We demonstrate an 80 Gbit/s, 5 Gsymbol/s 256 QAM coherent optical transmission by employing an injection-locked homodyne detection circuit based on both fiber lasers and LDs. With either circuit, low phase noise carrier-phase synchronization between the transmitted data signal and an LO were achieved with a phase noise variance of only 0.2 degrees. As a result, we have successfully transmitted an 80 Gbit/s data signal over 150 km with a simple receiver configuration. This is the highest QAM multiplicity yet realized with injection-locked homodyne detection.