Fiber-optic Code Division Multiple Access Research Articles

The combinatorial construction for a class of optimal optical orthogonal codes

Optical orthogonal code (OOC) has good correlation properties. It has many important applications in a fiber-optic code-division multiple access channel. In this paper, a combinatorial construction for optimal (15 p , 5, 1) optical orthogonal codes with p congruent to 1 modulo 4 and greater than 5 is given by applying Weil’s Theorem. From this, when v is a product of primes congruent to 1 modulo 4 and greater than 5, an optimal (15 v , 5, 1)-OOC can be obtained by applying a known recursive construction.

Temporal/spatial fiber-optic CDMA systems with post- and pre-optical amplification

We present a thorough analysis based on photon-counting techniques on temporal/spatial fiber-optic code-division multiple-access systems incorporating both post- and pre-optical amplifiers. In this analysis, we consider the effect of shot noise, thermal noise, and source extinction on system bit-error rate. Our results can be used to estimate the maximum tolerable amount of quantum fluctuations in the received signal.

The effect of grating‐position deviation and fiber dispersion in the fiber‐optic–CDMA network with the FBG encoder/decoder for incoherent 2D coding

AbstractThe effect of grating‐position deviation (GPD) and fiber dispersion in a fiber‐optic–CDMA network with a FBG encoder/decoder for incoherent 2D coding are studied here. For error‐free transmission, the GPD‐caused maximum optical pulse‐position deviation (PPD) must be below Δm, and the transmission distance must be below Lm. A modified encoder/decoder configuration is proposed to compensate for the fiber dispersion. With this proposed encoder/decoder, the user's distances in a network can be improved to L ± Lm. Error‐free transmission has been achieved through 9.5‐km SMF in the experiment with this design. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 35: 16–19, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10504

Statistical analysis of fiber-optic CDMA communication systems. I. Device modeling

In this paper, we present a new methodology for analysis of all-optical fiber-optic code-division multiple-access (FO-CDMA) networks. In this analysis, we propose statistical models, based on photon-counting techniques, for some basic elements of the network, such as splitters, combiners, star couplers, and FO-CDMA passive encoders/decoders. By following the statistical variation of the photon-count of the string of pulses that constitutes the address sequence in an FO-CDMA network, we will be able to reveal the quantum-limited optical signal-to-noise ratio (OSNR) required at the transmitter output to meet the performance limits. Moreover, considering receiver thermal noise and source extinction ratio, we explore the role of using an optical preamplifier before the detector and the dependence of the performance on the transmitted power and the weight of the employed optical orthogonal codes (OOCs).

Multistage decoding for internally bandwidth efficient coded Poisson fiber-optic CDMA communication systems

We consider the structure and performance of a multistage decoding scheme for an internally bandwidth efficient convolutionally coded Poisson fiber-optic code division multiple access (CDMA) communication system. The decoder is implemented electronically in several stages in which in each stage, the interfering users' coded bit decisions obtained in the previous stage is applied for computing the likelihood of the coded symbols of the desired user. The first stage is a soft-input Viterbi decoder for the internally coded scheme, in which the soft-input coded symbol likelihood values are computed by considering the multiuser interference as a noise signal. The likelihood of coded symbol computed in each stage is then entered into the convolutional decoder for the next bit decisions. The convolutional codes that are used for demonstrating the performance of the multistage decoder are super orthogonal codes (SOCs). We derive the bit error rates (BERs) of the proposed decoder for internally coded Poisson fiber-optic CDMA systems using optical orthogonal codes (OOCs) along with both ON-OFF keying (OOK) and binary pulse position modulation (BPPM) schemes. Our numerical results indicate that the proposed decoding scheme substantially outperforms the single-stage soft-input Viterbi decoder. We also derive the upper bound on the probability of error of a decoder for the known interference case, which is the ultimate performance of a multiuser decoder, and compare the result with that of the soft-input Viterbi decoder.

Statistical analysis of fiber-optic CDMA communication systems. II. Incorporating multiple optical amplifiers

For pt. I see ibid., vol.20, no.8, p.1304-16 (2002). In this paper, we study and elaborate on the architectural consideration of using multiple optical amplifiers in an all-optical fiber-optic code-division multiple-access (FO-CDMA) network. Our study and considerations are based on obtaining the statistical characteristic functions for photon-counts of a string of output pulses that constitutes the address code in a FO-CDMA network. Optical orthogonal codes (OOCs) with minimum auto-and cross correlation are employed as the address or signature sequence codes. Our analysis is based on chip-synchronous interference only, which provides an upper bound on the system performance. Shot noise, thermal noise, and extinction ratio are considered in this analysis. Bit error rate (BER) results are obtained using two methods, namely, saddle-point and Gaussian approximations. From our BER analysis, we develop some insight into the optimum distribution of optical amplifiers and their corresponding gain setting parameter. Moreover, the effects of source power, code weight, number of users, bit rate, and amplified spontaneous emission noise are investigated.

