Need of fiber optic communication

  • Fiber optics deals with the learning of propagation of light through the transparent dielectric wave-guide.
  • Fiber optic system currently used extensively as the transmission line between terrestrial hardwired systems.
  • The fiber optics are used for transmission of data from point to point location, So its very accurate result in fiber optic system.
The fiber Optic Communication system has emerged most important fiber optic communication system. This article is very helpful for daily need of fiber optic communication compared to the traditional system because of following requirements :
  • In the long-haul transmission system, it is need of low loss transmission medium
  • It is need of compact and least weight transmitters and receivers
  • There is need for increased span or area of transmission
  • There is need for increased bit rate-distance product
A fiber optic communication system fulfills these requirements, hence most widely accepted. So most of the telephone company are currently using optical fiber communication for long distance communication and many more advantages of optical fiber communication.

Evolution of fiber optic system

  • Fiber optics deals with the learning of propagation of light through the transparent dielectric waveguide. Fiber optic system currently used extensively as the transmission line between terrestrial hardwired systems.
  • The fiber optics are used for transmission of data from point to point location, so its very accurate result in fiber optic system, Now a day there are many advance technology upgraded in fiber optic system.
  • Optical fiber are very helpful for daily need of fiber optic communication compared to the traditional system because of the long-haul transmission system, as well as low loss transmission medium
  • This article is very helpful for how a new technology upgraded in the year of 1975 to 2015. So now let us talk about the evolution of fiber optic system.
1. First generation :

 The first generation of lightwave systems uses HaAs semiconductor laser and the operating region was near 0.8 micrometers. Other specifications of this generation are as under :
  • Bitrate  : 45 Mb/s
  • Repeater spacing: 10 km
2. Second generation :
  • Bit rate : 100 Mb/s to 1.7 Gb/s
  • Repeater spacing : 50 km
  • Operating wavelength : 1.3 micro meter
  • Semiconductor : In GaAsP
3. Third generation :
  • Bit rate : 10 Gb/s
  • Repeater spacing : 100 km
  • Operating wavelength :  1.55 micro meter
4. Fourth generation :

 The fourth generation uses WDM techniques.
  • Bit rate : 100 Tb/s
  • Repeater spacing : >10000 km
  • Operating wavelength: 1.45 to 2.62 micrometer
5. Fifth generation :

 The fifth generation uses raman amplification techniques and optical solitiors.
  • Bitrate : 40-60 Gb/s
  • Repeater spacing: 24000 km – 35000 km
  • Operating wavelength: 1.53 to 1.57 micrometer

LED structures

Light emitting diode (LED) is a component that converts the electrical signal into a corresponding light that be injected into the fiber. Basically, the light emitter is a key element in any fiber optic system. Essentially LED is a PN junction diode.

Heterojunctions


A (heterojunction) is an interface between two adjoining single-crystal semiconductors devices with a different bandgap.

Heterojunctions are of two types, Antisotype ( p-n ) or,  Isotype (n-n or p-p)

Double Hetero-junctions (DH) 


In order to achieve efficient confinement of emitted radiation double hetero-junctions are used in LED structures. In double hetero-junction, the crosshatched regions represent the energy levels of free charge recombination occurs only inactive InGaAsP layer.

A hetero-junction is a junction formed by dissimilar semiconductors. Double heterojunction ( DH) is formed by two different semiconductors on each side of the active region. Figure shows double heterojunction (DH) light emitter.

Double heterojunction emitter ( DH )
The two materials have different refractive indices and different bandgap energies. This structure can change in bandgap energies create the potential barrier for both holes and electrons and also some free charges can recombine only in narrowband, well defined active layer side.

A double hetero-junction structure will confine both holes and electrons are narrow active layer. So in DH junction under forwarding bias, it will be a large number of carriers injected into active regions where they are efficiently confined. One of the most advantages of LED serves to narrow the output beam.

