Direct Sequence Code Division Multiple Access Research Articles

Spectral Efficiency Comparison of Asynchronous MC-CDMA, MC DS-CDMA and MT-CDMA with Carrier Frequency Offset

Multicarrier code-division multiple access (MC-CDMA), multicarrier direct-sequence CDMA (MC DS-CDMA) and multitone CDMA (MT-CDMA) are three flavors of spread spectrum multicarrier communications. Multiple access interference, inter-carrier interference, self-interference and noise are the major factors that deteriorate their spectral efficiency. A novel joint analysis of spectral efficiency is presented for asynchronous MC-CDMA, MC DS-CDMA and MT-CDMA in a frequency-selective Rayleigh fading environment. This allows us to compare performance of the three schemes in indoor and outdoor environments in the presence of interferers, carrier frequency offset, multipath fading and noise.

LuxSteg: First Practical Implementation of Steganography in VLC

Visible-light communication (VLC) is a new technique for high-speed, low-cost wireless data transmission services. One of the areas in which VLC is considered superior to traditional radio-based communication is security. The common slogan summarizing the VLC security features is WYSIWYS- “What You See Is What You Send.”However, the broadcast nature of downlink VLC makes it possible for eavesdroppers to easily intercept the light communication in various settings, e.g., offices, conference rooms, plenum spaces, and so on. Similarly to radio-based data broadcasting systems, such as Wi-Fi, VLC opens the possibilities of hiding information in the public channel. In this paper, we describe (for the first time, to the best of our knowledge) the implementation of steganographic data transmission in a VLC system called LuxSteg. This VLC system utilizes pulse position modulation and direct sequence-code division multiple access modulation. In our implementation, multiple steganographic data streams are mixed with the spreading codes and combined with the overt data stream. We achieve a steganographic transmission rate of approximately 1 Mb/s hidden in a 110-Mb/s data stream. We analyze the influence of the spreading factor, spreading code type, number of hidden data streams, and the amplitude on the achievable transmission rate, undetectability, and robustness.

Asynchronous Detection and Identification of Multiple Users by Multi-Carrier Modulated Complementary Set of Sequences

The correct detection of arrivals is a crucial step in any communication system, in order to achieve frame synchronization. This step is even more relevant in underwater acoustic communications, as the medium is characterized by hazardous phenomena such as long multipath delays and non-uniform Doppler shifts, which hinder the detection process. All these phenomena make the detection of arrivals and identification of multiple users in underwater communication systems a challenging task. In this paper, a multi-user detector is presented. The detection is based on the direct-sequence code-division multiple access technique, by using a combination of two sequences as a preamble: a Zadoff-Chu sequence that provides candidate peaks and frame synchronization, and a complementary set of sequences (CSS) that validates the true Zadoff-Chu arrival while at the same time identifies the user. Both sequences are modulated using multi-carrier techniques, and the users can transmit asynchronously to the channel. In the case of CSS, multi-carrier modulation techniques allow performing channel equalization and demodulation at the receiver in a simple manner, enabling the correlation of the CSS at bit level, thus keeping its ideal correlation properties. Sea trials were conducted in February 2017 at Suruga Bay, Japan, in which the reception of two different users was analyzed under different noise and multipath conditions, as well as uniform and non-uniform Doppler shifts. The results show that, even under strong multipath and non-uniform Doppler shifts caused by roll and pitch motions, the receiver is able to detect and identify the user in 98% of the transmitted packets.

On a Subspace Training Method for Dispersive DS-CDMA Channels

A subspace training method has been proposed for dispersive direct-sequence code division multiple access (DS-CDMA) channels. Its performance was compared to a matched filter receiver. We think that it is not quite fair to compare a minimum mean square error (MMSE) based receiver to a matched filter receiver. For this reason, we further investigate the method demonstrating that its performance significantly falls short of the true MMSE receiver and that it suffers from error floor. We also illustrate that any attempt to fill the gap significantly increases the system complexity and drops the efficiency of the transmission, which were originally mentioned as the advantages of the proposed method.

