無線通訊技術的普及促使頻寬使用以及高傳輸速率(data rate)行動通訊的需求越來越大，再加上目前頻譜資源的短缺和其低使用率的情形，讓傳統頻譜分配的制度重新受到檢視。頻譜池(spectrum pooling)的觀念是將數個已授權頻段(licensed-band)組成共同的頻譜池(spectrum pool)，讓出租用戶(rental user)能在授權用戶(licensed user)的允許下存取這些授權頻段進而提升頻譜的使用率(spectral utilization)。然而，頻譜池的觀念需要改善原本授權用戶所使用的控制機制和硬體設備，因此降低了實做上的彈性(flexibility)和可行性(feasibility)。相較之下，感知無線電(cognitive radio)因為能讓這些未授權的使用者自發性地使用已被授權但空出來的頻譜、而不需經過授權用戶的允許，因此實行上比頻譜池更為方便且更能提高頻譜的使用率(spectrum efficiency)。 除此之外，感知無線電結合正交分頻多工(orthogonal frequency division multiplexing, OFDM)或正交分頻多工存取(orthogonal frequency division multiple access, OFDMA)的傳輸技術因為能夠彈性且適應性地存取頻譜，理論上能達到最大的頻譜使用率，此多載波的傳輸技術也有高傳輸速率和可靠傳輸的優點。因此，一個基於正交分頻多工/正交分頻多工存取傳輸的感知無線電系統儼然被認為是解決目前頻譜壅塞不足最有效的一個方法。
在本論文中，我們主要探討發展基於正交分頻多工/正交分頻多工存取傳輸的感知無線電系統所面臨的問題，進而提出相對應的解決方法。為了解決在頻譜感測(spectrum sensing)時，由授權用戶造成的強訊號干擾(strong interference)問題，提出一個適用於寬頻(wideband)環境、基於自由度(degree of freedom)的頻譜偵測器(spectral detector)，此偵測器可利用從頻譜估測(spectral estimation)所得到的自由度改善偵測效能(detection performance)，此外，所提出的偵測器亦可針對強訊號干擾的環境執行兩階段不同頻譜解析度(spectral resolution)的頻譜感測，此二階段分別為粗寬頻感測(coarse wideband sensing)和細窄頻感測(fine narrowband sensing)，用以提高對隱藏性主要用戶(hidden primary user)的偵測率，相較於傳統其他的方式如能量偵測(energy detection)或基於特徵值偵測(eigenvalue-based detection)，在寬頻強訊號干擾的環境中，所提出的偵測器有更強健(robust)的偵測效能表現。
另外，為了在感知無線電系統中達到可靠(reliable)且有彈性(flexible)的正交分頻多工傳輸，多工存取和同步(synchronization)，這兩個關鍵的技術和其問題就必須被考慮。在本論文中，我們提出了一個適用於多使用者的正交分頻多工存取系統，同時考量了上行(uplink)和下行(downlink)兩種傳輸情況。在下行部份，所提出的接收機(receiver)包含了載波漂移(carrier frequency offset, CFO)和取樣頻率偏移(sampling frequency offset, SFO)的估測及補償機制、通道估測及補償、符元時間同步，以及取樣增加或刪除(add/drop)這些功能。至於上行傳輸部份，我們提出一個可配置(configurable)於單輸入單輸出(single-input single output, SISO)以及多輸入多輸出 (multiple-input multiple-output, MIMO)的基頻處理器，此處理器能以正交分頻多工或者正交分頻多工存取來傳輸，且具有效率功耗(power efficient)的優點。為了解決在多使用者傳輸下的載波漂移問題，提出一個基於不同載波間干擾消除(inter-carrier interference-based)的載波漂移估測器，利用迭代搜尋最大訊號對干擾雜訊比(signal-to-interference-noise ratio, SINR)的方式分別估出各個使用者的載波漂移量，相較於其他演算法，此載波漂移估測器對於多輸入多輸出和多使用者傳輸的傳輸結構(transmission configuration)所遭受的載波漂移大小有較強建的估測效果。此外，針對複雜的不同載波間干擾以及多重擷取干擾(multiple access interference, MAI)消除等運算，使用泰勒展開式(Taylor series expansion)對干擾重建做化簡，提出一個比直接實作減少78%硬體複雜度的硬體架構。通道估測則採取簡單的二維線性估測法，有效地幫助載波漂移估測以及追蹤時變的多通道(time-variant multipath channel)。在多輸入多輸出偵測器(MIMO detector)方面，採用兩種不同演算法，貝爾實驗室垂直式分層時空(vertical Bell Laboratory layered space-time, V-BLAST)方法和貝爾實驗室垂直式分層時空搭配最大似然法(maximum likelihood, ML)的簡化最大似然偵測法(V-ML)，分別運用在載波漂移估測和資料偵測兩階段，以同時達到減少輸出延遲(output latency)和接近理想最大似然法的位元錯誤率(bit-error-rate, BER)偵測效能的結果。上行接收機裡主要的功能模組(包含不同載波間干擾以及多重擷取干擾的消除器、載波漂移估測器、通道估測器和多輸入多輸出偵測器等)透過台積電(Taiwan Semiconductor Manufacturing Company, TSMC)的0.13 μm 1P8M製程下線，製作成專用積體電路(application-specific integrated circuit, ASIC)，量測結果顯示，所製作的晶片有高達2.31 Mbps/mW的功耗效率以及相較於理論值低於1.5 dB的實做誤差(位元錯誤率的效能)。整個所提出的上行收發機(transceiver)更整合於系統單晶片(system-on-chip, SoC)平台做最後的驗證和展示以證明所提出系統的高度整合效力及其實用性。

Ubiquitous wireless applications drive the growing demand for more spectrum access opportunity and higher data rate mobile communications. The scarcity of spectrum resources and the poor spectrum utilization urges the re-examination of old spectrum allocation policies. The concept of spectrum pooling enables public access to a common spectrum pool which is comprised of several licensed frequency bands hosted by licensed users. Rental users can access the unused frequency bands in the common pool with the permission of the licensed users. However, spectrum pooling requires the improvement of control and infrastructure framework for the licensed users, which makes the implementation inflexible. On the contrary, cognitive radio (CR), which allows unlicensed users (or usually called secondary users in CR networks) to access the available spectrum autonomously, provides a more feasible solution to improve spectrum efficiency. Moreover, a CR system incorporating orthogonal frequency division multiplexing
