Showing papers on "Spread spectrum published in 1988"

Interference rejection techniques in spread spectrum communications

Abstract: It is argued that the ability of a spread-spectrum system to withstand interference, both intentional and unintentional, is probably its greatest asset. Any spread spectrum receiver can only suppress a given amount of interference; if the level of interference becomes too great, the system will not function properly. Even under these latter circumstances, however, other techniques, which enhance the performance of the system over and above the performance improvement that comes automatically to systems from using spread spectrum, are available for use. These techniques typically involve some type of additional signal processing and are examined here. Two general types of narrowband interference suppression schemes are discussed and an overview is presented for several other techniques. The two classes of rejection schemes emphasized are (1) those based on least-mean-square estimation techniques, and (2) those based on transform-domain processing structures. >

Secondary channel for digital modems using spread spectrum subliminal induced modulation

Abstract: A system and method are provided for use of a secondary channel in high speed (greater than 2400 bits per second) wire line modems to provide low speed (around 110 bits per second) non-interfering communications for control or monitoring functions. The secondary channel is used to monitor, configure and test the modems in the network using commands sent over the secondary channel. Secondary channels are also used in low duty cycle applications such as in alarms or teletype data transmission. The invention provides a spread spectrum secondary channel which occupies the same bandwidth as the primary data channel and is immune to jamming tones that would interfere with conventional frequency shift keying secondary channels. The spectrum spreading is achieved through use of the primary channel's trellis encoder. Phase shifts of 0°, 90°, 180° or 270° are induced depending upon the trellis state. This rotation simplifies decoding which as a result can be done in the reference system of the decoded primary trellis signals instead of the baseband equalizer reference system. The primary trellis decoder automatically resolves the 90° references. There is no penalty in terms of additional circuitry or software because the transmitter spreading eliminates the need for additional rotation in the receiver which is required for non-spread signals.

Single-user detectors for multiuser channels

Abstract: Optimum decentralized demodulation for asynchronous Gaussian multiaccess channels is considered. It is assumed that the receiver is the destination of the information transmitted by only one active user, and single-user detectors that take into account the existence of the other active users in the channel are obtained. The problem considered is one of signal detection in additive colored nonGaussian noise, and attention is focused on one-shot structures where detection of each symbol is based only on the received process during its corresponding interval. Particular emphasis is placed on asymptotically optimum detectors for weak interferers, for CDMA (code-division multiple-access) signature waveforms with long spreading codes, and for low background Gaussian noise level. >

Performance of DS/SSMA communications in impulsive channels. II. Hard-limiting correlation receivers

Abstract: For pt.I see ibid., vol.COM-35, no.11, p.1179-88 (1987). In part I it was demonstrated that impulsive channel noise can be a serious detriment to the performance of direct-sequence spread-spectrum multiple-access (DS/SSMA) communications when conventional linear correlation reception is used. Here, a hard-limiting correlator as an alternative for reception of multiple-access transmission in impulsive channels is considered. For K asynchronous binary PSK DS/SSMA users sharing a linear channel corrupted by impulsive noise that is modeled at the output of the front-end filter of the receiver, techniques are developed for analyzing bit error probabilities of this hard-limiting receiver by exact computation for short spreading sequences, by approximation for longer spreading sequences, and by asymptotic limits for infinitely long spreading sequences. Performance is compared to that of the linear correlator under a variety of conditions, showing that hard-limiting correlation reception can offer substantial improvement over conventional systems in impulsive channels. However, the linear receiver is more effective against multiple-access noise only, and so a tradeoff emerges between rejection of impulsive noise and rejection of multiple-access interference. >

Using spread-spectrum in a high-capacity fiber-optic local network

Abstract: A local fiber-optic communications network capable of supporting tens of thousands of simultaneous users, each requiring on the order of 10-Mb/s continuous data rate, is proposed. The system uses coherent optical techniques to fully utilize the vast bandwidth offered by single-mode optical fibers (tens of thousands of GHz) a spread-spectrum technique is used to circumvent the problem caused by the instabilities of present-day semiconductor lasers. These include difficulty in reliably setting a laser's frequency with an accuracy better than several hundred GHz, and phase noise in the laser output, which would otherwise result in excessive amounts of interference among the various users. The method proposed is a variant of code-division multiple access (CDMA) that is called random-carrier (RC) CDMA, since the modulated carriers can be assumed to be completely randomly placed in the available optical band. >

