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Structure of an OFDM Packet


 

What does an OFDM signal look like?

An OFDM signal contains a header (that is called a cyclic prefix). That message bits are placed. Then again, a cyclic prefix and other message bits are located. The length of the cyclic prefix is a fraction of the length of the message bits. The structure is called a packet containing cyclic prefixes and message bits.
Let's assume I need to transmit 16 bits as an OFDM symbol. If the length of the cyclic prefix is 4, then the original OFDM symbol, for example, is 1001100101010110. The OFDM symbol with the cyclic prefix will be 01101001100101010110, where the last 4 bits of the OFDM symbol are copied to the front of the original symbol.
 
Fig: OFDM Modulation and Demodulation


What are the roles of cyclic prefixes in an OFDM packet?

  1. It mitigates the inter-symbol interference (ISI)
  2. It works as training bits
  3. It helps in equalization

The process of sending an OFDM packet practically

Firstly, we apply inverse fast Fourier transform (IFFT) on the OFDM packet's bits to define it in the time domain. Oppositely, on the receiver side, we apply a fast Fourier transform (FFT) to recover the original OFDM packet.

For example, if you are transmitting 100 OFDM symbols, each symbol contains 64 bits, and the number of subcarriers is 64, then there will be 64 subchannels in parallel. 
 
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