Code Division Multiple Access System Research Articles

Analysis of deep multilayer perceptron neural network in MWC coded optical CDMA system with LDPC code

Abstract In this paper, applying the Deep Multilayer Perceptron Neural Network (MLPNN) to the Sum-Product Algorithm (SPA) for decoding the Modified Welch-Costas (MWC) coded Optical Code Division Multiple Access (OCDMA) system with Low-Density Parity-Check (LDPC) code is analyzed. The goal is to train the MLPNN-SPA through the stochastic gradient descent (SGD) to learn and optimize the weights to each edge of the neural network decoder. Once these parameters have been trained, the decoding complexity of the MLPNN-SPA is similar to that of the SPA. Furthermore, from the simulation results, it has been shown that the MLNN-SPA can improve the system performance without additional decoding complexity as compared to the SPA.

Экспериментальное исследование многополосной акустооптической фильтрации при декодировании спектрально-кодированных сигналов в некогерентных системах OCDMA

Acousto-optical multi-band filtering has been experimentally proven to be basement for designing optical code division multiple access (OCDMA) systems. The interference caused by used for encoding / decoding sets of spectral lines which shape close to the sin (x) / x function was investigated. To study decoding of the total signal from 2-5 transmitters, it was emulated with just one filter using novel acousto-optical technique. The measured Signal-to-Interference Ratio for 5 transmitters was more than 11 dB and decreased due to the emulation error by less than 3 dB.

Effect of adverse weather conditions and pointing error on the performance of 2-D WH/TS OCDMA over FSO link

Abstract In this paper, the performance of two-dimensional (2-D) wavelength-hopping/time-spreading (WH/TS) optical code division multiple access (OCDMA) system over free space optical (FSO) channel is analyzed in the presence of pointing error and different weather conditions. Prime code scheme is employed for both wavelength-hopping and time-spreading to address user code-matrix. The operating central wavelength of 1550 nm is considered to demonstrate the bit error rate (BER) performance of the proposed system as a function of various system parameters. The required optical power of the proposed system is determined to maintain a BER value of 10−9. The numerical evaluation interprets that the BER performance is highly dependent on transmission length, transmitted power, pointing error angle as well as the number of simultaneous user. It is also observed that the 2-D OCDMA system over free space needs minimum required optical power in case of rainy atmospheric condition, but it is maximum for foggy atmospheric condition.

Development of a secured optical code-division multiple access system by implementing hybrid 2D-modified Walsh code

We present the development of a multi-user cross-layer security network implemented on the optical code-division multiple access (OCDMA) with newly designed hybrid 2D-modified Walsh code (MWC) along with an algorithmic approach in cryptography domain. Minimum BER value is evaluated on the order of 10 − 50 using our designed MWC, which is comparatively lower than other existing codes. The security performance of the network was analyzed using the probability detection parameter, which was found to be superior apart from few other available existing codes. The enhancement of the quality and grade of security in this network eavesdropper can intercept the designed optical code of OCDMA in the physical layer. It compels one to break the security in the data-layer to access the original code. We also calculated the probability of successful data interception as a function of time complexity. Furthermore, we presented the simulation of the hybrid system using the OptiSystem simulator.

A Direct Construction of Optimal ZCCS With Maximum Column Sequence PMEPR Two for MC-CDMA System

The high peak-to-mean envelope power ratio (PMEPR) is a major problem in multicarrier code-division multiple-access (MC-CDMA) system. In this letter, we have proposed a new optimal Z-complementary code set (ZCCS) using higher-order (≥ 2) generalized Boolean functions (GBFs). The proposed ZCCS has maximum column sequence PMEPR of 2 and it achieves the theoretical upper bound of optimality.

Construction and performance analysis of a new SAC-OCDMA code based on Latin square matrix

In this paper a family of novel spreading code called Latin square code (LSC) is proposed for spectral amplitude coding-optical code division multiple access (SAC-OCDMA) system. The main feature of the proposed code is the zero cross-correlation which eliminates both multiple access interference (MAI) and phase induced intensity noise (PIIN). The code construction can be easily accomplished using Latin square matrix (LSM) for any weight and number of users. The simplicity in the construction code has made it a compelling candidate for future OCDMA applications. SAC-OCDMA system employing direct decoding is mathematically analyzed and then numerically simulated using Matlab and OptiSystem respectively. The results show the improvement given by the LSC code to the SAC-OCDMA system compared to the other codes such as: ZCC (zero cross-correlation) and MMS (modified multiservice) by allowing high cardinality and improving BER. Furthermore, the transmission quality, so that the BER does not exceed the value of $10^{-9}$, is ensured by the LSC code with a lower effective source power of -14.5 dBm, a bit rate of 4.6 Gb/s and a cardinality exceeding ZCC and MMS by 1.7 and 1.22 times respectively. The simulation results validate the mathematical analysis and show that the system makes it possible to increase the transmission distance without affecting QoS (quality of service)

