Showing papers on "Spread spectrum published in 1998"

Impulse radio: how it works

Abstract: Impulse radio, a form of ultra-wide bandwidth (UWB) spread-spectrum signaling, has properties that make it a viable candidate for short-range communications in dense multipath environments. This paper describes the characteristics of impulse radio using a modulation format that can be supported by currently available impulse signal technology and gives analytical estimates of its multiple-access capability under ideal multiple-access channel conditions.

Wideband DS-CDMA for next-generation mobile communications systems

Abstract: Wideband wireless access based on direct sequence code division multiple access aimed at third-generation mobile communications systems is reviewed. W-CDMA is designed to flexibly offer wideband services which cannot be provided by present cellular systems, with various data rates as high as 2 Mb/s. The important concept of W-CDMA is the introduction of intercell asynchronous operation and the pilot channel associated with individual data channels. Intercell asynchronous operation facilitates continuous system deployment from outdoors to indoors. Other technical features of W-CDMA include fast cell search under intercell asynchronous operation, fast transmit power control, coherent spreading code tracking, a coherent RAKE receiver, orthogonal multispreading factor forward link, and variable-rate transmission with blind rate detection. The introduction of the data-channel-associated pilot channel allows W-CDMA to support interference cancellation and adaptive antenna array techniques that can significantly increase the link capacity and coverage. This article presents the radio link performance evaluated by computer simulation. Field experiment radio link performance results are also presented.

Improved parallel interference cancellation for CDMA

Abstract: This paper introduces an improved nonlinear parallel interference cancellation scheme for code-division multiple access (CDMA) that significantly reduces the degrading effect on the desired user of interference from the other users that share the channel. The implementation complexity of the scheme is linear in the number of users and operates on the fact that parallel processing simultaneously removes from each user a part of the interference produced by the remaining users accessing the channel the amount being proportional to their reliability. The parallel processing can be done in multiple stages. The proposed scheme uses tentative decision devices at the multiple stages to produce the most reliably estimated received data for generation and cancellation of user interference. Simulation results are given for a multitude of different situations, in particular, those cases for which the analysis is too complex.

On the energy capture of ultrawide bandwidth signals in dense multipath environments

Abstract: A quasi-analytical experimental analysis is described in this paper to quantify the tradeoff between energy capture and diversity level in a RAKE receiver using measured received waveforms obtained from ultrawide bandwidth signal propagation experiments.

The role of synchronization in digital communications using chaos. II. Chaotic modulation and chaotic synchronization

Abstract: For pt. I see ibid., vol. 44, p. 927-36 (1997). In a digital communications system, data are transmitted from one location to another by mapping bit sequences to symbols, and symbols to sample functions of analog waveforms. The analog waveform passes through a bandlimited (possibly time-varying) analog channel, where the signal is distorted and noise is added. In a conventional system the analog sample functions sent through the channel are weighted sums of one or more sinusoids; in a chaotic communications system the sample functions are segments of chaotic waveforms. At the receiver, the symbol may be recovered by means of coherent detection, where all possible sample functions are known, or by noncoherent detection, where one or more characteristics of the sample functions are estimated. In a coherent receiver, synchronization is the most commonly used technique for recovering the sample functions from the received waveform. These sample functions are then used as reference signals for a correlator. Synchronization-based coherent receivers have advantages over noncoherent receivers in terms of noise performance, bandwidth efficiency (in narrow-band systems) and/or data rate (in chaotic systems). These advantages are lost if synchronization cannot be maintained, for example, under poor propagation conditions. In these circumstances, communication without synchronization may be preferable. The theory of conventional telecommunications is extended to chaotic communications, chaotic modulation techniques and receiver configurations are surveyed, and chaotic synchronization schemes are described.

