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Boolean functions naturally induce binary sequences of length 2 m and a large number of such orthogonal sequences is required in the design of code-division multiple-access (CDMA) systems. In this paper, Boolean functions are used to construct nonlinear phase orthogonal sequence sets for CDMA communications. For even m, employing carefully designed an m-variable Boolean function with five-valued Walsh spectra, one can get 16 different orthogonal sequence sets with sequence length 2 m . These sequence sets are assigned to a lattice of regular hexagonal cells, and we can ensure the orthogonality of adjacent cells. Moreover, the cross-correlation values between the sequences in a given cell and the sequences in non-neighbouring cells belong to { 0 , ± 2 m 2 , ± 2 m 2 + 1 } . On the other hand, the cardinality of the sequences sets is 2 m - 3 thus implying a trade-off between the quality of communication and the number of users assigned to each cell. This method can be improved so that the number of users is increased to 2 m - 2 in one half of the network while preserving the orthogonality between adjacent cells and the same level of low cross-correlation values to the non-neighbouring cells. [ABSTRACT FROM AUTHOR]

In this paper, we employ the so-called semi-bent functions to achieve significant improvements over currently known methods, regarding the number of orthogonal sequences per cell that can be assigned to a regular tessellation of hexagonal cells, typical for certain code-division multiple-access systems. Our initial design method generates a large family of orthogonal sets of sequences derived from vectorial semi-bent functions. A modification of the original approach is proposed to avoid a hard combinatorial problem of allocating several such orthogonal sets to a single cell of a regular hexagonal network, while preserving the orthogonality to adjacent cells. This modification increases the number of users per cell by starting from shorter codewords and then extending the length of these codewords to the desired length. The specification and assignment of these orthogonal sets to a regular tessellation of hexagonal cells have been solved, regardless of the parity and size of m (where 2^{m} is the length of the codewords). In particular, when the re-use distance is D=4 , the number of users per cell is 2^m-2 for almost all m , which is twice as many as can be obtained by the best known methods. [ABSTRACT FROM AUTHOR]

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Copyright of Military Technical Courier / Vojnotehnicki Glasnik is the property of Military Technical Courier / Vojnotehnicki Glasnik and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The conventional minimum mean-square error (MMSE) multiuser receiver requires that each particular user transmits a sequence of training symbols known to the receiver and the receiver estimates the user signatures using this knowledge. However, in the presence of multiaccess interference (MAI) during the training period and/or in scenarios with short training sequence length, the signature estimates can be erroneous, and the performance of the MMSE multiuser receiver can degrade substantially. In this paper, we propose a new blind multiuser receiver that is robust against the effects of erroneously presumed desired user signature and short data length. Our approach is based on the explicit modeling of possible mismatches in the mean-square error cost function and worst-case performance optimization. We show that this approach leads to a multiuser receiver which uses the data covariance matrix with an adaptive diagonal loading. The proposed method has simple implementation with a computational complexity comparable with that of the fixed diagonal loading-based multiuser receiver. Simulation results show performance improvements achieved by our approach relative to existing techniques. [ABSTRACT FROM AUTHOR]

