ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING

ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING:

Orthogonal Frequency Division Multiplexing or OFDM is a modulation format that is finding tremendous use in today's radio communications scene. OFDM has been adopted in the Wi-Fi arena where the 802.11a standard uses it to provide data rates up to 54 Mbps in the 5 GHz ISM (Industrial, Scientific and Medical) band. In addition to this the recently ratified 802.11g standard has it in the 2.4 GHz ISM band. In addition to this, it is being used for WiMAX and is also the format of choice for the next generation cellular radio communications systems including 3G LTE and UMB. Orthogonal frequency-division multiplexing (OFDM) effectively mitigates Inter Symbol Interference (ISI) caused by the delay spread of wireless channels. Therefore, it has been used in many wireless systems and adopted by various standards.

Orthogonal frequency division multiplexing (OFDM) has become a popular technique for transmission of signals over wireless channels. OFDM has been adopted in several wireless standards such as digital audio broadcasting (DAB), digital video broadcasting (DVB-T), the IEEE 802.11, local area network (LAN) standard and the IEEE 802.16, metropolitan area network (MAN) standard. OFDM is also being pursued for dedicated short-range communications (DSRC) for road side to vehicle communications and as a potential candidate for fourth-generation (4G) mobile wireless systems.

Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. The technology was first conceived in the 1960s and 1970s during research into minimizing interference among channels near each other in frequency.

In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk, among the channels and symbols.

Orthogonal frequency-division multiplexing (OFDM) is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, DSL broadband internet access, wireless networks, and 4G mobile communications. Orthogonal Frequency Division Multiplexing (OFDM) is a method that allows to transmit high data rates over extremely hostile channels at a comparable low complexity.  Orthogonal FDM’s (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the “orthogonality” in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multi path-channels (i.e. the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information. OFDM is sometimes called multi-carrier or discrete multi-tone modulation.

OFDM is a combination of modulation and multiplexing. Multiplexing generally refers to independent signals, those produced by different sources. So it is a question of how to share the spectrum with these users. In OFDM the question of multiplexing is applied to independent signals but these independent signals are a sub-set of the one main signal. In OFDM the signal itself is first split into independent channels, modulated by data and then re-multiplexed to create the OFDM carrier. OFDM is a special case of Frequency Division Multiplex (FDM). As an analogy, a FDM channel is like water flow out of a faucet, in contrast the OFDM signal is like a shower. In a faucet all water comes in one big stream and cannot be sub-divided. OFDM shower is made up of a lot of little streams.

APPLICATIONS:

cognitive radio, digital media broadcasting, in ultrawideband, highly secure transmission & payload circuitry of unmanned ariel vehicle.

CATHODE RAY OSCILLOSCOPE (CRO)

CRO is a very essential and versatile lab instrument used for display , measurement and analysis of waveforms having electrical nature. This is a X-Y plotter which display input signal versus another input signal or versus time. The stylus of this plotter is a luminous spot which moves in accordance to input voltage. The luminous spot  is produced by electron beam striking the flourescent screen.
The normal form of CRO uses a horizontal input voltage which is an internally generated ramp voltage. This horizontal voltage move the luminous spot in horizontal direction on the screen. The vertical input voltage is the voltage under investigation. This vertical voltage moves the spot up - down in accordance with its instantaneous value. Thus a waveform of input voltage is traced on screen wrt time.

CRO basically consist of CRT ( cathode ray tube) which has some additional circuitry. The main parts of CRT are electron gun assembly , deflection plate assembly , flourescent screen , glass envelope and base. Electron gun is used to produce focussed and accelerated beam of electrons. The deflection plate assembly is used to move beam in vertical as well as horizontal direction.
There are numerous applications of CRO. They can accept multiple input signals and waveforms can be traced and analysed easily. These oscilloscopes employ time sampling and are able to measure singals of 20GHz.The complete waveform can be stored and can be displayed as stationary signal.       

