PERFORMANCE EVALUATION &
OPTIMIZATION OF ERROR FREE
WDM RADIO OVER FIBER LINK
KUMARI KALPNA
Department of Instrumentation & Control Engineering, College of Engineering & Management Kapurthala, Punjab-144601, India
BINDIYA JAIN
Department of Electronics & Communication Engineering, DAV institute of Engineering & Technology Jalandhar, Punjab, India
Abstract:
In this paper, we introduce WDM radio over fiber (RoF), which is one of enabling technologies for 3G and beyond. To minimize the problem of high attenuation, power loss and to improve the efficiency of frequency resuse, the Performance Evaluation of WDM radio over fiber transmission system (RoF), based on various performance measures such as Q-factor, eye opening, BER and jitter has been made at different data rates.
Keywords: WDM RoF System; BER; Q factor; Eye Opening; Timing jitter.
1. Introduction
Nowadays communications target to transmit a variety of services. Those are classical telephony, facsimile transmission, but also the Internet traffic, data transmission, radio and television broadcasting etc. Consequently, various transmission media are used as metal and fiber cables, and microwave, millimeter wave, and optical free space communication links. However, owing to top performance of contemporary optical fibers there is a tendency to exploit optics as far as possible. Thus fibers are used not only for digital voice or Internet traffic transmission, but also for expanding Radio-over-Fiber transmission applications that exploit the optical carrier wave amplitude modulation with a microwave carrier, including analogue cable television transmission. The next generation of access networks is rushing the needs for the convergence of wired and wireless services to offer end users greater choice, convenience, and variety in an efficient way. This scenario will require the simultaneous delivery of voice, data, and video services with mobility feature to serve the fixed and mobile users in a unified networking platform. In other words, new telecom systems require high-transmission bandwidths and long haul with reliable mobility [1]. Radio over Fiber (RoF) application has attracted much attention recently because of the increasing demand for capacity/coverage and the benefits it offers in terms of low-cost base station deployment in macrocellular system. RoF systems are now being used extensively for enhanced cellular coverage inside buildings such as office blocks, shopping malls and airport terminal. RoF is fundamentally an analog transmission system because it distributes the radio waveform, directly at the radio carrier frequency, from a central unit to a Radio Access Point (RAP) [2].
Central Station (CS) and Base Station (BS) connected by an optical fiber link or network. Modulated radio signals have to be available at the input end of the RoF system, which subsequently transported them over a distance in the form of optical signals.
The RoF basic concept is to distribute the radio-frequency (RF) signals by optical transmission to radio access points (RAPs) so that the RAPs are not required to perform complicated functionalities such as modulation, coding, up/down conversion and multiplexing. RoF systems can provide specialized coverage of wireless services by using an extended optical backbone. These systems are suitable for variety applications, such as in-building coverage, outdoor cellular systems, and broadband fixed and mobile wireless access. They are entirely transparent to the system frequency, protocol, and bit rate. This characteristic makes them extremely interesting for the convergence of optical and mobile systems. RoF technology has been investigated by many Research Groups in the last years. However, the great majority of works published in literature are based on simulations and/or experiments carried out in laboratories.
RoF technology is a technology by which microwave (electrical) signals are distributed by means of optical components and techniques [7]. RoF systems depicted in Fig. 1 are used to transport microwave signals. Radio over Fiber (RoF) application has attracted much attention recently because of the increasing demand for capacity/coverage and the benefits it offers in terms of low-cost base station deployment in macrocellular system. RoF systems are now being used extensively for enhanced cellular coverage inside buildings such as office blocks, shopping malls and airport terminal. RoF is fundamentally an analog transmission system because it distributes the radio waveform, directly at the radio carrier frequency, from a central unit to a Radio Access Point (RAP). Note that although this transmission system is analog, the radio system itself may be digital such as GSM. Mainstream optical fiber technology is digital. Telecommunication networks use synchronous digital hierarchy transmission technology in their cores. Fiber-based data networks such as fiber distributed data interface and gigabit Ethernet all use digital transmission. Fiber transmission links to base stations in mobile communications systems are digital. Digital optical fiber transmission links are therefore ubiquitous in telecommunications and data communications, constituting a high volume market worth billions of dollars worldwide.
Fig 1: WDM ROF Transmission system
The electrical signal may be baseband data, modulated IF, or the actual modulated RF signal to be distributed is used to modulate the optical source. The resulting optical signal is then carried over the optical fiber link to the remote station where the data is converted back into electrical form by the photo detector.
