Sennur Ulukus's scientific contributionswhile working at Loyola University Maryland, Baltimore, Maryland, United States and other institutions

Publications (270)

  • Sennur Ulukus · Ezio Biglieri · Moe Z. Win
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  • A. Yener · R. D. Yates · S. Ulukus
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  • Onur Kaya · Sennur Ulukus
    Abstract: with E(b2i) = 1, si = (si1;¢¢¢ ;siN) > denotes the signature se- quence, p hi denotes the random and continuously distributed channel gain, and pi denotes the transmit power; n is a zero- mean Gaussian random vector with covariance æ2IN. We as- sume that the receiver and all of the transmitters have per- fect knowledge of the channel states of all users represented as a vector h = (h1;¢¢¢ ;hK) > . The transmitters are then able to choose their powers as a function of the channel state... Show More
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    Sennur Ulukus · Roy D Yates
    Abstract: For wireless communication systems, iterative power control algorithms have been proposed to minimize transmitter powers while maintaining reliable communication between mobiles and base stations. To derive deterministic convergence results, these algorithms require perfect measurements of one or more of the following parameters: (i) channel gains of some or all the users, (ii) the mobile's signal to interference ratio (SIR) at the receiver, (iii) the interference experienced by the mobile,... Show More
    Full-text available · Article ·
  • S. Ulukus · R. Yates
    Article ·
  • Sennur Ulukus · Roy D Y Ates
    Abstract: In this paper, we c haracterize the user capacity, i.e., the maximum number of supportable users at a common SIR target level for a xed processing gain, of a single cell symbol asynchronous CDMA system. Based on the delay proole of the users, we identify a class of optimum signature sequences that achieve a l o wer bound on the total squared asynchronous correlation (TSAC) among the users. When the users' signatures achieve this lower bound, the user capacity o f a single-cell asynchronous... Show More
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    Ravi Tandon · Sennur Ulukus
    Abstract: The idea that no more dependence can be consumed than produced was used to obtain an outer bound to the capacity region of the single output two-way channel in (1). A parallel channel extension of this bound was used to obtain an outer bound of 0.64628 bits/transmission for the sum-rate point of the binary multiplying channel (BMC) which improved over Zhang- Berger-Schalkwijk bound of 0.64891 bits/transmission (2). It is not obvious that this selection of parallel channel would yield the... Show More
    Full-text available · Article ·
  • S. Ulukus · R. D. Yates
    Article · Jan 1989
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    Sennur Ulukus · Roy D Yates
    Abstract: Power control algorithms assume that the receiver struc-ture is xed and iteratively update the transmit powers of the users to provide them with an acceptable quality of service while minimizing the total transmitter power. Multiuser detection on the other hand optimizes the re-ceiver structure with the assumption that the users have xed transmitter powers. In this study, we combine the two approaches and propose an iterative and distributed power control algorithm which iteratively updates... Show More
    Full-text available · Article · Jan 1997
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    Sennur Ulukus · R.D. Yates
    Abstract: Power control algorithms assume that the receiver structure is fixed and iteratively update the transmit powers of the users to provide them with an acceptable quality of service while minimizing the total transmitter power. Multiuser detection on the other hand optimizes the receiver structure with the assumption that the users have fixed transmitter powers. In this study, we combine the two approaches for a CDMA cellular system and propose an iterative and distributed power control... Show More
    Full-text available · Conference Paper · Jul 1997
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    Sennur Ulukus · Roy D. Yates
    Abstract: The optimum multiuser detection problem was shown to be NP-hard, i.e., its computational complexity increases exponentially with the number of users (Verdu 1986, 1989). In this letter, we show that the optimum multiuser detection problem for a synchronous code-division multiple access (CDMA) system is equivalent to the minimum capacity cut problem in a related network and propose an optimum multiuser detection algorithm with polynomial computational complexity for a certain class of... Show More
    Full-text available · Article · May 1998 · IEEE Communications Letters
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    Sennur Ulukus · Roy D. Yates
    Abstract: Power control algorithms assume that the receiver structure is fixed and iteratively update the transmit powers of the users to provide them with an acceptable quality of service while minimizing the total transmitter power. Multiuser detection on the other hand optimizes the receiver structure with the assumption that the users have fixed transmitter powers. In this study, we combine the two approaches and propose an iterative and distributed power control algorithm which iteratively... Show More
    Full-text available · Article · May 1998 · Wireless Networks
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    Sennur Ulukus · Roy D. Yates
