J.E. Salt

Effects of filtering on the performance of QPSK and MSK modulation in D-S spread spectrum systems using RAKE receivers

Direct-sequence spread spectrum systems utilizing RAKE receivers provide inherent diversity for in-building mobile CDMA radio. A new compact expression for the SNR of the decision variable of a RAKE receiver is derived for generalized offset quadrature carrier modulation in this multipath environment. This expression also applies to BPSK, QPSK, and MSK modulation. The novelty of the derived expression is that it includes the effects of filtering. Filtering must be used to eliminate out-of band radiation and can also be used to shape the in-band spectrum, It is shown that bandlimiting filters have a significant effect on the SNR. A comparison of bandlimited O-QPSK and MSK reveals that, under the same co-user load, MSK has a lower SNR-0.917 times that of O-QPSK. However, MSK has an offsetting advantage of being a constant envelope-type modulation that can be very efficiently generated using direct modulation of a power RF oscillator. It is further shown that the O-QPSK SNR can be increased by a factor of 1.63 by whitening the in-band spectrum. >

BER analysis of GMSK with differential detection in a land mobile channel

The BER performance of a land mobile radio communication system with GMSK modulation is analyzed using one-bit differential detection. In particular, the effect of the IF filter bandwidth on the BFR is analyzed. The optimum bandwidth for the IF filter is found to be a function of signal-to-noise power ratio, signal-to-co-channel-signal power ratio, and signal-to-delayed-signal power ratio. Several graphs are presented to illustrate the effect of the IF bandwidth in the presence of Doppler spread and frequency-selective fading. >

An algorithm for locating microseismic events

Monitoring seismic activity in mines, produced by high stress faults in the vicinity of the mining operations, is an important issue for mine safety. A seismic event produces a short-time duration acoustic pressure wave that travels through the rock. This low-energy seismic activity in mines is typically referred to as microseismic events. The location of a microseismic event can be estimated using the pressure wave signals recorded at a set of sensors distributed throughout the mine. The classical process for locating a radiating source involves two steps: an estimation of the time difference of arrival between all sensor pairs followed by the localization, requiring the solution of a set of non-linear equations. This traditional localization process has limited success when applied to microseismic events, since they have a short-time duration, and thus generating accurate arrival time estimates is a challenging task in a noisy environment. An alternate approach to traditional localization, that avoids time-delay estimation, is to search over a grid of hypothesized source locations to find the one that best explains the observed measurements. Here, this approach is used with a performance function that is the greatest energy calculated from the sum of the sensor signals, each of which is time-shifted by an amount consistent with the hypothesized location of the event. The result is a very robust algorithm that works well with short-time duration signals and given the recent advances in low-cost computational power can be implemented in real time. The paper describes the location algorithm. Results are presented for computer generated signals as well as actual signals produced by a microseismic event that occurred one kilometer below the surface in a potash mine near Saskatoon, Saskatchewan.

A two-phase genetic K-means algorithm for placement of radioports in cellular networks

This paper proposes a two-phase hybrid algorithm for optimal placement of radioports. The placement strategy minimizes the dynamic range of the channel, and thus reduces the peak transmit power needed by a wireless handset. The performance of the proposed algorithm is compared with that of an exhaustive search, as well as with that of a simplex algorithm. Simulation results show that the two-phase hybrid algorithm, although suboptimal, outperforms the simplex algorithm with only a modest increase in computation time. It has the added advantage of being quite insensitive to the seed solution.

Near-optimal range and depth estimation using a vertical array in a correlated multipath environment

This paper proposes a near-optimal procedure to localize a single stationary source in a two-path underwater acoustic environment. The investigation is for an M-element vertical array with omnidirectional sensors. The range and depth estimators are developed using a linear least-squares technique when a set of auto- and cross-correlators is used for time difference of arrival (TDOA) estimates. A weighting matrix is derived to achieve the approximate maximum likelihood (ML) performance of the weighted least-squares range and depth estimators. The expressions for error variances and covariances of the range and depth estimates are derived with a small error analysis technique. It is verified analytically that the error covariance matrix of the weighted least-squares solutions reaches the Cramer-Rao lower bound in the small error region. The correlation of the range and depth estimation errors is investigated. Results show that the range and depth estimation errors are highly correlated in a multipath environment. The accuracy properties of the proposed multipath localization procedure are analyzed using different array configurations. The results show that the performances of the range and depth estimators are significantly improved if the linear-dependent TDOA estimates are included for localizing and that the unweighted range and depth estimators, using the entire set of TDOAs, are approximately optimal for most of the applications. The theoretical development of error variance and covariance expressions of the range and depth estimates, which incorporates the correlation in the TDOA estimates, is corroborated with Monte Carlo simulations.

