Synchronous Optical Code-division Multiple-access Research Articles

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.

A New Code Family for QS-CDMA Visible Light Communication Systems

Visible light communication (VLC) is a promising technology for wireless communication networks. Optical code division multiple access (OCDMA) is a strong candidate for VLC-based applications. The predominant source of bit error in OCDMA is the multiple access interference (MAI). To eliminate MAI in synchronous OCDMA, zero cross correlation (ZCC) codes have been proposed. However, synchronization problems and multipath propagation introduce relative non-zero time delays. Therefore, the zero correlation zone (ZCZ) concept was introduced. In this paper, we propose a new method for generating ZCC codes. The proposed construction can accommodate any number of users with flexible Hamming weight. The numerical results obtained show that the proposed codes significantly reduce MAI, compared to ZCC, as well as ZCZ codes.

Design of double-weight code for synchronous OCDMA systems with power control

Double-weight optical code division multiple access (OCDMA) systems are proposed for studying differentiated quality-of-service transmission. Based on quadratic congruence code (QCC), we construct a one-dimensional double-weight code family, which can be well utilized in incoherent synchronous double-weight OCDMA networks. By introducing algebraic transformation to code sequences of QCC in level 1, we obtain multiple double-weight codes with cross-correlation 1. Under the same-bit-power assumption, the performance of low-weight codes can be significantly improved and is always superior to that of high-weight codes in double-weight OCDMA systems with power control. This property is contrary to previous conclusions under the same-chip-power assumption.

Enhancing spectral efficiency and capacity in synchronous OCDMA by transposed-MPC

This paper investigates a newly proposed spreading code-set based on the prime code (PC) families referred to as ‘transposed modified prime codes (T-MPC)’ for the enhancement of spectral efficiency of synchronous incoherent optical code-division multiple-access (OCDMA) networks. This code family increased the code-set cardinality up to twice of existing PC families’ size. This also implies that a greater number of users can be accommodated by the network. Since there is no longer a time-shift feature in T-MPC like in conventional modified prime codes (MPC), the code is not predictable and thus even more secure. Since the code structure is similar to MPC, its deployment in a system/network already running MPC will not require hardware modification. Due to the higher code utilization factor of T-MPC, a greater number of users are accommodated under certain bit-error rate (BER) resulting in remarkable improvement in the spectral efficiency (SE) and capacity. The T-MPC compatibility with low-weight energy-efficient MPC construction is also investigated. The BER and SE performances are analyzed and compared with existing code families. The results indicated that the T-MPC employment can improve up to 50% higher spectral efficiency.

Direct-detection synchronous O-CDMA system with interference estimation and cancellation

An M-ary coded synchronous optical code-division multiple-access (O-CDMA) system with pulse-position modulation (PPM) is investigated. One novel class of optical spreading codes based on combinatorial designs is adopted in the multiplexing process. Their ideal correlation properties facilitate the discrimination between desired signals and jamming. However, the multiple-access interference (MAI) with high intensity significantly deteriorates the system performance even if the number of interferers is small. In this paper, we present an interference reduction technique for direct-detection O-CDMA to suppress the noise effect and increase the system capacity. The MAI from reference signals can be estimated by utilizing the uniform cross-correlation (CC) among its sequences and considerably cancelled out after the photodetection process. The upper bound on the error probability of optical synchronous PPM-CDMA is then derived. The proposed system is shown to be effective to improve the bit error performance and to alleviate the error floor when the number of simultaneous users and the received optical power are not appreciably small.

Synchronous optical code-division multiple access systems with constant multi-user interference

A synchronous optical code-division multiple access system with balanced encoder instead of on-off-keying (OOK) encoder is proposed. In the system, the multi-user interference is maintained constant during a frame, and the receiver estimates the interference frame by frame instead of bit by bit. The data rate of the proposed system is no longer severely limited by the electronic processing time in estimating the interference, and the structure of the proposed receiver is simpler than those in OOK systems. In the receiver, both linear and non-linear threshold estimators are introduced. Numerical results show that the proposed system has smaller bit error rate than OOK systems, especially when the number of training bits is increased.