Earth station capable of effectively using a frequency band of a satellite

Abstract: For use in a satellite communication system which carries out communication through a satellite by the use of an up-link frequency band and a down-link frequency band, each of the up-link and the down-link frequency bands has a plurality of frequency subbands spaced apart from one another with frequency gaps interposed between the frequency subbands. An earth station comprises a modulating arrangement (47) for modulating a selected one of the frequency subbands of the up-link frequency band by a first input signal into a subband transmission signal, a first transmitting arrangement (51) coupled to the modulating arrangement for transmitting the subband transmission signal through the selected one of the frequency subbands, a spread spectrum processing arrangement (50) for processing a second input signal into a spread spectrum transmission signal in the up-link frequency band, and a second transmitting arrangement (51) coupled to the spread spectrum processing arrangement for transmitting the spread spectrum transmission signal through the up-link frequency band.

Self-synchronous spread spectrum transmitter/receiver

Abstract: The present invention relates to a self-synchronous spread spectrum transmitter and an associated remote spread spectrum receiver which communicate with each other by the transmission of both (1) a spread spectrum Pseudo Noise Code (PNC) sequence signal, and (2) a combined PNC sequence plus the data information (PNC+data) signal. The two signals can be transmitted concurrently using either different frequency bands or on a quadrature carrier; or the two signals can be transmitted with a time offset between signals. At the receiver the PNC and the PN+data signals are separately recovered for the case of the concurrent transmission techniques, and directly mixed to despread the received signal and recover the data signal at baseband. For the time offset technique, the delay provided in one of the time offset signals is again introduced to the previously delayed signal, and the delayed and undelayed signal portions of the received combined signal are directly mixed and then low pass filtered to recover the data signal. Such techniques eliminate the necessity for providing PNC acquisition and tracking circuits in associated spread spectrum transmitter and receiver combinations.

Selective fading on LOS microwave links: classical and spread-spectrum measurement techniques

Abstract: A survey is presented of techniques for measuring selective fading on line-of-sight (LOS) microwave links, particularly over a wide band. Classical pulse methods, including pulse compression, as well as the much used frequency-sweep approach are covered. Methods utilizing both frequency-hopping and direct-sequence spread-spectrum transmissions are also discussed. An analysis of this latter method (used by the US National Bureau of Standards and by the authors) is included and a relationship is established between the true channel transmittance and that estimated by this approach. The advantages and disadvantages of the various schemes are outlined with the aim of assisting an experimenter in the choice of technique in a given experimental context. >

Initial synchronization in spread spectrum receiver

Abstract: An initial synchronization system for spread spectrum communication including a despreader-demodulator circuit for despreading and demodulating an input signal according to a despread control signal, a decoder circuit for decoding a data signal demodulated by the despreader-demodulator circuit and outputting a code synchronization signal when code synchronization is taken, a correlation detector circuit in response to the input signal for detecting the correlation of the input signal and generating the despread control signal and a correlation detection signal, and a control circuit for outputting an initial synchronization start signal to the correlation detector circuit so as to cause the correlation detector circuit to start the initial synchronization if the code synchronization signal is not generated within a predetermined period after the correlation detection signal was generated.

Delay-locked loop circuit in spread spectrum receiver

Abstract: Spread spectrum receiver includes a delay locked loop (DLL) circuit for holding the synchronization of a despread signal with a received spread spectrum signal. The DLL circuit comprises a subtracter for generating a signal representative of a subtraction between first and second correlation signals, a positive feedback amplifier for amplifying the subtraction representative signal and a window comparator for producing a lock signal when the subtraction representative signal is positioned within a window width.

