An Overview Of Radio Over Fiber Technology Computer Science Essay

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Abstract - To accommodate high traffic demands and high data rates, some solutions were proposed such as reduction in cell size and operating in new frequency bands. But these solutions have high costs associated with them. After that a new cost effective technology "Radio over fiber (RoF)" was proposed. It is an integration of wireless and fiber optic technologies. Comparisons of different RoF technologies are discussed with their pros and cons. RoF has advantages of large bandwidth, low power consumption, high reliability and security. It has also disadvantages of chromatic and modal dispersion. Multiplexing techniques like Sub carrier and wave length division multiplexing are used for RoF. Network architectures for different applications of RoF have been proposed. Some of these architectures are discussed for in building, rural areas, road vehicle systems and Reconfigurable RoF. RoF has applications in mobile networks, fixed networks and satellite communication networks.

Index Terms- Radio over fiber, Subcarrier multiplexing, external modulation, optical heterodyning, frequency up conversion, wavelength division multiplexing, chromatic dispersion, modal dispersion.

INTRODUCTION

In early days of telecommunication, people were interested in voice communication only. Speech signals use less bandwidth ranges from 300 Hz to 3100 Hz.

GSM that was the key technology at that time, is not capable of sending high data rates. But with the passage of time people are not only limited to low data rates. Now

they are more interested in broadband communication. Broad band communication is the need of the hour. With this, it is not only possible to send voice signals but also data signals and multimedia signals and interestingly at the same time. Users are demanding services that will provide them a faster transmission, flexible and anytime anywhere solutions. For example, WiMax and WiMAN, use bandwidth up to 1Gbps with wide coverage area. [1] On one side, wireless operators are facing challenges to provide such high data rates to users and on other side numbers of users are increasing day by day.

Wireless operators are increasingly challenged to accommodate a great diversity of data oriented mobile services and the growing number of end users. [2] So a question was raised to handle this problem. To enhance traffic capacity and users high data rates demand at the same time is a big problem. Some solutions were proposed for this problem. As we know that our conventional cell ranges from 1-2Km. If cell size is reduced up to 300 meters then network will have large number of cells. Such large number of cells increases the capacity. These small cells are termed as pico cells or micro cells. But unfortunately large numbers of base stations are required for this purpose. Such large base stations will increase our network cost. Secondly maintenance of these base stations is not an easy task.

To solve this problem a new technology was proposed that is called Radio over Fiber (RoF) technology. The name suggests that it

is an integration of Wireless and optical fiber technologies. To find the secret behind combination of wireless and optical fiber, pros and cons of both technologies must be known. The biggest advantage of wireless includes mobility. It means transmitter and receiver can move around anywhere in the network and still able to avail all services offered by the network but increasing number of users is a big problem. Fiber on the other hand, has advantage of high capacity but it lacks mobility. So the basic idea behind this technology is an amalgamation of both wireless and fiber that will lead to high data rate, high capacity and mobility solution.

SYSTEM LAYOUT

RoF is an analog technology. The basic principle behind RoF is that the light signal is modulated by the radio signal. Radio signal is converted into optical signal. It is then transmitted via optical fiber to receiver side. At the receiver, the optical signal is again converted into radio signal and then transmitted via air interface. Thus the connectivity between central station and base station is based on optical fiber and the connectivity between base station and remote mobile unit is based on air. Figure 1 [1] is showing the system topology for RoF system.

fig1: RoF system lay out [1]

RoF TECHNOLOGIES

In RoF there are different ways of signal generation and distribution. These include:

External modulation of continuous wave laser light in the central station [3]

Optical heterodyning

Frequency Up conversion.

Intensity Modulation Direct Detection (IMDD) is the simplest technique. It is further divided into Direct Modulation and External Modulation. IMDD is used for low/medium RF range. Chromatic dispersion is a big problem in IMDD. It is because of the production of double side bands at the output of modulator. If single side band modulation is used then this problem can be solved. To overcome chromatic dispersion, some compensation methods can be used such as fiber Bragg grating. Figure 2. [1]

Fig 2. IM-DD Technique [1]

a)Directly modulated RF b) Externally modulated RF

Optical heterodyning is good in a sense that chromatic dispersion decreases but it has problem of phase noises. It is because the spectrum of the multiplexed signal lies very close to one another and it results in Phase noise. To overcome phase noise problem, a technique Optical Injection Locking is used.

