Simulative analysis of BER and Q-factor for sub-carrier multiplexing (SCM) based Bi-directional Radio over fiber communication system.

Singh, Satbir ; Singh, Amarpal

Introduction

Radio over fiber (RoF) is a hybrid technology design to enable the efficient and cost effective transport of wireless signals over optical fibers [1]. In radio over fiber (RoF) scheme optical signal is modulated at radio frequencies and transmitted via the optical fiber [2]. RoF makes it possible to centralize the RF signal processing functions in one shared location (head end), and then to use optical fiber, which offers low signal loss (0.2 dB/km for 1550 nm, and 0.5 dB/km for 1310 nm wavelengths) to distribute the RF signals to the remote antenna units (RAUs) [3]. At the RAU, the transmitted RF signal is recovered by direct detection in the PIN photo detector (PD). The signal is then amplified and radiated by an antenna. This method of transporting RF signals over the fiber is called Intensity Modulation with Direct Detection (IM-DD), and is the simplest form of the RoF link [4]. There are two ways of modulating the light source: First, the RF signal directly modulates the laser diode's current. The direct modulation scheme is simple but suffers from a laser--frequency chirp effect, this chirp effect results in severe degradation of the RoF system performance. Secondly, to operate the laser in continuous wave (CW) mode and then use an external modulator such as the Mach-Zehnder Modulator (MZM), to modulate the intensity of the light. After transmission through the fiber and direct detection on a photodiode, the photocurrent is a replica of the modulating RF signal applied either directly to the laser or to the external modulator at the head end. The photocurrent undergoes transimpedance amplification to yield a voltage that is in turn used to excite the RF antenna. IM-DD radio over fiber (RoF) systems use single mode fiber for distribution of RF signals [5]. The biggest advantage of IM-DD is that amplitude modulation (AM) and multi-level modulation formats such as x QAM may be transported. Sub-Carrier Multiplexing (SCM) can also be used in Radio over fiber systems [6]. The performance of radio over fiber (RoF) system depends on following parameters: method used to generate the optically modulated RF signal, optical fiber chromatic dispersion, laser and RF power level ,nonlinearity due to an optical power level, bit rate and modulation format used[7].

Literature Survey

Various research papers have been investigated for the analysis of the performance of radio over fiber system in terms of bit error rate and quality factor improvements. M.G.Larrode and A.M.J.Koonen demonstrated and studied the optical frequency multiplication (OFM) technique for the design of reliable RoF system for increased cell capacity allocation and multi-standard support [8]. Kosuke Uegaki et al. investigated convergence of communication and broadcasting which makes use of Radio over Fiber (RoF) network [9]. He focused that in RoF networks, although system costs can be reduced by employing sub-carrier multiplexing (SCM), the influence of nonlinear distortion becomes large. G.M.Sil et al. presented a novel BER estimation method for sub carrier multiplexed signals usig QAM modulation that accounts for joint impact of noise and intermodulation distortions on the bit error rate (BER) of the RoF system [10]. David Wake discussed the RoF distribution system for cellular communication [11]. M. Garcia Larrode et al. demonstrated the feasibility of generating two QAM radio signals simultaneously at 17.3 GHz and 17.8 GHz after 4.4 km of multimode fiber in an optically frequency multiplication radio over fiber link for wireless multi standard support at the antenna site [12]. Jianxin Ma et al. investigated the influence of the modulation index of MZM modulator on radio over fiber link with ASK millimeter-wave signal [13]. Hai-han Lu et al. proposed and demonstrated four wavelengths CATV/RoF transport system based on DFB laser source. The system performance evaluated in terms of BER, Carrier to Noise ratio (CNR) and composite triple beat (CTB) [14]. In this paper we will investigate BER and Q-factor improvements under the influence of sub-carrier multiplexing (SCM) on Radio over Fiber (RoF) system in uplink as well as in downlink connections..

System Set Up for Radio Over Fiber System

Radio over Fiber (RoF) technology is proposed as a solution for reducing cost and providing highly reliable communication services. The RoF system is very cost-effective because the localization of signal processing in central station and also use a simple base station. Radio over fiber system realizes the transparent transform between RF signal and optical signal.

