The invention relates to an adaptive modulation (AM) method design and performance evaluation for a 
wireless distributed multi-antenna 
system (DAS). Under the constraint condition of a target 
bit error rate (BER), the 
system frequency spectrum efficiency is maximized to respectively design two AM schemes of a discrete rate and a continuous rate. By aiming at the discrete rate AM, an 
adaptive switching threshold based on precise BER is provided. Compared with the traditional AM based on an approximate threshold, the AM method disclosed by the invention can obtain 
high frequency spectrum efficiency. A channel model comprising 
path loss, 
shadow fading and 
Rayleigh fading is designed by considering the practical situation of DAS. On the basis, the computation expression of a continuous rate AM 
frequency spectrum efficiency, a discrete rate AM 
frequency spectrum efficiency and the average BER is given so as to provide an 
effective method for 
system performance evaluation. An 
Matlab (matrix laboratory) 
simulation platform shows that the designed AM method can satisfy the target BER requirement, and the continuous rate AM method is better than a discrete rate AM method. In addition, due to the providing of an improved threshold, the discrete rate AM can be used for realizing the 
high frequency spectrum efficiency if being compared with the traditional AM method. In addition, the provided frequency spectrum efficiency has good consistency with the average BER calculation and 
simulation, and the system performance can be effectively evaluated.