Orthogonal Frequency Division Multiplexing (OFDM)

Orthogonal Frequency Division Multiplexing (OFDM)

      Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier modulation technique. OFDM provides high bandwidth efficiency because the carriers are orthogonal to each other and multiple carriers share the data among themselves. The main advantage of this transmission technique is their robustness to channel fading in wireless communication environment. 

     OFDM is an attractive modulation scheme used in broadband wireless systems that encounter large delay spreads. OFDM avoids temporal equalization altogether, using a cyclic prefix technique with a small penalty in channel capacity.

  

      

Where Line-of-Sight (LoS) cannot be achieved, there is likely to be significant multipath dispersion, which could limit the maximum data rate. Technologies like OFDM are probably best placed to overcome these, allowing nearly arbitrary data rates on dispersive channels.

     OFDM signals have a large peak-to-average power ratio (PAPR) due to the superposition of all subcarrier signals. Therefore, in each Transmitter the power amplifier will limit the OFDM signal by its maximal output power. This also disturbs the orthogonality between subcarriers, leading to both intercarrier and out-of-band interferences, which is unacceptable.

The major advantage of OFDM is its robustness against multi path propagation. Thus, it is suitable to be implemented in wireless environments. The introduction of cyclic prefix made OFDM system resistance to time dispersion. OFDM symbol rate is low since a data stream is divided into several parallel streams before transmission. This make the fading is slow enough for the channel to be considered as constant during one OFDM symbol interval.

   

Cyclic prefix is a crucial feature of OFDM used to combat the inter-symbol interference (ISI) and inter-channel-interference (ICI) introduced by the multi-path channel through which the signal is propagated. The basic idea is to replicate part of the OFDM time-domain waveform from the back to the front to create a guard period. The duration of the guard period Tg should be longer than the worst-case delay spread of the target multi-path environment. The use

of a cyclic prefix instead of a plain guard interval, simplifies the channel equalization in the demodulator.

      One of the major disadvantages of OFDM is its requirement for high peak-to average power ratio. This put high demand on linearity in amplifiers.

     Second, the synchronization error can destroy the orthogonality and cause interference. Phase noise error and Doppler shift can cause degradation to OFDM system. A lot of effort is required to design accurate frequency synchronizers for OFDM.

Summery for Adv. & DisAdv. of OFDM :

   Advantages:

             • N narrowband transmissions are done, this way is easily to accomplish that Ts (of     each channel) is larger than T (of the channel), this means the transmission is not affected by the channel.

            • High transmission bitrates.

            • Chance to cancel any cannel if is affected by fading.

    Disadvantages:

           • High synchronism accuracy.

           • Multipath propagation must be avoided in other orthogonallity not beaffected.

           • Large peak-to-mean power ratio due to the superposition of all subcarrier signals, this can become a distortion problem.