[FIGURE 1 OMITTED]

Fig (1) shows the scheme for the system set up. The system setup shows implementation approach for transmitting sub carrier multiplexing (SCM) encoded multiple data channels (analog and digital channels) over a bidirectional single mode optical fiber.This radio over fiber link is set up by the simulation software Optisystem[TM] For the downlink simulation link, a narrow bandwidth continuous wave (CW) from laser diode (power = 6dBm and linewidth of 10 MHz and dynamic noise of 3 dB) at the wavelength of 1550 nm is modulated via a LiNb[O.sub.3] Mach-Zehnder modulator (MZM) having extinction ratio of 30 dB and insertion loss of 5 dB. The 10 GHz RF sinusoidal wave is amplitude modulated by pseudo-random bit sequence (PRBS) data format (NRZ) with sequence length of [2.sup.9]-1. The 10 Gb/s downlink data signal with PRBS length of [2.sup.9]-1is mixed with 10GHz local oscillator signal (sine wave) and a carrier generator having number of RF sub-carriers. The numbers of analog channels used by carrier generator are 78 at a frequency of 49.25 MHz with a channel frequency spacing of 6 MHz. An ideal EDFA (power = 10 dBm) pre-amplifies the optical carrier before o/e conversion at the receiver section (at PIN-PD having thermal noise of 1e-22W/Hz)) with a center frequency of 193.4 THz is used for the analysis. A bidirectional reflective filter (Gaussian type and of 4th order) with the center frequency of 193.1 THz and having reflection of 99% is used for the simulative analysis. The optical signal sent over different lengths of single mode fiber. In the receiver section, the optical signal is detected by a PIN-photodiode having responsivity of 0.9 A/W and amplitude demodulated. The downlink microwave signal was boosted by an electrical amplifier (EA) with a gain of 15 dB and noise power of--60 dBm.

For the uplink connection, the optical spectrum and waveform of the remaining optical carrier used with the help of a bidirectional reflective filter with an insertion loss 0 dB. This optical carrier was given to amplitude modulator driven by 10Gbps uplink PRBS data with a sequence length of [2.sup.9]-1. The uplink optical sidebands produce crosstalk when uplink data was detected at the control station. The crosstalk can be reduced with the help of Bessel optical filter having bandwidth of 10GHz with depth of 100dB. The eye diagrams, BER and Q-factor values of the signals are measured by the BER analyzers I and II at the base station and control station for downlink and uplink connections. Table (1) shows the values of various simulation parameters used for the RoF system approach.

Results and Discussion

The multiple data signals are analyzed with the help of optical and RF spectrum analyzers for radio over fiber (RoF) approach. The signals are analyzed at central station and at receiving end. Fig (2) and (3) show power vs. frequency relationship for optical and RF spectrum analysis. The Fig (3) depicts the RF spectrum analysis for signal and then (signal+noise) at central station and at receiving end of the radio over fiber system. The figure shows the reduction of noise level with increase in frequency. The figures (4) and (5) focus on bit error rate analysis for improving quality of the radio over fiber system. For uplink and downlink analysis of radio over fiber system, three different optical fiber lengths have been chosen i.e (1) 2 km (2) 8 km and (3) 12 km.

[FIGURE 2 OMITTED]

[FIGURE 3a OMITTED]

[FIGURE 3b OMITTED]

Fig (2) & (3): Optical spectrum and RF spectrum shows the maximum power and minimum power in dBm with a resolution bandwidth of 0.01nm at central stage. The measured values of maximum and minimum powers are 1.0147dBm and -104.81dBm at a center frequency of 193.1THz.RF spectrum analysis shows power vs. frequency relation for radio over fiber system. The maximum level of power for fig.3 (a) is--17.55dBm and minimum level is -103.92dBm at a center frequency of 7.99GHz. (b) signal with noise at base station.

[FIGURE 4 OMITTED]

Figure (4): Maximum Eye amplitudes at receiver section at 193.1 THz at a bit rate of 10 Gbps with NRZ modulation format and at a frequency of 10 GHz for sine generator with dispersion of 16.75ps/nm/km at optical fiber lengths of (a) 2 Km (b) 8 Km and (c) 12 Km for uplinking Radio over Fiber (RoF) communication system

[FIGURE 5 OMITTED]

Figure (5): Eye amplitudes at receiver section at 193.1 THz at a bit rate of 10 Gbps with NRZ modulation format and at a frequency of 10 GHz for sine generator with dispersion of 16.75ps/nm/km at optical fiber lengths of (a) 2 Km (b) 8 Km and (c) 12 Km for down linking Radio over Fiber (RoF) communication system.

