Research Issues

There are various research issues for practical applications of OFDM technology. Some examples are as follows:

• Sync Detection (especially for burst Tx mode)
  Detection is a very classical problem in most communication systems. There are already a lot of detection algorithms which are known to show good performance. However, they require some modifications according to a communication system to which those algorithms are applied.
   

• Timing and Frequency Synchronization
  Timing and frequency synchronization techniques can be classified into “Post-FFT” and “Pre-FFT” approaches. Synchronization is closely related to the design of training symbols; however, in “Pre-FFT” approach, the correlation method by using cyclic prefix has been considered. Since OFDM is very sensitive to frequency offset, frequency synchronization is essential part in the design of an OFDM system
   

• Training Symbol/Sequence Design
  Training symbols/sequence may be required for timing synchronization, frequency synchronization or channel estimation. It is very important to design the training symbols/sequence which should be robust against timing error, frequency error and channel distortion. In addition, all training symbols must have low PAPR.
   

• Channel Estimation
  Channel information is essential for coherent detection, adaptive bit loading / modulation / power distribution, coherent space-time decoding, etc. Channel estimation can utilize both frequency and time correlation properties of the channel to achieve better estimation performance by using 2D Wiener (or MMSE) filter. It is also associated with training symbol design problem. Channel energy leakage due to non-sample-spaced delay may be one of major obstacles to achieve the accurate channel estimation.
   

• PAPR Reduction
  High PAPR is very critical problem in OFDM. There have been various researches on reducing PAPR of OFDM such as changing the phase of subcarrier or the order of transmit symbol so as to reduce PAPR, or applying various coding techniques. Predistortion technique is also one of PAPR reduction technique.
   

• Adaptive Modulation/Bit Loading/Power Allocation
  In maximizing the channel capacity, water filling problem can be solved by adaptive technology such as adaptive modulation/bit loading/power allocation. In this viewpoint, OFDM is favorable due to multiple narrowband subcarriers, but requires various strategies to adapt the time-varying channel.