Performance analysis of atmospheric optical CDMA systems

The atmospheric two-dimensional optical code-division multiple-access (CDMA) systems using pulse-position modulation (PPM) and Turbo-coded were presented. We analyzed the bit-error rate (BER) of the proposed system using pulse-position modulation (PPM) with considering the effects of the scintillation, avalanche photodiode noise, thermal noise, and multi-user interference. We showed that the atmospheric two dimensional (2D) optical PPM CDMA systems can realize high-speed communications when the logarithm variance of the scintillation is less than 0.1, and the turbo-coded atmospheric optical CDMA system has better bit error rate(BER) performance than the atmospheric optical PPM CDMA systems without turbo-coded. We also showed that the turbo-coded system has better performance than the multi-user detection system.

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.

A parallel combinatory (PC)-hybrid pulse interval modulation-optical code division multiple access (hPIM-OCDMA) system is proposed. In the PC-hPIM-OCDMA system, a user chooses e among K code sequences assigned to the user and sends e2b-ary PIM symbols for a given frame, where b is the number of bits per PIM symbol. The PC-hPIM-OCDMA system can offer higher bit rate than hPIM-OCDMA systems, and then it can reduce the effect of MAI, without reducing the bit rate. The bit error rate (BER) of PC-hPIM-OCDMA is derived, by considering that the dominant interference is MAI, which is caused by cross-correlation of strict optical orthogonal codes (SOOCs). Numerical results show that the PC-hPIM-OCDMA system can offer lower BER and accommodate more users than hPIM-OCDMA systems.

The encoding/decoding scheme based on Fiber Bragg Grating (FBG) for Optical Code Division Multiple Access (OCDMA) system is analyzed and the whole process from transmitting end to receiving end is researched in detail. The mathematical mode including signal transmission, summing, receiving and recovering are established respectively. One of the main sources of Bit Error Rate (BER) of OCDMA system based on FBGs is the unevenness of signal power spectrum, which leads to the chip powers unequal with each other. The Signal to Interfere Ratio (SIR) and BER performance of the system are studied and simulated at the case with uneven distribution of chips’ powers.

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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.

We compared the theoretical bit-error-rate (BER) and throughput performance limits of 2-D time-wavelength and coherent time-spreading optical code division multiple access (OCDMA) communication systems. The results show that the BER performance of the coherent time spreading OCDMA scheme is much superior to 2-D time wavelength scheme in the multiple access interference (MAI) noise limited case without considering beat noise, and the beat noise affects the system performance seriously and appears as a dominant source of noise for the two kinds of OCDMA systems. Nevertheless, the 2-D time wavelength scheme has better tolerance to beat noise than coherent time spreading OCDMA scheme, and the superiority is more apparent with the increasing of the code length N. While the coherent time spreading scheme has higher peak throughput, and it supports a higher throughput than the 2-D time wavelength scheme at higher offered loads.

We propose a multi-rate and multi-BEP transmission scheme using adaptive overlapping pulse-position modulator (OPPM) and optical power controller in optical code division multiple access (CDMA) networks. The proposed system achieves the multi-rate and multi-BEP transmission by accommodating users with different values of OPPM parameter and transmitted power in the same network. The proposed scheme has advantages that the system is not required to change the code length and number of weight depending on the required bit rate of a user and the difference of bit rates does not have so much effect on the bit error probabilities (BEPs). Moreover, the difference of transmitted powers does not cause the change of bit rate. We analyze the BEPs of the four multimedia service classes corresponding to the combinations of high/low-rates and low/high-BEPs and show that the proposed scheme can easily achieve distinct differentiation of the service classes with the simple system configuration.

A detailed analysis of the scalability of two-dimensional optical codes is presented and two network deployment architectures for optical code division multiple access (OCDMA) are discussed. We present the architecture and code design for a highly scalable, 2.5 Gb/s per user OCDMA system. The system is scalable to 100 potential and more than 10 simultaneous users, each with BER of less than 10-9. The system architecture uses fast wavelength-hopping, time-spreading codes. Unlike phase sensitive coherent OCDMA systems, this architecture utilizes standard on-off-keyed optical pulses allocated in the time and wavelength domains. This incoherent OCDMA approach is compatible with existing WDM optical networks and utilizes off-the-shelf components. We discuss a novel optical subsystems design for encoders and decoders that enable the realization of a highly scalable incoherent OCDMA system with rapid reconfigurability. We demonstrate the operation of 4 simultaneous users operating at OC-48 (~2.5 Gbit/s) with a power penalty of <;0.5dB and the reduction of multiple-access interference using ultra-fast all-optical sampling with newly proposed TOAD-based OCDMA receiver.

