利用數位訊號處理與分集技術於高通量可見光通訊系統

Abstract

近年來由於多媒體影音服務的蓬勃發展，其所需要的傳輸資料量也越來越大，因此對於現有的射頻訊號帶寬來講，因其所使用頻帶較小且受到國際規章…等等的限制，已漸漸不能夠滿足消費者的需求。因此光學無線通訊系統在近年來被提出，用來取代或彌補現有的無線通訊設施，因其本身具有頻寬廣闊、不受電磁波干擾．．．等等優點，且在近年來受到熱烈的研究與討論。 並且由於發光二極體固態照明技術的蓬勃發展，其夾帶著使用壽命長、高發光效率、低成本、體積小與反應快…等等優點，其已漸漸取代傳統白熾與螢光燈泡，並且也已與我們日常生活密不可分，已被廣泛利用在各式平面顯示器，照明設備，交通號誌與指示燈上。由於其使用涵蓋範圍廣泛，因此近年來可見光通訊便受到了很大的青睞，其期望在現有或未來的照明設備系統上不僅可以提供照明的效果，並同時能夠賦予無線通訊傳輸的功用，達到照明與互聯網的功能。 可見光通訊對於下世代家庭式區域無線網路與短距離無線傳輸也帶來許多的優點，如其被廣泛的使用性，高隱密性，不受電磁波干擾，可使用頻段寬廣，且不受國際規章限制…等等優點。其可以有效利用在一些易受電磁波干擾或危害的場所，如醫院,煉油廠,或機場等等，也可以利用在一些高隱密性環境，如軍事基地或金融機構…等等，並且由於可見光能夠被人眼所察覺，因此不像紅外線此類通訊頻段，會因過度強光造成人眼的傷害，並且在使用上也不會造成現今無線射頻通訊系統上多餘的負擔。 基於以上優點，可見光通訊對於下世代無線區域網路的發展有著很大前瞻性。現今可見光通訊系統的研究上大多是利用白光發光二極體做為傳輸的媒介，因為對於照明設備而言，白光發光二極體其有著較為普遍的利用性的特點，其主要是產生利用人眼無法察覺的高速明暗的閃爍訊號來達到傳輸的效果，然而對於可見光通訊系統來講，因其傳輸頻寬受到發光二極體本身可調變頻寬的限制，僅僅只有幾十兆赫(MHz)而已，這對於實際上要達到高品質影音無線傳輸服務，仍然是一個需要去克服的問題。 因此在本論文中，我們主要致力於研究應用各種高頻譜效率調變技術與數位訊號處理技術，來達到傳輸資料量的提升，並且我們也經由實驗證明其可行性。其中已成功達到 500 Mbps-2 m 以上的室內可見光通訊傳輸系統。而在於個人無線網路系統上，我們也成功利用光分波多工技術達到3 Gbps 以上的可見光無線傳輸。最後我們並討論多路徑可見光通訊所帶來的影響與問題，其中成功利用了空時分組碼(space-time block code)處理技術，解決了由傳輸線路所引起多路徑干擾與功率衰落的問題，並成功運用在3.2 公尺的室內可見光系統上。

In recent years, due to the increasing demand for wireless communication, the bandwidths of radio frequency (RF) communication gradually become very crowded, and are not enough to use for high-speed services. Therefore, optical wireless communications are proposed to compensate or replace the deficiencies of RF communication due to the advantage for widely available bandwidth. Recently, due to the rapid development of solid-state lighting, light emitting diode (LED) has attracted lots of attention to use in next generation illumination due to the advantages, such as longer lifetime, lower cost, smaller size, lower power consumption, and higher modulation speed. Therefore, visible light communication (VLC) is used as a novel wireless technique, which is used with LED to provide communication and illumination simultaneously. With broad deployment of LEDs, the ubiquitous communication will become feasible. However, in order to pursue the high brightness of LEDs to illumination, it is still a technically challenged to avoid high diffusion capacitance and resistance, which limit the modulation bandwidth of LEDs to tens or hundreds of megahertz. Therefore, high spectrally efficient modulation format is a straightforward way to increase the capacity under the limited bandwidth. In this thesis, the spectrally efficient modulation formats, such as carrier-less amplitude and phase (CAP) modulation, discrete multi-tone (DMT) modulation, single-carrier frequency division multiplexing (SC-FDM) are introduced to use in the VLC system. Moreover, in order to enhance the performance, several digital signal-processing techniques are also used such as the adaptive equalizers and the bit- and power-loading algorithm. Moreover, we have experimentally demonstrate the indoor LED-based VLC over 2-m wireless transmission, and the data rate up to 500 Mbps can be achieved. In addition, due to the widely available bandwidth of visible spectra, the wavelength division multiplexing (WDM) is also used in the VLC system, and the aggregated data can be achieved over 3.22 Gbps with 25 cm wireless transmission. In general, the performance of indoor VLC systems is very dependent on the arrangement of the relative locations of transmitters and receivers. In order to realize full-range illumination with wireless communication of consistent performance, multiple LEDs would be required to place in the appropriate positions. Therefore, the different lengths of electrical cables for the LED transmitted system may induce a serious interference. In order to mitigate the interference, the Alamouti STBC is proposed to use in the multi-LEDs system to avoid the serious power fading.