In this paper, we present two novel low-complexity transmission schemes for optical ground station (OGS)-satellite free space optical (FSO) uplink. Specifically, the TCAMscheme which is a joint dual-branch threshold combining and adaptive modulation scheme and the SCAM-scheme which is a joint dual-branch selection combining and adaptive modulation
scheme. In both schemes, the data rate is adjusted according to the instantaneous received combined SNRs at the satellite. We analyze the performance of the proposed schemes in terms of the outage probability, the average spectral efficiency and average bit error rate, while considering both subcarrirer intensity modulation/direct detection (SIM/DD) and coherent FSO detection techniques. Numerical results show that both TCAM and SCAM schemes provide better performance as compared to a single FSO uplink, while SCAM scheme enjoys better performance than TCAM scheme. This comes at the expense of increased complexity of the SCAM scheme. Also, coherent FSO detection technique shows better performance than the SIM/DD FSO detection technique for both schemes. The choice of which scheme and which
FSO detection technique to use is a compromise of required quality of service and complexity.