A Low-complexity Hardware Implementation of Discrete-time Frequency-selective Rayleigh Fading Channels

A low-complexity hardware implementation method is proposed for discrete-time frequency-selective Rayleigh fading channels. the proposed method first employs the Sum-of-Sinusoids method to generate multiple independent flat fading channel responses, then utilizes a simple weight-delay-sum filtering method to incorporate the fractionally delayed multipath rays into inter-tap correlated tap gains. It thus achieves accurate correlation properties in both inter-tap correlation and temporal correlation (or Doppler spectrum). the proposed method is implemented by an Altera Stratix II FPGA development kit and the results show excellent performance match with those by MATLAB software simulations. ©2009 IEEE

Optimum Multi-hop Transmission Strategies for Energy Constrained Wireless Sensor Networks

This paper presents optimum multi-hop transmission strategies (MHTS) for energy constrained wireless sensor networks (WSNs). Nodes in a multi-hop WSN need to transmit their own information and to relay each other\u27s information to a base station (BS), and there are usually multiple available paths between a node and the BS. the optimum MHTS derived in this paper answers three questions: 1) how should a node divide its limited energy between the transmission of the self-information and the relay-information? 2) whether a single path or a combination of multiple paths should be used to route the information from a node to the BS? and 3) if multi-path routing is used, how should a single data stream be divided among the multiple paths? the answers to these questions are obtained by minimizing the energy per bit, or equivalently, by maximizing the amount of information delivered to the BS under certain energy constraints. Two different scheduling strategies are considered, the fair equal information strategy that requires all the nodes deliver the same amount of information to the BS, and the unfair maximum information strategy that maximizes the total amount of information delivered to the BS. the optimum MHTS for these two strategies are derived, with either convex optimization or analytical expressions, under a per node energy constraint and a total energy constraint, respectively. © 2012 IEEE

Layered Frequency-domain Equalization for Single Carrier MIMO Systems with Multiple Carrier Frequency Offsets

In this paper, carrier frequency offsets (CFOs) are considered in a layered frequency-domain equalization (LFDE) architecture for a single carrier (SC) broadband wireless multiple-input multiple-output (MIMO) system. at each layer of detection, a group of best data streams are selected to be equalized and reliably detected via removing the phase and amplitude distortion caused by the multiple CFOs. the estimated phase rotations and amplitude scalars are required to be compensated to the detected data streams in order to reconstruct the interference signals which are canceled out from the received signals. Instead of direct estimation of CFOs, our algorithm is to estimate the phase rotations and amplitude scalars in a group-wise fashion, which dramatically reduces the computational complexity at receiver. Simulation results demonstrate that the proposed LFDE with phase compensation architecture can provide good performance for a MIMO system impaired by CFOs over unbalanced multi-path channels with long delay spread. We also show that the performance will be improved if more layers are used at receivers

Bandwidth-efficient MIMO Underwater Acoustic Communications with Frequency-domain Equalization

This paper proposes a bandwidth-efficient frequency-domain equalization (FDE) for single carrier (SC) underwater acoustic (UWA) communications with multiple transducers and hydrophones. the proposed algorithm implements an overlapped-window FDE by partitioning a large block into small subblocks. a decision-directed channel estimation scheme is also proposed to track the channel variation by the detect symbols. the proposed algorithm is tested by undersea data collected during the Rescheduled Acoustic Communications Experiment (RACE) in March 2008. the experimental results demonstrate the proposed receive algorithm effectively tracks the time variation of the channel impulse responses and significantly improves the performance of uncoded bit error rate (BER). Compared with traditional SC-FDE system, the proposed overlapped-window FDE achieves 74.4% and 84.6% average BER reduction for the 400 m and 1000 m range systems, respectively at the same data efficiency and has only 8.4% transmission overhead, which is much smaller than over 20% of other existing UWA OFDM and SC-FDE systems at the same BER level. © 2010 IEEE

A New Variable Step-size Fractional Lower-order Moment Algorithm for Non-Gaussian Interference Environments

A new variable step-size fractional lower-order moment (FLOM) algorithm is proposed for non-Gaussian interference suppression. the Normalized FLOM algorithm is a generalization of the normalized least mean square (NLMS) algorithm with a lower order p ≤ 2 and p = 2 corresponding to the NLMS algorithm. with a fixed step size, the smaller the order p, the faster the convergence of the NFLOM algorithm, but the higher the excess mean square errors (MSE) after convergence. the proposed vs.S-FLOM provides a compromise solution to the two conflicting goals of low excess MSE and fast convergence. Simulation results for a system identification application in Gaussian and compound K interference environments show that the proposed vs.S-FLOM algorithm outperforms the fixed step-size NFLOM and vs.S-LMS algorithms in non-Gaussian interference environments. ©2009 IEEE

