Ultra Dense Wavelength Division Multiplexing (UDWDM) is a promising technique for access PON, introducing the wavelength-to-the-user concept. The key enabling technology will be a new cost-effective coherent detection scheme, providing high splitting ratios and long-reach extension. This also will allow implementing low-cost, yet effective, coherent terminals suitable for mass deployment. Employing a unique laser diode as transmitter and receiver local oscillator at the ONU to realize a cheap terminal implies to use the same wavelength for both up and down directions; therefore, in Full-Duplex (FD) transmission mode the Rayleigh Backscattering (RB) crosstalk will drastically perturb the reception. We propose and study a Half-Duplex (HD) solution to avoid the RB disturbance and analyze the different scenarios of reach and splitting ratios.