hi ranjeet
i am attaching a link which can be very helpful to you. It is a paper which should help you in getting some idea.
http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=6013167&abstractAccess=no&userType=inst
In this paper a new structure of PON system based on coherent heterodyne receivers is presented. The coherent detection enables a filterless ultra-dense WDM system. here they have simulated a 64 channel system with 10GHz channel spacing, offering symmetrical 1 Gbit/s sustained data rate per user both downlink and uplink by single fiber. The quadrature phase-shift keying (QPSK) modulation format is applied to downstream data for higher spectrum efficiency, while use of Amplitude Modulation (AM) on upstream data to reduce the system cost. Hope it helps you.
Regards