Practical LPI Communication with Noise-Shaped Signaling

The next-generation wireless personal and mobile communication systems are expected to accommodate not only high-quality voice services, but also a broad range of other multirate services. Of the various multiaccess techniques, wide-band code-division multiple access (CDMA) has been regarded as an important part of the third-generation wireless communication systems because of its high frequency utilization efficiency and suitability for handling multimedia and multirate services. In this paper, we consider a system with a simplified receiver structure for direct-sequence CDMA (DS/CDMA) wireless communication handsets, in which improved performance is demonstrated when compared to a conventional DS/CDMA system with a RAKE receiver at the mobile station. We arrive at this system by finding the optimal solution to a general multiple-input single-output (MISO) DS/CDMA smart antenna system. We find that this solution reduces to a pre-RAKE with space transmit diversity system under the assumption that a simple one-finger matched filter is used at the receiver. This system combines the advantages of pre-RAKE diversity and transmit antenna diversity. It is shown that significant system performance and capacity improvements are possible. The numerical results also reveal that this system is not too sensitive to channel estimation errors.

A DS/CDMA (direct-sequence code division multiple access) system using BPSK (binary phase shift keying) modulation is simulated to determine the bit error probabilities for various numbers of users with a processing gain close to that of a cellular network. The results are compared to results obtained using the Gaussian approximation equations. The results obtained are to the feasibility of a DS/CDMA cellular network. It is concluded that DS/CDMA cellular networks appear to offer higher subscriber capacities than existing analog or TDMA (time division multiple access) formats. In addition, DS/CDMA offers additional advantages such as inherent privacy through the use of spreading codes and the ability to combat problems such as multipath fading and narrowband noise. >

A CDMA scheme based on perfect Gaussian integer sequences

We introduce a novel merger of direct sequence code division multiple access (DS-CDMA) and smart antenna arrays. With regard to the DS-CDMA scheme, we employ carrier interferometry DS-CDMA (CI/DS-CDMA), a novel implementation of DS-CDMA where chips are decomposable into N narrowband frequency components. With regard to the antenna array, we deploy the oscillating-beam smart array. Here, applying proper time-varying phases to the array elements, we create small movement (oscillation) in the antenna array's pattern, while steering the antenna pattern main lobe to the position of the intended user. The oscillating antenna pattern creates a time-varying channel with a controllable coherence time. This, in turn, provides transmit diversity in the form of a time diversity gain at the mobile receiver side. At the receiver, three stages of combining are available: combining time components of the received signal within symbol duration Ts (each experiencing a different fade) to enhance performance via time diversity; combining frequency components which make up the CI/DS-CDMA chip to enhance the performance via frequency diversity; and combining across chips to eliminate the interfering users on the system. Merging CI/DS-CDMA with the oscillating-beam smart antenna at the base station, we achieve very high capacity via the merger of SDMA (available through directionality of the antenna array) and code division multiple access (inherent in CI/DS-CDMA), and very high performance via the construction of receivers that exploit both transmit diversity and frequency diversity. We present the performance gains of the proposed merger.

Chaotic sequences have good correlation properties and they can be used as address sequences in Spread Spectrum Communication. Chaotic functions are highly sensitive to initial condition and exhibit non-linear behavior. In Chaotic spread spectrum communication systems, different user may be assigned different sequences generated with different initial conditions. Set of binary sequences with good pair wise cross- correlation property finds applications in Direct Sequence- Code Division Multiple Access (DS-CDMA) systems. Security in Direct Sequence-Code Division Multiple Access systems is relatively limited. To enhance security in Direct Sequence- Code Division Multiple Access systems, chaotic sequences are generated. In this paper we analyze the generation of chaotic binary sequences and study of auto correlation and cross correlation properties.

This paper presents an open and flexible radio interface architecture based on Code Division Multiple Access (CDMA) which is particularly tailored to the requirements of third-generation mobile communication systems (FPLMTS/UMTS). The proposed multi-rate radio interface is based on direct sequence CDMA with variable spreading factors, multiple chip rates and multiple RF bandwidths. The basic concept of the multi-rate radio interface is elucidated and some indicative figures as to key parameters of the radio interface are given.

We consider a combined space-time (ST) block coding and direct-sequence code division multiple access (DS/CDMA) scheme for downlink transmissions over multipath fading channels, in which ST block coding is performed at the chip level, i.e. after code spreading. For this scheme, we develop low complexity and low decoding delay linear ST single-user detection algorithms. In addition, we demonstrate that by exploiting the signal structure imposed by the chip-level ST block coding, blind channel multipath estimation (with only a scalar ambiguity) is feasible, even with the assignment of a single code vector to each user.

A direct sequence optical code division multiple access technique is investigated. This paper introduces, first, the principles definitions of Code Division Multiple Access in radio communications. After that, the performances of Optical Code Division Multiple Access are evaluated in association with a realistic amplified optical channel model.

Blind detection of a desired user's signal in a code-hopping (CH) direct sequence code-division multiple access (CDMA) system is considered. In CH CDMA systems each user switches between a predetermined set of short-code sequences. A user signal in such systems may be treated as the superposition of several virtual short-code signals. A code-constrained inverse filter criterion (IFC)-based blind detector for short-code CDMA signals in asynchronous multipath channels to detect a desired user's signal was recently presented by Tugnait and Li (2001). A novel approach combining the code-constrained IFC and a penalty function is proposed to simultaneously extract all virtual users associated with a given CH user. Global minima of the proposed cost function are analyzed. An extension of an existing subspace-based approach is also investigated. An illustrative simulation example is provided where the proposed algorithm is compared with a clairvoyant matched filter receiver, two linear minimum mean-square error (MMSE) receivers with channels known, and the subspace-based approach.