Employment of Generalized Receiver in DS-CDMA with STBC over Fading Channels: Capacity and Error Probability

The capacity and error probability of orthogonal space-time block codes (STBCs) are considered for the pulse-amplitude modulation (PAM), phase shift keying (PSK), quadrature-amplitude modulation (QAM) in fading channels. The suggested approach is based on employment of the generalized approach to signal processing in noise over the equivalent scalar additive white Gaussian noise channel with a channel gain proportional to the Frobenius norm of the matrix channel for the STBC. Using this effective channel, the capacity and probability of error expressions are derived for the PSK, PAM, and QAM modulation with space-time block coding. The Rayleigh, Rician and Nakagami fading channels are considered. As an application, these results are extended to obtain the capacity and probability of error for a multiuser direct sequence code-division multiple-access (CDMA) system constructed on the basis of the generalized approach to signal processing in noise employing the space-time block coding.

Design improvement to reduce noise effect in CDMA multiple access optical systems based on new (2-D) code using spectral/spatial half-matrix technique

Abstract This paper presents for non-coherent Optical Code Division Multiple Access (OCDMA) systems a new optical code namely Two-Dimensional Half Spectral/Spatial Zero Cross Correlation (2D-HSSZCC) code based on a One-Dimensional Zero Cross Correlation (1D-ZCC) code already developed using block matrices characterized by a high capacity. The results of simulation show that the use of the new (2D-HSSZCC) code eliminates totally the Multiple Access Interferences (MAI) due to the zero cross correlation flexibility, and less complexity of the code construction which produces a very low bit error rate of closely (4×10−18) at 1 Gbps for four users with a low power source of −12.60 dBm to reach a high data rate and high number of simultaneous users upper to closely 149, save an effective power around −1.35 dBm, −3.3d Bm compared between those provides by (Two-Dimensional dynamic cyclic shift (2D-DCS) code and Two-Dimensional Dimensional Diluted Perfect Difference (2D-DPD) and (1D-ZCC) code, and increase the cardinality percentage upper to 1.58, 2.19, 2.33 and 3.9 times comparing to (2D-DCS) code, 2D-DPD code, 1D-ZCC code and Two-Dimensional Flexible Cross Corelation/Modified Double Weight (2D-FCC/MDW) code. On the other hand, 2D-HSSZCC code is comparied with other codes which has it same property namely Two-Dimensional zero cross correlation/multi diagonal (2D-ZCC/MD) and (2D-MD) codes where the increased percentage in system capacity was 1.38 and 1.05 times, respectively. Finally, the results obtained in part 1 (with Matlab software) were confirmed and validated with the Optisystem software, the proposed system gave a better BER minimum value around 10−21 and a maximum value of the Q factor of around 9.4 at 622 Mbps of data rate when the number of simultaneous users increases.

Signal Processing by Generalized Receiver Using DS-CDMA with STBC Technology in Multidimensional Systems over Fading Channels: Capacity and Error Probability

The capacity and error probability of orthogonal space-time block codes (STBCs) are considered for the pulse-amplitude modulation (PAM), phase shift keying (PSK), quadrature-amplitude modulation (QAM) in fading channels. The suggested approach is based on employment of the generalized approach to signal processing in noise over the equivalent scalar additive white Gaussian noise channel with a channel gain proportional to the Frobenius norm of the matrix channel for the STBC. Using this effective channel, the capacity and probability of error expressions are derived for the PSK, PAM, and QAM modulation with space-time block coding. The Rayleigh, Rician and Nakagami fading channels are considered. As an application, these results are extended to obtain the capacity and probability of error for a multiuser direct sequence code-division multiple-access (CDMA) system constructed on the basis of the generalized approach to signal processing in noise employing the space-time block coding

Development and performance improvement of a novel zero cross-correlation code for SAC-OCDMA systems

Abstract In order to improve the performance of the Spectral Amplitude Coding-Optical Code Division Multiple Access (SAC-OCDMA) system, a zero cross-correlation (ZCC) code named double weight multi-diagonal (DW-MD) is proposed with constant weight (CW) or variable weight (VW). Mathematical results illustrate that it is feasible to reduce the number of filters without sacrificing system performance by using the CW DW-MD code instead of the multi-diagonal (MD) code. And for the VW DW-MD code, the supportable number of users for lower code weight W L (3) at bit error rate (BER) of 10−3 (voice) is 43, medium code weight W M (4) at BER of 10−9 (data) is 29, and higher code weight W H (5) at BER of 10−12 (video) is 32. Finally, the analysis about the position of chips “1” is given, and the simulation setups show that the CW DW-MD code outperforms the MD code when Gaussian optical filter is used.

