Canopy height

How can the backscatter coefficient of sentinel-1 be computed to directly estimate canopy height?

There is no empirical model to apply, especially because the structure of the forest strongly varies. But there are studies which have tested this approach, mostly in combination with other satellite data. You need ground truth data to calibrate your model.

I have field data of forest height area 100 x 100 m, If subset sentinel-1 same area with field data by already preprocessing and got backscatter coefficient (sigma0VV_db,sigma0VH_db).But i just want to show how canopy height of field data according with sentinel-1 without using model.Is it possible,or do you have any suggestion by using polarization to calculate.

yes, you can import the field data as csv or shp and then use the Pin Manager to extract the backscatter values at these points to create a correlation plot.