- Research Issues
- Cross Layer
- SDR 3000
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 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
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
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.
Mobile & Portable Radio Research Group
Tel: (540) 231-2971
FAX: (540) 231-2968