Novel QPSK Modulation for DWDM Free Space Optical Communication System

Novel QPSK Modulation for DWDM Free Space Optical Communication System

Bijayananda Patnaik International Institute of Information Technology

Bhubaneswar, Gothapatna, PO: Malipada Bhubaneswar, Odisha, India, Pin: 751003

e-mail: [email protected]

P. K. Sahu Indian Institute of Technology Bhubaneswar

Samantapuri, Bhubaneswar Odisha, India, Pin: 751013 e-mail: [email protected]

Abstract— A novel 1.28Tbps dense wave length division multiplexing system for free-space optical communication is proposed here. It is a line-of-sight system and uses novel coherent optical quadrature phase shift keying (QPSK) modulation technique. The system consists of 32-channels with bit-rate of 40-Gbps/channel and adjacent channel spacing of 200GHz. The system is analyzed for various modulation formats such as return-to-zero, non-return-to-zero and coherent optical QPSK modulation technique. The performance of the proposed system is analyzed in terms of bit error rate, Q-factor and eye opening etc. For an input power level of 10dBm, the coverage distance observed for the system is 1360km, for coherent optical QPSK modulation technique.

Keywords - Free-space optical communication; dense wave length division multiplexing; coherent optical quadrature phase shift keying; bit error rate; Q-factor.

I. INTRODUCTION Optical communication provides huge bandwidth and

very high speed data transfer, thus it meets the today’s ever-increasing demand for broadband traffic, mostly driven by internet access, video on demand and high definition television (HDTV) broadcasting services etc [1]. For 40Gbps bit-rate, if the frequency spacing between the channels of the multiplexed system is less than or equal to 200GHz, then it is called dense wave length division multiplexing (DWDM) system [2]. DWDM technology is used to make full use of huge bandwidth resources of optical systems. Here the optical signals of different channels are coupled and transmitted through a single media. The most common type of DWDM system uses optical fiber but today the system is also designed for free-space optics (FSO) systems [1].Optical transmission over the atmosphere refers to FSO communication. It uses either light emitting diode (LED) or laser as an optical source. The advantage of the proposed system over radio frequency (RF) communication is (i) large bandwidth, which allows very high data rate (ii) no licensing requirements (iii) low power consumption (iv) immunity from interference (v) the system is small, lightweight and has compact dimensions (vi) easy to install comparing to optical fiber etc [3]. Based on the detection technique of FSO system, it can be either non-coherent (intensity modulation/direct detection, (IM/DD)) or coherent (heterodyne detection) system. In coherent system, at the receiver, the local oscillator (LO) field mixes the incoming optical field and then using a photodetector, it produces the desired electrical

signal. Here the LO field should be temporally and spatially coherent with the received field [4]. Hence coherent systems are difficult to implement comparing to IM/DD systems. But the coherent systems are more flexible as any kind of modulation such as amplitude, frequency or phase modulation can be used, hence significant performance enhancements is observed due to spatial temporal selectivity and heterodyne gain [5]. The performance of the FSO system is greatly affected by the propagation medium. Random fluctuations are observed in the received signal owing to atmospheric turbulence of the medium, which is called scintillation. Thus in the design process, selection of proper modulation technique is a vital role [6]. The IM/DD scheme is the simplest and extensively used technique, but it does not offer immunity to the turbulence-induced fading channel. The behavior of turbulence level, which is non-predictive, produces random fluctuation of the optical intensity level at the receiver. Hence, it requires an adaptive thresholding for optimal performance. But, this adaptive thresholding technique is complex to implement and practically not suitable [7]. Since the optical intensity level is affected by the scintillation effects, it will be better to use the modulation techniques that carry the information in the phase or frequency of the carrier signal. The phase shift keying (PSK) based modulation requires no adaptive thresholding scheme, thus offers superior performance compared with the IM/DD in the presence of the atmospheric-turbulence [7]. With partial phase compensation, Belmonte and Kahn have developed a model to characterize the combined effects of turbulence induced amplitude fluctuation and phase distortion on the performance of coherent receivers [8]. They again investigated the spectral efficiency and the outage probability of coherent FSO systems with multiple receiver apertures [9]. E. Ciaramella et al. designed a 32-channel 40-Gbps DWDM system using IM/DD technique and they demonstrated that wavelength division multiplexing (WDM) transmission system for FSO technology is suitable for a range of high quality of service (QoS) as well as high capacity applications [1]. A single channel FSO system using coherent optical QPSK

