新世代光纖載微波系統與技術之整合研究-子計畫一：W-頻帶微波光纖通信系統

Abstract

無線通信的歷史非常的悠久，在約100 年前，Marconi 提出無線通信的概念之後，在100 年內，由於 IC 的進步，終於使得個人化的無線通信得以實現。隨著對無線傳輸的需求大量 的增加，對於能夠提供大量傳輸無線信號的系統逐漸受到大家廣泛的注意。在最近幾年對於 超高速的無線通信系統 (> 1Gbps)的研究更是在加速的進行。以60 GHz 的系統為例，最近所 提出正在制定的標準就有: IEEE 802.15.3c, ECMA TC48 (ECMA 387), wireless HD/HDMI, Wigig, IEEE 802.11 VHT 等等。而對60 GHz 的 IC 研究，不論是在以 SiGe 或是以 CMOS，都有明顯 顯著的進步。在商品化方面，有已經有產品問世 (http://www.sibeam.com)。但是60 GHz 的 主要應用是短距離的無線傳輸 (<20m)，因此對一般距離 (1~2 km)需要大量資訊量的無線傳 輸，仍然是十分缺乏。因此以 W‐band 為主的傳輸系統，不但能夠傳輸HD 畫質的影音信號， 無線傳輸距離也可以達到 1‐5 km。就成為能夠滿足此一需求的最佳系統。 而有線截取網路 (wireline access network) 的演進是由xDSL、到各種規格的 PON (passive optical network, broadband‐PON, Gigabit‐PON, and Ethernet‐PON)。其目標是為了提供使用者所 謂的triple‐play services (voice, video and internet)。然而有線網路雖然能提供寬頻服務，但是 無法滿足使用者需要 anytime, anywhere 的機動性 (mobility) 要求。另外一方面，無線網路 雖然能提供非常好的機動性，但是由於頻寬的限制，無法完全滿足使用者所需要的寬頻服務。 為了因應此發展，最好的解決方法之一就是使用光纖當作傳輸媒介，同時傳送有線與無線的 擷取服務。讓網路能更接近使用者，來解決頻寬不足的困境。以光纖為主幹的通訊系統不但 可以有效的舒緩目前無線通訊頻寬不足的窘境，也是目前所知最能滿足未來頻寬要求 (future proof) 的傳輸媒介。因此整合光纖到家 (fiber‐to‐the‐home, FTTH 提供 triple‐play services, voice, video and internet) 以及radio‐over‐fiber (ROF, 提供無線的語音以及網路服務) 的混成 光纖擷取網路架構 (hybrid optical access network) 已經逐漸被大家所認可是最具有經濟效益 的網路架構。因此在本計畫中，我們將提出W‐band RoF 的系統實驗，來驗證並且建立一個 高速 (> 4Gbps)、遠距 (>2 km) 的無線傳輸系統。

The tremendous increase in the necessary bandwidth of wireless data‐transmission has attracted attention on ways to use the millimeter wave (MMW) bands at 60 GHz (V‐band) or above 100 GHz (W‐band) as the carrier frequency for the realization of systems with very high transmission data rates over many gigabits‐per‐second. Several ways to realize such a system have been proposed. By using the mature CMOS integrated‐circuit (IC) technology, a V‐band transceiver module, which includes a printed‐circuit antenna, a 60GHz power amplifier, a local oscillator (LO), and an envelope detector, has been demonstrated (http://www.sibeam.com). However, a large propagation loss of the MMW signal occurs in the W‐band or V‐band frequencies, whether in free space or in coaxial cable and the connection or synchronization between different transmitters thus become a serious issue by use of such technique especially when there are several emitters in the system. A promising solution to overcome such problem is the radio‐over‐fiber (ROF) technique, in which the MMW LO signal and data are both distributed through a low‐loss optical fiber and then radiated over the last‐mile to the end‐user. In this project, to generate MMW signals that support even higher frequency applications, a filterless optical MMW signal generation system with frequency octupling is proposed. Two cascaded frequency quadrupling systems are keys to the proposed octupling system. The W‐band 100‐GHz MMW signals are experimentally demonstrated in this work. Since no narrow band optical filter is required, the proposed system can also be used in WDM systems. The idea of utilizing the nonlinearity of high‐speed PDs to serve as the optoelectronic (OE) mixer, such as UTC‐PDs, and realize this up‐conversion process is very attractive, because this could eliminate the necessity of the high‐frequency electronic mixer or E‐O modulator. Compared to the p‐i‐n PD based OE mixer, the UTC‐PD based OE mixer has much lower up‐conversion loss and higher up‐converted RF power due to its superior high‐speed and high‐power performance to p‐i‐n PDs. However, such a technique requires the UTC‐PD to be under zero‐bias and the AC voltage amplitude swings the device into forward bias regime, which may seriously limit the modulation bandwidth. By inserting an additional p‐type charge layer into the collector layer to control the electrical field, a higher electron drift‐velocity, higher saturation‐current bandwidth product, than those of reported UTC‐PD have been achieved. Furthermore, NBUTC‐PDs based OE mixers and photonic transmitter‐mixers have demonstrated low up‐conversion loss, wide modulation bandwidth, and QPSK wireless data transmission at W‐band (100 GHz) under bias modulation without using the forward bias operation to enhance the nonlinearity as for the case of UTC‐PD. In this project, we will demonstrate a >4Gbps, > 2 km 100 GHz RoF system based on UTC‐PD with integrated antenna design.