Orthogonal Frequency Division Multiplexing

Orthogonal Frequency Division Multiplexing (OFDM) For Powerful Optical Transmission

Orthogonal Frequency Division Multiplexing or OFDM was an innovative technique that was adapted for enhancing optical transmission in a better way, even better than RZ-OOK. This technology increased the efficiency of transmission using significant improvements that made it 2.9bits/Hz efficient than the older RZ-OOK. This was a much needed technology because “optical transmission” is one of the intensively researched fields, where researchers are always in the heed of creating something extraordinary that would support the power of transmission. With a high speed optical transmission the system can be linked to various chromatic dispersion mode and polarization mode that increases the transmission capacity of the system. However, to increase this capacity Orthogonal-FDM holds narrow channels that surpass the transmission wave so that it can be modulated properly.

Simple Block Diagram of OFDM

This is definitely a special case that was created to act as a multicarrier of transmission using which substantial information bearing system can be transmitted at a lower rate to the sub channels. Initially, this technology was first seen in digital broadcasting and later it became popular for wireless LANs too. The technological standard of this system is IEEE 802.11 approved which makes it one of the best systems for optical transmission.

Working of Orthogonal Frequency Division Multiplexing

As mentioned before, OFDM is a perfected FDM where the transmission is responded in an orthogonal manner. Talking about the working of this system, the orthogonal system first chooses a sub carrier frequency; now one thing should be noted here that these frequencies are orthogonal to each other. Thus, the selected frequency gets simplified where the structure of transmitter and receiver are just the same. This orthogonality increases the “spectral efficiency” giving birth to a huge transmission rate equivalent to  “Nyquist Rate” where the entire double side physical band length can be used for transmission.  This means that for optimal working of Orthogonal-FDM it’s crucial to have the frequency in sync along the transmitter and the receiver, as if the frequency deviates then the system will no longer remain orthogonal. This leads to an inter-carrier interference (ICI) that disrupts the path of transmission. 

OFDM Signal  OFDM Spectrum

The common offset of frequency is mainly caused by the oscillations of the receiver that gets mismatched with the transmission rate this is known as the “Doppler Shift” which is a little difficult to correct. When the Doppler shift occurs then the system needs to be stabilized, if the system is not stabilized then it starts forming a multi-path which is only going to worsen the working of the system.

The system holds an efficient demodular and modular protocols that are enhanced with FFT algorithms. These algorithms are basically used at the receiver side and are inversely depended to the transmission side. Though these algorithms were established in the late 1960s, yet the technology of OFDM that is adapted today has low cost digital signal processing benefit and even has the ease of use. Along with that the components that are used in the making of the system can efficiently calculate the algorithms so that they can function in a better way.

What are the Applications of OFDM?

There are a plethora of applications of Orthogonal Frequency Division Multiplexing that can be seen today. There are broadly categorized into two distinct types which are Wireless systems and cabled systems.

Wireless Systems

  • This is used in Wireless LAN (WLAN) along with that for radio interfaces too.
  • Works for terrestrial television system such as ISDB-T and DVB-T.
  • Works for terrestrial mobiles and even MediaFLO links
  • It holds major personal area network (PAN) functions and is even used for ultra-wide band too.
  • It’s used for broadcasting 4G or 4G cellular speed internet in various mobile phones.

Cabled Systems

  • OFDM forms a major part of the power line communication.
  • It is even used in VDSL and ADSL broadband access in the form of copper wiring via POTS.
  • It stabilities the modem power of broadband.
  • It is even used for home networking.

Advantages of Orthogonal Frequency Division Multiplexing    

  • This is an adaptive system that can easily amalgamate with any sort of server conditions without any equalization.

MIMO OFDM

  • Improves the spectral efficiency in a better way, compared to the other sidebar broadband.
  • Offers a narrow co-band interface that allows the transmission to occur in a subtle manner.
  • Is finely tuned with the sub channel filters so that it does not need any other type of conventional filters (as needed by other systems of FDM).
  • Uses Fast Fourier Transmission for efficient implementation.
  • There is no time sync errors caused because of low sensitivity of the system.
  • Works against the inter symbol interference so that it can fade the errors that are caused due to multi path propagation.
  • Has the ability to work with a single frequency network.

 

Disadvantages of Orthogonal Frequency Division Multiplexing    

  • It is quite affiant to the Doppler Shift that destroys the system.
  • Without the right frequency sync the entire system may not function.
  • Efficiency of the system is reduced when cyclic prefix or guards occur.
  • For high peak power transmission a linear transmitter is needed so that the system can function without any complications.

At last, OFDM is a powerful system that not only increases the optical transmission of any modulations but at the same time it even simplifies the level of dispersion by using the sub carrier for multiplexing the system.

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