The BER analysis shows that for the given radio over fiber system, the performance level of Rf signals degraded up to some extent when the optical fiber distance increased for fixed no. of channels and for a particular power level (in dBm) of laser diode and EDFA. This bit error rate analysis is for short/medium transmission distance (<12Km).The dynamic noise for CW laser is 3dB. These results are reasonable for the direct detection scheme based on the external modulation in a medium transmission distance for a macrocell. From the above analysis, it is concluded that there is a considerable degradation in the output signal as we increase the optical fiber length which is because of the spreading of the optical pulse, which is directly proportional to the length of fiber. The bit error rate and quality factor (Q) of the system can be increased by decreasing the power and line width of optical source (CW laser) and also by increasing the power of EDFA. As from the results we can conclude that the resultant RF power decreases as the length of the fiber increases. The BER can be improved for the above model by carefully choosing the simulation parameters like fiber core area, laser power and linewidth , EDFA power ,number of channels in the carrier generator etc. For making more clarity about the performance of RoF system, we have plotted graphs showing how the different parameters like Q-factor, power level vary according to different fiber lengths at a frequency of 193.1 THz at a bit rate of 10 Gbps with 78 channels at a constant dispersion level of 16.75ps/nm/km in case of uplink and downlink RoF system.

[FIGURE 6 OMITTED]

Figure (6): Comparison charts at receiver section at 193.1 THz at a bit rate of 10 Gbps with NRZ modulation format and at a frequency of 10 GHz for sine generator with dispersion of 16.75ps/nm/km obtained by plotting with respect to different fiber lengths (a) Q-factor and (b) Received Power level in case of uplink and downlink radio over Fiber system.

Figure 6(a) compares the variations in the Q-factor with the optical distance for bit error rate analyzers (I) & (II).It is investigated from the figure that the Q values shows a considerable fall with the increase in the length of optical fiber. For BER analyzer (II), there is a quick fall in Q-factor after 8 km of fiber length. From 10-12 Km of fiber length, Q-factor values from both the BER analyzers superimposed (Q=6.5dB at fiber length of 10 km). Figure 6(b) shows the comparison of decreasing power levels with respect to increasing optical distance. This analysis reveals that with increasing the optical fiber distance from the central station to the base station, the performance of the resultant RF signal degraded. The BER and quality (Q) factor of the RoF system decreases. This gives to large amount of jittering.

[FIGURE 7 OMITTED]

Figure (7): Plots for receiver section at 193.1 THz at a bit rate of 10 Gbps with different dispersions values obtained by plotting over single mode optical fiber of length 10 Km. (a) Total Received Power for downlink connection and (b) Q factor for uplink connection for bidirectional RoF system.

The maximum value of the Q-factor is 9.61dB in case of downlink and its minimum value is 8.27dB in case of uplink connection .The received power level varies from -66.72dBm to -34.72dBm respectively for uplink and downlink connection for RoF system. It is investigated from the figure 7(b), that the Q values show a maximum value (max. value of Q=8.91 dB with min.BER <10-17) when dispersion level approaches to 16.75ps/nm/km. The figure 7(a) shows the corresponding decreasing power (dBm) level for the given dispersion values. The overall analysis of these parameters shows a significant change in the performance of radio over fiber (RoF) system for macro cell area ([approximately equal to] 8-12 Km). Hence these systems are beneficial for providing mobile communication as well as cable TV/radio transmission services in a cell area having radius of 2-15 km with single mode optical fiber cables.