2012 Wireless Advanced (WiAd)

978-1-4577-2194-6/12/$31.00 ©2012 IEEE 170

modulation technique is also proposed for bit-rate of 1 Gbps to 100Gbps [6]. As an extension to this work, here we have proposed a high capacity 1.28Tbps, multiplexed FSO system using the same coherent optical QPSK modulation technique. As per authors’ knowledge, such a high capacity FSO system using coherent optical QPSK modulation technique is proposed for the first time. The performance of the system is also compared with the conventional IM/DD techniques.

II. COHERENT OPTICAL QPSK MODULATION TECHNIQUE The block diagram of the coherent optical QPSK modulation technique is shown in Fig. 1. The number of bits per symbol considered is 2. It consists of PSK sequence generator, which generates the in-phase (I) and quadrature signals (Q) as given in equation (1) and (2) respectively. Table I shows the details of I & Q signal as per the incoming symbol. The output of the PSK sequence generator is given to the M-array pulse generator, where M = 4, as it is a QPSK modulation [10]. The optical signal is fed to the Mach-Zehnder modulator using a coupler, both I and Q signal is combined using an optical power combiner at the end, as shown in Fig. 1. The I and Q signal is given by, [10] Ii = cos(�i) (1) Qi = sin (�i) (2)

where �i = 2� (i-1)/M, M=4 and i=1, 2, 3, 4.

III. PROPOSED SETUP The block diagram of the proposed DWDM system is given in Fig. 2. It consists of transmitter, FSO channel, receiver and few visualizers. Transmitter consists of 32 number of optical QPSK modulators and a multiplexer.

Fig. 1 Block diagram of coherent optical QPSK transmitter

Table I: I and Q signal as per the incoming symbol

Symbol I Signal Q Signal 00 1 0 01 0 110 0 -111 -1 0

The output of the transmitter is given to the FSO channel, which consists of a transmitter telescope, the free space communication channel and the receiver telescope. The output of the receiver telescope is given to the de-multiplexer and then to the optical QPSK demodulator, which coherently detects the optical signal using a local oscillator (laser source). Output of the demodulator produces the desired data in the electrical domain. Then the electrical signal is applied to the signal conditioner which consists of amplifier, low pass filter, M-array threshold detector and a PSK decoder. Here the PSK decoder is used to produce fresh 0s and 1s from the received distorted signal. Then the BER analyzer is used to observe the bit error rate, eye opening and eye diagram etc. Optical spectrum analyzer (OSA) is used to observe the optical signal spectrum. For the LOS system, the received power is given by [11], PR = PT �T �R GT GR LT LR (�/4�z)2 (3) where: PR :Received power PT :Transmitted power �T : Optics efficiency of the transmitter �R : Optics efficiency of the receiver GT : Transmitter gain GR : Receiver gain LT : Transmitter pointing loss factor LR : Receiver pointing loss factor � : Operating wavelength z : Distance between transmitter and receiver

IV. SIMULATION The proposed DWDM system, for free-space optical communication as given in Fig. 2 is designed and simulated with centre channel frequency of 193.1THz. The details regarding the simulation parameters used for the system are given in Table II. The parameters are considered as per the practical scenario of FSO system. Free space path loss is also taken into consideration. The local oscillator power used is maximum of 10dBm [12]. Data rate considered is 40Gbps as per Telecommunication standardization sector of the International Telecommunications Union (ITU-T) guide lines [2].

V. RESULTS AND DISCUSSION At first, the setup shown in Fig.2 is simulated for RZ data format and for an input power level of 10dBm. For this data format, we observed a coverage distance of 614km. For this coverage distance the Q-factor obtained for all the channels are greater than or equal to the forward error

Optical QPSK Signal

Optical QPSK Tx

PRBS Gen.

