William W. Jones

Narrowband interference suppression using filter-bank analysis/synthesis techniques

The authors consider the application of multirate digital filter banks to the suppression of narrowband interference in direct sequence spread spectrum. In addition to providing a general framework from which to study and design transform domain interference suppression systems, these banks are a particular time-frequency distribution, making them applicable to a larger class of jamming scenarios. The authors exploit the banks perfect reconstruction property to mitigate on/off-type jammers. Quite practical filter banks are shown to provide near-perfect reconstruction while, due to superior filtering, effectively mitigating the spectral leakage problem with nearly complete removal of the interference. Bit error performance results are presented which demonstrate the superiority of the filter banks for a number of jamming scenarios. The mathematical-equivalent, computationally efficient processing structure known as polyphase filter networks is put forth and shown to possess only a slight complexity increase from windowed transform processing.<<ETX>>

Multi-scale wavelet modulation

The purpose of this paper is to provide the fundamental characterization of a general multidimensional modulation format which utilizes wavelet basis functions as pulse shapes. This type of modulation is particularly effective in distorting channels since it allows for frequency selective processing. In addition to developing the signal format, power spectral density, bandwidth efficiency and error rate performance, an efficient digital implementation utilizing digital filter banks is put forth. Finally, the use of wavelets in communication is seen to generate new pulse shape designs. A practical solution discussed in the paper applies the Smith-Barnwell two-channel filter bank design.<<ETX>>

An all digital 650-Mbps demonstration receiver for NASA's high data rate satellite applications

A demonstration receiver that tracks and demodulates 650-Mb/s QPSK (quadrature phase-shift keying) data has been developed, utilizing a parallel processing VLSI architecture and multirate digital signal processing, to meet upcoming NASA satellite communication requirements. Data rates of 650 Mb/s were achieved with low implementation loss and two-sample/symbol operation using high-speed digital GaAs logic in the front and back end, with the majority of the computationally intensive signal processing implemented in CMOS VLSI. Using a parallel architecture combined with multirate digital signal processing means that receiver speed is no longer limited to the speed of the signal processing hardware elements, but only by the speed of the analog-to-digital converters. In addition to unsurpassed data rate levels, superior bit error rate (BER) performance has been achieved using a joint feedforward and feedback timing estimator and data recovery algorithm. Laboratory performance results for the hardware demonstration receiver are presented as well as a detailed discussion of the hardware implementation and signal processing algorithms.<<ETX>>

Joint time and frequency estimation in TDMA burst receivers using the generalized semicoherent statistic

This paper develops joint parameter estimates of time and frequency offsets using the generalized semicoherent statistic with application to TDMA burst receivers. Joint operation enables shorter preambles and reduces hardware complexity. Excellent performance is achieved at low SNR with estimation intervals as low as 32 symbols. Comparisons of the frequency estimate are made with the widely used least squares estimate of the frequency which suffers a significant thresholding at mid and low levels of SNR. The frequency estimate from the semicoherent approach does not suffer from this problem. Consequently, the semicoherent approach can be used in high gain coding applications, e.g., in support of frame relay and ATM.

Efficient link quality estimation in DS spread systems

The author describes an algorithm for generating a link quality estimate (LQE) for direct-sequence (DS) phase-shift-keyed (PSK) signals that simultaneously possess good estimator properties, low-computational requirements, and a form easily incorporated into a digital receiver architecture. The algorithm exploits the nonlinear magnitude operations, available in the noncoherent and data automatic gain control (AGC) hardware. Taking the ratio of the mean value of two signals obtainable from the AGC circuitry produces a monotonically increasing function of signal-to-noise ratio (SNR) usable as a measure of link quality. Inverting this function provides an estimate of SNR. Extensive Monte Carlo simulation results are presented to quantify the LQE and SNR estimators performance. The results show that under reasonable conditions both estimators have an asymptotic unbiased behavior with error variance monotonically decreasing with sample size, suggesting consistency. The performance of the SNR estimator is shown to be superior to that of an alternative SNR estimator previously reported, while possessing reduced complexity.<<ETX>>