Hermitian symmetry in OFDM systems
The Hermitian symmetry is used to obtain a real-valued time-domain signal. It is a special case of OFDM called discrete multitone (DMT). It exploits a property of the discrete Fourier transform (DFT), namely that the DFT of a real-valued signal has Hermitian symmetry. The motivation is usually the channel: if the signal shall be transmitted over a low-pass channel without additional radio-frequency upconversion (which requires an oscillator at the transmitter and a local oscillator at the receiver) the transmit signal must be real-valued.

Given an OFDM system with $N$ subcarriers the following conditions must hold
$$
c_{-k} = c^*_k\\
c_0\in \mathbb{R}
$$
where $c_k$ is the complex value of subcarrier with index $k\in[-N/2\ldots N/2-1]$. This can be achieved by mapping $N/2$ complex symbols to subcarriers $0$ to $N/2-1$ and then assigning the respective complex conjugate value to the according subcarrier with negative index. This would normally take place after the mapper and before the inverse DFT (IDFT) block.