The smallest time-frequency grid for downlink transmission is called a Resource Element, which is one symbol on one sub-carrier. A group of 12 contiguous sub-carriers in frequency and one slot in time form a Resource Block (RB) as shown in the following figure. Data is allocated to each User Equipment (UE) in units of RB.

Fig. 1-3 LTE resource grid

When using normal Cyclic Prefix (CP), a RB spans 12 consecutive sub-carriers at a sub-carrier spacing of 15 kHz, and seven consecutive symbols over a slot duration of 0.5 ms as shown in Fig. 1-1.

A CP is added to each symbol as a guard period. Thus, an RB has 84 resource elements (12 sub-carriers x 7 symbols), corresponding to one slot in the time domain and 180 kHz (12 sub-carriers x 15 kHz spacing) in the frequency domain.

The size of an RB is the same for all bandwidths, therefore the number of available physical RBs depends on the transmission bandwidth. In the frequency domain, the number of available RBs can range from 6 (when transmission bandwidth is 1.4 MHz), to 100 (when transmission bandwidth is 20 MHz).

Note that the number of symbols in a slot depends on the CP length. For a normal CP, there are seven OFDM/SC-FDMA symbols per slot. For extended CP there are six OFDM/SC-FDMA symbols per slot. In a TDD special subframe the number of OFDM symbols allocated for Uplink/Downlink depends on the special sub frame configuration.

Table 1-5 Number of symbols per slot and number of sub-carriers per resource block for different cyclic prefix type