No abstract

termines the necessary bandwidth, is analyzed under the amplifiergain condition G ϭ 30 dB.It can be seen that a BER lower than 10 Ϫ4 is achieved for standard access, low-power level, and low-cost filtering conditions with R b ϭ 155 Mbits/s, P 1 ϭ Ϫ54 dBm (before amplification), and B o /B e ϭ 100). This value can easily improved by using low-cost forward correcting codes (FECs), such as the ReedSolomon FEC [11].In Figure 3, the effect of multiaccess users on the BER is depicted. The graphs establish the relation between the MAI and optical noise. The MAI effects are limited by the noise introduced during the transmission via the optical channel, and the upper bound of the associated error probability is limited by the error probability of the optical channel. Indeed, the BER that results from 29 simultaneous users is identical to the BER obtained for an equiprobable optical sequence, in the same physical condition, without using CDMA. Thus, in our optical model, it is demonstrated that the effects of the MAI on the BER are conditioned and limited by the signal distortions generated by optical noise. CONCLUSIONThe performances of an optical CDMA system associated with a Gaussian model of an optical amplified channel have been presented. The results demonstrate that appropriate time-domain optical codes (PSs) can be used to reduce and control the effect of multiuser interference.An analytic expression of the error probability has been developed. This formulation includes the most important parameters of the optical channel, and arises the effect of filtering conditions by pointing out the benefit of a low optical-to-electrical bandwidth ratio on the resultant BER. The simulations carried out thereafter confirm the theoretical expression of the error probability.On the other hand, our model simultaneously takes into account the MAI and optical noise generated by the optical channel, and demonstrates that the effect of the MAI and optical codes on error probability is directly related to the optical noise introduced by the components of the optical channel.

We investigate different techniques to improve the spectral efficiency of systems based on the DVB-S2 standard, when the transmitted signal bandwidth cannot be increased because it has already been optimized to the maximum value allowed by transponder filters. We will investigate and compare several techniques involving different sections of the transceiver scheme. The techniques that will be considered include the use of advanced detection algorithms, the adoption of time packing, and the optimization of the constellation and shaping pulses. The low-density parity-check codes recently proposed for the evolution of the DVB-S2 standard will be considered, as well as the adoption of iterative detection and decoding. Information-theoretical analysis will be followed by the study of practical modulation and coding schemes.

Abstract-Noncoherent sequence detection algorithms, recently proposed by the authors, have a performance which approaches that of coherent detectors and are robust to phase and frequency instabilities. These schemes exhibit a negligible performance loss in the presence of a frequency offset, provided this offset does not exceed an order of 1% of the signaling frequency. For higher values, the performance rapidly degrades. In this paper, detection schemes are proposed, characterized by high robustness to frequency offsets and capable of tolerating offset values up to 10% of the signaling frequency. More generally, these detection schemes are very robust to rapidly varying phase and frequency instabilities. The general case of coded linear modulations is addressed, with explicit reference to -ary phase shift keying and quadrature amplitude modulation.

control loop can be done for the three modulations considered. Design guidelines are given that account for the presence of the AFC loop by properly using some performance results derived in the assumption of perfect frequency tracking.