A Class of Errorless Codes for Overloaded Synchronous Wireless and Optical CDMA Systems

In this paper, we introduce a new class of codes for overloaded synchronous wireless and optical code-division multiple-access (CDMA) systems which increases the number of users for fixed number of chips without introducing any errors. Equivalently, the chip rate can be reduced for a given number of users, which implies bandwidth reduction for downlink wireless systems. An upper bound for the maximum number of users for a given number of chips is derived. Also, lower and upper bounds for the sum channel capacity of a binary overloaded CDMA are derived that can predict the existence of such overloaded codes. We also propose a simplified maximum likelihood method for decoding these types of overloaded codes. Although a high percentage of the overloading factor degrades the system performance in noisy channels, simulation results show that this degradation is not significant. More importantly, for moderate values of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> /N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (in the range of 6-10 dB) or higher, the proposed codes perform much better than the binary Welch bound equality sequences.

Unipolar codes for optical spectral-amplitude code-division multiple-access systems based on combinatorial designs

A novel class of optical signature codes based on combinatorial designs is proposed for optical spectral-amplitude code-division multiple-access (CDMA) systems. It is applicable to both synchronous and asynchronous incoherent optical CDMA and is compatible with both frequency-encoded and time-spreading schemes. Simplicity of construction, larger code cardinality and larger flexibility in cross-correlation (CC) control make the proposed code family an interesting candidate for future optical CDMA applications that require a large number of simultaneous users. It has been shown that the system performance can be significantly improved by using the proposed codes with ideal in-phase CC in preference to Hadamard codes.

Performance enhancement of perfect difference codes with spectral amplitude coding using sub‐optimal decision threshold

AbstractThis paper applies perfect difference code (PDC) to develop an optical code‐division multiple‐access (OCDMA) system with spectral amplitude coding (SAC). The differential receiver of the proposed systems exploits the cross‐correlation property of PDC to remove the multiple‐access interference (MAI) without reducing the power of the received signal. The threshold applied in the data decision process has a crucial influence on the bit error rate (BER). Hence, this study investigates the effects of several different decision rules. The numerical results show that the BERs of the synchronous OCDMA system using PDC proposed by Weng and Wu are 100 times and 17 times greater than those of the current system for code sizes of 183 and 307, respectively, under conditions of equal power budgets. To achieve the same BERs, the theoretical analysis reveals that the transmission power required by the Weng's system is around k/(k‐1) times greater than that of the current system, where k denotes the code weight of the signature code employed. Additionally, under conditions of reliable communication (i.e. BER ≤ 10−9), the capacity of the proposed system is almost one and half times greater than that of Weng's system. Regarding the data decision threshold, this study proposes a realisable sub‐optimal decision threshold as a replacement for the optimal decision threshold. The performance of the proposed system using this sub‐optimal threshold is found to be only marginally poorer than that obtained when the optimal decision threshold is employed. Therefore, the proposed sub‐optimal threshold provides a suitable means of improving the system performance of SAC OCDMA systems using signature codes with a fixed in‐phase cross‐correlation property. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Optical spectral amplitude coding CDMA systems using perfect difference codes and interference estimation

The spectral amplitude coding (SAC) technique is applied to a synchronous optical code-division multiple-access (OCDMA) system, using perfect difference code (PDC). The use of the SAC technique removes the codeword synchronisation requirement of time-spreading OCDMA systems using PDCs. In the proposed system, the unity cross-correlation property of PDCs is exploited by the interference estimator of the proposed system to estimate the number of active users simultaneously transmitting bit 1. This estimate is then employed to modify the threshold used to differentiate between transmitted bits 1 and 0. In contrast to the variance expansion of the decision statistics used in synchronous OCDMA systems with interference cancellation, the proposed system preserves the characteristics of the received signal statistics, thereby avoiding data decision errors. As the estimation rules and the decision thresholds are important factors influencing the estimates and the bit-error rates, the effects of these two factors are thoroughly investigated. A second SAC-PDC OCDMA system is developed, which mitigates multiple-access interference (MAI) by means of an interference cancellation technique and makes data decisions based on the estimated MAI. The numerical results reveal that the two systems provide a better performance and a larger capacity than the synchronous OCDMA system.

