Several transmitter and receiver concepts for the generation and detection of OFDM signals for optical communications applications have been proposed. OFDM signal generation, (de)multiplexing and processing can be performed in the electronic and optical domains. OFDM signal generation using digital signal processing in electronics is very popular due to its extraordinary flexibility in selecting desired
modulation formats, data rates, guard band values (cyclic prefix), up-conversion to desired frequencies and combination of lower-speed data streams into a single high- speed data signal [69]. However, bandwidth and speed limitations of the available DACs and ADCs limit OFDM signal generation using DSP and electronic circuitry to moderate aggregate data rates [70, 89, 90]. However, OFDM signal formation in the optical domain relaxes ADC/DAC requirements, which allows the generation of higher data rate channels and their combination into superchannels [69]. As OFDM signalling gained significant research and industry attention in recent years, many combinations and variations of the electronic and optical OFDM schemes have been suggested and implemented. However, the proposed OFDM concepts can be classified based on the transceiver configuration into all-optical OFDM and DSP based OFDM.
1.3.3.4.1 DSP Based Optical OFDM
Figure 1.15 shows typical schematic designs of a DSP based OFDM optical systems. Figure 1.15(a) depicts DSP based coherent optical OFDM. At the transmitter side, data is encoded using the desired modulation format and parallelised before being processed by the electronic IDFT. After the addition of the cyclic prefix and parallel-to- serial conversion, the OFDM signal is encoded onto the optical carrier using an IQ modulator (i.e. Mach-Zehnder IQ modulator) after being converted from the digital to analog domain with an appropriate DAC. At the receiver, the inverse process includes optoelectronic down-conversion, i.e. coherent detection, digitalisation of the received analog signal by an ADC, serial-to-parallel conversion, cyclic prefix removal and DFT processing. Digital signal processing is used after DFT to retrieve the transmitted data. Capacities higher than 1 Tb/s have been experimentally achieved using multi- band DSP based coherent optical OFDM [91, 92]. Also, the real-time implementation of the single channel DSP based coherent optical OFDM up to data rates of 100 Gb/s have been reported [93, 94]. The typical implementation of the coherent optical OFDM is in metro and core networks [89, 91, 93].
Figure 1.15(b) depicts DSP based electro-optical OFDM (E/O OFDM). The difference in the transmitter design compared to the coherent optical OFDM is the possibility to use an intensity electro-optical modulator instead of an IQ modulator for data encoding onto the optical carrier [90, 95]. At the receiver side, instead of using coherent detection, direct detection is used for simple E/O down-conversion. Several
possible configurations of the electro-optical OFDM systems have been proposed [70, 90]. In contrast to the coherent optical OFDM which requires two DACs and ADCs (for single polarisation detection), the E/O OFDM configuration which employs a single DAC and ADC have been reported [90]. The electro-optical OFDM (E/O OFDM) is usually implemented in optical access and metro networks due to its simple receiver configuration which does not require a coherent receiver and a high power low linewidth laser at user premises [31]. Nevertheless, experimental long-haul implementations of this system have been proposed [90, 96, 97]. The real-time implementation of the E/O OFDM at data rates up to 40 Gb/s have been reported [98- 100].
Figure 1.15. Transmitter and receiver concepts for optical OFDM. (a) DSP based coherent optical OFDM and (b) DSP based electro-optical OFDM.
In general, the maximum available capacity of the DSP based optical OFDM is limited by the bandwidth and speed of DAC and ADC [101]. The system configuration shown in Figure 1.15 illustrates the single channel systems, which can be extended to the
WDM case by simple scaling of the number of transmitters and receivers. However, that would significantly increase the total cost of the system.
1.3.3.4.2 All-Optical OFDM
All-Optical OFDM (AO-OFDM) has been proposed in order to overcome electronic circuit bandwidth limitations. Even though electronic circuits have been rapidly developed to provide higher speed and bandwidth, and lower cost implementations, they still represent a bottleneck in achieving very high data rates. Therefore, unlike DSP based OFDM which employs digital (de)multiplexing of an OFDM signal, AO- OFDM systems employ IDFT and FFT in the optical domain. Capacity scaling beyond DSP based OFDM applications was enabled using multicarrier OFDM where carriers are all-optically (de)multiplexed using optical IDFT/DFT. In practice, AO-OFDM has been implemented using several scenarios, which may differ in transmitter and receiver design but all target to benefit from orthogonality between carriers [101-107]. In the case of AO-OFDM every optical carrier represents a single OFDM subcarrier. Similar to the case when digital IDFT/DFT is used, the orthogonality condition is satisfied when carrier frequencies are spaced at multiples of the inverse of the symbol periods. Each optical carrier is modulated with the desired modulation format whose symbol rate equals the frequency separation between adjacent optical carriers in order to satisfy the orthogonality condition. Additionally, the symbols in the modulated carriers should be time-aligned [16, 104]. Carrier orthogonality can be preserved, even with arbitrary modulation formats as long as the modulation periods are equal and synchronised [101]. Furthermore, a phase correlation between all of the optical carriers is desirable in order to mitigate crosstalk between optical channels [101, 104, 106]. Therefore, multicarrier optical sources which have phase-locked optical carriers are typically used in AO-OFDM systems [92, 103, 107]. A set of free-running continuous-wave (CW) lasers can also produce AO-OFDM carriers as long as the laser frequencies are precisely tuned and locked to satisfy the orthogonality condition [101, 104]. However, in this configuration the carriers are not phase correlated. The lack of phase correlation between optical carriers might cause additional OSNR penalty as the impact of ICI may not be eliminated by digital signal processing at the receiver. The impact of ICI results in fast beat noise fluctuations due to fast phase rotations of free-running sources [101].
The modulated optical carriers are applied to an optical IDFT whose function is to multiplex optical carriers and generate the optical OFDM signal as shown in Figure 1.16. Instead of IDFT at the transmitter, a passive combiner can be used. However, it is shown that the ICI introduced by imperfect WDM multiplexers (passive combiner) can be significantly reduced by using IDFT instead [108]. A number of IDFT/DFT designs have been reported recently: cascaded delay-line interferometers (DIs) based IDFT/DFT [87], AWG based IDFT/DFT [101] and LCoS based IDFT/DFT design [109]. The optical DFT differs from its electronic counterpart by its continuous mode of operation. In an electronic implementation, the optical signal is sampled and the DFT is computed from all samples. In the optical domain, the DFT is computed continuously. However, in both cases sampling must be performed in synchronisation with the symbol over the duration of T/N [87]. Demultiplexed optical carriers are coherently detected and digital signal processing is used to retrieve the transmitted data.
Figure 1.16. Transmitter and receiver concepts for all-optical OFDM.