Chad C. Lau

Controlling instabilities in manipulation requires specific cortical-striatal-cerebellar networks

Dexterous manipulation requires both strength, the ability to produce fingertip forces of a specific magnitude, and dexterity, the ability to dynamically regulate the magnitude and direction of fingertip force vectors and finger motions. Although cortical activity in fronto-parietal networks has been established for stable grip and pinch forces, the cortical regulation in the dexterous control of unstable objects remains unknown. We used functional magnetic resonance imaging (fMRI) to interrogate cortical networks engaged in the control of four objects with increasing instabilities but requiring constant strength. In addition to expected activity in fronto-parietal networks we find that dexterous manipulation of increasingly unstable objects is associated with a linear increase in the amplitude of the BOLD signal in the basal ganglia (P = 0.007 and P = 0.023 for 2 compression tasks). A computational regression (connectivity) model identified independent subsets of cortical networks whose connection strengths were mutable and associated with object instability (P < 0.001). Our results suggest that in the presence of object instability, the basal ganglia may modulate the activity of premotor areas and subsequent motor output. This work, therefore, provides new evidence for the selectable cortical representation and execution of dynamic multifinger manipulation for grasp stability.

Generalized Multi-carrier Chaotic Shift Keying

Chaotic shift-keying, perhaps better described as chaotic carrier shift keying (CSK), encodes information in the selection of a time-synchronized orthogonal spread spectrum signal and decodes that same information by comparing the output of parallel despreaders. The general performance of CSK waveforms lacks that of coherent single-carrier modulations due to each despreader generating an independent noise statistic and potential for inter-correlations between assumed orthogonal carriers. However, the multiple carriers can be employed to significantly increase the throughput capacity of the spread waveform, permitting novel modulations that also take advantage of other phase or amplitude manipulations, much like an OFDM system. This paper generalizes the analytical and simulation results for arbitrary multi-carrier CSK systems, as well as reports on measured hardware results of a lower-order hardware CSK implementation.

Quantization Effects in Digital Chaotic Communication Systems

Recent work in developing digital chaotic communication systems has validated the potential to increase low probability of interception/detection (LPI/D) performance levels over direct sequence spread spectrum (DSSS) comms, but at the expense of a multi-bit resolution spreading signal with relatively high peak-to-average power ratio (PAPR). This paper quantifies the LPI/D and communications performance trades that come from reducing the precision of the multi-bit resolution signal, with the objective of optimizing hardware utilization via reduced-precision chaotic signals and quantifying performance trades for disadvantaged platforms. Sensitivities of the digital chaos generation process, baseband transmitter processing, and matched filter reception are considered independently, preliminary measurements for compressed HPA distortion artifacts are also provided. All analyses leverage Matlab simulations and targeted measurements from four prototype hardware digital chaotic communication systems.

Performance of CSSS Signals with Orthogonal Overlay Codes

The use of orthogonal overlay codes in spread spectrum communication systems is relatively well known practice, most often to mitigate the interference of other narrowband communication signals in a shared channel [1] or to facilitate space-time coding in multi-carrier code division multiple access (MC-CDMA) [2] communications. This paper proposes an alternative usage of a variable duration wavelet-like overlay code that helps ensure optimal correlation in Gaussian distributed digital chaotic sequence spread spectrum (CSSS) systems. This process is shown analytically, through fixed precision simulations, and through hardware measurements to improve the effective signal-to-noise ratio (SNR) performance of receivers for these Gaussian chaotic signals, ultimately validating a computationally efficient mechanism for building sequence based carrier shift keying (CSK) modulations from non-constant envelope spreading signals.

Space-time processing for MIMO-OFDM using DFT-based complementary sequences

In this paper, a new space-time signaling scheme is proposed for Orthogonal Frequency Division Multiplexing (OFDM) using complementary sequences derived from the rows of the DFT matrix. The autocorrelative properties of the complementary sequences allows multiple complex data signals at the transmitter with an arbitrary number of antennas to be perfectly separated and reconstructed at the receiver without prior channel knowledge while achieving full-rate. This new method is proposed and derived for multiple MIMO-OFDM systems with multipath fading; at the receiver, symbol estimation is effected via maximum likelihood estimation (ML).

Spatio-temporal scheduling of complementary sequences with application to MIMO-OFDM

In this paper, a new method of space-time processing is proposed for Orthogonal Frequency Division Multiplexing (OFDM) using complementary sequences derived from the rows of the DFT matrix. The autocorrelative properties of the complementary sequences allows multiple complex data signals at the transmitter with an arbitrary number of antennas to be perfectly separated at the receiver without prior channel knowledge while achieving full-rate. This new method is proposed and derived for multiple MIMO-OFDM systems with multipath fading; at the receiver, symbol estimation is effected via maximum likelhihood estimation (ML).

Space-time processing for OFDM using complementary Golay sequences

In this paper, a new method of space-time processing is proposed for Orthogonal Frequency Division Multiplexing (OFDM) using complementary pairs of Golay sequences. Space-time processing with complementary Golay sequences provides diversity at the transmitter which in turn helps improve performance in multipath fading channels without the need for channel knowledge at the transmitter. The autocorrelative properties of complementary Golay pairs allows multiple data signals at the transmitter to be perfectly separated at the receiver. Further, these properties significantly boost the signal energy at the receiver, leading to a higher SNR and thus better bit error performance. Building on work done in the field of orthogonal space-time block codes, the new Golay method is proposed and derived for both a 2×1 and 2×2 MIMO-OFDM system with a channel exhibiting Rayleigh fading; at the receiver, symbol estimation is effected via minimum mean-square estimation (MMSE).

Reduced dimension equalizer and interference canceller for MIMO-OFDM

In this paper, an interference cancellation and equalization method is proposed for a multiple-input, multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) system. Standard equalization techniques such as zero-forcing (ZF), and adaptive array processing techniques such as per-tone or time-domain equalization, may require large amounts of training and memory for storage of the weight coefficients. In a MIMO-OFDM system with N carriers, ZF considerations help dictate required equalizer lengths, which are characterized by the solution space associated with an underdetermined system of linear equations. In the practical case where the cyclic prefix length, and hence the channel length, is a fraction of the OFDM frame duration, it is shown that the respective equalizers to all tones (across antennas) are characterized by the basis of a data-independent, low-dimensional subspace. A structured per-tone technique is proposed that utilizes the extra degrees of freedom afforded by having multiple antennas at the receiver in order to cancel interfering signals.

Equalization for OFDM with Multiple Delay-Doppler Paths using Conjugate Gradients with Chebyshev Preconditioning

In this paper we consider the performance of iterative solution techniques when applied to equalization for orthogonal frequency division multiplexing (OFDM) with multiple delay-doppler paths. The use of a minimum mean-squared error (MMSE) equalizer in this problem yields satisfactory bit error rate (BER) performance, but requires the inversion of a high-dimensional matrix. Reduced complexity is achieved by applying the method of conjugate gradients (CG). Proper use of preconditioning is used to accelerate convergence of CG in terms of reducing the number of steps required for good performance. We analyze the performance and efficiency of a polynomial-based preconditioner wherein an approximate inverse of the MMSE equalizer is formed from a linear combination of matrix-valued Chebyshev polynomials. The results of a simulation study are presented, demonstrating that Chebyshev-preconditioning substantially reduces the number of CG steps required to achieve performance close to that obtained with the MMSE estimator using the full inverse.