Capacity and mutual information of broadband multipath fading channels

Abstract: We investigate the capacity and mutual information of a broadband fading channel consisting of a finite number of time-varying paths. We show that the capacity of the channel in the wideband limit is the same as that of a wideband Gaussian channel with the same average received power. However, the input signals needed to achieve the capacity must be "peaky" in time or frequency. In particular, we show that if white-like signals are used instead (as is common in spread-spectrum systems), the mutual information is inversely proportional to the number of resolvable paths L/spl tilde/ with energy spread out, and in fact approaches 0 as the number of paths gets large. This is true even when the paths are assumed to be tracked perfectly at the receiver. A critical parameter L/spl tilde//sub crit/ is defined in terms of system parameters to delineate the threshold on L over which such overspreading phenomenon occurs.

Direct sequence spread spectrum method, computer-based product, apparatus and system tolerant to frequency reference offset

Abstract: A system employs a digital receiver (Fig. 9) (or transceiver) to receive, digitize and process a direct sequence spread spectrum signal using digital signal processing components. A radio front end portion (905) of the receiver receives and digitizes the signal (907), and a digital signal processing portion downconverts (909) and despreads (911) the signal by applying a pseudorandom noise (PN) code, used at a transmitter to spread a data signal contained in the direct sequence spread spectrum signal, to the received signal. A timing and state control mechanism (921) is included to provide time reference correction information to the signal processing components of the receiver for alignment of the PN code with the direct sequence spread spectrum signal, allowing the receiver to be compatible with transmitter using inaccurate frequency references which impart a significant frequency ambiguity in the received signal. Computer-based synchronization methods and mechanism suitable for use for low performance digital signal processors, and power management mechanisms (925) are employed to enable long-term operations using battery power, enabling utilization in network setting.

Highly bandwidth efficient communications

Abstract: A discrete multitone stacked-carrier spread spectrum communication method is based on frequency domain spreading including multiplication of a baseband signal by a set of superimposed, or stacked, complex sinusoid carrier waves. In a preferred embodiment, the spreading involves energizing the bins of a large Fast Fourier transform (FFT). This provides a considerable savings in computational complexity for moderate output FFT sizes. Point-to-multipoint and multipoint-to-multipoint (nodeless) network topologies are possible. A code-nulling method is included for interference cancellation and enhanced signal separation by exploiting the spectral diversity of the various sources. The basic method may be extended to include multielement antenna array nulling methods for interference cancellation and enhanced signal separation using spatial separation. Such methods permit directive and retrodirective transmission systems that adapt or can be adapted to the radio environment. Such systems are compatible with bandwidth-on-demand and higher-order modulation formats and use advanced adaptation algorithms. In a specific embodiment the spectral and spatial components of the adaptive weights are calculated in a unified operation based on the mathematical analogy between the spectral and spatial descriptions of the airlink.

Analytic alpha-stable noise modeling in a Poisson field of interferers or scatterers

Abstract: This paper addresses non-Gaussian statistical modeling of interference as a superposition of a large number of small effects from terminals/scatterers distributed in the plane/volume according to a Poisson point process. This problem is relevant to multiple access communication systems without power control and radar. Assuming that the signal strength is attenuated over distance r as 1/r/m, we show that the interference/clutter could be modeled as a spherically symmetric /spl alpha/-stable noise. A novel approach to stable noise modeling is introduced based on the LePage series representation. This establishes grounds to investigate practical constraints in the system model adopted, such as the finite number of interferers and nonhomogeneous Poisson fields of interferers. In addition, the formulas derived allow us to predict noise statistics in environments with lognormal shadowing and Rayleigh fading. The results obtained are useful for the prediction of noise statistics in a wide range of environments with deterministic and stochastic power propagation laws. Computer simulations are provided to demonstrate the efficiency of the /spl alpha/-stable noise model in multiuser communication systems. The analysis presented will be important in the performance evaluation of complex communication systems and in the design of efficient interference suppression techniques.

