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IEEE Communications Letters

Impact of the beat noise on the performance of 2-D optical CDMA systems

L. Tancevski, Leslie Ann Rusch

Published Aug 1, 2000• 110 citations
Publisher

Abstract

The system performance of 2-D wavelength hopping/time spreading CDMA systems is found to be limited by the occurrence of the beat noise between the components of the signal and the multiple user interference. The variance of the noise is determined and the probability of error is calculated for a general system employing asymmetric prime-hop sequences. A comparison with an idealized system neglecting the beat noise reveals that the system performance is seriously affected by the beat noise.

DOI
10.1109/4234.864189

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The system performance of 2-D wavelength hopping/time spreading CDMA systems is found to be limited by the occurrence of the beat noise between the components of the signal and the multiple user interference. The variance of the noise is determined and the probability of error is calculated for a general system employing asymmetric prime-hop sequences. A comparison with an idealized system neglecting the beat noise reveals that the system performance is seriously affected by the beat noise.
Enhanced security has often been cited as an important benefit of optical CDMA (O-CDMA) signaling. However, the quality and degree of security enhancement has not been closely examined in the literature. This paper examines the degree and types of security that may be provided by O-CDMA encoding. A quantitative analysis of data confidentiality is presented for O-CDMA encoding techniques that use both time spreading and wavelength hopping. The probability of successful data interception is calculated as a function of several parameters, including signal-to-noise ratio and fraction of total available system capacity. For reasonable choices of system and encoding parameters, it is shown that increasing code complexity can increase the signal-to-noise ratio (SNR) required for an eavesdropper to "break" the encoding by only a few dB, while the processing of fewer than 100 bits by an eavesdropper can reduce the SNR required to break the encoding by up to 12 dB. The overall degree of confidentiality obtainable through O-CDMA encoding is also compared with that obtainable through standard cryptography. time-spreading/wavelength-hopping in particular, and O-CDMA in general, are found to provide considerably less data confidentiality than cryptography, and the confidentiality provided is found to be highly dependent on system design and implementation parameters.
The performance of coherent time-spreading (TS) and asynchronous TS phase coding optical code division multiple access (OCDMA) systems is investigated and analyzed. Particular consideration is given however to both beat noise (BN) and multiple-access interference (MAI) effects, and the impact of decision threshold on the bit-error-rate (BER) performance is evaluated. Improvement of the BER performance can be obtained by use of optimum threshold detection, and the improvement is more apparent in the asynchronous TS OCDMA scheme for a low number of simultaneous users with long code length.
Multi user interference in optical CDMA systems comes from the property that users' signature sequences are not completely orthogonal, which will severely affect the performance of OCDMA systems, thereby directly limiting the number of users which simultaneously use the system to transfer data. So it is very important to try our best to minimize the multi user interference. In this paper several solutions for reducing multi user interference are introduced, and their performance is discussed.