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With PAM4, which is closest to the symbol rate (symbols per second) when sending a signal of about 106 Gb/s per wavelength (including error-correction coding, etc.)?

  1. AAbout 53 GBd
  2. BAbout 106 GBd
  3. CAbout 27 GBd
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Answer: A. About 53 GBd

PAM4 carries 2 bits per symbol using four levels, so the symbol rate is half the bit rate, about 53 GBd (gigabaud). Sending the same bit rate with NRZ (two levels) would require about 106 GBd, demanding correspondingly wider-bandwidth modulators, photodetectors and electrical circuits. On the other hand, because PAM4 divides symbols into four levels, the spacing between adjacent levels is about one-third that of NRZ, making it more susceptible to noise. About 27 GBd is the value for a 16-level modulation sending 4 bits per symbol.

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