Integrating digital data with perceptible signals

Abstract: A method for combining digital data with a perceptible program signal. The data stream is modulated onto an electromagnetic signal encoding the perceptible signal; the modulating employs a spread spectrum encoding of the data stream. The modulated program signal is transduced into perceptible form. A capture device receives the perceptible signal, and a decoder extracts the spread spectrum encoded data from the received signal. The method is particularly useful in encoding purchase information or watermarking information into the signal.

Blind adaptive interference suppression for direct-sequence CDMA

Abstract: Direct sequence (DS) code division multiple access (CDMA) is a promising technology for wireless environments with multiple simultaneous transmissions because of several features: asynchronous multiple access, robustness to frequency selective fading, and multipath combining. The capacity of DS-CDMA systems is interference-limited and can therefore be increased by techniques that suppress interference. In this paper, we present developments in interference suppression using blind adaptive receivers that do not receive knowledge of the signal waveforms and propagation channels of the interference, and that require a minimal amount of information about the desired signal. The framework considered generalizes naturally to include additional capabilities such as receive antenna diversity. The most powerful application of the methods described here is for linearly modulated CDMA systems with short spreading waveforms (i.e., spreading waveforms with period equal to the symbol interval), for which they provide substantial performance gains over conventional reception. Implications for future system design due to the restriction of short spreading waveforms and directions for further investigation are discussed.

A multi-user communication system architecture with distributed transmitters

Abstract: A new signal processing architecture for base stations and gateways (124, 126) used in spread spectrum communication systems (100) that simplifies data transfer, reduces required bus capacity, and does not require special synchronization of signals that are to be combined. A series of transmission modules (5081-508M) are used to transfer data to corresponding ones of a series of analog transmitters (4121-412M) used to form communication circuits for each system user. Each transmission module (5081-508M) employs a series of encoders (502MR) and modulators (504MS) to form spread communication signals, using appropriate PN spreading codes. Spread spectrum communication signals from each module (508) for each system user (D), are summed together (5101-510M) and transferred to a single analog transmitter (412) associated with that module. The signals being combined are automatically synchronized by common timing signals used for elements within each module. The number of processing elements within each module is such that at least one processing path is available for each user or user channel over which it is desired to transmit information through the connected analog transmitter (4121-412M). Data is output from the modules (5081-508M) at a greatly reduced transfer rate which can be more easily accommodated using current technology. This is very useful for satellite based communication systems, or high capacity cellular systems, and this system architecture can be accomplished cost effectively using a series of easily manufactured circuit modules.

Local area multiple object tracking system

Abstract: A radio frequency positioning system is described that determines identity and positional data of numerous objects. The system includes a plurality of spread spectrum radio transceivers where at least one transceiver is positioned on each of the numerous objects. At least three spread spectrum radio transceivers transmit to and receive signals from the plurality of radio transceivers. A signal processor is coupled to the spread spectrum radio transceivers and determines the identity and the positional data of the objects.

System capacity, latency, and power consumption in multihop-routed SS-CDMA wireless networks

Abstract: Ad-hoc wireless networking presents challenges that are different from those of tethered networks in several significant ways. In addition to high error rates and constantly varying channels, mobile communication imposes new constraints, including limited energy supplies, and the need for portability. A system for wireless networking utilizing code division multiple access (CDMA), in conjunction with spread spectrum (SS) modulation is presented. By combining SS, automatic power control and local coordination, a "collisionless," energy and spectrum efficient system can be created which is capable of simultaneously providing high bandwidth and low latency communications. A new routing method, minimum consumed energy routing is evaluated. This new method is shown to reduce latency by 75%, reduce power consumption by 15%, and avoid congestion, in comparison with minimum transmitted energy routing. A simulator, SSNetSim, was developed to simulate the performance of these networks. By taking into account factors such as station placement, traffic patterns, routing strategies, and path loss, network performance, in terms of SNR, throughput, latency, and power consumption, is computed.

