Zhang Shu,Liu Deming,Wu Guangsheng

(College of Optoelectronic Science and Engineering,Huazhong University of Science and Technology,Wuhan 430074,P.R.China)

Abstract:Convergence of optical and wireless networks is a promising development for future access network architecture.A Radio over Fiber(ROF)-based network that converges Ethernet Passive Optical Network(EPON)and Worldwide Interoperability for Microwave Access(WiMAX)technologies makes it possible to simultaneously transmit EPON baseband signals and WiMAXwireless Radio Frequency(RF)signals.This article elaborates on uplink and downlink transmission,redundancy protection,and roaming features of such a network.

B roadband access technology has come into widespread use in line with wideband-consuming services such as Internet Protocol Television(IPTV)and Video on Demand(VoD).To further optimize resources,cut down costs,and provide wider and more flexible broadband access services,convergence of optical and wireless networks is possible in future architecture[1-2].At present,application areas of Ethernet Passive Optical Network(EPON)[3]are being expanded because of its low cost,wide bandwidth and structural Ethernet advantages.On the other hand,Worldwide Interoperability for Microwave Access(WiMAX)technology specified by IEEE 802.16[4]is becoming a mainstream wireless broadband access technology.Compared to Wireless Local Area Network(WLAN)access technology,WiMAXprovides wider bandwidth,broader coverage and better Quality of Service(QoS).It also provides better data access service compared to cellular technology.

Convergence of EPON and WiMAX therefore avoids the drawbacks within each individual technology,making full use of the wide bandwidth of optical access technology and flexibility of wireless technology.This gives users a better experience,and also decreases construction and maintenance costs across the entire network.

1 Current Solutions to EPONand WiMAX Convergence

There are currently 2 solutions to EPON and WiMAXconvergence.One solution is baseband optical fiber transport technology,which transports digital baseband signals directly through optical fiber.The Optical Network Units(ONUs)of EPON and WiMAXBase Stations(BSs)are directly connected through the Ethernet interface,and Ethernet frames are transmitted via optical fiber.This scheme can be easily implemented,and has wide coverage.It also has good bandwidth allocation and QoSperformance[5].However,the BSs are required to convert digital baseband signals into wireless RF signals,which may be very expensive especially when wireless signals are high frequency.Large numbers of BSs are required for wide coverage and to compensate for the quick fading of wireless signals.

The other solution is Radio over Fiber(ROF)technology[6].An ROFsystem uses optical fiber as the transmission link between BSand Central Station(CS)and uses an opticalcarrier to transmit RFsignals.Optical fiber is only used for transmission,while the CSis responsible for switching,control and signal regeneration.The BSperforms optical-electricalconversion.In this way,complex and expensive devices can be placed at the CSto be shared by multiple remote BSs,and power consumption costs can be reduced.Many technical papers have been written on WLAN and ROF convergence,as well as on successful commercial cases of GSM and ROF convergence.Research into transmission of WiMAXin Synchronous Optical Networks(SONET)is currently being undertaken[7-8].

2 Architecture of a ROF-Based EPONand WiMAXConverged System

In contrast to a standard EPON system,a new Optical Line Terminal(OLT)and an Optical Network Unit Base Station(ONU-BS)are introduced in a ROF-based EPON and WiMAX converged system.

For the sake of reducing costs and concentrating processing,the devices originally on the BSWiMAXphysical layers are moved to the central node OLT.The IEEE 802.16 physical layer defines a working frequency of 2-66 GHz and 2 modulation modes:single-carrier modulation and Orthogonal Frequency Division Multiplexing(OFDM)modulation.OFDM is used on 2-11 GHz frequency.Supporting Non-Line of Sight(NLOS)transport,OFDM technology resists attenuation and multipath.OFDM modulation and demodulation devices are used in this architecture.

There are 3 usable frequencies under 11 GHz:2.5 GHz,3.5 GHz,and 5.8 GHz.Considering WiMAX deployment,this architecture adopts 3.5 GHz.Since there is no specifically defined carrier bandwidth,a WiMAX system can work at a bandwidth between 1.25-20 MHz.This architecture adopts a bandwidth of 20 MHz,but ignores the problem of co-channel interference.In actual application,frequency reuse and sector partitioning technologies can make better use of frequency resources and system throughput.In the deployment of an ROFsystem,direct modulation is usually adopted if the RFsignals are below 10 GHz,and external modulation is above 10 GHz.Direct modulation is used in this architecture.

