Spatial Diversity for Mitigating Near-far Problem in Wireless Optical CDMA Systems

This study explores a data collection system using an optical wireless code-division multiple access (CDMA) scheme. The system's transmitters asynchronously send data to a base station receiver. In CDMA systems, communication performance may degrade because of the near-far problem caused by variations in propagation path length between each transmitter and the receiver. To solve this problem, this paper proposes a new base-station receiver structure that employs hard limiters to restrict the amplitude of strong signals and successive interference cancellers to eliminate the effects of residual signals, and shows the advantages of this new combination method over existing methods that use hard limiters or successive interference cancellers independently.

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.

We present a novel optical code division multiple access (OCDMA) scheme based on spatial optical heterodyning for free-space optical communication systems. In this technique, in particular, the decoding process is established by means of a spatial optical heterodyne receiver. The spatial heterodyning OCDMA introduced can be considered to be in the class of spread-space techniques, which implies that there is no structural limitation on the shape of time domain signals, e.g., digital or analog modulation. For the sake of simplicity and practicality we considered on-off keying modulation. However, there is no limitation on using more advanced modulation schemes such as pulse position modulation. In this scheme a set of independent orthogonal spatial channels at both transmitting and receiving apertures is needed. Thus this class of OCDMA is applicable only to near-field free-space systems. The structures of the encoder and decoder are presented, and the wave propagation is completely analyzed by means of the Fresnel approximation for the free-space kernel. Only the effect of multiuser interference is considered. The effects of background noise and atmospheric turbulence on the performance of the system are avoided in order to highlight the main capabilities and features of spatial optical heterodyning OCDMA. A thorough statistical analysis is carried out, and the characteristic function of the sampled output is computed. The bit error rate is obtained by means of Gaussian and saddle-point approximations. Results show that the signal-to-interference ratio is approximately a function of processing gain and the number of users, as expected. The optical spatial heterodyning OCDMA technique can prove to be of importance in optical switching and free-space optical communication systems.

We analyze the basic principles of the spread-space holographic code division multiple access (CDMA) technique. We describe the structure of the encoder and decoder and present a basic mathematical analysis based on spatial harmonic decomposition for the receiver's processing module, which is comprised of a Fourier transform lens and a holographic matched filter. Subsequently, we study two applications of optical holographic CDMA, namely, free-space holographic CDMA photonic switch and free-space (wireless) multiaccess optical (infrared) indoor communications. For both techniques, we describe the two-dimensional imaging techniques, namely user's code transmission, and highlight the basic parameters and components that need to be optimized. To obtain the bit-error rate (BER) of the proposed application, we first evaluate the probability density function of multiuser interference and then evaluate the BER as a function of the processing gain, number of users, the received power, and the optimum threshold. For BER /spl ap/ 10/sup -9/, the result shows an extraordinary number of users that can be supported via holographic CDMA, for both applications.

Green's function calculating of electric fields in surface-channel charge-coupled devices : R. M. Barsan. Int. J. Electronics44, (2) 177 (1978)

Code division multiple access (CDMA) promises to become the enabling technology for the future generation of satellite communications. For instance, CDMA is being used in Globalstar, Skybridge and is also proposed for the satellite component of UMTS. The evolution of satellite systems towards multimedia communications, necessitates not only wideband CDMA but also requires a hybrid code and time division multiple access (CTDMA) technique because of the packet nature of the data. In this paper, we give an overview of the challenges in designing a wideband CTDMA modem. As a first challenge, the wideband nature of the CDMA modem dictates that the tracking loops of the modem can no longer be executed in software but must be fully integrated in VLSI. Second, the packet based nature of the communication in a CTDMA modem requires ultra fast acquisition for each packet. The extremely low signal-to-noise ratio in satellite communications largely complicates this fast acquisition task.

We experimentally demonstrate what we believe to be a novel detection scheme for interfacing asynchronous optical code division multiple access (CDMA) signals with an electronic clock and data recovery system that operates only at the baseband bandwidth. This allows using a large optical bandwidth expansion factor in which the optical chip rate is much larger than the bandwidth of the optoelectronic receiver. The received optical CDMA signal is launched into a four-wave-mixing-based wavelength-aware all-optical front end that rejects multiaccess interference, followed by an amplitude-noise suppression stage comprised of a semiconductor optical amplifier. The clean signal is then converted into a non-return-to-zero-like signal by a baseband receiver. Using the proposed detection scheme, asynchronous transmission and detection of optical CDMA signals is implemented. With the novel detection scheme, the classic CDMA near-far problem is mitigated, and error-free detection is easily obtained.