Advantages of LED structure :

  • Carrier recombination occurs in a small region 
  • light emission occurs in an optical waveguide, which serves to narrow the output beam.

Laser Diodes

  • The laser is a basic device which amplifies the light, hence the LASER full form is an acronym for light amplification by stimulated emission of radiation.
  • The operation of the device may be described by the formation of an electromagnetic standing wave within a usage a cavity (optical resonate) which provides an output of monochromatic highly coherent radiation.
  • There are different types of LASER used like single-Mode Lasers and Modulation of Laser Diodes, depends on which types of application is used.
  • The output of laser diodes characteristics is depends on the drive current passing through it. In laser diode operating at low drive current, it operates as an inefficient LED full form when driving current crosses threshold value and lasing action begins.
  • An injection laser is also known as a laser diode or diode laser. It is a semiconductor device, Nowadays it can be used many application or devices like optical fiber, compact disk, remote control device etc.
  • In LASER diode the power current characteristics, the threshold current only spontaneous emission is emitted hence there is a small increase in optic power with drive current.

Principle :

  • The material absorbs light rather than emitting. Three fundamental processes occur between the two energy states of an atom. 
1. Absorption 
2. Spontaneous emission 
3. Stimulated Emission. 
  • Laser Action is the result of three process absorption of energy packets (photons) spontaneous emission, and stimulated emission. (These processes are represented by the simple two-energy-level diagrams) 
Where,
  • E1 is the lower state energy level.
  • E2 is the higher state energy level. 
  • Quantum theory states that any atom exists only in certain discrete energy state, absorption or emission of light causes them to make a transition from one state to another. The frequency of the absorbed or emitted radiation f is related to the difference in energy E between the two states.
  • If E1, is lower state energy level and E2, is higher state energy level.
E = E1 - E2 = h.f.
You may also know advantages and disadvantages of laser diode know more details about laser diode.

Advantages of Laser Diode

  • Simple economic design.
  • Better modulation capability.
  • It gives high optical power.
  • Laser diode is smaller size as compared to other types of laser.
  • Production of light can be precisely controlled.
  • Ability to transmit optical output powers between 5 and 10 mW
  • Low spectral width (3.5 nm).
  • Laser diode has high coupling efficiency.
  • It can be used at high temperatures.
  • Laser diode is the cheaper device to produce laser output.
  • Ability to maintain the intrinsic layer characteristics over long periods.

 Disadvantages of Laser Diode 

  • A speckle pattern appears as two coherent light beams add or subtract their electric field depending upon their relative phases at the end of the fiber.
  • In laser diode use of large drive current produces unfavorable thermal characteristics and necessitates the use of cooling and power stabilization.
  • It produces more divergent laser beam
  • Expensive
  • Poorly absorb in hard tissue and hydroxyapatite

Advantages and disadvantages of LASER diode

The terms stand for LASER full form Light Amplification by Stimulated Emission of Radiation. It can produce coherent radiation in the visible or in an infrared spectrum when current passes through it. It can be used in optical fiber systems, compact disk players and also some remote control devices. Here this post gives information about the advantages and disadvantages of LASER diode to know more about it.

Advantages of LASER diode :

  • Simple economic design
  • It has low power as compared to other types of laser diodes
  • Better modulation capability
  • The laser diode has a high coupling efficiency
  • It can be used at high temperatures
  • It gives high optical power
  • In this diode cheaper device to produce laser output  
  • It has a smaller size as compared to other types of laser diodes
  • Production of light can be precisely controlled
  • Laser diode to transmit optical output powers between the range of about 5 and 10 mW
  • Low spectral width (3.5 nm)
  • Compact
  • It is easily manufactured in arrays
  • The delivered system not as expensive as hard tissue laser diode
  • Ability to maintain the intrinsic layer characteristics over long time periods

Disadvantages of  LASER diode :

