使用過取樣低解析度數位類比轉換器之手機發射機

Abstract

此論文展示了一基於過取樣高速數位類比轉換器之手機射頻發射機設計、實踐和驗證於第四代及第五代行動通訊技術。隨著通訊信號的進步，其所支援的傳輸頻寛亦發增加，並產生更高的峰均功率比。此一趨勢使得發射機效率和線性度下降、且增加設計難度與硬體成本。因此在無線射頻發射機上便有了研究的空間，並使其符合現代第四代及第五代行動通訊技術 。 在首次的研究上，本論文提出了一個應用於第四代寛頻手機通訊之低解析度雙相位脈衝調變極座標發射機。藉由極座標發射機於信號上的特性，配合雙相位的設計以消除奇次諧波並有效的降低極座標發射機所需要的解析度，其在不需要額外的數位預失真技術下得以通過第四代無線通訊的線性度要求。基於此架構下，接著驗證與現行第五代無線通訊的相容性。第五代無線通訊相較於第四代無線通訊有著較高的峰均功率比與更為嚴格的線性度規範，故此論文深度探討了設計雙相位脈衝調變極座標發射機所需要的過取樣率、切換頻率及數位類比轉換器的解析度。其中亦探討了極座標發射機與傳統發射機於線性度與解析度上的差異。此架構得以通過第五代無線通訊的線性度要求，其於低解析度設計下與無其它線性化技術時，誤差向量幅度可達-29.89 dB。 本論文接續提出一使用抖動技術之直接數字合成射頻發射機，其大幅改善傳統I/Q調變發射機於低解析度時的非線性現象。傳統射頻發射機中的數位類比轉換器操作於低解度時會產生嚴重的量化誤差，進而使基頻通訊信號產生互調失真而導致射頻發射機的線性度下降。基於直接數字合成與演算法之特性，此架構同時支援多個5G頻段。本論文提出之架構使用一低解析度射頻數位類比轉換器做為射頻發射機的核心、數字合成系統來推動一寛頻功率放大器完成驗證。相較於傳統I/Q調變發射機，此架構無需任何的數位校正技術，且具有較小面積、低功耗及低複雜度之特性，並同時符合第五代無線通訊系統於多個頻段的線性度要求。 本論文最後提出一基於直接數字合成抖動技術之極低解析度雙相位脈衝調變極座標發射機。相比於先前提出之雙相位脈衝調變極座標發射機，此架構使用一低漣波脈衝調變技術使得調變後的基頻訊號具有一較窄的頻寛，並降低使用非寛帶功率放大器其窄頻匹配網路產生的雜訊。從模擬和實際頻譜分析儀得知，此架構在使用一對二位元數位類比轉換器情況下可通過5G手持式行動裝置嚴格的線性度要求。 最後，本論文所提出之架構將有益於低成本射頻發射機的開發，其特點相當符合物聯網之應用與建置。

This thesis presents several new transmitter architectures based on oversampling high-speed digital-analog converters (DACs). With the advancement of the communication signal, both the transmission bandwidth and peak average power ratio have increased. This trend has reduced the efficiency and linearity of the transmitters significantly and increased design difficulty and hardware cost. Therefore, new transmitter architectures that meet the linearity requirements of new communication standard are in great demand, which has led to this thesis. The first part of the thesis proposes a low-resolution dual-phase pulse-modulated polar transmitter (PMPT) using a wideband orthogonal frequency-division multiplexing (OFDM) signal, which is compatible with 4G/5G mobile applications. Combined with the characteristics of the pulse-modulated signal and the dual-phase design, this work can eliminate odd harmonics and effectively reduce the DACs’ resolution in the PMPT architecture. The pulse switching frequency, oversampling ratio (OSR), and the resolution of the DACs are further discussed in this section. The difference between the PMPT and the conventional transmitter in linearity and resolution is also explained and examined on the testbed. This architecture can meet the linearity requirements of the 5G standard without additional linearization techniques. The second part of the thesis proposes a direct digital synthetic (DDS) RF transmitter using dithering technique, which dramatically improves the non-linear behavior of low-resolution conventional transmitters. The resolution of baseband I/Q DACs mainly determines the conventional RF transmitters' linearity. The severe quantization errors introduce intermodulation distortion to low-resolution DACs and decrease the RF transmitter's linearity. The intermodulation error can be effectively suppressed by using the dithering technique and improving signal quality. The architecture utilizes a low-resolution RF DAC in addition to a wideband power amplifier (PA) to achieve multiband application. Compared with the conventional I/Q transmitter, this architecture does not require digital pre-distortion (DPD) technology. It has the advantages of a small area, lower power consumption, and low complexity while supporting multiple 5G frequency bands. The third part of the thesis proposes an ultra-low-resolution DDS PMPT using dithering technology. Compared with the previous dual-phase PMPT, this architecture uses a Gibbs-reduction digital pulse-width modulation (GRDPWM) signal to achieve a smaller frequency bandwidth requirement. It reduces the noise caused by a narrow-band input matching network. This architecture utilizes a pair of 2-bit RF DACs for the verification testbed and meets the rigorous linearity requirements in 5G mobile applications. The architectures presented in this thesis will benefit the development of low-cost RF transmitters, whose characteristics are especially suitable for Internet of Things (IoTs) applications where cost and power consumption are the main interests.