The transmission of both optical code division multiple-access (OCDMA) and wavelength division multiplexing (WDM) users on the same band is investigated. Code pulses of spectral amplitude coding (SAC)/optical code division multiple-access (CDMA) are overlaid onto a multichannel WDM system. Notch filters are utilized in order to suppress the WDM interference signals for detection of optical broadband CDMA signals. Modified quadratic congruence (MQC) codes are used as the sig- nature codes for the SAC/OCDMA system. The proposed system is sim- ulated and its performance in terms of both the bit-error rate and Q-factor are determined. In addition, eavesdropper probability of error-free code detection is evaluated. Our results are compared to traditional nonhybrid systems. It is concluded that the proposed hybrid scheme still achieves acceptable performance. In addition, it provides enhanced data confiden- tiality as compared to the scheme with SAC/OCDMA only. It is also shown that the performance of the proposed system is limited by the interference of the WDM signals. Furthermore, the simulation illustrates the trade- off between the performance and confidentiality for authorized users.

To provide Quality of Service (QoS) and data rate differentiation, we investigate in this paper a 2 dimensional optical code division multiple access (OCDMA) technique. A 2D multi-weight coding scheme with parallel mapping technique is developed. The system enhancement is assessed thanks to the use of Forward Error Correction (FEC) based on Low Density Parity Check (LDPC) codes. In order to have an efficient correction power, we develop an adaptation of the LDPC decoding algorithm to the specific multiple access interference distribution in a multimedia OCDMA network. The proposed network permits a high number of users to communicate with high data rates adapted to a multimedia transmission (voice, data and video) with corresponding QoS (10-3, 10-6, 10-9).

<div class="section abstract"><div class="htmlview paragraph">Aviation industry is striving to leverage the technological advancements in connectivity, computation and data analytics. Scalable and robust connectivity enables futuristic applications like smart cabins, prognostic health management (PHM) and AI/ML based analytics for effective decision making leading to flight operational efficiency, optimized maintenance planning and aircraft downtime reduction. Wireless Sensor Networks (WSN) are gaining prominence on the aircraft for providing large scale connectivity solution that are essential for implementing various health monitoring applications like Structural Health Monitoring (SHM), Prognostic Health Management (PHM), etc. and control applications like smart lighting, smart seats, smart lavatory, etc. These applications help in improving passenger experience, flight operational efficiency, optimized maintenance planning and aircraft downtime reduction. Intra Aircraft WSNs (IAWSN) used for such applications are expected to provide robust and reliable communication performance. However, IAWSNs, when deployed, must co-exist with other wireless devices and networks based on Wi-Fi and Bluetooth technologies. Sharing of the ISM frequency band (2.4 GHz) among these networks makes the co-existence problem more challenging and has a significant impact on the Quality of Service (QoS) in terms of throughput, latency and Packet Error Rate (PER) of IAWSN. In addition, they are subjected to additional electromagnetic interference from other electronic and avionic systems onboard aircraft. There are various channel management, priority-based scheduling and time-sharing techniques that are deployed currently to address the co-existence problem. However, these methods perform a trade-off among one or more operational parameters like channel bandwidth, number of nodes per channel, throughput, latency, PER, etc. of IAWSN. Code Division Multiple Access (CDMA) technique is a proven one in cellular networks for providing reliable interference free communication performance especially in large scale networks. This paper evaluates the feasibility of deploying CDMA for IEEE 802.15.4 based IAWSN onboard aircraft. A CDMA based communication schema is proposed and simulated for IAWSN operating as per IEEE 802.15.4 protocol. The communication performance of the CDMA based IAWSN is evaluated in comparison with the performance of a standard IEEE 802.15.4 protocol implementation in the presence of interference from the co-nodes of the IAWSN and Wi-Fi devices by analyzing the QoS parameters like signal to noise ratio (SNR), operational bandwidth, Bit-Error Rate (BER) and Process Gain through simulation. Simulations results are evaluated against the desired performance level for a reference use case application. Improvement opportunities are identified and areas for future research are proposed.</div></div>

In this article, a network structure of passive optical network (PON) in conjunction with optical code division multiple access (OCDMA) technique is theoretically investigated using Modified Quadratic Congruence (MQC) codes as a signature sequence, and taking into account the effects of various noises such as shot noise, phase-induced intensity noise (PIIN), thermal noises and multiple access interference (MAI) noises. The system also has been simulated with 9 users simultaneously over a 20km channel length, utilizing spectral amplitude coding (SAC) detection scheme with fiber Bragg grating filters. The results show that, the system based on SAC scheme and MQC codes at 622Mb/s, was achieved the error free transmission (bit error rate BER <;10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> ) for a fully loaded PON. Hence, the system shows good performances when various system parameters applied such as the optimum transmit power, channel length are varied.

