Summary: Radio on fiber (RoF) – distributed antenna system (DAS) over wavelength division multiplexing – passive optical network (WDM-PON) with multiple – input multiple – output (MIMO) has been proposed as a next generation radio access network (RAN). The system employs optical time division multiplexing (OTDM) over one WDM channel as a backhaul for RAN to flexibly transmit various types of radio air interfaces. To cover a wider wireless service area, the WDM-PON has a combination of double and bus topologies. This paper analyses the channel capacity in the MIMO cell provided by the RoF-DAS over WDM-PON with computer simulation considering noise power added in the RoF link, and discusses the trade-off between losses in RoF and wireless channel appeared in the channel capacity. Then, this paper clarifies a method to derive the optimal cell size to obtain the highest channel capacity.