A new class of block coordinate algorithms for the joint eigenvalue decomposition of complex matrices

Several signal processing problems can be written as the joint eigenvalue decomposition (JEVD) of a set of noisy matrices. JEVD notably occurs in source separation problems and for the canonical polyadic decomposition of tensors. Most of the existing JEVD algorithms are based on a block coordinate procedure and require significant modifications to deal with complex-valued matrices. These modifications decrease algorithms performances either in terms of estimation accuracy of the eigenvectors or in terms of computational cost. Therefore, we propose a class of algorithms working equally with real- or complex-valued matrices. These algorithms are still based on a block coordinate procedure and multiplicative updates. The originality of the proposed approach lies in the structure of the updating matrix and in the way the optimization problem is solved in CN×N. That structure is parametrized and allows to define up to five different JEVD algorithms. Thanks to numerical simulations, we show that, with respect to the more accurate algorithms of the literature, this approach improves the estimation of the eigenvectors and has a computational cost significantly lower. Finally, as an application example, one of the proposed algorithm is successfully applied to the blind source separation of Direct-Sequence Code Division Multiple Access signals.

Computationally Efficient Chaotic Spreading Sequence Selection for Asynchronous DS-CDMA

Abstract The choice of the spreading sequence for asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems plays a crucial role for the mitigation of multiple-access interference. Considering the rich dynamics of chaotic sequences, their use for spreading allows overcoming the limitations of the classical spreading sequences. However, to ensure low cross-correlation between the sequences, careful selection must be performed. This paper presents a novel exhaustive search algorithm, which allows finding sets of chaotic spreading sequences of required length with a particularly low mutual cross-correlation. The efficiency of the search is verified by simulations, which show a significant advantage compared to non-selected chaotic sequences. Moreover, the impact of sequence length on the efficiency of the selection is studied.

Gold-MUSIC based DOA estimation using ULA antenna of DS-CDMA sources with propagation delay diversity

Direction of Arrival (DOA) estimation using gold-MUSIC is achieved for Uniform Linear Array (ULA) antenna based Direct-Sequence Code Division Multiple Access (DS-CDMA) sources having different Propagation Delay. The DOA is estimated by different MUSIC (Multiple Signal Classification) algorithms for M signal sources with Phase Shift Keying (PSK) transmission in presence of Additive White Gaussian Noise (AWGN). Spatial spectrum, computational efficiency and Root Mean Square Error (RMSE) of DOA estimation using gold-MUSIC over MUSIC and Root-MUSIC algorithms are studied meticulously. The effects of number of ULA elements, Data length and arrival angle on RMSE of DOA are also conferred here.

A comprehensive performance analysis of relay-aided CDMA communications over dissimilar fading channels

In this paper, bit error probability (BEP), outage probability (OP) and channel capacity (CC) of direct-sequence code-division multiple access systems with amplify-and-forward relaying are presented for different fading scenarios. In the first scenario, the source-destination link is assumed to experience Rayleigh fading while it is subject to Nakagami-m fading in the second scenario. The source-relay and relay-destination channels are considered to have Nakagami-m fading conditions in two scenarios. First, analytical expressions for the end-to-end probability density function (PDF) are derived by using the convolution integral. Then, BEP, OP and CC are obtained based on these PDFs in terms of infinite series. Truncation error analyses are presented for different parameter values in order to show that truncation error arising from the infinite series is negligible. Simple and easy-to-compute asymptotic expressions are also introduced for BEP and OP in order to simplify the performance analysis in high signal-to-noise ratio region. Simulation results are provided to show the accuracy of the proposed approximate and asymptotic expressions.

Blind spreading sequence estimation algorithm for long‐code DS‐CDMA signals in asynchronous multi‐user systems

Despreading signal at receiver side requires prior knowledge of the spreading sequences in direct-sequence code division multiple access (DS-CDMA) system. However, the knowledge of spreading sequences is always unknown to the receiver in non-cooperative communication. Therefore the receiver has to estimate the spreading sequences in a blind manner. This study presents a novel algorithm to estimate the spreading sequences for asynchronous long-code (LC) DS-CDMA signals. By investigating the projection characteristics of the signal subspace, the authors treat the spreading sequence estimation problem as a discrete optimisation search problem that can be solved efficiently by modern optimisation search methods. Moreover, they design a successive subspace deflation scheme to alleviate interference from other users. Numerical experiments demonstrate that the proposed algorithm provides good estimation of the spreading sequences for LC-DS-CDMA signals. Compared with the existing algorithms, the proposed algorithm exhibits much better performance in large system load scenario with low signal-to-noise ratio.

