optical communication link in free space

which modulation scheme i should use for laser communication and should i go for fpgas for implementing transmitter and receiver

Everything depends upon the link budget. You should specify your receiver's minimum input power (or noise level), bandwidth, distance, like when you design a radio link. Then, with the available transmitter power and known propagation loss, you should calculate receiver S/N ratio. If your budget results not positive, you should consider increasing your antenna (lens) size to send more power density into a narrower beam.

There are many factors can affect the telecomm bit rate. You can ref many live system of optical telecomm. MIT LL have conducted 10Gb/s from GEO satellite to ground. Japan also exceeded 1Gb/s.
Anyway, it's up to the whole system design and budget. Also the component cost is important factor. Such as BMDO STRV-2 system, it's only 2X500Mb/s. So I guess it's about 256QAM modulation.

I am doing project on "Diversity gain and Outage probability for MIMO free space optical link with misalignment"..i am not able to start coding for this..I saw you posts in this topic so i am writing this expecting help from you all.

thank you in advance..