Most studies on OFDM-based communication systems consider that multipath channels consists of a limited set of taps in the discrete-time domain. However, this only holds if the tap delay is a multiple of the sampling period Ts = 1/B. In case the tap delays are not integer multiples of Ts, they will leak to all neighboring samples in the discrete channel impulse response, e.g. as in Fig. 2.2. here.
When simulating such fractional delays (I also did measurements and I got very similar results), the samples from one OFDM symbol leak into the next one, causing intersymbol interference (ISI). Due to the sinc-shaped decay of this interference caused by the band-limited nature of the OFDM system, this ISI becomes relatively small in the actually evaluated OFDM symbol, that is, after removing cyclic prefix (CP).
My question is regarding channel estimation. If I use a block pilot symbol (i.e., all subcarriers are used), this interference seems to be sufficiently suppressed and I get decent simulation and measurement results. However, if I use comb pilots and interpolate them to get the full channel frequency response (CFR), the experienced ISI seems to significantly affect the edges of the obtained CFR. The effect I observed is basically the same when I cut the estimated CIR with a block pilot, extract only the first N_cp samples (N_cp is the length of the CP) of it and generate a CFR via ZP-FFT - which would obviously goes wrong since the leaking of the tap over all samples would be neglected, but correcting this fractional delay also does not help here. The only solutions I found so far are either using block pilots with all subcarriers for channel estimation, or using comb pilots and a guard band at the edges of the spectrum (composed of non-zero subcarriers) that is discarded before pilot interpolation. Any thoughts on how to avoid the need for the guard band in the comb pilot case?