Statistical Analysis of Multiple Access Interference in Chaotic Spreading Sequence Based DS-CDMA Systems

This paper presents a statistical analysis of multiple access interference (MAI) in Direct Sequence Code Division Multiple Access (DS-CDMA) communication systems based on different types of chaotic spreading sequences. The probability distribution of the interference in a system with K users causing the MAI is studied using MATLAB simulation. For chaotic spreading sequence generation six different 1-D chaotic maps are used: modified Bernoulli, modified Tent, Gauss, Sine-Circle, Cubic and Pinchers map. A brief statistical analysis of the cross-correlation properties of the chaotic sequences generated by the aforementioned maps is also presented.

Outage performance of cooperative DS‐CDMA systems with best path selection over α − η − μ fading channels

Outage probability (OP) of amplify-and-forward direct-sequence code-division multiple-access (DS-CDMA) systems is analysed with best path selection over α − η − μ fading channels. First, cumulative distribution function (CDF) of α − η − μ fading is derived and then, lower bound of the OP is obtained for the considered system. An asymptotic CDF expression is also proposed in order to get more insights on system behaviour. The OP performance is presented for different relay and fading parameter configurations. Analytical derivations are validated by the close agreement with simulations.

A CDMA scheme based on perfect Gaussian integer sequences

Both m-sequences and Gold sequences have been applied as the pseudonoise (PN) code in the code division multiple access (CDMA) scheme to enable distinct users to simultaneously share the available capacity of a common channel, which is called direct sequence CDMA (DS-CDMA). However, this scheme does not provide the required ideal periodic cross-correlation function property among various members of sequences within the same set. This paper proposes a new perfect Gaussian integer sequence (PGIS)-based CDMA scheme called PGIS-CDMA; in this scheme, the PN code of DS-CDMA is replaced by a set of PGISs. The intrinsic orthogonal property of PGIS can be applied to a CDMA system to achieve perfect cochannel interference separability under the ideal synchronism; it can also be adapted at the receiver to obtain diversity gain and improve the carry-to-interference ratio. These two concepts are further developed to implement two PGIS-CDMA configurations, and they can outperform DS-CDMA on the basis of Gold sequences that possess a favorable cross-correlation property for the CDMA scheme.

Performance of Direct-Oversampling Correlator-Type Receivers in Chaos-Based DS-CDMA Systems Over Frequency Non-selective Fading Channels

In this paper, we present a study on the performance of direct-oversampling correlator-type receivers in chaos-based direct-sequence code division multiple access systems over frequency non-selective fading channels. At the input, the received signal is sampled at a sampling rate higher than the chip rate. This oversampling step is used to precisely determine the delayed-signal components from multipath fading channels, which can be combined together by a correlator for the sake of increasing the SNR at its output. The main advantage of using direct-oversampling correlator-type receivers is not only their low energy consumption due to their simple structure, but also their ability to exploit the non-selective fading characteristic of multipath channels to improve the overall system performance in scenarios with limited data speeds and low energy requirements, such as low-rate wireless personal area networks. Mathematical models in discrete-time domain for the conventional transmitting side with multiple access operation, the generalized non-selective Rayleigh fading channel, and the proposed receiver are provided and described. A rough theoretical bit-error-rate (BER) expression is first derived by means of Gaussian approximation. We then define the main component in the expression and build its probability mass function through numerical computation. The final BER estimation is carried out by integrating the rough expression over possible discrete values of the PFM. In order to validate our findings, PC simulation is performed and simulated performance is compared with the corresponding estimated one. Obtained results show that the system performance get better with the increment of the number of paths in the channel.

