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IEEE Transactions on Signal Processing 01/2013; 61(4):828-842. · 2.63 Impact Factor
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ABSTRACT: Orthogonal frequency division multiplexing (OFDM) communication systems require accurate estimation of timing offset and channel impulse response in order to achieve desirable performance. In this paper, we consider the optimal placement of pilot symbols over the OFDM subcarriers in order to minimize a function of the Cramer-Rao bound on these parameters. Previous work has investigated this problem for channel estimation only, and found that equi-spaced, equi-powered pilots are optimal. We show that when the time-delay must be simultaneously estimated, the optimal pilot distribution is often quite different, with more pilot energy distributed to the edges of the signal bandwidth. Upper and lower bounds for the required number of optimal pilots are also presented for the case where the variance on the time-delay estimate is minimized.
Acoustics, Speech and Signal Processing (ICASSP), 2011 IEEE International Conference on; 06/2011 · 4.63 Impact Factor
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ABSTRACT: In narrowband multiple-input multiple-output (MIMO) communication systems with perfectly known channel state information (CSI), the singular value decomposition (SVD) is commonly used to decompose the MIMO channel into independent single-input single-output subchannels. In theory, optimal interference-free data multiplexing may then be carried out using the subchannel power levels provided by the well-known waterfilling solution. In practice, however, when finite codebooks are used and perfect CSI is unavailable, adaptations to power levels and bit-loading schemes are often needed to maintain reasonable performance. In this paper, we use expressions for the per-subchannel signal-to-interference- and-noise ratio for the imperfect CSI case to derive approximately optimal subchannel power levels and thresholds on the amount of CSI imperfections and noise tolerable in SVD-based multiplexing systems using M-ary quadrature amplitude modulation. Numerical simulations demonstrate the usefulness of the derived expressions.
Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
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ABSTRACT: Unmanned air vehicles (UAVs) are playing an increasingly prominent role in both military and civilian applications. We focus here on the use of multiple UAV agents in a target tracking application where performance is improved by exploiting each agent's maneuverability. Local time-delay and Doppler measurements made at each UAV are used as inputs to an extended Kalman filter (EKF) which tracks the target's position and velocity. Two simple metrics are defined to quantify the accuracy of the tracking algorithm, and heading feedback to the UAVs is used to minimize the metric and improve tracking performance. A simplified version of one of the algorithms that reduces computational complexity is also presented. Simulations demonstrate the significant improvement that results when the UAV sensors are allowed to be optimally positioned during tracking.
IEEE Transactions on Aerospace and Electronic Systems 02/2010; · 1.10 Impact Factor
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ABSTRACT: The performance of a mobile multiple-input multiple-output orthogonal-frequency-division multiplexing (MIMO-OFDM) system depends on the ability of the system to accurately account for the effects of the frequency-selective time-varying channel at every symbol time and at every frequency subcarrier. Typically, pilot symbols are strategically placed at various times over various subcarriers in order to calculate sample channel estimates, and then these estimates are interpolated or extrapolated forward to provide channel estimates where no pilot data was transmitted. Performance is highly dependent on the distribution of the pilots with respect to the coherence time and coherence bandwidth of the channel, and on the chosen channel parameterization. In this paper, a vector formulation of the Cramer-Rao bound (CRB) for biased estimators and for functions of parameters is used to derive a lower bound on the channel estimation and prediction error of such a system. Numerical calculations using the bound demonstrate the benefits of multiple antennas for channel estimation and prediction and illustrate the impact of modeling errors on estimation performance when using channel models based on calibrated arrays.
IEEE Transactions on Signal Processing 06/2009; · 2.63 Impact Factor
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ABSTRACT: In narrowband multiple-input multiple-output (MIMO) communication systems, when the channel state information (CSI) is known perfectly at the transmitter and the receiver, techniques such as waterfilling may use the singular value decomposition to separate the MIMO channel into independent single-input single-output subchannels. The signal-to-noise ratios of these subchannels are easily found, and, therefore, so are the subchannel bit allocations. In practice, perfect CSI is difficult to obtain. Imperfect CSI results in subchannel coupling and co-channel interference. In this paper, simple first-order expressions are presented for the signal and interference/noise powers for each subchannel for the imperfect CSI case. These expressions may be used to obtain more realistic subchannel bit allocations, allowing for fewer channel outages. Numerical simulations demonstrate the applicability and usefulness of the derived expressions.
Acoustics, Speech and Signal Processing, 2009. ICASSP 2009. IEEE International Conference on; 05/2009 · 4.63 Impact Factor
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ABSTRACT: A distributed double integrator discrete time consensus protocol is presented along with stability analysis. The protocol will achieve consensus when the communication topology contains at least a directed spanning tree. Average consensus is achieved when the communication topology is strongly connected and balanced, where average consensus for double integrator systems is discussed. For second order systems average consensus occurs when the information states tend toward the average of the current information states not their initial values. Lastly, perturbation to the consensus protocol is addressed. Using a designed perturbation input, an algorithm is presented that accurately tracks the center of a vehicle formation in a decentralized manner.