LIGHT EMITTING DIODES

LEDs are p-n junction diodes which can emit radiation in the visible range (370nm to 770nm) under forward biased condition.

In a p-n junction electrons from n side diffuses to p side and holes from p side moves towards n side forming a depletion region. So no mobile charges present in the depletion region.When sufficient voltage is applied movement of electrons across junction takeplace. These electrons when recombined with holes , a quantum of electromagnetic energy is emitted in the form of photon of light with frequency charateristic of semiconductor material ( generally GaAsP , GaP). 

Characteristics:
1. These give adequate power.
2. These give low noise.
3. LEDs are compatible with optical fibre.
4. The light from LED can be varied or modulated by altering applied voltage or current which is helpful in transmitting information.

Applications:
1. They can be used as cheap source of light.
2. In optical fibre applications.
3. They can be used as sensor as in case of optical mouse of computer.

COMMUNICATION SYSTEM

Communication is the process of exchanging information. It is the process of stablishing link between two points for information exchange.
The electronic equipment used for communication purpose are known as communication equipment. These communication equipment are assembled togather to form a communication system.
Examples: line telephony , line telegraphy , mobile communication , radio broadcast ,tv broadcast etc.

Communication System Elements :
1. Information Source:The source which generate useful information is termed as information source.
2. Transducer: It is a device which is used to convert one form of energy to other form. We can say that it converts information signal into electrical pulses. For example microphone converts sound waves into electrical signals.
3. Transmitter : In this signal processing is done so as the signal can be transmitted easily and safely. In this amplification and modulation of signal takes place.
4. Channel: It is the medium through which information travels from transmitter to reciever. It can be wirelines , microwave links , optical fibre cables etc.
5. Reciever: It is used to reproduce the signal in original form. The reproduction of signal is done with the help of demodulation.

TRANSISTOR

A transistor is formed by sandwitching p-type semiconductor material in between two n-type semiconductor material or n-type semiconductor material in between two p-type semiconductor material. It is also known as Bipolar junction transistor as in this device conduction takes place by motion of both polarity particles ie electrons as well as holes.
Every transistor has three terminals:
1.EMITTER : This terminal supplies large number of majority carriers and is heavily doped. This terminal is always forward biased wrt base in order to supply high majority carriers.
2.BASE : The middle sections forms two pn junctionsbetween emitter and collector is called base.It is lightly doped.
3.COLLECTOR : This section collects major portion of majority carriers supplied by emitter. The collecor base junction is always reverse biased. It is moderately doped.

Linear Variable Differential Transformer

Linear Variable Differential Transformer is used as inductive transducer to convert linear motion into electrical signals. The transformer consist of primary and two secondary windings wound on a cylinderical former. The secondary windings have equal number of turns and placed on either side of primary winding. The primary winding is alternatively connected to alternating current source. A movable soft iron core is inside former and displacement is measured with the help of movement of core.The output voltage obtained is the differential voltage of two secondary windings.
USES:
1. LVDT is used in all applications which include displacement ranging from fraction of mm to few cm.
2. It is used to measure force , weight and pressure etc.
3. Pressure of liquid in tank.

TRANSDUCER

Transducer is a device which converts one form of energy into other. As in measurement system , measurand can be any physical quantity like displacement , stress , pressure etc but it is calculated in terms of electrical signals so that they can be processed, calibrated and recorded easily. So transducer is used to convert these physical signals into electrical signals.We can also say that transducer is a device which converts mechanical force into electrical signal.
Few Examples of Transducer:
1. Thermocouple which converts heat into electrical voltage.
2. Photoconductor which converts light intensity into change in resistance.
3. LVDT converts displacement into voltage change.
Classification of Transducers:
Passive Transducer: These devices derive power from auxiliary power source for transduction of signal.
Example POT
Active Transducer : These devices do not require any auxiliary power source for transduction of signal.
Examples thermocouple , tachogenerators, photovoltic cells.