Chaotic spreading sequences can increase secrecy and resistance to interception in signal transmission. Chaos-based CDMA systems have been well investigated in the case of flat fading and noise presence in the channel. However, these systems operating in wideband channels, characterized by the frequency selective fading and white Gaussian noise, have not been investigated to the level of understanding their practical applications. This paper presents a detailed mathematical model of a CDMA system based on chaotic spreading sequences. In a theoretical analysis, all signals are represented in the discrete time domain. Using the theory of discrete time stochastic processes, the probability of error expressions are derived in a closed form for a multi-user chaos based CDMA system. For the sake of comparison, the expressions for the probability of error are derived separately for narrowband and wideband channels. The application of the system interleaving technique is investigated in particular, which showed that this technique can substantially improve probability of error in the system. The system is simulated and the findings of the simulation confirmed theoretically expected results. Possible improvements in the probability of bit error due to multipath channel nature, with and without interleavers, are quantified depending on the random delay and the number of users in the system. In the analyzed system, a simplified version of the wideband channel model, proposed for modern wideband wireless networks, is used. Introduction Over the past years, the demand for wireless communications has increased substantially due to advancements in mobile communication systems and networks. Following these increasing demands, modern communication systems require the ability to handle a large number of users to process and transmit wideband signals through complex frequency selective channels. One of the techniques for transmission of multi-user signals is the Code Division Multiple Access (CDMA) technique which allows communications of multiple users in the same communication system. This is achieved in such a way that each user is assigned a unique code sequence, which is used at the receiver side to discover the information dedicated to that user. These systems belong to the group of communication systems for direct sequence spread spectrum systems. Traditionally, CDMA systems use binary orthogonal spreading codes. In this paper, a mathematical model and simulation of a CDMA system based on the application of non-binary, precisely speaking, chaotic spreading sequences. In their nature, these sequences belong to random sequences with infinite periodicity, and due to that they are appropriate for applications in the systems that provide enhanced security against interception and secrecy in signal transmission. Numerous papers are dedicated to the development of CDMA systems in flat fading channels. This paper presents the results of these systems analysis for the case when frequency selective fading is present in the channel. In addition, the paper investigates a possibility of using interleaving techniques to mitigate fading in a wideband channel with the frequency selective fading. Basic structure of a CDMA communication system and its operation In this paper, a CDMA system block schematic is ppresented and the function of all blocks is explained. Notation to be used in the paper is introduced. Chaotic sequences are defined and explained in accordance with the method of their generation. A wideband channel with frequency selective fading is defined by its impulse response function. Theoretical analysis of a CDMA system with flat fading in a narrowband channel A narrowband channel and flat fading are defined. A mathematical analysis of the system is conducted by presenting the signal expressions at vital points in the transmitter and receiver. The expression of the signal at the output of the sequence correlator is derived. Then the expression for the probability of bit error is derived in a closed form and discussed from both the number of users and the spreading factor point of view. Theoretical model of a CDMA sistem with frequency selective fading in a wideband channel A wideband channel and frequency selective fading are defined. All signals in the system are presented in the discrete time domain. In order to use a finite length of chips, each chip is represented by identical samples. A mathematical analysis of a wideband CDMA system is conducted. The mathematical expressions of the signals at vital points in the transmitter and receiver are presented. The expression for probability of bit error is derived in a closed form. The principles of the transmitter and receiver operations with and without interleavers are separatelly analyzed. Simulation model of the CDMA system and the simulation results The following system simulations are presented in this chapter: simulation of the system with a narrowband channel and flat fading; simulation of the system with a wideband channel and frequency selectiive fading without interleaving and simulation of the system with a wideband channel and frequency selectiive fading with interleaving. It was shown that the probability of error curves obtained by the simulation follow the curves obtained by the theoretical analysis. Conclusion The basic aim of this research, the results of which are shown in this paper, is to investigate the properties of CDMA systems in the case when frequency selective fading is present in a wideband channel. The developed theoretical model of the system is presented and the expressions of the probability of error are derived in a closed form., The case when the block interleaving technique is applied in the system is analysed in particular. The derived theoretical expressions for the probability of error show that it is possible to achieve significant improvement in the quality of signal transmission using this technique. Due to the random nature of chaotic sequences, the secrecy of information transmission is improved. For the same reason, the system's protection against interception is achieved. The modelled system is simulated in Matlab. The results of the simulation confirmed the results of the theoretical analysis.

For various quasi-synchronous (QS) CDMA systems, to reduce or eliminate the multiple access interference and multipath interference, it is required to design a set of spreading sequences which are mutually orthogonal within a designed shift zone. In this article, we demonstrate that a concept of irregular spatial assignment, with flexibility to assign different number of users to different cells, can be used to provide the maximal number of orthogonal sequences in any three adjacent cells in networks with a regular tessellation of hexagonal cells. We first consider p-phase spreading sequences of length p m (thus nonbinary p-valued sequences) suitable for synchronous (S)-CDMA applications, for p > 3 , and give an efficient design method for reaching the maximal cardinality achievable (being p m ). A simple solution for a flexible assignment of our orthogonal sets of spreading sequences to the cells in hexagonal networks is given. To address QS-CDMA applications as well, an efficient method to combine these orthogonal sequences with Zadeoff–Chu sequences is proposed for the purpose of designing sets of zero correlation zone (ZCZ) sequences (within a certain shift zone) with optimal parameters, thus reaching the Tang–Fan–Matsufuji bound. A similar design framework, based on the use of some special classes of Boolean functions, is then employed for the binary case to provide the maximum cardinality of pairwise orthogonal sequences of length 2 m through this irregular spatial assignment. This improves upon the best known results achieved in Zhang et al. (IEEE Trans Inf Theory 62:3757–3767, 2016), which assigns 2 m - 2 orthogonal sequences (users) per cell, by doubling the number of users in one third of the network. [ABSTRACT FROM AUTHOR]

