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

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...