What is interference in communication

The performance of a cellular system is highly affected by interference. Interference is the main problem in increasing capacity. basically, there are two types of interference in the cellular system.

1. Co-channel interference
2. Adjacent channel interference

Co-channel interference :

In the frequency reuse method, several cells use the same set of frequencies in the given service area. The cell using the same set of the channel are called co-channel cells. The interference between signals from these cells is called as co-channel interference.

While the order to reduce co-channel interference, the co-channel interference cell are actually separated by the smallest distance to provide isolation.

The minimum distance that allows the same frequency to be reused depends on factor like a number of co-channel cells near the center cell.

Now let us discuss how to find frequency reuse distance D  in co-channel interference  

D= √3N  R 

N =  i²  + i j +  j²

Here we have to calculate different values of D  for given values of N.

D=3 , N=3 (i=1, j=1)

D=4.58R, N=7(i=1, j=2)

D= 6R, N=12(i=2, j=2)

If all the cell transmits the same power, then N increases and the frequency reuse distance D increases. This increases D reduces the probability interference can occur.

The ratio D/R is called the co-channel reuse ratio. It is expressed as,

                                Q = D/R = √3N

A small value of Q provides large capacity as the cluster size N is small. A large Q, i.e a large value of D/R indicates that the spatial separation between the co-channel cell relative to the coverage distance of a cell is increased.

   

Adjacent channel interference :       

While the interference resulting from the channel that is adjacent in frequency to the desired channel is called adjacent channel interference.

If the user that is using an adjacent channel and is transmitting in close range to subscriber receiver then while the receiver tries to receive a base station on the desired channel the problem can be critical. It is called the near-far effect.

Be proper filter and channel assignment the adjacent channel interference can be minimized. As each cell is given only part of the available channels a cell needs to assign channels that are adjacent in frequency.

In adjacent channel interference is frequency reuse when the frequency reuse is high there are more chances for interference because of neighboring channels and because of separation concept. 

Let us now talk about the basic source of interference :

• Any other mobile in the same cell.
• A call in always progress in the neighboring of a cell.
• In most of the other base station operating on the same frequency.
• Any non-cellular system which leaks energy into the cellular frequency band.

Channel assignment strategies

The channel assignment strategies can be classified as fixed or dynamic depending on the system performance in managing cell calls when a mobile user is handed off from one cell to another.

Fixed channel assignment : 

In the fixed channel assignment method each cell is assigned a predetermined set of voice channel.

Any effort that is done to call a number within the cell can be serviced by the unused channels in that particular cell.

In case if all the channels in that cell are used, then the cell is blocked. such a situation the subscriber does not  get service.

Borrowing techniques are used from an adjacent cell if all of its channels are occupied. This method is called as borrowing strategies.

Dynamic channel assignment :

In dynamic channel assignment strategy there is no permanent allocation of channels. Every time a call request is done, the BS full form (base station) requests a channel from MSC full form is mobile switching center.

Mobile switching center only allocates the channel after verifying that the channel is not currently in use in the cell or any other cell that comes within the minimum restricted distance of cellular frequency reuse to avoid co channel interference.

Dynamic channel assignment strategy reduces the probability of blocking and increases the trunking capacity of the system. These strategies need MSC to collect real time data. 

Difference between co-channel interference and adjacent channel interference

The performance of the cellular system is highly affected by interference. Interference is the main problem in increasing capacity. basically, there are two types of interference in the cellular system called : (i) Co-channel interference (ii) adjacent channel interference. 

Let us now talk about the difference between co-channel interference and adjacent channel interference to know more details about Interference.

Co-channel interference :

• The interference between the signal from co-channel cells is called co-channel interference.
• Co-channel interference mainly occurs on the same channel.
• Co-channel cells are physically separated by a minimum distance to reduce co-channel interface and also have to provide sufficient isolation.
• A large co-channel reuse ratio improves the transmission quality because of the smaller level of co-channel interference. 