Multiuser communication techniques are used in modern wireless networks like cellular and wireless sensor networks. The design of the physical layer for these networks was based on the application of binary spreading sequences and a correlation receiver. This paper presents a design of a multiuser code division transceiver using a new method of communication based on the application of chaotic spreading sequences that can increase security of the system, and on a new technique of detecting the received signal using a support vector machine and a traditional correlation receiver. The two receivers are theoretically analyzed, including the derivation of the probability of error expression in a closed form for the fading channel, and then implemented in DSP technology on a 32-bit floating-point TigerSHARC DSP platform. The comparison of two implemented systems showed that the support vector machine solution improves the robustness of the system, sacrificing the speed of processing. The achievable bit rate for the correlator receiver was up to two times higher than for the support vector machine receiver. / Технология многопользовательской системы коммуникации с кодовым распределением каналов (CDMA) используется в современных беспроводных сетях, таких как сотовые, радиотелефонные сети и беспроводные сенсорные сети. Развитие передачи сигнала на физическом уровне в сети, основано на применении двоичных последовательностей в расширении зоны сигнала и применении корреляционных приемников. В данной работе описана разработка приемопередатчика CDMA сигналов, при применении новых коммуникационных методов, основанных на хаотической последовательности в расширении зоны сигналов, обеспечивая тем самым защиту передачи сообщений в системе, применяя методы новых технологиях обнаружения сигналов при использовании машины опорных векторов в приемнике (SVM – приемник) и традиционного применения корреляционных приемников. Проведен теоретический анализ обеих моделей приемников, включая вывод вычисления вероятности битовых ошибок в каналах с замираниями (федингом) сигналов, затем произведено внедрение в цифровые технологии (DSP) на 32-битовую процессорную платформу TigerSHARC. Сопоставление этих систем показало, что решение применения машины опорных векторов (SVM – приемник) улучшает робастность системы за счет снижения скорости потока. В то время как осуществленный поток битов корреляционного приемника в два раза превышает поток битов приемника машины опорных векторов. / Tehnike višeučesničkih komunikacija sa kodnom raspodelom kanala (CDMA) koriste se u modernim bežičnim mrežama kao što su mobilne radio-telefonske mreže i bežične senzorske mreže. Razvoj prenosa signala na fizičkom nivou u ovim mrežama zasnovan je na primeni binarnih sekvenci za proširenje signala i primeni korelatorskih prijemnika. U ovom radu prikazan je razvoj primopredajnika za prenos CDMA signala korišćenjem nove komunikacione metode zasnovane na primeni haotičnih sekvenci za proširenje signala, čime se može povećati zaštita u prenosu poruka u sistemu, i razvojem nove tehnike detekcije signala korišćenjem mašina sa podržavajućim vektorima u prijemniku (SVM prijemnik) i tradicinalno korišćenog korelatorskog prijemnika. Oba razvijena prijemnika su teorijski analizirana, uključujući izvode izraza za verovatnoću greške bita u zatvorenom obliku u slučaju kanala sa fedingom, a zatim je izvršena njihova implementacja u digitalnoj tehnologiji (DSP) na 32-bitnoj procesorskoj platformi TigerSHARC. Poređenje ova dva sistema pokazalo je da rešenje sa mašinom sa podržavajućim vektorima (SVM prijemnik) poboljšava robustnost sistema po cenu smanjenja brzine procesiranja. Međutim, ostvarljivi protok bita kod prijemnika sa korelatorom bio je dva puta veći nego kod prijemnika koji koristi mašinu sa podržavajućim vektorima.

In this paper, we present a robust decentralized method for jointly performing channel estimation and closed loop power control for the reverse link of CDMA networks. Our method, based on linear quadratic Gaussian (LQG) control systems theory and Kalman filtering, does not require any training symbols for channel or signal to interference ratio (SIR) estimation. The main interest of this new scheme is that it improves the performance of current SIR based power control techniques while avoiding the problem of power escalation, which is often observed in current systems.