A Report On Transmission Systems Communications Essay

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1.0 Introduction

Radio over Fibre (RoF) system transmit the signal through fibre for facilitate the wireless network. The development of mobile and wireless devices increases the demand and now a day it is the integral part of the society. To fulfill the demand of the user the service provider receiving pressure to increase the system capacity. The Radio Frequency (RF) signals get around is by something called waveguides [1]. In waveguide RF signals passes from one point to another as invisible waves through a pipe with a rectangular cross section. Though this technology is older and very expensive, it is still used for special case. For these reasons the waveguide technologies are not used so frequently.

In modern communication systems RF and microwave signals used to transmit wireless transmission of information [2]. The light signal in RoF technology is modulated by radio signal. The development of feeder network and raising the carrier frequency can be the better solution but it increases the cost. The technology radio-over-fibre (RoF) seems to be cost effective because it reduces the cost and simplifies the remote sites. The costly radio equipments installed at the Switching Centres (SC) or Central Sites/Stations (CS) and enhanced the sharing capability [11].

2.0 Fibre Optic Cable

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In the recent years fibre optic cables are mostly using for transmission communication signal instead of copper cable. For transmission data, electronic pulses are transmitted through copper wires, but in fibre optic information transmitted as light pulses. Fibre optic cable can transmit information to long distance with lowest attenuation and highest transmission speed. The electronic pulse coming from the copper to fibre, first this signal converted to light signals. For generating the light pulse, Light Emitting Diodes (LED) or laser can be used and funnelled into optical fibre medium.

The core of optical fibre can be made of plastic or glass. Silica is the main material of the fibre but chalcogenide, fluoroaluminate and fluorozirconate are used for special applications. For commercial use there are some standard wavelengths to ensure the equipment from different manufactures is compatible. The standard wave length, where the performance is better is called windows [4]. There are three windows, 850nm, 1300nm and 1550nm. The 850nm wave length has maximum loss and used short distance transmission up to 10 km because less expensive and easy to install. Due to lower loss and higher bandwidth, 1300 nm and 1550 nm wave lengths are used for long distance communications.

2.1 Types of Fibre

2.1.1 Single Mode Fibre Optic cable

Single mode fibre optic cable has a core diameter of 8.3 to 10 microns with propagation of 1310 nm or 1550 nm wave lengths. With compare to multimode fibre optic, single mode can transfer long distance and higher bandwidth but needed narrow spectral light sources. One single mode cable can carry data at multi frequency. For the lowest attenuation and highest transmission speed, single mode fibre optic is used for long distance communications. Wave Division Multiplexing (WDM) is used to send data over single mode in many applications.

2.1.2 Multimode Fibre Optic Cable

The multimode fibre optic has bigger core than single mode and the core diameter is 50 to 100 micron but 62.5 micron is commonly used.

The light waves passes through numerous paths with 850 nm for short distance or 1300nm wave lengths for long distance. By using multimode fibre optic cable medium distance can covered. For the medium distance up to 2 km communication multimode is suitable with high bandwidth and high speed.

3.0 System Design Factors

For the high quality transmission, so many factors are to be considered. The following factors and considerations should be taken for the better transmission systems through fibre optic.

Table-2 Factors for Evaluating Fibre Optic System Design [5]

Sl. no

System Factor

Consideration/Choice

1

Fibre type

Single mode or Multimode

2

Dispersion

Regenerators or Dispersion Compensation

3

Fibre Nonlinearities

Fibre Characteristics, Wavelengths and Transmitter Power

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4

Operating Wavelength

850, 1310, 1550 and 1625 nm

5

Transmitter Power

Decibel (dBm)

6

Type of Source

LED or Laser

7

Sensitivity of the Receiver

Decibel (dBm)

8

Type of Detector

PIN Diode, APD or IDP

9

Modulation Code

AM, FM, PCM or Digital

10

BER-Bit Error Rate

10-9 and 10-12

11

Signal-to-Noise Ratio (SNR)

Decibels (dB)