Average symbol error rate and outage probability of DS-CDMA systems with AF relaying over asymmetric fading channels

In this paper, approximate outage probability (OP) and average symbol error rate (SER) of cooperative direct-sequence code-division multiple access (DS-CDMA) systems with amplify-and-forward (AF) relaying are evaluated over asymmetric fading channels. Practically, the channels between source (S), relay (R) and destination (D) nodes in a DS-CDMA system can be subject to different fading due to the nature of wireless medium. Motivated by this reality, the S–D path is assumed to be Nakagami-m distributed which is a general fading model for different types of channel conditions while S–R and R–D paths are considered to experience Rician fading in order to include line of sight conditions. First, closed-form solutions are obtained for both the cumulative distribution function and the probability density function of S–R–D paths. Then, a closed-form total moment generating function (MGF) of the considered system is derived. The lower bound of the OP is calculated with the help of the inverse Laplace transform of the derived MGF expression while average SER is analyzed by evaluating the integral over the derived MGF. Finally, an asymptotic MGF expression is proposed for high signal-to-noise ratio (SNR) analysis of AF DS-CDMA systems. The simulation results are provided to verify the correctness of the analytical derivations. It is shown that the approximate OP and average SER results are in well agreement with the simulations and the asymptotic results are tight from medium to high SNR regime.

Simulation of an optimized technique based on DS-CDMA for simultaneous transmission of multichannel biosignals.

Telemedicine is becoming increasingly, with applications in many areas of healthcare, such as home telecare of the elderly, diagnosis at a distance and robotic surgery. The simultaneous transmission of several leads of biomedical signals should be considered in telemedicine, given the many benefits it brings. Code division multiple access (CDMA) is a multiple access technique that enables users to transmit independent information simultaneously within the same bandwidth. The direct sequence CDMA (DS-CDMA) is a variant of the CDMA technique in which a pseudorandom sequence having a higher bandwidth than the information signal is used to modulate the information signal directly. Biomedical signals are confidential; thus, their transmission must be secured. In this paper we propose a protocol similar to DS-CDMA for the simultaneous transmission of all of the leads of some multichannel biomedical signals. We assigned orthogonal codes to different leads of a signal. The convolution of each lead with the code gives a signal spread over a broad frequency band. All of the spread signals are then mixed to produce a single composite signal. This composite signal is frequency modulated, amplified and transmitted. At the reception, inverse functions to the previous are developed to perform demodulation, demultiplexing and extraction of the physiological signals transmitted. We used the discrete Walsh functions as codes. The results obtained are satisfactory, even in situations where the noise disturbances are significant.

Cooperative MIMO relay beamforming in overloaded DS/CDMA multicast networks

A new beamforming scheme is proposed for multi-input multi-output (MIMO) non-regenerative relays in an overloaded synchronous direct sequence code division multiple access (DS-CDMA) network in which the number of users is larger than the processing gain. In this scheme, several transmitters multicast their data to their groups of receivers through MIMO relays and DS-CDMA technique is utilised to decrease the level of multiple access interference (MAI) at the relays and the receivers. At the relays, the data of transmitters are despread, beamformed and re-spread. The aim of the beamformer is to minimise the total relays transmission power subject to the signal to interference and noise ratio (SINR) constraints at the receivers. This optimisation problem is an indefinite quadratic (non-convex) programming. Semi-definite programming (SDP) and second-order cone programming (SOCP) are two existing methods to solve the problem. However, they, respectively, suffer from high computational complexity and performance degradation when the number of antennas or users increase. To effectively mitigate these deficiencies, we propose a novel DC (difference of two convex functions) programming method. Our simulation results demonstrate the computational advantage of the proposed DC programming over SDP, while a better performance over SOCP is achieved. Moreover, the superiority of the proposed MIMO-CDMA scheme over other multiplexing schemes is shown by several scenarios.

