Sharif Digital Repository

In this paper we study the performance of spectrally-encoded spread-time (SE/ST) technique when applied to ultrawideband (UWB) signals in the context of wireless code-division multiple-access (CDMA) communication. In this technique UWB pulses are spectrally encoded with pseudorandom codes that are assigned uniquely to each user. We shall show that the SE/ST ultrawideband system provides an appropriate multi-user capacity in realistic UWB channels, along with the ability of narrowband interference (NBI) suppression. To this end, after obtaining a suitable statistical model for spectrally encoded spread-time signals, we investigate the performance of the system in the presence of... 

In this study, the authors study a fibre-optic local area network deploying optical code division multiple access technique with differential M-ary pulse position modulation (DPPM). The authors present the performance analysis of DPPM signalling using a double optical hardlimiter receiver structure considering the impairment caused by interference of other users. The analysis assumes a set of bit asynchronous transmitters employing generalised optical orthogonal codes for their DPPM signalling (GOOC-DPPM). Using mathematical analysis and simulations, the authors have demonstrated that GOOCDPPM improves both frame error probability and the maximum achievable network throughput compared with... 

The demands for mobile communication services are growing rapidly. In heavily populated areas, cell splits are necessary to increase the capacity of cellular systems. Cell splitting causes a cellular system to have mixed cell sizes. Calculation of the reverse link capacity of CDMA cellular systems with mixed cell sizes and imperfect power control is presented in this paper. When a macro cell is split into micro cells, the capacity of a non-uniform cellular system has been calculated, together with the calculation of micro and neighboring macro cell capacity. The computer simulation results show that with the reduction in radius of a micro cell, its capacity increases, while that of the... 

In this paper, we propose a new scheme to reduce Peak-to-As erage Power Ratio (PAPR) in a MC-CDMA system. In this scheme, some codes among the family of signature codes used in the system are utilized only for PAPR reduction. A set of spreading codes and only one despreading code are assigned to each user. The user selects its own spreading code among the assigned set based on which one proides lower PAPR. The numerical results show that despite of its simplicity, our proposed approach substantially reduces the PAPR of MCCDMA system at the expense of not capturing some portions of the transmitted energy in the receiver for some users © 2002 IEEE  

This paper describes a study of the performance of various configurations for placing multiple optical amplifiers in a typical coherent ultrashort light pulse code-division multiple access (CULP-CDMA) communication system using the additive noise model. For this study, a comprehensive performance analysis was developed that takes into account multiple-access noise, noise due to optical amplifiers, and thermal noise using the saddle-point approximation technique. Prior to obtaining the overall system performance, the input/output statistical models for different elements of the system such as encoders/decoders, star coupler, and optical amplifiers were obtained. Performance comparisons... 

In this paper, 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... 

In this study, the authors explore a generalised scheme for the synchronous code division multiple access (CDMA). In this scheme, unlike the standard CDMA systems, each user has different codewords for communicating different messages. Two main problems are investigated. The first problem concerns whether uniquely detectable overloaded matrices (an injective matrix, i.e. the inputs and outputs are in one-to-one correspondence depending on the input alphabets) exist in the absence of additive noise, and if so, whether there are any practical optimum detectors for such input codewords. The second problem is about finding tight bounds for the sum channel capacity. In response to the first... 

In this paper, we propose a new method for creating more distinguishable QoS levels in OCDMA networks. The key idea behind the proposed scheme is to adaptively control the transmit power level in each chip based on OCDMA channel state. The structure of the transmitter and receiver are proposed. We provide performance analysis of the recommended scheme and show that it can reduce the system BER and provide better QoS differentiation in a multiclass network. We provide simulation results and demonstrate that they have a good match with the results obtained from mathematical analysis  

In this paper, we analyze and obtain performance of a multilevel Optical Code Division Multiple Access (OCDMA) system with Variable-Weight Optical Orthogonal Codes (VW-OOCs) under the same-bit-energy assumption. The exact error-probability of this system is derived for the Generalized VW-OOCs (codes with arbitrary cross-correlation) which is necessary for the optimum design of multilevel OCDMA systems. We show that the bit power has an important role on determining the performance of codes with different weights. Furthermore, the results reveal that the performance of low-weight codes depends on the weight of the low-weight codes and the code-weight ratio of two classes. Moreover, our exact... 

Due to the low cardinality of optical orthogonal codes (OOCs) with a correlation value of 1, the capability of an internal channel-coding scheme cannot be applied effectively for an increasing number of simultaneous users in a typical fiber-optic code division multiple-access (FO-CDMA) system. In this paper, we consider applying the internal channel-coding scheme to the framed time-hopping (FTH) FO-CDMA system. Due to the extremely high cardinality of the spreading sequence in FTH FO-CDMA, the technique highlights the ability of the internal channel-coding scheme to increase the effective number of users in an FO-CDMA system. For the FTH FO-CDMA system, we only consider a Poisson... 

