A.M. Haimovich

MIMO radar: an idea whose time has come

It has recently been shown that multiple-input multiple-output (MIMO) antenna systems have the potential to improve dramatically the performance of communication systems over single antenna systems. Unlike beamforming, which presumes a high correlation between signals either transmitted or received by an array, the MIMO concept exploits the independence between signals at the array elements. In conventional radar, target scintillations are regarded as a nuisance parameter that degrades radar performance. The novelty of MIMO radar is that it takes the opposite view; namely, it capitalizes on target scintillations to improve the radars performance. We introduce the MIMO concept for radar. The MIMO radar system under consideration consists of a transmit array with widely-spaced elements such that each views a different aspect of the target. The array at the receiver is a conventional array used for direction finding (DF). The system performance analysis is carried out in terms of the Cramer-Rao bound of the mean-square error in estimating the target direction. It is shown that MIMO radar leads to significant performance improvement in DF accuracy.

Performance of MIMO radar systems: advantages of angular diversity

Inspired by recent advances in multiple-input multiple-output (MIMO) communications, this paper introduces the statistical MIMO radar concept. The fundamental difference between statistical MIMO and other radar array systems is that the latter seek to maximize the coherent processing gain, while statistical MIMO radar capitalizes on the diversity of target scattering to improve radar performance. Coherent processing is made possible by highly correlated signals at the receiver array, whereas in statistical MIMO radar, the signals received by the array elements are uncorrelated. It is well known that in conventional radar, slow fluctuations of the target radar cross-section (RCS) result in target fades that degrade radar performance. By spacing the antenna elements at the transmitter and at the receiver such that the target angular spread is manifested, the MIMO radar can exploit the spatial diversity of target scatterers opening the way to a variety of new techniques that can improve radar performance. In this paper, we focus on the application of the target spatial diversity to improve detection performance. The optimal detector in the Neyman-Pearson sense is developed and analyzed for the statistical MIMO radar. An optimal detector invariant to the signal and noise levels is also developed and analyzed. In this case as well, statistical MIMO radar provides great improvements over other types of array radars.

New approaches for cooperative use of multiple antennas in ad hoc wireless networks

The paper explores the interaction between cooperative diversity techniques, in the physical layer, and routing, in the network layer. Three approaches are proposed: relay-by-flooding; relay-assisted routing; relay-enhanced routing. In relay-by-flooding, the tradeoff between achieved rate and required power is studied for three selective decode-and-forward schemes: simple relay; space-time-coded relay; best-select relay. In relay-enhanced routing, cooperation is applied to each link of an existing route. Two relay selection approaches are proposed: best-select in the neighbor set; best-select in the decoded set. Best-select in the decoded set is shown to improve the performance significantly.

Link-failure probabilities for practical cooperative relay networks

In this paper, practical cooperative diversity schemes are proposed and investigated. The outage probability is used to evaluate four constant-power decode-and-forward cooperative schemes: pre-select one relay, best-select relay, simple relay, and ST-coded relay. Two new methods for improving these approaches, called simple distributed power allocation and m-group ST-coded relay, are proposed. It is shown that the performance of simple relay and ST-coded relay is improved significantly by employing the proposed power allocation without an increase in implementation complexity. Similarly, m-group ST-coded relay gains in lower complexity with only a slight degradation in performance. Performance of the various schemes in a network is evaluated through the link-failure probability.

Rejection of narrow-band interferences in PN spread spectrum systems using an eigenanalysis approach

A new adaptive technique is suggested for rejecting narrow-band interferences in spread spectrum communications. When data is coded using a pseudo-noise code, the received signal consists of a wide-band signal with an almost white spectrum, and correlated narrow-band interference. The conventional approach to the interference suppression has been to exploit this correlation property to minimize the mean square error between predicted values of the signal and actual observations. The optimal solution is given by the Wiener filter. A different approach is suggested by the eigen-analysis of the data across the filter taps. While the energy of the spread spectrum signal is distributed across all the eigenvalues of the data correlation matrix, the energy of the interference is concentrated in a few large eigenvalues. The corresponding eigenvectors span the same signal subspace as the interference. The proposed method derives an error prediction filter with the additional constraint of orthogonality to these eigenvectors. The eigen-analysis based interference cancellation is sub-optimal for known correlation matrix, but is superior to the Wiener filter when the correlation matrix is estimated from a limited amount of data.<<ETX>>

