Experimental Demonstration of a Direct-Detection Constant Envelope OFDM system Jair A. L. Silva1, Tiago Alves2, Adolfo Cartaxo2, and Marcelo Eduardo V. Segatto1 1. Laboratório de Telecomunicações, Universidade Federal do Espírito Santo, Vitória, Brasil 2. Instituto de Telecomunicações, Instituto Superior Técnico,1049-001 Lisboa, Portugal E-mail:
[email protected]
Abstract: We propose and experimentally demonstrate an optical constant envelope OFDM system based on electrical phase modulation. We transmit 1.4 Gbps data onto 16-QAM symbols in 500 MHz bandwidth. Our system shows good tolerance to the optical modulator intermodulation effects. ©2010 Optical Society of America [9-point type] OCIS codes: (060.2330) Fiber optics communications; (060.4230) Multiplexing. 1.
Introduction
Tolerance towards polarization mode dispersion (PMD) and chromatic dispersion (CD) are foremost reasons that make optical orthogonal frequency division multiplexing (OFDM) a potential technique for long-haul optical transmission [1]. Systems operating at 100 Gbps have been reported and demonstrated experimentally for both coherent optical (CO)-OFDM and direct-detection optical (DDO)-OFDM [1,2]. (DDO)-OFDM are simpler and cheaper when compared with (CO)-OFDM but presents lower spectral efficiency due to the use of guard interval and lower power efficiency. An important challenge for optical OFDM systems is the use of peak-to-averagepower (PAPR) reduction techniques in order to increase their tolerance to optical modulator intermodulation and fiber nonlinearities impairments [3-5]. In this paper, we introduce a direct detected optical constant envelope (DDO-CE)-OFDM system that reduces the PAPR of electrical OFDM waveform to 3 dB by modulating the phase of an electrical carrier. We report experimental results of 1.4 Gbps using 16-quadrature amplitude modulation (16-QAM) and 500-MHz signal bandwidth. The results show an improved tolerance to Mach-Zehnder modulator (MZM) nonlinearity effects. 2.
System Design and Experimental Setup
The setup used for the DDO-CE-OFDM transmission experiment is shown in Fig 2. An OFDM signal with 384 data subcarriers and its conjugates, transferred into a time domain real-valued electrical signal by an IFFT size of 1024, was generated offline. The 1.4 Gbps OFDM signal uses 16-QAM symbol mapping, 1.08 µs for the OFDM symbol duration and it is oversampled and windowed by a raised cosine filter. Multiplied by 2h = where h is the phase modulation index, it modulates the phase of an electrical carrier centered at F c = B = 500 MHz. After 64 samples cyclic prefix extension, the CE-OFDM signal produced at the output of the CE-OFDM transmitter block is loaded in a 5 Gsamples/s Tektronix AWG7052 and transmitted in the continuously mode for a Sumitomo Mach-Zehnder modulator biased at quadrature point (Vbias = 0.5V) where V ~ 3.5 V is its switching voltage. A JDSU DFB centered at 193.1 THz with 10 MHz linewidth and 10 dBm output power is used as laser source.
windowing
Phase Modulator
OFDM Tx
CE-OFDM TX CP
Experimental Setup AWG 7052 5GS/s Tektronix
MZM 10 dBm
2h
windowing
OFDM Rx
Fc
CE-OFDM TX
CE-OFDM RX
Phase Demodulator
(.) arg
FDE
CP-1
DSO 40 GS/s Agilent
CW
EDFA
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APEX OSA
EVM
-Fc
Fig. 1. Constant envelope OFDM transmitter and receiver design of the experimental setup.
OF
PD CE-OFDM RX
A variable optical attenuator (VOA) and an erbium-doped fiber amplifier (EDFA) are used for OSNR tuning. Before photodetection, an optical filter with a 50 GHz bandwidth is used to reduce the optical noise. The photocurrent is then sampled at 12 GHz and recorded by a real-time 40 Gsample/s Agilent DSO81204A oscilloscope. The recorded signal goes to the CE-OFDM receiver. The general receiver structure for CE-OFDM consists of a phase demodulator, to undo the transformation (phase modulation), followed by a standard OFDM demodulator as shown in Fig. 1. 3.
Experimental Results and Discussion
The performance of the (DDO-CE)-OFDM system proposed in Fig. 2 was analyzed in terms of the Optical Modulation Index (OMI) defined as OMI = (Vin)rms/V, where (Vin)rms is the root mean square of the input MZM voltage signal [5]. We have used 100 CE-OFDM symbols (153600 bits) and an OSNR of 32 dB. Fig. 2a) depicts the equalized-CE-OFDM waveform with PAPR = 3 dB. The power spectral density (PSD) of the constant envelope received signal is shown in Fig. 2.b and the received constellation is shown in Fig. 2.c.
Fig. 2.a)The generated CE-OFDM waveform, b) PSD of the received CE-OFDM signal, c) 16-QAM symbols scatterplot of 100 CEOFDM received symbols, d) EVM versus OMI system performance.
Fig. 2.d shows the measured Error Vector Magnitude (EVM) as a function of OMI. The maximum reached OMI value is 0.14 due to the 2 Volts peak-to-peak AWG7052 output signal limitation. Although limited by the equipment limits, our results reveal that the proposed DDO-CE-OFDM system has a good tolerance to MZM distortions due to its nonlinear characteristic. This can be explained considering the definition OMI2 = PAPR(V2/Vp2) presented in [5]. In this case Vp is the OFDM peak amplitude. In the proposed system, the PAPR is constant and equal to 3 dB, consequently much lower than the conventional OFDM. 4.
Conclusions
A direct-detection optical constant envelope (DDO-CE)-OFDM system has been proposed and experimentally demonstrated. We have showed that it is possible to reduce the PAPR and consequently improve the system performance. References [1] William Shieh and Ivan Djordjevic, Orthogonal Frequency Division Multiplexing for Optical Communications (Elsevier, 2010). [2] Brendon J. C. Schmidt et al., “100 Gbit/s Transmission using Single-Band Direct-Detection Optical OFDM”, OFC/NFOEC, PDPC3, (2009). [3] Steve C. Thompson et al., “Constant Envelope OFDM”, in Transactions on Communications (IEEE, 2008). [4] Jochen Leibrich et al., “Impact of Modulator Bias on the OSNR Requirement of Direct-Detection Optical OFDM” in Photonics Technology Letters (IEEE, 2009). [5] Wei-Ren Peng et a.l, “Theoretical and Experimental Investigations of Direct-Detected RF-Tone-Assisted Optical OFDM Systems” in Journal of Lightwave Technology (IEEE, 2009).
