Discriminating two coherent states is a fundamental problem in both classical and quantum communications. The optimal quantum detection for discriminating two coherent states requires real-time feedback loops and high control complexity. Therefore, it is of practical importance to study near-optimum quantum detection techniques having simple implementation structures. Our research aim is to study one promising near-optimum quantum detection, the optimally displaced threshold detection (ODTD) for a generalized Kennedy receiver. We first analytically study the performance of the Kennedy-like receiver with ODTD in a realistic optical fiber communication (OFC) system with noises and imperfect devices. Then we propose a heuristic greedy search algorithm to design these parameters. We prove that the ODTD asymptotically approaches the Kennedy receiver with threshold detection when the signal power is large. We also clarify the connection between the generalized Kennedy receiver with threshold detection and the one-port homodyne detection. We show that the ODTD can outperform the homodyne detection in a realistic OFC system. To embed the ODTD into a free-space optical communication system, we propose a conditional dynamics based Kennedy (CD-Kennedy) receiver combined with the optical combining technique and the diversity technique. We analytically study the error probability of the CD-Kennedy receiver. Numerical results show that both the original Kennedy receiver and the optimized displacement receiver fail in the presence of either turbulence or thermal noise. We show that the proposed CD-Kennedy receiver can outperform the homodyne receiver in either weak turbulence or small thermal noise. At last, we combine the ODTD with the binary modulated continuous variable-quantum key distribution (CV-QKD) based on the post-selection scheme. We derive the secret key rate for a binary modulated CV-QKD using ODTD and post-selection under both individual and collective beamsplitter attacks. We optimize the post selected area and the coherent amplitude for different transmission efficiency of the channel under different thermal noises. Numerical results show that the secret key rate using ODTD can surpass that using the classical coherent detection under different transmission efficiencies and different thermal noises.