Implementation and simulation of basic structure of the radio over fiber link

CHAPTER 1: INTRODUCTION

1.1 WIRELESS COMMUNICATION SYSTEMS

Wireless communication has gone through enormous growth in the past ten years. Less than a percent of world population had access to cellular technology before early nineties, and by the start of this millennium approximately every one in a five people has a mobile phone. In the same period different countries across the globe have increase the mobile network technology over ninety percent and future forecast says that by the end of 2010 there will be more than 1700 million mobiles users across the world. [1][2]

Apart from cellular technology WLANs has also seen phenomenal growth during the past ten years. These WLAN hotspots can be used in public places such as airports, cafes, hotels and restaurant etc.

YEAR	WLAN	Frequency	Modulation	Bit-Rate (MAX)
1997	IEEE 802.11	2.4 GHz	Frequency Hopping and Direct Spread Spectrum	2 Mbps
1998	ETSI Home RF	2.4 GHz	Wideband Frequency Hopping	1.6 Mbps
1999	IEEE 802.11b	2.4 GHz	Direct Sequence Spread Spectrum	11 Mbps
1999	IEEE 802.11a	5 GHz	OFDM	54 Mbps
2000	ETSI HiperLAN2	5 GHz	OFDM Connection-Oriented	54 Mbps
2003	IEEE 802.11g	2.4 GHz	OFDM compatible with 802.11a	54 Mbps

Table 1.1 Evolutions of WLAN Standards [3]

The rapid growth in wireless communication achieved more fame due the ease of installation as compared to the fixed network. The first generation (1G) mobile system were analogue, discovered in 1980s. The second generation (2G) known as global system for mobile communication (GSM) came on the scene in 1990s, which has been very successful and has achieved extreme success across the globe. GSM is currently the major mobile communication system which is used worldwide. [1]

The graph above shows the relationship between coverage and capacity of communication systems. By looking at the graph the cell size of WPAN is of few meters but there transmission rate may go upto 10 Mbps. While considering 2G and 3G systems, there cell sizes may vary upto several kilometres but that are limited to less than 2Mbps. WiMAX technology can provide high bit rate mobile services using frequency span between 2 – 11 GHz. [6]

FREQUENCY	WIRELESS COMMUNICATION SYSTEMS
2 GHz	UMTS/ 3G Systems
2.4 GHz	IEEE 802.11 b/g WLAN
5 GHz	IEEE 802.11 a WLAN
2-11 GHz	IEEE 802.16 WiMAX
17/19 GHz	Indoor Wireless (radio) LANs
28 GHz	Fixed Wireless Access – Local point to multi point (LMD)
38 GHz	Fixed Wireless Access – Picocellular
58 GHz	Indoor Wireless LANs
57-64 GHz	IEEE 802.15 WPAN
10-66 GHz	IEEE 802.16 WiMAX

Table 1.2 Frequencies for Wireless Communication Systems [2]-[5]

1.2 CLASSIFICATION OF WIRELESS NETWORK

Wireless networks can be categorized into different groups depending on the area they are applied to. As a result high numbers of standards have been making to public for the development of new techniques in order to increase the spectrum efficiency and perfect utilization of spectrum, which is scarce natural resource.

Wireless networks can be divided into three classes;

1.2.1 Wireless Private Area Network (WPAN)

Devices of such networks can communicate in the range of tens of metres. Infrared (IR) and Bluetooth are the two implementation of this principle.

1.2.2 Wireless Local Area Network (WLAN)

It is computer network that connects devices which are distributed over a local area (e.g office, house, mall, and airport). IEEE 802.11 which is commonly known as Wi-Fi, is an example of WLAN.

1.2.3 Wireless Metropolitan Area Network (WMAN)

Such a network covers a geographic area such as city or village. IEEE 802.16 which is commonly known as WiMAX, is an example of WMAN.

Depending upon the application, there are licensed and unlicensed frequency bands in which wireless systems can operate.