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

Optical CDMA systems have received much attention in fiber optic networks where the high-speed, the huge band width, the secure and immune communication, asynchronous and simultaneous access for multiple users on the same channel. Those features nominate strongly Optical CDMA for future optical multimedia networks. However, multiple access interference (MAI), which is originated from other simultaneous users, severely limits the capacity of the system. In this paper, an efficient transceiver architecture has been proposed using MAI cancellation to improve the performance of the existence simple transceiver without MAI cancellation. The strategy of the proposed transceiver is based on an estimation and cancellation of MAI in Optical CDMA system with employing a new spreading code named double padded modified prime code (DPMPC) and an efficient pulse-position modulation (PPM) scheme. In addition, for further improvement of the bit-error rate (BER) performance for the proposed transceiver Manchester coding (source coding) is used in signaling the transmitted data. The MAI estimation is achieved by pre-reserving one of optical spreading code sequences at the receiver based on the correlation property of the DPMPC. The proposed receiver estimates the interference and cancels it out from the received signal after the photo-detection process. The performance of the proposed transceiver architecture and its improvement will be compared with the performance of simple transceiver without MAI cancellation. The performance of various codes like modified prime code (MPC), new modified prime code (NMPC) are applied and compared with the proposed DPMPC to demonstrate its potential. An upper bound on the BER for the proposed system is derived and compared with a lower bound on the BER from the system without cancellation. The simulated computer results reveal that the performance, in terms of the BER, of the proposed system is significance improved compared with that of the system without cancellation.

We proposed the multipolar inserting zeros (MIZ) code for direct-sequence optical code division multiple access (OCDMA) systems. This code is designed to correspond with the judgment based on linear convolution. Simulation results showed that it can provide a lower multiuser interference than the conventional Gold sequence. With the same number of users, the bit error rate in the OCDMA system coded by MIZ can be reduced by several decuples than that coded by Gold, and the source power needed to reach a BER of 1×10?9 can also be reduced by several decibels. So the performance of OCDMA can be improved.

In this paper we introduce a successive interference cancellation (SIC) scheme for direct sequence optical code division multiple access (DS-OCDMA) systems using pulse position modulation (PPM). Considering double-padded modified prime code (DPMPC) as a signature sequence code, results show that the system has better performance in terms of both capacity and bit error rate (BER) as compared to the one without cancellation scheme, where the system with SIC scheme can support up to 88 users while the system without SIC scheme can support only 38 users at similar BER=10

In this paper, a theoretical analysis is presented to evaluate the influence of polarization mode dispersion (PMD) on the bit error rate (BER) performance of direct sequence optical code division multiple access system. In this analysis, intensity modulation direct detection technique is employed in single mode fiber operating at 1550nm, and optical orthogonal codes are used as address sequence. Optoelectronic conversion is performed by an avalanche photodiode in an optical correlator receiver. The system BER performance is determined on account of receiver, optical amplifier, and multiuser access interference noises. The power penalty suffered by the system is evaluated at BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> as a function of number of simultaneous users, chip rates, and PMD co-efficient. The system performance is found to degrade more at higher chip rate, and longer fiber length due to the PMD.

We present three detector/estimators (DEs) which allow multiuser detection and parameter estimation without a side channel in a dynamic asynchronous code-division multiple-access (CDMA) system in which users are entering and leaving the system. These DEs optimally detect a new user given only the chip rate and the spreading factor of the new user. Two of these DEs, the maximum-likelihood detector/estimator (MLDE) and the generalized maximum-likelihood detector/estimator (GMLDE), produce maximum-likelihood estimates of the new user's signature sequence, delay, and amplitude, which are then incorporated into a multiuser detector. The third DE, the cyclic detector/estimator (CDE), is the most computationally efficient of the three processors. This DE detects the new user by testing for cyclostationarity and then uses suboptimal schemes to estimate the new user's signature sequence, delay, and amplitude. Simulations indicate that all three DEs reliably detect a new user for an E/sub s///spl sigma//sup 2/ (symbol-energy-to-noise ratio) of 5 dB. The MLDE and GMLDE produce signature sequence and delay estimates with probability of error less than 0.07 for an E/sub s///spl sigma//sup 2/ of 10 dB, and the CDE produces signature sequence and delay estimates with probability of error less than 0.13 for an E/sub s///spl sigma//sup 2/ of 15 dB.