Showing papers on "Spread spectrum published in 1994"

MMSE interference suppression for direct-sequence spread-spectrum CDMA

Abstract: We consider interference suppression for direct-sequence spread-spectrum code-division multiple-access (CDMA) systems using the minimum mean squared error (MMSE) performance criterion. The conventional matched filter receiver suffers from the near-far problem, and requires strict power control (typically involving feedback from receiver to transmitter) for acceptable performance. Multiuser detection schemes previously proposed mitigate the near-far problem, but are complex and require explicit knowledge or estimates of the interference parameters. In this paper, we present and analyze several new MMSE interference suppression schemes, which have the advantage of being near-far resistant (to varying degrees, depending on their complexity), and can be implemented adaptively when interference parameters are unknown and/or time-varying, Numerical results are provided that show that these schemes offer significant performance gains relative to the matched filter receiver. We conclude that MMSE detectors can alleviate the need for stringent power control. In CDMA systems, and may be a practical alternative to the matched filter receiver. >

Analysis of a simple successive interference cancellation scheme in a DS/CDMA system

Abstract: Compensating for near/far effects is critical for satisfactory performance of DS/CDMA systems. So far, practical systems have used power control to overcome fading and near/far effects. Another approach, which has a fundamental potential in not only eliminating near/far effects but also in substantially raising the capacity, is multiuser detection and interference cancellation. Various optimal and suboptimal schemes have been investigated. Most of these schemes, however, get too complex even for relatively simple systems and rely on good channel estimates. For interference cancellation, estimation of channel parameters (viz. received amplitude and phase) is important. We analyze a simple successive interference cancellation scheme for coherent BPSK modulation, where the parameter estimation is done using the output of a linear correlator. We then extend the analysis for a noncoherent modulation scheme, namely M-ary orthogonal modulation. For the noncoherent case, the needed information on both the amplitude and phase is obtained from the correlator output. The performance of the IC scheme along with multipath diversity combining is studied. >

Performance of orthogonal multi-carrier CDMA in a multipath fading channel

Abstract: A DS-CDMA multi-carrier system is proposed. The transmitted data bits are serial to parallel converted to a number of parallel branches. Each bit on each branch is DS-SS modulated and transmitted with a number of orthogonal and overlapping carriers. Given that sufficient interleaving is provided, this procedure provides many advantages. The system performance is compared to that of the conventional RAKE receiver. It is shown that the multi-carrier system over-performs the RAKE receiver when the system parameters are selected properly. The system utilizes a small number of carriers to solve the intersymbol interference (ISI). The data on each carrier is spread by a lower rate PN code. This reduces the effect of inter-chip interference (ICI). This technique is shown to provide the following advantages: the spread spectrum processing gain is increased, the effect of multipath interference is removed, and frequency/time diversity is achieved.

Chaotic direct-sequence spread-spectrum communication system

Abstract: The use of chaotic sequences as spectral spreading sequences in direct-sequence spread-spectrum (DS/SS) communication systems is proposed. The error probabilities of such systems are investigated and shown to be, for all practical purposes, identical to the conventional DS/SS systems which use binary signature sequences. Among the advantages of the use of chaotic sequences in DS/SS are the availability of a great number of them, the ease of their generation, as well as their inherent improvement in the security of transmission. >

A chaotic direct-sequence spread-spectrum communication system

Abstract: The use of chaotic sequences as spectral spreading sequences in direct-sequence spread-spectrum (DS/SS) communication systems is proposed. The error probabilities of such systems are investigated and shown to be, for all practical purposes, identical to the conventional DS/SS systems which use binary signature sequences. Among the advantages of the use of chaotic sequences in DS/SS are the availability of a great number of them, the ease of their generation, as well as their inherent improvement in the security of transmission. >

Adaptive receiver structures for asynchronous CDMA systems

Abstract: Adaptive linear and decision feedback receiver structures for coherent demodulation in asynchronous code division multiple access (CDMA) systems are considered. It is assumed that the adaptive receiver has no knowledge of the signature waveforms and timing of other users. The receiver is trained by a known training sequence prior to data transmission and continuously adjusted by an adaptive algorithm during data transmission. The proposed linear receiver is as simple as a standard single-user detector receiver consisting of a matched filter with constant coefficients, but achieves essential advantages with respect to timing recovery, multiple access interference elimination, near/far effect, narrowband and frequency-selective fading interference suppression, and user privacy. An adaptive centralized decision feedback receiver has the same advantages of the linear receiver but, in addition, achieves a further improvement in multiple access interference cancellation at the expense of higher complexity. The proposed receiver structures are tested by simulation over a channel with multipath propagation, multiple access interference, narrowband interference, and additive white Gaussian noise. >