Optical orthogonal code acquisition in fiber-optic CDMA systems via the simple serial-search method

We analyze, for the first time, the performance of a serial-search algorithm for optical code-division multiple access (CDMA) systems using optical orthogonal codes (OOC). For our analysis, we use advance photon-counting methods and we consider the effect of shot noise and dark current and multi-access interference (MAI). We introduce a new method for modeling the effect of MAI, then we use two approaches to consider this effect on the performance of the synchronization system. For the first approach, we consider the average effect of interference in our analysis, while for the second approach two specific forms of interference are taken into account where one form represents a heavily localized interference pattern and the other shows a uniformly distributed one. The effects of different parameters of the system including system threshold, number of active users in the network, and mean number of photons in each optical pulse on the performance of the synchronization system are evaluated and discussed.

BER analysis of synchronous fiber‐optic CDMA networks with suboptimal decision threshold

AbstractThe bit‐error rate (BER) of synchronous optical CDMA networks with a fixed (suboptimal) decision threshold is derived. The strategy used offers the advantage that the network need not dynamically monitor the number of users. The numerical results are satisfactory when compared with previous literature based on an optimal threshold. © 2002 John Wiley & Sons, Inc. Microwave Opt Technol Lett 32: 293–296, 2002.

Performance Analysis of Synchronous Fiber-optic Code Division Multiple Access Networks with Consideration of Receiver Noises and Laser Chirping

Article Performance Analysis of Synchronous Fiber-optic Code Division Multiple Access Networks with Consideration of Receiver Noises and Laser Chirping was published on August 1, 2002 in the journal Journal of Optical Communications (volume 23, issue 4).

Synchronous Fiber-Optic PPM-CDMA Systems with Interference Reduction

In this paper, we propose a multi-user interference estimation and cancellation technique for direct-detection fiber-optic code division multiple-access (FO-CDMA) communication systems employing pulse position modulation (PPM). Interference estimation is based on the unique grouping property of modified prime code sequences. By having a code unused in each group of the code sequence, the estimated interference can be canceled out from the received signal after the photo-detection process. We have used APD photo-detector in our proposed system. An upper bound on the bit-error rate (BER) for the proposed system is derived and compared with a lower bound on the BER for the system without cancellation. In the presence of multiple users interference (MUI), shot noise, thermal noise, dark current noise and surface leakage current, our results clearly indicate that the performance, in terms of the bit error probability, of the proposed system is significantly improved compared with that of the system without cancellation. In our proposed system, we have also used Manchester codes in signaling the transmitted data to further improve the bit error rate (BER).

Multiwavelength self-seeded Fabry-Perot laser with subharmonic pulse-gating for two-dimensional fiber optic-CDMA

A new scheme using self-seeding in a gain-switched Fabry-Perot (F-P) laser with subharmonic pulse gating in an external cavity consisting of a string of fiber Bragg gratings (FBG) for incoherent two-dimensional fiber-optic code division multiple access applications is proposed and demonstrated experimentally. Three-wavelength optical pulses are generated sequentially with pulsewidths of 78 ps at a repetition rate of 250 MHz, and side-mode suppression ratio better than 20 dB. The FBG position deviation has been confirmed not to affect the laser operation if the resulting pulse position deviation is within the gating pulsewidth. The generated optical pulses are much more stable and intense than those obtained directly from the gain-switched F-P laser.

Analysis of the higher order signals in fiber optic code-division multiple access networks using series coupler access encoder and series coupler access decoder

Fiber optic code-division multiple access (FO-CDMA) networks using optical series coupler access encoders (SCAEs) and series coupler access decoders (SCADs) are analyzed considering the higher order signals of the optical matched filter. By mathematically interpreting the inevitable interference effect of the higher order signals up to the third order, we found that the performance of the FO-CDMA networks could be seriously degraded due to the sidelobe peaks increased by the higher order signals. We also found that the interference signal intensities increase according to the increase of coupling coefficient (?). Compared with the conventional analysis without considering the higher order signals, the peak-to-sidelobe ratio considering the higher order signals is degraded, for example, by 3.75 dB for N(coupler number) = 5, ? = 0.5, and the code length of 341.