Interference suppression to aid acquisition in direct-sequence spread-spectrum communications

Abstract: The acquisition and tracking systems of a spread-spectrum receiver are probably the most critical components of the receiver, since if they fail to function properly, it is doubtful that the desired signal can be successfully detected. This means that the affect of interference (such as jamming) on the receiver while it is attempting to learn the correct phase position of the incoming code might be especially harmful, since the interference might not allow the receiver to acquire the signal. To address this problem, a narrow-band interference suppression filter is used to enhance the performance of a serial search acquisition scheme for a direct-sequence spread-spectrum receiver. Analytical expressions for the probabilities of error in both the search and lock modes are derived, and numerical results are used to illustrate the sensitivity of the receiver to various system parameters. It is shown that the presence of the rejection filter can be significantly improves the performance of the acquisition system. >

Performance of a class of parallel spread-spectrum code acquisition schemes in the presence of data modulation

Abstract: An acquisition procedure for data-modulated direct-sequence spread-spectrum systems is investigated. The correlation time is partitioned into subintervals, and the integration results in these subintervals are noncoherently combined for detection. The tradeoff between noncoherent combining loss and data modulation degradation guides the optimum choice of the number of subintervals. Two forms of data modulation are considered, namely, the alternate-data and random-data cases. The parallel acquisition schemes discussed allow multiple code-phase offsets to be examined at each test. The circular state diagram approach is used to analyze the performances of these schemes. The theory presented is valid for a class of such parallel schemes. >

Diversity combining in FH/BFSK systems to combat partial band jamming

Abstract: A combiner for a frequency-hopped binary frequency-shift keyed (FH/BFSK) system, called the product-combining receiver (PCR), is introduced. The performance of the PCR is evaluated for the cases of an on/off partial-band noise with optimum-jamming fraction, and worst-case, partial-band tone jamming. The performance of PCR is shown to be comparable to that of the clipper receiver. The effect of diversity combining, along with convolutional coding and ratio-threshold technique, is analyzed. Whereas the clipper requires the knowledge of signal-to-noise ratio for threshold adjustments, the PCR does not require this knowledge for this operation. >

Differential correlator for spread spectrum communication system

Abstract: A spread spectrum communications system includes an encoder for differentially encoding a spread spectrum spreading code sequence in accordance with an input data signal. Each chip of the spreading code sequence is inverted, or not inverted, relative to the polarity of a corresponding chip of the spreading code sequence a fixed time delay previously, depending on whether the input data is a logic one or zero, respectively. At the receiver, the data is recovered in a differential data decoder wherein the presently received chip of the spread spectrum signal and a corresponding previously received chip of spread spectrum signal, received a fixed time delay previously, are compared one chip at a time. Since it is the spreading code sequence that is differentially encoded and differentially decoded, there is no need for a synchronized code sequence generator at the receiver, and data synchronization is achieved after one cycle time of the received spread spectrum spreading code sequence has elapsed.

Automatic decision threshold level control (ADTLC) in direct-sequence spread-spectrum systems based on matched filtering

Abstract: An automatic decision threshold level control (ADTLC) algorithm for direct-sequence spread-spectrum systems is described and analyzed. Two parameters of interest, probability of false alarm P/sub a/ and probability of correct signal detection P/sub d/, are controlled simultaneously. As an example, the implementation of the algorithm in several practical direct-sequence spread-spectrum receivers is discussed. A considerable advantage of this algorithm compared to the classical approach (constant false alarm probability control) to the problem is demonstrated. >

Error-correcting codes and nonlinear diversity combining against worst case partial-band noise jamming of frequency-hopping MFSK systems

Abstract: Error probability analyses are performed for a coded M-ary frequency-shift keying system (MFSK) using L hops per M-ary word frequency-hopping spread-spectrum waveforms transmitted over a partial-band Gaussian noise jamming channel. The bit error probabilities are obtained for a square-law adaptive gain control receiver with forward-error-control coding under conditions of worst-case partial-band noise jamming. Both thermal noise and jamming noise are included in the analyses. Performance curves are obtained for both block codes and convolutional codes with both binary and M-ary channel modulations. The results show that thermal noise cannot be neglected in the analysis if correct determinations of the optimum order of diversity and the worst-case jamming fraction are to be obtained. It is shown that the combination of nonlinear combining, M-ary modulation, and forward-error-control coding is effective against worst-case partial-band noise jamming. >

Computationally efficient bounds for the performance of direct-sequence spread-spectrum multiple-access communications systems in jamming environments