1.3 WIRELESS APPLICATIONS

Now we will discuss wireless standards along with the overview of their applications:

1.3.1 Bluetooth – WPAN

Bluetooth is a radio standard, which operates in the unlicensed Industrial Scientific and Medical (ISM) band at 2.4 – 2.485 GHz. Frequency Hopping Spread Spectrum (FHSS) is used in order to minimize interference and fading. In order to make the transceiver architecture as simple as possible, binary modulation is used. The bit rate is up to 3 Mb/s. The benefits of Bluetooth include low power consumption and low cost, therefore they are used in devices such as laptops, mobile phones and PDAs. [7]

Power Class	Maximum Output Power	Minimum Output Power
1	100mW(20dBm)	1mW(0dBm)
2	2.5mW(4dBm)	0.25mW(-6dBm)
3	1mW(0dBm)	–

Table 1.3 Bluetooth classes and power levels [7]

1.3.2 Wi – Fi – WLAN

The Wi-Fi alliance, the Institute of Electrical and Electronics Engineers (IEEE) and the European telecommunications standard Institute (ETSI) are the three organizations which influenced the standardization of WLAN. The IEEE WLAN standard is referred as 802.11. At the moment, the most used techniques are defined by the IEEE 802.11a, b and g standards. [8]

Standard	Release date	Operating frequency	Maximum Data Rate
802.11a	1999	5.15 – 5.35 GHz 5.725 – 5.825 GHz	54 Mbps
802.11b	1999	2.4 – 2.5 GHz	11 Mbps
802.11g	2003	2.4 – 2.5 GHz	54 Mbps

Table 1.4 IEEE 802.11a, b and g standards [8]

1.3.3 WiMAX – WMAN

WiMAX is an abbreviation for Worldwide Interoperability for Microwave Access. The WiMAX Forum is a non profit association. The aim and objective of the WiMAX technology is to provide fixed, portable or mobile connectivity to the users even if they located up to 6 miles away from base station and it is not necessary to be in line of sight. WiMAX can operate on any frequency below 66 GHz, as operating frequency may change for different countries depending on local regulation. It is possible replacement for mobile/cellular technologies such as GSM and CDMA. It has been considered to be the wireless backhaul technology for 2G, 3G and 4G networks. The limitations associated with WiMAX is that it can either provide high data rates or it can transmit data over longer distances but not both simultaneously. [9]

1.3.4 Distributed Antenna Systems and Radio Over Fiber

Distributed Antennas Systems (DAS) are used for several applications in the mobiles and wireless communications. It can be installing over indoor and outdoor sites. DAS can be implemented on those areas where there is lack of signals such as tunnels, underground stations etc. in order to extend the coverage of mobile network.

Radio over fibre consists of remote unit and central unit. Remote unit is kept very simple since it only consists of devices for reception of radio frequency signals and optoelectronic conversion. All expensive and complex equipments are located at central unit and functions such as modulation and up/down conversion etc. are done. This resulted in increase in efficiency and maintenance cost because as compared to central units, remote units are numerically high in numbers and often remote units are located in sites that are not easy to get in touch with. [10]

1.4 FLOW CHART OF THE DISSERTATION

1.5 AIMS AND OBJECTIVES

The aim of the dissertation is to implement and simulate the basic structure of the radio over fiber link using OFDM transceiver with the help of MATLAB/SIMULINK. The MATLAB version 7.8.0 (R2009a) is used for model implementation. Basically two models are designed: model number 1 consists of OFDM transceiver linked with a gain which represents the length of the fiber channel. Actually it is based on the theoretical fact that fiber has 0.2db loss per kilometre. For example 25km length fiber will be represented as – 5 dB(-ve sign to show loss). Later on simulations are carried out by varying the length of fiber and results are deduced. Model 2 consist of OFDM transceiver as well but linked with laser diode model, fiber channel model and photodiode model as these are the fundamental components of RoF link. Some additional parameters of measuring the transmitted and received power and bit error rate calculation are also introduced to enhance the diversity of the project.

1.6 DISSERTATION OUTLINE

The dissertation consists of six chapters:

Chapter 1 is the introduction chapter in which wireless communication systems and wireless applications have been discussed briefly.

Chapter 2 consist of the theory of radio over fiber which includes the need of RoF system, what RoF technology is, advantages and disadvantages of RoF system and applications of RoF technology.