A near‐Nyquist rate transmission: A new minimum‐bandwidth direct‐sequence code division multiple access scheme

SummaryBandlimited direct‐sequence code division multiple access (DS‐CDMA) attracts much attention for its compact spectrum and the ability to suppress inter‐symbol interference. Among the various bandlimited DS‐CDMA systems available, minimum‐bandwidth DS‐CDMA (MB‐DS‐CDMA) is the only realizable Nyquist rate transmission system. But, MB‐DS‐CDMA only applies to certain kinds of spreading codes. Accordingly, this study proposes a modified DS‐CDMA structure which extends the application of MB‐DS‐CDMA to all common spreading codes at the expense of a negligible reduction in the transmission rate. Additionally, the bit error rate of the proposed schemes adopting either single‐user or multi‐user detection receiver is analyzed and compared with that of the commonly‐used raised‐cosine‐pulsed DS‐CDMA over multipath fading channels. The numerical results show that given a sufficiently large number of users, the bit error rate performance of modified MB‐DS‐CDMA is comparable to that of the raised‐cosine‐pulsed DS‐CDMA scheme; meanwhile, the realizable modified MB‐DS‐CDMA approaches the ultimate transmission rate.

Buffer-aided distributed space-time coding schemes and algorithms for cooperative DS-CDMA systems

In this work, we propose buffer-aided distributed space-time coding (DSTC) schemes and relay selection algorithms for cooperative direct-sequence code-division multiple access (DS-CDMA) systems. We first devise a relay pair selection algorithm that can exploit all possible relay pairs and then select the optimum set of relays among both the source-relay phase and the relay-destination phase according to the signal-to-interference-plus-noise ratio (SINR) criterion. Multiple relays equipped with dynamic buffers are then introduced in the network, which allows the relays to store data received from the sources and wait until the most appropriate time for transmission. A greedy relay pair selection algorithm is then developed to reduce the high cost brought by the exhaustive search that is required when a large number of relays are involved in the transmission. The proposed techniques effectively improve the quality of the transmission with an acceptable delay as the buffer size is adjustable. An analysis of the computational complexity of the proposed algorithms and the delay and a study of the greedy algorithm are then carried out. Simulation results show that the proposed dynamic buffer-aided DSTC schemes and algorithms outperform prior art.

Run‐length method for determining the segmentation length based on the segmentation long‐code blind estimation

In this study, the blind despreading of long-code direct sequence spread spectrum signals (LC-DSSS) is considered. Qiu et al. presented a heuristic segmentation-based method to estimate the spreading code of LC-DSSS signals, which divided the received signal into far shorter temporal windows than the symbol duration, then spreading code was reconstructed from these short windows by computing the related covariance matrices. However, Qiu et al. did not consider the criterion of determining the segmentation length. By combining the matrix perturbation theory and run length property, the authors propose a run-length method for determining the segmentation length based on the segmentation idea, and this method can also be applied to long-code direct-sequence code division multiple access (LC-DS-CDMA) signals. Monte Carlo simulations for both LC-DSSS signals and LC-DS-CDMA signals are presented to demonstrate the effectiveness of the proposed method.

Three Natural Computation methods for joint channel estimation and symbol detection in multiuser communications

This paper studies three of the most important optimization algorithms belonging to Natural Computation (NC): genetic algorithm (GA), tabu search (TS) and simulated quenching (SQ). A concise overview of these methods, including their fundamentals, drawbacks and comparison, is described in the first half of the paper. Our work is particularized and focused on a specific application: joint channel estimation and symbol detection in a Direct-Sequence/Code-Division Multiple-Access (DS/CDMA) multiuser communications scenario; therefore, its channel model is described and the three methods are explained and particularized for solving this. Important issues such as suboptimal convergence, cycling search or control of the population diversity have deserved special attention. Several numerical simulations analyze the performance of these three methods, showing, as well, comparative results with well-known classical algorithms such as the Minimum Mean Square Error estimator (MMSE), the Matched Filter (MF) or Radial Basis Function (RBF)-based detection schemes. As a consequence, the three proposed methods would allow transmission at higher data rates over channels under more severe fading and interference conditions. Simulations show that our proposals require less computational load in most cases. For instance, the proposed GA saves about 73% of time with respect to the standard GA. Besides, when the number of active users doubles from 10 to 20, the complexity of the proposed GA increases by a factor of 8.33, in contrast to 32 for the optimum maximum likelihood detector. The load of TS and SQ is around 15–25% higher than that of the proposed GA.