Wavelet Notch Filter Design of Spread-Spectrum Communication Systems for High-Precision Wireless Positioning

Spread-spectrum communication systems are now commonly used in the field of cellular telephone positioning. However, wireless positioning systems by low-power spread-spectrum communication are extremely vulnerable to high-power interference, which limits achievable measurement precision. In this paper, a bandwidth variable wavelet notch filter design method is proposed to suppress a large number of jammers in multiple locations with noise interfering with spread-spectrum systems. The filter uses combinations of Gaussian wavelets with optimal time-frequency localization and computational efficiency for real-time operation of denoising. The performance of the adaptive filter has been evaluated by experiments associated with a spread-spectrum communication system model employing a reliable noise detector to locate the filter notch. Experimental results demonstrate that the proposed wavelet notch filter removes the narrow-band interference in accordance with the corrupted frequency contents while minimizing signal distortion and information loss, which leads to high-precision wireless positioning.     

Active interference suppression in CDMA overlay systems

Communication networks involving the overlay of spread-spectrum systems an narrower band services are of increasing interest as a means of producing greater efficiencies and flexibility in the use of the radio spectrum. Although spread-spectrum systems enjoy a natural immunity to interference from narrowband sources, their performance in the presence of such interference can be significantly enhanced by active suppression techniques. The study of this problem has elicited a very rich body of methodology, which has progressed over nearly 25 years from some of the simplest signal processing paradigms to some of the most advanced. This paper provides an overview of a number of these techniques, most of which have been developed over the past decade. In particular, a progression of techniques is described, in which successively more information about the spread-spectrum signal and interference is used to make improvements on the interference suppression capability via more advanced signal processing methods. These include linear predictive methods that make use of the spectral properties of the spread-spectrum and narrowband signals, nonlinear predictive methods that make use of the spectra and first-order probability distribution of these signals, linear code-aided methods that make use of the spreading codes of the signals of interest and the second-order statistics of the narrowband interference, and finally, a maximum-likelihood code-aided technique that makes use of essentially all that is known about the useful signals and interference. Performance comparisons show that moving up this progression of improved modeling is rewarded with performance gains that can be quite significant     

A spread-spectrum based synchronization technique for digitalbroadcast systems

A technique that has been verified experimentally to improve the synchronization efficiency of digital communication systems over present systems without sacrificing reliability is presented. This technique is called the spread-spectrum system pilot technique (SSPT). Incorporated into this technique are elements of tone-calibrated techniques (TCT) and spread-spectrum systems. In the SSPT system, the desired pilot tone is modulated by a pseudorandom binary sequence (PRBS) generator of finite duration and then linearly added to the source (customer) data that is actually being sent. At the receiver, the same finite PRBS sequence is used to decode the received signal. The system works because it provides an accurate version of the actual data carrier to be used in demodulation of the received signal. Based on the research presented, it is expected that this type of system will lead to better performance than currently used transparent-tone-in-band and other techniques used in multipath fading environments, which are especially problematic in mobile broadcast and communication applications     

Triple correlation analysis of binary sequences for codeworddetection

It is well known that multiple correlations of deterministic signals allow information about those signals to be extracted. The paper describes the application of triple correlation techniques to identify characteristics of several binary cyclic sequences commonly used in communications systems, especially those used in spread-spectrum systems. It is shown that the feedback function for a linear feedback shift register which generates an m-sequence can be found through the use of triple correlation. Examples of how to identify the feedback function when noise and data are present on the spread-spectrum signal are given     

A comparison of slow-frequency-hop and direct-sequencespread-spectrum communications over frequency-selective fading channels

