OFDMA : Orthogonal Frequency division multiple access.
LTE
uses OFDM for the downlink –that is, from the base station to the terminal.
OFDM meets the LTE requirement for spectrum flexibility and enables
cost-efficient solutions for very wide carriers with high peak rates. OFDM uses
a large number of narrow sub-carriers for multi-carrier transmission.
The
basic LTE downlink physical resource can be seen as a time-frequency grid. In
the frequency domain, the spacing between the subcarriers, Δf, is 15kHz. In
addition, the OFDM symbol duration time is 1/Δf + cyclic prefix. The cyclic
prefix is used to maintain orthogonality between the sub-carriers even for a
time-dispersive radio channel.
One
resource element carries QPSK, 16QAM or 64QAM. With 64QAM, each resource
element carries six bits.
The
OFDM symbols are grouped into resource blocks. The resource blocks have a total
size of 180kHz in the frequency domain and 0.5ms in the time domain. Each 1ms
Transmission Time Interval (TTI) consists of two slots (Tslot).
In E-UTRA, downlink modulation schemes QPSK,
16QAM, and 64QAM are available