Adjacent channel interference :

• The interference that results from the signal that is adjacent in frequency to the required signal called adjacent channel interference.
• Adjacent channel interference occurs from a different channel.
• By proper filter and channel assignment strategy, the adjacent channel interference can be minimized.
• Another cause for adjacent channel interference is frequency reuse and when the frequency reuse is high there are more chances for interference because of operation.

Cellular frequency reuse

What is cellular frequency reuse ?

A group of radio channel is allocated a separate circular base station to be used within a small geographic area called cell.

The design method of selecting and allocating channel groups for all cellular base station within a system called as frequency reuse.

Here we have to use hexagonal geometry is used in the design of cellular system.

we have to better to understand frequency reuse concept in cellular system having S duplex channel then, and if n channel is less than S (n<S) and is S channel are divided among N cell into unique and disjoint channel groups with each having the same number of channels, then the total number of available radio channel is define as : 

            S = nN

Total number of duplex channel  C can be defined as :      

             C = mnN = mS

The capacity of cellular system is directly proportional to the number of times a cluster repeated in a fixed service area, the factor N is called as the cluster size 

For maximum capacity, the smallest possible value of N is desired. 1/N is called as the frequency reuse factor as each cell within the cluster is only assigned 1/N times of the total available channels.

The geometry of hexagon is such that the number of cell per cluster N can have values that satisfy the equation, 

N = i² +ij + j²

Where i and j are non negative integer
So here we have to calculate the possible cluster size above this equation :
i=1, j=1 so N=3
i=2, j=0 so N=4
i=2, j=2 so N=12
i=2, j=3 so N=19

Basic cellular system

Before we learn about the cellular system first let we check it out what is the cellular system. In a cellular system, replaced the single high power transmitter with many numbers of low power transmitters (Base station) each signal providing coverage to only a fraction of service area (Cell). Each base station is allocated a portion of the total number of channels and nearby BS full form (base station) are assigned different group of channels. Basically hexagonal is close approximation the circular radiation pattern in an omnidirectional base station antenna.

A basic cellular system can be comprised of three units :

1. Mobile unit
2. Cell site
3. A mobile telephone switching office

• Mobile unit :

It comprises a control unit, a transceiver, and an antenna system.

• Cell site :

It is an interface between the mobile unit and the full form of MTSO. It comprises a control unit, antennas, power plant, and data terminals.

• Mobile telephone switching office :

It is the central co-coordinating unit for all cell sites. It comprises of the cellular processor and the cellular switch.

One of the most important points is that the MTSO is responsible for controlling the call processing operation and handling the billing activities of the subscriber.

In general cellular, the switch can be analog or digital form. It can connect the mobile subscriber to the other mobile subscriber to the nationwide telephone network.

Cellular system

What is cellular system

In a cellular system, replaced the single high power transmitter with any numbers of low power transmitters (Base station) each signal providing coverage to only a fraction of service area (Cell). Each base station is allocated a portion of the total number of channels and the nearby base station (BS) are assigned different group of channels.

The neighboring base station is assigned a different group of channels that the interference between the base station is minimized in the cellular group.

In a Basic cellular system, replaced the single high power transmitter with any number of low power transmitters (Base station) each signal providing coverage to only a fraction of service area (Cell).

Some advantages and disadvantages of the cellular system :

Advantages of the cellular system :

• High capacity
• Reduced in  process
• Less transmission power
• Reduced set up times
• It reduced the interference which increases the system capacity
• It improved the S/N ratio
• Reduced the cluster size
• More robust against the failure of single components

Disadvantages of the cellular system :

• Handover is needed 
• Good infrastructure needed
• Frequency planning should be good
• Increases the number of an antenna in each base station 

Quantum efficiency of photo detector

In photo detector quantum efficiency is defined as the number of electron hall pair generated per incident photon of energy hv and is given as ȵ

ȵ = Number of electron hall pair generated /  number of incident photons

    =   I_P / q  /  P_in / hv

 I_P = Average photo current

 P_in = Average optical power incident on photo detector

Absorption coefficient of material determines the quantum efficiency. Quantum efficiency ȵ < 1 as all photons incident will not generate e-h pair. It is normally expressed in percentage.