  • It produces a more divergent laser beam
  • They require big and costly optics for a large source of size
  • It has a critical heating problem
  • Hight drive current to drive the large laser for pellets
  • Expensive
  • A speckle pattern appears as two coherent types of light beams to added or subtracts their electric field depending upon their relative phases at the end of the fiber optic device
  • Poorly absorb in hard tissue and hydroxyapatite
  • Current produces unfavorable thermal characteristics 
  • Necessitates the use of cooling and power stabilization

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Features of LED

Nowadays the light is collected from the edge of the full form of LED, in order to reduce the losses caused by absorption in the active layer and to make the beam more directional. Such a device is known as edge-emitting LED or LED. This article informs to know about more features include in ELDE and how to use LED in daily life.

Features of LED : 
  • A linear relationship between optical output and current. 
  • While we are using LED, Spectral width is around 25 to 40 nm or lambda is equal to 0.8 - 0.9 µm.
  • The modulation bandwidth is much large. 
  • Not affected by catastrophic gradation mechanisms hence LED is more reliable. 
  • Better coupling efficiency than the surface emitter.
  • Less temperature is sensitive. 
Usage :
  • It is mostly used for short-range narrow and also used medium bandwidth links. 
  • Long-distance analog links.
  • Suitable for digital systems up to 140 Mb/sec.

Advantages and disadvantages of LED

The term LED full form stands for a light-emitting diode is used in suited for short-range narrow and medium bandwidth links. LED can be suitable for the digital system technology up to 140 Mb / sec. Here this post gives information about the advantages and disadvantages to know more about LED.

Advantages of LED :

  • LEDs produce more light per watt
  • LED is more efficient 
  • Lower initial cost
  • Simple design
  • LED doesn't change their light tint as the current passing through them is lower 
  • Ease of manufacture
  • Simpler fabrication 
  • Simple drive circuitry
  • Simple system integration
  • Linear light output against current characteristics.
  • High reliability
  • Less temperature dependent
  • LED will achieve full brightness in few microseconds
  • LED does not contain mercury, unlike compact fluorescent lamps
  • Excellent CRI ( Color rendering index )
  • Environmental friendly
  • Quick turn ON and OFF
  • Good adaptability to coherent laser operation
  • No warm-up period 
  • Not affected by cold temperature 

Disadvantages of LED :

  • Refraction of light at semiconductor/air interface
  • The average lifetime of radiative recombination is only a few nanoseconds, therefore, modulation BW is limited to only a few hundred megahertz
  • Low coupling efficiency-usually low optical power coupled into the fiber (W)
  • Large chromatic dispersion
  • Driving LED hard in ambient temperature may result in overheating of the LED package, eventually leading to device failure
  • Incoherent light source
  • Spontaneous emission and non-linear output characteristics
  • Little effective in a wide area

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Number of channel in TDMA system

The number of channel slots that can be provided in a TDMA system is found by multiplying the number of TDMA slots per channel by the number of channels available.

N = m (Bt -2 Bguard) / Bc

m = Maximum number of TDMA users supported on each radio channel 
Bc =  Channel bandwidth
Bt = Total specturm allocation
Bguard = Guard band allocated at the edge of allocated spectrum

Frame structure of GSM

Each GSM user transmits data burst during a time slot that is assigned to it. The transmission of bursts occurs in a particular format called the GSM frame.

There are five types of data bursts for controlling the traffic bursts.

Normal burst :

The normal burst is used for TCH and the DCCH transmissions on the forward and reverses link. It consists of 148 bits that are transmitted at a rate of 270.83 Kbps. Only 114 bits are information bits that are transmitted as two sequences of 57 bits.

FCCH burst :

It consists of 3 start bits 3 stop bits with 142 bits between the start bits and stops bits. The stop bits are followed by a guard period of 8.25 bits.
RACH burst : 

 It consists of 8 start bits and 3 stop bits. It 41 bits for synchronization of transmitter and receiver, 36 bits of encrypted data an extended guard period of 68.25 bits.

