The UL slot structure is similar to the DL slot structure. Differences are mainly due to simplifications of reference symbols, robustness, and physical UL channel multiplexing. A radio frame lasts 10 ms and is divided into 10 subframes with two slots of 0.5 ms duration as a DL radio frame is split into subframes and slots. Figure 1 shows an UL radio frame with its subframe and slot structure. An UL RB has 12 subcarriers on the frequency axis and on the time domain it has, seven SC-FDMA symbols per slot when normal CP is used, and six SC-FDMA symbols when extended CP is used (see Figure 2).
Figure 1 Uplink radio frame and subframe with two slots including PUSCH, PUCCH, PRACH, DMRS, and SRS. Reproduced with permission from Nomor
UL synchronization signals are not used because all UL signals from UEs transmitting within the cell are time aligned at the eNB with an UL timing control procedure. The eNB signals a Timing Advance (TA) command to each UE to track the UL alignment. Timing varies due to the different special distribution of UEs within a cell region. Signals are delayed because of the propagation delay. Firstly, during the random access procedure a total timing offset to the UE is transmitted. After this there is a control loop just tracking and signaling differential timing offsets in steps of 0.52 μs (16 × Ts).
The PRACH always uses six consecutive RBs on the frequency axis and is time-wise one subframe wide. Which six RBs are used is variable and set in SIB type 2. The PRACH configuration index is signaled in SIB type 2 as well, which defines the subframes which carry the PRACH's six RBs. This subframe configuration applies for either even or any radio frame. There are 64 possible PRACH configuration index permutations.
Resources at the upper and lower edges of the system bandwidth are used to carry the PUCCH. The frequency resources in between the PUCCH bands are designated to PUSCH transmission.
UL reference signals are used for UL channel estimation. DL reference signals are spread over frequency and time as single REs, which leads to a two-dimensional spheric channel estimation. UL reference signals are similar to TDMA pilots in the middle of each time slot. LTE defines reference signals in the middle of each slot (on the fourth OFDM symbol assuming normal CP duration) and spanning via the complete allocated frequency range of each UE.
The eNB can instruct UEs to transmit special reference signals over the complete system bandwidth, or parts of it, independently of UL data transmission either on PUSCH or PUCCH. These reference signals are called Sounding Reference Signals (SRSs). SRSs are used to estimate UL channel quality of a wider frequency range in order to optimize frequency-selective UL scheduling.