For example,

On an GaAs photo detector 

Number of photon emitted = 6  × 10⁶ 

Average e-h pair generated = 5.4 × 10⁶

ȵ  = 5.4 × 10⁶  / 6  × 10⁶ 

      = 0.9

      

      = 90%

Cut-off wavelength of photo detector

A cut of the wavelength of any photodetector can be defined as if any particular semiconductor can be absorbed photon over a limited wavelength range. The highest bandwidth is known as cut-off wavelength.

Cut off wavelength is determined by band gap energy Eg of a material.

      Cut of wavelength = hc / E_g  = 1.24 / E_g  

Where,

E_g  = Electron volts (eV) 

A typical value of cut of wavelength for silicon is 1.06 micrometer and germanium is 1.6 micrometer.

For example :

In PIN photo diode  E_g  = 1.43 eV, hc = 1.24 , find cut of wavelength = ?

 Cut of wavelength = hc / E_g  = 1.24 / E_g

                                                

                                               _{= 1.24 / 1.43}

                    

                                                _{= 0.867 micro meter}

                       

                                                _{= 867 nm}

WiMAX features

A wireless broadband solution that offers various features with a lot of flexibility in terms of potential service offering is WiMAX technology.

• WiMAX support multipath.
• IT provide up to 50 km of service area range.
• WiMAX uses OFDM technology.
• WiMAX support TDD and FDD.
• WiMAX offer modulation and error correction.
• Adaptive modulation enables while using WiMAX system to optimize the throughput based on the propagation conditions.
• Very scalable bandwidth and high-speed data rate.
• Flexible and dynamic per-user resource allocation.
• Support for advanced antenna techniques.
• Support for mobility.
• Portable internet usage.
• Quality of service support.
• It is IP based architecture.
• WiMAX is always best connected.

WiMAX architecture

As we know that the WiMAX system consists of two major parts: Main part is to be (i) A WiMAX base station (ii) And a WiMAX receiver.

Let us take a look at the WiMAX architecture to shown in the figure.

A WiMAX base station :

WiMAX base station is very similar to accessing a wireless access point in a Wi-Fi network, but the coverage is greater than to another network.

WiMAX base station can provide a very large area up to a radius of around 6 miles. A WiMAX base station(BS) consists of indoor electronics and as we know that WiMAX tower similar look like to a cell phone tower.

Each base station(BS) can provide wireless coverage over an area called a cell. The WiMAX base station can also use a multiple antennas point in a different direction. This area covered by one antenna signal called sector.

The uplink and also a downlink channel are shared among the many subscriber stations in a given sector. WiMAX can also have support bursty data and also provide high-quality telephone and high volume multimedia.

The uplink and also for downlink channels are divided into slots of equal size. A WiMAX frame takes multiple slots. Different frame takes a different number of slots. The downlink channel easy to subdivide into connection as only the base station sends on that channel.

A WiMAX receiver : 

A WiMAX receiver side may have a separate antenna or could be a stand-alone box or a PCMCIA card present in your laptop or any other device. It is called as customer premise equipment (CPE).

In most of the cases, a simple plug and play terminal, similar to a DSL modem provides connectivity as shown in the figure given below.

For customer located several kilometers from the WiMAX base station, a self-install outdoor. The antenna may be required to improve transmission quality. 

For customer requesting a voice in addition to broadband services, specific CPE will allow the connection of standard or VoIP phones device.

WiMAX architecture

WiMAX standard

We all know that the WiMAX system has been supporting IEEE 802.16, This standard is called IEEE  called 802.16, it was associated with 802.16a / REV d / e standard.

