1. say, the sampling rate of transmission is 40MSPS. so, the baseband width is 40MHz. while u only uses the central 20MHz bandwidth, thus u need add 64 pt zeros. where to add? u may lookup the principle of FFT implementation. ideally, u need add 32 zeros
to the left, say, [-64, -33] and 32 zeros to the right, say, [32, 63]. as in C/C++, there is no negative index, [-64, -33] are mod to 128 resulting [64, 95], so the negative sub-carriers are located in [96, 127].
2. in the receive procedure, u have 40MSPS, too. in software implementation, samples are first down-sampled to 20MSPS (of course in software) to reduce the computation complexity.
Danial.F