Adaptive OFDM_Orthogonal Frequency Dvision Multiplexing_ Continuation part_3

Adaptive OFDM SYSTEM

Presented byHarish Rajula

P14EC009Sv_Nit-surat

harish.rajula91@gmail.com

Continuation on AOFDM_presentation3

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

A.SYSTEM MODEL

Contd…

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.

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.

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

1.Fixed threshold adaptation algorithm

2.Subband BER estimator adaptation algorithm

3.Constant throughput adaptive algorithm

Contd…

Simulation…