Performance Study of 1 Tbits/s WDM Coherent Optical OFDM System ()
1. Introduction
The demand for high data rate and high capacity in the optical communications field has motivated researchers to try different modulation formats that can support this demand. Among this was Coherent Optical OFDM which got special attention due to its tolerance to Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD) [1]. In addition, CO-OFDM has a great potential when it comes to receive sensitivity and spectral efficiency. Tbits/s transmission rate is available through the WDM (Wavelength Division Multiplexing) transmission system; but, this system has a low spectral efficiency due to wavelength spacing [2-4]. However, integrating WDM with CO-OFDM will produce a system with high spectral efficiency; better tolerance to PMD and CD; and, significantly high data rate. Because of the great potential of CO-OFDM, it is considered the solution to upgrade todays’ 10 Gbits/s transmission rate to over 100 Gbits/s [5-8].
This paper demonstrates the architecture of Tbits/s WDM-CO-OFDM system. In this experiment, we studied a WDM system by using CO-OFDM with 4-QAM (Quadrature Amplitude Modulation). 20 WDM channels are used with a 50 GHz wavelength space and 20 OFDM signals, each with 50 Gbits/s to produce a net data rate of 1 Tbits/s. To study the performance of the system, we focused on the constellation diagram of the system and the relationship of the BER (Bit Error Rate) and the OSNR (Optical Signal to Noise Ratio) with regard to transmission distance.
2. System Design
The WDM CO-OFDM system is simulated and studied using an OptiSystem V.11 simulation tool. The simulation diagram is shown in Figure 1. The design consists of three main parts: CO-OFDM Tx (Transmitter), optical fiber link and CO-OFDM Rx (Receiver). In the WDM system, 20 channels with 50 GHz channel spacing are used to support 20 OFDM bands, each with a 50 Gbits/s bitrate to reach 1 Tbits/s data rate. Important simulation parameters are shown in Table 1.
2.1. CO-OFDM Tx Design
Figure 2 shows the CO-OFDM transmitter design; the bit stream is generated by a PRBS generator and mapped by a4-QAM encoder. The resulting signal is modulated by an
Table 1. Simulation global parameters.
OFDM modulator; the parameters are shown in Table 2. After that, the resulting electrical signal is modulated to the optical signal using a pair of Mach-Zehnder modulators (MZM). Figure 3 shows the in-phase and quadrature parts of the OFDM signal, where Figure 4 shows the signal after the two MZMs which will be fed to the optical link. The laser source has a line width of 0.15 MHz and launch power of −5 dBm [9,10].
2.2. Optical Fiber Link
The optical link consists of 15 spans of 100 Km SMF, with a dispersion coefficient of 16 ps/nm/Km, nonlinearity coefficient of 2.6 × 10−20; and, attenuation of 0.2 dB/Km. SMF parameters are shown in Table 3. Fiber dispersion is compensated by the Dispersion Compensation Fiber (DCF) of 20 Km with a -80 ps/nm/Km coefficient in each span; DCF parameters are shown in Table 4. The attenuation of SMF and DCF are balanced by optical amplifiers with 4 dB noise figure in each span.
2.3. CO-OFDM Rx Design
Figure 5 shows the CO-OFDM receiver design; to recover the I/Q component of the OFDM signal, two pairs of balanced PIN photodetectors and LO (Local Oscillator) lasers are used. The balanced detectors perform the I/Q optical to electrical detection and help perform the noise cancellation. Electrical amplifiers are used to adjust the signal intensity [11,12]. After the balanced detectors the
Figure 4. OFDM Signal after the two MZMs.
resulting signal is demodulated using the OFDM demodulator with similar parameters as the OFDM modulator, the guard interval is removed. After that the signal is fed into a 4-QAM decoder, and the BER is calculated at the end [12-16].
3. Results and Discussion
Figure 6 shows the RF spectrum of the signal at the transmitter side, where the power of the RF is approximately −12 dBm. Figure 7 shows the RF spectrum at the receiver side after 1800 Km SMF. The power of the RF is decreased to −22 dBm, this decrease in power is because of the increase in fiber length which increases the attenuation.