    Abstract: For wireless communication systems, iterative power control algorithms have been proposed to minimize the transmitter power while maintaining reliable communication between mobiles and base stations. To derive deterministic convergence results, these algorithms require perfect measurements of one or more of the following parameters: (1) the mobile's signal-to-interference ratio (SIR) at the receiver; (2) the interference experienced by the mobile; and (3) the bit-error rate. However, these... Show More
    Full-text available · Article · Jul 1998 · IEEE Transactions on Communications
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    Sennur Ulukus · Roy D. Yates
    Abstract: For single cell synchronous code division multiple access (CDMA) systems, both the information theoretic capacity and the network capacity have been identified. In both cases, it was shown that if the number of users, N , is no more than the processing gain, L, then orthogonal signatures are optimal while if N ? L, then signatures which satisfy the Welch bound on the total squared correlation with equality (called WBE sequences) are optimal. This paper presents an algorithm which iteratively... Show More
    Full-text available · Article · Oct 1998
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    Sennur Ulukus · R.D. Yates
    Abstract: The decorrelating detector is known to eliminate multiaccess interference when the signature sequences of the users are linearly independent, at the cost of enhancing the Gaussian receiver noise. We present a blind adaptive decorrelating detector which is based on the observation of readily available statistics. The algorithm recursively updates the filter coefficients of a desired user by using the output of the current filter. Due to the randomness of the information bits transmitted and... Show More
    Full-text available · Article · Nov 1998 · IEEE Journal on Selected Areas in Communications
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    Aylin Yener · Roy D. Yates · Sennur Ulukus
    Abstract: The optimum bit detector for multiuser CDMA systems has an exponential complexity in the number of users. Many suboptimum receivers have been developed to achieve good performance with less complexity. We approximate the solution of the optimum multiuser detection problem using nonlinear programming relaxations. We observe that some popular suboptimum receivers correspond to relaxations of the optimal detection problem. In particular, one approximation method yields to iterative solutions... Show More
    Full-text available · Conference Paper · Feb 1999
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    Christopher Rose · Sennur Ulukus · Roy Yates
    Abstract: Motivated by the emergence of programmable radios, we seek to understand a class of communication system where pairs of transmitters and receivers can adapt their modulation/demodulation method in the presence of interference to achieve better performance. Using signal to interference ratio as a metric, we present a class of iterative distributed algorithms for synchronous systems which results in an ensemble of optimal waveforms for multiple users connected to a common receiver (or... Show More
    Full-text available · Article · Apr 1999
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    Aylin Yener · Roy D. Yates · Sennur Ulukus
    Abstract: CDMA systems are limited by the interference that users create for each other. Several methods of controlling and/or suppressing the interference through power control, multiuser detection (temporal filtering) and beamforming (spatial filtering) are studied to increase the capacity of CDMA systems. We investigate the capacity increase that is possible by combining power control with intelligent temporal and spatial receiver filter design. The SIR maximizing joint temporal-spatial receiver... Show More
    Full-text available · Conference Paper · Aug 1999
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    Sennur Ulukus · Roy D. Yates
    Abstract: The capacity of Code Division Multiple Access (CDMA) systems has been studied in two different contexts in [3, 4]. In [3] and [4], signature sequence sets that maximize the information theoretic capacity and user capacity, respectively, of CDMA systems were identified. In both cases, it was shown that if the number of users is less than the processing gain then orthogonal signature sequences are optimal and if the number of users is larger than the processing gain then Welch bound equality... Show More
    Full-text available · Article · Nov 1999
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    Aylin Yener · Roy D. Yates · Sennur Ulukus
    Abstract: Among the ambitious challenges to be met by the third generation systems is to provide high capacity flexible services. W-CDMA emerges as a promising candidate to meet these challenges. It is well known that CDMA systems are interference limited, and interference management is needed to maximally utilize the potential gains of this access scheme. Several methods of controlling and/or suppressing the interference through power control, multiuser detection (temporal filtering) and receiver... Show More
    Full-text available · Article · Nov 1999

Publications citing this author (7510)

    • Later, convergence properties were improved by using averaging in [10]. The problem of joint power control, multiuser detection, and diversity combining is addressed in [11] and [12], where the effect of using antenna diversity is studied. Naturally, on the one hand, mobile users prefer to transmit at a lower power for a fixed SIR, and, on the other hand, for a given transmitter power, they prefer to obtain a better SIR.
