在传统OFDM系统中实现自适应调制需要传输大量的有关子载波调制参数的信息，导致传输效率的下降。本文分析了采用数据扩展技术的自适应调制OFDM系统在全自由度条件下的特征，说明最优的算法就是在原始数据符号之间平均分配比特和功率。在此基础上提出了以用户需求为目标的最小化总功率的自适应调制算法，原始数据符号最多采用两种调制阶数，并且可以在原始数据符号之间按顺序分配。这样不仅降低了算法复杂度，而且大大减少了有关调制参数信息的传输。仿真结果表明，该算法具有良好的性能，在复杂度相近的前提下，比采用自适应功率控制的传统OFDM系统节省功率约2dB。

In traditional orthogonal frequency division multiplexing （OFDM） systems, the realization of adaptive modulation needs to transfer a great deal of sub-carrier modulation parameters, which may decrease the transmission efficiency. In order to solve this problem, this paper analyzes the characteristics of adaptive modulation OFDM systems with data spread, and points out that an optimal algorithm is to assign the bits and power among original data symbols averagely. From this viewpoint, an adaptive modulation algorithm orientated to users＇ requirements with a minimum total power is proposed. In this new algorithm, two different constellations are adopted at most to assign the original data symbols in order. Thus, the complexity of the algorithm is greatly reduced and the transmission of modulation parameters is obviously decreased. Simulated results indicate that the proposed algorithm is good in performance, and that it saves about 2 dB power （represented by the average signal-to-noise ratio of the sub-carrier） , as compared with the traditional adaptive power control-based OFDM system with similar complexity.