Channel interference reduction using random Manchester codes for both synchronous and asynchronous fiber-optic CDMA systems

In the underwater passive positioning systems (UPPS), it involves a key technology of multi-user communication that the beacons synchronously send positioning messages to the targets. Interferences from other users, referred to multi-access interferences (MAIs), can induce significant bit errors for the desired user. In this paper, we designed an underwater acoustic Code Division Multiple Access (CDMA) system using the coded cyclic shift key (CCSK) modulation, and proposed a hybrid MAI cancellation scheme based on cyclic shift inverse matrix transform (CSIMT) and recursive least square (RLS) in the CDMA system. The hybrid scheme can suppress MAI better than RLS or CSIMT, and meets the requirements of strong stability to support reliable transmission for UPPS. The CSIMT interference cancellation performs well under the far-near effects, which doesn’t depend on the accuracy of the estimation of transmitted data, and the strong MAI can be reduced by transformation of the zero-forcing correlation values using m sequence cyclic shift inverse matrix. The RLS interference cancellation is used to mitigate MAI form the detected users with comparable power, by using the estimated the channel impulse response between the receiver and the detected users, and the regenerated signal of detected users form decoder output, which allows of strong error-correcting codes, and it’s robust in an asynchronous environment. The theoretical analysis and experimental result show that the hybrid MAI cancellation scheme can effectively suppress MAI better. The shallow water sea trial was carried out near Dalian Tiger Beach Ocean Park in China, at a distance of about 5 km, the data rates of 100bps, where three users could communicate asynchronously with low failure ratio and can match the UPPS application.

A new interference cancellation technique using time division reference signal is proposed for optical synchronous code-division multiple-access (CDMA) systems with modified prime sequence codes. In the proposed system one user in each group is not allowed to access the network at each time, and this unallowable user's channel is used as a reference signal for other users in the same group at the time. The performance of the proposed system using an avalanche photodiode (APD) is analyzed where the Gaussian approximation of the APD output is employed and the effects of APD noise, thermal noise, and interference for the receiver are included. The proposed cancellation technique is shown to be effective in improving the bit error probability performance and in alleviating the error floor when the number of users and the received optical power are not appreciably small.

For high-speed code division multiple access (CDMA) systems, interchip interference that is caused by multipath propagation becomes severe and cannot be ignored. Together with the multiple access interference due to nonideal conditions in CDMA, they become major obstacles to overall system performance. In this paper, both batch and adaptive algorithms based on linear prediction are proposed for joint blind equalization and multiuser detection for asynchronous CDMA systems. It is shown that the new methods are insensitive to estimation error of propagation delay or chip timing. In addition, the adaptive algorithm is computationally efficient. Simulation results show that the proposed methods are also near-far resistant and compare favorably to many existing methods.

Optical CDMA systems have been investigated widely to apply for high speed LAN, because they allow multiple users to access the network simultaneously. For optical CDMA systems, both asynchronous and synchronous systems have been studied. With synchronous CDMA, the available number of signature codes is larger than that of asynchronous CDMA systems. For a given value of bit error rate, synchronous CDMA systems can accommodate more simultaneous users than asynchronous CDMA systems. Furthermore, synchronous CDMA can be efficiently used in conjunction with TDMA and WDMA on multimedia communication networks.This chapter discusses sequences, modulations, system architectures used in optical synchronous CDMA systems and their performance.

We report the design of optical fiber asynchronous code-division multiple-access (CDMA) systems using fully optical orthogonal codes (FOOCs), which can be employed to support future airborne real-time communication services. The use of incoherent optical CDMA (IO-CDMA) can result in lower complexity and more reliable implementation than that of the coherent version, so this makes IO-CDMA more adequate for operation in the severe aircraft environment. When FOOCs are used in the IO-CDMA systems, the peaks of all the cross-correlation functions and all the sidelobes of any auto-correlation function are then limited to "1" (i.e., the minimum correlation constraint for incoherent optical processing) in fully asynchronous transmissions. Moreover, proposed systems can effectively support multi-rate and variable-rate data communications with no violation of the minimum correlation constraint and no change of IO-CDMA encoders/decoders. This characteristic can be used to improve the system flexibility and availability. A basic experiment is also demonstrated to verify the characteristics of FOOCs.

