Broadband CDMA for personal communications systems

Multipath propagation in a broadband CDMA environment is described. A propagation model for broadband spread-spectrum signals is presented. Experimental results relating to the sharing of the band by fixed service microwave users and mobile personal communications network (PCN) users are discussed. Field tests indicate that PCN systems can provide high-quality communications when sharing the spectrum with fixed-service microwave systems in suburban and urban areas     

A methodology for modeling the distributions of medical images and their stochastic properties

The probabilistic distribution properties of a set of medical images are studied. It is shown that the generalized Gaussian function provides a good approximation to the distribution of AP chest radiographs. Based on this result and a goodness-of-fit test, a generalized Gaussian autoregressive model (GGAR) is proposed. Its properties and limitations are also discussed. It is expected that the GGAR model will be useful in describing the stochastic characteristics of some classes of medical images and in image data compression and other applications.     

Homogeneous Catalytic Isomerization of Allylbenzene and Phenylbutenes by Some Platinum-Metal Oomplexes

The isomerization of Allylbenzene, p-allylanisole, 4-phenylbut-1-ene and trans-1-phenylbut-2-ene catalyzed by various ruthenium, rhodium and iridium complexes has been investigated. At the boiling point of the olefins the reaction does not require co-catalysts. The activity of the complexes studied decreases in the order Ru > Ir > Rh. PPh₃ and AsPh₃ increase the initial rates of the Rh (III), catalyzed reactions, decrease the activities of Rh (I)- and Ir (I)-complexes and do not affect the Ru (II)-compound. Different mechanisms for the rhodium- and ruthenium-catalyzed isomerizations are suggested.     

Spread spectrum goes commercial

The use of spread-spectrum techniques to achieve more efficient utilization of available frequency spectra is examined. The two main spread-spectrum techniques, direct sequence and frequency hopping, are explained. In frequency hopping, the transmitter repeatedly changes (hops) the carrier frequency from one frequency to another. Direct-sequence transmission spreads the spectrum not by periodically changing the frequency but by modulating the original (information) baseband signal with a very wide-baseband digital signal. The wideband modulating signal's amplitude changes continually between two states, high and low, arbitrarily called +1 and -1, respectively, with the sequence of highs and lows being pseudorandom. Applying spread spectrum to code-division multiaccess (CDMA) communication, in which each user is assigned an identification code (a distinct sequence of frequencies for frequency hopping or +1 s and -1 s for direct-sequence modulation) is discussed. The use of CDMA for cellular radio, where it promises a capacity of over 1000 users per cell, by the authors' calculations, is considered. The potential of spread spectrum for relieving spectrum congestion is addressed     

On the feasibility of a CDMA overlay for personal communicationsnetworks

Because of the continually increasing demand for mobile communications, it has been suggested that personal communication networks (PCNs) be established in the 1850-1990 MHz range. However, that band of frequencies is currently occupied by various microwave signals transmitted by users ranging from utility companies to state and local agencies. In order to allow both sets of users to occupy these frequencies as well as improve the spectral efficiency of this band, a spread-spectrum overlay is proposed, whereby a code-division multiple-access (CDMA) PCN would share the spectral band with the existing narrowband microwave traffic. The results of several field tests which have been designed to demonstrate the feasibility of an overlay of this type are discussed     

Information efficiency of multihop packet radio networks with channel-adaptive routing

This paper analyzes the benefit of adaptive routing based on knowledge of the channel state information in multihop, ad hoc wireless networks that use direct-sequence code-division multiple access. Cross-layer, channel-adaptive routing exploits the inherent spatial diversity of multihop wireless networks to select links with favorable channel conditions. The information efficiency, an extension of a previously used measure called expected progress, is used to evaluate performance. Results show that, combined with adaptive modulation, adaptive routing can improve performance in ad hoc networks by a factor of four to five in channels with Rayleigh fading and lognormal shadowing. The lack of position information in the routing decision would reduce performance by 25%. New approaches to channel-adaptive routing that enable rapid adaptivity to channel conditions are discussed.     

Convergence analysis of a CM array for CDMA systems

The incorporation of directional sensitivity, provided by so-called adaptive antennas is useful in suppressing interfering signals that arise from spatially distinct mobile sources. The problem is that in a cellular radio environment where multipath exists, the standard adaptive antenna using reference signals may not properly lock on the desired signal. This is because the signal correlation matrix processed by the antenna may then be close to singular and standard algorithms fail. Also, most standard algorithms need to cooperate with the receiver for the spatial discrimination of signals. A smart antenna utilizing a blind algorithm is of interest since the antenna may not need to get any feedback from a receiver for the adjustment of weight coefficient for spatial processing and can stand alone to be plugged into any kind of receiver structure.In this paper, we address the convergence property of a Constant Modulus Algorithm which is a blind algorithm and, if employed, can provide no need for an antenna to cooperate with a receiver attached. By identifying a relationship between the weight coefficients and output signal amplitude, we also evaluate the performance of such a stand-alone antenna plus a CDMA matched filter reception. Our results show that for a three element CM array, the BER of a desired user with the other interfering users is much better than a conventional correlation receiver for a single user case since the array suppresses interferences and achieves array gain in SNR.     

Analysis of Integrated Voice/Data Multiplexing

-A model of a moving-boundary, fixed frame length, integrated multiplexor is proposed and analyzed. The assignment of slots within a frame to the voice and data sources is made by an allocation function. The joint distributions of queue length and expected waiting time are derived.