Jitter Performance Analysis of SOCDMA-Based EPON Using Perfect Difference Codes

Recently the passive optical network (PON) has attracted a lot of attention because it is an excellent solution for broad-band first mile services. Combining Ethernet frame with PON (EPON) and using synchronous optical code-division multiple-access (SOCDMA) technology are very promising for this application. In this paper, we analyze the jitter performance of SOCDMA-based EPON communication systems using perfect difference codes (PDC). We propose the joint jitter estimator and canceller (JJEC) to reduce the impact of jitter and multiple-user interference (MUI). The bit error rate (BER) of the system with JJEC is formulated analytically based on the correlator and the simple sampler receiver structures. The numerical results show that the proposed scheme can significantly suppress the MUI and improve the system performance.

Synchronous Optical-CDMA Systems Using Tunable Hard Limiters

In this paper, a structure of tunable optic hard limiter (TOHL) is proposed for the synchronous optical code-division multiple access (OCDMA) system. In the proposed OCDMA system, the multiple-access interference is estimated and cancelled by the TOHL. The OCDMA system using the modified prime code as the spreading code with the proposed TOHL is introduced. The avalanche photodiode (APD) noise and thermal noise are taken into account in the performance analysis. The numerical results show that using TOHL in the receiver can effectively reduce the bit error rate of the OCDMA system.

Unipolar codes with ideal in-phase cross-correlation for spectral amplitude-coding optical CDMA systems

Unipolar codes with ideal in phase cross-correlation are important in spectral amplitude-coding optical code-division multiple-access (CDMA) systems since these codes eliminate multiuser interference and also suppress the effect of phase-induced intensity noise. However, very little research work has been done on such codes although codes with ideal cross-correlation have been studied for many years. In the paper by Zhou et al.(see Electron. Lett., vol.36, p.728-29, 2000), such a code has been introduced without a clear construction method. In this paper, we firstly review this old code, and then construct two new codes with ideal in-phase cross-correlation in algebraic ways. Both of the proposed new codes are obtained by modifying former codes with ideal cross-correlation. It has been shown that the system performance can be improved significantly by using codes with ideal in-phase cross-correlation instead of the Hadamard code. These codes can also be used in synchronous optical CDMA systems for multiuser interference cancellation.

Codes for spectral-amplitude-coding optical CDMA systems

Codes with ideal in-phase cross correlation (CC) are required in spectral-amplitude-coding optical code-division multiple-access (CDMA) systems since these codes eliminate multiuser interference and also suppress the effect of phase-induced intensity noise. However, little research work has been done on such codes although codes with ideal CC have been studied for many years. In a paper by Zhou et al. (2000), such a code has been introduced, but no clear construction method has been presented. In this paper, we first review the construction method of this old code and then construct two new codes with ideal in-phase CC in algebraic ways. Both of the proposed new codes are obtained by modifying former codes with ideal CC. It has been shown that the system performance can be improved significantly by using codes with ideal in-phase CC instead of the Hadamard code. These codes can also be used in the synchronous optical CDMA systems for multiuser interference cancellation.

Performance analysis of direct-detection optical asynchronous CDMA systems with double optical hard-limiters

Performance of optical asynchronous code-division multiple-access (CDMA) systems with double optical hard-limiters is analyzed under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Optical orthogonal codes (OOC's) are employed as signature sequence codes. In the analysis, chips are assumed to be synchronous among users, that is, the chip synchronous case, because the effect of the interference is largest in the chip synchronous case and thus the performance in the chip synchronous case results in the upper bounds on the performance of the asynchronous system. The performance is evaluated under average power and bit rate constraints. The results show that, differing from the optical synchronous CDMA systems with double optical hard-limiters, the optical asynchronous CDMA systems with double optical hard-limiters have good performance even when the number of simultaneous users is large.

Direct-detection optical synchronous CDMA systems with double optical hard-limiters using modified prime sequence codes

An optical code-division multiple-access (CDMA) system with double optical hard-limiters is proposed where the optical hard-limiters are placed before and after an optical correlator. Moreover, the-effect of the optical hard-limiter on the performance of the optical synchronous CDMA systems using modified prime sequence codes as signature codes is analyzed under the assumption of a Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. We evaluate the performance under average power and bit rate constraints. Our results show that using the single optical hard-limiter slightly degrades the performance of the optical CDMA systems under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Moreover, we show that the optical CDMA systems with double optical hard-limiters have better performance than other conventional CDMA systems with and without the optical hard-limiter when the number of simultaneous users is not so large.