The future wireless mobile communication systems will be required to support high-speed transmission rate and high quality of service. Direct sequence code division multiple access (DS-CDMA) is an important scheme for high-rate wireless communication. The capacity of DS-CDMA can be impaired by two problems; near-far effect and multiple-access interference (MAI). The use of conventional-matched filter detector for multiple users in DS-CDMA fails to combat any of these problems. The performance degradation caused by MAI can be overcome using multiuser detection (MUD). The use of maximum likelihood (ML) sequence estimation detector provides excellent results, but involves high computational complexity. In this paper, we propose a new meta-heuristic approach for MUD using honeybees mating optimization (HBMO) algorithm to detect the user bits based on the ML decision rule for DS-CDMA systems in additive white-Gaussian noise and flat Rayleigh fading channels. In order to improve the solutions generated by the HBMO, a second meta-heuristic method simulated annealing is used. By computer simulations, the bit error rate performance and the complexity curves show that the proposed HBMO-SA MUD is capable of outperforming the other conventional detectors and genetic algorithm detector.

Innovations of modern digital radio technology has enabled many large mobile and distributed information systems, such as inventory tracking systems, to provide readily accessible voice and data services to end users despite mobility of data servers. These systems often contain components that are equipped with radio frequency identification (RFID) tags and interrogators for wireless connectivity. However, there are usually large number of these tagged items in these systems that are highly mobile. The wireless connections among them are also prone to failure. In this paper, we evaluate and compare the performance of two RF tag data access protocols in a single channel per cell implementation: slotted ALOHA Time Division Multiple Access (slotted ALOHA/TDMA) and Direct Sequence Code Division Multiple Access (DS/CDMA).We use a discrete event simulation model to study the impact of these protocols on data service under a diversity of highly mobile operating conditions. Mobile conditions are captured separately by various levels of burst error, uplink and downlink disconnections, and tag population. The results show that performance of DS/CDMA dominates in all mobile conditions. In the most general cases, DS/CDMA outperforms slotted ALOHA/TDMA by 1.5 times faster message time. DS/CDMA is particularly superior in the case of pure uplink disconnections. For burst error and pure downlink disconnections, DS/CDMA renders as much as 2.0 times and 4.5 times faster message time respectively.

Since cognition has become an important topic in Electronic Warfare (EW) systems, Electronic Support Measures (ESM) are used to monitor, intercept and analyse radar signals. Low Probability of Intercept (LPI) radars is preferred to be able to detect targets without being detected by ES systems. Because of their properties as low power, variable frequency, wide bandwidth, LPI Radar waveforms are difficult to intercept with ESM systems. In addition to intercepting, the determination of the waveform types used by the LPI Radars is also very important for applying counter-measures against these radars. In this study, a solution for the LPI Radar waveform recognition is proposed. The solution is based on the training of Support Vector Machine (SVM) after applying Principal Component Analysis (PCA) to the data obtained by Time-Frequency Images (TFI). TFIs are generated using Choi-Williams Distribution. High energy regions on these images are cropped automatically and then resized to obtain uniform data set. To obtain the best result in SVM, the SVM Hyper-Parameters are also optimized. Results are obtained by using one-against-all and one-against-one methods. Better classification performance than those given in the literature have been obtained especially for lower Signal to Noise Ratio (SNR) values. The cross-validated results obtained are compared with the best results in the literature.

The effect of signal quantisation on noncoherent matched filter code acquisition in a direct-sequence code division multiple access (CDMA) system is considered. The signal structure is applicable to a wideband CDMA system. The effect of quantisation is derived mathematically and verified using computer simulations with several users. The results indicate that the effect of quantisation is not significant, i.e. from the point of view of acquisition, only a few bits are needed. The effect of signal quantisation can be modelled as a noise increase.

The main focus of this paper is to present an analytical method for calculate the correlation between OQPSK (Offset Quadrature Phase-Shift Keying) signals modulated by several spreading sequence families. Our analytical model includes nonlinearities, namely by treating the transmitter amplifier as highly nonlinear. Grossly nonlinear power amplification is highly desirable as it allows power-efficient low-cost receivers, with applications in satellite, underwater or deep space communications. OQPSK-type modulations include as special cases CPM (Continuous Phase Modulation) schemes and can be designed to have low envelope fluctuations or even a constant envelope, essential in many communications systems (e.g., satellite). This model and the derived analytical results for the correlation of sequences in such a nonlinear environment are then applied to a DS-CDMA (Direct Sequence - Code Division Multiple Access) system. We present simulation results for three types of spreading sequences: ML (Maximum-Length), Kasami and TCH (Tomlinson, Cercas, Hughes) sequences. The simulation results are presented to compare the performance of those types of sequences in the linear and nonlinear DS-CDMA systems.

We address the possibility of applying a new type of a frequency hopping system in order to deliver information from roadside beacons to passing vehicles. The system is based on being able to divide the FH addresses into two parts and use one of them as a group address and the other as individual address. Families of FH codes suitable for this application are given. The information class CDMA in the context of FH is also explained and some examples given.