2. Benefits of ROF systems:
2.1 Low Attenuation Loss
Electrical distribution of high frequency microwave signals either in free space or through transmission lines is problematic and costly. In free space, losses due to absorption and reflection increase with frequency. Use of optical fibers, offer much lower losses. These losses are much lower than those encountered in free space propagation and copper wire transmission of high frequency microwaves. Therefore, by transmitting microwaves in the optical form, transmission distances are increased several folds and the required transmission powers reduced greatly [5].
2.2 Large Bandwidth
Optical fibers offer enormous bandwidth. There are three main transmission windows, which offer low attenuation, namely the nm 850, nm 1310 and nm 1550 wavelengths. The high optical bandwidth enables high speed signal processing that may be more difficult or impossible to do in electronic systems [5].
2.3 Immunity to Radio Frequency Interference
Immunity to electromagnetic interference is a very attractive property of optical fiber communications, especially for microwave transmission. This is so because signals are transmitted in the form of light through the fiber [5].
2.4 Easy Installation and Maintenance
In RoF systems, complex and expensive equipment is kept at the CSs, thereby making remote base stations simpler. For instance, most RoF techniques eliminate the need for a local oscillator and related equipment at the Remote Station (RS) [5].
2.5 Reduced Power Consumption
Reduced power consumption is a consequence of having simple RSs with reduced equipment. Most of the complex equipment is kept at the central SC [5].
3. Performance Measures:
The right choice of the performance evaluation criteria for the characterization of optical transmission links represents one of the key issues for an effective design of future long-haul optical systems [8]. The evaluation criteria should provide a precise determination and separation of dominant system limitations, making them crucial for the suppression of propagation disturbances and a performance improvement. The most widely used performance measures for performance evaluation are the Q-factor, BER and jitter, eye opening [10].
3.1 Q-factor
Q-factor represents the signal-to-noise ratio at the receiver decision circuit in voltage or current unit.
3.2 BER
The BER can be estimated from following Equation. The BER gives the upper limit for the signal because some degradation occurs at the receiver end [8].
BER =
Π
−
≈
2
2
2
exp
)
2
(
2
1
Q
Q
Q
erfc
3.3 Eye opening
Considering only samples at the optimum sampling instant, it is the difference between the minimum value of the samples decided as logical ‘‘1’’ and the maximum value of the samples decided as logical ‘‘0’’.
3.4 Jitter
Jitter value is evaluated as the standard deviation of the position of the maximum of the received signal referred to the bit frame.
4. System Description and Results:
Fig 2: WDM RoF Transmission link considered for simulation. Data source 101010 DPSK Modulator Electro Absorption Modulator CW Lorentzian laser DPSK Demodulator Data Channel1 [IF1] Optical Splitter Data source 101010 DPSK
Modulator Absorption Electro Modulator CW Lorentzian laser Optical amplifier Optical amplifier
Fiber link 2 at 10 Km
Fiber link 1 at 1Km Optical Combiner SOA MZI Optical Filter 2 Optical Filter 1 Photo diode Electrical Filter Electrical Signal Multiplier DPSK Demodulator BER estimator Eye diagram analyzer Q Factor Jitter Data Channel2 [IF2] BER estimator Eye diagram analyzer Q Factor Photo
diode Electrical signal multiplier Central Station
(CS)
Remote Node (RN)
Remote Antenna Station (RAS)
5. Results:
Table 5.1 & 5. 2: Comparative study of the performance metric indices of WDM RoF System at various data rates.
# At Section A:
S.No. Parameters Q value [dB] Jitter [ns] BER Eye opening [a.u.]
Data rate[Gbps]
1. 1.5 9.686512 0.0312685 0.00114414 3.39938e-007
2. 2.0 10.206633 0.0318283 0.000618246 3.17119e-007
3. 3.5 10.060838 0.0313109 0.000764695 3.60716e-007
4. 4.0 10.371890 0.0316091 0.0004985 4.76325e-007
5. 5.5 10.620311 0.0316927 0.000365147 8.48817e-007
6. 6.5 9.632132 0.0316949 0.00122341 1.97337e-008
7. 7.0 9.722221 0.0313288 0.00112138 6.57325e-008
8. 8.5 9.836501 0.0316139 0.000957761 3.62001e-007
9. 9.0 10.560282 0.0317983 0.000370819 4.28472e-007
10. 10.0 11.000574 0.0319599 0.000199306 4.63927e-007
Table 5.1: Simulation results at different data rates on WDM RoF Remote Antenna Station A.
# At Section B:
Table 5.2: Simulation results at different data rates on WDM RoF Remote Antenna Station B.
