多輸入多輸出與適應性調變技術在正交分頻多工系統的應用

Abstract

多輸入多輸出(Multiple-Input Multiple-Output, MIMO)為使用多天線於傳送和接收端的可靠通訊技術，並被認為是符合第四代高速通訊需求的最佳方案之一。近來MIMO技術的發展趨勢主要分為兩類：一為空間分集，另一為空間多工。另一方面，正交分頻多工（Orthogonal Frequency Division Multiplexing, OFDM）為一種具高頻譜效益，並能有效克服多路徑衰落效應的調變技術。在本論文中，吾人將探討結合MIMO與OFDM之通訊系統架構。基於OFDM上子通道等效於平坦式衰落通道之特性，MIMO所提供的分集與多工將可以多種形式呈現在OFDM系統上。此外，在無線寬頻傳輸中，頻率選擇效衰落應促使OFDM中相隔大於同調頻寬的子通道可擬似於相隔大於半波長的天線。基於此一觀點，吾人提出一種簡易的空時頻區塊編碼技術，藉以有效地利用頻率分集。吾人更進一步針對MIMO-OFDM提出一種適應性傳收架構及位元負載程序，使其能夠隨時間動態地在頻率與空間通道上調整傳輸參數，如：調變階數、編碼率以及傳輸能量，以便充分地利用空間、時間以及頻率通道上的特性以維持系統的目標錯誤率。此種架構的特色之一在於可視需要彈性地獲取分集與多工兩種增益。最後，吾人藉由電腦模擬驗證上述架構在室內無線環境中具有優異的傳輸表現。

Multiple-input multiple-output (MIMO) is a promising technique suited to the increasing demand for high-performance 4G broadband wireless communications with multiple antennas at both transmitter and receiver side. The recent researches on MIMO techniques can be categorized into two major types: one is spatial diversity (SD) and the other is spatial multiplexing (SM). On the other hand, orthogonal frequency division multiplexing (OFDM) is a high spectral efficiency modulation technique that could deal efficiently with multipath fading effects. In this thesis, we consider a new wireless communication system combining MIMO and OFDM, called the MIMO-OFDM system. Base on the flat fading channels introduced by OFDM tones, SD and SM may come in various formats in the OFDM systems. Besides, in wideband transmission, due to frequency selective fading, the equivalence between antennas and OFDM-tones holds for those well-separated tones (> coherence bandwidth) and antennas (> half wavelength). From this point of view, we propose a type of simple space-time-frequency code that can extract the frequency diversity effectively. Furthermore, to fully exploit the channel properties in the space-time-frequency wireless channels, we propose an adaptive MIMO-OFDM transceiver architecture along with a designed loading procedure to dynamically adjust the transmission parameters such as modulation order, coding rate and transmit power over spatial and frequency channels, according to the instantaneous channel statistics, to meet the target BER. Such transceiver scheme enjoys both the diversity and multiplexing gain in a flexible manner. Finally, we evaluate the performance of the proposed systems, and confirm that they work well in a typical indoor environment.