OFDM
Orthogonal Frequency Division Multiplexing
OFDM is multi carrier system uses multiple carrier
modulation to transmit the data rather than the traditional single carrier
modulation (FDM) .OFDM divides single carrier frequency into multiple small
frequencies called sub carriers. These group of sub carriers are called channel.
 |
| fig 1 |
 |
| fig2 |
The IEEE 802.16e/ WiMax use OFDMA as Multiple access technique.
For Bandwidth options 1.25, 5, 10, or 20 MHz,Entire bandwidth is divided
into 128, 512, 1024 or 2048 sub carriers. For 20 MHz bandwidth with 2048 sub
carriers has 9.8KHz spacing between sub carriers.
Orthogonality of sub
carrier allows their overlapping while disabling the cross talk.figure
below shows four sub carriers from one OFDM signal. In this example, all sub carriers have the phase and amplitude, but in practice the amplitudes and
phases may be modulated differently for each sub carrier. Note that each sub carrier has exactly an integer number of cycles in the interval T , and the
number of cycles between adjacent sub carries differs by exactly one. This
properly accounts for the orthogonality between sub carriers.
 |
| fig 3 |
to be continued...