12

Number of Connectors

Loss increases with connectors

13

Number of Splices

Loss increases with splices

14

Environmental Requirements

Humidity, Temperature, Exposure to sunlight

15

Mechanical Requirements

Flammability, Indoor/outdoor Application

4.0 Radio over Fibre Technology

The radio frequency signal or microwave electrical signals in communication systems are transmitted over fibre optic cable and technology. The main properties of a radio over fibre, is allow high bandwidth for simultaneous transport of a wide range of air interface standard [9]. There are Central Site (CS) and Remote Site (RS) for radio over fibre systems. By RoF system high frequency is transmitted over fibre link. In Global System for Mobile (GSM) application network, the CS is called Mobile Switching Centre (MSC), RS is the Base station (BS) and for wireless Local Area Networks (WLAN), the CS is called headend and Radio Access point (RAP) acts as the Remote Sites (RS) [11].

In figure-5 it is shown that the microwave signal modulated first and applied to the input to the radio over fibre system. Then the high frequency millimeter wave (mm-wave), microwaves and low frequency signal passes through fibre as optical signals. The microwave signal modulation, processing and frequency conversion are radio systems functionalities. The baseband data and modulated IF or RF signals transmitted as electrical signal [11]. For modulate the optical source the electrical signal is used and sending through fibre optic to the destination area. In the receiver end the data is again converted to electrical signal as according to the wireless operator's specifications like GSM, UMTS and WLAN. Like macro diversity for seamless handover, the RoF installed and managed the high frequency expensive equipment at the main location and simple in remote sites.

5.0 Advantages of Radio over Fibre Technology

5.1 Enormous Potential Bandwidth

The optical fibres have a potential bandwidth with frequency range 1013 to 1016 Hz, whereas the bandwidth of coaxial cable is 500MHz. The bandwidth of fibre is not fully used. Due to the capability of information transfer the fibre is the superior, for transmitting microwave signals. The high bandwidth offers better signal processing which is impossible in electronic systems. It also supports filtering, mixing and up-down conversions. Mach Zehnder Interferometer (MZI) is used for filtering electrical signal to optical signal. Several optical signals with different wavelengths can be transmitted same time through same fibre. On the basis of nature optical fibre windows can categorize in to three broad parts- 850nm, 1310nm and 1550nm wavelengths which offer low attenuation. The optical fibre has low dispersion. Due to high gain, large bandwidth and low noise Erbium Doped Fibre Amplifier (EDFA) used 1550nm window and Optical Time Division Multiplexing (OTDM) with Dense Wavelength Division Multiplex (DWDM) techniques are used for multiplexing.

5.2 Low Attenuation Loss

Transmission of high frequency microwave electrical signals in free space or transmission lines is problematic and costly [11]. Regenerator equipment is needed for long distance electrical transmission because impedance rises with the frequency. The base band signals or low intermediate frequencies (IF) can be transmit to base station via central station and to perform better each base station needs own oscillator. Optical fibre shows very low attenuation or transmission loss with compare to the copper conductor. Fibres have been fabricated with losses as low as 0.2 dB per km [3]. The losses are too low with compare the conventional copper wire transmission for the high frequency. The fibre can transmit long distance without intermediate repeaters; it reduces the required transmission power.

5.3 Immunity to Frequency Interference and Crosstalk

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The transmitted signal passes as light signal, so it is free from electromagnetic interference (EMI) and radio frequency interference (RFI) which is an important property of fibre optic communications. Because of immunity, optical fibre provides more security and privacy.

5.4 Less Power Consumption

Most of the high frequency expensive and complex equipment of radio over fibre have setup in the main location and simple in remote sites, it reduces the equipments and cost for the practical uses the antenna sites are operated in passive mode, 5 GHz fibre radio systems employing very small pico cell can have the RSs (BSs) operate in passive mode [11]. So the power consumption for the RS is reduces.

5.5 Abundant Raw Materials and low cost

The silica is the principle material of which optical fibre are made [7]. The silica is available in ordinary sand and inexpensive. The actual price is depends on how this raw materials fabricated and it offer low cost communication compare with copper.

