Adaptive OFDM_Orthogonal Frequency Dvision Multiplexing_ Continuation part_3

1. Presented by
Harish Rajula
P14EC009
Sv_Nit-surat
harish.rajula91@gmail.com
Continuation on AOFDM_presentation3

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.

5. The performance comparison for
different subcarrier allocation
schemes

6. IV. Adaptive Modulation of OFDM
 A. SYSTEM MODEL
 B. CHANNEL MODEL
 C. CHANNEL ESTIMATION
 D. CHOICE OF MODULATION SCHEME

7. A.SYSTEM MODEL

8. Contd…

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

12. 1.Fixed threshold adaptation algorithm

14. 2.Subband BER estimator adaptation algorithm

17. Contd…

18. Simulation…