Chapter 3 purely consist of theory related to OFDM technology. Sub topics include in this chapter are principles of OFDM, history, advantages and disadvantages and applications of OFDM. Fourier transform is also discussed in this particular chapter.

Chapter 4 consist of methodology of the dissertation. It consists of the models implemented using MATLAB/SIMULINK and the brief study of the essential blocks used in the models.

Chapter 5 is the chapter of simulations and results.

Chapter 6 includes the conclusion and future work regarding radio over fiber and OFDM.

CHAPTER 2: RADIO OVER FIBER

2.1 INTRODUCTION

Radio-over-fiber (RoF) is a communication technology for delivering broadband applications to wireless users such as satellite communications, mobile-radio communications, broadband access radio, multipoint video distribution and broadband mobile services. RoF technologies make use of optical and radio communication media for providing above mentioned broadband services. The optical part is used to transmit microwave signals between a central radio base station and a remote radio antenna and on the other hand radio part provides coverage to wireless users. In RoF system radio frequency (RF) signal is transmitted through an optical network in an easier way by directly modulating the intensity of the light source with the RF signal to be transmitted and on the receiving end direct detection of the signal at photo detector. The modulating of the laser-diode light intensity with electrical signals at multiple frequencies causes a number of problems such as relative intensity, noise chirp and inter modulation distortion. The main sources of non-linearity in a system are the laser-diode light source, the optical fiber and the photo detector. [27]

2.2 NEED FOR RADIO OVER FIBER SYSTEMS

For the future prerequisite multimedia services and broadband over wireless media, some distinctive characteristics are needed such as cell size reduction in order to accommodate more users and to operate in the millimetre wave (mm-wave) frequency bands to overcome spectral clogging. Such a system would demands a large number of base stations to cover large geographical coverage area and base station should be cost effective as well, then only such a system would be successful in market. In such a competitive market, this necessity has led to the evolution of system architecture where microwave functions such as signal processing, signal routing, handover, modulation, protocols setting and frequency allocation etc. are performed at central control station (CS) rather than at remote station or base station (BS). This type of centralized arrangement allows complex, sensitive and expensive equipments to be positioned in safer environment and shared among several BSs or RSs (Remote Stations). Now the question arises how to link the central station (CS) with BS. In such type of radio network, the use of optical fiber is the most suitable choice for the linking of CS with BSs, as fiber is cheaper in cost, has low loss, immune to Electromagnetic Inter Modulation (EMI) and provides wider bandwidth. By keeping the BSs as simple as possible and by sharing the resources provided by CS among several BSs, can effectively minimizes the cost of entire network and thus maintenance cost. Modulation of RF sub carriers onto an optical carrier over an fiber is known as Radio over Fiber (RoF) technology. Typically RoF network consist of central CS, where functions like switching, routing, medium access control (MAC) and frequency management takes place whereas at BSs functions like optical to electrical and vice versa are performed. [32]

2.3 RADIO OVER FIBER TECHNOLOGY

Radio over fiber system consists of a Radio Base Station (RBS) and Radio Access Point (RAP) which are connected by an optical fiber link. Optical fiber link is used to distribute RF signals from a RBS to RAP. RAP only contains optoelectronic conversion devices and amplifiers. In GSM technology RBS could be referred as Mobile Switching Centre (MSC) and RAP as Base Station (BS). The frequency used by the RoF systems usually lies under GHz region depending on the nature of application. Basically RoF systems were used to transmit microwave signals and to achieve mobility functions in RBS. Therefore modulated microwave signals had to be available at the input end of the system, which are then delivered to the RAP as optical signals. Signals at RAP are re-generated and radiated by antennas. Due to the advancement of technology, RoF systems are designed to perform added radio system functionalities other then transportation and mobility functions. The functions include are data modulation, signal processing and frequency conversion (up and down). The electrical signal at the input of the multifunctional RoF system may be baseband data, modulate IF or actual modulated RF signal for distribution. The modulated optical signal is carried over the optical fiber link to the remote station. At the receiving end, demodulation of the signal is carried out by the photo detector and the optical signal is converted back to electrical signal. [12] [13]

