How to specify the input data bit width for fixed point implementation in windowed OFDM?
Depends on which processor platform you want to implement it. If it is an ASIC (Application Specific Integrated Circuit) chip, they come with ARM processor cores in most cases and real time computation require fixed-point implementation. There is also memory limitation in such processors due to smaller footprint of device. If you implementation is on processor which is 32 or 64-bit wide and has large amount of memory at its disposal (such as on Laptop, Desktop), you can use 32-bit fixed point notation (even though floating point would just work fine like IPP DSP Library from Intel). In your case, if your I and Q data each are 16-bit fixed point numbers, I don't think you have to worry too much. I am not sure if there is a specific advantage but almost all fixed point convention I have come across use 1 sign bit and rest fractional bit. In this case Q1.15 format. In that case you need to normalize all your symbols, channel co-efficients etc so that they are within +/-1.