OFDM block diagram

In OFDM full form is basically two parts as transmitter and receiver.

In this system at the transmitter side error correction coding, interleaving process is done over the data stream. The data stream that is sent is in the form of a symbol that has undergone quadrature amplitude modulation. Then the symbol stream is assigned to one of the subcarriers with an assigned symbol. Every subscriber will operate at a particular frequency. 

In the frequency domain, the symbol frame is formed with a group of symbols from all subcarriers.

OFDM is a multi-carrier transmission technology. OFDM is used to obtain a higher data rate in the multipath fading environment of mobile communication.

The every symbol frame is transformed into a time frame with the help of N point "inverse Fourier transformation.

OFDM block diagram

The digital data stream is generated by this time of frames.

Finally, the signal is amplified and transmitted to the antenna side. Convolution coding method with CRC error detection is used to provide high data security for the system.

OFDM is sensitive to phase and frequency noise. To minimize its correct pulse shape needs to be selected.

OFDM is possible to use maximum likelihood decoding with reasonable complexity, so it is one types of OFDM advantages.

In OFDM receiver side the symbol frame received is detected. so it has to adjust automatic gain control to the desired level. A robust algorithm is to be used to withstand phase noise and also a frequency offset of the receiving unit that will interface with the synchronization process.

OFDM definition

Before we learn about OFDM, first let we check it out what is full form of OFDM. OFDM is a modulation format used for the latest wireless communications technology. 

OFDM is a multi-carrier transmission technology. OFDM is used to obtain a higher data rate in the multipath fading environment of mobile communication.

In mobile communication, the selection of digital modulation is dependent on three parameters. They are listed below :

• Narrower bandwidth 
• Efficient power utilization
• Elimination of intermodulation products

Features of OFDM

• A symbol is modulated on to orthogonal sub-carrier.
• Modulation is done by using IFFT.
• High spectral efficiency.
• Demodulation is done by using FFT. ( Demodulation of received signal ) 
• Low efficiency to time synchronization techniques.
• Better resultant quality of fading.
• Very sensitive to time-frequency synchronization.
• Simplified channel equalization.
• Immunity to intersymbol interference. 
• No inter-carrier guard band.
• Orthogonal carrier and control overlapping band.
• Improved quality of narrowband interference.
• Robust against intersymbol interference.
• Fading caused by multi-path propagation.
• Decoding and de-interleaving. 
• Orthogonality is maintained during channel transmission.

Applications of OFDM

OFDM technique is the most prominent technique in a recent era so now we can check some applications of OFDM one by one below. 

Applications of OFDM : 

• Digital audio broadcasting
• Digital video broadcasting over the terrestrial network. Digital terrestrial television broadcasting. In this broadcasting, 2000 to 8000 subcarriers are used.  
• UMTS
• Wireless LANs. OFDM is used in HIPERLAN phase II which supports 20 Mbit/s in propagation environments with delay spreads up to 1 µsecond. 
• HDTV
• The optical wireless communication system
• Uni-polar communication system
• Wireless ATM transmission system
• Digital video broadcasting
• Wireless LAN networks
• UMTS terrestrial radio access
• Some ultra wideband system
• Power line communication
• Very high-speed digital subscriber lines
• Point to point and point to multipoint application
• The flash OFDM cellular system
• Mobile broadband wireless access
• Used in Wi-Fi and WiMAX
• High bit rate digital subscriber lines
• A global standard for asymmetric subscriber line
• Metropolitan area network standard also include in OFDM
• 5.3.1 HIPERLAN/2
• IEEE 802.16 Broadband
• IEEE 802.11a and IEEE 802.11g
• Used in LTE and LTE advanced

Advantages and disadvantages of OFDM

OFDM stands for orthogonal frequency division multiplexing, it has been used in many high data rates wireless communication system because of the many advantages it provides. This article gives information about the advantages and disadvantages of OFDM  to know more details about it.

Advantages of OFDM :

• More resistance to frequency selective fading than single carrier system because it divides to the overall channel into multiple narrowband signals that are affected individually as flat fading sub-channels
• Interference appearing on a channel may be bandwidth limited and in this way will not affect all the sub-channels this not all the data is lost
• Very resilient to inter-symbol and inter-frame interference
• It is possible to recover symbols lost due to the frequency selectivity of the channel and  also a narrow-band interference using adequate channel coding and interleaving
• Maor efficient use of the available spectrum using close-spaced overlapping sub-carriers
• Using multiple sub-channels, the channel equalization becomes much simpler that is cause issue with a CDMA method
• Simple implementation by the mean of FFT
• Eliminates ISI and IFI by the use of a cyclic prefix
• Provide good protection against co-channel interference
• High transmission bit rates
• A chance to cancel any channel so it affected by fading device
• Low sensitivity to time synchronization error
• Facilitates single frequency networks ( SFN )
• Tuned subchannel receiver filters are not required
• It is able to protect  total energy loss at the frequency domain

Disadvantages of OFDM :

• Require highly linear amplifier
• Sensitive to carrier offset and drift
• Sensitive to frequency synchronization problems
• Inter-carrier interference between the sub-carrier
• Loss of efficiency caused by cyclic prefix or guard of interval 
• High power transmitter amplifier needs linearization
• High peak to average power ratio

Explore more information:

1. Advantages and disadvantages of FDM