    [Show abstract] [Hide abstract] ABSTRACT: Distributed power control is an important issue in wireless networks. Recently, noncooperative game theory has been applied to investigate interesting solutions to this problem. The majority of these studies assumes that the transmitter power level can take values in a continuous domain. However, recent trends such as the GSM standard and Qualcomm's proposal to the IS-95 standard use a finite number of discretized power levels. This motivates the need to investigate solutions for distributed discrete power control which is the primary objective of this paper. We first note that, by simply discretizing, the previously proposed continuous power adaptation techniques will not suffice. This is because a simple discretization does not guarantee convergence and uniqueness. We propose two probabilistic power adaptation algorithms and analyze their theoretical properties along with the numerical behavior. The distributed discrete power control problem is formulated as an N-person, nonzero sum game. In this game, each user evaluates a power strategy by computing a utility value. This evaluation is performed using a stochastic iterative procedures. We approximate the discrete power control iterations by an equivalent ordinary differential equation to prove that the proposed stochastic learning power control algorithm converges to a stable Nash equilibrium. Conditions when more than one stable Nash equilibrium or even only mixed equilibrium may exist are also studied. Experimental results are presented for several cases and compared with the continuous power level adaptation solutions.
    Full-text · Article · Sep 2008
    • The MMSE decoder is a special case of the RLS decoder when the regularizer ? is set to 1 SNR . Another popular heuristic that enhances the performance of the detector is the box relaxation [3][4][5], that is solving the RLS problem when constraining the variables to be in [?1, 1] n instead of {?1, 1} n . The error probability of RLS, particularly the MMSE, is not known in exact expression, even at the asymptotic regime.
    [Show abstract] [Hide abstract] ABSTRACT: This paper investigates the problem of recovering an n-dimensional BPSK signal x0 ∈ {−1, 1}^n from m-dimensional measurement vector y = Ax+z, where A and z are assumed to be Gaussian with iid entries. We consider two variants of decoders based on the regularized least squares followed by hard-thresholding: the case where the convex relaxation is from {−1, 1}^n to R^n and the box constrained case where the relaxation is to [−1, 1]^n. For both cases, we derive an exact expression of the bit error probability when n and m grow simultaneously large at a fixed ratio. For the box constrained case, we show that there exists a critical value of the SNR, above which the optimal regularizer is zero. On the other side, the regularization can further improve the performance of the box relaxation at low to moderate SNR regimes. We also prove that the optimal regularizer in the bit error rate sense for the unboxed case is nothing but the MMSE detector.
    Full-text · Conference Paper · Mar 2017 · IEEE Transactions on Information Forensics and Security
    • In MC-CDMA, higher system capacity is hypothetically promised by passing on multiple CDMA spreading signatures to each user at the transmitters and by applying sophisticated multiuser detection (MUD) schemes at the receivers and innate frequency diversity of wideband channels is well subjugated by scattering each symbol across multiple sub carriers [19]. Unique signature waveform is assigned for each user, which are used to modulate their information bits in CDMA [4]. For digital communication it is a spread spectrum modulation technique and for military applications it is used either to provide resistance to hostile jamming, or to hide the signal by transmitting it at low power [9].
    [Show abstract] [Hide abstract] ABSTRACT: To attain large information rates, code division multiple access (CDMA) is a well-built aspirant for the downlink of cell phone communications. However, while transmitting any signal over fading channel, the performance of CDMA system is highly affected. For that reason, Multiuser detection (MUD) and channel estimation play a significant function for overcoming the interference and characterizing the channel. In this paper, biogeography base optimization (BBO) algorithm introduced for multiuser detection CDMA system. The objective of BBO algorithm is used to minimize the error rate of the user transmitting signal. According to immigration rate and emigration rate, the optimal solution of the detection problem is decided. Therefore, the user detection complexity and the interference of user transmitting signal are resolved. Proposed multiuser detection algorithm is appropriate for run time user identification process; since the detection complication and the required time are concentrated. The proposed approach is implemented in MATLAB working platform and the performance measures are examined. The error rate of proposed algorithm is compared with harmony search algorithm, enhanced harmony search algorithm, and without multiuser detection procedure.