Frequency Up conversion does not need any modulator at the transmitter side but it requires high quality local oscillators for synchronization which enhance the cost. Secondly high power is required to send the data signal with the optical reference signal.

IV. COMPARISON OF RoF TECHNOLOGIES

RoF technologies can be compared with one another. Different experiments were performed for this purpose. Results of these experiments are summarized in Table 1 [4]

Table 1

Techniques

RF band (GHz)

IF Band (GHz)

Bit Rate (Mbps)

Modulation

Fiber Length (Km)

Optical heterodyning

64

60

-

-

155

155.52

OQPSK

DPSK

12.8

25

External Modulation

59.6

60, 59.6

2.6

3, 2.6

156

156

DPSK

DPSK

50

85

Up and Down Conversion

40

40

3.0

3.0

156

155 (down)

51.8 (up)

DPSK

BPSK

50

40

BENEFITS OF RoF

RoF has large number of advantages. It is transparent to bandwidth and modulation techniques [4]

Large Bandwidth

For a single SMF optical fiber, the combined bandwidth of the three windows is in the excess of 50 THz [15].In presence of such high BW, we can transmit not only voice and data signals but also video signals and interestingly all these formats on single fiber.

Centralized Network

RoF adopts a concept of centralized network. One central station (CS) is shared by many remote antenna units (RAU). As a result all signals processing and routing is performed at CS due to which size of RAU has reduced. Thus it can be placed anywhere in the environment. Cost associated with site acquisition, site leasing has also reduced.

Reduced Power Consumption

Conventionally, copper cables were used to feed signals to antennas. These copper cables are very lossy in nature. Signal power reduces during its transmission from 3dB to 10dB transmitter to receiver. To increase the SNR, signal power is enhanced. High power amplifiers are required for this purpose which is expensive on one side and less efficient on other side. Use of fiber optic instead of copper, results in lossless transmission. No or less power losses so there are no requirement of air conditioning.

Low Attenuation

Fiber optics has very less attenuation. Due to fiber optic the use of repeaters also reduces. Transmission loss reduces from 0.3dB/Km to 0.5dB/Km.

High Immunity

It has high immunity against cross talk which results in secure and reliable transmission.

Dynamic Allocation of BW

There is no requirement of static BW allocation. Control is centralized so every user is entertained according to the use. Thus make it possible to allocate the BW dynamically. Dynamic allocation is achieved by using wavelength division multiplexing (WDM)

Capacity Enhancement

Capacity of the system is increased by using RoF technology. It is because of the improvement in trunking efficiency. Through trunking network access is provided to many users by sharing set of lines or frequencies instead of allocating them individually.

High reliability

RoF provides high reliability and security. It is because of the reason that fiber optics are made of insulators. Thus the optical signals do not react with any atmospheric constituents which results in secure transmission.

LIMITATIONS OF RoF

RoF has also some limitations.

Chromatic Dispersion

Optical fiber is used to connect CS with BS. Chromatic dispersion increases with the increase in fiber length. As the fiber length increases, signal degrades and SNR reduces to undesirable level. Thus it is not possible to introduce long spacing between transmitter and receiver. This chromatic dispersion is associated with the single mode fiber.

Modal Dispersion

Multimode fiber is used for short distances. It has problem of modal dispersion. To overcome dispersion, some dispersion compensation methods are employed but it can increase our network cost.

Laser

The laser is usually a significant source of noise and distortion in an ROF link, and laser diode normally exhibits nonlinear behavior [3] Laser behaves non linear when current is driven above the threshold value. This non-linearity causes noise due to which dynamic range decreases.

MULTIPLEXING TECHNIQUES IN RoF

Multiplexing is a process where single medium is shared by many users to increase the capacity.

Sub-carrier Multiplexing (SCM) in RoF Systems

SCM is a technique that allows different fiber optics signals to multiplex into single fiber. It is a mature, simple and cost effective approach. In this scheme, different signals are modulated with different sub carriers. These modulated signals are then multiplexed. This multiplexed signal is again modulated. This signal is then transmitted to receiver. Advantage of SCM is that it supports traffic of different modes.

