10Gbit/s MSK Modulation for Radio-over-Fiber System

Ning Zhang, Chunning Hou, Li Tao, Bo Huang, Xinying Li, Nan Chi Department of Communication Science and Engineering , and State Key Lab of ASIC & System

Fudan University, Shanghai 200433, China nanchi@fudan.edu.cn

AbstractIn this paper, We propose a single sideband RoF system based on MSK modulation. One sideband of terahertz wave undergoes the single sideband MSK modulation while the terahertz wave is obtained via carrier-suppression double sideband modulation. The BER performance achieves [ and the sensitivity of the receiver is -17.8dBm with a data rate of 5Gb/s after 40km transmission.

Keywords-RoF; MSK; SSB; THz; homodyne detection

I. INTRODUCTION The increased demand of bandwidth for fast data

transmission applications requires the transformation of communication systems to higher carrier frequencies. THz-wave radio transmission will probably be restricted to short-range pico-cellular indoor communication systems, due to the high absorption by water vapor in the atmosphere and increased free-space losses [1]. Radio over fiber (RoF) is one of the most attractive technologies for these systems with its numerous advantages including high reliability, huge bandwidth, low loss and being immune to interference [2].

The optical Minimum-Shift Keying (MSK) modulation format belongs to the class of continuous-phase modulation. The phase of MSK signal is continuous over time and changes

constant. Due to these features in MSK signal, it exhibits narrower power spectrum when comparing with a binary phase-shift keying (BPSK). And its high spectrum efficiency (SE) is comparable to DQPSK, while retaining the same receiver sensitivity as differential binary phase shift keying (DPSK) [3].

In this paper, we present a novel scheme of RoF system to reduce dispersion of RF power penalties when transmitting data at THz frequencies while obtaining higher SE with MSK. In the scheme, THz-wave is obtained via carrier-suppression DSB modulation, and one of the sidebands undergoes the second baseband SSB modulation in the process of data modulation in MSK format, which achieves cascade SSB modulation [2]. As to MSK, we introduce a heterodyne detection scheme for its better resistance to the phase noise caused by the laser in the receiver and the simpler configuration. It is proved by simulation that this scheme of RoF system is effective to overcome the chromatic dispersion and nonlinear effects.

II. SIMULATION AND RESULTS DISCUSSION

A. Modeling and system configuration Figure 1 shows the proposed scheme of RoF system with

MSK modulation. Figure 2 presents the details of our MSK scheme.

Figure 1. MSK-RoF system simulation setup

Figure 2. MSK modulator for one of the sidebands

A dual-arm LiNbO3 Mach-Zehnder modulator (LN-MZM) biased at V and driven by two complementary 5 GHz clocks is used to generate optical mm-wave via sub-carrier multiplexing (SCM), including one sub-carrier modulated in MSK format. After a span of standard single mode fiber (SSMF) and corresponding dispersion compensation fiber (DCF) transmission, the beating RF signal is generated at 10 GHz by the photodiode detection in BS (electrical spectrum shown in Figure 1). For bit-error-rate (BER) measuring, the local oscillator (LO) of 10 GHz and mixers are used to down-convert the electrical mm-wave signal to baseband signal.

447978-1-4244-7113-3/10/$26.00 2010 IEEE

In the MSK modulator, the RoF sideband signal and triangular wave drive the first MZ modulator. Then the output is divided into two branches, each of which drives one MZ modulator with one of the two branches of the precoded data signal. And the two MZ modulators produce the I and Q components respectively. MSK signal is obtained by coupling the two components together.

As to the homodyne detection of MSK, the MSK signal is coupled with the local laser by the 90-degree hybrid coupler. With the different phase shift for the In-phase and Quadrature components, the phase information contained in the MSK signal can be transferred to the intensity information. Then the original data can be recovered with appropriate thresholds setting in I and Q components respectively.

B. Simulation and analysis

Figure 3. Spectrums of the optical signals in the system

Figure 3 shows the spectrums of the optical signals in the system.

We measure the BER curve versus received optical power in back-to-back link, 20km and 40km fiber link transmission respectively (in Figure 4). It is hopeful that the RoF link could be over 30km to maintain the power penalty less than 0.5dB without DCF. And we can tell from the eye diagrams that only slight transmission impairments are introduced in case of MSK signal being transmitted through 40km SMF and the phase distortion is not very serious. This is due to the high SE of MSK format which reduces the impact of nonlinear effects in the fiber.

Figure 4. BER performance in the case of (i) back to back, (ii) 20km and (iii) 40km transmission respectively, the insets are the eye diagrams of I and Q branches for each case

III. CONCLUSION In this paper, we demonstrate an effective MSK scheme for

RoF system. BER is measured under different received power and transmission length. The BER achieves 1x10-9 while the sensitivity of the receiver is -17.8dBm with a data rate of 5Gb/s in a transmission span of 40km. Only about 0.5dB power penalty is incurred after 40km SMF span. It is obvious that MSK is a competitive candidate for the promising RoF systems in the near future.

REFERENCES

[1] P. Radoslaw, J. Christian, M. Daniel, K. Thomas, K. Martin, K. Thomas, !" #$% '"* +666 #ransactions on Antennas and Propogation, papers 55(11), 3002-3009 (2007).

[2] C. Hou, Y. Shao, X. Liu, X. Zheng, X. Li, S. Zou, N. Chi, Dual-level optical single side band modulation scheme for 0.1 tera Hz radio-over-Fiber systems*, Proceedings of SPIE, v 7632, 2009.

[3] G. Luo, S. Takahide, C. Akito, K. Tetsuya, M. Tetsuya, H. Kaoru, I. Junichiro, 80-Gb/s optical MSK generation using a monolithically integrated quad-mach-zehnder IQ modulator*, OFC/NFOEC 2010, 2010.

[4] J. Mo, Y. Wen, Y. Wang, C. Lu, W. ; 6 -shift keying with direct ?\\^*, APOC 2003, pp. 5281-23, Nov. 2003.

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