PSK sequence Generator

M-array Pulse Gen.

M-array Pulse Gen.

Mach- Zehnder

Mod.

Mach- Zehnder

Mod.

Power Comb-iner

CW

Laser

Fiber coupler

171

Fig.2 Block diagram of the proposed DWDM FSO system correction (FEC) threshold value that is 6.8 (BER < 10-12). Then the system is simulated for NRZ data format and we obtained a coverage distance of 798km. Fig. 3 and Fig.4 show the eye diagram and Q-factor of the setup for channel No. 32 for RZ and NRZ data format respectively. End channel, that is channel No. 32 is observed here for analysis, because in DWDM systems the end channel performance is worse comparing to the centre channels, as its operating frequency is more deviated from the centre channel operating frequency of the system (193.1THz). The performance of NRZ format is better than RZ format as it contains more energy than RZ format [13]. Then the system is simulated for coherent optical QPSK modulation technique for an input power level of 10dBm. Fig. 5 shows the optical spectrum of the modulator of the optical QPSK transmitter of channel number 32 of the setup. The power transmitted is observed to be approximately -10dBm at 1550nm (193.1THz). Fig. 6 shows the multiplexed optical signal that is being transmitted through the channel. Fig. 7 shows the multiplexed optical spectrum at the receiver. The power level received is observed to be decreased to approximately -52dBm at a distance of 1360km. The received power at the receiver of the proposed system is also calculated theoretically using equation (3) for these parameters, which is observed to be same, as obtained from the simulation.

From Fig. 8, the eye opening observed is as desired and the Q-factor obtained is 2242.64. Here a high value of Q-factor is observed as fresh 0s and 1s are reproduced at the receiver, from the distorted received signal. Fig. 9 and Fig. 10 show the electrical signal transmitted, and received at a distance of 1360km respectively. Comparing Fig. 9 with Fig. 10, we can conclude that there is a strong similarity between the input signal with the output signal, as fresh 0s and 1s are reproduced at the receiver. Further increasing the distance, we observed a distorted signal at the receiver and Q-factor obtained is 0. Hence we can conclude that the coverage distance of the setup is 1360km. Fig.11 shows the coverage distance of the system for varying input power and modulation formats. From Fig. 11 we can conclude that comparing to IM/DD techniques (RZ & NRZ), the coherent optical QPSK modulation technique provides more coverage distance for the DWDM system for free-space optical communication applications. It is because, the information is contained in the phase of the carrier for the coherent optical QPSK modulation technique and hence it’s tolerance to the random fluctuation of atmospheric turbulence is more compared to IM/DD techniques.

OSA

OSA

.

.

. . .

.

.

.

. .

FSO Channel

Optical QPSK Modulator (� 1)




Coherent Optical QPSK Demodulator (� 1)




M U X

DE MUX

B E R A n a l y z e r

S i g n a l C o n d i t i o n e r

172

Table II: Simulation parameters of the DWDM system for free-space optical communication

Parameters Values

FSO type Line of sight (LOS)

Capacity 32-channel, 40Gbps

Modulation RZ, NRZ and Coherent optical QPSK

Centre channel frequency of the DWDM system

193.1THz

Transmitter power 0dBm to 10dBm

Local Oscillator power at receiver

Same as transmitter power

Sequence length 64

Samples per bit 256

Line width of Laser 0.1MHz

Dark current 10nAmp

Responsivity of PIN 1A/W

Transmitter aperture diameter 150mm

Receiver aperture diameter 150mm

Transmitter optics efficiency 0.8

Receiver optics efficiency 0.8

Transmitter pointing error 1.1μrad

Receiver pointing error 1.1μrad

Additional losses (Pointing loss, Synchronization loss etc)

1dB

V. CONCLUSION In our work, we have analyzed in detail about a high capacity and high speed 32-channel 40Gbps/channel DWDM system for free-space optical communication applications. The performance of the system is compared among various modulation formats. Coherent optical QPSK modulation technique is proved to be the best among all. For an input power level of 10dBm, the coverage distance of the LOS system is found to be 1360km for coherent optical QPSK modulation technique. The system may further be

analyzed for more number of input channels, diffused link (DL) setup and also for higher data rates.