SCH burst : 

It consists of 3 start bits, 39 bits of encrypted data, 64 training bits, 3 stop bits and a guard period of 8.25 bits.
                 
Dummy burst : 

 It consists of 3 starts and 3 stop bits. It contains two parts of 58 mixed bits that are separated by 26 training bits and a guard period of 8.25 bits.

The frame structure of GSM :



GSM frame structure 

TCH in GSM

TCH stands for a traffic channel in GSM.
This article describes the GSM traffic channel. Traffic channel carries digitally encoded user data or user speech on the forward and reverse link.
TCH support two types of information rates, they are
  • Full rate ( TCH/F )
  • Half rate ( TCH/H )
TCH is a group of 26 consecutive TDMA frame, called multi-frame of the 26 TDMA frames, 24 are used as TCH frames, one frame (thirteen ) is for SACCH and 26th is the unused or idle frame.
TCH data transmitted in 26 consecutive frames

1. Full rate TCH  (TCH/F)

Full rate speech channel  (TCH/FS) - At 13 Kbps the full rate speech channel is digitized. After channel coding, full rate speech channel carries 22.8 Kbps.

Full rate data channel (TCH/F9.6), Full rate data channel (TCH/F4.8), Full rate data channel (TCH/F2.4) channel carries the data at the speed of 9.6 Kbps, 4.8Kbps, 2.4Kbps respectively.

After channel coding is applied is converted to data transferred rate at up to 22.8 Kbps.

2. Half rate TCH (TCH/H)


Half rate speech channel (TCH/HS) - At 6.5 Kbps the half rate speech channel is digitized. After channel coding, half rate speech channel carries up to 11.4 Kbps.

Hull rate data channel ( TCH/H4.8), Full rate data channel (TCH/H2.4) channels carry the data at the speed of 4.8 Kbps, 2.4 Kbps respectively.

After channel coding is applied is converted to data transferred rate at up to 11.4 Kbps.
Explore more information:

  1. CCCH in GSM
  2. BCCH in GSM
  3. AGCH in GSM
  4. FCCH in GSM
  5. RACH in GSM
  6. DCCH in GSM
  7. SACCH in GSM
  8. SDCCH in GSM
  9. FACCH in GSM

CCCH in GSM

CCCH stands for the common control channel in GSM.

GSM  common control channel ( CCCH ) is used for conveying from network to the mobile subscriber and provide access to the mobile subscriber,

There are three types of different channels  : (I) PCH, (II) AGCH, (III) RACH

Refer the following link to know details of the PCH, AGCH, RACH channel, and their processing.
  • GSM Paging channel ( PCH )
  • GSM Access grant channel ( AGCH )
  • GSM Random access channel ( RACH )

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BCCH in GSM

BCCH stands for the broadcast control channel. This article learns to GSM broadcast control channel.

The BCCH  is a forward channel and also is a unidirectional base to mobile channel that provides information about the network, the cell in which the mobile is currently located and adjacent cells. BCCH is transmitted in 51 frames multi frame structure in a frame next to SCH on TS0. It broadcast a list of the channel that is currently in use within the cell.

The BCCH includes two channels, they are : (I) FCCH  (II) SCH

Refer the following link to know details of the PCH, AGCH, RACH channel, and their processing. 
  • Frequency correction channel ( FCCH )
  • Synchronization Channel ( SCH )

FCCH in GSM

FCCH stands for the Frequency correction channel in GSM.
It is a base to mobile channel that provides information for carrier synchronization. It is a special data burst that occupies the TS0 slot for the first GSM frame. It is repeated every ten frames within a control channel multi-frame.


The FCCH allows the mobile subscriber to synchronize its internal frequency standard with the frequency of the base station.


In the first burst of FCCH, all zero bits are sent to indicates the unmodulated carrier.

FCCH burst structure

AGCH in GSM

AGCH stands for Access grant channel in GSM. 