In the downlink direction,this architecture adopts Subcarrier Multiplexing(SCM)technology at the OLTend in order to simultaneously transmit EPON wireline baseband signals and wireless RFsignals,and also to distinguish RFsignals that belong to different BSs.This architecture defines the EPON baseband signals at 0-2.5 GHz,and up-converts the radio signals to subcarriers above 3.5 GHz.The bandwidth of each subcarrier is 20 MHz,and the central frequency interval is 0.1 GHz.One subcarrier corresponds to one BS;for an EPON system with a branching ratio of 1:16,there are 16 subcarriers in total.The laser is modulated after the baseband signal and modulated subcarrier have been integrated.Since the baseband signal and subcarrier remain at different frequencies,no interference is produced.Baseband and radio signals are simultaneously transported in the downlink.When the remote ONU-BS receives mixed signals from the OLT,it demultiplexes the baseband and subcarrier signals,uploads the baseband signal to related devices for processing,and performs down-conversion of the subcarrier signal.The frequency of the local oscillation and the converter used is the same as that of OLT.After moving through the band pass filter,the radio RFsignal—originally belonging to the local BS—is demodulated and sent out through the antenna.That is,the BS only performs down conversion and requires no other devices.Costs are therefore lowered.

The ONU-BSuses Time Division Multiple Access(TDMA)mode to upload data in the uplink direction.In this mode,the baseband and radio RF signals are mixed together.The EPON MAC and WiMAXMAC cooperate with each other to allocate transmission timeslots of uplink data for every ONU-BS.When a designated timeslot appears,the ONU-BSsends data according to the authorized OLT window size.In this way,uplink data of all ONU-BSs can be transported in a pre-defined sequence without conflict after they have reached the shared optical fiber.Performing up-conversion on the radio RFsignal to convert it into a subcarrier signal at the ONU-BSend is unnecessary.Different ONU-BSs are located in different transmission timeslots and their uplink RFsignals do not conflict with each other.Moreover,the radio RFsignal has a frequency of 3.5 GHz while that of the baseband signal is below 2.5 GHz.Therefore,the baseband signaldoes not conflict with the radio RFsignal,and they can be transmitted at the same time.With this design,the OLTreceiver can be simplified by using only an electrophotonic detector and a WiMAX OFDM demodulator.No local oscillation or converter is needed since there is no cross-interference between subcarriers.

3 Features of the ROF-Based Convergence Solution

The architecture of this ROF-Based EPON and WiMAXconverged system has the following features:

(1)The BSperforms down conversion only.Devices responsible for radio signal processing and some upper-layer functions are placed at the OLTend.For ease of description,the OLTend shown in Figure 1 uses N OFDM modulators(one for each BS),which in practise can be combined into one.At the OLTend,all BSs can share one set of OFDM modulation and demodulation devices.

▲Figure 1.ROF-based EPONand WiMAXconverged system architecture.

(2)N+1(rather than N+N)protection is implemented because alldevices are moved to the OLTend.N BSs share just one spare BS.In current radio networks,BSs cannot support redundancy protection,or can only provide 1+1 protection.In other words,one spare BSis needed for every BS.In an ROF-based converged network,system stability is improved and cost is reduced.

(3)OLTserves as a centralcontroller that manages the wireless resources of all BSs and knows the real-time radio resource information of all BSs.In standard WiMAXarchitecture,wireless resource management is performed at different BSs and information cannot be exchanged within the necessary timeframe.This limits the system’s work efficiency.With the help of an algorithm,OLTdynamically allocates wireless resources for every BSin order to make full use of the wireless resource and to balance the loads among BSs.System efficiency is accordingly improved,and can be further improved if Multiple-Input and Multiple-Output(MIMO)and adaptive modulation technologies are employed.

(4)Roaming among BSs is much simpler.OLTprocesses all data and knows all information,so the control channelbetween OLTs and every BSis no longer necessary.To support user mobility,roaming in different WiMAX BSs must be taken into consideration.BSs cannot connect with each other,and in any ordinary networking scheme,a control channelis used—between OLTand every WiMAXBS—to exchange controlinformation in real time.When a user roams from one BS to another,OLTsends a command via this control channel to make the original BSdisconnect the user and at the same time add them to the local BS.However,in converged architecture,when OLT senses that a user is roaming between different BSs,it simply switches over to a new subcarrier frequency to send data to the user.

The conveged system,however,is not without some weaknesses.It is susceptible to fiber attenuation,dispersion,and non-linear effect in opticalfiber,as well as cross interference between subcarriers.It also places high requirements on modulation and detection technology,especially when the radio frequency is high.Moreover,as it takes time for the radio signal to be transported over opticalfiber,the longer transmission distance impacts MAC performance[9]more significantly.Further research is required into the bandwidth allocation algorithm so that uplink transmission timeslots can be better allocated for wireline baseband data and radio RF signals.

4 Conclusion

Architecture that converges optical fiber and wireless networks has a bright future.An ROF-based EPON and WiMAXconverged network simultaneously transmits the EPON baseband signal and WiMAXradio RF signal via optical fiber.Such a system combines fixed line and mobile networks,and brings about more satisfactory user experience.Moreover,it can greatly reduce network construction and maintenance costs.