LTE downlink transmission is based on Orthogonal Frequency Division Multiplex (OFDM). Radio frames in LTE are transmitted in both time and frequency domain. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in Figure below.
Each resource element corresponds to one frequency sub-carrier during one OFDM symbol interval.
For LTE, the downlink sub-carrier spacing has been chosen to be = 15 kHz.
In addition to the 15 KHz sub-carrier spacing, a reduced sub-carrier spacing flow = 7.5 KHz is also defined for LTE. This reduced sub-carrier spacing is specifically used for scenarios such as MBSFN- based multicast/broadcast transmission.
Grouping of 12 sub carriers:
As illustrated in Figure above, in the frequency domain, the downlink sub-carriers are grouped into resource blocks (RB), where each resource block consists of (12) twelve consecutive sub-carriers. The LTE physical-layer specification allows for a downlink carrier to consist of any number of resource blocks, ranging from NRB-min = 6 for 1.4 MHz and upwards.
There is also an unused DC-sub-carrier in the center of the downlink spectrum. The reason for not using the DC carrier is that it is assumed it can be severely interfered, (e.g., due to local-oscillator leakage at the transmitter and/or the receiver).