Advanced Modulation Formats

From the beginning of the previous decade it was clear that the requirements for the network capacity due to the trends in the network traffic growth could not be fulfilled with simple scaling of the existing technology, which was mainly based on 10 Gb/s non-return-to-zero (NRZ) OOK modulation format. Due to the expenses involved in deploying new fibre optic links, the main focus was to upgrade the existing network using more advanced underlying technology. Even though the possibility of the practical implementation of 40 Gb/s OOK systems was demonstrated [50], it was realised that the obtained spectral efficiency would not be sufficient for the future optical networks. Furthermore, the difficulties in managing the transmission impairments at higher data rates, such as chromatic and polarisation mode dispersion

[51], motivated researchers to revive coherent detection which was the subject of intensive research during the 1980’s [44, 52].

Before the coherent systems were fully revived, higher order modulation formats such as differential phase shift keying (DPSK) and DQPSK have been considered for 40 and 100 Gb/s interface solutions [10, 53]. The field trials employing these modulation formats at 40 and 100 Gb/s data rates have been successfully demonstrated [24, 54]. At the time, DPSK and DQPSK formats were attractive solutions to increase system capacity mainly due to the direct detection based receiver which can be realised with low complexity as key components such as the optical delay-interferometer become mature. Furthermore, monolithic PDM-DQPSK receivers which allow high bit rate signal reception and automatic polarisation tracking have been proposed [55-57]. Both DPSK and DQPSK modulation formats have been deployed in carrier networks. DQPSK format at 100 Gb/s data rate is typically deployed on 100 GHz frequency grid. For transition to 50 GHz channel spacing, the data rate per channel needs to be reduced to avoid overlapping of the adjacent channels which causes significant performance degradation. In [53] the spectral efficiency of 3.4 bit/s/Hz was achieved with 84.5 Gb/s PDM-RZ-DQPSK signal on 50 GHz grid.

With the advances in speed and bandwidth of electronic circuitry and digital signal processing (DSP), the basis for development of coherent receivers and coherent reception of optical signals was founded. The digital coherent receiver allows linear mapping of the optical field and enables the detection of the real and imaginary components of the complex amplitude of both polarisations of the optical field and, therefore, quadruples the spectral efficiency. With the significant cost reduction of key components for optical coherent systems, such as high quality optical sources, optical field modulators (IQ Mach-Zehnder modulator), phase and polarisation diversity 90° optical hybrids and analog-to-digital and digital-to-analog converters (ADC/DACs), higher spectral efficiencies have been achieved with QAM formats [16] (see Figure 1.10(a)). QAM formats are often denoted as m-QAM, where m represents the number of states per symbol. With increasing of number of bits per symbol, spectral efficiency increases accordingly for a given bit rate, and the baud rate (symbol rate) reduces which allows for the use of lower cost electro-optical devices and readily available electronic circuitry. However, with the increase of the number of bits per symbol, the more stringent the requirements on the SNR of the received signal become, as shown in Figure 1.10(b). Therefore, depending on the application a specific m-QAM format would be used. 4-QAM, which is also known as QPSK is typically used for long reach

applications as it has the lowest SNR requirements among m-QAM formats [58]. Maximum spectral efficiency achievable with QPSK format per-polarisation is 2 bit/s/Hz. For the capacity oriented applications where reach is not of crucial importance, 16-QAM [59, 60], 32-QAM [61, 62] and 64-QAM [62, 63] modulation formats can be considered. The maximum achievable spectral efficiency per- polarisation for these modulation formats are 4 bit/s/Hz, 5 bit/s/Hz and 6 bit/s/Hz respectively. As shown in Figure 1.10(b), transition from QPSK to 16 QAM (which doubles the spectral efficiency and system capacity), comes at the requirement of a 3.7 dB higher SNR per bit, or 6.7 dB higher optical SNR (OSNR) [64] at fixed symbol rate [16]. A further doubling in spectral efficiency and capacity, from 16-QAM to 256- QAM, however, comes at the expense of an additional 8.8 dB in required SNR per bit (see Figure 1.10(b)), which is not feasible to accommodate without reducing system reach [16]. The implementation of 512-QAM format and transmission over 150 km has been reported in [65]. Besides SNR requirements for increasing modulation order (number of bits per symbol), the purity of optical sources, by means of phase noise and optical linewidth characteristics, becomes the limiting factor [44].

Figure 1.10. (a) Experimentally obtained per-polarisation spectral efficiencies in single (red) and dual-polarisation (blue) experiments. (b) Spectral efficiency versus received SNR per bit (per-polarisation). The Shannon limit for a linear, additive white Gaussian noise channel is

shown together with the theoretical performance of various square QAM formats (blue circles). Red squares represent experimental results where numbers indicate QAM

constellation sizes. After [16].

It has been shown that the QPSK format has the strongest bandwidth requirements, and simultaneously the lowest requirements for the resolution of DAC/ADCs [66]. With the increase of modulation order, the requirements on bandwidth relax, whilst the requirements on the DAC/ADC resolution increase [66]. The trade-off between the bandwidth of electronic circuitry and resolution of ADC/DACs has resulted in 16-QAM

being the optimum modulation format to achieve the highest per-interface data rate over longest distances [66]. Indeed, the highest interface data rate of 320 Gb/s at 80 GBaud, polarisation multiplexed to a single-carrier 640 Gb/s was achieved with 16 QAM modulation format [67]. Interface data rate of 450 Gb/s and transmission over 800 km was achieved with polarisation multiplexed 32-QAM [61].