3. Some subcarriers are severely distorted by frequency
selection fading and therefore cause high bit error rate
Leaving out weak subcarriers and/or adapting the
modulation level individually resulting in a better
BER performance.
Constant data rate case
Fixed information carrying subcarriers.
Variant data rate case
With the threshold design, the data are transmitted with the robust
Subcarrier.
III. Adaptive Subcarrier Allocation of OFDM
4. Subcarrier allocation schemes
leaving out weak subcarrier depends on ranking
the received subcarrier amplitude
leaving out weak subcarriers depend on phase
difference between demodulated signal and estimated
signal in the signal constellation
leaving out weak subcarriers depend on Euclidean
distance between demodulated signal and decided
signal in the signal constellation
leaving out weak subcarriers depend on ranking
the SNR value of received subcarrier.
If the channel is time variance, this allocation scheme
has to be repeated for each data transmission.
9. B.CHANNEL MODEL
Each impulses is faded by obeying a Rayleigh distribution
of a normalized maximal Doppler frequency of
Fd’=1.235*10^-5 where the normalization time duration
was the length of the OFDM symbol, rather than the input
bit duration.
10. C.CHANNEL ESTIMATION
The channel estimation on the basis of the received symbol
can be performed by pilot symbol assisted modulatio(PSAM),
or upon invoking more sophisticated methods, as decision-
directed channel estimation.
We will initially assume perfect knowledge of the channel
transfer function during the received timeslot.
11. D.CHOICE OF MODULATION
Three modulation scheme allocation algorithms were
investigated in the subbands…
1.Fixed threshold adaptation algorithm
2.Subband BER estimator adaptation algorithm
3.Constant throughput adaptive algorithm