3 January 2018

What is orthogonal frequency division multiplexing

OFDM is a modulation format that is being used for many of the latest wireless and telecommunication technology. First, let us check it out the full meaning of OFDM.

OFDM has been adopted in the Wi-Fi arena where the standards like 802.11a and many more. It has also been adopted by cellular telecommunication standards LTE and LTE-A. It also used for a number of broadcast standards such as DAB digital radio to digital video broadcast standards. 

Definition of OFDM : 

OFDM is a special form of multi-carrier modulation, patented in 1970 is particularly suited for transmission over a dispersive channel. An OFDM signal consists of a number of closely spaced modulation carrier. When modulation in the form of voice or data is applied to a carrier, then sidebands spread out either side. As a result, when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and must be a guard band between them. 

It is used to obtain a higher data rate in the multipath fading environment of mobile communication. It also used in Wi-Fi, DSL internet access, 4G wireless communications, and digital television and radio broadcast services. 

The selection of digital modulation is dependent on three parameters are listed below :

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

How OFDM works? 

Orthogonal means "unrelated or independent of each other". The subcarriers for each radio channel are orthogonal to one each other and spaced close to each other without guard bands. To know how OFDM works, first of all, we will look at the receiver. The receiver acts as a demodulators bank, translating each carrier down to DC. This resulting signal is integrated over the symbol period to regenerate the data from that carrier. When the carrier spacing equal to the reciprocal of the symbol period so that they will have a whole number of cycles in the symbol period and their contribution will sum to zero thus there is no interference contribution. 

Detailed Analysis : 

OFDM uses data bursts to reduce the intersymbol interference many symbols are sent in parallel. Hence, the symbol rate decreases.

When the symbol duration increases, the intersymbol interference effects that were produced by Rayleigh fading are reduced.

The principle of orthogonality can be obtained by dividing the carrier signal with an integer multiple that is capable of making the inverse of symbol duration of the parallel bit stream.

The OFDM method is applied to reduce the frequency selective fading and burst errors generated by a wideband fading signal in wireless communication. It is robust in a narrow band interference environment.

Block diagram of OFDM :

In OFDM block diagram basically two parts : Transmitter and Receiver.

Assume that the system is operating on the downlink so that the transmitter is in the BS and receiver is in the mobile. BS is sending streams of bits to different mobile. It modulates each bit stream independently, using a different modulation scheme then passes each symbol stream through a serial-to-parallel converter to divide it into sub-streams. The number of sub-streams for each mobile depends on the data rate. The resulting information is the amplitude and phase of each sub-carrier in the form of a function of frequency. We can compute the in-phase and quadrature components of the corresponding time-domain waveform by passing it through an inverse FFT. This can be digitized, filtered and mixed up to radio frequency for transmission. The mobile reverse the process, starts by sampling the incoming signals, filtering it, and converting it down to baseband. After that passes the data through a forward FFT to recover the amplitude and phase of each sub-carrier. Using this knowledge, the mobile selects the require sub-carriers, recover the transmitted information and discarding the reminder. 

Block diagram of OFDM

Advantages of OFDM : 

  • Immunity to selective fading 
  • Resilience to interference
  • High spectrum efficiency 
  • Very resilient to inter-symbol and inter-frame interference 
  • Adequate channel coding and interleaving possible to recover symbols which are lost due to frequency selectivity of the channel and narrow-band interference. 
  • Simpler channel equalization 

Disadvantages of OFDM : 

  • High peak to average power ratio 
  • Sensitive to carrier offset and drift 
  • Loss of efficiency caused by cyclic prefix or guard interval 
For detailed information : 

OFDM variants : 

  • COFDM : 
COFDM stands for coded orthogonal frequency division multiplexing. It is the form of OFDM where error correction coding is incorporated into the signal. 
  • Flash OFDM : 
This variant was developed by Flarion. It is a fast hopped form of OFDM. 
  • OFDMA : 
OFDMA stands for orthogonal frequency division multiple access. It is used to provide multiple access capability for applications such as cellular telecommunications. 
  • VOFDM : 
VOFDM stands for vector OFDM. It uses the concept of MIMO technology. It is being developed by CISCO systems. 
  • WOFDM :
WOFDM stands for wideband OFDM. It uses a degree of spacing between the channels that is large enough that any frequency errors between transmitter and receiver don't affect the performance. This variant was particularly applicable to Wi-Fi systems.