    Full-text · Article · Dec 2015
    • Rather than being concerned with the stability analysis of power control, the studies [6][9] focus on developing extra techniques of reducing the impact of randomness encountered by power control in wireless networks, where these techniques includes matched filter receiver, decision feedback receiver, and stochastic approximation. To the best of our knowledge, it still lacks of systemic study on stability analysis of typical power-control laws taking into account the randomness existed in practical wireless networks while the power control does not use extra randomness reducing techniques proposed by [6][9]. We shall emphasize here that, this kind of study is indispensable, because, on the one hand, it could attentively reveal the inherent attributes of the typical power-control laws when the randomness exists (but no randomness reducing technique is involved) so as to make the stability theory of typical power-control laws complete, on the other hand, extra technique of reducing randomness may not necessarily be available in practical systems due to the objective factors such as realtime processing demands so that engineers have to be aware of the stability of power control in randomness environments without any randomness reducing technique.
    [Show abstract] [Hide abstract] ABSTRACT: Owing to the requirements from realistic wireless networks, the stochastic stability analysis for discrete-time power control, which concerns the randomness brought by the fading channels and noise of wireless systems, is of practical significance. By developing a norm-inequality-based framework of analyzing the stochastic stability of linear systems with random parameters, we show that a typical powercontrol law with linear system model is stable in the sense of the pth-moment stability. Several conditions of achieving the pth-moment stability for the considered power-control law are obtained, which can easily applied to realistic wireless networks. Besides, within this study, the stability analysis of power control for the first time takes into account the effect of multiple-access methods.
    Full-text · Article · Oct 2016
    • In [12], Verdú studied the capacity region of an uplink time-invariant CDMA system with inter-symbol interference (ISI) by exploiting the asymptotic properties of Toeplitz matrices. In [13, 14], the authors studied user and sum capacities of a symbol-asynchronous CDMA system but with chip-synchronous transmission (the timing of the chip epochs are aligned) assumption, which made the analysis tractable using a discrete-time model. In [15], the spectral efficiency of an asynchronous CDMA system has been considered while neglecting the ISI by assuming a large spreading factor.
    [Show abstract] [Hide abstract] ABSTRACT: This paper investigates the capacity limit of an uplink WCDMA system considering a continuous-time waveform signal. Various realistic assumptions are incorporated into the problem, which make the study valuable for performance assessment of real cellular networks to identify potentials for performance improvements in practical receiver designs. An equivalent discrete-time channel model is derived based on sufficient statistics for optimal decoding of the transmitted messages. The capacity regions are then characterized using the equivalent channel considering both finite constellation and Gaussian distributed input signals. The capacity with sampling at the receiver is also provided to exemplify the performance loss due to a typical post-processing at the receiver. Moreover, we analyze the asymptotic capacity when the signal-to-noise ratio goes to infinity. The conditions to simultaneously achieve the individual capacities are derived, which reveal the impacts of signature waveform space, channel frequency selectivity and signal constellation on the system performance.
    Full-text · Article · Dec 2015
    • The transmit power level is set according to the distance of the mobile device to the base station, and to the neighboring cells. For example, for 3rd generation (3G) systems which adopt the code division multiple access (CDMA) technique, power control is one effective way to avoid the so-called near-far problem, thus increasing system capacity and reducing intra-cell and inter-cell interferences [5]. In Long Term Evolution (LTE) systems, the uplink power control mitigates inter-cell interference, which is the dominant interference source [6].
    [Show abstract] [Hide abstract] ABSTRACT: The utilization of a burst-mode power amplifier (PA) together with pulse-width modulation (PWM) is a promising concept for achieving high efficiency in radio frequency (RF) transmitters. Nevertheless, such a transmitter architecture requires bandpass filtering to suppress side-band spectral components to retrieve the wanted signal, which reduces the transmit power and the transmitter efficiency. High efficiency can only be expected with the maximum transmit power and signals with low peak-to-average-power ratios (PAPRs). To boost efficiency for signals with high PAPRs and signals at variable transmit power levels, the burst-mode multilevel transmitter architecture has been widely discussed as a potential solution. This paper presents an efficiency optimization procedure of burst-mode multilevel transmitters for signals with high PAPRs and signals at variable transmit power levels. The impact of the threshold value on the transmitter efficiency is studied, where the optimum threshold value and the maximum transmitter efficiency can be obtained according to input magnitude statistics. In addition, the relation between the threshold value and the efficiency expression of burst-mode multilevel transmitters and those of Doherty PAs is investigated. It is shown that the obtained optimum threshold value, although originally designed for burst-mode transmitters, can also be applied to Doherty and multistage Doherty PAs to achieve maximum transmitter efficiency. Simulations are used to validate the efficiency improvement of the optimized burst-mode multilevel transmitters compared to two-level and non-optimized multilevel transmitters.
