**6. Conclusions**

362 Optical Communication

respectively.

electrical/optical)

all-optical methods such as all-optical WPT-OFDM.

system based on the Haar WPT with the wavelet function

filter coefficients g[n] and h[n] given by eqs. (27), (28) [13], [19].

**5.3. Hierarchical architecture and performance of the WPT based OFDM system** 

The DSP in CO-OFDM systems is carried out by the algorithms realized with the field programmable gate array (FPGA) and application specific integrated circuit (ASIC) electronic processors. Their computational power is limited with the operation rate of the VLSI electronic elements. For this reason, the electronic "bottleneck" can be eliminated and the system operation rate can be improved if high data rates signal processing is realized by

We proposed a novel hierarchical architecture of the 1Tb/s transmission system based on WPT-OFDM in order to reduce the computational complexity of the DSP algorithms [15]. The hierarchical architecture concept is based on the separation of low bit rate and high bit rate signal channels, unlike the system discussed in Ref. [13]. We used an IDWPT/DWPT

The WPT-OFDM hierarchical transmitter and receiver are shown in Figures 13 and 14,

The high data rate bands are multiplexed using all-optical IDWPT. The transmitter includes IQ modulator. QAM 16, QAM 4 and other multilevel modulation formats can be used for each subband. Subbands are multiplexed in electrical domain also by utilizing the IDWPT. At the receiver side, the multiband signal is demultiplexed into the 8 bands using all-optical

**Figure 13.** Hierarchical architecture of the WPT-OFDM transmitter (S/P - serial/parallel; E/O -

*t* , scaling function

*t* , and

> We discussed in this chapter the structure, operation principle and basic properties of the all-optical high SE CO-OFDM systems. The CO-OFDM is the most promising modulation

method in the modern optical systems combining the advantages of the coherent detection and OFDM modulation. However, the high data rate transmission is strongly influenced by the optical channel chromatic dispersion and PMD. As a result, the transmission system performance significantly deteriorates due to ISI and ICI. The dispersion influence can be mitigated by an appropriately chosen CP. Unfortunately, a long enough CP would decrease the system SE. The problem can be solved and the necessity of CP may be eliminated if a generic DFFT based CO-OFDM is replaced with a WPT-OFDM since WPs are localized both in time and frequency domains. The simulation results show that WPT-OFDM system provides a 500 km transmission distance at the FEC level of <sup>2</sup> 2 10 without CP, with a small 5% GI. We proposed a novel hierarchical architecture of the WPT-OFDM system based on the separation of the low data rate and high data rate signal processing. The former ones are processed in the electrical domain, while the latter ones undergo the alloptical processing. The numerical simulations show that such an approach improves the WPT-OFDM system performance which is demonstrated by the constellation of a signal with QAM 16 modulation. The WPT-OFDM all-optical signal processing units can be implemented by using the passive SOI waveguide components. The passive components of this architecture are the wavelet filters realized by the SOI waveguide based MZIs. The change of the Si refraction index is realized by using the thermo-optic effect. We presented the numerical simulations of the Haar wavelet filters for all-optical signal processing based on such MZIs. The modal profile of a SOI waveguide in a single mode regime for each polarization of the optical wave has been calculated.
