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 caseFixed information carrying subcarriers.
Variant data rate caseWith the threshold design, the data are transmitted with the robustSubcarrier.
III. Adaptive Subcarrier Allocation of OFDM
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.
The performance comparison for different subcarrier allocation schemes
IV. Adaptive Modulation of OFDM
A. SYSTEM MODEL B. CHANNEL MODEL C. CHANNEL ESTIMATION D. CHOICE OF MODULATION SCHEME
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.
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.
D.CHOICE OF MODULATION
Three modulation scheme allocation algorithms were investigated in the subbands…