Subcarriers are used in this technique. These sub carriers are of low frequencies. This is why their components are easily available.

b) Wavelength Division Multiplexing (WDM) in RoF Systems

WDM is an analog multiplexing technique to combine different wavelengths. A composite signal is obtained after multiplexing that is sent to the receiver side and then Demultiplexer is used to separate the composite signal into separate signals. The application of WDM in RoF networks has many advantages including simplification of network topology by allocating different wavelengths to individual BSs, enabling easier network and service upgrades and providing simpler network management. Thus WDM in combination with optical mm-wave transport has been widely studied. [5]- [8]

Dense Wavelength Division Multiplexing (DWDM) is also extensively using in RoF networks. Due to DWDM system capacity has increased tremendously. Instead of 4 channels now system can accommodate 128 channels while channel spacing has reduced from 500 GHz to 50 GHz. But there are some challenging issues related with this integration of RoF and WDM. Most important is number of wavelengths that are required for one BS. Full duplex communication is possible if one wavelength is used for both downlink and uplink. However wavelength reuse technique has been proposed in [7] to solve this problem.

NETWORK ARCHITECTURES

Many Network architectures have been proposed for different applications of RoF.

RoF based architecture for In-Building Networks

In [9], architecture was proposed for in-building networks. In this architecture a new technique 'Optical Frequency Multiplying' (OFM) is implemented. Due to this technique it is possible to provide high capacity wireless services to users according to their demands. This architecture is based on multimode fiber. Multimode fiber is easy to install and cheap but it has limitation of modal dispersion. Due to OFM it is possible to reduce modal dispersion. OFM makes it possible to use multimode fiber In- building.

Dynamic Capacity Allocation

For dynamic radio capacity allocation FBG based wavelengths add drop multiplexers were used to demonstrate the concept.

.

fig 3. Wavelength routed radio over multimode fiber

(ADM- Add drop multiplexer, MMF- Multimode fiber, FBG - Fiber Bragg Grating)

ii) Results

For this purpose, multimode mode fiber of 50um was used. Three wavelengths were selected i.e. 1304, 1310 and 1315 nm. It was implemented for ring topology.

Successful conversion and transport of RoF signals was achieved for 950 meters of silica graded index fiber. It was achieved for RoF signals of 18.3 GHz that can carry data up to 36 Mbits/s by utilizing 16- QAM on 52 subcarriers of OFDM.

RoF based architecture for Road Vehicle Communication Systems

In [10], a system is proposed for road vehicles communication systems. To increase data rates, cell size has been reduced. This small cell size results in high mobility. High mobility results in large number of handoffs. Management of such system is not an easy task. So a Media Access Control (MAC) scheme is proposed featuring fast handovers based on dynamic TDMA.

Media Access Control

This MAC protocol scheme has advantages of simple and fast handovers. It is also capable of dynamic bandwidth allocation. Frame structure for MAC protocol is shown in the figure 4.

fig. 4 Frame Structure

Each frame belongs to a certain BS. Frame has a beacon field. This field contains BS identification number and slot assignment map specifying the starting position of slot and its length for each MH. After beacon field, it has field of reservation minislots. It is further divided into minislots for hand over requests and for new connection requests. It is used by those MH which have not reserved any slots but they have data to transmit. Then there is broadcast field. This field contains data about previous reservation trials. This data is broadcasted through this field. Then there are fields for downlink and uplink. Uplink also contains one bit handover indication flag for fast handover.

A MH first scans the RF channels. Then a request for bandwidth is sent to the CS using reservation mini-slots. If CS has enough bandwidth then it is assigned to MH in next super frame.

Types of handovers

Different types of handovers are discussed for this architecture.

Intra-VCZ handover: Handover takes place between two BSs. These BSs belong to the same VCZ.

Inter- VCZ handover: Handover takes place between two BSs. These BSs belong to different VCZ.

Inter- CS handover: Handover occurs between two BSs that are controlled by two different CS.