A pseudo random sequence length of bits taken one bit per symbol is used to obtain realistic output values at the receiver. Firstly, to observe the impact of data rate upon system performance, simulation results are obtained for different data rates varying from 1.5Gbps to 10Gbps. It was observed that for the data rate up to 10Gbps, Q factor for the system remains nearer to 10dB and also BER at data rate 6.5 Gbps is at minimum value & jitter remains almost nearer to the value of 0.0307 that shows a good performance of WDM RoF system. As we increases the data rates further, impact upon the Q factor, jitter, eye opening etc. comes into play. It is investigated that system provides optimum results at data rate of 9.5 Gbps (refer Table 5.1 & 5.2). The eye diagrams obtained for the system at various data rates are shown in figures (refer Fig. 3 to 12).
S.No. Parameters Q value [dB] Jitter [ns] BER Eye opening [a.u.] Data
rate[Gbps]
1. 1.5 9.742236 0.0314526 0.00107178 3.27156e-007
2. 2.0 9.714900 0.0317134 0.00115153 3.21517e-007
3. 3.5 9.605962 0.0308473 0.00125388 1.52769e-006
4. 4.0 9.312404 0.0310956 0.00173913 2.29042e-007
5. 5.5 8.474493 0.0309313 0.00398179 2.0033e-007
6. 6.5 10.209459 0.0317742 0.000661857 6.52316e-007
7. 7.0 9.020807 0.0311503 0.00241097 3.52251e-007
8. 8.5 9.053125 0.0307204 0.00229256 6.3256e-007
9. 9.0 9.018063 0.0308247 0.00263116 3.19057e-007
3(a) 3(b)
Fig. 3(a) Eye Diagram at 1.5 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm. & 3(b) Eye Diagram at 1.5 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm.
4(a) 4(b)
Fig. 4(a) Eye Diagram at 2.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm & 4(b) Eye Diagram at 2.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm
5(a) 5(b)
6(a) 6(b) Fig. 6 (a) Eye Diagram at 4.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &
6 (b) Eye Diagram at 4.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm
7(a) 7(b)
Fig. 7 (a) Eye Diagram at 5.5 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm & 7 (b) Eye Diagram at 5.5 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm
8(a) 8(b)
9(a) 9(b)
Fig. 9 (a) Eye Diagram at 7.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm & 9 (b) Eye Diagram at 7.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm
10(a) 10(b)
Fig. 10 (a) Eye Diagram at 8.5 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm & 10 (b) Eye Diagram at 8.5 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm
11(a) 11(b)
Fig. 11 (a) Eye Diagram at 9.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &
12(a) 12(b)
Fig. 12 (a) Eye Diagram at 10.0 Gbps for Section A for standard SM fiber at 10km and wavelength of 1550nm &
12 (b) Eye Diagram at 10.0 Gbps for Section B for standard SM fiber at 10km and wavelength of 1550nm 6. Conclusion:
In this paper, we have proposed a novel WDM RoF system and experimentally demonstrated the simultaneous generation and transmission of the 1.5 to 10 Gbps, 0 to 50-GHz signals over 10-km Standard SM fiber and wavelength of 1550 nm with less power penalties. It is investigated that system provides optimum results at data rate of 9.5 Gbps The system is of low cost & can be scaled to higher data rates.
7. References:
[1] David J. T. Heatley, “Optical Wireless:The Story So Far”, IEEE 1998, pp 72-82.
[2] Michel Goloubkoff, “I Outdoor and Indoor Applications for Broadband Local Loop with Fibre supported mm-wave Radio Systems”, IEEE, 1997,pp 31-34.
[3] Mohammad Shaifur Rahman, Jung Hyun Lee, Youngil Park, and Ki-Doo Kim, “Radio over Fiber as a Cost Effective Technology for Transmission of WiMAX Signals” World Academy of Science, Engineering and Technology 56 (2009).
[4] Hoon Kim, “Radio-over-Fiber Technology for Wireless Communication Services”, Samsung Electronics Oct. 13, 2005. [5] Anthony Ngoma, “Radio-over-Fiber Technology for Broadband Wireless Communication Systems”, 2005
[6] W. D. Jemisona, “Fiber radio: from links to networks”, IEEE, 2001, pp 169-172 [7] Kwansoo Lee, “Radio over Fiber for Beyond 3G”, IEEE, July 2005.
[8] Vishal Sharma, Amarpal Singh, Ajay K.Sharma, “Simulative investigation of the impact of EDFA and SOA over BER of a single-tone RoF system” Available online at w.w.w.sciencedirect.com in Elsevier Science Direct, International Journal for Light and Electron Optics, Optik, Germany, 10th Jan., 2009.
[9] Hyoung-Jun Kim, and Jong-In Song, “Full-Duplex WDM-Based RoF System Using All-Optical SSB Frequency Upconversion and Wavelength Re-Use Techniques” IEEE Transactions on Microwave Theory and Techniques, July 2010.