5.6 Easy to Operate and Maintenance

The fibre optic has lower loss and for the long distance communication it reduces the intermediate repeaters for amplifying the transmitted signal. Due to the reduce numbers of repeaters the installation and maintenance is easier with compare with the conventional copper wire based systems. In radio over fibre systems the expensive and complex modulation and switching equipments installed at the centre position and the remote stations has the simple. Both switching centre and remote stations managed easily with low cost and short time. Multi-operator and multi-service traffic can be transmitted through the same network and modulation, switching and other technical part done in switching centre, it reduces the cost. The GSM traffic can be controlled during the peak and off peak hours.

5.7 Millimetre Waves

Milimetre wave application is another important because it cannot be distributed through electrically due propagation loss. In fibre low attenuation loss and large bandwidth reduces the electronic bottleneck for generation of mm-wave in electrical systems. It makes the cost effective. The micro and pico cells small sizes radio systems are used to cover a large number of users but it needed numerous base stations.

6.0 WLAN Technology

A Wireless Local Area Network (WLAN) is the addition to LAN and feed from it. In WLAN radio frequency and Infrared (IR) are used as transmission technology. The Network Interface Adapter (NIA) links between mobile device and wireless LAN and base station links between wired and wireless LAN. The radio Access Point (AP) acts as a bridge of them.

The IEEE 802.11 wireless LAN speed is 2Mbps and 802.11b up to 11 Mbps with 2.4 GHz band. The product compatible with 2.4 GHz are available in market and 5GHz devices will be available soon. The European Telecommunication Standards Institute (ETSI) used OFDM with 54Mbps data rates. The High Performance Local Area Network type2 (HIPERLAN/2) is ESTI standard. The following table compare the wireless LAN-

Sl

Standard Parameter

IEEE 802.11b

IEEE 802.11a

HIPERLAN/2

1

Operating Frequency Bands

2.4 - 2.4835 GHz

2.471-2.497 GHz

5.150-5.350 GHz

5.725-5.825 GHz

5.150-5.350 GHz

5.470-5.725 GHz

2

Data rates vs. range

11 Mbps (60m)

2 Mbps (100m)

24 Mbps (30m)

6 Mbps (60m)

(Max. 54 Mbps)

Same

3

Modulation vs. Data rate

DBPSK (1 Mbps)

DQPSK (2 Mbps)

BPSK (6, 9 Mbps)

QPSK (12, 18 Mbps)

16-QAM (24, 36 Mbps)

64-QAM (54 Mbps)

Same

4

Occupied Bandwidth

26 MHz

(per logical channel)

16.6 MHz (per carrier)

Same

5

Allocated bandwidth

83.5 MHz

300 MHz

455 MHz

7.0 System Requirements for RoF

7.1 Bandwidth Requirements

The optical fibre has enormous bandwidth which is sufficient to meet the present requirements. To get the better performance the capacity of feeder network must be increased. From the table-3 it is seen that the IEEE 802.11a and HIPERLAN/2 wireless LAN has maximum bandwidth 455 MHz, so it can transmit up to this value. Presently OFDM uses 5GHz and for the future need more bandwidth (60 GHz).

7.2 Modulation Format

The radio signal can be transmitted over fibre many ways, modulate the light intensity of the optical source directly by the radio signal is the easiest way. At remote antenna site, photodector and bandpass amplifier converts the received optical signal to radio signal to be radiated by the antenna [8]. The radio over fibre must have the appropriate modulation technique. The common modulation techniques are Phase Shift Keying (PSK) and Quadrature Amplitude Modulation (QAM) and the future modulation or multiplexing techniques is OFDM. The following table shows the parameters of this.

Table-4 Main Parameters of the OFDM standard [11]

Sl. No

Items

Specifications

1

Data Rate (Mbps)

6, 9, 12, 18, 24, 36, 48, 54

2

Modulation

BPSK, QPSK, 16-QAM, 64-QAM

3

Coding Rate

½, 2/3, 3/4

4

Number of Sub-Carriers

52

5

Number of Pilots

4

6

OFDM Symbol Duration

4 µs

7

Guard Interval

800 ns

8

Sub-Carrier Spacing

312.5 kHz

9

-3 dB Bandwidth

16.56 MHz

10

Channel Spacing

20 MHz

7.3 Microwave Generation Technique

Radio over fibre is capable of high frequency carrier applications like 60 GHz and multimode fibre based systems are suitable. For operate higher bandwidth new system Polymer Optical Fibre (POF) needed and capable to generate microwave carrier beyond 5 GHz.

