In the code division multiple access ( cdma ) system , the multiple access interference ( mai ) mitigation capability as well as system capacity are determined , to a great extent , by the correlation properties of spreading sequences ; while adaptive modulation and coding ( amc ) with high spectral efficiency is the key technique to obtain high speed and low symbol error rate ( ser ) performance in mobile data transmission 在码分多址( cdma )系统中，扩频序列的特性在很大程度上决定了抗多址干扰( mai )能力和系统容量，而高频谱效率的自适应调制编码( amc )技术则是获得高速率和低误码率( ser )移动数据传输性能的关键因素。
Firstly , in wireless environment , receiver has no know the time delay between transmitter and itself . in ofdm system , ofdm data is transmitted as one symbol , so in order for correct demodulation , the correct start of symbol should be got from ofdm system to reduce the effect of isi . secondly , the high frequency utilization in ofdm is based on the orthogonal properties of sub carriers . if frequency offset is generated , the orthogonal properties among sub carriers will lost , that will cause inter carrier interference ( ici ) and high ber of system Ofdm技术也存在着一些缺陷,首先,在无线环境中传输的ofdm信号对定时要求高,为了减少码间串扰( isi )影响,必须从接收信号中提取出正确的符号起始位;其次, ofdm系统对频率偏移敏感, ofdm技术的高频谱利用率和传输可靠性均以子载波的正交性为基础,如果接收机和发射机之间发生频率偏移,子载波之间就会失去正交性,导致严重的子载波间干扰( ici ) ,降低系统性能。
