  The coherent-state qubit is a promising candidate for optical quantum
information processing due to its nearly-deterministic nature of the Bell-state
measurement (BSM). However, its non-orthogonality incurs difficulties such as
failure of the BSM. One may use a large amplitude ($\alpha$) for the coherent
state to minimize the failure probability, but the qubit then becomes more
vulnerable to dephasing by photon loss. We propose a hardware-efficient
concatenated BSM (CBSM) scheme with modified parity encoding using coherent
states with reasonably small amplitudes ($|\alpha| \lessapprox 2$), which
simultaneously suppresses both failures and dephasing in the BSM procedure. We
numerically show that the CBSM scheme achieves a success probability
arbitrarily close to unity for appropriate values of $\alpha$ and sufficiently
low photon loss rates (e.g., $\lessapprox 5\%$). Furthermore, we verify that
the quantum repeater scheme exploiting the CBSM scheme for quantum error
correction enables one to carry out efficient long-range quantum communication
over 1000 km. We show that the performance is comparable to those of other
up-to-date methods or even outperforms them for some cases. Finally, we present
methods to prepare logical qubits under modified parity encoding and implement
elementary logical operations, which consist of several physical-level
ingredients such as generation of Schr\"odinger's cat state and elementary
gates under coherent-state basis. Our work demonstrates that the encoded
coherent-state qubits in free-propagating fields provide an alternative route
to fault-tolerant information processing, especially long-range quantum
communication.