(OFDM)/orthogonal frequency division multiple access (OFDMA) transmission can perform spectrum access in a highly flexible and adaptable manner to maximize spectrum efficiency; meanwhile, the multicarrier modulation technique can also achieve a reliable high data rate transmission. Therefore, an OFDM/OFDMA-based CR system
has become a promising solution to solve the spectrum crowding and spectrum deficiency problems.
In this dissertation, design challenges of an OFDM/OFDMA-based CR system are investigated. To overcome the interference problem caused by licensed users (or usually called primary users in CR networks) in spectrum sensing, a degree of freedom (DoF)-based spectral detector that is applicable to wideband strong interference environments is proposed. By exploiting the DoF obtained from spectral estimation, the proposed detector provides a higher DoF for the improvement of detection performance. Besides, the detector can perform two-phase sensing with adjustable spectral resolution. This makes the system robust to strong interference environments and compatible with wideband spectrum sensing applications.
Moreover, to achieve a reliable and flexible OFDM transmission in a CR system, multiple access schemes and synchronization issues should be considered. In this dissertation, a multiuser OFDMA system, which can offer both uplink and downlink communications, is presented. For downlink communications, the proposed receiver consists of carrier frequency offset (CFO)/sampling frequency offset (SFO) estimation/compensation, channel estimation/equalization, and sampling add/drop function for symbol timing synchronization. As for uplink communications, a configurable (for single-input singleoutput (SISO)/multiple-input multiple-output (MIMO) and OFDM/OFDMA communications) and power efficient multiuser MIMO-OFDMA baseband processor will be considered. To solve the CFO problem in multiuser transmission, an inter-carrier interference (ICI)-based CFO estimator is implemented based on an iterative search criterion of maximum signal-to-interference-plus-noise ratio (SINR). Compared to state-of-theart methods, the proposed CFO estimator is more robust to transmission configurations
(MIMO and multiuser) and CFO variations. Moreover, the authors propose an efficient architecture that reduces the hardware complexity by 78% compared to the direct implementation architecture by employing Taylor series expansion for ICI/multipleaccess interference (MAI) cancellation. Meanwhile, a 2D linear channel estimator is also proposed to assist the CFO estimator and track the time-variant multipath channel. Two kinds of MIMO detector, vertical Bell Laboratory layered space-time (V-BLAST) and
V-BLAST with maximum likelihood (V-ML), are adopted to minimize output latency and achieve the best ML bit-error-rate (BER) performance. An application-specific integrated
circuit (ASIC) fabricated by 0.13 μm 1P8M CMOS technology is measured with 2.31 Mbps/mW power efficiency and less than 1.5 dB implementation loss. In addition, the whole transceiver is integrated and verified by a system-on-chip (SoC) platform to demonstrate its efficacy.

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