Hello
If I have OFDM system but I'm only interested with the receiver so I need a signal that passes through AWGN channel and then multiply it with the receiver array gain in frequency domain but the signal is modulated using OFDM what the parameters that I need to consider?
As Omid suggested, number of subcarriers in the channel is one important parameter. Another parameter is the guard interval (or cyclic prefix), between symbols in each subcarrier.
The more the subcarriers, the slower the symbol rate needs to be in each subchannel to aggregate to a given channel capacity, the longer you can make the guard interval without severely impinging on overall channel capacity. The longer is the guard interval, the less likely multipath distortion will cause inter-symbol interference. This is intuitive, right? If symbols are relatively far apart in time, then variable delay in symbol arrival times, caused by multipath, won't result in symbols running together, within each subchannel. So that the receiver can successfully decode the signal.
On the other hand, the more subcarriers you have, the closer together the subcarriers are in the frequency spectrum. So that if the OFDM is used in a mobile device, which is likely to experience doppler shift, the receiver will eventually become confused, which subchannel is which. That's why in mobile applications, you'll see fewer subchannels in use.
Then of course, there is the modulation used in the subchannels. Usually some form of QAM, QPSK being the most robust, but it also is least spectrally efficient. Then you have 16-QAM, 64-QAM, or even higher, for increased spectral efficiency but more fragile transmissions.
Then you have the forward error correction (FEC), usually a combination of convolutional coding concatenated with block codes like Reed-Solomon. The convolutional code is expressed as a fraction, such as 2/3, 3/4, 7/8, and so on. The fraction indicates the number of information bits available compared with the number of bits transmitted. So, 2/3 would say that you get 2 good bits of information when 3 bits are transmitted. Helps to correct errors on the fly.
Instead of using a traditional convolutional code, the newest OFDM schemes today use Low Density Party Check (LDPC), which is supposed to be more efficient. Closer to the Shannon limit. And then often still concatenated with Reed Solomon block code. The LDPC is still expressed as a fraction, same as before, to indicate how strong the FEC is, and at what price in channel capacity.
So: overall channel width, number of subcarriers, guard interval, modulation used in subchannels, FEC in use, are the main parameters you'll see mentioned, with OFDM.
Another item you'll see mentioned is interleaving of the symbols sent through subchannels. This helps protect against sharp notches in the transmission channel, to allow the FEC to work more effectively. If symbols are interleaved, even in subchannel location, then a burst in noise will clobber fewer consecutive symbols, allowing the FEC to fix the distorted symbol stream better.
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