The impact of the timeliness of information on the performance of multihop best-select

Cooperative relaying enables nodes to actively cooperate to deliver packets to their destination. This cooperation allows nodes to take advantage of the diversity provided by variations in the channel gains between nodes. Best-select, a particular type of cooperation, has been shown to result in significant gains in the performance of source-to-destination communication. However, this increase in performance is achieved by exchanging channel gain measurements, which requires overhead. One way to reduce this overhead is to exchange channel gain measurements less frequently. This paper examines the trade-off between performance and the frequency of exchanging channel gains. This investigation focuses only on the channels that are impaired by multipath fading and shadow fading

Training and signal cancellation in adaptive radar

It is well known that the performance of adaptive arrays is affected by calibration errors. In particular, target cancellation occurs when there are calibration errors and/or the target signal is present during the computation of the array covariance matrix. Due to the mismatch between the presumed steering vector and the true signal vector, the target is not properly protected by the steering vector, it is interpreted as an interference, and the array proceeds to cancel it. Signal cancellation effects are particularly evident when the SMI method is employed. Researchers have suggested one of two main approaches to mitigate the signal cancellation problem: (1) modification of the beamformer constraints to widen the desired signal protection region and lessen the effects of calibration errors, and (2) modifications of the steering vector. In this work we analyze the robustness of eigenanalysis-based adaptive beamforming and compare its performance to the SMI method. It is shown that the eigenanalysis-based method is more robust to array pointing errors and to the presence of the target signal during training. Analytical expressions are developed and the results are illustrated using realistic scenarios generated by the Rome Laboratory RLSTAP algorithm development tool.

Sidelobe mitigation in MIMO radar with multiple subcarriers

This paper presents the studies on the reduction of peak sidelobe level in distributed MIMO radar with multiple subcarrier signals. Multiple subcarriers with sufficient frequency spacing become an alternative to increasing the number of sensors for sidelobe reduction. It is shown that the multiple subcarrier signals are most effective in reducing sidelobes at locations far from the target. Two signaling methods, namely continuous carrier transmission and Gaussian-OFDM signals are studied with respect to the sidelobe mitigation properties. The paper also presents an upper bound to the peak sidelobe level considering the non-coherent combining. It is shown that with non-coherent combining, the peak sidelobe of the localization metric scales down as 1/MNLsin(3π/2L) where L is the number of subcarriers, and M, N are the number of transmit and receive sensors. While there are grating lobes present in the metric with non-coherent combining, there is a grating lobe free region around the mainlobe, lower bounded by ρ = ±Lρ0/2B . With coherent processing, multiple subcarriers are effective in reducing the sidelobes as well as grating lobes.

A stochastic gradient-based decorrelation algorithm with applications to multicarrier CDMA

A stochastic gradient-based decorrelation algorithm is suggested for separation of an unknown linear mixture of signals. It is shown that while the decorrelation algorithm is similar in cost to the LMS algorithm, its rate of convergence is significantly faster, making it more attractive for signal separation. Analysis of the decorrelator algorithm shows that the faster speed of convergence is a consequence of the eigenvalue spread associated with the decorrelation problem, which is smaller than the spread associated with the corresponding mean square problem. Operation of the algorithm is illustrated as an adaptive multiuser detector in a multiple carrier CDMA system.

Generalized DFT waveforms for MIMO radar

Recently, the Generalized Discrete Fourier Transform (GDFT) with nonlinear phase was introduced to improve the traditional Discrete Fourier Transform (DFT). We take advantage of the phase shaping function (PSF) of GDFT framework to optimize auto- and cross-correlation properties of OFDM frames used to generate radar waveforms. The superior performance of the GDFT MIMO radar waveforms over multifrequency complementary phase coded (MCPC) and Oppermann code families are presented in this paper. GDFT MIMO radar waveforms may be employed in the future as an efficient upgrade to the DFT based technologies.