## 1 Introduction  
Free-space optical (FSO) systems have emerged over the last few years as a valuable solution for broadband networks, providing performance features with high data rate applications and high security transmission due the narrow optical beam [1]. Analogue intensity modulation (AIM) technique comprises a low cost and simple modulation technique for optical communications, which finds applications in cable TV (CATV) distributions, radio-over-fiber (RoF), radio-on-free-space optics (RoFSO) etc. [2]. RoFSO systems constitute a relatively new wireless technology with the similar aim to the RoF technique of distributing radio frequency (RF) signals among central base stations (CBS) and remote antenna units (RAU) (Figure 1) [3, 4].  
However, FSO links experience the detrimental influence of atmospheric-related phenomena such as atmospheric turbulence, pointing errors (PE), beam wander, atmospheric attenuation etc., [5, 6, 7, 8, 9, 10, 11, 12, 13]. Turbulence-induced irradiance fluctuations are studied statistically and many statistical distribution models have been proposed over the years, according the specific turbulence conditions. In the weak turbulence regime the log-normal and the gamma distribution models have been proven that describe accurately the statistics of the irradiance fluctuations. From weak up to strong turbulence conditions the gamma-gamma model constitutes a very accurate model while for strong and saturate turbulence conditions the K and the negative exponential models are the most suitable [5]. Recently, the _M_(alaga) model has been proposed in the literature, which has the significant advantage of unifying almost all the previous-mentioned distribution models as special cases [6]. Additionally, PE impairments are also considered a serious degradation factor for FSO systems [7, 8]. In this work, we employ a new generalized PE model that approximates the Beckmann distribution with a modified Rayleighdistribution. In this model, nonzero boresight (NZB) displacement and different spatial jitters for the axes of the receiver are taken into account [9].  
In order to mitigate the influence of the atmospheric turbulence and the PE, diversity schemes have been proposed and used [1; 2; 14; 15; 16; 17]. Spatial diversity technique can be realized when a multitude of transmit or receive apertures are employed [1; 2]. In the present work, we assume a RoFSO system with spatial diversity on the receivers and multiple laser sources at the transmitter. Each one of the laser sources is directed towards a specific receiver. The objective of this configuration is to harness the maximum optical radiation for each diversity branch.  
Code-division multiple access (CDMA) is a widely used multiple access technique, where multiple users communicate using the same band of frequency. CDMA is based on the spread spectrum (SS) technique where each user is provided with a unique spreading code. CDMA was commercially adopted in the cellular standards of IS-95, IS-2000 and UMTS, [18]. It is also considered, for potential applications in the future cellular standards, in combination with orthogonal frequency division multiple access (OFDMA), [4].  
Thus, in this work we investigate a RoFSO system with receivers' spatial diversity for transmission of CDMA signals. The specific CDMA RoFSO system is studied in both, the forward and the reverse link. The performance evaluation is carried out with the average bit error rate (ABER) and the outage probability (OP) estimation, taking into account nonlinear effects related to the optical link, the multiple access interference (MAI), the atmospheric turbulence effect modeled by the \(M\)-distribution and the NZB PE. Closed-form expressions are extracted for the aforementioned performance metrics and numerical results are illustrated using the derived expressions.