Power distribution grid communication system

Abstract: A communication system arrangement uses powerline distribution systems to broadcast a signal from the distribution station to individual households. The signal is coded, using a spread spectrum technique and the receiving arrangement is designed to process the signal quickly for transmission to downstream households. In a preferred embodiment equipment, within each household is able to initiate a communication to a predetermined computer for reporting in accordance with an instruction signal received over the powerline system. With this system, a narrow band broadcast signal arrangement is realized, which does not require extensive modification to the distribution system.

Successive interference cancellation for multiuser asynchronous DS/CDMA detectors in multipath fading links

Abstract: There has been an increasing interest in the use of code-division multiple access (CDMA) in cellular mobile and wireless personal communications. The choice of such multiaccess technique is attractive because of its potential capacity increases and other technical factors such as privacy and multipath rejection capabilities. However, it is well known that the performance of CDMA can be significantly degraded due to cochannel interference (CI) and the near-far effects. We consider the performance of direct-sequence (DS)-based CDMA over fading channels that are modeled as slowly varying Rayleigh-fading discrete multipath channels. Specifically, we propose and analyze an adaptive multistage interference cancellation strategy for the demodulation of asynchronous DS spread-spectrum multiple-access signals. Numerical results show that the proposed multistage detector, which alleviates the detrimental effects of the near-far problem, can significantly improve the system performance.

Interleaved FDMA-a new spread-spectrum multiple-access scheme

Abstract: A new spread-spectrum multiple-access scheme applicable for both the uplink and the downlink of a mobile radio communications system is proposed, which is termed "interleaved frequency-division multiple-access". This scheme combines the advantages of spread-spectrum and multicarrier transmission. Moreover, it can be viewed as a special kind of a multicarrier spread-spectrum scheme where user discrimination is carried out using frequency-division multiple-access. The multiple-access capability is achieved by assigning to each user a different set of orthogonal subcarriers. The orthogonality between the different users is maintained at the receiver, even in time-dispersive channels resulting in no multiple-access interference. However, the cost to avoid multiple-access interference is to allow for intersymbol interference and therefore equalization is required at the receiver.

Linear receivers for the DS-CDMA downlink exploiting orthogonality of spreading sequences

Abstract: We address the problem of downlink interference rejection in a DS-CDMA system. Periodic orthogonal Walsh-Hadamard sequences spread different users' symbols followed by scrambling by a symbol aperiodic base-station specific overlay sequence. This corresponds to the downlink of the European UMTS wideband CDMA proposal. The point to point propagation channel from the cell-site to a certain mobile station is the same for all downlink signals (desired user as well as the interference). The composite channel is shorter than a symbol period for some user signals, while other users can have significant ISI owing to a faster transmission rate. In any case, orthogonality of the underlying Walsh-Hadamard sequences is destroyed by multipath propagation, resulting in multiuser interference if a coherent combiner (the RAKE receiver) is employed. We propose linear zero-forcing (ZF) and minimum mean-squared-error (MMSE) receivers which equalize for the estimated channel, thus rendering the user signals orthogonal again. A simple code matched filter subsequently suffices to cancel the multiple access interference (MAI) from intracell users.

Software programmable radio and method for configuring

Abstract: A software programmable radio (200) receives information to configure a reconfigurable resource (208) to perform an operation based on the information. A processor (210) within the reconfigurable resource performs a software program in accordance with the operation, such as a waveform for communicating via a spread spectrum technique. The method includes the steps of checking (304) for a valid license granted to the radio to determine when the radio is authorized for use in the network. A controller (204) configures the radio to perform the operation based on the information.

Digital interference suppression system for radio frequency interference cancellation

Abstract: A digital signal processing system that produces an adaptive cancellation arrangement which nulls out all types of concurrent interference and/or jamming signals received by Global Positioning System (GPS) or spread spectrum receiver (7) from diverse antennas. In the present arrangement, orthogonal components of the composite received signal are separated by the receive antenna arrangement (3) and adjusted in the digital network (5) between the antenna (3) and the receiver (7) in phase and amplitude to optimally cancel components. The arrangements can be synergistically combined with digital adaptive transversal filter technology which is primarily used to supplement suppression performance by reducing narrowband interference in the band. The orthogonal received signal components from the GPS satellite constellation and from interference sources are combined in the present arrangement to adaptively create a null that attenuates interference sources while slightly modifying the GPS received signals.