Fig. 3 Eye diagram and Q-factor of channel No. 32 for RZ data format at a distance of 614km

Fig. 4 Eye diagram and Q-factor of channel No. 32 for NRZ data format at a distance of 798km

173

Fig. 5 Optical spectrum of the optical QPSK modulator

Fig. 6 Multiplexed optical spectrum at the transmitter

Fig.7 Multiplexed optical spectrum at the receiver

Fig. 8 Eye diagram and Q-factor of channel No. 32 for optical QPSK modulation at a distance of 1360km

174

0 1 2 3 4 5 6 7 8 9 100

200

400

600

800

1000

1200

1400

Input Power (dBm) -------->

Cov

erag

e di

stan

ce (

km)

----

----

->

Coherent optical QPSKNRZRZ

Fig. 9 Electrical signal transmitted at the transmitter

Fig. 10 Electrical signal received at a distance of 1360km

Fig. 11 Coverage distance of the setup for varying input power and modulation format

V. References

[1] E. Ciaramella, Y. Arimoto, G. Contestabile, M. Presi, A. D. Errico, V. Guarino, and M. Matsumoto, "1.28 Terabit/s (32x40 Gbit/s) WDM Transmission System for Free Space Optical Communications," IEEE Journal on selected Areas in Communications, Vol. 27, No. 9, pp. 1639-1645, December 2009.

[2] A. Sheetal, A. K. Sharma, R. S. Kaler, "Simulation of high capacity 40Gb/s long haul DWDM system using different modulation formats and dispersion compensation schemes in the presence of Kerr's effect," Optik 121, pp. 739-749, 2010.

[3] D. Kedar and S. Arnon, “Urban optical wireless communication networks: the main challenges and possible solutions,” IEEE Commun. Mag., vol. 42, no. 5, pp. S2–S7, May 2004.

[4] S. M. Aghajanzadeh and M. Uysal, "Diversity–Multiplexing Trade-Off in Coherent Free-Space Optical Systems With Multiple Receivers," J. OPT. COMMUN. NETW, Vol. 2, No. 12, pp. 1087-1094, December 2010.

[5] V. W. S. Chan, “Free-space optical communication,” J. Lightwave Technol., vol. 24, no. 12, pp. 4750–4762, Dec. 2006.

[6] B. Patnaik and P. K. Sahu, "Design and study of high bit-rate free-space optical communication system employing QPSK modulation," Int. J. Signal and Imaging Systems Engineering (in press).

[7] S. M. Haas and J. H. Shapiro, "Capacity of wireless optical communications," IEEE Journal on Selected Areas in Communications, Vol. 21, October, pp.1346–1357, 2003.

[8] A. Belmonte and J. M. Kahn, “Performance of synchronous optical receivers using atmospheric compensation techniques,” Opt. Express, vol. 16, no. 18, pp. 14151–14162, Sept. 2008.

[9] A. Belmonte and J. M. Kahn, “Capacity of coherent free-space optical links using diversity-combining techniques,” Opt. Express, vol. 17, no. 15, pp. 12601–12611, July 2009.

[10] S. Benedetto, E. Biglieri, V. Castellani, "Digital Transmission Theory," Prentice-Hall, 1987. [11] S. Arnon, “Performance of a satellite network with an optical

preamplifier," J. Opt. Soc. Am. A, 2005, 22(4), pp. 708-715. [12] I. Kim, G. Goldfarb, and G. Li, "'Electronic wavefront correction for

PSK free-space optical communications," Electronics Letters, Vol. 43 No. 20, 27th September 2007.

[13] H. Taub, D. Schilling and G. Saha, "Principles of Communication Systems," Third Edition, McGraw-Hill, chapter 5, pp. 274-278.

175

Documents

Novel QPSK Modulation for DWDM Free Space Optical Communication System