Figure 2
From: Identifying time-varying channels with aid of pilots for MIMO-OFDM
Overview of the pilot schemes studied. The left subplot depicts the Comb-type I pilot structure; the middle subplot the Comb-type II pilot structure, and the right subplot the Block-type pilot structure. Each rectangle corresponds to one OFDM symbol interval and contains OFDM symbols from each transmit antenna. Inside the rectangle, the zero pilots are represented by circles; the non-zero pilots by crosses, and the data symbols by squares.