Ultrafast all-optical code-division multiple-access (CDMA) fiber-optic networks

Abstract With the advanced development of fiber-optics, the large bandwidth-expansion required by spread spectrum code-division multiple-access (CDMA) can now be accomodated by using a fiber-optic channel for transmission and incoherent optical signal processing for code generation and correlation. Optical synchronous CDMA (S/CDMA), a synchronous transmission format, is introduced and compared with optical (asynchronous) CDMA. Using modified prime sequence codes, S/CDMA is shown to accomodate a larger number of subscribers and more simultaneous users than CDMA. A two-user experiment demonstrating the feasibility of optical S/CDMA is performed at 10 Mbit/s (500 Mchip/s), using optical signal processing. (A chip is a bit-representation component; please see Section 2.) An environment in which S/CDMA would be particularly suited is also discussed. In addition, recent research shows that the feasibility of optical CDMA is also determined by the architecture of the optical encoders/decoders. The coding architecture affects, for example, the amount of power loss and length of optical delays associated with code sequence generation and correlation, which, in turn, affect the power budget, size, and cost of an optical CDMA system. Therefore, a new, modified 2 n coding architecture is proposed and compared with prime and 2 n coding architectures. Study shows that the modified 2 n architecture performs the best and is particularly suitable for ultrafast all-optical processing and waveguides implementation for the future all-optical CDMA networks. A 100 Mbit/s optical CDMA experiment in free space demonstrating the feasibility of the modified 2 n coding architecture, using a serial combination of 50/50 beam splitters and retroreflectors, at 10 Tchip/s (i.e., 100,000 chip/bit) with 100 fs laser pulses is also reported.

The design and performance analysis for several new classes of codes for optical synchronous CDMA and for arbitrary-medium time-hopping synchronous CDMA communication systems

New families of spread-spectrum codes are constructed, that are applicable to optical synchronous code-division multiple-access (CDMA) communications as well as to arbitrary-medium time-hopping synchronous CDMA communications. Proposed constructions are based on the mappings from integer sequences into binary sequences. The authors use the concept of number theoretic quadratic congruences and a subset of Reed-Solomon codes similar to the one utilized in the Welch-Costas frequency-hop (FH) patterns. The properties of the codes are as good as or better than the properties of existing codes for synchronous CDMA communications: both the number of code-sequences within a single code family and the number of code families with good properties are significantly increased when compared to the known code designs. Possible applications are presented. To evaluate the performance of the proposed codes, a new class of hit arrays called cyclical hit arrays is recalled, which give insight into the previously unknown properties of the few classes of number theoretic FH patterns. Cyclical hit arrays and the proposed mappings are used to determine the exact probability distribution functions of random variables that represent interference between users of a time-hopping or optical CDMA system. Expressions for the bit error probability in multi-user CDMA systems are derived as a function of the number of simultaneous CDMA system users, the length of signature sequences and the threshold of a matched filter detector. The performance results are compared with the results for some previously known codes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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With the advanced development of fiber-optics, the large bandwidth-expansion required by spread spectrum code-division multiple-access (CDMA) can now be accomodated by using a fiber-optic channel for transmission and incoherent optical signal processing for code generation and correlation. Optical synchronous CDMA (S/CDMA), a synchronous transmission format, is introduced and compared with optical (asynchronous) CDMA. Using modified prime sequence codes, S/CDMA is shown to accomodate a larger number of subscribers and more simultaneous users than CDMA. A two-user experiment demonstrating the feasibility of optical S/CDMA is performed at 10 Mbit/s (500 Mchip/s), using optical signal processing. (A chip is a bit-representation component; please see Section 2.) An environment in which S/CDMA would be particularly suited is also discussed. In addition, recent research shows that the feasibility of optical CDMA is also determined by the architecture of the optical encoders/decoders. The coding architecture affects, for example, the amount of power loss and length of optical delays associated with code sequence generation and correlation, which, in turn, affect the power budget, size, and cost of an optical CDMA system. Therefore, a new, modified 2n coding architecture is proposed and compared with prime and 2n coding architectures. Study shows that the modified 2n architecture performs the best and is particularly suitable for ultrafast all-optical processing and waveguides implementation for the future all-optical CDMA networks. A 100 Mbit/s optical CDMA experiment in free space demonstrating the feasibility of the modified 2n coding architecture, using a serial combination of 50/50 beam splitters and retroreflectors, at 10 Tchip/s (i.e., 100,000 chip/bit) with 100 fs laser pulses is also reported.