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Journal of Optical Communications

Theoretical Analysis of Optimum Threshold Detection in Coherent Time-spreading OCDMA Systems

Xiaogang Chen, Shengqin Feng, Dexiu Huang

Published Jan 1, 2009• 0 citations
Publisher

Abstract

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.

DOI
10.1515/joc.2009.30.3.166

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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.
We propose a novel quasi-synchronous coherent time-spreading optical CDMA (TS-OCDMA) system, where the synchronization among users can be controlled within permissible time delay. Relationship between the system performance and code parameters is analyzed. In quasi-synchronous coherent TS-OCDMA system, beat noise (BN) and multiple-access interference (MAI) are depend- ent on the mean intensity of aperiodic cross-correlation within permissible time delay. LA sequence is employed in this network, and code performance is analyzed. Within permissible time delay, the mean intensity of aperiodic cross-correlation equals zero, and hence the BN and MAI can be eliminated in quasi-synchronous coherent TS- OCDMA system. When the time delay is out of zero correlation zone, crosstalk value of LA sequence is less than that of Gold sequence. Therefore, quasi-synchronous coherent TS-OCDMA using LA sequence is beneficial in a low-user-load OCDMA network.
The performance of WDM-based coherent time-spreading OCDMA system is studied. Upper bound and lower bound of normalized throughput are obtained with 8 wavelength channels and 127 length and 511 length Gold codes. It is shown that when all simultaneous users are equally allocated to different wavelength channels, WDM+ TS OCDMA has much better performance. However, if there is no central control to allocate wavelength channels equally, WDM+ TS OCDMA system has the in-between performance of normalized throughput.
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The performance of optical code-division multiple-access (CDMA) communication systems utilizing number-state light field is evaluated. Lossy direct detection optical channels are assumed. Both on-off keying (OOK) and pulse-position modulation (PPM) schemes are investigated. For OOK, the exact bit error rate is evaluated taking into account the effect of both multiple-user interference and transmission loss. Upper and lower bounds on the bit error probability for PPM-CDMA systems are derived under the above considerations. The effect of both background and thermal noise is neglected in the analysis. A comparison between the performance of the number- and coherent-state OOK/PPM-CDMA networks is also presented. The results demonstrate that the number-state system requires less than half the energy consumed by the coherent-state one in order to attain the same performance.>
The performance of optical code-division multiple-access (CDMA) communication systems utilizing number-state light field is evaluated. Lossy direct-detection optical channels are assumed. Both on-off keying (OOK) and pulse-position modulation (PPM) schemes are investigated. For OOK, the exact bit error rate is evaluated taking into account the effect of both multiple-user interference and transmission loss. Upper and lower bounds on the bit error probability for PPM-CDMA systems are derived under the above considerations. The effect of both background and thermal noise is neglected in our analysis. Performance comparison between the number-state and coherent-state OOK/PPM-CDMA networks is also presented. Our results demonstrate that the number-state system requires less than half the energy consumed by the coherent-state one in order to attain the same performance. Lower bounds on the maximum number of simultaneous users are derived for both number- and coherent-state PPM-CDMA systems with asymptotically zero error rate.>