Multicarrier orthogonal CDMA signals for quasi-synchronous communication systems

Abstract: Proposes a multicarrier orthogonal CDMA signaling scheme for a multiple-access communication system, such as the reverse channel of a cellular network, as an alternative to the multi-user interference cancellation approach. The average variance of cross-correlations between sequences is used as a measure for sequence design. The authors search for sets of sequences that minimize the probability of symbol detection error, given that there is imperfect synchronization among the signals, that is, the signals are quasi-synchronous. Orthogonal sequences based on the Sylvester-type Hadamard matrices (Walsh functions) are shown to provide a significant improvement over the case where a Hadamard (orthogonal) matrix is chosen at random. Computer searches suggest that this set of codes is optimal with respect to the above measure. The issue of chip pulse shaping is investigated. Optimal pulses designed to minimize multiple-access interference in quasi-synchronous systems are obtained for various bandwidths and are shown to provide a large improvement over the raised cosine pulses. A multicarrier signaling scheme is introduced in order to reduce chip level synchronization offsets between the users. >

Spread spectrum clock generation for the reduction of radiated emissions

Abstract: A method is presented for reducing the radiated emissions of an electronic device by frequency modulating (FM) the system clock. This method, referred to as spread spectrum clock generation, or SSCG, is applicable to most microprocessor based systems. A unique waveform used to frequency modulate a digital clock signal results in a spectrum with sideband harmonics that are nearly uniform in amplitude when measured with an EMI receiver. This has the effect of spreading the energy of a discrete frequency harmonic over a wider bandwidth, thereby reducing the amplitudes of the harmonics. Attenuation as high as 13 dB is presented using an experimental setup, an actual SSCG integrated circuit, and theoretically computed results. >

Medium access control of wireless LANs for mobile computing

Abstract: To provide high-speed seamless services for mobile computing, an effective medium access control capable of dealing with mobility issues in multicell wireless local area networks is needed. The physical layers discussed include direct sequence spread spectrum (DS-SS), frequency hopped spread spectrum (FH-SS), and diffuse infrared. Both spread spectrum transmissions are in the 2.4 to 2.4835 GHz ISM band, according to FCC part 15.247. >

Spread spectrum clock generator and associated method

Abstract: A clock circuit includes an oscillator for generating a reference frequency signal, and a spread spectrum clock generator cooperating with the oscillator for generating a spread spectrum clock output signal having a fundamental frequency and reduced amplitude EMI spectral components at harmonics of the fundamental frequency. The spread spectrum clock generator preferably includes a clock pulse generator for generating a series of clock pulses, and a spread spectrum modulator for frequency modulating the clock pulse generator to broaden and flatten amplitudes of EMI spectral components which would otherwise be produced by the clock pulse generator. The spread spectrum modulator frequency modulates the clock pulses with specific profiles of frequency deviation versus the period of the profile. Electronic devices including the spread spectrum clock circuit and associated method are also disclosed.

Decision feedback equalization for CDMA in indoor wireless communications

Abstract: Commercial interest in Code Division Multiple Access (CDMA) systems has risen dramatically in the last few years. It yields a potential increase in capacity over other access schemes, because it provides protection against interference, multipath, fading, and jamming. Recently, several interference cancellation schemes for CDMA have been proposed but they require information about all interfering active users or some channel parameters. The authors present an adaptive fractionally spaced decision feedback equalizer (DFE) for a CDMA system in an indoor wireless Rayleigh fading environment. This system only uses information about the desired user's spreading code and a training sequence. An analysis on the optimum performance of the DFE receiver shows the advantages of this system over others in terms of capacity improvements. A simulation of this system is also presented to study the convergence properties and implementation considerations of the DFE receiver. Effects on the performance because of sudden birth and death of users in the CDMA system and bit error rate performance of the DFE receiver is also presented. >

Narrowband interference suppression in CDMA spread spectrum communications

Abstract: Spread spectrum (SS) communications offers a promising solution to an overcrowded frequency spectrum amid growing demand for mobile and personal communications services. The proposed overlay of spread spectrum signals on existing narrowband users implies strong interference for the SS system. The paper discusses how system performance can be improved by preprocessing to suppress narrowband interference. Linear prediction filters have been proposed since the 1980s for suppression of narrowband interference. Vijayan and Poor (1991) proposed nonlinear methods of suppressing the narrowband signal with significant increase in the SNR improvement. The present authors derive an enhancement to this nonlinear prediction and achieve further improvement by applying the technique to interpolating filter structures. Finally, they extend results to the case of multiple spread spectrum users and demonstrate how nonlinear filtering can dramatically outperform linear filtering. >