These standards were issued by IEEE standard 802.16 into the subgroup called  (802.16a/REV d/e) standard that originally covered the Wireless local loop (WLL) technologies with radio spectrum from 10 to 66 GHz. Recently were extended below 10 GHz.

In the year of January 2003, the approved IEEE standard 802.16a as an amendment to IEEE  standard 802.16 defining line of sight capability.

In the year of 2004, the IEEE 802.16 standard Revd was introduced to support basically indoor customer premises equipment through additional radio capabilities like antenna beamforming and OFDM sub channeling.

In the year of 2005, an IEEE standard 802.16e variant was developed for supporting mobility device.

Following are the details of various IEEE 802.16 standard related to WiMAX system listed below.

1. 802.16

• Spectrum - 10-66 GHz
• Configuration  - Line of sight
• Mobility - Fixed
• Channel bandwidth - 20,25 MHz
• Typical cell radius - 1-3 miles
• Modulation - QPSK-16 QAM 64 QAM
• Completion  - December 2001

2. 802.16a

• Spectrum - 2-11 GHz
• Configuration  - Non-Line of sight
• Mobility - Fixed
• Channel bandwidth - Selectable, 1.25-20 MHz
• Typical cell radius - 3-5 miles
• Modulation - total number of 256 subscribers, OFDM using QPSK, 16 QAM, 64 QAM, 256-QAM
• Completion - January 2003

3. 802.16e

• Spectrum - <6 GHz
• Configuration  - Non-Line of sight
• Mobility - Fixed
• Channel bandwidth - 5 MHz planned
• Typical cell radius - 1-3 miles
• Modulation - total number of 256 subscribers, OFDM using QPSK16, QAM, 64 QAM, 256-QAM
• Completion - 2nd half of 2005

GSM handover

Definition of handover : 

Handover is the process of transferring a mobile station from one channel to another channel side.

This system can be defined as a mechanism to hand over the mobile device to the neighboring cell.

Basically, handover is a core element that can be planned and deployed cellular system based network. It must be efficiently done using different strategies.

In the handover process, if the mobile device moves out the range of one cell, a different base station provides it with a use of the stronger signal.

In this process, if the channel of the base station is busy then the nearby base station can provide service to the device.

Types of handover :

There are two types of handover :

1. Hard Handover
2. Soft Handover

1. Hard handover :

In hard handover, the radio link must be dropped for a small amount of time before it can be taken over by another base station.

So the call process not from only one base station (BS) to another base station but from current transmits to receiver frequency pair to another frequency pair. A break in call transmission is called as a call drop.

In general, the handover occurs in a very few milliseconds. The GSM system performs hard handovers.

2. Soft handover :

While in soft handover indicates an MS full form is a mobile station at the boundary of two adjacent cells does not call drop because of handover in the boundary region. 

A mobile can be connected easily to several base station (BS full form) is base stations simultaneously. The 3D CDMA support soft handover.

Photo detector

The photodetector is also called an optical receiver. It converts the variation in optical power into a corresponding variation in the electric current. As compare to the optical transmitter the design of optical receiver is more complicated just because the receiver must detect weak, distorted signals and then make decisions on what type of data was sent based on an amplified version of this totally distorted signal.

Photodiodes are one type of photodetector, is capable of converting light into either voltage or current. That depends upon the mode of operation. In photodetector cut off wavelength and quantum efficiency can be defined as λc and ȵ

The requirement of the ideal photodetector 

• High sensitivity at the operating wavelength 
• Spectral response: This parameter also describe the efficiency of the photodetector as a function of wavelength.
• Size: Photodetector size must be small
• Voltage: It must be work on small voltage 
• High fidelity: The received signal must be reproduced as it as far as possible
• Cost: Cost of the photodetector must be less
• Noise: Minimum noise must be introduced by the detector device
• Stability: Performance characteristics of photodetector should not change with a change in ambient conditions

Types of photodetector 

• LDR
• the p-n junction semiconductor photodetector
• Traveling wave photodetector
• PIN diode
• Avalanche photodiode
• Phototransistor

Full form of LED

What is the full form of LED?