2.4 ADVANTAGES OF RADIO OVER FIBER

2.4.1 Low Attenuation

It is observed that high frequency signals when transmitted in free space or through transmission lines are expensive and sometimes due to different reasons challenging as well. In free space, losses are directly proportional to frequency due to absorption and reflection. Increase in frequency also gives rise in impedance when signal is delivered through transmission line. Therefore in order to overcome these issues, expensive signal regenerating equipment is required to distribute radio signal electrically over long distances. The cheaper solution is to use optical fibers which offer lower losses. Single Mode Fiber (SMF) made from glass (silica) has attenuation losses below 0.2dB/km and 0.5dB/km in the 1.5um and 1.3um windows respectively. [11]

2.4.2 Larger Potential Bandwidth

Larger bandwidth is being offered by optical fibers. Larger bandwidth provides high capacity for transmitting high frequency signals and also enables high speed signal processing which is difficult to achieve in electronics systems. Basically there are three main transmission windows, namely 850nm, 1310nm, and 1550nm wavelengths, which offer low attenuation. Anyhow optical system has to combine with electronic system in order to perform different tasks. But bandwidth mismatch of the systems create problem which is known as ‘electronic bottleneck’. The solution to this problem is the use of effective multiplexing techniques such as OFDM, DWDM and SCM. [11]

2.4.3 Easy Installation And Maintenance

The plus point of RoF system is the Switching Centre (SC), which are less in numerical quantity because one SC is shared by several Remote stations (RSs), which are equipped with all the expensive and complex equipments and RSs are kept simpler which includes only photo detector, amplifier and an antenna, thus reducing system installation and maintenance cost. [11]

2.4.4 Reduced Power Consumption

As discussed earlier centralized SCs are equipped with complex equipment and RSs are kept simpler with less equipments thus resulting in reduced power consumption. Thus RSs can be operated in passive mode. [11]

2.4.5 Immune To Interference And Crosstalk

As we know that optical fibers form a dielectric waveguide therefore there are no concepts as electromagnetic interference (EMI), radio frequency interference (RFI), or switching transients giving electromagnetic pulses (EMP). In fact it doesn’t require shielding form EMI. Hence optical signal can be transmitted through electrically noisy environment unaffectedly. The optical fiber can be used underground or overhead as it is not disposed to lightening strike. [11]

2.4.6 Signal Security

In RoF system, optical signals are transmitted in the form of light, which doesn’t radiate drastically, thus providing high degree of signal security. Therefore it is widely used in military, banking and general data transmission applications. [11]

2.5 DISADVANTAGES OF RADIO OVER FIBER

RoF systems can be called as analog communication system. Therefore signal impairments such as noise and distortion are worth considering in RoF. These impairments tend to limit Noise Figure (NF) and Dynamic Range (DR) of the RoF links. Chromatic dispersion may limit fiber link length when considering SMFs RoF. Modal dispersion can limit the available link bandwidth and distance when considering MMFs RoF system. Relative Intensity Noise(RIN), lasers phase noise, photodiode’s shot noise, amplifier’s thermal noise and fibre’s dispersion are few examples of noise sources in analog optical fibre links.[10]

2.6 APPLICATIONS OF RADIO OVER FIBER

Listed below are the few applications regarding RoF:

2.6.1 Mobile Communication Network

A mobile network is a useful application of RoF technology. In the past decade the numbers of mobile subscribers coupled with the increasing demand of broadband service have been keeping massive pressure on the mobile service provider to provide vast capacity to the end user. [11]

2.6.2 Video Distribution Systems (VDS)

VDS is one of the major applications of RoF systems. In this case the Multipoint Video Distribution Service (MVDS) is used for mobile terrestrial transmission. In MVDS the transmitter serves the coverage area based on tall building. Gunn oscillators and heat pipes are used for frequency stabilization while a fiber link can be used for feeding a TWT or solid state amplifiers. This system provides reduction in weight and wind loading of transmitter. [11]

2.6.3 Cellular Broadband Services

Due to the very high bit rates of nearly 155 Mbps, carrier frequency is pushed into mm-waves. For this purpose frequency band in 66 GHz frequency band have been allocated. The 62-66 GHz band is used for downlink while 65-66 GHz frequency band can be used for uplink transmission. [11]

