Secured relay-assisted atmospheric optical code-division multiple-access systems over turbulence channels
The authors propose a relay-assisted atmospheric optical code-division multiple-access (AO/CDMA) system for secured, multiuser optical communications. Chip detect-and-forward (CDF) scheme is used at relay nodes so that the complex multiuser decoding process can be avoided. The proposed system performance, in terms of bit-error rate (BER) and transmission confidentiality, is analysed over atmospheric turbulence channel taking into account channel loss due to atmospheric attenuation and beam
... ation and beam divergence. Multiple-access interference and background noise are also included in the analysis. In addition, a quantitative analysis of data confidentiality is further examined in the study. The numerical results show that the relay transmission is an efficient solution to improve the system performance. Thanks to this solution, AO/CDMA systems can achieve low BER, long distance, and a large number of users. Moreover, high confidentiality can be attained by properly configuring system parameters, such as limiting the transmitted power and/ or reducing the signal beam width. Introduction Atmospheric optical (AO) or free-space optical (FSO) communication, a data transmission technology based on the propagation of light over atmospheric channel, has recently received much attention thanks to its advantages of high data rates, unregulated spectrum, and flexible deployment . Since AO communication is capable of providing Gigabits per second (Gbps) data rates, it is a promising candidate for broadband access environments  . To allow multiple users to simultaneously share the same resource of atmospheric channels, AO/code-division multiple-access (AO/CDMA) systems have been proposed and attracted much interest thanks to additional benefits including asynchronous access, scalability, and inherent security       . It has been shown in the literature that the performance of AO/ CDMA systems is severely limited by the effect of atmospheric turbulence, which is caused by the variations in the refractive index due to inhomogeneities in temperature and pressure changes  . Atmospheric turbulence, together with background noise and multiple-access interference (MAI), causes the increase of bit-error rate (BER) and significantly limits the transmission distance of AO/CDMA systems. Several techniques have been proposed to deal with this problem, including pulse-position modulation (PPM), spectral phase encoding, and forward error correction (FEC). PPM offers a number of advantages, including power-efficient and non-threshold detection      . PPM-based AO/ CDMA system, which requires the transmission of short pulse, is however significantly affected by pulse broadening effect in the case of transmitting high data-rate and long distance  . Spectral phase-encoded optical CDMA also offers better spectral efficiency and performance  . It is, nevertheless, relatively complicated since the use of coherent sources is required. The study in  shows that FEC can effectively combat with physical layer impairments. The use of FEC is still limited as it imposes high delay and reduces the transmission efficiency.