Spread-spectrum modulation can provide protection from the selective fading that is typically encountered in mobile radio networks. Because the methods of combating frequency-selective fading are quite different for slow-frequency-hop (SFH) and direct-sequence (DS) spread spectrum systems, these two types of modulation perform very differently. The purpose of this paper is to compare the performance of SFH and DS systems under identical conditions for several models of the wide-band fading channel. Each system has the same bandwidth, transmits over the same frequency-selective Gaussian wide-sense-stationary uncorrelated-scattering channel, and uses error-correction coding to combat thermal noise and fading. The probability of bit error at the output of the decoder is determined for each system by a combination of analysis and simulation. Results are presented for systems with a single transmitter-receiver pair and for networks with multiple simultaneous transmissions (i.e., multiple-access communications). The multiple-access network is distributed, so that control of power by a central terminal, such as a base station, is not possible. The results illustrate the tradeoffs in performance between SFW and DS spread-spectrum systems as a function of the parameters of the signals and the channel model. The performance of the SFH system is shown to be less sensitive to the exact characterization of the channel delay spectrum than the DS system. For most of the channels considered in this paper, SFH spread spectrum gives better performance than DS spread spectrum     

One-coincidence sequences with specified distance between adjacentsymbols for frequency-hopping multiple access

Frequency hopping spread-spectrum systems have been popular in multiple-access communication systems. In such frequency-hopping multiple-access systems, multilevel sequences are used to specify which frequency will be used for transmission at any given time. One-coincidence sequences with specified distance between adjacent symbols for frequency-hopping multiple access are described. In such sequences, the distances between any adjacent symbols in any sequence are more than some specified amount. A theoretical bound is derived for the sequences, and two theorems on the construction of the sequences are given     

A class of frequency hop codes with nearly ideal characteristicsfor use in multiple-access spread-spectrum communications and radar andsonar systems

The problem of constructing frequency hop codes for use in multiuser communication systems such as multiple-access spread-spectrum communications and multiuser radar and sonar systems is addressed. Previous frequency hopping techniques are reviewed. The construction of a new family of frequency hopping codes called hyperbolic frequency hop codes is given. The concepts of multiple-access spread-spectrum communication systems and multiuser radar and sonar systems are reviewed, and it is shown that the hyperbolic frequency hop codes possess nearly ideal characteristics for use in both types of system. Specifically, in multiple-access communications the codes achieve minimum error probability, while in radar and sonar systems the codes have at most two hits in their auto- and cross-ambiguity function. Examples of address assignment for multiple-access communications systems and radar and sonar auto- and cross-ambiguity functions are also given     

飞行器的测控及遥感图像传输体制与系统

本文讨论飞行器常用的三种测控及图像传输体制，即分散体制，统一载波体制，统一扩频体讨论的重点是后两种体制，文中简述了各种体制的原理，比较了各种体制的优缺点，讨论了关键问题，及多种派生技术。     

Broadcast capability of direct sequence and hybrid spread spectrum

Two forms of spread-spectrum signaling, namely direct-sequence and hybrid (direct-sequence/frequency-hopped) are shown to provide high broadcast capability, especially when used in conjunction with forward-error-control coding schemes. The broadcast capability is defined as the maximum number of simultaneous distinct messages that can be transmitted to distant receivers from a single transmitter at a given bit-error rate. The quantity provides a useful measure of the capacity of hub-to-mobile or satellite-to-earth-station links of communication networks. When bursty data or voice traffic is dominant in such networks, the above forms of spread-spectrum CDMA provide a viable alternative to FDMA or TDMA. Ways of multiplexing the direct-sequence and hybrid signals are presented that use, respectively, distinct carriers, distinct pairs of orthogonal carriers, and only two orthogonal carriers for broadcasting the different messages. Systems with chip-synchronous signals and systems with random delays between the signals are considered. The average error probability of all systems is evaluated using the characteristic-function and Gaussian-approximation techniques. Besides the uncoded systems, systems using Reed-Solomon and convolutional codes are analyzed. A comparison of the broadcast capability of the different schemes is presented     

