The full-duplexFull-duplex relay assisted ultraviolet (UV) communications can achieve longer communication distances and higher efficiency of time-frequency utilization compared with direct UV communications. However, due to the strong scattering effectScattering effect, serious inter-relay-interference (IRI) is inevitably introduced in multi-hop full-duplexFull-duplex relay links. In this chapter, we introduce an alternate iterative-Newton method (AINM) for mitigating the impacts of IRI by optimizing jointly the relay placementRelay placement and transmit powers of each relay. We first introduce the system model of relay-assisted multi-hop UV communication in Sect. 5.1. Then we derive the bit-error rate (BER) and the achievable information rate (AIR) of the consider communication system model in Sect. 5.2, where a space-division coupled full-duplexFull-duplex relay configuration is further proposed to mitigate the influence of IRI. In Sect. 5.3, we introduce the AINM to jointly optimizing the relay placementRelay placement and the transmit power. In Sect. 5.4, we present some numerical results to explore the performance of relay-assisted UV communication systems. Numerical results show that the AINM can significantly decrease the BER and increase the AIR; and the space-division scheme can further decrease the BER but sacrifices about half AIR. At last, we conclude this chapter in Sect. 5.5.