    Full-text · Article · Jul 2013
    • Many receivers based on MUD techniques exist, which are currently capable of decoding multiple signals and maximizing the signal-to-interference-plus-noise ratio (SINR) of each signal. These MUD techniques make it possible to decode the compound signals from multiple transmitters at the receiver side [1,22]. For the received signal in (1), the constellation of the received signal has a number of densely distributed points for the signals received from multiple transmitters.
    [Show abstract] [Hide abstract] ABSTRACT: Driven by advances in signal processing and multiuser detection (MUD) technologies, it has become possible for a wireless node to simultaneously receive multiple signals from other transmitters. In order to take full advantage of MUD in multi-packet reception (MPR) capable wireless networks, it is highly desirable to make the compound signals from multiple transmitters more separable on its constellation at the receiver by coordinating both the transmit power level and carrier phase offsets of the transmitters. In this article, we propose a feedback-based transmit power and carrier phase adjustment scheme that estimates the symbol energy and the carrier phase offset for each transmitter’s received signal, computes the optimal received power level and carrier phase shift to maximize the minimum Euclidean distance between the constellation points, and finally feeds the optimal transmit power level and phase shift information back to the transmitters. We then evaluate the performance of the proposed transmit power and carrier phase adjustment scheme and subsequently show that the proposed scheme significantly reduces the error probability in a multiuser communication system having MPR capability.
    Full-text · Article · Dec 2013
    • The effect of cooperation on the achievable rates for other communication models with secrecy constraints can be found in [18]–[21]. Z-IC without secrecy and without cooperation: In [4], lower bounds on the capacity region of the Gaussian Z-IC for the weak and moderate interference regimes are derived. In [22], it is shown that superposition encoding with partial decoding is optimal for a certain class of Z-IC.
    [Show abstract] [Hide abstract] ABSTRACT: This paper derives outer bounds on the secrecy capacity region of the 2-user Z interference channel (Z-IC) with rate-limited unidirectional cooperation between the transmitters. First, the model is studied under the linear deterministic setting. The derivation of the outer bounds on the secrecy capacity region involves careful selection of the side information to be provided to the receivers and using the secrecy constraints at the receivers in a judicious manner. To this end, a novel partitioning of the encoded messages and outputs is proposed for the deterministic model based on the strength of interference and signal. The obtained outer bounds are shown to be tight using the achievable scheme derived by the authors in a previous work. Using the insight obtained from the deterministic model, outer bounds on the secrecy capacity region of the 2-user Gaussian Z-IC are obtained. The equivalence between the outer bounds for both the models is also established. It is also shown that secrecy constraint at the receiver does not hurt the capacity region of the 2-user Z-IC for the deterministic model in the weak/moderate interference regime. On the other hand, the outer bounds developed for the Gaussian case shows that secrecy constraint at the receiver can reduce the capacity region for the weak/moderate interference regime. The study of the relative performance of these bounds reveals insight into the fundamental limits of the 2-user Z-IC with limited rate transmitter cooperation.
    Full-text · Article · Apr 2016
    • URING the past several years there have been extensive studies of the multiple-access Gaussian vector channel motivated by system developments such as code-division multiplexing , multiple-input multiple-output communications, cooperative/relay transmissions, and broadcasting. Researchers were attracted by intriguing versions of the problem where the dimensionality of the vector Gaussian channel is less than the number of accessing vector signals [1], [2], [3], the signals are correlated [4] or undergo fading in which case ergodic sumcapacity characterization is sought [5], feedback is present as in [6] for the white channel, or the multiple access channel is studied as a dual to the broadcast channel [7]. The sum capacity optimal signal covariance matrix under white Gaussian noise is examined in [8] and the capacity region when each user is allowed to modulate a set of orthonormal waveforms is derived in [9].