Conclusion

The bit error rate (BER) and quality factor improvements for the radio over fiber system at (i) different optical fiber lengths and (ii) different dispersion levels have been investigated for a sub carrier multiplexing (SCM) based bidirectional radio over fiber system. With increasing the optical fiber distance from the central station to the base station, the performance of the resultant RF signal degraded due to large amount of jittering. We can achieve the maximum value of Q-factor and corresponding BER at optimal optical distance with optimal dispersion value (16.75ps/nm/km) in case of bidirectional single mode fiber. The maximum value of the Q-factor is 9.61dB in case of downlink connection and minimum value of 8.27dB in case of uplink connection has been reported. It is also noticed that a maximum value of Q=8.91 dB with min. BER < [10.sup.-12] has been achieved when dispersion level approaches to 16.75ps/nm/km in case of uplink connection. Hence the 2 -10 km radius micro/macro cells give excellent performance in the improvements of BER as well as Q-factor for providing RoF communication services.

References

[1] M. G. Larrode and A. M. J. Koonen, "Towards a reliable RoF infrastructure for broadband wireless access" Proceedings Symposium IEEE/LEOS Benelux chapter, 2006, Eindhoven , pp. 183-184.

[2] S. Z. Pinter and X. N. Fernando, "Fiber-Wireless solution for broadbamd multimedia access", IEEE Canadian Review-summer 2005.

[3] M. Garcia Larrode et al. "RF bandwidth capacity and SCM in a RoF link employing optical frequency multiplication", IEEE conf., 2006.

[4] G. M. Sil, Hadrien Louchet & Andre Richter, "Efficient BER estimation for Radio over Fiber systems" OSA 1-55752-830-6.

[5] H. Al Raweshidy, "Radio over Fiber Technologies for Mobile Communication Networks", Artech house, Inc, USA, 2002.

[6] Kosuke Uegaki et al, "A novel nonlinear distortion suppression method in RoF systems using optical filter" IEEE conf., 2005.

[7] David Wake, "Trends and prospects for radio over Fiber picocells", Microwave Photonics, International topical meeting on, 5-8 Nov, 2002, W3-1, pp. 21-24.

[8] B. Charbonnier et al., "Upcoming perspectives and future challenges for RoF", IEEE conf., 2007, pp.21-23.

[9] S. Sari and J. E. Mitchell, "Performance analysis of multi-wavelength RoF system applying double sideband suppressed carrier DSB-SC modulation", Electronic letters, 1995.

[10] Hai-Han Lu et al., "CATV/RoF transport systems based on one DFB LD with main and side modes injection-locked", Optical Fiber Technology 14, pp. 232-236, Elsevier, 2008.

[11] M.Fabbri and Pier Faccin, "Radio over Fiber Technologies and systems:New Opportunities", IEEE,ICTON,2007.

[12] T. S. Cho, Changho Yun, Jong-In Song, "Analysis of CNR penalty of radio over fiber systems including the effects of phase noise from laser and RF oscillator", IEEE J. Lightwave Technology, vol 23, No.12, pp.4093-4099, December, 2005.

[13] Jianxin Ma et al. "Influence of the modulation index of Mach-Zehnder modulator on RoF link with ASK millimeter wave signal", Optics & laser Technology, volume 41, pp: 11-16, Elsevier, 2008.

[14] O. K. Tonguz and H. Jung, "Personal communications access networks using subcarrier multiplexed optical links," IEEE J. Lightwave Technology, vol 14, pp. 1400-1409, June 1996.

Satbir Singh (1) and Amarpal Singh (2)

(1) Deptt. of Electronics & Communication Engineering, G.N.D.U, Regional Campus, Gurdaspur, Punjab, India. E-mail: [email protected]

(2) Deptt. of Electronics & Communication Engineering, B.C.E.T, Gurdaspur, Punjab, India. E-mail: [email protected]

Table 1: Simulation Parameters for the experiment setup for RoF system

Effective fiber core area           78 [micro][m.sup.2]

Reference wavelength                1550 nm

Attenuation                         0.22 dB/Km

Dispersion                          16.75ps/nm/km
GVD parameters: [[beta].sub.2]      -20 [ps.sup.2]/km and
and [[beta].sub.3]                  0.08[ps.sup.2]/km

PMD Coefficient                     0.5 ps/[(Km).sup.2]

Optical Fiber Length                varying

Frequency Shift                     11 GHz

EDFA Power                          10 dBm

No. of Channels (sub-carriers)      78
Channels frequency                  49.25 MHz

Frequency Spacing                   6 MHz

Laser Power                         6 dBm

Line width                          10 MHz

PIN Responsivity                    0.9 A/W