2.6.4 Vehicle Control And Communication

For vehicle communication and system the frequency band between 63 – 64 GHz and 76-77 GHz frequency band has been allocated. They are used to provide continuous mobile communication coverage in major areas for the purpose of intelligent transport systems which includes road to vehicle communication (RVC) and inter vehicle communication (IVC). These can be made simple and cost effective by feeding them through RoF system. [11]

CHAPTER 3: ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

3.1 THE PRINCIPLES OF OFDM

Orthogonal frequency division multiplexing is a multi carrier technique which divides the bandwidth into several carriers. Each carrier is modulated by a low rate data stream. OFDM has the ability to use the spectrum efficiently by spacing the channels close to each other. Closeness of the channels can result in the interference therefore to prevent interference all carriers are orthogonal to each other which means all carriers are independent to each other. [14]

In FDMA a single channel is allocated to each user to transmit information. The bandwidth of each channel is about 10 kHz-30 kHz for voice communications. In order to prevent channels from interfering with one another, the allocated bandwidth is made wider than the minimum amount required. This extra bandwidth or spacing between channels is wasting about 50% of the total spectrum. As the channel bandwidth becomes narrower the problem becomes worst. [14]

In TDMA multiple users access the same channel or utilized the full bandwidth in different time slots. Many low data rates users can be combined to transmit in a single channel thus bandwidth or spectrum can be used efficiently. There are two problems associated with TDMA. Firstly the symbol rate of each channel is high resulting in multipath delay spread. Secondly at the start time of each user to use bandwidth for data transmission, a change over time has to be allocated in order to prevent from propagation delay variations and synchronization errors. This change over time is a loss, limiting the number of users that can be accommodated efficiently in each channel. [14]

OFDM is solution to both the problems occurring in FDMA and TDMA. Actually OFDM splits the available bandwidth into many narrow sub channels. As the carriers are orthogonal to each other which means they are purely independent of each other therefore they can be spaced very close to each other. Any time full utilization of bandwidth is possible in OFDM, therefore there is no need for users to be time multiplex and no more switching of the users for bandwidth. Users can send and receive data at any time unlike TDMA. [14]

3.2 OFDM HISTORY

The concept of OFDM was first developed in 1950’s. A US copyright was issued in January 1970. The evolution of OFDM took place in order to use the available bandwidth or spectrum more efficiently. [15][16]

OFDM was first implemented in military communications just like CDMA. KINIPLEX [17] and ANDEFT [18] are two examples of OFDM application in high frequency military system. AN/GSC-10(KATHRYN) variable rate data modem was the early application of OFDM which was built for high frequency radio.

In 1980’s, OFDM had been studied for high speed modems, digital mobile communications and high density recording. OFDM techniques for multiplexed QAM using DFT was discover by Hirosaki [19]. He has also designed 19.2 kbps voice band data modem which uses QAM modulation.

In 1990’s, OFDM has been exploited for data communication over mobile radio FM channels, high bit rate digital subscribers lines(HDSL), very high speed digital subscriber lines(VHDSL), digital audio broadcasting(DAB), digital television, HDTV terrestrial broadcasting and asymmetric digital subscriber lines(ADSL).[14]

OFDM has been considered more towards mobile communication due to its robustness to multipath propagation. Recently OFDM has been put into practice in audio broadcasting applications such as DAB and DVB. And it has been successfully implemented in wireless LAN applications as well. [14]

3.3 FOURIER TRANSFORM

The application of OFDM was not very practical in 1960’s. Quite a few numbers of oscillators were needed to generate the carrier frequencies for sub channel transmission. At that time it was a bit difficult to make it practical, that is why OFDM scheme was said to be impracticable.

Complexity of the OFDM scheme was eliminated with the evolution of Fourier Transform where harmonically related frequencies are generated by Fourier and Inverse Fourier Transforms used to implement OFDM systems. Fourier Transform can be used in linear systems analysis, antenna studies, optics, random process modelling, probability theory, quantum physics and boundary-value problems.