Radio frequency communication network having adaptive parameters

Abstract: Improved apparatus for a radio communication network having a multiplicity of mobile transceiver units selectively in communication with a plurality of base transceiver units which communicate with one or two host computers for storage and manipulation of data collected by bar code scanners or other collection means associated with the mobile transceiver units. The radio network is adaptive in that in order to compensate for the wide range of operating conditions a set of variable network parameters are exchanged between transceivers in the network. These parameters define optimized communication on the network under current network conditions. Examples of such parameters include: the length and frequency of the spreading code in direct-sequence spread spectrum communications; the hop frame length, coding, and interleaving in frequency-hopping spread spectrum communications; the method of source encoding used; and the data packet size in a network using data segmentation. The invention is preferably to be applicable as an upgrade of an existing data capture system wherein a large number of hand-held transceiver units operate over an extensive area to gather data in various places, requiring the use of multiple base stations. In a variety of such installations such as warehouse facilities, distribution centers, and retail establishments, it may be advantageous to utilize not only multiple bases capable of communication with a single host, but with multiple hosts as well.

Spread spectrum telemetry of physiological signals

Abstract: A system for the transmission of physiological signals from a patient to a display analysis and/or recording device using a spread spectrum transmission technique to reduce interference with the detection of the transmitted physiological signal wherein multiple channels of the physiological signals are transmitted over a bandwidth of approximately 3 MHz

Train location and control using spread spectrum radio communications

Abstract: Communication devices and communication network techniques are employed to meet safety-critical, operational requirements for train location and control in railroad and public transit applications. On-board and trackside communication radios participate in a synchronous, time-slotted communication network, providing contentionless and highly responsive access to all participants. Encrypted, validated, and error protected communication links provide reliable and redundant transfer of information among the train, trackside, and control station radios. Spread spectrum communication techniques enable range measurements between all participants. These wireless communication links relay train range measurements to control or processing stations and relay control information (principally speed commands) from the processing stations to the trains. High resolution position location is determined along elevated, at-grade, and in-tunnel tracks using wireless methods and sparse device placement.

System and method for orthogonal spread spectrum sequence generation in variable data rate systems

Abstract: A method and system for allocating a set of orthogonal PN code sequences of variable length among user channels operative at different data rates in a spread spectrum communication system is disclosed herein. PN code sequences are constructed that provide orthogonality between users so that mutual interference will be reduced, thereby allowing higher capacity and better link performance. In an exemplary embodiment, signals are communicated between a cell-site and mobile units using direct sequence spread spectrum communication signals. Information signals communicated on the cell-to-mobile link channels are encoded, interleaved, and modulated with orthogonal covering of each information symbol. Orthogonal Walsh function codes of varying length are employed to modulate the information signals. Code assignments are made on the basis of channel data rates in a manner which results in improved utilization of the available frequency spectrum. A substantially similar modulation scheme may be employed on the mobile-to-cell link.

Neural network techniques for adaptive multiuser demodulation

Abstract: Adaptive methods for performing multiuser demodulation in a direct-sequence spread-spectrum multiple-access (DS/SSMA) communication environment are investigated. In this scenario, the noise is characterized as being the sum of the interfering users' signals and additive Gaussian noise. The optimal receiver for DS/SSMA systems has a complexity that is exponential in the number of users. This prohibitive complexity has spawned the area of research on suboptimal receivers with moderate complexity. Adaptive algorithms for detection allow for reception when the communication environment is either unknown or changing. Motivated by previous work with radial basis functions (RBF's) for performing equalization, RBF networks that operate with knowledge of only a subset of the system parameters are studied. Although this form of detection has been previously studied (group detection) when the system parameters are known, in this work, neural network techniques are employed to adaptively determine unknown system parameters. This approach is further bolstered by the fact that the optimal detector in the synchronous case can be implemented by a RBF network when all of the system parameters are known. The RBF network's performance (with estimated parameters) is compared with the optimal synchronous detector, the decorrelating detector and the single layer perceptron detector. Clustering techniques and adaptive least mean squares methods are investigated to determine the unknown system parameters. This work shows that the adaptive radial basis function network attains near optimal performance and is robust in realistic communication environments. >

Mobile access to an ATM network using a CDMA air interface

Abstract: The paper presents a possible integrated system concept for a direct sequence spread spectrum CDMA radio access system suitable for third-generation mobile radio. The system has been conceived to take account of such diverse services as low bit rate voice and quasi-broadband services at rates of up to 256 kb/s. Broadband services imply the use of the ATM transmission technique, and particular attention is paid to the mutual impact of CDMA and ATM. An efficient automatic repeat request technique is described which gives a suitably low overall error rate and a soft capacity limit. The proposed solution represents a quantum advance on today's CDMA solutions and integrates well with the ATM fixed network. >