Performance analysis of DS-UWB multiple access under imperfect power control

Ultra-wideband (UWB) technology is characterized by transmitting extremely short duration radio impulses. To improve its multiple-access (MA) capability, UWB technology can be combined with traditional spread-spectrum techniques. So far, much of the research has focused on perfect power-control conditions and on employing time-hopping spread-spectrum with impulse radio using pulse-position modulated signals. In this letter, we outline the attractive features of direct-sequence (DS) UWB MA systems employing antipodal signaling and compare it with time hopping. Performance of DS-UWB communication systems, in terms of MA capability and bit-error rate performance, are evaluated under both perfect and imperfect power-control conditions. An upper bound on the total combined bit-transmission rate is derived.     

Low Probability of Intercept Performance Bounds for Spread-Spectrum Systems

An important application of spread-spectrum (SS) in military communication systems is that of making the signal difficult to intercept by unauthorized receivers. Error-correction coding techniques may be employed to reduce the required input signal-to-noise ratio at the receiver, allowing the system to operate at a lower power level, hence resulting in a lower probability of interception. In the absence of error-correction coding, a specified message bit rate, transmitted signal bandwidth, and required bit-error probability at the receiver will yield a required signal-to-noise ratio for each type of spread-spectrum system. When coding is employed, a specified decoder delay time implies a specified code complexity, and can be used to determine bounds on the required receiver signal-to-noise ratio. These performance bounds are evaluated as functions of the specified parameters. In addition, one may specify a burst interference environment in that the system must operate. Presented is the evaluation of the increase in the upper bound on signal-to-noise ratio as a result of the specification of a correctable single-burst time. This increase indicates an antiintercept /antijam tradeoff.     

Trellis-coded direct-sequence spread-spectrum communications

This paper considers the application of trellis coding techniques to direct-sequence spread-spectrum multiple-access (DS/SSMA) communication. The unique feature of the trellis codes considered is that they are constructed over the set of possible signature sequences rather than over some standard 2-D signal constellation. The resulting codes have a small number of signals per dimension. We present several examples of these trellis codes, and suggest possible methods of implementation. We also present a detailed error analysis for this system, which employs techniques developed by Lehnert and Pursley (1987, 1989)) to accurately model the multiple access interference. We generate numerical results for several examples and conclude that the proposed trellis coded systems yield significant performance improvements over binary antipodal DS/SSMA systems. In addition, the new trellis codes perform better than standard error control techniques with the same complexity and code rate. Analytic results are verified with simulations     

Spread-spectrum techniques in optical communication using transformdomain processing

A method of applying spread-spectrum techniques to optical communication is presented. The interference-suppression capability of spread-spectrum systems is shown to be enhanced by optical transform domain processing, and a design for spectral coding at optical frequencies is given. The encoding system is based on optical pulse compression and shaping. Several possible implementations of this system are suggested, and applications to fiber optics, laser radar, free-space optical communications, and other systems are discussed     

Coexistence of chaos-based and conventional digital communication systems of equal bit rate

Chaos-based communication systems represent a new category of spread-spectrum communication systems, whose working principle differs significantly from conventional direct-sequence and frequency-hopping spread-spectrum systems. However, like all other kinds of spread-spectrum systems, chaos-based systems are required to provide reasonable bit error performance in the presence of a narrow-band signal which can be generated from an intruder or a coexisting conventional communication system. In particular, the frequency band of this foreign narrow-band signal can fall within the bandwidth of the chaos-based system in question. Such a scenario may occur in normal practice when chaos-based systems are introduced while the conventional systems are still in operation. It is therefore important to examine the coexistence of chaos-based and conventional systems. The objective of this paper is to evaluate the performance of the chaos-based system when its bandwidth overlaps with that of a coexisting conventional system. In particular, the chaos-based systems under study are the coherent chaos shift keying (CSK) system and the noncoherent differential CSK (DCSK) system, whereas the conventional system used in the study employs the standard binary phase shift keying scheme. Also, both the chaos-based and conventional systems are assumed to have identical data rates. Analytical expressions for the bit-error rates are derived, permitting evaluation of performance for different noise levels, power ratios and spreading factors. Finally, results from computer simulations verify the analytical findings.     