The AACH is used by the base station to provide forward link communication to the mobile. It carries data that instructs the mobile to operate in a specific channel with a particular dedicated control channel.

It is the message sent by the base station before a subscriber is moved off the control channel.

The AGCH is used by the base station to respond to RACH sent by the mobile station in the earlier CCCH frame. 

RACH in GSM

RACH stands for Random access channel in GSM.

It is a reverse link channel. With the help of RACH, the mobile station originates a call, send signaling message when not on call, acknowledges message.

It uses a slotted ALOHA access method. All mobile must access or respond to a PCH alert within TS0 of a GSM frame.

At the BTS each frame will accept the RACH transmissions from the mobiles during TS0. The base station responds to the RACH transmission by allocating a channel and a stand-alone dedicated control channel for signaling in the presence of a call. The connection is confirmed by the BS full form ( base station ) over the access grant channel.

Random access burst

DCCH channel in GSM

DCCHs stands for dedicated control channels in GSM. 

The dedicated control channel comes into the picture after a call is established. The dedicated control channels are bidirectional. They have the same formats and function in the forward and the reverse links.

Refer the following link to know details of the PCH, AGCH, RACH channel, and their processing.
  • GSM Fast associated control channel ( FACCH )
  • GSM Slow associated control channel ( SACCH )
  • GSM Stand dedicated control channel ( SDCCH )

SACCH in GSM

SACCH stands for Slow associated control channel GSM. 

It is related to the traffic channel or SDCCH.

On the forward link, it is used to send regularly changing information like: 
  • Transmit power channel 
  • Broadcast messages
  • Specific timing advance instruction for each user
On the reverse link, it carries information about 
  • Received signal strength 
  • Quality of TCH and BCH
  • Measurement results from neighboring cells
  • It is used to inform the base of power measurements made by the mobile of signal strength in the adjacent cells
  • It is transmitted during the total 13th frame of every dedicated control channel multi-frame
  • SACCH exchanges control information between the base station and mobile station a call or call set up

SDCCH in GSM

The full form of SDCCH is Stand-Alone Dedicated Control Channel GSM. 

The Standalone dedicated control channel is allocated for every mobile station.

It contains the signaling data that follows the connection of the mobile with the base station, prior to the allocation of TCH by the base station ( BS ) to the MS.

It guarantees that the mobile station, as well as the base station, will remain connected. The base station and mobile switching center will verify the subscriber unit and assign the needed resources to the mobile.

This channel that accepts the newly completed call from the BCH. It holds the traffic while waiting for the base station to assign a TCH channel.

The SDCCH is also used to transmit authentication and alert messages as the mobile synchronize itself with the frame structure and wait for TCH.

SDCCH can be assigned its own physical channel. Also, they can occupy the TS0 slot of the BCH if there is a low demand for BCH or CCCH traffic. 

FACCH in GSM

FACCH full form is Fast associated control channel.

 FACCH carries important messages. It is used for the exchange of time-critical information between the mobile and the base station during the process of the cell.

 The FACCH  is assigned whenever SDCCH is not dedicated to a user and there is an urgent message like a handoff request.

 The FACCH was transmitted control information by stealing capacity from the associated traffic channel. It is done by setting two special bits called stealing bits. In the TCH forward channel burst.

 If a stealing bit is set, the time slot contains FACCH data for that frame.

Synchronization channel

This channel acts as a base to the mobile channel. The mobile channel carries the information for identification and frame synchronization of the base station transceiver.

It also comprises the frame number in relation to the hyper frame and the BASIC ( Base station identity code ).

In the frame that after the FCCH, in time slot TS0 BTS transmit with SCH. The SCH has a unique burst structure. It comprises of 64-bit binary sequence that is common throughout the GSM networks.

The BASIC is assigned to each BTS in the GSM system. The SCH is transmitted once every ten frames within the control channel multi-frame.
Synchronization burst structure

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