Answer :

• Light Emitting Diode

What does LED mean?

LED is a component is its convert 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 technology. Nowadays the light has collected the edge of LED, in order to reduce the losses caused by absorption the active layer and to make the beam more directional, such a device is known as edge emitting LED or ELED.

Properties of light

Light is electromagnetic radiation that has wave properties. Light waves can travel in a straight line. The light rays travel at the speed 3×10⁸ or 186000 miles/sec. in free space. Light is a basic form of energy produced by the luminous object. It can travel through a vacuum. A light source needs characteristics of light must be possible to operate the device continuously at a variety of temperatures for many years.

Let us now check it out the primary definitions which we will be using while discussing optical fiber.

1. Refractive Index :

Refraction is the bending of a light ray that occurs when a light ray passes from one medium to another.

So it usually happens when the light to them from one medium to the next. The measure of how much light refract in a medium call index of refraction.

Refractive index can be defined as, n of an optical parameter of a material or substance as the ratio of the speed of light(c) in the air to the speed of light(v) in another medium, such as glass, water etc.

Where, 

Refractive index  n = Speed of light in air (c) / speed of light in a material (glass-v)

n = Refractive index

c = Speed of light,  speed of light in free space = 3×10⁸ or 186000 miles/sec

v = speed of light in a material

Now will discuss the index of refraction in a different material

• Air - 1.003
• Vacuum - 1
• Water - 1.33
• Glass fiber - 1.5-1.9
• Diamond - 2.0 - 2.42
• Silicon - 3.4
• Gallium arsenide - 2.6

Due to the refraction of light, we have to observe the various effect in our daily life. The following some example of them.

• A stick partially dipped in water seems to be sent
• A clear pool of water always appears to be shallower than compare it actually is.

2. Reflection :

The law of reflection state that when the light ray incident upon a reflective surface at some of the incident angle i from imaginary perpendicular normal, the ray will be reflected from the surface at some angle r^r from normal which is equal to the angle of incident.

The angle of reflection =  r^r  = the angle at which the light is reflected from a surface is called the angle of reflection

                                                                  i =  r^r 

                           ^{The angle of incidence  is the same as = the angle of reflection}

The angle at which the light strikes a surface of the mirror with respect to the normal is called the "angle of incidence". The figure shows that the law of refraction and reflection.

 

3. Diffraction :

Diffraction is the slight bending of light as it passes around the edge of an object side. It refers to various phenomena that occur when a wave encounters an obstacle or silt. It is also defined as the bending of light around the corners of an obstacle or aperture into the region of the geometrical shadow of the obstacle. every cloud is a silver lining is the example of diffraction.

4. Dispersion :

The process on which light is separated into its colors due to the different degree of refraction, so the separation of visibility into its different colors is known as dispersion.

Full form of SONET

What is the full form of SONET?

Answer :

• Synchronous  Optical Network

What does SONET mean?

SONET is standing for the synchronous optical network. It is most commonly transmit data at a speed between the range of around 155 megabits per second and 2.5 gigabits per second.  To build these high bandwidth data streams, SONET  multiplexer together channels having bandwidth as low as 64 kilobits per second into data frames sent at fixed intervals.

SONET interface defines all layers, from the physical to the application layers, while SDH is a set of standard interfaces in a network of elements that conform to these interfaces.

Advantages and disadvantages of SONET

SONET has commonly transmitted data at speed between 155 megabits per second and 2.5 gigabits per second. To the build these high bandwidth data streams, for SONET multiplexer techniques together channels having bandwidth as low as 64-kilobits per second into data frames sent at fixed intervals. So now let us check out the advantages and disadvantages of SONET to know more details about SONET.

