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

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 system based on the Haar WPT with the wavelet function *t* , scaling function *t* , and filter coefficients g[n] and h[n] given by eqs. (27), (28) [13], [19].

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

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 electrical/optical)

**Figure 14.** Hierarchical architecture of the WPT-OFDM receiver (O/E - optical/electrical, WS-window synchronization, S/P-serial/parallel, P/S-parallel/serial)

filters and consequently demultiplexed into the subbands in the electrical domain by the DWPT. The performance of the WPT-OFDM communication system based on the hierarchical architecture has been investigated theoretically taking into account the 10% non-ideality of the system devices and the white Gaussian noise model. We used the modulation format QAM 16, 8 level decomposition, the bit rate of 500Gb/s. The 500Gb/s multiple band consists of eight 62.5Gb/s bands. The simulated constellation of the transmitter-receiver "back-to-back" system, i.e. without optical fiber link is shown in Figure 15.

**Figure 15.** Constellation diagram for all-optical WPT-OFDM with a 500Gb/s bit rate, QAM-16 modulation format and 8 level decomposition

The constellation clearly shows the high performance of the transmitter-receiver system.