This paper studies detecting matrices in random dense and sparse Code Division Multiple Access (CDMA) systems. Detecting matrices were originally introduced in the coin weighing problem. Such matrices can be used in CDMA systems in over-loaded scheme where the number of users is greater than the number of chips. We drive some conditions in the large system limit for binary and bipolar random CDMA systems to ensure that any random matrix is a detecting matrix. Furthermore, we extend our results to sparse random ternary matrices that have been using in the sparse CDMA literature. Finally, a construction method for the sparse detecting matrices is introduced.

Code division multiple access (CDMA) systems are actively being considered for use in mobile environments. New semianalytical expressions for the bit error rate of an asynchronous direct sequence CDMA system employing noncoherent binary demodulation formats in a Nakagami fading multipath channel are derived. Two binary demodulation schemes are considered, namely, the differential phase shift keying and the square-law demodulation with orthogonal spreading sequences. In each case the analysis assumes that an arbitrary number of independent but nonidentical resolvable paths are combined by the RAKE receiver. The results show that the effect of nonidentical fading on system performance cannot be ignored in such systems.

The problem of blind despreading of short-code direct sequence spread-spectrum signal in asynchronous or synchronous system is considered. A novel blind estimation algorithm for time offset, spreading and information sequences is presented in this paper. Firstly, the received signal is divided into two-information-period-length temporal vectors overlapped by one-information-period and accumulates these vectors one by one to form the accumulated matrix. The algorithm exploits an unbiased estimation of user’s time offset to realize synchronization blindly, and an operation of singular value decomposition is applied to the intercepted matrix, which is intercepted from the accumulated matrix according to user’s time offset. Lastly, the spreading sequence is recovered from a right singular vector blindly; therefore, the shortcomings of traditional algorithms, such as phase ambiguity when make use of two vectors to form the spreading sequence, are avoided. And the information sequence is then recovered blindly from the left singular vector. Simulation results show that the new method is capable of retrieving time offset, the spreading and information sequences with higher accuracy than previous methods.

In this paper, the performance of a binary phase shift keyed random\ntime-hopping impulse radio system with pulse-based polarity randomization is\nanalyzed. Transmission over frequency-selective channels is considered and the\neffects of inter-frame interference and multiple access interference on the\nperformance of a generic Rake receiver are investigated for both synchronous\nand asynchronous systems. Closed form (approximate) expressions for the\nprobability of error that are valid for various Rake combining schemes are\nderived. The asynchronous system is modelled as a chip-synchronous system with\nuniformly distributed timing jitter for the transmitted pulses of interfering\nusers. This model allows the analytical technique developed for the synchronous\ncase to be extended to the asynchronous case. An approximate closed-form\nexpression for the probability of bit error, expressed in terms of the\nautocorrelation function of the transmitted pulse, is derived for the\nasynchronous case. Then, transmission over an additive white Gaussian noise\nchannel is studied as a special case, and the effects of multiple-access\ninterference is investigated for both synchronous and asynchronous systems. The\nanalysis shows that the chip-synchronous assumption can result in\nover-estimating the error probability, and the degree of over-estimation mainly\ndepends on the autocorrelation function of the ultra-wideband pulse and the\nsignal-to-interference-plus-noise-ratio of the system. Simulations studies\nsupport the approximate analysis.\n

The design of code-division multiple-access (CDMA) transmission systems for satellite communications requires an appropriate consideration of the distortion effects due to on-board nonlinear amplification. The aim of this paper is to provide an analytical framework for the evaluation of the in-band nonlinear distortion effects on the performance of CDMA systems. Both synchronous systems with orthogonal codes and asynchronous systems are considered. It is first shown that, when the users accessing the channel have the same power and their number is sufficiently large, the nonlinear distortion in the decision variables at the receiver can be simply described by a complex scale factor, which depends on the high-power-amplifier (HPA) characteristics only, and an additive noise, which is uncorrelated to the useful signal. Moreover, an analytical formulation of the bit error probability and the total degradation as a function of the output back-off and number of users is given. In the results, which are obtained for three classes of HPA models (i.e., the traveling wave tube amplifier, the solid-state power amplifier, and the amplifier with ideal predistortion), the performance and the capacity of power-limited systems is also discussed.