Figure 5 is showing example of proposed architecture. This architecture is comprised of CS which is connected with five BSs. It has two VCZ. Three types of handover are also taking place in cell number 2, 3 and 4. Cell 2 and 3 are involved in intra- VCZ handover. Inter VCZ handover is occurring between 3 and 4 cells. While cell number 4 and 5 are involved in Inter - CS handover.

fig 5. An example of the proposed architecture where cell 1,2,3 and 4,5 constitute VCZs (Virtual Cellular Zone) respectively

Limitations

This architecture has limitation related to resource allocation. To improve the Quality of Service (QoS) successful handover management is necessary. A guard channel concept was proposed where some bandwidth is reserved for handovers. To imply this concept for VCZ, some modifications are trying to do. With these modifications bandwidth is allocated to VCZ instead of each cell. Research efforts have been made to solve this issue.

c) RoF based architecture for Rural Areas

In [11], radio over fiber based broadband wireless access network architecture is proposed for rural areas. Though urban areas have always been a hot spot for communication but demands for services in rural areas are continuously increasing. Tunable transmitters and tunable receivers are used in this system. However WDM is used to increase the capacity. All base stations are tuned to certain wavelengths for flexible bandwidth allocation. Capacity is limited but the central station structure is very simple.

Architecture Description

This system operates in TDMA mode. Transceiver supports the BS only if it knows the wavelength and RF channel used by the BS. It is assumed that RF channels are already determined by BS. RF channels for each BS are fixed. This assignment is fixed and based on frequency reuse technique. Note that the bandwidth provided to any BS is limited by the capacity of the TRX. Only one wavelength is allocated to each BS to avoid wavelength collision. This architecture has two silent features. First it provides flexible bandwidth allocation. Secondly, system capacity can be enhanced easily. It is because of the reason that system capacity is directly proportional to the TRXs. By increasing number of TRXs, system capacity can increased easily.

Fig6. A proposed radio over fiber access network architecture consisting of K transceivers (TRXs) and N base stations (BSs)

To avoid collision, frames are fragmented in such a way that no wavelength collision can occur. For K TRX, K super frames are generated. There is a scheduler at CS. This scheduler controls each TRX.

Fig7. Packing of five frames into two super frames

Limitation

It has limitation associated with dynamically changing bandwidth capability of CS. CS can change the time and frequency bands at any time. It leads to adjacent channel interference because adjacent channels cannot use same frequencies at the same time.

Reconfigurable Radio over Fiber (RoFnet)

In [12], a novel architecture is proposed. WDM is used to simplify the network architecture. Due to this network becomes more flexible. Single fiber is used to fed the signals to different BSs. Each wavelength is intended for different BS. To make network architecture more simple subcarrier multiplexing (SCM) is integrated with WDM. To utilize air interface properly sectorization of cells through antennas is utilized. Single side band modulation scheme is used to decrease the chromatic dispersion. Wavelength allocation is done according to the users requirements.

Architecture Description

Connectivity between CS and BS is via fiber optic. Each BS is connected with fiber network via two fibers. One fiber is used for uplink transmission and other fiber is used for downlink transmission. Fiber access network is employing WDM technology. Antennas using by BSs are sectored antennas. Due to WDM, our network becomes simple. Reflective Semiconductor Optical Amplifiers (RSOA) devices are used because of their cheapness. It eliminates the use of expensive devices.

fig 8. Overall RoFnet Architecture

The base station schematic is shown in the figure.

Fig9. Schematic of Base Station

Limitations

The performance of RoF systems may be severely impaired by fiber chromatic dispersion. [13] The optical single side band (OSSB) modulation is used for this purpose.

RoF APPLICATIONS

RoF has also large number of applications in different areas.

Access to dead zones

Due to RoF it is possible to provide wireless services to those areas where wireless backhaul link is not possible. These are dead zones like tunnels, secluded places like jungles, areas behind buildings or mountainous regions.

Mobile Networks

Mobile networks are one of the applications of RoF. Mobile subscribers are increasing gradually with their high traffic demands. RoF has capability to fulfill their needs cost effectively.

Fixed Networks

Fixed networks are those networks where user lacks mobility. RoF technology has its applications in fixed networks. It is because of its capability to provide broadband services to not only mobile users but also fixed network users. It supports high data rates of about 155Mbps per user.

Satellite Communications

Due to light weight, fiber optics has become very attractive in satellite communications. RoF is based on centralized system and this centralization helps in satellite communication where high frequency equipment can be centralized to reduce the costs.

CONCLUSIONS

RoF has become a key solution for broadband communication. Different methods are used to transport signals in RoF. To imply its uses in different areas, different architectures have been proposed. SCM and WDM are used in RoF systems. By integrating WDM and SCM in RoF networks, number of users can be further increased. That is why it has large applications in mobile, fixed and satellite communications.

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