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

Abstract

SUMMARYThis 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=ðk À 1Þ 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 4 10 À 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.

Performance Analysis of Spectral Amplitude Coding Based OCDMA System with Gain and Splitter Mismatch

“…The analysis is carried out in ideal conditions and is based on some serious assumptions such as the gain of APDs used in receiver of proposed systems are perfectly matched and that the power is split Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/26/15 1:00 PM uniformly in the splitters used at transmitter and receiver side. In what follows we analyze the system proposed in [5] considering practical environment and attempt to critically analyze the obtained results and also propose some ways to minimize or overcome the losses.…”

“…However, if the data bit is space "0," no pulses are transmitted during the time frame. The system presented in [5] proposes the use of PDC to overcome the limitation of codeword synchronization and power loss incurred in the spectral amplitude coding (SAC)-OCDMA Systems.…”

“…In [10], it is shown by the authors that such approximations in practice may be an overestimate or underestimate of the actual probability of error. In this paper, we extend our analysis presented in [10] and include the impact of APD mismatch [2,5] along with splitter's uniformity loss [5] in the OCDMA systems based on the perfect difference codes (PDC). We also use SNR criterion to propose the optimal value of APD gain.…”

“…In the most research on OCDMA systems based on PDCs [2,5], the evaluation of error probability is based on the ideal assumptions such as the gains of APDs used in information extracting and MAI cancelling branches of optical code-division multiple-access (OCDMA) systems receiver are matched or the multiplexers used at transmitter and receiver uniformly splits the input and received power. In [10], it is shown by the authors that such approximations in practice may be an overestimate or underestimate of the actual probability of error.…”

“…Designing an OCDMA system, however, imposes a challenge to minimize the influence of multiple-access interference (MAI). Spectral amplitude coding (SAC) techniques are widely been considered to address this issue as the MAI can be theoretically cancelled when code sequences with fixed in-phase crosscorrelation (such as perfect difference codes or Hadamard code) are used [5].…”