Maximum-likelihood synchronization of a single user for code-division multiple-access communication systems

Abstract: Code-division multiple access (CDMA) has emerged as an access protocol well-suited for voice and data transmission. One significant limitation of the conventional CDMA system is the near-far problem where strong signals interfere with the detection of a weak signal. Multiuser detectors assume knowledge of all of the modulation waveforms and channel parameters, and exploit this information to eliminate multiple-access interference (MAI) and to achieve near-far resistance. A major problem in practical application of multiuser detection is the estimation of the signal and channel parameters in a near-far limited system. We consider maximum-likelihood estimation of users delay, amplitude, and phase in a CDMA communication system. We present an approach for decomposing this multiuser estimation problem into a series of single-user problems. In this method the interfering users are treated as colored non-Gaussian noise. The observation vectors are preprocessed to be able to apply a Gaussian model for the MAI. The maximum-likelihood estimate (MLE) of each user's parameters based on the processed observation vectors becomes tractable. The estimator includes a whitening filter derived from the sample covariance matrix which is used to suppress the MAI, thus yielding a near-far resistant estimator.

Chaotic complex spreading sequences for asynchronous DS-CDMA. Part II. Some theoretical performance bounds

Abstract: For pt.I see G. Mazzini et al., vol.44, pp.937-47 (Oct. 1997). This paper and its companion (Part I) are devoted to the evaluation of the impact of chaos-based techniques on communications systems with asynchronous Code Division Multiple Access. In Part I, a performance index was introduced and exploited to a priori estimate the performance of DS-CDMA communications systems based on chaotic spreading sequences, and to compare it to that of conventional systems. Here, tools from nonlinear dynamical system theory are employed to give a formal ground for those results. Analytical bounds on the expected partial cross correlation between spreading sequences obtained by quantizing and repeating a chaotic time series are derived, ensuring general applicability of such a technique in a real environment. Further analytical arguments guarantee that, when particular chaotic generators are used, expected performance is not worse than that of a well-behaving communications system. This analysis ensures also that, unlike conventional sequences, chaotic spreading codes can be generated for any number of users and allocated bandwidth.

A linear receiver for direct-sequence spread-spectrum multiple-access systems with antenna arrays and blind adaptation

Abstract: A linear receiver for direct-sequence spread-spectrum multiple-access communication systems under the aperiodic random sequence model is considered. The receiver consists of the conventional matched filter followed by a tapped delay line with the provision of incorporating the use of antenna arrays. It has the ability of suppressing multiple-access interference (MAI) and narrowband interference in some weighted proportions, as well as combining multipath components without explicit estimation of any channel conditions. Under some specific simplified channel models, the receiver reduces to the minimum variance distortionless response beamformer, the RAKE receiver, a notch filter, or an MAI suppressor. The interference rejection capability is made possible through a suitable choice of weights in the tapped delay line. The optimal weights can be obtained by straightforward but computationally complex eigenanalysis. In order to reduce the computational complexity, a simple blind adaptive algorithm is also developed.

Adaptive detection for DS-CDMA

Abstract: A review of adaptive detection techniques for direct-sequence code division multiple access (CDMA) signals is given The goal is to improve CDMA system performance and capacity by reducing interference between users The techniques considered are implementations of multiuser receivers, for which background material is given Adaptive algorithms improve the feasibility of such receivers Three main forms of receivers are considered The minimum mean square error (MMSE) receiver is described and its performance illustrated Numerous adaptive algorithms can be used to implement the MMSE receiver, including blind techniques, which eliminate the need for training sequences The adaptive decorrelator can be used to eliminate interference from known interferers, though it is prone to noise enhancement Multistage and successive interference cancellation techniques reduce interference by cancellation of one detected signal from another Practical problems and some open research topics are mentioned These typically relate to the convergence rate and tracking performance of the adaptive algorithm

