What is light emitting diodes

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

A basically conventional p-n junction is called as homojunction as the same semiconductor material.

The electron-hole recombination occurs in a relatively wide layer = 10 micrometers. As the carriers are not confined to the immediate vicinity of a junction, hence high current densities cannot be realized.

The carrier confinement problem can be resolved by sandwiching a thin layer of 0.1 micrometers between the p-type and n-type layers. 

 When the carrier confinement occurs due to bandgap discontinuity of the junction. Such a junction is called heterojunction and its device is called a double heterostructure. 

In an optical communication system when the requirement is an LED is 

1) In LED bit rate of 100-200 Mb/sec.
2) In LED optical power in tens of microwatts. 

LED Structures


Heterojunctions


A (heterojunction) is an interface between two adjoining single crystal semiconductors device with different band gap.

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. The figure shows double heterojunction (DH) light emitter.


Double heterojunction emitter ( DH )
The two materials have different refractive indices and different band gap energies. The changes in band gap energies create the potential barrier for both holes and electrons. The free charges can recombine only in narrow, well defined active layer side.

A double hetero-junction (DH) structure will confine both electrons and holes a narrow active layer. Under forward bias, it will be a large number of carriers injected into the active region where they are efficiently confined. 

Main LED materials or Type :


  • Gallium Arsenide ( GaAs ) - infra-red
  • Gallium Phosphide ( GaP ) - Red, yellow and green
  • Aluminum Gallium Phosphide ( AlGaP ) - Green
  • Gallium Indium Nitride ( GaInN ) - near ultraviolet, bluish-green and blue
  • Aluminium Gallium Arsenide Phosphide ( AlGaAsP ) - High brightness red, orange-red, orange and yellow
  • Silicon Carbide ( SiC ) - Blue as a substrate
  • Gallium Nitride ( GaN ) - green, emerald green
  • Zinc Selenide ( ZnSe ) - Blue
  • Aluminium Gallium Nitride ( AlGaN ) - Ultraviolet

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
  • Smaller Size
  • Light in weight
  • Longer lifetime
  • Operate very fast
For detailed information

Application of LED diode :

  • Sensor application
  • Mobile application
  • Sign application
  • Indicator
  • Remote  control
  • Optoisolator
  • LED signal
  • Illumination
  • Automotive uses
For detailed information
Read more >> Application of LED

Characteristics of Light Source for Communication

A light source needs the following characteristics is given below :

Characteristics of Light source for communication:

  • It must be possible to operate the device continuously at a variety of temperatures for many years.
  • It must be possible to modulate the light output over a wide range of modulating frequencies.
  • For fiber links, the wavelength of the output should coincide with one of the transmission windows for the fiber type used.
  • To couple a large amounts of power into an optical fiber device the emitting area should be very small.
  • To reduce material dispersion in an optical fiber link, the output spectrum should be too narrow.
  • The optical output power must be directly modulated by varying the input current to the device.
  • High coupling efficiency.
  • Low weight and low cost.
  • Better linearity to prevent harmonics and inter-modulation distortion.
  • High optical output power.
  • High reliability.
  • The power requirement for its operation must be low.
  • The light source must be compatible with the modern solid-state device.
Two types of light sources used in fiber optics are full form of LEDs and laser
diode (LD). 

Advantages and disadvantages of optical fiber

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. Here are the main advantages and disadvantages of optical fiber to learn or know more details about optical fiber.

Advantages of optical fiber :

  • Small size
  • Higher bandwidth
  • Less signal attenuation
  • Light weight
  • Immune to cross-talk
  • Fiber cable are more strength
  • Optical fiber have long life more than 100 or above years
  • Grater immune to tapping
  • Resistance to corrosive material
  • Long distance transmission is possible
  • Immunity to electromagnetic interference 
  • Low power loss so less signal degradation 
  • Much thinner and lighter than metal wires
  • Difficult to tap so they do not radiate electromagnetic energy and thus emission cannot be intercepted 
  • Long lifespan about 100 years

Disadvantages of optical fiber :

  • Unidirectional propagation
  • High initial cost
  • Optical fiber more tensile stress than copper cables
  • Installation and maintenance
  • Fiber joining process is very costly and require skilled menpower
  • Difficult to splice
  • Expensive to install 
  • Highly susceptible
  • It can only be used on ground mostly not used in mobile communication so limited applications

Element of optical fiber transmission link

In this block diagram that are a different block can be used in optical fiber communication. General block diagram of optical communication system consists of following important blocks :
  •     Transmitter
  • .    Information channel
  • .    Receiver
Block diagram of OFC systems

Message origin :

  • Generally, message origin is from a transducer that converts a nonelectrical message into an electrical signal.
  • Common examples microphones for the room a transducer that converts a non-electrical include microphones for converting sound waves into currents and video (TV) cameras for convert images into the current.
  • In the data transfer between computers, the message already in electrical form.

Modulator :


The modulator has two main functions :

  • Modulator converts the electrical message into the proper format
  • Modulator impresses this signal onto the wave generated by the carrier source
  • Two distinct categories of modulation are used i.e. analog and digital modulation system

Carrier source :

  • Carrier source generates the wave on which the information is transmitted. This wave is called the carrier.
  • In fiber optic system, a laser diode ( LD ) or a light emitting diode ( LED ) is used for the carrier.

Channel coupler :

  • Coupler feeds the power into the information channel. In channel coupler the  atmospheric optical fiber system is a lens used for collimating the light emitted by the transmitter side and  also have directing this light towards the receiver.
  • This channel coupler must be efficiently transfer the modulated light beam from the source to the optic fiber device.  
  • One of the advantages of this design is that it possibility of high losses.

Information channel :

  • The information channel is the path between the transmitter and receiver. In fiber-optic communications, a glass or plastic fiber is the channel.
  • Amplifiers are needed in very long links to provide sufficient power to the receiver. Repeaters can be used only for digital systems.

Optical detector :

  • The information being transmitted is detected by a detector. In the fiber optic system, the optic wave is converted into an electric current by a photodetector.
  • The current developed by the detector. this current is proportional to the power in the incident optic wave. Detector output current contains the transmitted information.
  • This detector output is then filtered to remove the constant bias and then amplified.

Signal processing :

  • Signal processing includes filtering, amplification. Proper filtering maximizes the ratio of signal to unwanted power.
  • For a digital system, decision circuit is an additional block. The bit error rate (BER) should be very small for quality communications.

Message output :

  • The electrical form of the message emerging, from the signal processor, are transformed into a sound wave or visual image.
  • Sometimes these signals are directly usable when computers or other types of  machines are connected through a fiber system.

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 )
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