In this study, the performance of a spectral-amplitude-coding optical code division multiple access (SAC-OCDMA) system in the asynchronous regime is evaluated using a Gaussian approximation of the decision variable for codes with fixed cross-correlation used in SAC-OCDMA systems. The authors consider the effect of phase-induced intensity noise (PIIN), thermal noise and shot noise. Moreover, the validity of the Gaussian approximation is confirmed by a Kolmogorov-Smirnov fitness test. For sake of comparison, the bit error rate (BER) of the asynchronous SAC-OCDMA system is also plotted numerically in comparison with the BER of the synchronous SAC-OCDMA. They show that a SAC-OCDMA system without... 

The objective of this study is to design sub-optimal signature matrices for binary inputs for an overloaded code division multiple access (CDMA) system as developed by this author group. In this study, the authors propose to use the sum capacity, the bit error rate and distance criteria as objective functions for signature matrix optimisation. Three optimisation techniques, the genetic algorithm, the particle swarm optimisation and the conjugate gradient (CG) are exploited in this work. Since the optimisation computational complexity increases by matrix dimensions, it is practically impossible to directly optimise the large signature matrices. In order to address this problem, a method is... 

In this paper, we analyze the sensitivity degradation of a system due to various orders of chromatic dispersion. To do this, the power penalty of the uncoded and encoded pulse of a coherent ultrashort light pulse Code Division Multiple Access (CDMA) communication system due to various orders of chromatic dispersions are analyzed. Analysis of uncoded pulse shows that the power penalty is unacceptably high when second order dispersion is not compensated. However we can obtain acceptable level of power penalties while compensating second and third order dispersion. Also, due to the high order of dispersion, the power of encoded pulse decreases as it propagates through the fiber which leads to a... 

Optical code division multiple access (OCDMA) is a promising multiple-access technique for future all optical networks. In this manuscript, the usefulness of exploiting bistable optical elements as threshold devices in coherent OCDMA systems is analytically investigated, in which the bistability effect is modeled by a simple square loop  

In this study the authors present an in-depth study, analysis and discussion on maximum likelihood (ML)-based receiver for a typical spectrally encoded spread-time CDMA in the presence of multiple narrowband interference (NBI) signals in an additive white Gaussian noise channel. Furthermore, the authors demonstrate that by combining useful properties of ML-based receiver and three-level codes, that is, codes with values of -1, 0, +1, the authors can introduce a new strategy in which superior performance with respect to previous receiver structure based on two-level codes, that is, codes with values of -1, +1, can be attained. With the help of an example, the authors drive, first, the... 

In this paper we present and investigate a digital design and implementational details of a wireless optical CDMA (OCDMA) system based on generalized optical orthogonal codes (OOCs). A detailed digital implementation analysis and transmitter/receiver architectures in the context of the above design are fully studied. We analyze various subsystems for a wireless OCDMA system including detector and receiver structure, acquisition, and tracking blocks as well as evaluating their performance in a typical wireless OCDMA communication system. By analyzing two main parts of the system, namely, chip-level detector and OCDMA receiver blocks we evaluate the performance of the overall system.... 

A novel packet-based photonic label switching router for a multirate all-optical switch using generalized multiprotocol label switching is proposed. The idea is based on using optical code-division multiple access (OCDMA) as multiplexing technique and treating OCDMA codes as labels. The system can coexist with current wavelength division multiplexing systems on the same infrastructure. The concept of switch fabric is introduced. Label processing and label swapping functionalities of the switch are discussed. In-depth analyses are made for spectrally phase-encoded OCDMA (SPE-OCDMA) due to its capabilities of supporting high data rates, large code cardinality, and its secure transmission.... 

We propose and consider using a class of multiple-access sequences, namely, optical orthogonal codes (OOCs) in atmospheric optical code-division multiple-access systems. We obtain analytical solutions to the error probability for various channel models using positive-intrinsic-negative diode and avalanche photodiode photodetectors. In our analysis, the effects of atmospheric turbulence, ambient light, thermal noise, and multiuser interference are considered, in the context of a semiclassical photon-counting approach. The performance of the systems taking advantage of space diversity and error-correcting codes are also evaluated. Two common and widely used optical modulations, on-off keying... 

In this paper, we introduce a new and advanced algorithm, namely, multiple-shift algorithm for code acquisition in optical code-division multiple access (CDMA) systems using unipolar optical orthogonal codes (OOCs) as signature sequences. We analyze the performance of the newly proposed algorithm and obtain a bound on its performance and show its advantage in reducing the mean time of synchronization when compared with other synchronization methods. The algorithm can be used with many different receiver structures, like active or passive correlator with or without hardlimiter(s). However, in this paper, we only consider the simple active correlator structure for further discussions and... 

A unified multiple-access performance analysis and comparison of three multicarrier spread-spectrum multiple-access schemes, namely MC-CDMA (multicarrier code-division multiple-access), MC-FH (multicarrier frequency hopping) and a hybrid of the above systems, called DS-MC-FH (direct-sequence MC-FH), in synchronous and asynchronous multi-rate environments, where each user can have several multi-rate services, with a maximum-ratio combining (MRC) receiver is provided. Near-far and power-control-error (PCE) effects on the systems' performances have also been investigated. Our analyses indicate that MC-CDMA systems with Walsh codes outperform the other schemes. DS-MC-FH systems with Walsh codes...