When you increase the spacing between pilot symbols in OFDM, the receiver has fewer reference points to estimate the channel response. Although interpolation is used, if the channel varies between pilots, the estimation becomes less accurate. This can lead to underestimating the channel’s effect, which reduces the amplitude of the equalized received signal and increases the bit error rate.
Essentially, sparser pilots make channel estimation less reliable, especially in frequency-selective channels, which would explain both the amplitude drop and higher BER you observed.
