Showing papers by "Arogyaswami Paulraj published in 1995"

A blind adaptive transmit antenna algorithm for wireless communication

Abstract: A method is proposed for forming an adaptive phased array transmission beam pattern at a base station without any knowledge of array geometry, path angles or mobile feedback. Estimates of receive vector channels are used to form a transmit weight vector optimization problem. The authors provide closed form solutions for both the single user case and the multiple user case. They show through simulation of a multiple user cellular network that the cooperative transmission network algorithm is capable of improving network frequency re-use capacity by a factor of 5 to 8.

Adaptive beam forming for transmitter operation in a wireless communication system

Abstract: A method for forming an adaptive phased array transmission beam pattern at a base station without any knowledge of array geometry or mobile feedback is described. The approach is immune to the problems which plague methods which attempt to identify received angles of arrival from the mobile and map this information to an optimum transmit beam pattern. In addition, this approach does not suffer the capacity penalty and mobile handset complexity increase associated with mobile feedback. Estimates of the receive vector propagation channels are used to estimate transmit vector channel covariance matrices which form objectives and constraints in quadratic optimization problems leading to optimum beam former solutions for the single user case, and multiple user case. The new invention in capable of substantial frequency re-use capacity improvement in a multiple user cellular network.

Method of subspace beamforming using adaptive transmitting antennas with feedback

Abstract: A method is disclosed for using feedback to reduce crosstalk between mobile receivers in a wireless communications system comprising an adaptive transmitting antenna array. Feedback signals from the receivers are time-averaged and used to calculate a set of weight vectors that minimize crosstalk and maximize desired signal without requiring burdensome feedback rates. Beamforming diversity is combined with feedback through the use of diversity vectors. The preferred embodiment includes a method for obtaining the required feedback by exploiting the SAT circuitry present in the AMPS system currently in use. Consequently, the method may be implemented without modification to existing cellular phone handsets.

Blind estimation of multiple digital signals transmitted over FIR channels

Abstract: Using oversampling and the finite-alphabet property of digital communication signals, it is possible to blindly identify an FIR channel carrying a superposition of such signals, provided they have the same (known) period, and certain rank conditions on the data and channel matrices are satisfied. In particular, this technique allows separation of finite alphabet signals, removal of intersymbol interference, and synchronization of the signals. An algorithm is proposed and tested on simulated data. >

Blind identification of FIR channels carrying multiple finite alphabet signals

Abstract: The finite alphabet property of digital communication signals, along with oversampling techniques, enables the blind identification and equalization of an unknown FIR channel carrying a superposition of such signals, provided they have the same (known) period. Applied to multi-user wireless communications, the same framework allows the blind separation of multiple finite alphabet signals received at all arbitrary antenna arrays through an unknown multipath propagation environment with finite delay spread. An algorithm is proposed and tested on simulated data.

Forward link antenna diversity using feedback for indoor communication systems

Abstract: An approach to mitigate forward link signal fading in a FDD communication system by using an adaptive transmit antenna array and feedback on the reverse link is presented In a personal communication system environment, multipath propagations can lead to severe space selective fading Cordless phones and similar devices which cannot conveniently provide multiple antennas at the receiver can suffer from long term fading and therefore have unacceptable quality Using multiple adaptive transmit antennas, we can adjust the transmission weights to ensure the user is kept out of deep fades This is achieved by using feedback of the received signal level on the reverse link and adapting the transmission weights Simulation shows that significant gain against the fading characteristics can be achieved

Recursive adaptive beamforming for wireless CDMA

Abstract: The increasing demand for mobile communication services without corresponding increases in RF spectrum allocation requires new techniques to improve spectrum utilization. Spatial processing using antenna arrays at the base-station has been proposed to reduce co-channel interference and reduce fading of the desired signal, leading to an increase in system capacity. The use of base station antenna, arrays in wireless CDMA is studied. A recursive, fast, and numerically stable algorithm estimating the array response vector and the optimum beamforming weights is derived. Our approach uses a space-time processing approach for estimating-the array response vector, and hence the optimum beamforming weights. Simulation results are also presented.

