適用於毫米波通訊系統之波束成型以及前端數位接收器之演算法實現

Abstract

近年來高畫質影音的需求呈現爆炸性的成長，對於無線通訊系統所需要的傳輸速率也大幅地增加到每秒數個Giga位元等級。在需求如此高速的傳輸系統下，必須要有足夠的頻寬才能夠提供如此高的傳輸速率。而毫米波頻帶具有足夠提供數個Giga-Hz寬的傳輸頻帶的優勢，因此在最新的IEEE 802.11 ad無線通訊標準便將載波頻率設定在此頻段。然而，毫米波最大的缺點是，在空氣中傳輸時，具有高耗損的問題。除此之外，當傳輸頻寬到達Giga 位元等級，取樣時脈也變成非常高速。如何可以精準地同步接收機的符元時間以及頻率飄移，是一個非常高難度的問題。因此，在毫米波頻段的傳送以及接收機系統設計上面，有許多難題需要被解決。 此論文主要提出了適用於毫米波頻段基頻接收機之低複雜度及高效能的演算法設計，其中包含了通道估測、訊雜比估測、精密同步以及波束成形等技術，來解決目前在毫米波基頻接收機所碰到的各種難題。 針對通道估測以及訊雜比估測的問題上面，提出了兩個應用於毫米波頻帶之單載波系統通道及訊雜比的估測方法。先前方法提出利用互補格雷碼(complementary Golay sequences)的特殊特性來估測通道響應。之後在通道估測序列的欄位中，再利用這估測到的通道響應來得到訊雜比。為了能更進一步地改善估測精準度，第一個提出的估測方法是利用特殊的通道互補格雷碼的估測序列排列方式，然後得到多一組的互補格雷序列應用於通道響應估測。因此，和先前方法比較起來，此方法可以理論上減少25%的最小均方根估測誤差。除此之外，透過進一步地理論推導，我們發現在現有的雜訊比估測方法中和理論值會有偏移量，而嚴重地降低訊雜比的準確性。因此，為了解決這個問題，提出的第二個方法就是透過理論推導計算出這個對於偏移量更正值得理論值，然後再來補償。模擬結果也顯示出估測出通道響應的最小平方誤差(minimum square error, MSE)符合理論推導值。另外，藉由應用所提出的技巧，位元錯誤率的效能與先前方法相比可以大幅地減少。 對於應用在毫米波的多天線波束成型技術，主要是利用低硬體複雜度的波束編碼簿波束成型的(code-book beamforming)，利用此技術的好處就是對於需要高天線數目的毫米波通訊系統中，編碼簿波束成型的方法不需要很高複雜度的數位運算電路以及DAC/ADC(數位類比轉換器以及類比數位轉換器)。然而，相比於最佳的波束成型的演算的效能，波束編碼簿的效能有明顯地差異。因此，我們提出了一個基於碼書波束成型技術下之估測出發角度(angle-of-departure, AoD)以及入射角度(angle-of-arrival, AoA)的技術。利用估測出來的角度，來補償碼書波束向量，進而改善波束編碼簿的問題。經過模擬實驗證明，此一方法可以大幅地增加波束編碼簿的效能，甚至可逼近理想效能。 另一方面，在毫米波頻帶中所採用的兩種調變技術中，其中一種是單載波團塊傳輸(single-carrier block transmission, SCBT)的調變技術。在現有文獻的方法，對於估測剩餘載波頻率偏移(residual carrier frequency offset)以及符元取樣偏移(symbol timing offset)，是需要在傳送的資料中塞入額外的單一字元(unique word)來輔助接收機。但是，塞入額外的單一字元會降低系統的傳輸能力(throughput)。因此，此論文也針對單載波團塊傳輸系統，提出了一個新的聯合盲目估測(joint blind estimation)演算法(稱作simple weighted least square for single carrier systems, SWLS-SC)來估測剩餘載波頻率以及符元取樣的偏移。分析的結果顯示，此演算法可以在不需要單獨字元的輔助時，表現得非常接近理想估測。因此，利用此演算法可以增加單載波團塊傳輸系統的傳輸能力。

In this dissertation, high-performance and low-complexity front-end modem designs for SC-FDE systems operating in the mmWave band are proposed with emphasis on channel estimations, received SNR estimations, fine synchronizations and codebook-based beamforming issues. Regarding channel and SNR estimations, an efficient algorithm specifically for mmWave communication system is proposed. Existing algorithms firstly perform channel estimations and then extract SNR information by utilizing special properties of specifically arranged Golay sequences defined in latest system standards. The proposed algorithms bring up two methods that significantly enhance estimation accuracies of channel response and SNR information. The first method fully utilizes two defined sets of complementary Golay sequences for the estimations. Therefore, comparing with previous works only using one set of Golay sequence, the proposed method can theoretically reduce 25% of estimation error. In addition, through theoretical derivations, an estimation bias of SNR information is proved to be present in previous works. As a result, the second proposed method is derived targeting at removing the existing estimation bias. Hence, BER performance due to the proposed methods is greatly enhanced. Due to the nature of high propagation loss in mmWave band, high-performance and efficient beamforming are crucial technologies to meet high-throughput requirement in mmWave bands. Among existing techniques, optimal beamforming can provide the optimal antenna gain with the cost of a high number of ADCs/DACs and complicated matrix computations implemented in the digital circuitry. Hence, codebook-based beamforming (CB) is a better solution requiring only one set of DAC and ADC at both transmit and receive sides, respectively. However, codebook-based beamforming suffers performance degradation compared with the optimal one due to its lack of accurate AoD/AoA (angle-of-departure/arrival) information. In this dissertation, the proposed CB-AE (codebook-based beamforming, angle estimation) method estimates AoD/AoA information based on codebook-based beamforming so that the information can be used to calculate compensation terms for the beamforming vectors defined in the codebook. Simulation results show that the performance of CB-AE is very close to the optimal one with much less hardware cost. Finally, a fine synchronization algorithm for single-carrier frequency-domain equalization (SC-FDE) system, called simple weighted least-square for single carrier system (SWLS-SC) is proposed, which can accurately estimate symbol timing and residual carrier frequency offsets without additional unique word (UW) sequences. SC-FDE system is adopted as one of main modulation schemes operating in the mmWave band. Its main advantages over the OFDM system are lower PAPR, peak-to-average ratio, and robustness against carrier frequency offsets. However, fine synchronization schemes for SC-FDE systems are much less discussed in the literature than those for OFDM systems. Existing fine synchronization schemes for SC-FDE systems require additional UW sequences to estimate residual carrier frequency and symbol timing offsets. As a result, system throughput is decreased due to the additional insertion of UW sequences. Simulation results show that the proposed algorithm can perform very closely to the ideal estimation regardless of channel conditions. Theoretical estimation bounds are also derived to demonstrate its effectiveness.