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One of the benefit of the Radio-over-Fibre technology is able to present in long distance communication that are requires the use of optical fibre links to distribute RF signals from a Central Station (CS) to Base Stations (BS). In optical systems involving RoF technology, Sub-Carrier Multiplexing (SCM) is used to increase optical fibre bandwidth utilization. In SCM, several microwave subcarriers, which are modulated with digital or analogue data, are combined and used to modulate the optical signal, which is then carried on a single fiber. On the other hand, Wavelength Division Multiplexing (WDM) is a multiplexer at the transmitter to join the signals together, and a demultiplexer at the receiver to split them apart. In order to perform RoF system cost-effective this paper is proposed the performance analysis of 150km Single Mode Fiber (SMF) that are deploying in the SCM/WDM RoF system. The result is present higher bandwidth for long distance communication system (SMF, 150 km) by using SCM/WDM for Radio over Fiber. Therefore, the performance of 150km utilization is expected to be much better than conventional optical SCM.
Keywords: Optical Communication, Cellular Communication, SCM, WDM, SMF, RoF, Wireless Communication
The explosive growth of the cellular communication and wireless communication and the success of 3.5G systems with WIMAX have had a reflective impact on our perception of communication. The need of being communication (always on, communication anytime, anywhere) has impact on the bandwidth demand. The recent of services in cellular communication that provided by cellular operator that striving to provide the large bandwidth that reliable to occupied the demand of user. In other hand, the operator had to reduce the cost that are rises from the applied or extend the systems.
In order to overcome of bandwidth limitation in wireless and cellular communication Radio over Fiber has offering technology that is cost-effective. RoF technology requires the use of optical fibre links to distribute RF signals from a Central Station (CS) to Distributed Antenna Units (DAUs) or Base Station (BS). In optical and wireless communication, RF signal processing functions such as frequency up-conversion, carrier modulation, and multiplexing, are performed at the BS or the Radio Access Point (RAP), and immediately fed into the antenna. RoF makes it possible to centralize the RF signal processing functions in one shared location (CS), and then to use optical fibre, which offers low signal loss (0.3 dB/km for 1550 nm, and 0.5 dB/km for 1310 nm wavelengths) to distribute the RF signals to the DAUs. The centralization of RF signal processing functions enables equipment sharing, dynamic allocation of resources, and simplified system operation and maintenance.
Figure 1: Perspective of SCM/WDM RoF Technology
150km SCM/WDM Radio over Fiber Architecture
In this work, we propose the integration the deploying of 150km SMF into the SCM ââ‚¬" RoF techniques with WDM to analyze the performance of characterization. The goals are to improve the optimization of the SCM and WDM for bandwidth utilizing in the base stations (BS) and control station (CS). The system modeled by using commercial optical system simulation software, OptiSystem, with considering practical system parameters. The model was setup in two parts, is the transmitter and the receiver.
In analogue optical systems including RoF technology, Sub-Carrier Multiplexing (SCM) is used to increase optical fibre bandwidth utilization. In SCM, several microwave subcarriers, which modulated with digital or analogue data, are combined and used to modulate the optical signal, which is then carried on a single fibre , . This makes RoF systems cost-effective.
Basically the operation of the sub carrier multiplexing (SCM) was similar to Time Division Multiplexing (TDM), such that TDM is commonly used in digital transmission system. On other hand, SCM play an important role in analogue transmission system, however multiplexing more conveniently carried out in frequency domain.
The main idea of the SCM is combining two-step of modulation that is operating at different domain. First modulation occupied at RF part such that several low bandwidth RF channel carrying analogue or digital signal add up together by using multiplexer. Thus the signal will be very close to each other in the frequency domain depending to local oscillator frequency that applied in the modulation part. This combined signal actually modulated onto higher frequency microwave carrier. The up-converted signals are in different frequency bands therefore combined by a microwave power combiner forming a microwave subcarrier multiplexed composite signal. Second modulation occupied at optical domain, the modulated signal then convert to optical domain by using laser diode and optical modulator.
The use of Wavelength Division Multiplexing (WDM) for the distribution of RoF signals has gained importance recently. WDM enables the efficient exploitation of the fibre networkââ‚¬â„¢s bandwidth. However, the transmission of RoF signals is seen as inefficient in terms of spectrum utilization, since the modulation bandwidth is always a small fraction of the carrier signal frequency. Therefore, methods to improve the spectrum efficiency have been proposed 
Presently, RoF technology is applied mainly in SMF-based systems. For MMF-RoF systems, only the IM-DD approach for WLAN applications has been reported. The reported signals transmitted over such links are generally below 6 GHz. Therefore, considering the increasing importance of in-building coverage for wireless systems, the need for high-frequency signal distribution over multimode fibre, which constitutes the main in-building fibre infrastructure, becomes equally important.
