In systems that use a fixed modulation scheme the carrier modulation must be designed to provide an acceptable BER under the worst channel conditions. This results in most systems using BPSK or QPSK. These give a poor spectral efficiency (1-2 bits/s/Hz) and provide an excess link margin most of the time. Using adaptive modulation, the remote stations can use a much higher modulation scheme when the radio channel is good. Thus as a remote station approaches the base station the modulation can be increased from 1 bits/s/Hz (BPSK) up to 4-6 bits/s/Hz (16QAM - 64QAM), significantly increasing the spectral efficiency of the overall system. Preliminary results show that for a cellular network the system capacity can potentially be doubled using adaptive modulation.
There are several limitations with adaptive modulation. Overhead information needs to be transferred, as both the transmitter and receiver must know what modulation is currently being used. Also as the mobility of the remote station is increased, the adaptive modulation process requires regular updates, further increasing the overhead.
There is a trade off between power control and adaptive modulation. If a remote station has a good channel path the transmitted power can be maintained and a high modulation scheme used (i.e. 64 QAM), or the power can be reduced and the modulation scheme reduced accordingly (i.e. QPSK).
參考答案:在使用固定调制方案的系统中，即使在最坏的通信环境下，载波调制的误码率也必须控制在可接受的范围内。所以大多数的系统采用BPSK或是QPSK传输，但频谱效率很低（仅1-2 bits/s/Hz)，并且大多数时候产生过度的联接差。在使用自适应调制的情况下，远处工作站可以在无线电频道良好时运用高得多的调制方案。因此，当远处工作站向主站靠近时，调制可以从1 bits/s/Hz (BPSK) 提高到 4-6 bits/s/Hz (16QAM - 64QAM),从而显著提高整个系统的频谱效率。初步结果显示，使用自适应调制后，一个蜂窝网络系统的工作能力理论上可以翻倍。
使用自适应调制有几处不便。由于发射机和接收机都需要知道当前正在使用的制式，必须传递信息。另外，由于远程工作站的灵活性增加，自适应调制过程需要经常更新，因此极大增加了信息量。
另外还应该考虑到电控和自适应调制之间的相互关联。 如果远程工作站使用良好的频道传输路径，可以保证传输的电力和高级调制方案的运用（比如64QAM）；电力降低时，调制方案也会响应降低（例如QPSK）。