8.0 Applications of Radio over Fibre Technology

The main applications of radio over fibre are mobile radio communications, satellite communications, Mobile Broadband Systems (MBS), broadband access radio, wireless LANs, Multipoint Video Distribution Services (MVDS), vehicle communications and control [11].

8.1 Mobile Networks

The demand of broadband services rapidly and the mobile operators have pressure to increase the capacity. To fulfill the requirements RoF technology should be the solution. For the traffic control in mobile network like GSM and UMTS, this technology can be used.

8.2 Satellite Communications

The radio over fibre technology implemented some communication systems, but most common and first in satellite. In the satellite earth stations, small optical fibre link operated between 1GHz and 15GHz for remoting the antennas. To development and controlling the earth station radio over frequency is used and in this case the antenna placement may be outside the control area.

8.3 Video Distribution

Radio over fibre is also used in Multipoint Video Distribution Services (MVDS), which is a cellular terrestrial transmission system for video (TV) broadcast [11]. For the small area coverage MVDS with allocated band 40 GHz, can transmit and a portion can be received. More stations are required for coverage; otherwise the cell size is 5 km.

8.4 Mobile Broadband Service

Another important service of radio over fibre is Mobile Broadband Service (MBS) in which collaborates with Broadband Integrated Services Digital Network (B-ISDN). The future B-ISDN services supported by MBS and with 155 Mbps bit rates. The micro cell diameter size is 100m and 65-66 GHz and 62-63 GHz are allocated for uplink and downlink respectively. For the desired coverage, high density radio cell required and connected to the fixed B-ISDN network by optical fibre [11]. The base station is simple to generate mm-waves by using radio over fibre technology, which made the systems economically feasible.

8.5 Wireless LANs

The demands of mobile or portable user are increasing day by day, which created pressure to mobile broadband access to LANs with higher carrier frequencies. The IEEE 802.11b wireless LANs have maximum capacity of 11 Mbps and operated at 2.4GHz bands. Next generation broadband wireless LANs primed to 54Mbps per carrier and required 5 GHz band (IEEE802.11a) [11]. With micro and pico cells coverage RoF is the cost effective solution. For transmission 60 GHz from base station with data, this data to mm waves by oscillator frequency. It has great impact on base station design.

8.6 Vehicle Communication and Control

The vital application of radio over fibre is vehicle communication and control in Europe at 63-64 GHz and 76-77 GHz dedicated frequencies. Road-to-Vehicle Communication (RVC) and Inter-Vehicle Communication (IVC) are the two major applications of Intelligent Transport Systems (ITS) for instant mobile communication. By using ITS a cost effective and manageable system can be developed for traffic information, improve transportation efficiency and better environment.

9.0 Conclusion

The radio over fibre technology transmit radio frequency from the central station to the remote station through fibre link network. The RoF system the signal processing functions and modulation done in central location. Fibre optic cable can transmit information to long distance with lowest attenuation and highest transmission speed. The bandwidth, attenuation and wavelength should be considered for design the optical communication link. The optical fibre future developments are multi terabit transmission, transparency, optical network and technology refinements. Due to involvement of analogue modulation and transmission systems, noise and distortion are involved in the transmission systems. In the analogue link, thermal noise, dispersion and phase noise are the important factors to consider the link design. Chromatic dispersion and modal dispersion limits the fibre link in single mode and multimode respectively. In the digital systems Quadrature Amplitude Modulation (QAM) and Orthogonal Frequency Division Multiplexing (OFDM) are used. The QAM is a multi modulation format and frequently used in WLAN and UMTS. It also support ASK, BPSK and QPSK modulation format and also capable to transmit the mm-wave high frequency and multi carrier with OFDM multiplexing. A radio over fibre system is a cost effective and easy to maintenance systems. By using radio over fibre technology, a long distance optical signal seems very closer to the user.

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