    [Show description] [Show abstract] [Hide description] [Hide abstract] DESCRIPTION: For the general correlated multiple-access vec-tor channel, we derive algebraically the sum-SINR (signal-to-interference-plus-noise ratio) optimal orthonormal set of carriers for any given set of carrier amplitudes. If, in addition, the correlated multiple-access channel is assumed Gaussian, the sum-capacity optimal orthonormal carrier set design and the jointly sum-capacity optimal orthonormal carrier and power assignment, for a given power budget, follow readily.ABSTRACT: For the general correlated multiple-access vec-tor channel, we derive algebraically the sum-SINR (signal-to-interference-plus-noise ratio) optimal orthonormal set of carriers for any given set of carrier amplitudes. If, in addition, the correlated multiple-access channel is assumed Gaussian, the sum-capacity optimal orthonormal carrier set design and the jointly sum-capacity optimal orthonormal carrier and power assignment, for a given power budget, follow readily.
    File · Data · May 2012
    • where the entries of the diagonal matrix Q can be obtained as solution of a converging fixed point equation [11]. Under the above mentioned assumption that the matrices Γ í µí±˜ and Σ í µí±˜ are not known by the BS, we optimize the transmission for system model (5).
    [Show abstract] [Hide abstract] ABSTRACT: We propose a novel protocol for fast fading massiveMIMO systems in Frequency Division Duplex (FDD) mode and,in general, non-reciprocal channels. Our approach relies onDevice-to-Device (D2D) cooperation to avoid instantaneous chan-nel state information (CSI) feedback, unaffordable in practicalsystems. This protocol assumes that clusters of user terminals(UT) cooperate to create virtual MIMO at a target UT via D2Dcooperation. The base station (BS) has statistical knowledge of the downlink channel and it is aware of the cooperation but itis oblivious of the statistics of the D2D links and the transmitteris designed based on the downlink channel statistics solely. Theperformance of the proposed protocol is analyzed and comparedto state-of-art schemes based on instantaneous CSI feedback.
    Full-text · Conference Paper · Nov 2015 · IEEE Transactions on Information Forensics and Security
    • The main idea of this aggregation is based on prime numbers. The latter are characterized by the uniqueness of factorization of prime factors [7] which insures the aggregation and disaggregation operations and the forward of multicast packets only towards the concerned nodes. In the next paragraph, we present a mathematical reminder of prime numbers and consequences constituting our aggregation approach.
    [Show abstract] [Hide abstract] ABSTRACT: In this paper, we address the fundamental problem of forwarding table optimization in mobile ad hoc networks and we propose a new multicast tree aggregation mechanism based on the uniqueness property of prime numbers. Instead of creating, for any new session, a new entry in the routing table, our mechanism allocates a unique identity to each session in the multicast shared tree and ensures the delivery to concerned receivers. Evaluation performance results show the gain obtained when applying such a mechanism in multicast ad hoc networks by reducing significantly the entries number in the forwarding table although generating a low network overhead.
    Full-text · Conference Paper · Jan 2008
    • S. Ulukus is with the Department of Electrical and Computer Engineering, University of Maryland, College Park, MD, USA (e-mail: ulukus@umd.edu). This work was supported by NSF Grants ANI 02-05330, CCR 03-11311, CCF 04-47613, and CCF 05-14846; and ARL/CTA Grant DAAD 19-01-2- 0011, and presented in part at the Conference on Information Sciences and Systems, Baltimore, MD, March 2005 [1] and at the Vehicular Technology Conference, Stockholm, Sweden, May 2005 [2]. Digital Object Identifier 10.1109/TCOMM.2009.04.070001 order to achieve the capacity, either vector coding or parallel processing of scalar codes is needed.
