  A free-space optical communication system using non-mode-selective photonic
lantern (PL) based coherent receiver is studied. Based on the simulation of
photon distribution, the power distribution at the single-mode fiber end of the
PL is quantitatively described as a truncated Gaussian distribution over a
simplex. The signal-to-noise ratios (SNRs) for the communication system using
PL based receiver are analyzed using different combining techniques, including
selection combining (SC), equal-gain combining (EGC), and maximal-ratio
combining (MRC). The integral solution, series lower bound solution and
asymptotic solution are presented for bit-error rate (BER) of PL based
receiver, single-mode fiber receiver and multimode fiber receiver over the
Gamma-Gamma atmosphere turbulence channels. We demonstrate that the power
distribution of the PL has no effect on the SNR and BER performance of the PL
based receiver when MRC is used; and it only has limited influence when EGC is
used. However, the power distribution of the PL can greatly affect the BER
performance when SC is used. Besides, the SNR gains of the PL based receiver
using EGC over single-mode fiber receiver and multimode fiber receiver are
numerically studied under different imperfect device parameters; and the scope
of application of the communication system is further provided.