Hitless ultra small aperture terminal satellite communication network

Abstract: A satellite communication system between a hub terminal and remote terminals simultaneously uses two or more satellites for communication. Rapid signal acquisition for mobile terminals is achieved by incorporation of a Global Positioning System receiver to establish initial frequency and time references. High resolution time distribution is provided by use of spread spectrum coded signals. A combined Time Division Multiple Access/Code Multiplex architecture receives simultaneous signals of widely varying signal strengths. Rapid and numerous changes to the polling sequence is possible. The hub terminal incorporates two high directivity antennas each illuminating a separate geostationary satellite transponder with a spread spectrum coded signal, each orthogonal to the other. The remote terminals employ an antenna of sufficiently small aperture to enable simultaneous reception of both orthogonal transponder signals. Each remote terminal transmits one signal that illuminates both satellite transponders and is received separately by each hub terminal antenna. All transmissions are synchronized to a precision time frame and are spread spectrum encoded. Multiple remote terminals may transmit simultaneously using orthogonal codes. Block addressing reduces the data bandwidth of the hub terminal. The network architecture allows numerous blocks to be established, each block containing any number of remote terminals and each remote terminal may be assigned to be a member numerous blocks. A single command issued by the hub terminal to a particular block, thereby addresses the multiple remote terminals which are members of that block. In order to demodulate the data received from multiple remote terminals, the hub terminal generates a numerical database that fully represents the composite received signal. Iterative digital signal processing is performed in real-time to establish the code phase of each remote terminal. Initial processing of the database demodulates only strong signals. In an iterative manner, the successfully demodulated signals are subtracted from the database and processing is reperformed to extract the weaker signals.

DS/CDMA successive interference cancellation

Abstract: Conventional DS/CDMA detectors operate by enhancing a desired user while suppressing other users, considered as interference (multiple access interference, MAI) or noise. A different viewpoint is to consider other users not as noise but to jointly detect all users' signals (multiuser detection). This has significant potential of increasing capacity and near/far resistance. Optimal multiuser detection is, however, too complex to implement, thus motivating the search for suboptimal algorithms. Our objective is to underline the need for simplicity and to discuss what is a relatively simple form of multiuser detection (at the base station of a cellular system), successive interference cancellation. The cancellation scheme uses only components already present in a conventional detector. >

Admission policies for integrated voice and data traffic in CDMA packet radio networks

Abstract: The authors derive optimal admission policies for integrated voice and data traffic in packet radio networks employing code division multiple access (CDMA) with direct-sequence spread spectrum (DS/SS) signaling. The network performance is measured in terms of the average blocking probability of voice calls and the average delay and packet loss probability of data messages. The admission scheme determines the number of newly arrived voice users that are accepted in the network so that the long-term blocking probability of voice calls is minimized. In addition, new data arrivals are rejected if the mean delay or the packet loss probability of data exceeds a desirable prespecified level. A semi-Markov decision process (SMDP) is used to model the system operation. Then, a value iteration algorithm is used to derive the optimal admission control. Two models for the other-user interference of the CDMA system are considered: one based on thresholds and another based on the graceful degradation of the CDMA system performance, and their performance is compared. These admission policies find application in emerging commercial CDMA packet radio networks including cellular networks, personal communication networks, and networks of LEO satellites for global communications. >

A noise-whitening approach to multiple access noise rejection .I. Theory and background

Abstract: Minimum probability of bit error is difficult to achieve in a DS-CDMA receiver. Since multiple-access noise is the sum of many independent random processes, it is reasonable to approximate it by a Gaussian process of the same power spectral density. This leads to the criterion of maximizing signal-to-noise ratio (SNR). In this paper, receivers that maximize SNR in a particular DS-CDMA system model under various constraints are proposed and analyzed. The method proposed here does not require locking and despreading multiple arriving CDMA signals. The maximization of SNR is compared with the minimization of probability of error, when the receiver is constrained to operate bit-by-bit, in the absence of knowledge of the other users' spreading codes, timing, and phase. >

Multi-user spread spectrum communication system

Abstract: Multi-user communications system which combines means (12, 22) (12B, 22B) implementing the technique of spread spectrum coding SSC with means (14n, 24n) (14Bn, 24Bm) implementing the technique of orthogonal frequency division multiplexing OFDM. This makes it possible to enhance the performance of the SSC system by simplifying certain data acquisition and processing mechanisms. One favoured use is in systems for telephone communication between mobile stations (U1, U2) and base stations (B). Application: telephone communications system.

