## 1 Introduction  
Terrestrial FSO communication systems that establish line-of-sight optical wireless links by using lasers and photo-detectors as transmitter and receiver terminals, respectively, have had growing research and commercial attention in the last few years. This is mainly due to the very high capacity, along with a very high security level, they can achieve, their operation with low power consumption in the unlicensed optical spectrum, their relatively low operational and installation costs, their flexibility for deployment and redeployment, their immunity to electromagnetic interference and multi-path dispersion, and their compatibility with the very performance-effective optical fiber communication systems [1; 2; 3; 4]. Nevertheless, even in clear weather conditions and particularly over propagation distanceranges of the order of 1 km or longer, atmospheric turbulence attenuates the propagating signal and causes the so-called scintillation effect, which results in random variations of the received signal irradiance that significantly degrades the FSO performance and availability [4; 5]. Additionally, wind loads, earthquakes and thermal expansion of building frame parts bring about the sway of high-rise buildings, where the FSO terminals are usually installed. Therefore, the line of sight alignment between transmitter and receiveris diminished by building sway, which results in misalignment-induced fading of the received signal irradiance, commonly known as the pointing errors effect [6; 7]. Here, the pointing errors effect is investigated by taking into account both the fixed displacement between the beam center and center of the detector, which is called the boresight, and the random spatial jitter as well [8; 9]. All the above-mentioned effects significantly degrade the system's performance. Thus, in order to counterbalance their negative side-effects, many techniques have been proposed. One such effective technique is the use of relays, which broadens the useful coverage area of the optical wireless communication system and upgrades the link's performance characteristics [10; 11; 12; 13; 14; 15; 16; 17]. The relays can be used either in serial or parallel configuration, according the needs of the specific FSO system. More specifically, the serially relayed optical links can be used for the cases where the extension of the link length is the main requirement, while parallel configuration is more useful when the reliability and the availability of the link need to be improved. Consequently, a mixed architecture with parallel and serial relayed FSO links can significantly improve the performance of the system [17]. In this context, a recently published work [17] estimated the outage probability (OP) of a mixed decode-and-forward (DF) FSO system over weak turbulent channels, modeled through the compact gamma distribution [18], along with non-zero boresight pointing errors modeled, in turn, with the accurate approximation of Beckmann's distribution [19].According to the mixed DF FSO topology proposed in [17], the source node initiallytransmitsthe same signal to a specific number of DF relay nodes in a parallel configuration. Next, each one of them decodes the signal after direct detection, modulates it without-off keying (OOK), and retransmits it to a specific number of DF relay nodes in a serial configuration. Thus, at the destination node's side, as many copies of the same information signal as the number of the parallel multi-hop branches of thespecific mixed FSO systemare eventually arriving. In this way, it enhancesboth the reliability and the useful coverage area of the total FSO system. Motivated from the above, in this work we estimate, under weak to saturated turbulence channels, the outage performance of the link by means of both OP and mean outage duration time (MOD) metrics for various mixed DF relay-assisted FSO system configurations, along with the presence of non-zero boresight pointing errors.