Abstract: An approach for calculating upper and lower bounds for the probability of error for asynchronous multiple-access spread-spectrum communication systems using deterministic codes is presented. The techniques is then generalized to include multiple-tone jamming. The approach utilizes the cumulative distribution function of individual interference terms. The computational complexity of the technique is calculated to the polynomial-like. Results showing the multiple-access performance of gold codes of lengths 31 and 127 in the presence of jamming are shown. The fact that this computational technique gives upper and lower bounds is rigorously proved using the Riemann-Stieltjes integral. >

Time-frequency hop codes based upon extended quadratic congruences

Abstract: Time-frequency hop codes are developed that can be used for coherent multiuser echolocation and asynchronous spread spectrum communication systems. They represent a compromise between Costas codes, which have nearly ideal autoambiguity but not so good cross-ambiguity properties, and linear congruential codes, which have nearly ideal cross-ambiguity but unattractive autoambiguity properties. Extended quadratic congruential code words are shown to have reasonably good autoambiguity and cross-ambiguity properties across the whole class of code sets considered. A uniform upper bound is placed on the entire cross-ambiguity function surface, and bounds are placed on the position and amplitude of spurious peaks in the autoambiguity function. These bounds depend on the time/bandwidth product and code length exclusively and lead naturally to a discussion of the design tradeoffs for these two parameters. Examples of typical autoambiguity and cross-ambiguity functions are given to illustrate the performance of the new codes. >

Generalized minimum shift-keying modulation techniques

Abstract: The simultaneous data demodulation and phase tracking of an MSK signal using the Viterbi algorithm is described, and two variations of MSK modulation are studied. The MSK with overlay is a dual-rate modulation techniques in which low-rate and high-rate data are superimposed on an MSK signal. Here the demodulator uses the Viterbi algorithm to estimate both the low-rate and high-rate data simultaneously. The MSK with pseudorandom sequence spreading combats intentional or unintentional jamming. A simplified receiver for these spread-spectrum MSK signals is found that takes into consideration the effect of random phase perturbations. The performance of these demodulators is evaluated using transfer-function bounds for the bit error probability. For demodulation of the spread spectrum MSK signal, a simplified receiver is derived, and its performance in the presence of continuous jamming is evaluated. >

Spread-spectrum multiple-access performance of orthogonal codes: impulsive noise

Abstract: A direct-sequence spread-spectrum multiple-access (SSMA) communication system that assigned a set of M-orthogonal sequences to each user is analyzed. An accurate model is incorporated for the impulsive noise that characterizes the LF and MF bands, so that the SSMA receiver operates in a combination of multiple-access interference and impulsive (atmospheric) noise. The performance of a linear receiver operating in such an environment is analyzed, and probability-of-error curves are presented. The presence of impulsive noise motivates the derivation and analysis of a nonlinear receiver that use a variable-gain stage to suppress noise impulses. This receiver is effectively optimum when the signal amplitudes are below a certain bound and when the noise and interference samples are independent, or nearly so. However, the gain stage of this nearly optimum receiver depends on the noise model parameters including the various user delays. Consequently, a nonparametric receiver that incorporates a simple clipper is also analyzed. The asymptotic relative efficiency of both receivers is determined. >

Rejection of multiple narrow-band interference in both BPSK and QPSK DS spread-spectrum systems

Abstract: The performance of direct-sequence spread-spectrum systems using a suppression filter in the presence of multiple narrow band interference with rational spectral densities is analyzed. Both BPSK and QPSK systems are considered, and analytical expressions for both the means-square error of the filter output and the performance improvement are established. When the bandwidths of the narrow band components of the interference are all small, approximate expressions are obtained and used to provide further insight into the behavior of the system. Results are presented for the limiting case when the bandwidths approach aero (i.e. when multiple narrow band interference becomes multiple sinusoidal interference). >

Adaptive nonlinear suppression of interference

Abstract: A signal processing technique designed to suppress interference in direct sequence spread spectrum receiving systems is described. The basic idea is to optimize the detection process dynamically in the presence of current interference. This is accomplished by estimating the statistics of the interference and then using this information to derive a nonlinear transform to apply to the signal-plus-noise. As the statistics of the interference change, the measured distributions and the resulting transformations do so also. This adaptation is open-loop, thus avoiding convergence problems. It is concluded that large improvements (tens of dB) can be achieved for a wider range of interference modulations than can be handled by any previously realized technique known to the author. >

Receiver for spread spectrum communication

Abstract: A receiver for spread spectrum communication is disclosed, which is provided with two convolvers and in which a first and a second reference signal, each of which consists of a 4-phase-modulated signal formed on the basis of a first PN code, a second PN code and a first CW signal, are supplied to a second input of each of the convolvers, respectively, while a received signal is supplied to a first input of each of the convolvers so that the correlation output of each of the convolvers is decoded so as to obtain an information signal.

