We study and experimentally evaluate the modulation index influence on Mach-Zehnder Modulator in external modulation at low-radio frequency voltage (2.277 V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</inf> ) to improve the system performance of a coherent receiver based-on 3×3 coupler. The system achieves a sensitivity of −29 dBm in the error-free condition, after propagation through 60 km-long fiber link at a frequency carrier of 15 GHz, offering a cost-effective solution for centralized radio access network (C-RAN).