    [Show abstract] [Hide abstract] ABSTRACT: We consider the sum capacity of a multi-input multi-output (MIMO) multiple access channel (MAC) where the receiver has the perfect channel state information (CSI), while the transmitters have either no or partial CSI. When the transmitters have partial CSI, it is in the form of either the covariance matrix of the channel or the mean matrix of the channel. For the covariance feedback case, we mainly consider physical models that result in single-sided correlation structures. For the mean feedback case, we consider physical models that result in in-phase received signals. Under these assumptions, we analyze the MIMO-MAC from three different viewpoints. First, we consider a finite-sized system. We show that the optimum transmit directions of each user are the eigenvectors of its own channel covariance and mean feedback matrices, in the covariance and mean feedback models, respectively. Also, we find the conditions under which beamforming is optimal for all users. Second, in the covariance feedback case, we prove that the region where beamforming is optimal for all users gets larger with the addition of new users into the system. In the mean feedback case, we show through simulations that this is not necessarily true. Third, we consider the asymptotic case where the number of users is large. We show that in both no and partial CSI cases, beamforming is asymptotically optimal. In particular, in the case of no CSI, we show that a simple form of beamforming, which may be characterized as an arbitrary antenna selection scheme, achieves the sum capacity. In the case of partial CSI, we show that beamforming in the direction of the strongest eigenvector of the channel feedback matrix achieves the sum capacity. Finally, we generalize our covariance feedback results to double-sided correlation structures in the Appendix.
    Article · May 2009
    • The associate editor coordinating the review of this paper and approving it for publication was M. Sawahashi. This work was supported by NSF Grants ANI 02-05330 and CCR 03-11311; and ARL/CTA Grant DAAD 19-01-2-0011, and was presented in part at the Conference on Information Sciences and Systems, Princeton, NJ, March 2004 [1]. O. Kaya is with the Department of Electronics Engineering, I¸sıkI¸sık University, Istanbul, Turkey (email: onurkaya@isikun.edu.tr).
    [Show abstract] [Hide abstract] ABSTRACT: We characterize the optimum power control policies that achieve arbitrary rate tuples on the boundary of the capacity region of a power controlled, code division multiple access (CDMA) system in a fading channel with perfect channel state information (CSI). We propose a "generalized" waterfilling approach, and provide an iterative algorithm that solves for the optimum power allocation policy, for a given arbitrary rate tuple on the boundary of the capacity region. We then investigate the effects of limited feedback on the capacity region, and demonstrate that a good power control policy may require only a very low rate feedback
    Article · Dec 2006
    • The derivation of this filter entails designing an MMSE filter for the lth antenna branch observing z l given in (5) only. If the small-scale fading coefficients of all users are known at the receiver, this results in [17] c
    [Show abstract] [Hide abstract] ABSTRACT: A CDMA system performance can be enhanced by interference management techniques. We consider the uplink of a CDMA system that employs two such techniques: temporal multiuser detection and spatial antenna array beam forming. Temporal-spatial MMSE type receivers with several different criteria have been previously proposed for a static channel. We consider such MMSE filters assuming a multiple time-scale fading channel model. It is shown that under the given channel conditions and system assumptions, three MMSE type receivers; the unconstrained optimum MMSE detector, the constrained optimum MMSE detector and the cascaded temporal-spatial MMSE detector are identical and the resulting combined receiver employs an MMSE multiuser detector at the output of each array element (temporal MMSE filter) followed by an MRC (spatial matched filter)
    Conference Paper · Feb 2001
    • The downlink throughput optimization for the HSDPA multi-code CDMA system [10] considers the signature sequence and the power allocation for downlink users. 3GPP standardized an approach to spread the transmission symbols by using a given fixed set size of orthogonal variable spreading factor (OVSF) signature sequences.
    [Show abstract] [Hide abstract] ABSTRACT: This article proposes the use of system value-based optimization with a symbol-level minimum mean square error equalizer and a successive interference cancellation which achieves a system value upper bound (UB) close to the Gaussian UB for the high-speed downlink packet access system without affecting any significant computational cost. It is shown that by removing multi-code channels with low gains, the available energy is more efficiently used, and a higher system throughput is observed close to the system value UB. The performance of this developed method will be comparable to the orthogonal frequency division multiplexing-based long-term evolution scheme, without the need to build any additional infrastructure. Hence, reduce the cost of the system to both operators and consumers without sacrificing quality.
    Full-text · Article · Dec 2013

Institutions

    • Loyola University Maryland
      Baltimore, Maryland, United States
    • University of Maryland, College Park
      • Department of Electrical & Computer Engineering
    • Princeton University
      Princeton, New Jersey, United States
    • AT&T Labs
      Austin, Texas, United States
    • Rutgers, The State University of New Jersey
      • Department of Electrical and Computer Engineering

Publication Stats

8k Citations