多重輸入輸出正交分頻多工基頻收發機之設計與實現

Abstract

本研究探討了4×4多重輸入輸出正交分頻多工無線通訊系統與進階的接收技術，其中包含了抗I/Q不平衡效應的自動頻率調整器、訓練序列輔助的I/Q不平衡效應偵測、適應性通道偵測、以及數位波束成型。 為了偵測在I/Q不平衡效應干擾下的載波頻率偏移量，發展了一個基於虛擬載波頻率偏移技巧的載波頻率偏移偵測演算法。此演算法透過注入虛擬的載波頻率於三個連續的訓練序列來改善載波頻率偏移偵測的精準度。從模擬的結果中，演算法的偵測錯誤量大約是0.3 ppm，並且低於傳統載波頻率偏移偵測演算法。透過0.13-μm CMOS製程，此演算法實現在一個測試晶片上。此測試晶片的面積為3.3×0.4 mm^2，耗電量為10 mW。 在直接降頻的架構下，必須同時考慮頻率相關I/Q不平衡與載波頻率偏移效應。本文提出一個利用訓練序列來偵測載波頻率偏移干擾下的I/Q不平衡效應。在載波頻率偏移干擾下的頻率相關I/Q不平衡效應可透過資料子載波與干擾子載波之間的關係偵測出來。模擬與實驗平台的結果顯示提出的方法可以有效改善系統效能。此外本文所提出的方法也相容於目前的無線區網標準。 最近對於移動下的無線通訊需求有增加的趨勢。為了達到高效能的接收機，就需要具備快速的通道追蹤能力。越快取得精確的通道狀態資訊並達到成功的傳輸是非常重要的。針對4×4多重輸入輸出正交分頻多工無線通訊系統，我們發展一個在時變通道下的適應性頻域通道偵測器。此適應性頻域通道偵測器利用每四組符號來確保通道偵測的精準度。為了降低硬體複雜度，利用了Alamouti矩陣的特性來設計有效率的VLSI架構。最後透過0.13-μm CMOS製程，此適應性頻域通道偵測器的面積為3×3.1 mm^2，另外4×4多重輸入輸出正交分頻多工無線通訊系統晶片在1.2V電壓下耗電量為62.8 mW。 此外，本研究也探討了數位波束成型在多重輸入輸出正交分頻多工通訊系統上的應用。數位波束成型是一種方向性濾波的技術，能有效的消除不必要的干擾，並正確的接收訊號。因此，數位波束成型技術可用來增加系統的容量與傳輸距離。 本研究探討收發機設計與元件實作。此外也建構了一個基於軟體定義的收發機平台來提供快速的原型機驗證。透過整體的效能指標評估，可在實作權衡上有效取得平衡。

In this study, a 4×4 multi-input multi-output (MIMO) orthogonal-frequency division multiplexing (OFDM) communication system and advanced receiving techniques, including anti-I/Q mismatch (IQ-M) auto frequency controller, preamble-assisted IQ-M estimator and compensator, adaptive channel estimator, and digital beamforming are explored. In order to estimate the carrier frequency offset (CFO) value under the conditions of IQ-M for direct-conversion structures, a pseudo-CFO (P-CFO) algorithm is developed. The proposed P-CFO algorithm rotates three training symbols by adding extra frequency offset into the received sequence to improve CFO estimation. Simulation results indicate that the estimation error of the proposed method is about 0.3 ppm, which is lower than those of two-repeat preamble-based methods. The proposed scheme is implemented as part of an OFDM wireless receiver fabricated in a 0.13-μm CMOS process with 3.3×0.4 mm^2 core area and 10 mW power consumption. In direct-conversion receivers, the impact of frequency-dependent IQ-M with CFO must be considered. A preamble-assisted estimation is developed to circumvent the frequency-dependent IQ-M with CFO. The frequency-dependent IQ-M with CFO can be estimated by taking advantage of the relationship between desired sub-carriers and image sub-carriers. Both simulation and experiment results indicate that the proposed method can meet system requirements by preventing frequency-dependent IQ-M from significantly degrading the performance. Moreover, the proposed scheme is compatible with current wireless local area network standards. Recently, the request for wireless communication under mobile conditions is increased. The ability of fast channel tracking is therefore needed to achieve high performance receivers. For successful transmissions, obtaining accurate channel state information as soon as possible is extremely important. An adaptive frequency-domain channel estimator (FD-CE) for equalization of 4×4 MIMO-OFDM system in time-varying frequency-selective fading is developed. The proposed adaptive FD-CE ensures the channel estimation accuracy in each set of four MIMO-OFDM symbols. To decrease complexity, the rich feature of Alamouti-like matrix is exploited to derive an efficient VLSI solution. Finally, this adaptive FD-CE using an in-house 0.13-μm CMOS library occupies an area of 3×3.1 mm^2, and the 4×4 MIMO-OFDM modem consumes about 62.8 mW at 1.2V supply voltage. In addition, digital beamforming for MIMO-OFDM communications is also studied. Digital beamforming is a method of spatial filtering, which can eliminate unwanted interferences and receive desired signal accurately. Consequently, digital beamforming can be used to increase system capacity and transmission range. In this study, the transceiver design and circuit implementation are presented. A software-defined radio is also constructed for rapid verification and fast prototyping. Based on the overall system performance index, the implementation trade-offs can be balanced.