In order to overcome losses and attenuation, SMF was introducing as a backbone for cellular communication. In this link the types of the SMF determine how signal travels over the link with the level of quality. Choosing the incorrect and unsuitable fiber into the system can be so much attenuation and dispersion existence. Therefore, a single mode fiber was the perfect match for this system according to the characteristic of the fiber. A single mode fiber conducts only one mode and capable to eliminate higher order modes. Attenuation in a single mode fiber is smaller than in a multimode fiber because in the single mode fiber less light will encounter absorption and scattering effects. However, attenuation (macro bending effect) in single mode fiber increases as operating wavelength increases and bend radius decreases.
Figure 2: The SCM/WDM RoF System Model Communication Link
In this SCM/WDM RoF system model, we performs of each Î» contains 16 channel of Phase Shift Keying (PSK) with the bit rate is 1.8 Gbps, Mach-Zehnder Modulator with the bandwidth 20 GHz and 1550 nm of Laser Diode. The frequency of RF channel has set from 1.8 GHz to 27 GHz. The RF signal is used to directly modulate the laser diode in the Central Station (CS). The resulting intensity modulated optical signal is then transported to the WDM over the length of the fibre to the BS. In the fiber optic link, the signal then pre-amplified by EDFA (1-5m) to gained the better perform in the receiver. At the BS, the transmitted RF signal is recovered by direct detection in the PIN photodetector. The signal is then amplified and radiated by the antenna.
Results and Analysis
The systems are designed incorporate three part domain of the system. Mm-wave domains are generated by applying of RF channels as SCM, Optical signal carrier is generating by Mach Zehnder Modulator and Multiplexing are construct by Wavelength Division Multiplexing. The mm-wave consist of 16 RF channels and separate into 2 SCM group. MZM generates of 20 GHz optical signal. A WDM ideal has 50 GHz channel spacing. Those systems are considered in bandwidth present. By focusing how to increase the bandwidth, we proposed the combine of SCM techniques applied into WDM technology. In this simulation the parameters was setup to measure the capacity in the systems such as SNR, BER, Number of Carrier, etc. The intention of the SCM/WDM system afforded the bandwidth capacity of the fiber.
Figure 3 illustrates the SNR performance to the some various distance links 150 km. In 150 km SNR, some channels of SCM fall down from 30 dB closed to 5 dB over the distance link. It is meaning that an error probability for SNR is increases according to the fiber length. SNR in optical link communication was used to evaluate the minimum energy per pulse that is required to achieve a prescribed maximum bit-error rate. SNR depend on the total noise in the systems. Total noise is accumulated from shot noise, thermal noise, shunt noise and series noise. These parameters have an effect on BER and Q factor. So the Q factor is related to the signal-to-noise ratio required to the desire bit error rate.
Figure 3: The SNR performance of PSK Channel in 150km
Figure 4 shows the BER performances of all electrical carrier frequency across 1 km until 150 km with length of EDFA 5 m. The SCM-1 is illustrated in Figure 4 (a), for channel 1, 2 and 6 the optimum fiber length is 30 km. For channel 4 and 8 the optimum fiber length is 75 km. Note that the existence of EDFA somehow affected the performance of the system in term of BER. In the SCM-2, the channels are arbitrary (fluctuate) in unsystematic line due to highest bit-error rate, as shown in Figure 4 (b). The preamplifier in optical domain has a primary drawback that call as Amplified Spontaneous Emission (ASE) noise. This kind of noise not only affect the BER but able to degrade CNR of the system.
(a) BER of 8 PSK-SCM/WDM RoF Channels 1
(b) BER of 8 PSK-SCM/WDM RoF Channels 2
Figure 4: Minimum of BER of SCM/WDM ââ‚¬" RoF SMF 150km
The power of received signal however were to small and will contributed to the uneccepted Carrier-to-Noise Ratio (CNR). Therefore, an electrical amplifier encounter this problem that can be reserved the amplified output signal as shown in Figure 5 below.
Figure 5: 60 GHz of RF spectrum are detected by PIN photo detector
In this part, we present the attaining of the RF bandwidth outcomes from the SCM/WDM-RoF system. By employing an external modulation of using Mach-Zehnder Modulation, totally gain bandwidth of MZM is 20 GHz we derive 60 GHZ of RF bandwidth spectrum as shown in Figure 5. The total capacity of RF bandwidth was increased by utilize the number of channels.
The performance of the SCM/WDM for RoF is resolved by many factors such as BER, SNR, Power, distortion and attenuation. In this works 16 channels of SCM are employed to the WDM over 150km SMF fiber. An external modulation of MZ Modulator was utilized to obtain optical bandwidth. And the WDM employed multiplexing/demultiplexing of RF signal that carried by optical signal carrier to resolve the huge bandwidth. The outcomes of bandwidth was increased to 60 GHz by applying of 16 Channel of SCM combined with WDM in optical fiber link.
The authors acknowledge the Ministry of Science, Technology and Innovation Malaysia for the financial support through E-Science funding 01-01-06SF0064. Our gratitude also goes to the administration of Universiti Teknologi Malaysia (UTM) especially for Research Management Centre (RMC) for the financial support through vote number 79026.