In this paper, we applied genetic algorithm (GA) scheme as a suboptimal detection of a bit-synchronous code division multiple access (CDMA) systems over a Gaussian channel. The GA scheme attempts to search for the user's transmitted bit sequence that optimizes the correlation matrix employed. Simulation results showed that the Bit Error Rate (BER) performance of our proposed multiuser detector is better than those of conventional receivers like Minimum Mean Square Error(MMSE), Matched Filter(MF) and Rake Receivers.

Measurement and analysis of the temperature dependence of bulk and surface leakage currents in InAs avalanche photodiodes have been performed between 77 K and 290 K. At unity gain, SU-8 passivated InAs photodiodes have low dark current densities of 100 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 290 K and 150 nA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 77 K. An avalanche multiplication factor of 25 was measured at 13 V and 19.5 V at 290 K and 77 K, respectively. The photodiodes exhibit dynamic resistance-area products, calculated at 0.1 V of 34 Ω-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 290 K and 910 MΩ-cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 77 K. Our analysis showed that between the temperatures of 200 K and 290 K, the bulk leakage current is proportional to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> whereas the surface leakage current is proportional to <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> from 77 K to 290 K, where <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> is the intrinsic carrier concentration. The activation energies deduced were 0.36 eV and 0.18 eV suggesting diffusion dominated bulk current and generation and recombination dominated surface current.

Steady-state visual evoked potential (SSVEP)-based brain-computer Interface (BCI) has demonstrated the potential to manage multi-command targets to achieve high-speed communication. Recent studies on multi-class SSVEP-based BCI have focused on synchronous systems, which rely on predefined time and task indicators; thus, these systems that use passive approaches may be less suitable for practical applications. Asynchronous systems recognize the user's intention (whether or not the user is willing to use systems) from brain activity; then, after recognizing the user's willingness, they begin to operate by switching swiftly for real-time control. Consequently, various methodologies have been proposed to capture the user's intention. However, in-depth investigation of recognition methods in asynchronous BCI system is lacking. Thus, in this work, three recognition methods (power spectral density analysis, canonical correlation analysis (CCA), and support vector machine (SVM)) used widely in asynchronous SSVEP BCI systems were explored to compare their performance. Further, we categorized asynchronous systems into two approaches (1-stage and 2-stage) based upon the recognition process's design, and compared their performance. To do so, a 40-class SSVEP dataset collected from 40 subjects was introduced. Finally, we found that the CCA-based method in the 2-stage approach demonstrated statistically significantly higher performance with a sensitivity of 97.62±02.06%, specificity of 76.50±23.50%, and accuracy of 75.59±10.09%. Thus, it is expected that the 2-stage approach together with CCA-based recognition and FB-CCA classification have good potential to be implemented in practical asynchronous SSVEP BCI systems.

Synchronous digital systems have been presenting serious problems of implementation in DSM (deep sub-micron) VLSI technology, for which the asynchronous paradigm has become an interesting alternative. Unfortunately, the topic of synthesis of asynchronous digital systems has been ignored or presents only a few numbers of teaching classes in most courses in electrical engineering, due the difficulty of different styles of asynchronous design, what requires many hours of classroom teaching. In this paper we propose a methodology for synthesis of asynchronous systems that requires just few hours of classroom teaching, being interesting to be incorporated in the subject of logic design. Starting from the RTL (Register Transfer Level) description of synchronous systems, the proposed method synthesizes the asynchronous system in the “decomposition” style, which is defined by XBM_AFSM (extended burst-mode asynchronous finite state machine) + datapath synchronous. A simple procedure is used to convert the specification of SFSM to the XBM specification, which describes the XBM_AFSM. Through a case study, it is shown the simplicity of the methodology, resulting in high-performance circuits.

Due to the desire for the next generation of wireless communications systems to be able to support higher data rate, multimedia, traffic, it is of interest to determine if the use of a higher order alphabet size, in conjunction with code division multiple access (CDMA) signaling, will allow enhanced performance. We compare the performance of an asynchronous CDMA system when a 16-QAM alphabet is used with that of a CDMA system employing QPSK as the modulation format.