Decision and Control, 2008. CDC 2008. 47th IEEE Conference on; 01/2009
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ABSTRACT: The performance of a mobile MIMO-OFDM system depends on the ability of the system to accurately account for the effects of the frequency-selective time-varying channel at every symbol time and at every frequency subcarrier. In this paper, a vector formulation of the Cramer-Rao bound (CRB) for biased estimators and for functions of parameters is used to find a lower bound on the estimation and prediction error of such a system. Numerical simulations demonstrate the benefits of multiple antennas for channel estimation and prediction and illustrate the impact of calibration errors on estimation performance when using parametric channel models.
Sensor Array and Multichannel Signal Processing Workshop, 2008. SAM 2008. 5th IEEE; 08/2008
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ABSTRACT: This paper deals with the joint estimation of temporal (time- delay, Doppler frequency) and spatial (direction-of-arrival, DOA) parameters of several replicas of a known signal in an unknown spatially correlated field. Unstructured and structured models have been proposed in the literature. The former suffers from a severe performance degradation in some scenarios, whereas the latter involves huge complexity. It is shown how the extended invariance principle (EXIP) can be applied to obtain estimates with the quality of those of the structured model, but with the complexity of the unstructured one. We present a method to improve the quality of the time- delay and Doppler estimates obtained with an unstructured spatial model when an estimate of the DOAs is available. Exemplarily, simulation results for time-delay estimation for GPS (global positioning system) are included and confirm that our proposal approaches the Cramer-Rao lower bound (CRLB) of the structured model even when suboptimal DOA estimates obtained by ESPRIT are introduced.
Acoustics, Speech and Signal Processing, 2008. ICASSP 2008. IEEE International Conference on; 05/2008 · 4.63 Impact Factor
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ABSTRACT: A lower bound on the capacity of trained space-time modulation is presented for the case of a time-varying channel. We consider the case where the channel consists of two components of variable strength: a specular component from line-of-sight or strong coherent multipath, and a time-varying diffuse component due to a mobile receiver or scatterers. The training period and the data period are assumed quasi-static, with the time-autocorrelation function of the channel used to model the variation between each sub-block. The training signal, training signal length, power allocation and training frequency that optimize the capacity bound are derived. We compare the effective SNR of this bound to that of differential modulation at high SNR, and find it to be lower. We find the best number of antennas to use at high SNR with differential modulation, and compare it with the optimal value for trained modulation. For our time-varying channel model, trained modulation often has a higher achievable rate than differential modulation. Our results are supported by several numerical examples.
IEEE Transactions on Wireless Communications 10/2007; · 2.59 Impact Factor
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ABSTRACT: We present medium access controllers (MAC) for ad-hoc networks which include uplink and downlink multi-user physical layer models as well as multiple antennas. We note the advantages of these MACs over those in the literature, including a multi-antenna version of the CSMA/CA protocol used in the distributed coordination function (DCF) mode of the IEEE 802.11 MAC. Specifically, our MAC is among the first to explicitly consider multi-user techniques in an ad-hoc network, which results in superior rate performance. We include interference and network topology in the design of our protocols. We characterize MAC controllers in terms of transport capacity, and present numerical results which show the benefit of multi-user techniques in the physical layer.
Signals, Systems and Computers, 2006. ACSSC '06. Fortieth Asilomar Conference on; 12/2006
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ABSTRACT: The performance of MIMO-OFDM communication systems depends upon accurate channel estimation across frequency bands much wider than the coherence bandwidth of the channel. Obtaining channel estimates at all necessary frequencies typically involves interpolation of channel estimates obtained using pilots at a few distinct frequencies. This paper derives a performance bound for channel estimates obtained via frequency interpolation for slowly-fading frequency-selective channels. Numerical simulations of the bound indicate that normalized interpolation performance improves significantly as the number of antennas increases, suggesting that fewer pilots are needed as antennas are added to a MIMO-OFDM system.
Signals, Systems and Computers, 2006. ACSSC '06. Fortieth Asilomar Conference on; 12/2006
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ABSTRACT: We consider a team of miniature unmanned air vehicles (MUAVs) in a multi-static radar scenario. Time delay and Doppler measurements made at the UAVs are transmitted to a base station which is tracking a target. The base then transmits heading commands to the MUAVs to reduce the tracking error. Optimal solutions that attempt to minimise a function of the error covariance or maximize the observability of the system are computationally difficult to implement. We present a simpler approximate method that yields a closed-form solution and performs comparably to the optimal approaches
Acoustics, Speech and Signal Processing, 2006. ICASSP 2006 Proceedings. 2006 IEEE International Conference on; 06/2006 · 4.63 Impact Factor
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ABSTRACT: Knowledge of future channel conditions can increase the performance of many types of wireless systems. This is especially true for radio channels with multiple transmit and receive antennas, i.e., multiple-input multiple-output (MIMO) systems. This paper derives a performance bound for MIMO channel prediction. It is assumed that prediction is based upon estimating a model for the channel and then extrapolating that model to predict future values of the channel. A vector formulation of the Crame´r-Rao bound for functions of parameters is used to find a lower bound on the prediction error. Numerical evaluation of this bound shows that substantially longer prediction lengths are possible for MIMO channels than for single antenna channels. An intuitive interpretation of this result is that more of the channel structure is revealed when using multiple antennas at both ends. Finally, the longer prediction lengths for MIMO channels are confirmed by numerical results obtained by implementing a MIMO extension of a single-antenna prediction scheme.
