将射频技术应用到自由空间光通信具有独到的优势。为了研究自由空间光载射频技术，在四进制频移键控调制解调理论基础上，以自由空间光通信为应用背景，采用仿真，从时域角度对四进制频移键控系统进行了编码及解码研究，并推导了经过高斯噪声和瑞利信道后系统误比特率及信道容量。两种解调法的系统误比特率仿真结果表明，当信噪比为12dB时，过零检测法的系统误比特率达到10-15，明显优于相干解调法。

It takes advantages of free space optical communication(FSO)to transmit RF signal, which is proved to have special predominance. In order to research radio over free space optics technology, under the background of FSO, the simulation was adopted to study coding and decoding system based on the theory of 4 frequency-shift keying modulation and demodulation. The error rate and channel capacity after the RF signal passing Gaussian noise and Rayleigh channel were also analyzed. The simulation results on bit error rate(BER)of two demodulation methods show that the zero crossing detection system has the better BER than the coherent demodulation system when the signal-to-noise ratio is 12dB.