Which Is the Better Adaptive or Fixed Modulation


Adaptive or Fixed Modulation 

Wireless communications


Which better Adaptive or Fixed Modulation ?


This experiment to made to know which is the better  adaptive modulation or fixed modulation 
We assume that the channel conditions are known for a few milliseconds ahead in time.The experiment is applicable to both TDD and FDD systems,provided that accurate predictions of the channel conditions exist.For each prediction of the SNR at the receiver, the modulation scheme is selected for 512 consecutive symbols.This implies that the channel estimator and the predictor work at a rate of (BW/512) where BW is the channel bandwidth.The data bit stream is then modulated and transmitted over the noisy channel.white Gaussian Noise (AWGN) with varying variance is added to simulate good and bad channel conditions.
At the receiver, the signal is demodulated and the obtained bit stream is compared to the original one. The number of errors is counted, as well as the number of transmitted bits. 


The first three columns in the figure belong to three transmissions using 
adaptive modulation with different error probability Thresholds. As expected, the adaptive modulation approach results in a relatively constant (adjustable) error rate. On the other hand, the use of non-adaptive modulation results in high peaks in the error rate when the receiver encounters a fading dip.
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