Near-field 3-D Synthetic Aperture Radar Imaging Via Compressed Sensing

This paper successfully implements compressed sensing (CS) to a near-field wideband 3-D synthetic aperture radar (SAR) imaging system. SAR data are measured at a low percentage of random-selected positions on a uniform grid of planar aperture in the strip map mode. the near-field 3-D range migration algorithm (RMA) is used in combination with the CS principle to reconstruct the 3-D image via l 1 regularized least-square approach. Experiments were performed with Q-band stepped-frequency monostatic strip map SAR imaging system on a blue foam embedded with eight rubber pads and one copper square chip. the results of the experiments show near-field 3-D image of the specimen under test (SUT) can be reconstructed efficiently from low percentage of the full measurement positions, which largely lessens the data collection load. the reconstructed image was better focused and denoised. © 2012 IEEE

Oversampled Orthogonal Frequency Division Multiplexing in Doubly Selective Fading Channels

A new oversampled orthogonal frequency division multiplexing (OOFDM) scheme is proposed for doubly selective fading environment. the proposed OOFDM scheme employs oversampling in the time domain and linear processing in the frequency domain, both at the receiver without changing the structure of conventional orthogonal frequency division multiplexing (OFDM) transmitters. the time-frequency processing enables a two-dimensional Doppler-frequency grid of the fading channel, such that each data symbol is modeled equivalently as being transmitted at multiple subcarriers and various Doppler spreads simultaneously, while retaining the same spectral efficiency as conventional OFDM systems. Optimum combining ensures coherent combining of the data samples spread over the Doppler-frequency grid and non-coherent combining of inter-carrier interference (ICI) components caused by time varying fading. Theoretical error probabilities of the OOFDM systems are derived by analyzing the statistical properties of Doppler-frequency fading coefficients and noise sample correlations. Both theoretical analysis and computer simulation show that the new system with an oversampling factor of two outperforms the conventional OFDM system by as much as 7 dB. © 2011 IEEE

Frequency-domain Turbo Equalization with Soft Successive Interference Cancellation for Single Carrier Mimo Underwater Acoustic Communications

This paper proposes a low-complexity frequency-domain turbo equalizer (FDTE) combined with phase rotation compensation and soft successive interference cancellation (SSIC) for single carrier multiple-input multiple-output (MIMO) underwater acoustic (UWA) communications. Different from existing time-domain turbo equalizers in MIMO UWA systems, the proposed receiver implements low-complexity turbo equalization in the frequency domain to combat severe inter-symbol interference (ISI) and employs a layered structure to cope with unbalanced MIMO channels. Soft, rather than hard, successive interference cancellation (SIC) is employed in the layered iterative turbo detection to alleviate co-channel interference (CCI), and phase compensation is utilized within the layered FDTE to mitigate phase rotation. the proposed scheme was evaluated by the SPACE08 experiment conducted in a shallow area of the Atlantic Ocean in October 2008. the 2-transducer 12-hydrophone MIMO system communicated with quaternary phase shift keying (QPSK) and 8-phase shift keying (8PSK) modulation schemes over 200 m and 1000 m ranges, where the proposed FDTE-SSIC scheme achieved low bit error rates (BERs) with only a few iterations. Simulation results also demonstrated that the proposed FDTE-SSIC receiver provided lower BER than one-layer FDTE receivers with comparable complexity. © 2006 IEEE

A Variable Step-size LMP Algorithm for Heavy-tailed Interference Suppression in Phased Array Radar

A new variable step-size Least Mean p-norm (VSS-LMP) algorithm is proposed for phased array radar application with space-time adaptive processing to combat heavy-tailed non-Gaussian clutters. the algorithms automatically change the step size according to the estimated p-th and (2p - 2)-th moments of the error, where 1 ≤ p ≤ 2. the algorithm is evaluated via a space-slow-time STAP example and the excess Mean Square Error (MSE) and mis adjustment results show that the proposed vs.S-LMP converges fast and reaches lower steady-state error than the fixed step size LMP. It also provides a better compromise between convergence speed and low steady state error than existing vs.S Least Mean Square (LMS) algorithms in both Gaussian and Compound K clutter environments. ©2009 IEEE

Performances of Variable Step-size Adaptive Algorithms in Non-Gaussian Interference Environments

Two variable step-size normalized least mean square (VSS-NLMS) algorithms, namely the non-parametric vs.S-NLMS and Switched Mode vs.S-NLMS, are reformulated into complex signal form for STAP applications. the performances of these two vs.S NLMS algorithms in Gaussian and compound-K clutters are evaluated via a phased array space-slow-time STAP example. We find that the mis adjustment behaviors are inconsistent with the excess MSEs which is a better measure of STAP performance. Both vs.S-NLMS algorithms outperform conventional fixed step size (FSS) NLMS algorithms with fast convergence and low steady-state excess MSE. the SM-VSS-NLMS provides a better performance compromise than the NP-VSS-NLMS with much lower steady-state excess MSEs and slightly slower convergence speeds. the performance gain of both vs.S algorithms reduces in heavy-tailed clutter environments than that in Gaussian clutters. their robustness against impulsive interference is better than conventional FSS-NLMS. ©2009 IEEE