We propose a hardware architecture for 50G-PON LDPC decoder achieving high throughput and high error correcting capability while maintaining low level of resource utilization and implementation complexity. Our approach employs phased decoding as a key algorithm which effectively balances the competing goals of high throughput and low amount of resource utilization. We also propose a fixed-structured wiring network between variable nodes and check nodes, leveraging the quasi-cyclic property of parity-check matrix to significantly reduce implementation complexity. To further simplify the decoder structure and enhance the throughput, we propose a blockwise column cyclic shift in the parity-check matrix, along with rules for normalizing and quantizing messages generated in decoder. Our design also incorporates a pipelined structure of computation units. By integrating the proposed design schemes, we draw a sophisticated architecture for high-speed and low-complexity LDPC decoder that achieves the seamless decoding throughput of 49.7664 Gbps, even on a single FPGA board.