移动衰落信道中的自适应OFDM调制

在宽带移动OFDM(正交频分复用)系统中,不同的子信道经受不同的信道衰落,具有不同的传输能力。若采用固定速率调制方案,信道容量和发送功率未能获得充分应用。自适应调制技术能充分利用信道容量和信号功率,满足不同传输速率和服务质量的要求。研究了一种适用于宽带移动OFDM系统的自适应算法。在平均发送功率不变条件下,算法根据子信道的衰落特性,自适应地分配子信道中数据比特,选择不同的信号星座和发送功率,使得系统的功率和频谱效率达到最佳。首先推导了在平均发送功率受限条件下,瑞利衰落信道中最大频谱效率;其次,分析了在给定误比特率(BER)条件下,采用连续功率、连续星座MQAM(多进制正交幅度调制信号)调制的最大功率频谱效率,在此基础上提出适用于实际系统的离散功率、离散星座MQAM调制算法;最后进一步优化系统的功率密度谱。计算机模拟表明:该算法能明显地提高移动OFDM系统的频谱效率。

Adaptive Modulation in OFDM System over Mobile Fading Channel

In broadband Orthogonal Frequency Division Multiplexing (OFDM) system over mobile fading channels, each individual subchannel has its own attenuation, which has different channel capacity. If a fixed modulation scheme is applied to all subchannels, the whole system capacity and the transmitted power have not been full used. This problem can be mitigated if adaptive modulation schemes are employed for individual OFDM subchannels. In this paper, an adaptive modulation algorithm of MQAM in OFDM system over mobile fading channel is investigated. The algorithm distributes the information bits, the signal constellations and transmitting power adaptively to different subchannels according to channel instantaneous attenuations to satisfy average power and BER constraints. The maximum spectral efficiency in Rayleigh fading channel under average power constraint is first deduced. Secondly, the maximum spectral efficiency of MQAM with continuous constellation and power is investigated. Then, a modify scheme of discrete constellation and power MQAM modulation for a practical OFDM system is proposed. Finally, we further optimize the power distribution over the whole system. The simulations show that the adaptive scheme improves the spectral efficiency of OFDM system evidently comparing with the one using fixed modulation scheme.