Spread spectrum multiple access system with intrasystem interference cancellation

Abstract: Analyse theorique et resultats experimentaux d'une technique d'annulation des interferences intrasysteme des systemes a acces multiples et spectre etale afin d'ameliorer leurs performances en operations simultanees

Automatic gain control system

Abstract: A spread spectrum receiver includes, for example, a correlator consisting of two convolvers whose correlation outputs are applied to a demodulator respectively via two variable gain amplifiers whose gains are controlled in response to a demodulation output from the demodulator.

Wide-band packet radio for multipath environments

Abstract: A direct-sequence spread-spectrum packet radio is described that has versatile signal-processing and local-control capabilities designed to support the functions required of a robust mobile communications network. Noteworthy capabilities include eleven selectable data rates with accurate range measurements in a fading multipath channel. The radio uses a hybrid analog/digital signal processor and nonrepeating spreading codes for suppression of intersymbol interference and jamming. It incorporates two sets of monolithic surface-acoustic-wave convolvers as programmable matched filters with time-bandwidth products of 64 and 2000. The analog matched filters are coupled with binary postprocessing for the functions of detection, RAKE demodulations and ranging measurements over a wide multipath spread. The data rate can be varied in response to channel conditions from 1.45 Mb/s down to 44 b/s with an almost ideal tradeoff in signal-processing gain from 18 dB up to 61 dB prior to multipath combining. >

Spread spectrum multiple access performance of orthogonal codes in fading multipath channels

Abstract: Indoor wireless communications (IWC) is an attractive alternative to wire-based local area networks. However, the indoor environment is characterized by fading multipath channels. Direct-sequence spread-spectrum techniques, with their inherent resistance to multipaths, are promising for IWC. To allow multiple users within a limited bandwidth, code division multiple access (CDMA) is needed. The authors analyze one relevant CDMA technique. A set of M-orthogonal sequences of length N is assigned to each of K users. The sequences consist of phase-modulated signals on the same carrier frequency. The bandwidth efficiency achieved over fading multipath channels is the presence of additive white Gaussian noise and multiple-access interference is determined. >

An iterative detector for asynchronous spread-spectrum multiple-access systems

Abstract: Proposes and analyzes an iterative multiuser detector for coherent demodulation in an asynchronous spread spectrum multiple access (SSMA) system. This detector processes the sufficient statistics using an iterative algorithm which is based on a multiple access interference annihilation strategy. An efficient real-time implementation of the iterative algorithm with a fixed decoding delay is obtained. This implementation requires a computational complexity per symbol which is linear in the number of users. An exact probability of error analysis shows that even the two-stage detector is particularly well suited for near-far situations. Its performance approaches that of single-user communications as the interfering signals become stronger. It is shown that significant performance gains over the conventional detector are obtained for relatively high bandwidth efficiency situations. >

Rapid acquisition scheme for spread-spectrum radio in a fading environment

Abstract: A modified two-level scheme for coarse code acquisition of spread-spectrum signals in a fading environment is considered. The scheme uses a bank of passive correlators followed by a bank of active correlators. The passive correlators are matched to short sync prefixes, which areinterspersed in the incoming signal. The passive devices correlate over sliding windows (in time), providing a rapid search capability. Threshold exceedances of the passive correlators areused to initiate active correlation over longer time intervals so that high reliability of decision is obtained. The multiple sync prefixes reduce the probability of missing the signal in a fading environment. An analytical model based on queueing and detection theory results is developed, and optimisation of the scheme is treated. A Gilbert model is used to characterise the signal fading on the channel. The performance in the presence of background noise with noisejamming is analysed. Theoretical performance curves and a discussion of the results are also presented.