## 2 DMT Signal Generation and Reception  
DMT is a multi-carrier transmission technique with real-valued orthogonal subcarriers. The signal processing flow charts for a DMT transmitter and receiver are given in Figure 2. A pseudorandom bit sequence (PRBS) is generated to test the transmitter. The bits are then rearranged from serial into parallel according to the number of subcarriers and the information content in the respective subcarriers. Each parallel bit stream is then mapped onto its individual constellation (e.g., quadrature amplitude modulation--QAM). DMT gives us the flexibility to encode a different modulation format onto every subcarrier. Therefore, depending on the available SNR at each subcarrier the number of bits per symbol can be adapted to achieve the highest possible throughput, a feature called bit-loading. In our experiments, the modulation schemes ranged from binary phase shift keying (BPSK with 1 bit/symbol) to 64 QAM with 6 bit/symbol. The constellation diagrams of the modulation formats are shown in Figure 3. After bit mapping, the power of each subcarrier is optimized in a power-loading stage depending on the SNR profile, see Figure 2. This is done such that all subcarriers will end up with a relatively similar bit error ratio (BER) performance even if they have different modulation formats. The adaptive bit-and power-loading algorithm is based on [PERSON]'s method discussed in [38]. In a next step, an inverse fast Fourier transform (IFFT) with Hermitian symmetry is performed to obtain the real valued DMT symbols. A cyclic prefix is added to each DMT symbol before being serialized and loaded into an arbitrary waveform generator (AWG). The AWG converts the DMT signal to the analog domain before it is fed to an RF amplifier and encoded to the optical domain by means of an RSOA. Figure 2: Discrete multi-tone (DMT) signal generation and reception flowchart. A pseudorandom bit sequence (PRBS) is generated, which is then rearranged into a parallel bit-stream. Subsequently, adaptive bit mapping and power-loading are implemented. Inverse fast Fourier transform (IFFT) is then used to encode the quadrature amplitude modulation (QAM) symbols on the DMT subcarriers. Cyclic prefix (CP) is added on the DMT symbols before the signal is converted back to serial. The serial data is loaded onto an arbitrary waveform generator (AWG) which converts the digital signal to analog using an inbuilt digital to analog converter (DAC). On the receiver side the signal is detected by a photodiode and sampled by a real-time oscilloscope (RTO).Next, timing synchronization is performed to recover the DMT clock. Then the serial signal is converted into a parallel data stream. After CP removal, FFT is used to decode the DMT subcarriers. Frequency and phase equalization of the received QAM symbols is performed. Here, we measure the error vector magnitude (EVM) and signal-to-noise ratio (SNR). Then, the symbols on each subcarrier are demapped to recover the transmitted PRBS for bit error ratio (BER) measurements. In the receiver, a direct detection scheme is used to detect the optical signals. The electrical signals are then fed into a real-time oscilloscope (RTO). The RTO effectively digitizes the signal with a resolution of ~5.8 bits. In a first step, timing synchronization is performed using the non-data aided DMT timing estimation technique discussed in [39]. Subsequently, the signal is parallelized and the cyclic prefix is removed.The subcarriers within the electrical signal are then derived by the fast Fourier transform (FFT). To properly decode the bits within a subcarrier, blind phase and frequency offset estimation methods are applied on every subcarrier [40]. The error vector magnitude (EVM) of the QAM symbols on each subcarrier is calculated and converted into SNR [41]. Then, the QAM symbols are de-mapped to extract the received bit pattern. The BER is measured by comparing the transmitted and received bit patterns on each subcarrier. The overall BER is also measured by comparing the total transmitted and received bits.