Multiservice/multirate pure CDMA for mobile communications

Future mobile communication systems should be able to support a wide range of services with different bit rates. Spread-spectrum code-division multiple-access (CDMA) techniques have attracted much attention to be employed in such a system. Different techniques of CDMA could be used to map low-, medium-, and high-bit rates data into the same allocated bandwidth, including pure or wide-band CDMA, FDM/FH/CDMA, TDM/TH/CDMA, or a hybrid of these techniques. This paper investigates multirate pure CDMA using multiuser interference statistics derived for both Gaussian and Rayleigh fading channels. Approximation of multiuser/multipath interference, in general, helps in the theoretical approach to error performance evaluation and, in particular, is quite useful for simulation approach in a fading channel. Some results of a multirate pure CDMA system with two services (low- and high-bit rates), for both BPSK and DPSK modulation schemes, are presented and compared     

Bit-error probabilities of multicode direct-sequencespread-spectrum multiple-access systems

Many techniques for calculating bit-error probabilities (BEPs) of direct-sequence spread-spectrum multiple-access systems (DS-SSMA) have been reported. Among them are the following three techniques: (1) the standard Gaussian approximation; (2) the improved Gaussian approximation; (3) and the simplified improved Gaussian approximation. We extend these techniques to derive the BEPs of multicode DS-SSMA systems. We assess the three techniques by comparing their results with the result of computer simulation     

Performance Analysis of Narrow-Band Interference Rejection Techniques in DS Spread-Spectrum Systems

Linear least squares estimation (LLSE) techniques can provide an effective means of suppressing narrow-band interference in direct sequence (DS) spread-spectrum systems. In the results presented here, analytical expressions for bit error rate are derived for two DS spread-spectrum systems under the conditions of either tone or narrowband Gaussian interference. It is shown that the most common LLSE filter design can lead to performance inferior to that of various other filter designs. However, results are also presented demonstrating that an LLSE filter design motivated by the structure of the maximum-likelihood receiver leads to consistently superior performance. The performance of a system using this new design criterion is compared with that of an approximation to the maximum-likelihood (ML) receiver for the tone interference model and with that of the exact ML receiver for the Gaussian interference. Finally, it is shown that the bit error rate estimate obtained from application of a Gaussian approximation for the test statistic is overly pessimistic for the systems studied here.     

On the generation and correlation measurements of Gold codes†

Gold codes have many important applications, particularly in spread-spectrum code-division multiple-acess (CDMA) systems. The generation of Gold codes and the measurement of their correlation properties using some simple circuits are described in this paper. In particular, a circuit for the generation of m-sequences and Gold codes of 31 bits length and arbitrary phases, as well as a special digital matched filter circuit that is used to measure the correlation properties of these codes, are described in detail. Examples of the measured autocorrelation and cross-correlation functions of Gold codes are presented and compared with the corresponding computer generated results.     

Spread spectrum: Principles and possible application to spectrum utilization and allocation

Because of the continuing demand for more telecommunication capacity, to serve the world's need for commerce and public safety, there is a continuing need for more efficient ways of sharing the radio spectrum. The conventional way of allocating the spectrum is by frequency division; however, for many kinds of services this is inefficient. Hence, it seems desirable to reexamine whether alternative procedures might not be necessary if the benefits of telecommunications are to be assured in the face of increased demand. Spreadspectrum techniques, which are based on principles antithetic to those currently used in spectrum allocation for reducing necessary bandwidth, seem to offer benefits for spectrum sharing, for some applications, superior to those of frequency division. This paper provides a tutorial summary of some of the principles upon which spread-spectrum systems have developed. It is hoped the reader might consider what role such techniques may play in future spectrum sharing and allocation opportunities.