다중 사용자 마이크로웨이브 무선전력전송 시스템 설계와 전력전송 효율 평가

In this study, a wireless power transmission system supporting multiple users was built based on a power pilot. The power pilot transmits the authorization and power status information of the user to the power station. The power station first transmits power to the user in the lowest power state for the longest time. A code-division multiple access system was used for supporting multiple receiving users and a non-uniform time-division multiple access system, which allocates power charging time differently to each user, was employed. Consequently, power was transmitted according to the power priority of multiple users. Finally, the power efficiency of the retrodirective array antenna system based on a phase-locked loop was increased by conjugating the phase of the power pilot received from the user and transmitting it to the user. We designed these three systems and verified their performance through simulations to build a wireless power transmission system that supports multiple users.

A hybrid OFDM–CDMA-based robust image watermarking technique

Digital watermarking is a process of embedding hidden information called watermark into different kinds of media objects. It uses basic modulation, multiplexing and transform techniques of communication for hiding information. Traditional techniques used are least significant bit (LSB) modification, discrete cosine transform (DCT), discrete wavelet transform (DWT), discrete Fourier transform (DFT), code division multiple access (CDMA) or a combination of these. Among these, CDMA is the most robust against different attacks except geometric attacks. This paper proposes a blind and highly robust watermarking technique by utilizing the basis of orthogonal frequency division multiplexing (OFDM) and CDMA communication system. In this scheme, the insertion process starts by taking DFT of host images, permuting the watermark bits in randomized manner and recording them in a seed as a key. Then PSK modulation follows inverse DFT (IDFT) that gives watermark information as OFDM symbols. These symbols are spread using spreading codes and then arithmetically added to the host image. Finally, scheme applies inverse DCT (IDCT) to get watermarked host images. The simulation results of the proposed scheme are compared with CDMA-based scheme in DCT domain. The results show that the robustness of the proposed scheme is higher than the existing scheme for non-geometric attacks.

Construction of a two-dimensional-VWOOC based on time domain/frequency domain optical CDMA system

In this paper, a new two-dimensional variable weight optical orthogonal code (2D-VWOOC) for optical code division multiple access (OCDMA) system is constructed. It takes the C as the frequency hopping sequence and the strict variable weight optical orthogonal code (SVWOOC) as the time spreading sequence. The code can meet the different quality of service (QoS) requirements of different users. Under the same time spread sequence, the bit error rate (BER) of QCHC/SVWOOC is 3∼9 orders of magnitude lower than that of other similar 2D-VWOOCs. In Optisystem simulation, when the transmission rate of the system is 14 Gbits/s, the BER of users with code weight 4 is 7 orders of magnitude lower than that of users with code weight 2, and a clear and correct eye diagram can be obtained.

Synchronization of Two Chaotic Stream Ciphers in Secure CDMA Communication Systems

In the basic processing of Code Division Multiple Access (CDMA) systems using chaotic code sequences, a pair of chaos generators in the transmitter and the receiver are used to generate the same sequence, in order to synchronize and retrieve the transmitted signals. In addition, a filter with a simple structure should be integrated into chaotic signals to achieve the maximum of the signal to noise ratio and mitigate the harmful effects of multipath. Another effective property of chaos signal is that a wireless multipath channel does not change the amount of contained information. Considering those issues, the present paper describes a practical system for synchronizing two chaotic generators used in a digital CDMA. Chaotic generators were used to spread the data and provide security against attack. Both receiver and transmitter were implemented using two separate Spartan 6 FPGA boards. Experimental results proved the robustness of the proposed method which could contribute towards the synchronization of chaotic signals in secure CDMA communication systems.

Hopfield learning‐based and non‐linear programming methods for resource allocation in OCDMA networks

This study proposes the deployment of the Hopfield neural network (H-NN) approach to optimally assign power in optical code division multiple access (OCDMA) systems. Figures of merit such as the feasibility of solutions and complexity are compared with the classical power allocation methods found in the literature, such as sequential quadratic programming (SQP) and augmented Lagrangian method. The analysed methods are used to solve constrained non-linear optimisation problems in the context of resource allocation for optical networks, especially to deal with energy efficiency in OCDMA networks. The promising performance-complexity trade-off of the modified H-NN is demonstrated through numerical results performed in comparison with classic methods for general problems in non-linear programming. The evaluation is carried out considering challenging OCDMA networks in which different levels of service quality required were considered for large numbers of optical users. The numerical results demonstrated that the three power allocation methods attain suitable convergence for different network sizes, while both the mH-NN and SQP methods achieve suitable equilibrium with less complexity.

