Multidimensional Modulation Formats for Coherent Optical Communication Systems

Licentiate thesis, 2014

Coherent optical receivers have enabled the use of multilevel modulation formats with high spectral efficiencies and long transmission reaches. Traditionally, modulation formats utilizing the two dimensions spanned by the amplitude and the phase of the signal have been dominating. This thesis is devoted to novel modulation formats exploring the possibilities of modulation formats in higher dimensional signal spaces to find formats with a good tradeoff between spectral efficiency and sensitivity. The work included in this thesis can be divided into two parts, experimental and theoretical. The first part includes experimental demonstrations of several four- and eight-dimensional modulation formats where the sensitivity as well as the performance in terms of transmission reach is evaluated and compared to conventional modulation formats. 128-level set-partitioning QAM (128-SP-QAM) is demonstrated with 50 % increased transmission distance over polarization-multiplexed 16-ary quadrature amplitude modulation (PM-16QAM). Binary pulse position modulation in combination with QPSK (2PPM-QPSK) is shown to achieve 40 % increased transmission reach over PM-QPSK. Further, the eight-dimensional modulation format frequency and polarization switched QPSK (4FPS-QPSK) is shown to have 84 % increased transmission reach over polarization-multiplexed quadrature phase-shift keying (PM-QPSK) in a dual-carrier setup. The second part includes theoretical work where the spectral efficiency and asymptotic power efficiency is evaluated for modulation formats in high dimensional signal spaces. The high dimensionality is achieved by considering multidimensional position modulation, which is a generalization of pulse position modulation, in combination with QPSK and polarization-switched QPSK. The different dimensions can be achieved by time slots, polarizations, frequency slots, modes of multimode fibers or cores of a multicore fiber.