Advantages of SONET :

• Reduced cost
• It offers network survivability features
• It is compatible with legacy and future network
• Very high efficiency
• Allows transportation of all  forms of traffic
• Standard optical interference
• De-multiplexing is easy
• Remote operation capabilities. remotely provisioned, tested, inventoried, customized and also reconfigured
• Can we used in SONET networks for fast restoral and protection
• Out of band management system

Disadvantages of SONET :

• No interoperable standard
• Tributary services require SONET mux services
• Low cost effective for low channel numbers.
• SONET/SDH network management system not well equipped to handle the DWDM method and management
• Bandwidth efficiency is a problem at higher capacity
• More overhead is required

Application of SONET

SONET has commonly transmitted data at speed between 155 megabits per second and 2.5 gigabits per second. To the build these high bandwidth data streams, the SONET multiplexer together channels having the bandwidth as low as 64-kilobits per second into data frames sent at fixed intervals. So now let us check out the application of SONET to know more details about SONET.

• SONET was originally designed for the public telephone network.
• SONET acts as a carrier of multiple higher-level application protocol.
• SONET provides a layer 1 or interfaces layer technology also termed physical layer in the OSI model.
• SONET is used to Multipoint configurations, enhanced performance, monitoring, and enhanced and integrated OAM.

What is SONET

Nowadays carrier synchronous digital hierarchy  (SDH) is an international standard for high-speed telecommunication networks. It is a synchronous system which provides simple network infrastructure and it is a more flexible network. SONET is very similar to SDH. SONET is the standard used in the US where SDH is the standard used outside the US also. Before we started the details about SONET/ SDH first we will discuss why using SONET/ SDH and also a full form of SONET.

• In SONET yield thinner cable than copper
• The lower bit error rate
• Higher bandwidth per fiber
• Higher transmission reliability 

SONET network is a transmission interface originally proposed by the broadband ISDN network. Now in this article, we talk about some important characteristics, similarities, major difference between SONET and SDH, SONET network topology. first, now let us check it out some important point SONET as well as SDH.

• SONET is a synchronous network while SDH is also synchronous network with an optical interface.
• SONET interface defines all layers, from the physical to the application layers, while SDH is a set of standard interfaces in a network of elements that conform to these interfaces.

SONET has commonly transmitted data at speed between 155 megabits per second and 2.5 gigabits per second. To the build these high bandwidth data streams, SONET  is multiplexer together channels having the bandwidth as low as around 64-kilobits per second into data frames sent at fixed intervals.

This article gives information about both SONET and SDH  are based on a structure that has a basic frame format and speed. All the base signaling level called STS-1, SONET support 51.84 Mbps and this STS-1 frame can be carried in an OC-1 signal. The SDH level and frame format used by the synchronous transport module(STM).

SONET versus SDH :

• Bitrate and frame format organization
• Frame synchronization schemes
• Error control
• Multiplexing and demultiplexing rules

SONET and SDH rates :

SONET Signal - STS-1, Optical carrier - OC-1

Data rate - 51.84 Mbps

Payload rate - 50.112 Mbps

SONET signal - STS-3,Optical carrier - OC-3,

SDH ITU-T - STM-1

Data rate - 155.52 Mbps

Payload rate - 150.336 Mbps

SONET Signal - STS-9, Optical carrier - OC-9,

SDH ITU-T - STM-3

Data rate - 466.56 Mbps

Payload rate - 451.008 Mbps

SONET signal - STS-12, optical carrier- OC-12
SDH ITU-T - STM-4
Data rate - 622.08 Mbps
Payload rate - 601.344 Mbps

STS-1 - Synchronous transport signal -1 

OC - Optical carrier

STM - Synchronous transport module

ITU-T - International telecommunication union telecommunication standardization sector.

Application of SONET :

• Used for the public telephone network.
• SONET acts as a carrier of multiple higher-level application protocol.
• SONET provides a layer 1 or interfaces layer technology also termed physical layer in the OSI model.
• Used to Multipoint configurations, enhanced performance, monitoring, and enhanced and integrated OAM.

You may also check it out :

Advantages and disadvantages of SONET