*Optoelectronic Key Elements for Polymeric Fiber Transmission Systems DOI: http://dx.doi.org/10.5772/intechopen.86423*

#### **Figure 7.**

*Principle of DMT transmission over an optical IM/DD channel: N CP, length of CP.*

orthogonal frequency division multiplexing (OFDM). Unlike the OFDM, which is used in wireless communication systems such as wireless local area networks (WLAN), the DMT is widely employed as enabling technology for digital subscriber lines (DSL), e.g., asymmetric DSL (ADSL) and very high DSL (VDSL). The DMT-based transmission was also shown to be very beneficial for SI-POF communication by Joncic et al. [24], Diaz et al. [25], and Vinogradov et al. [10].

The DMT technique slices the frequency-selective channel into a large number of subchannels that can be considered to have a flat frequency response. Each subchannel is then used for transmission of a passband signal with quadrature amplitude modulation (QAM). The simultaneous transmission of the low-speed parallel streams reduces the influence of the ISI. Another important property of DMT is that it adapts the signal parameters (QAM size and power in each subchannel) to the characteristic of the communication channel.

The principle of the DMT transmission over an optical IM/DD channel is shown in **Figure 7**. A high-speed serial data stream is first divided into *N* parallel lower-speed streams, where *N* corresponds to the number of subcarriers. Every *M* bits in each stream are grouped together and mapped into a complex value corresponding to a point of the QAM constellation with 2 *<sup>M</sup>* size. The modulation of the complex values onto *N* different subcarrier frequencies is based on the inverse fast Fourier transform (IFFT). To obtain real-valued time domain signal samples at the output of the IFFT block, a 2*N*-point IFFE has to be carried out. Thereby, the second half of 2*N* IFFT inputs must be complex conjugate of the first half and symmetric around the middle of the input vector, a property referred to as a Hermitian symmetry. The real-valued samples of the DMT time signal are then parallel-to-serial converted, and a cyclic prefix (CP) is added as a guard interval at the beginning of each DMT frame to resist to inter-frame interference occurring in a band-limited channel. After digital-to-analog conversion, a bipolar continuoustime DMT frame is produced. It is then DC-biased to modulate the intensity of an electrical-to-optical converter.

At the receiver, the DMT waveform is direct-detected, analog-to-digital and serial-to-parallel converted, and demodulated using 2*N*-point fast Fourier transform (FFT). The first *N* complex outputs of the FFT block are QAMdecoded and parallel-to-serial converted to result in an output serial data stream.
