DMT vs. PAM for 100G Client-Side Transmissions
Using High-Speed Low-Resolution Interface

• Description

  Pulse Amplitude Modulation (PAM) and real-valued Orthogonal Frequency Division Multiplexing (OFDM), also called Discrete Multitone (DMT), are two competing alternatives for high-speed client-side transmissions [1]. PAM has the advantage of ease of implementation, while DMT presents the benefits of an improved spectral efficiency as well as link adaptation capability using power and bit loading.
  In this example, we investigate the differences between PAM and OFDM transmission at 1550 nm with intensity modulation and direct detection using an interface that is based in a high- baudrate and low-resolution DAC (3-bit and up to 60 Gbaud). The experimental setup is described in Figure 1. The PAM and DMT signals generated using VPIlabExpert are digitized and passed to a 3-channels bit pattern generator (SHF 12104A) feeding the SHF 613A DAC. The optical modulator is an MZ-based SHF 46215B. The MZM is biased for intensity modulation. 0/1/10 km SSMF links are considered; a PIN is connected to a Tektronix DSA 8300 sampling oscilloscope with 16 bits resolution and 64 GS/s speed. Communication with lab equipment, signal generation, visualization, decoding as well as filtering and BER estimation are performed using VPIlabExpert.

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• Typical Results

  Exemplary PAM and DMT signals as capture at the output of the DAC are reported in Figure 2. As both modulation formats are affected by chromatic dispersion (CD), we investigate the impact of Volterra equalization for OFDM and Decision Feedback Equalization (DFE) for PAM. Since most similar experiments are usually limited to specific fiber lengths, we used the experimental results to characterize the system and to perform simulations in order to evaluate the system capacity for OFDM and PAM for transmission up to 20 km SSMF.
  We report the results in terms of achievable capacity versus fiber length in Figure 3. The results for OFDM and Volterra are not provided as the OFDM signal is mainly limited by (quantization) noise and CD-induced power cancellation that cannot be mitigated at the receiver side.

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• Further Information

  Keywords: DMT, OFDM, PAM, DAC, 100G Ethernet, 400G Ethernet, SHF, DFE, Volterra, Equalizer
  
  Similar demonstrations are available in VPItransmissionMaker Optical Systems.
  
  [1] "IEEE P802.3bm 40 Gb/s and 100 Gb/s Fiber Optic Task Force Home Page," Mar. 14, 2014 [Apr. 26, 2015] Internet: http://www.ieee802.org/3/bm/
  [2] Nuno Sequeira Andre, Kai Habel, Hadrien Louchet, and Andre Richter, "Adaptive nonlinear Volterra equalizer for mitigation of chirp-induced distortions in cost effective IMDD OFDM systems" Opt. Express 21, 26527-26532 (2013)

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