Effects of diversity power control, and bandwidth an the capacity of microcellular CDMA systems

Abstract: We evaluate the capacity and bandwidth efficiency of microcellular CDMA systems. Power control, multipath diversity system bandwidth, and path loss exponent are seen to have a major impact on the capacity. The CDMA system considered uses convolutional codes, orthogonal signalling, multipath/antenna diversity with noncoherent combining, and fast closed-loop power control on the uplink (portable-to-base) direction. On the downlink (base-to-portable), convolutional codes, BPSK modulation with pilot-signal-assisted coherent reception, and multipath diversity are employed. Both fast and slow power control are considered for the downlink. The capacity of the CDMA system is evaluated in a multicell environment taking into account shadow fading, path loss, fast fading, and closed-loop power control. Fast power control on the downlink increases the capacity significantly. Capacity is also significantly impacted by the path loss exponent. Narrowband CDMA (system bandwidth of 1.25 MHz) requires artificial multipath generation on the downlink to achieve adequate capacity. For smaller path loss exponents, which are more likely in microcellular environments, artificial multipath diversity of an order of as high as 4 may be needed. Wideband CDMA systems (10 MHz bandwidth) achieve greater efficiencies in terms of capacity per MHz. >

Multi-path resistant frequency-hopped spread spectrum mobile location system

Abstract: A multi-path resistant frequency-hopped spread spectrum mobile vehicle or personal location system is described which provides low cost manufacture and low power operation while still enabling the accurate location of the mobile unit over long distances and in moderate to severe multi-path conditions. The frequency-hopped spread spectrum mobile vehicle or personal location system consists of a central station, a plurality of base stations and a plurality of mobile transmitters which transmit using a frequency-hopped spread-spectrum differential bi-phase shift keying communication signal. Frequency Shift Keying modulation may also be used. Each of the plurality of base stations include an array of receiving dipole antennas and employs special algorithms for retrieving very low power frequency-hopped spread spectrum signals in a noisy and multi-path environment. The base stations use computational algorithms for determining the phase difference between each of receiving dipole antennas to determine the direction of the transmitter relative to the location of the respective base station. The multiple direction of arrival angles of the received signal at each base station are corrected based on an n-dimensional ambiguity space to locate most probable angles of arrival. The ambiguity space plot is used to eliminate erroneous comparisons of dissimilar phases of the incoming signal. The most probable values are then tracked over multiple frequency hops and a histogram analysis of the strongest surviving angles of arrival is performed. The two peaks of the histogram are used as the two most probable directions of arrival. Each base station then communicates the two relative directions of the transmitter to a central station where the location of the transmitter is determined by triangulation. The direction of arrival angles from the multiple base stations are summed in a least mean square approach to find a single direction of arrival which is then plotted on a gnomonic projection to correct for the curvature of the earth.

Direct sequence spread spectrum matched filter acquisition in frequency-selective Rayleigh fading channels

Abstract: This paper presents the performance of a matched filter acquisition scheme in nonselective and frequency-selective Rayleigh fading channels. This acquisition scheme employs a reference filter to estimate the variance of the interference at the output of the detecting matched filter. Expressions for probability of detection and probability of false alarm are derived and the mean acquisition time is evaluated for various channel and system parameters. The results presented show the relationship between channel and system parameters and the system performance. >

Spread spectrum microwave overlay with notch filter

Abstract: A spread spectrum CDMA communications system for communicating data and/or digitized voice between a plurality of users to a plurality of PCN units. The spread spectrum communications system is located within a same geographical region as occupied by an existing microwave system. The spread spectrum can operate without causing undue interference to the microwave system and the microwave system can operate without causing significant interference, less than 1% total users affected, to the PCN system. The spread spectrum CDMA communications system includes a plurality of PCN-base stations and a plurality of PCN units. A PCN-base station has a device for converting the format of the data into a form suitable for communicating over radio waves, a spread spectrum modulator for spread spectrum processing the data, a notch filter, and a transmitter for transmitting the spread spectrum processed converted data from the PCN-base station to a PCN unit. The PCN-base station also has an antenna, and spread spectrum detectors for recovering data communicated from the PCN units. A PCN unit has an antenna, and a detector coupled to the antenna for recovering data communicated from the PCN-base station. The detector includes a spread spectrum demodulator. Also, the PCN unit has a device for converting the format of the data into a form suitable for communicating over radio waves, a spread spectrum modulator, an adjustable notch filter and a transmitter.