Dear Sanjana,

For any communication channel you have to estimate the energy per bit to the noise density ration Eb/ N0. This ratio is limiting of the quality of the transmission channel since it determines the bit error rate. It is so that the minimum required bit error rate depends on modulation technique and the type of the channel. It is so that the required Eb/N0 for certain bit error rate increases with increasing order M of the modulation techniue from PSK, 4QAM,, 8 QAM..etc. Eb/N0 for certain bit error rate increase also with type of channel from AWGN additive white Gaussian noise channel, to Rician to Rayleigh fading channel. If you have line of sight then your channel will be AWGN. and you will find in the literature error performance formulas and curves for all modulation techniques. The error performance can be improved by channel coding where parity bits are added to the data to detect and correct transmission errors. Also, the EB/ N0 gain for many channl coding techniques are also studied i the literature.

So, in order to decide for certain modulation order you have to measure Eb/N0 of the channel and assign lowest value for 4 QAM and highest value to higher order modulation where the threshold i satisfied.

To get exactly the required range you can consult the book ADVANCED ELECTRONICS COMMUNICATION SYSTEMS by G Tomasi or any other standard book in digital communication. You can also build your models and calculate the required threshold values of Eb/N0 for different modulation techniques.

wish you success 

I agree with the answer of Mr. Abdulhalim

How do I choose the adaptive modulation in visible light communication?

Amplitude and phase of optical carrier is modulated in optical communication systems. Adaptive modulation means to use a modulation level commensurate with BER in the receiver. If the received signal strength is large high-level modulation (16 level or higher) can be used. On the other hand, if received signal strength is weak, low level modulation (two level) can be used. Therefore, based upon the received signal strength a modulation scheme is selected.

What is the range of BER to be obtained by adaptive modulation?

The BER in receiver is generally set as 10-6 or better. Adaptive modulation helps to achieve this goal irrespective of change in received signal strength. For example, modulation scheme can be changed from on-off keying to 16 QAM to achieve required BER.