Time varying vector channel estimation for adaptive spatial equalization

Abstract: New blind adaptive spatial equalization techniques are proposed for severe multipath fading with co-channel interference. In rapidly fading environments, equalizer performance suffers with existing algorithms because the desired and interference channels change significantly over the update time interval. It is shown that for fast fading channels, conventional squared error equalizer cost functions can yield poor results. A new algorithm is proposed based on a variation of the SINR cost function coupled with optimal time varying channel estimation to improve signal tracking. In simulations, the algorithm achieves theoretically optimum time varying equalizer performance in extreme interference channels with narrow band FM signals and high velocity portables. These results are contrary to the widespread belief that high performance blind adaptive array processing is not possible for AMPS with severe combinations of Doppler spread, angle spread, and interference power.

Performance of DS/CDMA with M-ary orthogonal modulation cell site antenna arrays

Abstract: Code division multiple access (CDMA) has been shown to offer higher capacity than the existing FDMA or TDMA systems. Since no pilot is used in the reverse link for power efficiency considerations, M-ary orthogonal modulation with noncoherent reception is a suitable modulation technique. Spatial processing with base-station antenna arrays was previously proposed for improving coverage and increasing system capacity. The present authors study the performance of a base-station antenna arrays receiver structure for M-ary orthogonal modulation and noncoherent RAKE combining in the presence of multipath.

Performance enhancement and trade-offs of smart antennas in CDMA cellular networks

Abstract: In this paper, we study the improvement in the reverse link budget due to base-station smart antennas in CDMA cellular networks. Considering a typical multipath propagation environment, we quantify the excess energy per bit to interference plus noise ratio /spl gamma//sub b/ due to smart antenna processing at the base station as a function of the number of sensors, their separation, and scattering angle, With this excess /spl gamma//sub b/, we evaluate different trade-offs between different performance measures such as capacity, area coverage, and mobile transmitted power. Both theoretical and simulation results are presented.

On optimizing base station antenna array topology for coverage extension in cellular radio networks

Abstract: Use of higher frequencies (1.8 GHz) for the US upper tier PCS cellular service and the FCC regulations on the network build out have resulted in significant interest in improving coverage of cellular networks. Networks whose coverage is limited imply that thermal noise is the limiting factor. Also, since the forward link (base station to mobile) has higher power than the reverse link, cell coverage is usually limited by the reverse link. This coverage can be extended by improving the reverse link budget. Use of receive antenna arrays for boosting array gain on the reverse link is therefore of great interest. When receive antenna arrays are used at the base station, several conflicting choices affect system performance and cost. Some of these aspects are: the number of antenna elements (and channels) improves coverage but also increases system cost; the maximum span of the array increases diversity hut must be limited for convenient deployment on a tower; large inter-element spacing can increase diversity but cause grating lobes at the same time. These conflicting requirements mean that a careful design of the array topology can minimize the cost. In this paper, we study performance of linear and circular base station antenna arrays with different topologies, angle spread, and the number of elements. We compare alternate topologies using maximal ratio combining for narrowband systems such as AMPS and IS-54.

Spatio-temporal channel estimation techniques for multiple access spread spectrum systems with antenna arrays

Abstract: A major advantage of spread-spectrum communication systems is their ability to exploit the multipath structure of the received signal. A standard RAKE receiver estimates the time delay, attenuation, and angle of arrival of each path and uses a maximum-ratio combiner to recombine different path signals. Antenna array structures were proposed to mitigate the effects of multipath by separating the paths in both time and spatial domains. New techniques for estimating the spatio-temporal characteristics of the channel for coherent direct-sequence spread-spectrum systems are proposed. The simulation results present the performance of such a multichannel estimator.