3.4 OFDM REAL PARAMETERS

In the last 10 years, the usage of OFDM has increased to enormous extent. It has been proposed for radio broadcasting such as EUREKA 147 standard and Digital Radio Mondiale (DRM). Some of the useful parameters are listed below: [20]

· Data rate: 6Mbps to 48 Mbps

· Modulation: BPSK, QPSK, 16-QAM and 64 QAM

· Coding: Convolutional concatenated with Reed Solomon

· FFT size: 64 with 52 sub-carriers uses, 48 for data and 4 for pilots

· Sub carrier Frequency Spacing: 200 MHz divided by 64 carrier or 0.3125 MHz

· FFT Period / Spacing Period: 3.2usec

· Guard Duration: One quarter of symbol time, 0.8usec

· Symbol time: 4usec

3.5 ADVANTAGES OF OFDM

· Overlapping is used for efficient use of spectrum.

· OFDM systems are more often reluctant to freq selective fading by dividing the channel into narrowband sub channels.

· Cyclic prefix is used to discard ISI and IFI.

· The symbols lost due to selective fading can easily be recovered by using channel coding and interleaving.

· The use of single carrier systems makes channel equalization simpler by using adaptive equalization techniques.

· With reasonable complexity max likelihood decoding is possible.

· FFT techniques allow OFDM to be computationally efficient to the functions of modulation and demodulation.

· It can also be used for DAB systems and partial algorithms can be used for program selection.

· A channel estimator can easily be discarded with the use of differential modulation.

· As compared to single carrier systems OFDM is less sensitive to sample timing offset.

· OFDM gives extra protection concerning parasitic noise and co channel interference.

· In severe multipath orthogonality is preserved.

· OFDM is used in high speed applications and dynamic packet access is also supported.

· Transmitting and receiving diversity are supported. On the other hand OFDM also supports adaptive antenna arrays, space time coding and power allocation.

3.6 DISADVANTAGES OF OFDM

· The OFDM signal has a noise like amplitude with a very large dynamic range, therefore it requires RF power amplifiers with a high peak to average power ratio.

· It is more sensitive to carrier frequency offset and drift than single carrier systems.

3.7 PROBLEMS WITH OFDM

3.7.1 Peak To Average Ratio

PAR is an important OFDM parameter which is defined as the ratio of peak instantaneous value to average time. It can also determine parameters such as current, voltage, phase and power of the signal. Since OFDM is a summation of several carrier signals therefore results in high PAR. The RF power needs to be increased to overcome the problem of efficiency in PAR. In order to increase the radio frequency power an amplifier is needed which can increase the cost of the system as it is expensive equipment.

In order to solve the problems created by PAR, different encoding schemes should be used before the modulation. Also the improvement in the amplification stage of transmitter is needed such as post processing the time domain signal to reduce the peak to mean signal ratio. [21][22]

3.7.2 Synchronization

The performance of OFDM link can be optimized by using two kinds of synchronizations between transmitter and the receiver.

· Timing Synchronization: The timing offset of the symbol is not need to be determined and then the optimal timing instants.

· Frequency Synchronization: The carrier frequency of the received signal must be aligned at the receiving end.

Timing sync can easily be achieved because the degree of sync error in OFDM structure is more severe. The sync techniques can be achieved by using known pilot tones that are embedded in OFDM signal or by using guard interval. [21][22]

3.7.3 Co-Channel Interference

In mobile communications co channel interference can be overcome by combining techniques related to adaptive antenna systems. Receiver antenna beam can be focused by beam steering while co channel interferer’s are attenuated. This is useful as OFDM is sensitive to co- channel interference. [21][22]

3.8 APPLICATIONS OF OFDM

· High frequency modems used for military

· Voice band modems

· ADSL

· HDSL

· DAB

· Terrestrial Digital Video Broadcasting (DVB-T)

· Power line communication systems

· WLAN

· Cable modems

· Wavelength Division Multiplexing

CHAPTER 4: METHODOLOGY

4.1 INTRODUCTION

This chapter includes the in depth study of the models built on MATLAB/SIMULINK. MATLAB version 7.8.0 (R2009a) is used for the modelling. Basically two models are designed whic