A hybrid spreading/despreading function with good SNR performance for band-limited DS-CDMA

Code-division multiple-access (CDMA) implemented with direct-sequence spread spectrum (DS/SS) signaling is a promising multiplexing technique for cellular telecommunications services. The efficiency of a direct-sequence spread-spectrum code-division multiple-access (DS-CDMA) system depends heavily on the shape of the spectrum of the spread signal. Maximum efficiency is obtained with an ideal brick-wall bandpass spectrum. There are two approaches toward achieving such a spectrum. One is to use a simple spreader that produces a broad spectrum and then follow it with a precise, high order filter to band limit the spectrum. A second approach, which is the approach taken in this paper, is to use a spreader that produces a spectrum close to the ideal spectrum and then employ a simple filter to control the out-of-band power. The proposed spreader/despreader is based on a simple hybrid function and can be easily implemented. An analysis provides a compact expression for the signal-to-noise ratio (SNR) of a RAKE receiver. The expression includes the effects of baseband, intermediate frequency (IF) and RF filtering as well as the effects of the spectral densities of the spreading/despreading functions. The analysis shows that the proposed spreader/despreader yields superior performance over a conventional pseudo noise (PN) spreading/despreading mechanism.

Range and depth estimation using a vertical array in a correlated multipath environment

The effects of correlation between time-difference-of-arrival (TDOA) estimates on the localization of an underwater source in a direct-surface reflected propagation environment is investigated for a two-element vertical array with omnidirectional sensors. Expressions for the covariance matrix of the noisy TDOAs are derived. The results show that the estimated TDOAs are correlated and that the degree of correlation depends on the relative location of the source. Expressions for variances of range and depth estimators are developed using a least-squares technique. The results show that the expressions for variances of the range and depth estimators can be in error by as much as a factor 3.3 if the correlation among the TDOAs is ignored. The expressions for the covariances of the TDOAs and the expressions for the variances of the range and depth, which incorporate the correlation in the TDOAs, are corroborated with Monte Carlo simulations. >

Error Analysis of a Localization Algorithm for Finite-Duration Events

This paper proposes and examines the performance of an algorithm used to locate a finite time-duration acoustic source that may have a small time-bandwidth product. The localization algorithm hypothesizes a source location and aligns the sensor signals by removing the propagation delay for that hypothesized location. The energy of the sum of the aligned signals is calculated and the hypothesized source location with the maximum energy is the estimate of the source position. The primary contribution of this paper is the development of the expression for the variance of localization error in any specified direction. The variance expression is validated through a comparison with Monte Carlo simulations

Coarse timing recovery in burst mode OFDM

In burst mode orthogonal frequency division multiplex communication, the correct point in time to begin collecting samples for Fourier processing must be identified. It is estimated with precision timing estimators whose computational efficiency can often be improved if the algorithm is seeded with a good initial estimate obtained from a coarse timing estimator. This paper provides the analysis of a low complexity edge detection algorithm that serves as a coarse estimator of the beginning of the Fourier processing window. The algorithm requires zero overhead and very little computation effort. The probability density function of the detection variable is shown theoretically to be approximately Gamma distributed. This is substantiated with simulation results.

An efficient and practical spreading function for D-S spread spectrum CDMA systems

The capacity of a spread spectrum CDMA system is greatly influenced by the power spectrum of the function used to do the spreading. This paper introduces a new spreading function that has a power spectrum with a flat mainlobe, a null at the band edge, and low sidelobe power. With this spreading function there is no need for an inband spectral shaping. The proposed spreading function is a non-binary function yet can be incorporated into a system with simple switching multipliers. This spreading function is shown to yield a larger SNR and be more immune to distortion in the transmit and receive band limiting filters than a conventional PN spreading function.