Low-rate super-orthogonal channel coding for fiber-optic CDMA communication systems

In this paper, we consider using practical low-rate error correcting codes in fiber-optic code division multiple-access (CDMA) communication systems. To this end, a different method of low-rate channel coding is proposed. As opposed to the conventional coding schemes, this method does not require any further bandwidth expansion for error correction in fiber-optic CDMA communication systems. The low-rate channel codes that are used for demonstrating the capabilities of the proposed method are super-orthogonal codes. These codes are near optimal and have a relatively low complexity. We evaluate the upper bounds on the bit-error probability of the proposed coded fiber-optic CDMA system assuming both on-off keying and binary pulse position modulation schemes. It is shown that the proposed method significantly outperforms the uncoded systems for various receiver structures such as a correlator with and without hard-limiter and chip-level detector. Furthermore, the performance of the proposed coded fiber-optic CDMA system is also evaluated in the presence of different values of dark current.

Performance Evaluation of Fiber-optic Synchronous Code Division Multiple Access Networks with Parallel Cancellation Scheme for PIN+OA receiver

In this paper, performance of PIN+OA receiver with inter-user interference cancellation technique based on parallel cancellation scheme for synchronous code division multiple access (S/CDMA) network has been analysed. The expression for probability of error is derived taking into consideration the inter-user interference, optical amplifier noise and various sources of receiver noise. The numerical results are computed for practical values of parameters and the results are compared with APD receivers. It is observed that the PIN+OA receiver provides an improvement of about 6.5 dB in the received optical power level over APD receiver for a given requirement.

Perfect difference codes for synchronous fiber-optic CDMA communication systems

In this paper, we introduce the perfect difference sets and propose a synchronous fiber-optic code-division multiple-access system (CDMA) using these sets to generate the signature codes. Two interesting properties of these codes are discussed: (1) any two different codes are cyclic-shifted with each other; and (2) the cross correlation between any two different codes is exactly one. Thus, we may treat these codes as quasi-orthogonal codes. Using the first property of these codes, we can simplify design of the transmitter. In the receiver, we can use the second property to effectively eliminate the multiple-user interference (MUI) without reducing the number of usable codes. Based on the proposed transmitter and receiver, the system performance is derived. The numerical examples reveal that the proposed system can provide reliable communication even under heavy load. We believe that the proposed system using perfect difference codes outperforms any other synchronous fiber-optic CDMA systems.

Fiber-grating-based optical CDMA spectral coding with nearly orthogonal M-sequence codes

An optical spectral coding scheme is proposed for fiber-optic code-division multiple access (FO-CDMA) communications. The spectral coding is based on the pseudo-orthogonality of FO-CDMA codes properly written in the fiber Bragg grating (FBG) devices. An FBG decoder is configured on the basis of orthogonal correlation functions of the nearly orthogonal maximal-length sequence codes. With such a design, an intended receiver user that operates on the defined orthogonal correlation functions will reject interfering users and obtain orthogonality between the FO-CDMA users in the system.

Incoherent bidirectional fiber-optic code division multiple access networks

We propose bidirectional fiber-optic code division multiple access networks based on the multiwavelength optical code, and the feasibility of the network is demonstrated experimentally. Encoders/decoders are implemented with AWG routers and fiber delay lines, and incoherent light sources are used. Due to the loop-back configuration of the AWG router-based encoder/decoder, bidirectional transmission is possible in the proposed access network. Decoding of broad-spectrum encoded signals (/spl Delta/f=1.6 THz, /spl Delta//spl lambda/=12.8 nm at 1550 nm) after transmission over a 40-km fiber link was performed with dispersion compensation.

Fiber-optic code division add-drop multiplexers

We propose a new fiber-optic code division add-drop multiplexer (ADM) which not only possesses the inherent advantages of the fiber-optic code division multiple access (CDMA), but also is fully reconfigurable. Because incoherent optical signal processing is unipolar, the dropping process is accomplished by adding the inversion of the received code. The proposed ADM is applicable to both the bus and the ring topologies. The corresponding-system performance is calculated with the consideration of thermal noise, shot noise, and the amplified spontaneous emission (ASE) noise of the optical amplifier. Many interesting and useful conclusions such as the suboptimum value of various system parameters are presented.

Performance Analysis of Direct-detection Fiber-optic Synchronous CDMA Systems with Parallel Interference Cancellers

Article Performance Analysis of Direct-detection Fiber-optic Synchronous CDMA Systems with Parallel Interference Cancellers was published on August 1, 2000 in the journal Journal of Optical Communications (volume 21, issue 4).

Analytical comparison of various fiber-optic CDMA receiver structures

We study the performance of optical code-division multiple access (CDMA) systems using various receivers structures. Two general classes of receivers based on required electronic bandwidth are studied. Optical orthogonal codes (OOCs) are utilized as signature sequences and the performance studied in this paper takes into account the effect of all major noise sources, i.e., quantum shot-noise, dark current noise, and Gaussian circuit noise. Furthermore, this paper introduces a generalized method of analyzing the performance of various optical CDMA receiver structures. Required mean number of photon count per chip time for reliable transmission of data bits for various receiver structures is investigated. Finally, the advantages and disadvantages of various receiver structures are discussed.