Adequate spreading codes to reduce MAI in quasi‐synchronous MC‐DS‐CDMA system

In this study, the authors consider the effect of multiple access interference (MAI) on the performance of a quasi-synchronous multicarrier direct-sequence code-division multiple access (QS-MC-DS-CDMA) system. An analytical expression of the bit error rate (BER) is derived for a QS-MC-DS-CDMA system over additive white Gaussian noise channel. The BER performance degradation is proportional to the squares of the even correlation function (ECF) and odd correlation function (OCF) of the spreading codes. The obtained results show that the adequate spreading code is a set of binary zero correlation zone sequences with both zero ECF and OCF. The construction of the latter codes remains a challenge for researchers in the field of code design.

A Near-Optimal and Low-Complex Joint Multiuser Detection for QCSS-MC-CDMA System

Joint maximum likelihood (ML) multiuser detection for quasi-synchronous multicarrier code division multiple access (MC-CDMA) system has an excessive computational complexity, and it increases exponentially as the number of users and bits in each of the modulation symbols increases. Recently, sphere decoding (SD) has been adopted for multiuser detection whose complexity is polynomial in nature with the number of users, and it does not rely on the number of bits in the modulation. In this article, first we provide a construction of large size quasi-complementary sequence sets (QCSSs), and then a low-complexity optimum SD algorithm is proposed to detect the multiuser communication in QCSS-based MC-CDMA (QCSS-MC-CDMA) system. The complexity analysis has shown that the proposed SD-based QCSS-MC-CDMA receiver for multiuser communication provides lower complexity than that of ML-based QCSS-MC-CDMA receiver. We also show that peak to average power ratio of the proposed system is lesser than that of complete complementary code (CCC) based MC-CDMA (CCC-MC-CDMA) system. The proposed system overcomes the fractional delay problem which is noticed for CCC-MC-CDMA receiver. The bit error rate performance of the proposed SD-based QCSS-MC-CDMA system is compared with maximum ratio combining based MC-CDMA, ML-based MC-CDMA, frequency domain equalization based MC-CDMA, and CCC-MC-CDMA receivers over Rayleigh frequency selective fading channels. The work is validated through Monte Carlo simulation results.

A Generalized Construction of Multiple Complete Complementary Codes and Asymptotically Optimal Aperiodic Quasi-Complementary Sequence Sets

In recent years, complete complementary codes (CCCs) and quasi-complementary sequence sets (QCSSs) have found many important applications in multi-carrier code-division multiple-access (MC-CDMA) systems for their good correlation properties. In this paper, we propose a generic construction of multiple sets of CCCs over $\mathbb {Z}_{N}$ , consisting of sequences of length $N$ , where $N\geq 3$ is an arbitrary odd integer. Interestingly, the maximum inter-set aperiodic cross-correlation magnitude of the proposed CCCs is upper bounded by $N$ . It turns out that the combination of the generated CCCs results in a new set of sequences to obtain asymptotically optimal and near-optimal aperiodic QCSSs. The proposed construction includes a recent optimal construction of QCSSs with prime length as a special case and leads to asymptotically optimal QCSSs with new flexible parameters.

A Comparative Study of Asynchronous and Synchronous OCDMA Systems

This article presents a detailed survey on both asynchronous and synchronous Optical Code-Division Multiple-Access (OCDMA) systems based on 1-D, 2-D, and 3-D codes, distinctively. Accordingly, different coding schemes along with their multiplexing capacity, correlation features, advantages and drawbacks are addressed and benchmarked. Additionally, different detection techniques such as direct and coherent detection and their capability to handle Multiple Access-Interference are also discussed. Furthermore, the main OCDMA systems from the last two decades are summarized considering their coding schemes, performance, device components, and applications. Therefore, a new comparative study in terms of spectral efficiency and bit error rate for 1-D, 2-D, and 3-D asynchronous and synchronous OCDMA systems is carried out. Numerical results shown that the 3-D MD code stands out among the codes with superior performance as well as support to higher data rates and number of users.

Two-Dimensional Permutation Vectors' (PV) Code for Optical Code Division Multiple Access Systems.

In this paper, we present a new algorithm to generate two-dimensional (2D) permutation vectors’ (PV) code for incoherent optical code division multiple access (OCDMA) system to suppress multiple access interference (MAI) and system complexity. The proposed code design approach is based on wavelength-hopping time-spreading (WHTS) technique for code generation. All possible combinations of PV code sets were attained by employing all permutations of the vectors with repetition of each vector weight (W) times. Further, 2D-PV code set was constructed by combining two code sequences of the 1D-PV code. The transmitter-receiver architecture of 2D-PV code-based WHTS OCDMA system is presented. Results indicated that the 2D-PV code provides increased cardinality by eliminating phase-induced intensity noise (PIIN) effects and multiple user data can be transmitted with minimum likelihood of interference. Simulation results validated the proposed system for an agreeable bit error rate (BER) of 10−9.