Code-spread CDMA using maximum free distance low-rate convolutional codes

Abstract: In code division multiple-access (CDMA) systems, maximum total throughput assuming a matched filter receiver can be obtained by spreading with low-rate error control codes. Previously, orthogonal, bi-orthogonal and super-orthogonal codes have been proposed for this purpose. We present in this paper a family of rate-compatible low-rate convolutional codes with maximum free distance. The performance of these codes for spectrum spreading in a CDMA system is evaluated and shown to outperform that of orthogonal and super-orthogonal codes as well as conventionally coded and spread systems. We also show that the proposed low rate codes will give simple encoder and decoder implementations. With these codes, any rate 1/n, n/spl les/512, are obtained for constraint lengths up to 11, resulting in a more flexible and powerful scheme than those previously proposed.

Spread-spectrum signal receiving method and spread-spectrum signal receiving apparatus

Abstract: A spread-spectrum signal receiving method and apparatus in which a correlation operation is performed for obtaining a correlation between a base-band component of a received spread-spectrum signal and a spread code, so as to demodulate the received signal. In the method, a correlation operation between the spread code and the base-band component, and a correlation operation at a timing equal to a timing difference between the spread code and the base-band component in the former correlation operation step, the timing difference being 1/2 of a spread-code interval, are performed. Then, based on results obtained in these correlation operations, a correlation operation result at the timing point where a timing difference between the spread code and the base-band component is less than 1/2 of the spread-code interval, is estimated.

A linearly constrained constant modulus approach to blind adaptive multiuser interference suppression

Abstract: This article presents a linearly constrained constant modulus approach for the blind suppression of multiuser interferences in direct-sequence code division multiple access systems. The method performs the same as minimum mean square error receivers and outperforms existing blind approaches because it only requires a rough estimate of the desired user code and timing.

Spread spectrum indoor geolocation

Abstract: The design and performance results of indoor geolocation using VHF spread spectrum signals and plans for future improvements will be presented. The development of the system is funded by the US DOD to support small unit operations in urban environments. In the present system direct sequence spread spectrum signals at 258.5 MHz carrier frequency and 23.5 MHz chipping rate are generated and transmitted by a small hand held, battery operated transmitter inside the building. The signal is received by four or more receivers located external to the building. The signals are downconverted and digitized at 8 bits and 94 MHz. 5.5 ms of data is collected and averaged to a vector length of a single PRN sequence. The cross correlation of this vector average and the PRN sequence is calculated using m techniques. We then fit the leading edge of a template cross correlation obtained in the absence of multipath to the reading edge of the envelope of the measured cross correlation, in order to extract the time of arrival of the signal. Even in a modern office building with a corrugated steel roof where indirect path is orders of magnitude greater than the direct path, we are able to reliably track the leading edge of the direct path. In data collected thus far we are able to get horizontal accuracy within 3 meters of ground truth in 90% of the fixes. Time of Arrival (TOA) information is passed to a central computer for processing fixes in real time with 1.7 sec update rate via a Wireless LAN. In the future, we plan to implement the PRN generator in a programmable ASIC that will allow for chipping rates of up to 100 MHz. This higher chipping rate will allow us to better isolate direct and indirect paths and improve accuracy.

Multi-carrier spread spectrum and its relationship to single-carrier transmission

Abstract: The combination of orthogonal frequency division multiplexing (OFDM) and code division multiplexing (CDM) has found great interest. In contrast to a conventional OFDM system the information is spread over several subcarriers using orthogonal codes which leads to a diversity gain in frequency-selective channels. In this paper it is shown that such a system is technically equivalent to a single-carrier system with a guard interval if the columns of a Fourier matrix are used as spreading codes. The pros and cons of such a system compared to an OFDM-CDM system with arbitrary spreading matrix are described and discussed.