IEEE Transactions on Signal Processing 03/2006; · 2.63 Impact Factor
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IEEE Signal Processing Magazine 08/2005; 22(4):10- 11. · 4.07 Impact Factor
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ABSTRACT: Recent theoretical results describing the sum-capacity when using multiple antennas to communicate with multiple users in a known rich scattering environment have not yet been followed with practical transmission schemes that achieve this capacity. We introduce a simple encoding algorithm that achieves near-capacity at sum-rates of tens of bits/channel use. The algorithm is a variation on channel inversion that regularizes the inverse and uses a "sphere encoder" to perturb the data to reduce the energy of the transmitted signal. The paper is comprised of two parts. In this second part, we show that, after the regularization of the channel inverse introduced in the first part, a certain perturbation of the data using a "sphere encoder" can be chosen to further reduce the energy of the transmitted signal. The performance difference with and without this perturbation is shown to be dramatic. With the perturbation, we achieve excellent performance at all signal-to-noise ratios. The results of both uncoded and turbo-coded simulations are presented.
IEEE Transactions on Communications 04/2005; · 1.68 Impact Factor
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ABSTRACT: Recent theoretical results describing the sum capacity when using multiple antennas to communicate with multiple users in a known rich scattering environment have not yet been followed with practical transmission schemes that achieve this capacity. We introduce a simple encoding algorithm that achieves near-capacity at sum rates of tens of bits/channel use. The algorithm is a variation on channel inversion that regularizes the inverse and uses a "sphere encoder" to perturb the data to reduce the power of the transmitted signal. This work is comprised of two parts. In this first part, we show that while the sum capacity grows linearly with the minimum of the number of antennas and users, the sum rate of channel inversion does not. This poor performance is due to the large spread in the singular values of the channel matrix. We introduce regularization to improve the condition of the inverse and maximize the signal-to-interference-plus-noise ratio at the receivers. Regularization enables linear growth and works especially well at low signal-to-noise ratios (SNRs), but as we show in the second part, an additional step is needed to achieve near-capacity performance at all SNRs.
IEEE Transactions on Communications 02/2005; · 1.68 Impact Factor
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ABSTRACT: Multiple-input multiple-output (MIMO) communication techniques have been an important area of focus for next-generation wireless systems because of their potential for high capacity, increased diversity, and interference suppression. For applications such as wireless LANs and cellular telephony, MIMO systems will likely be deployed in environments where a single base must communicate with many users simultaneously. As a result, the study of multi-user MIMO systems has emerged recently as an important research topic. Such systems have the potential to combine the high capacity achievable with MIMO processing with the benefits of space-division multiple access. In this article we review several algorithms that have been proposed with this goal in mind. We describe two classes of solutions. The first uses a signal processing approach with various types of transmitter beamforming. The second uses "dirty paper" coding to overcome the interference a user sees from signals intended for other users. We conclude by describing future areas of research in multi-user MIMO communications.
IEEE Communications Magazine 11/2004; · 3.79 Impact Factor
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ABSTRACT: The primary issue in downlink beamforming for wireless communications is how to balance the need for high received signal power for each user against the interference produced by the signal at other points in the network. In this paper, we describe several approaches to this problem: channel inversion, regularized channel inversion, vector modulo pre-coding, channel block diagonalization and coordinated transmit/receive beamforming. While the basic idea behind these algorithms is the same, namely the use of channel information at the transmitter to predict and then counteract the interference produced at each node in the network, each of the algorithms is based on achieving a different performance objective. We compare the various goals of the above algorithms, and detail their respective advantages and disadvantages in terms of computational complexity, required transmit power, network throughput, and assumed receiver capabilities. The results of several simulation studies are presented to quantify these comparisons.
Sensor Array and Multichannel Signal Processing Workshop Proceedings, 2004; 08/2004
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ABSTRACT: The use of multi-user multiple input multiple output (MIMO) processing algorithms in wireless communication systems requires new channel allocation algorithms that can intelligently assign users to channels that can best take advantage of the spatial processing available at both transmitter and receiver. The availability of spatial processing at the receiver adds yet another variable to the classic channel allocation problem, making it very difficult to find the optimal solution for a particular set of users at a reasonable computational cost. We propose a two-step heuristic solution. The first step is the computation of a metric that quantifies the spatial compatibility of two users. The second step is to group the users into shared channels based on optimizing the sum of the compatibility metrics over all groups. we also propose a modified version of the algorithm in which the sub-channels (from different multipath components) of a single user are not required to share the same time-domain channel. In simulations, these algorithms come reasonably close to the optimal solution at a moderate computational cost.
Communications, 2004 IEEE International Conference on; 07/2004