A harmonic noise model for direction finding in colored ambient noise

Abstract: High-resolution algorithms for direction finding of narrowband sources require a good estimate of the noise correlation matrix. We derive a physically motivated linear noise model. It is based on a Fourier expansion of the ambient noise field and is applicable to arbitrary antenna arrays. We also show that for contrived cases, a linear model may not be uniquely identifiable. >

Blind estimation of multiple digital signals transmitted over multipath channels

Abstract: A challenging signal processing problem is the blind joint space-time equalization of multiple digital signals transmitted over multipath channels. This problem is an abstraction of a PCS wireless communication scenario. A number of authors have recently proposed algorithms for the blind separation and equalization of multiple co-channel digital signals transmitted through wireless environments with long delay spreads. These algorithms compute the coefficients of a joint space-time equalizer in which the outputs of multiple antennas are oversampled and linearly combined to produce estimates of the transmitted finite alphabet symbols. We present improvements in three directions, namely (1) a more efficient way to do the inherent subspace intersections, (2) a way to solve the problem for channels with largely differing and ill-defined delay spreads, and (3) algorithm-independent lower bounds for the number of antennas and amount of oversampling to provide sufficient resolution in the case of bandlimited signals. The improved algorithm is tested on experimental data.

Blind Adaptive Transmit Beamforming For Mobile Radio With Arbitrary and Unknown Antenna Array Geometries

Abstract: Note: inproceedings Reference LICOS-CONF-1995-002 Record created on 2006-12-11, modified on 2017-12-17

Blind estimation of multiple co-channel digital signals in vector FIR channels

Abstract: In this paper we examine the problem of detecting multiple co-channel signals arriving at a sensor array in a dispersive, slowly fading environment. This problem of separating and estimating co-channel signals has recently drawn much attention due to its applications in digital mobile radio. First, we propose a spatio-temporal approach for estimating both the channel and the signals in a delay-spread multipath fading environment. In this approach we exploit the discrete-alphabet property of some of the digital modulation schemes in this algorithm. This algorithm is applicable to an unknown array geometry and propagation environment, and hence it is especially attractive to digital mobile communication applications. Next we examine the matched filter bound for this channel environment. We obtain closed form expressions for the matched filter bound for the single user vector channel. Finally we present some preliminary simulation results to examine the performance of this algorithm and compare it to the single user matched filter bound.

Estimation of co-channel FM signals with multitarget adaptive phase-locked loops and antenna arrays

Abstract: A simple adaptive technique is proposed for separation and demodulation of multiple co-channel frequency modulated (FM) signals received at an antenna array. The proposed method, which for FM signals is embodied in an architecture referred to as a multitarget adaptive phase-lock loop (MADPLL), exploits known signal structure through a complete demodulation and remodulation of the signals. The two properties of the signal that are exploited here are the known bandwidth of the information signal and the constant-modulus (CM) property of FM signals. It is shown that the proposed method can lead to significant improvements in performance over methods that exploit only the CM property.

Performance of switched beam systems in cellular base stations

Abstract: We explore the performance of switched beam systems (SBS) under conditions of angular, delay, and Doppler spread. We offer a derivation of the probability of selecting a given beam when validation is done using TDMA color codes. This result is then used to derive the average CIR for a SBS using the given validation process. We simulate the performance of SBS using maximum received signal strength indicator (RSSI) validation and compare it to that of the standard sector antenna system. Finally, we simulate the performance of the color code validation scheme as compared to the maximum RSSI scheme.

Signal detection for time-varying vector channels

Abstract: We look at the problem of signal detection over time-varying channels using spatio-temporal processing. We first derive a matched filter bound for a fading vector channel. Next, we derive an upper bound on the pairwise probability of error in fading vector channels and show that for perfect channel estimation we do not observe error-flooring. We present a signal detection algorithm which approximates the maximum likelihood joint channel-data estimator for vector channels. Finally we present some simulation results of this algorithm for fast time-varying vector channels.

The Evolution of Mobile Communications

Abstract: One early application of the wireless telegraph developed by Marconi was ship-to-shore communications. This maritime version of mobile communications finally evolved into the ubiquitous cellular networks of today through a series of technology breakthroughs. This paper traces the evolution of this fascinating technology.