The figure shows one frame consisting of 64 OFDM symbols. The system operates in time division duplex (TDD) mode and thus the frame is divided in a CH transmission time interval (TTI) and a MR TTI. The first four symbols of the CH TTI are reserved for pilot symbols. Each CH transmits one common pilot symbol (CHSCH$_0$) at position 0 and one dedicated pilot symbol (CHSCH$_i$) at position i \in \{1,2,3\}. This way we can ensure orthogonality between the pilots of different CH received at one MR. The pilot symbols are followed by a broadcast channel (CHBCH). The rest of the CH TTI frame is reserved for the scheduled access channel (SACH), which carries the actual data.
The MR TTI contains the random access channel (RACH) with an associated pilot symbol (SCH$_0$). Each MR transmits one pilot symbol SCH$_i$, i \in \{1,2,3,4\}. This way we can ensure orthogonality between the pilots of different MR received at one CH. The pilot symbols are followed by the physical scheduled access channel (PSACH) and a broadcast channel (MRBCH) with an associated pilot symbol. The end of the CH and MR TTIs are protected by a guard interval of two symbols.
-- User.FlorianKaltenberger - 20 Mar 2008