVE COMMUNICATIONS: Fundamental Limits and Practical Implementation 

      Chakrabarti, Arnab; Sabharwal, Ashutosh; Aazhang, Behnaam (2006-01-01)
      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 ...