The strong scattering effectsScattering effect of UV signalsUltraviolet signals in the atmosphere enable the NLOS ability of UV communications, but introduces large time dispersion of the channel impulse response (CIR)Channel impulse response, which inevitably results in serious inter-symbol interferences (ISIs)Inter-symbol interference. The effects of ISIs on communication performance can be quantified by using an analytical CIR function. In this chapter, through analyzing the impacts of coplanar and non-coplanar geometries on CIRs, a modified Gamma function (MGF) is presented for the precise analytical characterization of CIRs in the presence of multiple scattering. We first introduce an analytical CIR, named MGF, related to the system geometries in Sect. 3.1. Then Based on the MGF model, we quantify the ISI in Sect. 3.2. Then we derive the bit-error rate (BER) and achievable information rate (AIR) for both on-off keying (OOK) modulation and 2-pulse-position modulation (2-PPM) in Sect. 3.3. Section 3.4 presents some numerical results to verify the derived BER and AIR. Numerical results indicate that the MGF model provides higher accuracy in fitting both CIRs and BERs compared to the traditional Gamma function model. At last, we conclude this chapter in Sect. 3.5.