Linearization of Terahertz Transmitter Based on Low Sampling Rate DAC and ADC
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摘要: 太赫兹(THz)频率高、带宽大，是6G移动通信中极具优势的潜在无线频谱资源。然而太赫兹器件的非线性失真，限制了功率转换效率与通信传输距离。若采用传统数字预失真(DPD)技术对其进行非线性校正，通常要求数模转换器(DAC)和模数转换器(ADC)的采样速率达到信号带宽的5倍，对于太赫兹频段难以应用。因此，该文提出一种低速率DAC和ADC的DPD算法对太赫兹发射机的非线性进行校正。该方法主要分为3个步骤：首先利用低采样率ADC获取的观测数据进行上采样，恢复出带宽受限的高采样率的观测信号，此时信号采样率为信号带宽的5倍，可以有效表征出5阶非线性失真；然后建立带宽受限的DPD模型，采用最小二乘算法提取DPD校正系数；最后对校正后的信号进行下采样送往DAC以校正发射通道的非线性失真。仿真结果表明，当DAC和ADC工作在1.25倍基带信号速率的采样率条件下，对于64-QAM调制信号，该方法可以把误差矢量幅值(EVM)从8.46%降低到2.27%，从而可以支持更高阶的调制方式。Abstract: TeraHertz (THz) with high frequency and large bandwidth is an advantageous potential wireless spectrum resource in Sixth Generation (6G) mobile communication. However, the nonlinear distortion of THz devices limits the power conversion efficiency and the communication transmission distance. If traditional Digital Pre-Distortion (DPD) technology is used for non-linear correction, the sampling rate of the Digital-to-Analog Converter (DAC) and Analog-to-Digital Converter (ADC) is usually required to reach 5 times the signal bandwidth, which is difficult to apply to the THz frequency band. Therefore, in this paper, a digital correction method is proposed to correct the nonlinearity of the THz transmitter using the low-rate DAC and ADC. This method is mainly divided into three steps: Firstly, the observation data obtained by the low sampling rate ADC is used, and the high sampling rate observation signal with limited bandwidth is recovered by up-sampling. At this time, the signal sampling rate is 5 times the signal bandwidth, which can effectively characterize the 5th order nonlinear distortion; Then the DPD model with limited bandwidth is established to extract the DPD correction coefficient; Finally, the corrected signal is down-sampled to the DAC to correct the nonlinear distortion of the transmitter channel. The simulation results show that when DAC and ADC work at the sampling rate of 1.25 times the baseband signal rate, for a 64-QAM modulated signal, the Error Vector Magnitude (EVM) can be reduced from 8.46% to 2.27%. Therefore, modulation schemes with higher spectrum efficiency can be adopted in THz communications.
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