Recently, free space optical (FSO) communication network has gained popularity because of its unlicensed spectrum, high data rate, low power consumption and ease of installation. Spectral amplitude coded optical code division multiple access (SAC-OCDMA) technique has become significant because of enhanced network capacity, privacy, and simultaneous use of multiple asynchronous users. However, the climatic turbulence causes signal fluctuations and degrades the quality of the received signal. Therefore, to improve the quality of the received signal in this work, SAC-OCDMA technique is integrated with orthogonal frequency division multiplexing (OFDM) technique and performance of such hybrid model is investigated. Two SAC-OCDMA codes are proposed in this work referred as triangular matrix-zero cross correlation (TM-ZCC) and pulse shifting substitution-zero cross correlation (PSS-ZCC) codes. The hybrid model uses the novel one dimensional (1D) SAC-OCDMA codes with two different types of receivers such as modified AND (M-AND), and single photodiode detection (SPD) receivers over Gamma-Gamma fading channel to evaluate the bit error rate (BER) performance of the proposed hybrid model. It is observed from the simulation results that the hybrid SAC-OCDMA-OFDM model with SPD detection receiver performs better in terms of bit error rate (BER) than that of M-AND detection receiver. Furthermore, it is also observed from the simulation results that the hybrid model based on the proposed TM-ZCC code performs better than the proposed PSS-ZCC code in terms of BER and higher FSO distance coverage. The performance of the proposed model is estimated for 20 and 30 active users simultaneously at various data rates. The TM-ZCC and PSS-ZCC codes are generated using MATLAB platform and the proposed hybrid SAC-OCDMA-OFDM model is validated using optisystem version 15 over the Gamma-Gamma fading channel.

With the growing number of autonomous vehicles equipped with pulsed scanning LIDAR operating close to each other at the same time, one pulsed scanning LIDAR might receive laser pulses from other pulsed scanning LIDAR. The reception of unwanted laser pulses from other pulsed scanning LIDAR is called mutual interference. Due to Lambertian reflectance of the laser pulse, a mixture of incoherent laser pulses arrives at each pixel from different paths apart from the direct path. This effects cause significant distortions in the estimation of depth, especially near corners and convex areas of the scene. We proposed a new pulsed scanning LIDAR, which measures a scene without mutual interference. The emitted pulse of each pixel is modulated pulses by direct sequence optical code division multiple access (DS-OCDMA) techniques. The modulated pulses include unique device identification number (ID), the pixel position in the line, and checksum. It emits the modulated pulse, and receives the return light to the detector. We modeled the entire of the pulsed scanning LIDAR operation in the RSoft OptSim and validated the results obtained in modeling and simulation.

This paper proposes a cost-efficient hybrid configuration based on optical code division multiple access (CDMA) system overlaid WDM grid. In the proposed hybrid system only 15 wavelengths with channel separations up to 25GHz have been used to carry out 120 users’ data with spectral efficiency of 1.6 b/s/Hz. In other words, a number of users can share one wavelength in an optical CDMA technique, at a 40Gbps of line rate for each wavelength. Hence, an improvement on the wavelength utilization efficiency can be achieved. Results show that the hybrid system supports 120 users over 90km of single mode fiber and 15.075km of dispersion compensating fiber. The results also show that optical CDMA, which is based on spread spectrum technology, has the ability to suppress the unavoidable nonlinear effect, i.e., four-wave mixing (FWM), where the optical energy of a bit is spread over signature sequence code. The effect of MAI in the proposed hybrid system is reduced due to the zero cross-correlation property of Multi-diagonal (MD) code; therefore the FWM light associated with MAI is reduced. Moreover, the squeezing of the chips interval into a quarter of bit duration guarantees the avoidance of inter-symbol interference (ISI).

This paper analyzes the performance of generalized optical orthogonal codes (OOC) in a wireless infrared system using code division multiple access (CDMA) technique. Since the emergence of generalized OOC s, they have been analyzed in optical fiber media, in which noise can be ignored with good approximation. But in an optical wireless system which inherently suffers from intense noise sources, generalized OOC behaves differently. The main issues of an optical wireless system are intense noise sources which degrades system performance and limited available bandwidth which is caused by multipath distortion in the channel. We believe that these analyses will help the designer of an optical wireless system, to select the best code family which satisfies the desired condition on bit error rate (BER) or required bandwidth.